WO2011050345A1

WO2011050345A1 - Catalyst materials for reforming carbon dioxide and related devices, systems, and methods

Info

Publication number: WO2011050345A1
Application number: PCT/US2010/053880
Authority: WO
Inventors: Giancarlo Corti; Timothy C. Cantrell; Miles F. Beaux; Tejasvi Prakash; David N. Mcilroy; Grant M. Norton
Original assignee: Gonano Technologies, Inc.
Priority date: 2009-10-23
Filing date: 2010-10-22
Publication date: 2011-04-28
Also published as: WO2011050345A8

Abstract

The present teachings relate to catalyst materials based on a composite of nanoscale components. These catalyst materials can be used for the photocatalytic reduction Of CO₂. The present teachings also relate to devices, apparatus, systems, and methods that incorporate a catalayst material based on a composite of nanoscale components. The devices, apparatus, systems, and methods can be used to reform CO₂ into partially reduced products (such as methanol or formaldehyde) at high conversion efficiences and selectivity.

Description

CATALYST MATERIALS FOR REFORMING CARBON DIOXIDE AND RELATED

DEVICES, SYSTEMS, AND METHODS

Cross Reference to Related Applications

[0001] This application claims priority to and the benefit of U.S. Provisional Patent Application Serial No. 61/254,205, filed on October 23, 2009, the disclosure of which is incorporated by reference herein in its entirety.

Field

[0002] The present teachings relate to photocatalytic reduction of carbon dioxide.

Background

[0003] Fossil fuels are expected to remain a primary source of energy into the foreseeable future. Unfortunately, carbon dioxide (C0₂) emissions generated from burning fossil fuels have increased drastically over recent years. Most scientists agree that atmospheric C0₂ concentrations will continue to increase at an alarming rate unless current and future energy systems can reduce their carbon emissions significantly.

[0004] In the United States, it is estimated that one third of all C0₂ emissions come from large single sources such as power or industrial plants. There is consensus in the

international community that a reduction of greenhouse gas (GHG) concentrations must occur to avoid future health and environmental damage.

[0005] Several approaches to stabilize and reduce GHG concentrations have been tested. Capture followed by sequestration and storage is the most common method in which C0₂ is sequestered. However, safety concerns have arisen due to possible underwater contamination and sudden C0₂ escape back into the atmosphere, which could have fatal consequences. Staving off increasing C0₂ emissions due to higher power consumption and larger restrictions to GHG emissions by world governments require alternatives to C0₂

sequestration.

[0006] Effective alternatives to physical sequestration of C0₂ must be capable of adapting to actual industry and power plant stacks without major modification, so that energy costs do not suffer a large increase.

[0007] Accordingly, the art continues to seek advances in the use of catalysts for reforming C0₂. Summary

[0008] The present teachings provide catalyst materials and related devices, systems, and methods for reforming carbon dioxide.

[0009] Despite active investigation of titanium oxide (Ti0₂) as a photocatalyst for the photoreduction of C0₂, typical conversion efficiencies remain at about 10% or less.

[0010] Therefore, it was unexpected when the inventors discovered that C0₂ could be photoreduced over supported Ti0₂ nanocrystals at much higher conversion efficiencies (e.g., at a -30% conversion rate) than previously reported. Specifically, the high conversion efficiencies were observed with catalyst materials comprising Ti0₂ nanocrystals

discontinuously deposited on a mat of nanostructures. Apart from the high conversion rates, these catalyst materials also can be used to reduce C0₂ into selective products, and photoreactors based on these catalyst materials can be integrated easily into existing industrial systems, where a waste stream of carbon dioxide can be converted on site into useful feedstocks such as methanol and formaldehyde.

[0011] In one aspect, the present teachings provide catalyst materials that can be used to reform carbon dioxide in a photoreduction reaction. The present catalyst materials include nanocrystals of a photocatalytic semiconductor material supported on a mat of nanostructures ("nanostructure mat"). The photocatalytic nanocrystals optionally can be doped and/or coated with metal nanoparticles to provide enhanced catalytic activity. In preferred embodiments, the present catalyst materials include optionally doped Ti0₂ nanocrystals conformally formed on individual nanostructures of the nanostructure mat as a discontinuous layer, where greater than about 90% of the Ti0₂ nanocrystals are in the anatase phase. For example, the Ti0₂ nanocrystals can be doped with a dopant selected from N, F, Fe³⁺, Mo⁵⁺, Ru³⁺, Os³⁺, Re⁵⁺, V⁴⁺, and Rh³⁺. In some embodiments, nanoparticles of a catalytic metal such as Pt, Ru, or Cu can be deposited on the Ti0₂ nanocrystals and/or the nanostructures to enhance the catalytic activity of the catalyst materials. In various embodiments, the nanostructure mat can include nanostructures (nanosprings and optionally nanowires) composed of Si0₂, and the Ti0₂ nanocrystals can be deposited on the surface of the Si0₂ nanostructures such that at least 10% of the surface of the Si0₂ nanostructures remains exposed. Individual Ti0₂ nanocrystals can be spaced apart at an average distance of about 20 nm to about 60 nm to increase the phase boundary between the Ti0₂ nanocrystals and the Si0₂ nanostructures. The Ti0₂ nanocrystals can have an average particle size of about 20 nm to about 80 nm.

[0012] The present catalyst materials can be prepared as follows. To prepare the nanostructure mat, one or more precursor materials of the nanostructures can be introduced into a heated chamber containing a substrate material that has been pre-treated to provide a surface coated with a catalytic thin film. The precursor materials typically are in gaseous form or have sufficiently low boiling points such that they can form a sustained vapor inside the chamber without heating to extreme temperatures. The gaseous precursor materials are absorbed by the liquid thin film, and nanostructures begin to grow once the precursor materials reach a critical concentration within the catalytic thin film. In certain embodiments, one or more reaction parameters can be controlled to favor the growth of helical

(nanosprings) over linear nanostructures (nano wires). In various embodiments, the nanostructure mat can include densely distributed nanosprings, where individual nanosprings can be formed from either a single nanowire or multiple nanowires. In some embodiments, the thickness of the catalytic thin film can be modulated to vary the surface coverage density and/or thickness of the nanostructure mat, and/or the dimensions (e.g., diameter) of individual nanostructures. Typically, the nanostructure mat can comprise a disordered array of nanosprings.

[0013] Subsequently, nanocrystals of a photocatalytic semiconductor material can be grown on the nanostructure mat. The nanocrystals can be deposited as a conformal thin film on individual nanostructures. In preferred embodiments, atomic layer deposition is used to provide an extremely conformal and uniform coating of the photocatalytic semiconductor material on individual nanostructures of the nanostructure mat. The photocatalytic semiconductor material can be polymorphous, and the composition of the nanostructures and/or one or more deposition parameters can be selected to favor a particular crystal form. Without wishing to be bound by any particular theory, it is believed that the use of the nanostructure mat as the support for the nanocrystals in itself also can favor their formation in a particular crystal form. In certain embodiments, substantially all the nanocrystals deposited on the nanostructures can be of a single crystal form. In addition, one or more deposition parameters can be controlled to ensure that the nanocrystals are of a narrow size range and/or to vary the surface coverage density of the nanocrystals on the nanostructures. In preferred embodiments, Ti0₂ nanocrystals are deposited by atomic layer deposition on a mat of Si0₂ nanosprings, where substantially all of the nanocrystals are in the anatase form. The nanocrystals can form both along the outer wall of individual nanosprings and along their internal wall (i.e., inside the coil), but can be sparsely distributed such that gaps are present between any two nanocrystals. The resulting anatase-coated nanospring mat can provide a network of highly accessible catalytic sites and phase boundary, thereby enabling

photoreduction of C0₂ with improved conversion efficiency and/or product selectivity.

[0014] In some embodiments, the nanocrystal-coated nanostructure mat can be modified further through the deposition of metal or metal alloy nanoparticles onto the surfaces of the nanocrystals and/or the nanostructure mat. The nanoparticles can be deposited through any number of means including chemical synthesis in solution (reduction of an aqueous precursor), chemical vapor deposition (optionally plasma-enhanced), and laser ablation. In particular embodiments, nanoparticles composed of a metal selected from Pt, Ru, and Cu can be deposited on the above-described Ti0₂ nanocrystals and/or Si0₂ nanosprings.

Modification with such nanoparticles can further enhance the photocatalytic activity of the Si0₂ nanospring-supported Ti0₂ nanocrystals.

[0015] In another aspect, the present teachings provide a photocatalysis device for reforming carbon dioxide, wherein the photocatalytic material described above, more specifically, the nanostructure mat, is attached to a porous (e.g., gas-permeable) substrate. In preferred embodiments, the nanostructure mat can be grown directly on the porous substrate. The porous substrate can be composed of a transparent material (e.g., glass). For example, suitable substrates can include fiberglass mesh and glass frit. In some embodiments, the porous substrate can have a pore size sufficient to allow water or other liquid to pass through. Depending on the structure of the porous substrate and its transparency (or opacity), the nanostructure mat can be deposited on the substrate or disposed within the substrate.

[0016] In yet another aspect, the present teachings relate to an apparatus (a photoreactor) for photoreducing carbon dioxide that includes the photocatalysis device described herein. The apparatus can be configured as a continuous flow reactor which can be of different geometries (e.g., tubular or planar). In most embodiments, the reactor defines a linear flow path, although more complicated gas flow patterns can be used. For example, the reactor can be an elongated vessel or a planar packed-bed reactor in which the photocatalysis device is disposed along the flow path. One or more inlet lines and outlet lines are provided respectively for introducing a reactant stream into the reaction chamber and extracting a product stream out of the reaction chamber. Inside the reactor, the photocatalysis device is positioned to be exposed to a light source that emits one or more wavelengths within the ultraviolet-visible spectrum. The light source can be external to the reactor (e.g., sunlight), in which case, the reactor includes a light transmittance surface (a window) positioned to expose the photocatalysis device to light. The window can be made of fused quartz or other materials transparent to UV or UV-Vis light. In some embodiments, the reactor can include an internal light source (e.g., a UV lamp). In some embodiments, the photoreactor can include both an internal light source and a window for transmitting light from an external light source. In some embodiments, heating elements can be disposed around or adjacent the reactor to provide temperature control inside the reactor.

[0017] In a further aspect, the present teachings relate to methods of reforming carbon dioxide. More specifically, the present methods involve the photoreduction of carbon dioxide using the present photocatalysis device via reaction with water and/or hydrogen gas. In various embodiments, the present methods can be practiced in a way that will lead to the selective formation of predefined products. For example, in some embodiments, the present methods can allow the selective formation of methanol, while in other embodiments, the present methods can allow the selective formation of formaldehyde. The present methods also can be implemented as a continuous process (as opposed to the conventional batch process), and can take place in mild conditions (at STP, or 1 atm and 25°C).

[0018] The present teachings also encompass a system including one or more photoreactors described above, where the one or more photoreactors are connected in-line with an existing industrial source of C0₂. For example, the reactors can be connected to an exhaust stack from a coal-fired power plant, or more preferably, to an existing scrubber system.

[0019] The foregoing as well as other features and advantages of the present teachings will be more fully understood from the following description, the accompanying drawings, and the appended claims.

Brief Description of Drawings

[0020] It should be understood that the drawings described below are for illustration purpose only. The drawings are not necessarily to scale, with emphasis generally being placed upon illustrating the principles of the present teachings. The drawings are not intended to limit the scope of the present teachings in any way.

[0021] Figure 1 is a schematic diagram illustrating an exemplary photoreactor

incorporating a photocatalysis device according to the present teachings. [0022] Figure 2 is a schematic diagram illustrating a photocatalysis device according to the present teachings, which comprises a mat of nanostructures coated with a discontinuous layer of photocatalytic nanocrystals, where the mat is attached to a porous substrate.

[0023] Figure 3 is a schematic diagram showing an array of photoreactors that could be implemented into an exhaust stream.

[0024] Figure 4 is a scanning electron microscopy image of a nanostructure mat.

[0025] Figure 5 is a transmission electron microscopy image showing a portion of a nanocrystal-coated nanostructure according to the present teachings, more specifically, a Ti0₂-coated Si0₂ nanospring.

[0026] Figure 6 is an X-ray diffraction (XRD) pattern confirming that Ti0₂ nanocrystals formed on a silica nanospring are mostly in the anatase phase.

[0027] Figure 7 is a scanning electron microscopy image of a nanostructure mat comprising densely packed Si0₂ nanosprings formed inside the pore structure of a porous glass compact ("frit").

[0028] Figure 8 is a representative mass spectrum confirming reduction of C0₂ into methanol (at about 40% conversion rate) over a photocatalysis device according to the present teachings upon exposure to UVA radiation at about 395 nm.

[0029] Figure 9 compares the amount of methanol reformed from C0₂ with 100 of water over a photocatalysis device according to the present teachings before and after UV light exposure.

[0030] Figure 10 compares the amount of methanol reformed from C0₂ with 400 mL over a photocatalysis device according to the present teachings before and after UV light exposure.

[0031] Figure 11 is a schematic diagram of a planar continuous flow reactor according to the present teachings.

[0032] Figure 12 is a representative gas chromatogram showing conversion of dissolved C0₂ in H₂0 into methanol.

[0033] Figure 13 is a representative gas chromatogram showing conversion of dissolved C0₂ and methanol in H₂0 into formaldehyde and formic acid.

[0034] Figure 14 shows the dependence of C0₂ conversion efficiences with respect to the weight ratio of Ti0₂ photocatalysis to the Si0₂ nanospring (NS) support. [0035] Figure 15 are gas chromatograms obtained in comparative experiments where instead of using photocatalytic materials according to the present teachings, a control catalyst that includes a mat of silica nanostructures coated with a contiguous layer of Ti0₂ nanocrystals was used.

Detailed Description

[0036] Described herein are catalyst materials, and related devices, systems, and methods for reforming carbon dioxide (C0₂). More specifically, the present materials, devices, systems, and methods can be used to achieve photoreduction of C0₂ at high conversion efficiencies and product selectivity. For example, in some embodiments, the present materials, devices, systems, and methods can be used to photoreduce C0₂ selectively into partially reduced products (i.e., compounds including at least one oxygen atom). In addition to high conversion efficiencies and product selectivity, the present teachings also provide advantages including mild reaction conditions, scalability, and facile integration into existing industrial sources of C0₂.

[0037] Throughout the application, where compositions are described as having, including, or comprising specific components, or where processes are described as having, including, or comprising specific process steps, it is contemplated that compositions of the present teachings also consist essentially of, or consist of, the recited components, and that the processes of the present teachings also consist essentially of, or consist of, the recited process steps.

[0038] In the application, where an element or component is said to be included in and/or selected from a list of recited elements or components, it should be understood that the element or component can be any one of the recited elements or components, or the element or component can be selected from a group consisting of two or more of the recited elements or components. Further, it should be understood that elements and/or features of a composition, an apparatus, or a method described herein can be combined in a variety of ways without departing from the spirit and scope of the present teachings, whether explicit or implicit herein.

[0039] The use of the terms "include," "includes", "including," "have," "has," or "having" should be generally understood as open-ended and non-limiting unless specifically stated otherwise. [0040] The use of the singular herein includes the plural (and vice versa) unless specifically stated otherwise. In addition, where the use of the term "about" is before a quantitative value, the present teachings also include the specific quantitative value itself, unless specifically stated otherwise. As used herein, the term "about" refers to a ±10% variation from the nominal value unless otherwise indicated or inferred.

[0041] At various places in the present specification, certain properties or characteristics are quantified in ranges. It is specifically intended that the description includes each and every individual subcombination of the members of such ranges.

[0042] It should be understood that the order of steps or order for performing certain actions is immaterial so long as the present teachings remain operable. Moreover, two or more steps or actions may be conducted simultaneously.

[0043] Photoreduction of C0₂ to organic compounds in the presence of a semiconductor catalyst was first reported in 1979. The reaction can be performed by either dissolving C0₂ in water (i.e., in liquid phase) or saturating C0₂ with a water vapor (i.e., in gas phase).

Various semiconductor materials have been used as the photocatalyst. Examples of such semiconductor materials include metal oxides such as titanium oxide (Ti0₂), zinc oxide (ZnO), zirconium oxide (Zr0₂), tin oxide (Sn0₂), tungsten oxide (W0₃), molybdenum oxide (M0O3), and cerium oxide (Ce0₂); metal chalcogenides such as zinc sulfide (ZnS), cadmium sulfide (CdS), cadmium selenide (CdSe), tungsten sulfide (WS₂), and molybdenum sulfide (MoS₂); ceramics such as silicon carbide (SiC); and gallium phosphide (GaP).

[0044] In particular, Ti0₂ has been extensively studied due to its high stability towards photocorrosion, cost effectiveness, and favorable band gap energy. Ti0₂ exhibits three distinct polymorphs, among which the brookite phase is not photocatalytic. The anatase phase possesses a band gap of 3.2 eV with the absorption edge approximately at 387 nm which lies in the near ultraviolet (UV) range. The rutile phase has a lower band gap of 3.02 eV with the absorption edge in the visible range at 416 nm. However, despite numerous studies using UV illumination both in gas-phase and in water, the C0₂ conversion rates reported with various forms of Ti0₂-based catalysts remain low. For example, as reviewed in Varghese et al, Nano Lett., 9(2): 731-737 (2009), the typical conversion yield is in the order of 10^"6 mol per gram of catalyst per hour (or μιηοΐ / (g cat. hr)), and the conversion efficiency is limited to about 10% or less. In addition, approximately 60-80%) of the C0₂ converted is methane, which is less desirable than partially reduced products such as methanol and formaldehyde.

[0045] Both methanol and formaldehyde are used extensively and highly desired by chemical, textile, resin, and paint industries. In addition, methanol has potential as a cleaner fuel than fossil fuels. Methanol is a key chemical feedstock that finds extensive use in the production of formaldehyde, acetic acid, and other chemicals, with formaldehyde production accounting for more than one-third of all methanol demand. Changing demand patterns have witnessed growth of methanol consumption in the fuel market. Currently, about 90% of the worldwide production of methanol is derived from methane, the main component of natural gas.

[0046] Current methods of producing methanol involve two stages. First, methane is converted into syngas (which is a mixture of primarily carbon monoxide and hydrogen) using one of three common processes, namely steam reforming, partial oxidation, or autothermal reforming (which combines the steam reforming process with the partial oxidation process). Then, a second catalyzed reaction is used to react carbon monoxide with hydrogen to produce methanol. Although both steps have become more efficient over time, the elimination of the syngas step could save money, given that it currently accounts for up to 70%> of the cost of making methanol. Furthermore, the production of methanol from C0₂ and CO instead of methane would decrease demand for natural gas.

[0047] Likewise, formaldehyde is mainly produced from methanol and accounts for up to 40% consumption of the worldwide methanol production. By producing separately methanol and formaldehyde, the demand for methanol in chemical industries therefore could be reduced by 40%> and the methanol resulting from the increase in supply could be used as an alternative clean burning fuel or for other applications in the chemical industry.

[0048] Thus, it was surprising when the inventors observed that when photoreduction of C0₂ was performed in the presence of a photocatalytic material that includes nanocrystals of Ti0₂ deposited on a mat of nanostructures, a conversion efficiency of about 30%> or even higher could be achieved (compared to less than 10% in prior art methods). It also was observed that a highly pure partially reduced product such as methanol or formaldehyde could be obtained by varying the composition of the reactants and/or reaction parameters.

[0049] Accordingly, in one aspect, the present teachings provide a photocatalytic material which can be used to reform carbon dioxide. The photocatalytic material generally includes a mat of nanostructures, where the individual nanostructures are coated with nanocrystals of Ti0₂.

[0050] As used herein, the term "nanostructures" is meant to encompass any structure having at least one dimension of about 300 nm or smaller, and the term "nanocrystals" is meant to encompass any crystal having at least one dimension of about 100 nm or smaller. A "mat of nanostructures" or a "nano structure mat" is meant to encompass all three- dimensional frameworks of nanostructures in which the nanostructures exhibit some degree of intertwining or entanglement. For example, a mat of nanostructures can be described alternatively as a mesh of nanostructures, a net of nanostructures, a matrix of nanostructures or the like.

[0051] The nanostructures described herein can have any suitable shape and/or dimensions. For example, the present nanostructures can include nanosprings, nanowires, nanorods, nanotubes, or any combination thereof. In addition, the nanostructures can be single-strand or bundled together (e.g., coiled or twisted around one another) to form multi-strand nanostructures. The nanostructures generally have a large aspect ratio. Accordingly, the nanostructures can range from less than about a micron to about 10 microns in length, and have a cross-sectional dimension from about 5 nm to about 500 nm. Within a mat or mesh, nanostructures having a substantial variation in cross-sectional dimensions can be present. For example, within a mat or mesh, nanostructures having a cross-sectional dimension from about 5 nm to about 300 nm, from about 5 nm to about 200 nm, from about 5 nm to about 150 nm, from about 5 nm to about 100 nm, from about 10 nm to about 300 nm, from about 10 nm to about 200 nm, from about 10 nm to about 150 nm, from about 20 nm to about 300 nm, from about 20 nm to about 200 nm, from about 20 nm to about 150 nm, from about 50 nm to about 300 nm, from about 50 nm to about 200 nm, or from about 50 nm to about 150 nm can be present. The nanostructures can have any suitable cross-sectional shape, for example, round, oval, hexagonal, elongated (e.g., ribbon-like), and the like. Further, individual nanostructures may or may not demonstrate an ordering within a nanostructure mat. In most cases, the nanostructures form an array of intertwined nanostructures that exhibits a high degree of disorder.

[0052] In some embodiments, the mat of nanostructures can include both nanowires (linear) and nanosprings (helical), with at least about 50%, at least about 60%>, at least about 70%>, at least about 80%>, or at least about 90%> of the nanostructures being nanosprings. Among the nanosprings, there can be both single-strand nanosprings and multi-strand (but usually < 10) nanosprings. The nano wires can have a width ranging between about 10 nm and about 100 nm. The nanosprings can have a diameter ranging between 30 nm and about 300 nm with a pitch generally in the same order as the diameter, that is, also between about 30 nm and about 300 nm. For example, the nanosprings can have a diameter in a range from about 30 nm to about 60 nm, from about 60 nm to about 90 nm, from about 90 nm to about 120 nm, from about 120 nm to about 150 nm, from about 150 nm to about 180 nm, from about 180 nm to about 210 nm, from about 210 nm to about 240 nm, from about 240 nm to about 270 nm, from about 270 nm to about 300 nm. In preferred embodiments, the nanosprings can have a diameter in a range from about 100 nm to about 300 nm.

[0053] The nanostructure mat typically includes a high surface coverage density (>90% coverage with nanostructures), thereby providing a very high surface area (-500-1000 m²/g) for the deposition of the photocatalytic nanocrystals. As used herein, a high surface area generally means a surface with at least about 10 m² of surface for every gram of material, and more specifically of about 100 m² to about 2,000 m² per gram of material. Yet, the nanostructure mat usually includes a large number of channels, gaps, openings, and/or other spacing both within an individual nanostructure and between nanostructures, through which reactant molecules of a specific chemical reaction can pass and thus reach the surface of the nanocrystals. Therefore, catalytic sites on the nanocrystals remain highly accessible despite the high surface coverage density of the nanostructures. Where the nanostructure mat includes a large percentage of nanosprings, additional spacing is provided between coils and by the annular space within each nanospring, thereby providing an even larger active surface area that is highly accessible to the reactant molecules. In particular, because of the coiling, nanosprings typically have orders of magnitude more surface area per unit length relative to a linear nano wire.

[0054] Both nanosprings and nanowires can be synthesized via what is known as vapor- liquid-solid (VLS) growth. See e.g., Mcllroy et al, J. Phys.: Condens. Matter, 16, R415- R440 (2004), the entire disclosure of which is incorporated by reference herein for all purposes. VLS growth occurs when a catalyst (e.g., a metal or a metal alloy) deposited on a substrate surface absorbs precursor materials (for forming the nanostructures) from a surrounding vapor. In conventional VLS growth, the catalyst is deposited onto the substrate as droplets of nanometer scale diameters. These droplets are isolated from other droplets of catalyst on the substrate, and as a result demonstrate a reduced melting point relative to a bulk material of identical composition. Once the droplets are deposited onto the material the pre-treated substrate is heated in a chamber with the precursor materials to a temperature sufficient to generate a sustained vapor pressure of the precursor materials (typically > 900°C). The gaseous precursors diffuse into the liquid metal droplet until a critical concentration (super-saturation) is reached, at which time excess materials are secreted out of the droplet base to the liquid (catalyst) / solid (substrate) interface, and a nanowire gradually forms beneath the droplet.

[0055] Nanosprings are obtained from the helical growth of one or more nanowires. To induce helical growth, some mechanism must exist that introduces an asymmetry to the growth of the nanostructure. Without wishing to be bound by any particular theory, it is believed that, in the case of nanosprings formed from a single amorphous nanowire, it is the existence of contact angle anisotropy at the interface between the nanowire and the catalyst that introduces the asymmetry. For nanosprings formed from multiple nanowires, it is postulated that competition between the multiple nanowires can provide the requisite asymmetry.

[0056] In preferred embodiments, the nanospring mat can be formed by a modified VLS growth process that is described in WO 2007/002369 and US 2009-0000192, the disclosure of each of which is incorporated by reference herein in its entirety. Briefly, instead of having the catalytic material deposited as nanodroplets, the catalytic material is deposited as a thin film on the substrate. Therefore, in some variations, a nanostructure mat can be formed by pre-treating the substrate by depositing a thin film catalyst on the substrate, heating the pre- treated substrate together with gaseous, liquid, and/or solid nanostructure precursor material or materials, and then cooling slowly under a relatively constant flow of an inert gas to room temperature. If more than one precursor material is used, the precursor materials can be added in a serial or parallel manner.

[0057] The concentration of precursor material(s) and/or heating time of the pretreated substrate together with the precursor material(s) can be varied to adjust properties of the resultant mat of nanostructures (e.g., mat thickness and/or nanostructure density). Typical heating times are from about 15 minutes to about 60 minutes. Molecular or elemental precursors that exist as gases or low boiling liquids or solids can be used so that processing temperatures as low as about 350°C (c.f. > 900°C) can be used. The processing temperature can be sufficiently high for the thin film catalyst to melt, and for the molecular or elemental precursor to decompose into the desired components. [0058] The thin film catalyst can be applied to the substrate using any suitable method. For example, thin films of metal or metal allow catalysts can be applied using a number of different techniques described below wherein the thickness and density of the catalytic coating can be controllably modulated. This type of surface pre-treatment (thin film deposition) can involve techniques such as plating, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), thermal evaporation, molecular beam epitaxy, electron beam evaporation, pulsed laser deposition (PLD), sputtering and reactive sputtering and various combinations thereof as known by those skilled in the art. In general, having the catalyst deposited as a relatively uniform thin film facilitates growth of the nanostructures in a nearly contiguous manner. The thickness of the thin film catalyst can be varied to tune properties of the resultant mat of nanostructures (e.g., mat thickness and/or density of the nanostructures). For example, the thickness of the thin film catalyst can be varied between about 5 nm and about 200 nm. Non-limiting examples of materials that can be used as the thin film catalyst include Au, Ag, Fe, FeB, B, Fe₃B, and Ni₃Si. In certain embodiments, the thin film catalyst layer can be formed as a patterned layer on the substrate (e.g., through the use of masking and/or lithography) to result in a correspondingly patterned mat of nanostructures. If a mask is used to pattern the catalytic thin film, the mask can be removed before or after growth of the nanostructures from the catalytic thin film. After the thin film catalyst layer has been applied to the substrate, the substrate is heated, in some cases so that the catalyst layer melts to form a liquid, and one or more nanostructure precursor materials are introduced in gasoues form so that they can diffuse into the molten catalytic material to begin catalytic growth of the nanostructures.

[0059] In some variations of these processes, a pre -treated substrate can be heated in a chamber at a relatively constant temperature to generate and maintain a vapor pressure of a nanostructure precursor element. In these variations, non-limiting examples of nanostructure precursor materials include SiH₄, SiH(CH₃)₃, SiCl₄, Si(CH₃)₄, GeH₄, GeCl₄, SbH₃, A1R₃ (where R is a hydrocarbon), Hg, Rb, Cs, B, Al, Zr, and In.

[0060] In other variations of these processes, a pre-treated substrate can be heated in a chamber together with a solid elemental nanostructure precursor at a relatively constant temperature that is sufficient to generate and maintain a vapor pressure of the nanostructure precursor element. In these embodiments, non-limiting examples of the solid elemental nanostructure precursor include C, Si, Ga, B, Al, Zr, and In. In some of these variations, a second nanostructure precursor can be added into the heated chamber, e.g., by introducing a flow or filling the chamber to a static pressure. Non-limiting examples of the second nanostructure precursor include C0₂, CO, NO and N0₂.

[0061] In still other variations of these processes, a pre -treated substrate can be heated in a chamber to a set temperature at least about 100°C, and a first nanostructure precursor material can be introduced into the chamber through a gas flow while the chamber is heated to the set temperature. After the chamber has reached the set temperature, the temperature can be held relatively constant at the set temperature, and a second nanostructure precursor material can be flowed into the chamber. In these variations, non-limiting examples of the first and/or second nanostructure precursor materials include SiH₄, SiH(CH₃)₃, SiCl₄, Si(CH )₄, GeH₄, GeCl₄, SbH₃, A1R₃ (where R is a hydrocarbon), C0₂, CO, NO and N0₂, N₂, 0₂, and Cl₂.

[0062] The processes described herein can be used to prepare nanostructures composed of various materials, non- limiting examples of which include glass (e.g., silica (Si0₂ or SiO_x)), ceramics (e.g., SiC, BN, B₄C or Si₃N₄), metal or ceramic oxides (e.g., A1₂0₃ or Zr0₂), and metals or semiconductors (e.g., Si, Al, C, Ge, CaN, GaAs, InP or InN).

[0063] For example, to prepare a mat including helical silica (Si0₂ or SiO_x) nanostructures (i.e., silica nanosprings), a substrate capable of withstanding at least about 350°C for about 15 minutes to about 60 minutes can be pre-treated by sputtering a thin, uniform layer of Au on the substrate (e.g., a layer about 15 nm to about 90 nm thick). To achieve the desired Au thickness, the substrate can be placed into a sputtering chamber at about 60 mTorr, and an Au deposition rate of about 10 nm/min can be used while maintaining a constant 0₂ rate during deposition. The substrate that has been pre-treated with Au can be placed in a flow furnace, e.g,. a standard tubular flow furnace that is operated at atmospheric pressure. A set temperature in the range of about 350°C to about 1050°C, or even higher, can be selected depending on the substrate used. During an initial warm up period in which the furnace is heated to the set temperature, a 10 to 100 standard liters per minute (slm) flow of SiH(CH₃)₃ gas can be introduced into the furnace for about 10 seconds to about 180 seconds, and then turned off. After the flow of SiH(CH ) is terminated, pure 0₂ can be flowed through the furnace at a rate of about 1 to 100 slm. The furnace then can be held at the set temperature for about 15 minutes to about 60 minutes, depending on the desired properties of the mat of silica nanostructures.

[0064] A range of densities of nanostructures on the substrate can be obtained with the methods described herein. The density of nanostructures on the substrate can be varied by varying the thickness of the thin film catalyst deposited on the substrate. If the thin film catalyst layer is relatively thick (e.g., 30 nm or thicker), the nanostructure mat can be very densely packed with nanostructures comprising groups of intertwined and/or entangled nanostructures (e.g., nanosprings) or a combination of nanostructures (e.g., nanowires and nanosprings). A relatively thin film catalyst layer (e.g., about 10 nm or thinner) can result in nanostructures that can be widely spaced apart (e.g., about 1 μιη apart or even farther). For example, an areal density of nanostructures on the substrate of about 5 x 10⁷ nanostructures per square cm to about 1 x 10¹¹ nanostructures per square cm can be achieved. In addition, the thickness of the thin film catalyst also can affect the diameter of individual

nanostructures. For example, when a gold catalyst thin film used to grow Si0₂ nanosprings is varied between about 15 nm and about 60 nm in thickness, it was observed that thinner films resulted smaller-diameter nanowires forming the nanosprings.

[0065] The areal density of nanostructures on a substrate can be estimated using the initial thickness of the thin film catalyst layer, and the average size of the catalyst particle or droplet left at the end of each nanostructure formed. The initial thickness of the thin film catalyst layer can be determined using an atomic force microscope, by examining a border between a catalyst-coated area (e.g., a gold-coated area) and an uncoated area of the substrate. The average catalyst size can be determined from the wavelength of the catalyst plasmon (e.g., the Au plasmon) obtained from the nanostructure mat. In some variations, multiple layers of nanostructures (e.g., nanosprings) can be formed by depositing a catalyst layer onto an existing nanostructure mat, whereby nanostructures can be grown on top of the existing mat by the previously described processes. In various embodiments, a single-layer nanostructure mat can have a depth of about 10 μιη, and multiple layers can be built up to provide a multilayer nanostructure mat that has a depth of about 20 μιη, about 30 um, about 50 μιη, about 80 μιη, about 100 μιη, or even thicker, e.g., about 200 μιη.

[0066] Many different types of substrates and substrate structures can be used because nanostructure mats according to the present teachings can be grown on any surface capable of withstanding the conditions for growing the nanostructures. However, the substrate material also should be able to withstand the conditions required for the subsequent deposition of the nanocrystals on the nanostructures as described below.

[0067] To provide the present catalytic material, Ti0₂ nanocrystals are formed in situ on the nanostructure mat. In particular, the nanocrystals can be deposited as a conformal thin film. While various thin film deposition techniques (e.g., ALD, CVD, PECVD) can be used, atomic layer deposition (ALD) is preferred because of its ability to produce very thin, extremely conformal films where the thickness and the composition of the films can be controlled at the atomic level.

[0068] The particle size and distribution of the nanocrystals on the nanostructures can change the catalytic activity of the catalyst material significantly. The inventors found that when photoreduction of C0₂ was performed over a nanostructure mat in which silica nanostructures are covered with a substantially contiguous coating of Ti0₂ nanocrystals, the conversion rate of C0₂ unexpectedly was close to zero. By comparison, high conversion rates were observed when the Ti0₂ nanocrystals are distributed on the silica nanostructures in a way such that gaps are present among the nanocrystals; in other words, when the Ti0₂ nanocrystals form a discontinuous (non-contiguous) layer on the nanostructures. Without wishing to be bound by any particular theory, it is believed that the phase boundary between the Ti0₂ nanocrystals and the silica nanostructure could increase the catalytic activity of Ti0₂. For example, the O atoms in the Ti0₂ nanocrystals that are bonded to both Ti and Si atoms can be the most catalytically active.

[0069] Accordingly, in the present catalytic materials, Ti0₂ nanocrystals can be distributed on a silica nanostructure in a way that leaves at least about 5-15% of the surface of the silica nanostructure exposed. Individual Ti0₂ nanocrystals can be spaced apart at an average distance of about 20 nm to about 60 nm, about 25 nm to about 60 nm, about 30 nm to about 60 nm, about 40 nm to about 60 nm, about 20 nm to about 50 nm, about 20 nm to about 40 nm, or about 30 nm to about 40 nm.

[0070] Because of the large population of nanocrystals and nanostructures present in the catalytic material, any particular nanostructure can include a small number of nanocrystals with at least one side abutting another nanocrystal. For example, less than about 30%, less than about 25%, less than about 20%>, less than about 15%, less than about 10%, or less than about 5% of the nanocrystals can have one or more sides abutting another nanocrystal.

However, microscopy images of the nanocrystal-coated nanostructures can be used to confirm that the nanocrystals generally form a discontinuous layer on the nanostructures. For example, the majority of the nanocrystals, that is, greater than about 70%, greater than about 75%, greater than about 80%, greater than about 85%, greater than about 90%, or greater than about 95% of the nanocrystals can have at least one side not in contact with another nanocrystal deposited on the same nanostructure, where the distance between a particular nanocrystal and its closest neighbor deposited on the same nanostructure can be at least about 20 nm (e.g., at least about 25 nm, at least about 30 nm, at least about 40 nm, at least about 50 nm, or at least about 60 nm).

[0071] Further, because deposition conditions that lead to less densely distributed nanocrystals often simultaneously reduce the particle size of the nanocrystals, the Ti0₂ nanocrystals can have an average particle size in the range of about 1 nm to about 100 nm, more particularly, between about 20 nm and about 80 nm, about 20 nm and about 70 nm, about 20 nm and about 60 nm, or about 20 nm and about 50 nm. As used herein, the particle size of a nanocrystal is defined as the longest length between any two points on the nanocrystal. Larger nanocrystals (e.g., particle size of about 200 nm or greater) usually provide smaller interparticle gaps (e.g., gaps on average that are less than about 10 nm), which translate into a larger coverage of the substrate surface. As used herein, "average" is meant to encompass any measure of a typical value of a distribution, for example, median, mode or mean. In terms of relative weight ratio, the present catalyst material can have improved catalytic activity (as measured by the conversion yield and/or efficiency rate of reducing C0₂) when the weight ratio of Ti0₂ nanocrystals to the silica nanostructures is between about 0.2: 1.0 and about 0.8: 1.0, for example, between about 0.25: 1.0 and about 0.75:1.0, between about 0.3: 1.0 and about 0.7: 1.0, or between about 0.35: 1.0 and 0.6: 1.0.

[0072] To achieve the distribution and particle size in the ranges described above, Ti0₂ nanocrystals can be deposited on a nanostructure mat comprising Si0₂ nanosprings using TiCl₄ and water vapor as precursors via atomic layer deposition. As in typical ALD processes, the precursors (i.e., T1CI₄ and water vapor) can be reacted with the nanospring mat one-at-a-time in a sequential manner. More specifically, TiCl₄ and water vapor alternately can be led to the substrate (i.e., the Si0₂ nanospring mat) by using nitrogen as the carrier gas. In some embodiments and as described in more details below, metal salts used as precursors for dopant ions can be introduced with the Ti precursors. After each precursor pulse, the reactor can be purged by pure nitrogen. Due to the characteristics of self-limiting and surface reactions, ALD film growth makes atomic scale deposition control possible. By keeping the precursors separate throughout the coating process, atomic layer control of film growth can be obtained as fine as -0.01 nm (or 0.1 A) per monolayer. The thickness of the Ti0₂ film typically ranges between about 5 nm and about 200 nm, and most typically in a range between about 20 nm and about 100 nm. By varying the thickness of the Ti0₂ film, the dimension and surface coverage density of the resulting nanocrystals can be modulated. The thickness of the Ti0₂ film can be varied by modifying the concentration of the Ti precursors, the flow rate of the carrier gas, the number of cycles of deposition, and/or other parameters as known by those skilled in the art. Specifically, few deposition cycles and/or low

concentration of Ti precursors could be used to achieve the distribution and particle size in the ranges described above.

[0073] As described above, Ti0₂ exhibits three distinct polymorphs. The catalytic activity of the present catalyst material can be enhanced by controlling the ratio of anatase Ti0₂ to rutile Ti0₂. For example, the anatase phase can be dominant at deposition temperatures between about 125°C and about 425°C. At higher temperatures, mixed anatase and rutile phases may result. Accordingly, to form largely anatase nanocrystals, a deposition temperature between about 145°C and about 350°C can be used. Without wishing to be bound by any particular theory, it is believed that the use of Si0₂ nanosprings as the support substrate and/or the use of ALD as the deposition technique can lead to the predominance of anatase crystals.

[0074] Certain dopants (whether incorporated into the crystal lattice during or after crystal growth) can increase the photoactivity of the nanocrystals by suppressing the growth of a less photocatalytic crystal phase and/or preventing the more photocatalytic crystal phase from transitioning into a less photocatalytic crystal phase. For example, the Ti0₂ nanocrystals can be doped (with cations and/or anions) to red-shift their absorption range and/or increase their photoactivity. In certain embodiments, Ti0₂ nanocrystals according to the present teachings can be doped with anions such as N or F and/or cations such as Fe , Mo , Ru , Os , Re , V⁴⁺, or Rh³⁺, where the anion dopants replace a small percentage of O, and the cation dopants replace a small percentage of Ti. These dopants typically are incorporated at less than about 3 atomic percent (at. %).

[0075] In certain embodiments, metal nanoparticles can be deposited on the Ti0₂ nanocrystals to enhance their photocatalytic activity. The metal nanoparticles can be deposited through any number of means including chemical synthesis in solution (reduction of an aqueous precursor), chemical vapor deposition (optionally plasma-enhanced), and laser ablation. In particular embodiments, nanoparticles composed of a metal selected from Pt, Ru, and Cu can be deposited on the above-described Ti0₂ nanocrystals using plasma-enhanced chemical vapor deposition using various precursors known in the art. For example,

(CH₃)3(CH₃CsFl₄)Pt can be used to prepare Pt nanoparticles. These metal nanoparticles typically have a dimension in the range of about 1 nm to about 25 nm. Typically, higher concentration of the metal precursors can lead to larger nanoparticles, as well as a higher concentration of the metal nanoparticles on the nanocrystals and generally a higher coverage density of the metal nanoparticles. Similar to the Ti0₂ nanocrystals, deposition parameters can be modulated to provide relatively narrow particle size range. The metal nanoparticles can be deposited on both the nanocrystals and the nanostructures, and can cover between about 5% and about 35% of the combined surface areas of the nanocrystals and the nanostructure mat. More specifically, the metal nanoparticles can provide a surface coverage density of greater than about 5%, greater than about 15%, greater than about 25%, or greater than about 35%.

[0076] The photocatalytic material described herein can be attached to a substrate to provide a photocatalysis device. The substrate can function as a mechanical support to which the photocatalytic material can be anchored, in which case, a large variety of substrate materials can be suitable. However, it can be advantageous to provide a photocatalysis device, where the photocatalytic material is attached to a porous substrate that optionally can be transparent. In some embodiments, the mat of nanostructures can be grown directly onto this porous substrate, so that a binder is not required to adhere the nanostructures to the substrate. Substrates with a large surface area on which nanostructure mats can be grown are generally desired. For example, honeycomb-structured substrates, coils or coiled substrates, undulated or fibrous substrates and/or substrates containing a variety of folds and bends are suitable. Alternatively, simple planar or other non-intricate substrate structures also can be used. The porous substrate typically has a pore size that is sufficient for gas (C0₂, H₂), and in some embodiments, water (or other liquid) to pass through at a constant flow rate. In most embodiments, the substrate also is optically transparent such that when the photocatalysis device is irradiated, regardless of its orientation and/or geometry, the photocatalytic nanocrystals are exposed to the irradiation without any significant absorption and attenuation by the substrate. For example, a transparent, filter-type substrate such as glass frits or fiberglass cloth can be suitable. Both glass frits and fiberglass materials are available in various porosities, which can be selected according to permeability and flow rate

requirements.

[0077] While the nanocrystals have been described herein as being composed of Ti0₂, nanocrystals composed of other photocatalytic semiconductors can be used without departing from the spirit of the present teachings. For example, nanocrystals of zirconium oxide (Zr0₂), zinc oxide (ZnO), tin oxide (Sn0₂), tungsten oxide (W0₃), molybdenum oxide (M0O₃), and cerium oxide (Ce0₂), zinc sulfide (ZnS), cadmium sulfide (CdS), cadmium selenide (CdSe), tungsten sulfide (WS₂), molybdenum sulfide (MoS₂), silicon carbide (SiC), or gallium phosphide (GaP) can be used. Similarly, while Ti0₂ nanocrystals were observed to exhibit enhanced catalytic activity when deposited on a mat of nanostructures composed of silica, the composition of the nanostructures can be modified without departing from the spirit of the present teachings. For example, the nanostructures can be composed of semiconductors such as zirconium oxide (Zr0₂), zinc oxide (ZnO), tin oxide (Sn0₂), tungsten oxide (WO3), molybdenum oxide (M0O3), and cerium oxide (Ce0₂), zinc sulfide (ZnS), cadmium sulfide (CdS), cadmium selenide (CdSe), tungsten sulfide (WS₂), molybdenum sulfide (MoS₂), silicon carbide (SiC), or gallium phosphide (GaP). Without wishing to be bound by any particular theory, the present catalyst material can provide improved C0₂ conversion efficiency when the nanocrystals and the nanostructrues are composed of different semiconductor materials, for example, different oxide semiconductor materials. In addition, the Ti0₂ nanocrystals can be supported on other nanoscale structures other than the ones already described. For example, effective photoreduction of C0₂ can be achieved over Ti0₂ nanocrystals supported on nanoparticles, where the nanoparticles can be about an order of magnitude larger than the Ti0₂ nanocrystals and the Ti0₂ nanocrystals form a discontinuous layer as described herein. In other variations, effective photoreduction of C0₂ can be achieved over Ti0₂ nanocrystals supported on silica materials that are microscale or larger.

[0078] The photocatalysis device can be implemented into a reactor for photoreducing C0₂ into desirable organic products. More specifically, the reactor can be configured as a continuous-flow reactor such that contact time between C0₂ and the catalyst material can be controlled.

[0079] Generally, the reactor can include a liquid- and gas-tight housing enclosing the photocatalysis device described herein. The housing can be of various geometries. For example, the housing can be tubular or planar. Inside the housing, the photocatalysis device is positioned to be exposed to a light source that emits one or more wavelengths within the ultraviolet-visible spectrum. The light source can be external to the housing (e.g., sunlight), in which case, the housing includes a light transmittance surface (a window) positioned to expose the photocatalysis device to light, where the window is composed of an optically transparent material that allows transmittance of various wavelengths within the UV-vis spectrum. In some embodiments, the reactor can include an internal light source, for example, a lamp or various light-emitting devices (e.g., light-emitting diodes), which emits light within the UV-vis (preferably UVA, UVB, and/or UVC) spectrum. Filters can be used to select the optimum wavelength(s) for generating catalytic sites on the photocatalytic material. While wavelengths in the near UV range (-350-380 nm) are often used to illuminate Ti0₂-photocatalyzed reactions, the present photocatalytic material was found to be able to photoreduce C0₂ at wavelengths larger than 380 nm (or wavelengths within the visible range), for example, at 395 nm, which allows the use of sunlight as the light source and make the photoreduction process more cost-effective. In certain embodiments, the reactor can include both an internal light source and a window for transmitting sunlight.

[0080] The interior of the housing provides a reaction chamber. The upper portion of the reaction chamber (i.e., the portion upstream of the photocatalysis device) is in fluid communication with one or more inlet lines for introducing reactants into the reaction chamber. Similarly, the lower portion of the reaction chamber (i.e., the portion downstream of the photocatalysis device) is in fluid communication with one or more outlet lines for extracting processed reactants (i.e., reduced products, unreacted reactants, and by-products) out of the reaction chamber. To establish a continuous flow within the reaction chamber, the reactor can include one or more components for controlling the flow rate across the photocatalysis device, specifically, by continuously charging a reactant stream into the reaction chamber and continuously extracting a product stream from the reaction chamber. For example, the reactor can include one or more valves, pumps, and/or monitoring device to measure and regulate pressure and/or flow rate.

[0081] The reactor can be configured to run gas-phase reactions, liquid-phase reactions, or both. For gas-phase reactions, the reducing agent can be hydrogen gas and/or a water vapor. When hydrogen gas is used, it can be pre -mixed with C0₂ or the two gases can be introduced via separate inlet lines into the reactor. Alternatively, carbon dioxide can be carried by a water vapor as a single reactant stream. Gas pump(s) can be used to control the flow rate of the reactant stream. For liquid-phase reactions, C0₂ can be dissolved in water and/or methanol, and a liquid-metering pump can be used to control the flow rate of the solution. Additionally, the outlet line can include an adjustable flow valve to provide further control of the flow rate. Other auxiliary components can include a mixer for mixing gases, heating elements (e.g., for generating the water vapor), a pressure gauge, and/or heat exchangers.

[0082] Figure 1 is a schematic diagram of an embodiment of a reactor according to the present teachings. The reactor 100 can include a housing 106 enclosing a photocatalysis device 108 which includes a photocatalytic material attached to a porous substrate 105. The reactor includes a window 102 composed of UV-transparent glass disposed above the photocatalysis device 108 such that the photocatalytic material can be exposed to an appropriate light source 101 (e.g., UV lamp or sunlight). A reactant stream 103 including carbon dioxide 104 and a reducing agent 109 (e.g., H₂) can be introduced via an inlet line, and led to pass through the photocatalysis device to provide a product stream 107 which is continuously extracted via an outlet line.

[0083] Figure 2 shows a schematic diagram of an embodiment of a photocatalysis device 200 according to the present teachings. In the embodiment shown, nanosprings 203 grown on the porous substrate 204 are conformally, but discontinuously coated with photocatalytic nanocrystals 206. Photoreduction of C0₂ can take place when the nanocrystal-coated nanostructures are irradiated by a light source 201 as carbon dioxide 202 and a hydrogen source (H₂ or water) 207 pass through the photocatalysis device. The photoreduction process can provide selective products by varying the source of hydrogen and/or the reaction parameters.

[0084] More specifically, a reactant stream including carbon dioxide and a source of hydrogen (hydrogen gas, water, and/or methanol) can be introduced via the inlet line into the reaction chamber. In some embodiments, the carbon dioxide can be introduced in substantially pure form. In other embodiments, the carbon dioxide can be introduced as a mixture with carbon monoxide. In some embodiments, the carbon dioxide (without or without carbon monoxide) can be pre -mixed with the hydrogen gas or water vapor prior to introduction into the reaction chamber. In other embodiments, the hydrogen or water vapor can be introduced into the reaction chamber via a different inlet line. In preferred

embodiments, the carbon dioxide is saturated with water vapor prior to introduction into the reaction chamber.

[0085] Once inside the reaction chamber (or after some mixing if needed), the reactants are exposed to UV and/or visible light irradiation as they flow through the photocatalysis device. While various reaction mechanisms have been proposed for the photoreduction of C0₂, it is generally agreed that the reaction begins with the creation of electron-hole pairs by the photocatalyst (Ti0₂) upon exposure to UV irradiation. Without wishing to be bound by any particular theory, it is believed that photohydrolysis occurs (in embodiments where water vapor is used), and free hydroxyl radicals are produced. A cascade of reactions then follows, including the production of atomic hydrogen and the reduction of carbon dioxide into carbon monoxide. The carbon monoxide then reacts with the atomic hydrogen to produce various reduced products which can include formic acid, formaldehyde, methanol, and methane. [0086] Due to the large active surface area and phase boundary provided by both the nanostructure mat and the discontinuous layer of Ti0₂ nanocrystals, the present catalyst material offers a high adsorption capacity and a large number of highly accessible catalytic sites for the reactants and other reactive species generated from the reactants, enabling efficient reactions between the adsorbed species. As a result, carbon dioxide (and carbon monoxide) can be photoreduced with improved efficiency (close to about 30%) as compared to prior art methods (limited to about 10% or less).

[0087] The product stream can be continuously collected via the outlet line. In some embodiments, the collected product stream can be reintroduced into the reaction chamber to allow multiple passes across the photocatalysis device, which can further increase the total conversion rate. Variation of reaction parameters such as the flow rate of the reactant gases, the relative ratio of the reactant gases, and/or the wavelength of the irradiation also can lead to increase in the conversion rate. In addition, the same or different parameters can be varied to optimize the selectivity of the gaseous products. Without wishing to be bound by any particular theory, it is believed that longer contact time between the reactants and the catalyst material can lead to more highly reduced products. Accordingly, it is believed that the content of the product gas can be selectively controlled by varying the flow rate of the input gas. More specifically, by ensuring a sufficiently high flow rate, desirable products such as methanol, formaldehyde, and formic acid can be obtained instead of methane. Among methanol, formaldehyde, and formic acid, formation of methanol, formaldehyde, and formic acid can be achieved by using an increasing flow rate. The present reactor, therefore, can be used to select a specific partially reduced product simply by adjusting the flow rate for the specific product.

[0088] In addition, it also was found that the composition of the product stream can be varied by changing the source of hydrogen. Specifically, when hydrogen gas is used, the primary product observed can be methane or carbon monoxide. When water is used, as much as 90%) of the reduced product was observed to be methanol. On the other hand, a mixture of formaldehyde and formic acid can be obtained by using methanol as the source of hydrogen.

[0089] Accordingly, in one variation, the present photocatalysis device can be used to photoreduce C0₂ with hydrogen (H₂:C0₂ = 1.5) at STP (1 atm, 25°C) using a low-pressure Hg lamp to provide UVC light (254 nm). The product observed is carbon monoxide. The reaction is believed to proceed via the reverse water-gas shift reaction, namely: C0₂ + H₂ = CO + H₂0

[0090] In another variation, the present photocatalysis device operated at STP and illuminated by UV light can be used to photoreduce C0₂ with water (by either saturating C0₂ in a water vapor or dissolving C0₂ in water) to produce methanol. The reaction can be described as:

[0091] In another variation, the present photocatalysis device operated at STP and illuminated by a low-energy UV lamp (395 nm) can be used to photoreduce C0₂ with methanol (for example, by bubbling CO₂ in pure methanol) to produce a

formaldehyde/formic acid mixture with high formaldehyde content. The reaction can be described as:

C0₂ + CH₃OH = HCOOH + HCHO

[0092] Another advantage of the present reactor is the ease of its incorporation into existing industrial processes that emit CO₂ as a pollutant. For example, no major modifications are required to incorporate the present reactor into existing power and industrial stacks. One or more reactors according to the present teachings can be placed in-line with an existing industrial source of CO₂. For example, the reactors can be connected to an exhaust stack from a coal- fired power plant, or more preferably, to an existing scrubber system. An exemplary system is illustrated in Figure 3. As shown, a system 300 for reforming CO₂ can include a plurality of photoreactors 301 which are connected in series and/or in parallel to each other by distribution lines 303 which also connect the photoreactors to a scrubber system 302. Each photoreactor can include a light source 305 to provide UV light 304 to each photoreactor. Outlet lines 306 of the photoreactors can be connected to a separation system 307, which separates the product stream into, for example, formaldehyde, formic acid, methanol, and methane, which respectively are stored in separate storage tanks 308, 309, 310, and 311.

[0093] The following examples are provided to illustrate further and to facilitate the understanding of the present teachings and are not in any way intended to limit the invention.

[0094] Example 1. Characterization of Nanostructure Mat

[0095] Referring to Figure 4, scanning electron microscopy images show that a typical nanostructure mat can include both helical nanostructures 401 (nanosprings) and linear nanostructures (nanowires). Some variance in diameter of the nanostructures can be observed, but in the embodiment shown, the nanostructures appear to have an average diameter of about 200 nm. The nanostructures are intertwined and form a disordered array.

[0096] Example 2. Characterizatioin of Nanocrystals

[0097] Figure 5 is a transmission electron microscopy image showing a portion of a silica nanospring 502 having a diameter of about 200 nm that has been coated with a discontinuous layer of Ti0₂ nanocrystals 501. The nanocrystals have a particle size of about 60 nm. The average distance between individual Ti0₂ nanocrystals can be measured from the image to be between about 20 nm to about 60 nm.

[0098] Example 3. Diffraction Data of Nanocrystals

[0099] XRD patterns of silica nanospring-supported Ti0₂ nanocrystals (deposited via atomic layer deposition) were obtained. Figure 6 shows a representative XRD pattern. As can be seen, the pattern is characterized by a narrow and dominant peak at (101) plane diffraction (2Θ = 25.4°), which confirms that as much as 99% or more of the Ti0₂

nanocrystals are in the anatase phase when deposited on Si0₂ nanosprings by ALD.

[0100] Example 4. Characterization of Porous Substrate

[0101] Figure 7 shows a scanning electron microscopy image of a porous glass compact ("frit"). Photocatalysis devices according to the present teachings were fabricated by growing a mat of silica nanostructures directly on the glass frit substrate 701, then depositing a discontinuous layer of Ti0₂ nanocrystals (Figures 5 and 6) via ALD by controlling the concentration of the Ti precursors.

[0102] Example 5. Mass Spectroscopy

[0103] Carbon dioxide was photoreduced with water over a photocatalysis device according to the present teachings. Upon illumination by UV light (395 nm), reduction of C0₂ by about 40% was observed, as confirmed by the mass spectroscopy data shown in Figure 8. The 40%) conversion rate was calculated based on the number of counts of the C0₂ signal before and after the UV light was switched on.

[0104] Example 6. Gas Chromatography

[0105] Carbon dioxide was photoreduced with water over a photocatalysis device according to the present teachings. In this experiment, the output flow rate was monitored continuously before and after the photocatalysis device was exposed to UV light (395 nm). [0106] In a first example, about 100 of water was mixed with C0₂. The initial output flow rate was about 20 ml/min. Samples from the product stream was analyzed by gas chromatography. Upon illumination, gas chromatograms showed a significant methanol peak. The output flow rate was reduced to 15 ml/min when photoreduction began to occur (i.e., after the UV light was switched on). It was observed that most of the methanol converted from C0₂ was in the liquid phase and gas chromatography could only detect the gas phase species. Therefore, it was estimated that about 25% of the C0₂ was converted into methanol.

[0107] Figure 9 compares the gas chromatograms from 1 ml samples before and after the UV light was switched on. It can be seen that the methanol peak is present only in the gas chromatogram with illumination. The gas chromatograph obtained before the UV light was switched on shows a flat line.

[0108] In a second experiment, about 400 mL of water was mixed with C0₂. The initial output flow rate was about 20 ml/min. Upon illumination, gas chromatograms showed a significant methanol peak and a small formaldehyde peak (Figure 10). The output flow rate was reduced to 17.2 ml/min when photoreduction began to occur (i.e., after the UV light was switched on). It was observed that most of the methanol converted from C0₂ was in the liquid phase and gas chromatography could only detect the gas phase species. Therefore, it was estimated that about 14% of the C0₂ was converted into methanol.

[0109] Examples 7 and 8 demonstrate the product selectivity of the present methods, devices, and systems for photoreducing carbon dioxide. Specifically, the reactions were carried out in a continuous flow reactor made of stainless steel plates. Figure 11 is a schematic diagram of the continuous flow reactor. The top half 1102 of the reactor provides a UV transparent (quartz) window 1102 and an inlet line 1105 that is designed for liquid phase and gaseous phase reactant mixtures. The bottom half 1103 of the reactor includes an outlet line 1104 for the collection of products. Disposed within the reactor and between the inlet port and the outlet port is a photocatalysis device 1106 comprising nanocrystals of anatase-phase Ti0₂ conformally deposited on a disordered array of silica nanosprings attached to a porous substrate, which is positioned to be exposed to a light source 1101.

[0110] Example 7. Selective Reduction of CO? into Methanol

[0111] Carbon dioxide was dissolved in water and the solution was introduced into the reactor at a flow rate of about 0.5 ml/hr. The illumination source was a 50 W Hg lamp equipped with an AM 1.5 G filter to simulate solar radiation. The operating conditions were 25°C and 1 atm. Product samples in volumes of 0.1 ml were collected by a syringe attached to the reactor and analyzed by a flame ionization detector (150°C with He as the carrier gas at a flow rate of 30 ml/min) on a HP5890 Series II gas chromatograph.

[0112] Figure 12 shows a representative chromatogram of conversion of dissolved C0₂ in H₂0 into methanol. The chromatogram indicates a conversion efficiency of about 3.17% of dissolved C0₂ being converted into methanol after 3 hours of light exposure. The space time yield was about 4.21 mmol / (g cat. hr), which is about an order of magnitude higher compared to the use of commercially available Degussa P25 Ti0₂ catalysts (which is reported to have a space time yield in the order of μιηοΐ / (g cat. hr)).

[0113] Example 8. Selective Reduction of CO? into Formaldehyde and Formic Acid

[0114] Carbon dioxide was dissolved in water and the solution was introduced into the reactor at a flow rate of about 0.5 ml/hr. Methanol was added to the C0₂ solution to provide a 1% solution. The illumination source was a 50 W Hg lamp equipped with an AM 1.5 G filter to simulate solar radiation. The operating conditions were 25°C and 1 atm. Product samples in volumes of 0.1 ml were collected by a syringe attached to the reactor and analyzed by a flame ionization detector (150°C with He as the carrier gas at a flow rate of 30 ml/min) on a HP5890 Series II gas chromatograph.

[0115] Figure 13 shows a representative chromatogram of conversion of dissolved C0₂ and methanol in H₂0 into formaldehyde and formic acid. The chromatogram indicates a conversion efficiency of about 12% of dissolved C0₂ being converted into formaldehyde (about 72.3%)) and formic acid (about 27.7%) after 3 hours of light exposure. It was observed that the conversion rate could be enhanced by using excess methanol.

[0116] Example 9. Effects of Relative Weight Ratio of Photocatalyst to Nanospring Support

[0117] Nanostructure mats with different Ti0₂ content were prepared and tested.

Specifically, the weight ratio of Ti0₂ nanocrystals to Si0₂ nanosprings was varied between about 0.35 and about 0.67. Figure 14 plots the C0₂ conversion efficiences against the relative weight ratio of Ti0₂ nanocrystals to Si0₂ nanosprings. As shown, it appears that the optimum ratio is in the range of about 0.4 to about 0.6 which indicates that the Si0₂ nanosprings provided enhancement in the photocatalytic conversion. [0118] Example 10. Effects of Particle Size and Distribution Pattern

[0119] Nanostructure mats coated with a contiguous layer of Ti0₂ nanocrystals were prepared and tested under conditions similar to those described in Examples 7 and 8.

Specifically, the Ti0₂ nanocrystals have an average particle size of about 200 nm. Carbon dioxide was dissolved in water and the solution was introduced into the reactor at a flow rate of about 0.5 ml/hr. The illumination source was a 50 W Hg lamp equipped with an AM 1.5 G filter to simulate solar radiation. The operating conditions were 25 °C and 1 atm. Product samples in volumes of 0.1 ml were collected by a syringe attached to the reactor and analyzed by a flame ionization detector (150°C with He as the carrier gas at a flow rate of 30 ml/min) on a HP5890 Series II gas chromatograph. No reduced product could be detected by gas chromatography (Figure 15) after 3 hours of UV light exposure. The nanostructure mats tested in this experiment had -18.5 mg of Ti0₂ nanocrystals deposited on them compared to similar size nanostructure mats tested in Examples 7 and 8 which had ~ 11 mg of Ti0₂.

[0120] As can be seen in Figure 15, the intensities of the peaks in the gas chromatograms obtained before and after the UV light was switched on are substantially the same.

[0121] The present teachings can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting on the present teachings described herein. The scope of the present teachings is thus indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.

[0122] What is claimed is:

Claims

1. A photocatalysis device for reforming carbon dioxide comprising a mat of

nanostructures attached to a porous substrate, wherein the mat of nanostructures comprises intertwined nanosprings and each nanostructure comprises a discontinuous layer of Ti0₂ nanocrystals deposited thereon.

2. The device of claim 1, wherein individual Ti0₂ nanocrystals are spaced apart at an average distance of about 20 nm to about 60 nm.

3. The device of claim 1 or 2, wherein the Ti0₂ nanocrystals have an average particle size of about 20 nm to about 80 nm.

4. The device of any one of claims 1-3, wherein at least about 10% of the surface of each nanostructure remains exposed.

5. The device of any one of claims 1-4, wherein greater than about 90% of the Ti0₂ nanocrystals are in the anatase phase.

6. The device of any one of claims 1-5, wherein the Ti0₂ nanocrystals are doped with a dopant selected from N, F, Fe³⁺, Mo⁵⁺, Ru³⁺, Os³⁺, Re⁵⁺, V⁴⁺, and Rh³⁺.

7. The device of any one of claims 1-6 further comprising metal nanoparticles deposited on the Ti0₂ nanocrystals and/or the nanostructures.

8. The device of claim 7, wherein the metal nanoparticles comprise a metal selected from Pt, Ru, and Cu.

9. The device of any one of claims 1-8, wherein the mat of nanostructures further comprises nano wires.

10. The device of any one of claims 1-9, wherein the nanostructures are composed of silica.

11. The device of any one of claims 1-10, wherein greater than about 50% of the nanostructures in the mat of nanostructures are nanosprings.

12. The device of any one of claims 1-11, wherein the porous substrate is composed of a transparent material.

13. The device of claim 12, wherein the transparent, porous substrate comprises a mesh of fiberglass.

14. The device of claim 12, wherein the transparent, porous substrate comprises glass frit.

15. The device of any one of claims 1-14, wherein the porous substrate is water- permeable.

16. The device of any one of claims 1-15, wherein the mat of nanostructures is deposited on the porous substrate.

17. The device of any one of claims 1-15, wherein the porous substrate is composed of a transparent material, and the mat of nanostructures is disposed within the transparent, porous substrate.

18. A continuous-flow photoreactor for reforming carbon dioxide comprising:

a photocatalysis device according to any one of claims 1-17;

a liquid- and gas-tight housing enclosing the photocatalysis device and defining an upper portion upstream of the photocatalysis device and a lower portion downstream of the photocatalysis device, wherein the housing comprises a window comprising a material that is optically transparent to one or more wavelengths within the ultraviolet- visible spectrum or wherein disposed within the housing is a light source emitting one or more wavelengths within the ultraviolet- visible spectrum, and wherein the light source or the window is positioned to expose the photocatalysis device to said one or more wavelengths;

an inlet line in fluid communication with the upper portion of the housing for introducing a reactant stream comprising carbon dioxide into the housing;

a flow rate controller for charging the reactant stream through the photocatalysis device to provide a product stream and establishing a continuous flow within the housing; and

an outlet line in fluid communication with the lower portion of the housing for collecting the product stream.

19. The photoreactor of claim 18 comprising a light source that emits light at one or more wavelengths within the ultraviolet- visible spectrum, wherein the light source is positioned within or external to the housing.

20. The photoreactor of claim 18 or 19, wherein the light source emits light at one or more wavelengths in the ultraviolet range.

21. The photoreactor of any one of claims 18-20, wherein the flow rate controller comprises a gas pump.

22. The photoreactor of any one of claims 18-20, wherein the flow rate controller comprises a liquid metering pump.

23. The photoreactor of any one of claims 18-22 further comprising an adjustable flow valve disposed in the outlet line.

24. The photoreactor of any one of claims 18-23, wherein the housing is tubular.

25. The photoreactor of any one of claims 18-23, wherein the housing is planar.

26. The photoreactor of any one of claims 18-25 further comprising a mixer upstream of the inlet line.

27. A system for reforming carbon dioxide comprising a photoreactor of any one of claims 18-26 and a scrubber system comprising a gas line connected to the inlet line of the photoreactor.

28. The system of claim 27 further comprising a storage tank connected to the outlet line of the photoreactor.

29. The system of claim 27 or 28 comprising a plurality of photoreactors according to any one of claims 18-26 connected in series and/or in parallel.

30. A photocatalytic material comprising Ti0₂ nanocrystals deposited on a mat of nanostructures, wherein the mat of nanostructures comprises intertwined nanosprings composed of silica, and wherein the Ti0₂ nanocrystals have an average particle size of less than about 100 nm.

31. The photocatalytic material of claim 30, wherein at least 80% of the Ti0₂

nanocrystals comprise at least one side not in contact with another Ti0₂ nanocrystal deposited on the same nanostructure.

32. The photocatalytic material of claim 30 or 31 , wherein at least 80% of the Ti0₂ nanocrystals deposited on a particular nanostructure are spaced apart from the closest adjacent Ti0₂ nanocrystal at an average distance of about 20 nm to about 60 nm.

33. A method of reforming carbon dioxide, the method comprising:

continuously feeding a reactant stream comprising carbon dioxide into a continuous- flow reactor; contacting supported nanocrystals comprising anatase phase Ti0₂ within the reactor with the reactant stream comprising carbon dioxide under conditions effective to reform said carbon dioxide; and

continuously extracting a product stream away from the continuous-flow reactor.

34. A method of reforming carbon dioxide, the method comprising:

exposing the photocatalysis device of any one of claims 1-17 or the photocatalytic material of any one of claims 30-32 to a light source that emits light at one or more wavelengths within the ultraviolet-visible range;

causing a reactant stream comprising carbon dioxide and a reducing agent to pass through the photocatalysis device; and

continuously removing a product stream across the photocatalysis device.

35. The method of claim 33 or 34, wherein the reactant stream comprises hydrogen gas.

36. The method of claim 33 or 34, wherein the reactant stream comprises water or water vapor.

37. The method of claim 36, wherein the product stream comprises methanol.

38. The method of claim 33 or 34, wherein the reactant stream comprises methanol.

39. The method of claim 38, wherein the product stream comprises formaldehyde.

40. The method of any one of claims 33-39, wherein both the reactant stream and the product stream are in gas phase.

41. The method of any one of claims 33-40, wherein the reactant stream is led across the Ti0₂ nanocrystals at a flow rate selected to alter the composition of the product stream.

42. The method of any one of claims 33-41, wherein the Ti0₂ nanocrystals are exposed to sunlight.

43. The method of any one of claims 33-41, wherein the Ti0₂ nanocrystals are exposed to UV light.