WO2013016841A1

WO2013016841A1 - Nanoparticles

Info

Publication number: WO2013016841A1
Application number: PCT/CN2011/001251
Authority: WO
Inventors: Xinhe Bao; Zhiqiang Yang; Xiulian Pan; Shujing GUO
Original assignee: Dalian Institute Of Chemical Physics, Chinese Academy Of Sciences; Bp P.L.C.
Priority date: 2011-08-01
Filing date: 2011-08-01
Publication date: 2013-02-07

Abstract

A composition is described which comprises a component including at least one nano-sized confinement space, and the composition further comprises at least one particle confined within the confinement space. In some examples, the particle comprises iron nitride. The composition may be used in a process for the hydrogenation of CO in which the process includes the step of passing a feed stream over a catalyst including the composition.

Description

NANOPARTICLES

This invention relates to compositions including nanoparticles. Aspects of the invention relate to nanoparticles in a confined environment. In examples described, the nanoparticles are encapsulated inside nanostructures, for example fullerene nanostructures. In examples described herein, a confined environment is provided by carbon nanotubes (CNT).

Examples described herein relate to the use of confined nanoparticles in some applications. For example use of confined nanoparticles as a catalyst in chemical processes is described. Other uses and applications for the confined nanoparticles are also possible.

Carbon nanotubes (CNT) are carbon materials having a generally cylindrical structure. CNTs can be envisioned as being formed of rolled-up graphene layers which roll into a cylindrical structure forming well-defined tubes or channels. The diameters of the CNT channels are typically of the order of 1 nm or less to about 100 nm. These channels enable encapsulation of nanomaterials as described in "Probing the Electronic Effect of Carbon Nanotubes in Catalysis: NH₃ Synthesis with Ru Nanoparticles" S. Guo, X. Pan, H. Gao, Z. Yang, J. Zhao and X. Bao, Chem.-Eur. J , 2010, 16, 5379-5384 in which the investigation of confinement in the inside of the channels CNTs of a Ru-based ammonia synthesis catalyst is described.

According to an aspect of the invention there is provided a composition comprising a component including at least one nano-sized confinement space, the composition further comprising at least one particle confined within the confinement space, the at least one particle comprising a metal nitride.

As discussed in more detail below, aspects of the invention find particular, but not exclusive application where the particles are nano particles. Preferably the particles have a size such that at least one dimension is not more than 100 nm, preferably not more than 50 nm, for example less than 20 nm.

In some examples, the metal nitride comprises iron nitride. Thus in some examples of the invention, the at least one particle comprises iron nitride. As discussed further below, in other aspects of the invention, the particles may have a different composition. The inventors have identified that confinement of the particles inside the nano-sized confinement spaces of the component can have benefits.

For example, the confinement of the particles comprising iron nitride may stabilize a cubic phase of the iron nitride. Thus in examples of the invention, the confined particles may include cubic phase iron nitride. The presence of the cubic phase may be beneficial in some applications.

For example, the confinement of the particles inside the component may enable control or restriction of the particle size of the particles in the confinement space.

For example, and as discussed further below, where the particles comprise catalytically active material, the performance of the catalytically active material may be improved by confinement of the particles in the component.

Where reference is made herein to particles being confined the particles are preferably retained within the confinement space, although they may or may not be bound within the space. Thus the particles may be free or partially free to move in the containment space. In some examples, the confinement space may be enclosed or partially enclosed. For example, the confinement space may be formed by a tube or channel.

The confinement space may have at least one dimension which is 100 nm or less, preferably between from 0.1 and 100 nm. The confinement space may have at least two dimensions each being 100 nm or less, preferably between from 0.1 and 100 nm.

The confinement space may have three dimensions less than about 100 nm, for example between 0.1 and 100 nm. Thus the confinement space may be nano-sized in fewer than all dimensions. For example, and as discussed in more detail below, the confinement space may comprise a nano-tube, or a nano-channel.

The component may be of any appropriate composition. For example, the component may comprise any material, structure or composition which includes one or more nano-sized formations which are suitable for confining a particle. Such component includes, but not exclusively, materials having pores, or cage or shell structures in which openings are present in the structure, which openings are suitable for confining one or more particles. The particles may be confined on fewer than all sides. For example, the confinement space may comprise a tube or channel.

The component may have any structure which includes openings or spaces suitable for confining a particle, for example the component may comprise a laminate material comprising a layer structure, confinement spaces being provided by spaces between the layers.

The component may for example include a zeolite or other molecular cage structure, or for example a clay or nano-porous material, or mesoporous material for example mesoporous silica and mesoporous carbon materials such as MCM-41, SBA-15, CMK-3.

Where reference is made to a component being for example nano-sized, it should be understood that preferably at least one of the dimensions of the component is for example not more than 100 nm , for example between from 0.1 nm and 100 nm. In some examples, the component may have more than one dimension being not more than 100 nm, for example between from 0.1 nm and 100 nm.

The component may include a nanostructure which contains the confinement space. The component may comprise a nanostructure. An aspect of the invention provides particles of a metal nitride, for example iron nitride, for example cubic phase iron nitride confined within a nanostructure. Aspects of the invention are applicable for other metal nitrides, for example cobalt or molybdenum nitrides.

The nanostructure may include any structure having one or more nano-sized spaces for containment of the particles. For example, the nanostructure may include a nanomesh, nanoshell, nanotube, nanosphere and/or nanocage or other similar structure.

The component may comprise a fullerene. The component may comprise a material including a graphene structure. The graphene structure may for example include spheres, balls, ellipsoids and/or tubes. In examples described herein, the component comprises a carbon nanotube.

In examples described herein the component comprises a carbon nanotubes and at least one particle is confined in a space in the nanotube.

Thus an aspect of the invention provides a composition comprising carbon nanotubes, and particles confined in the nanotubes, wherein preferably the particles include metal nitride, for example iron nitride.

The carbon nanotubes (CNT) may be any appropriate type, for example single walled, double walled or multiwalled.

Where the component is a fullerene, the fullerene may include substituents, modifiers, contaminants, and/or other components. The graphene for example may have substituents, or modifications. The component, for example the nanostructure may include endohedral or exohedral substituents or modifications.

The at least one particle confined in the space preferably has an average particle size not more than 50 nm, preferably not more than 20 ran.

Preferably the average particle size of the confined particles is not more than about

10 nm or 6nm. In some examples, at least 90 number % of particles have a size less than 15 nm, for example less than 1 Onm.

The composition may further include some particles outside of the confinement space. Preferably only a relatively small number of the particles are outside of

confinement spaces.

Preferably at least 50 number % of the particles, for example nanoparticles, of the composition are confined in one or more spaces of the component. For example, at least 70% of the particles, for example 80% or more of the particles, may be confined in spaces in the nanostructure.

Also provided by the invention is a composition comprising a component including at least one nano-sized confinement space, the composition further comprising at least one particle confined within the confinement space, the at least one particle comprising a metal nitride, metal carbide or metal phosphide. Thus aspects of the invention may find application where the particles comprise metal carbide and/or metal phosphide.

Aspects of the invention may be applied to metal nitrides other than iron nitrides.

The component may include carbon nanotubes.

The inventors have identified that confinement of iron nitride particles can have particular benefits. For example, confinement of iron nitride inside CNTs may benefit stabilization of the cubic FeN phase of the nitride particles. As discussed further below, the inventors have also identified that confined particles may exhibit higher catalytic activity compared with non-confined particles. For example, CNT-confined nitride particles (for example iron nitride) may in some cases exhibit a higher activity in CO hydrogenation compared with nitride particles which are not confined, for example particles on the outer walls of CNTs. The confined nitride particles may also have improved activity compared with CNT-confined reduced iron catalyst. In examples described, cubic FeN nanoparticles were synthesized through

incorporation in carbon nanotube (CNT) channels. In examples, the confined particles were used as a catalyst in CO hydrogenation.

The invention also provides the use in a chemical process of a composition as described herein. Compositions described herein may find application in a chemical process, and may for example be used as a catalyst. Other applications are also envisaged, for example in relation to fuel cell and energy storage, for example lithium battery and supercapacitor technology.

In some examples described herein, the compositions are comprised in a catalyst for CO hydrogenation. The compositions may be comprised in other catalysts. For example they may be utilised in water gas shift catalysts (WGS).

It has previously been reported, for example in W. Chen, X. Pan, M.-G. Willinger, D. S. Su and X. Bao, J Am. Chem. Soc, 2006, 128, 3136-3137, that confinement inside CNTs can lead to facilitated reduction of a series of metal oxides such as iron. Other studies have shown facilitated reduction of ruthenium and cobalt inside CNT channels, and retarded oxidation of metallic iron inside CNT channels with respect to their counterparts located on the outer walls of CNTs. This was attributed in the studies to the interactions of the confined materials with the interior surface of the CNTs and the spatial restriction on the particle aggregation under reaction conditions.

CO hydrogenation via Fischer-Tropsch synthesis (FTS) has been investigated as an option for producing clean transportation fuels and chemicals by conversion of natural gas, coal or biomass. This has become important in the energy and environmental fields. Iron- based catalysts are widely applied in FTS because they are relatively cheap, flexible to changes in temperature, pressure and different H₂/CO ratios. However, those iron catalysts are often relatively non-resistant to deactivation due to the oxidizing effect of water under reaction conditions. Iron can be nitrided with varying contents of nitrogen, which exhibit different lattice structures. With increasing nitrogen content, iron nitride transforms from fee y'-Fe₄N to hep s-Fe_xN (2 < x < 3) and to orthorhombic ζ-Ρε₂Ν. These nitrides have been reported to have a significantly enhanced catalytic activity in FTS with respect to the reduced iron (see A A Hummel et al, J Catalysis, (1988) 1 13, 236-249). The resistance to oxidation has been proposed to be responsible for their higher activity upon incorporation of nitrogen into the Fe lattice. A cubic FeN phase with a higher nitrogen content of 50 at.% N was theoretically predicted and recently synthesized in the form of a thin film by reactive magnetron sputtering, molecular beam epitaxial growth and ion bombardment.

A further aspect of the invention provides a catalyst composition comprising a component including at least one nano-sized confinement space, the composition further comprising at least one particle confined within the confinement space, the at least one particle comprising a metal nitride. The at least one particle may comprise an iron nitride.

The invention also provides a process for the hydrogenation of CO, comprising the steps of passing a feed stream over a catalyst, the catalyst comprising a composition as described herein.

The invention further provides a process for the hydrogenation of CO, the process including the step of passing a feed stream over a catalyst, the catalyst including a composition comprising a component including at least one nano-sized confinement space, the composition further comprising at least one particle confined within the confinement space, the at least one particle comprising a metal nitride.

The composition may have further features as described herein in relation to other aspects of the invention.

The composition may include a metal nitride confined in a nanostructure, for example a carbon-based nanostructure, for example a fullerene. Without wishing to be bound by any particular theory, it is considered that interaction between the walls of the nanostructure and the particles may give improved catalytic properties. The component may comprise carbon nanotubes. The particle may comprise iron nitride, for example cubic- iron nitride.

At least 50% by number of particles of metal nitride may be confined in the component. Preferably at least 50% of particles of catalyst are confined in the component, preferably at least 70%, or 80% or more.

The invention also provides a method of producing a composition as defined herein. The invention further provides a method of producing a confined metal nitride, the method including the steps of introducing a composition comprising the metal to a component including at least one nano-sized confinement space such that at least one particle including the metal is confined within the confinement space.

The method may further include the step of carrying out a nitriding treatment to form a metal nitride in the confinement space. The introduced composition may for example include a metal oxide, metal chloride, metal acetate or metal complexes. The introduced composition may include a metal salt.

The metal nitride formed may for example comprise iron nitride. In some preferred examples, the metal nitride includes cubic- iron nitride. Preferably cubic iron nitride is formed in the confinement space, for example during the nitriding treatment.

The confinement space may have at least one dimension 100 nm or less, preferably between from 0.1 and 100 nm. The component may include a nanostructure including the confinement space.

The component may comprise a fullerene. The component for example may comprise a carbon nanotube, wherein at least one particle is confined in the nanotube.

The introduction of the metal in to the component may be carried out at elevated temperature. The nitriding treatment may include treatment using a source of nitrogen, for example ammonia or nitrogen. The nitriding treatment may be carried out at elevated temperature.

The elevated temperature for the nitriding treatment may be at least 400 degrees C, for example at least 450 degrees C, for example 500 degrees C or more.

The nitriding step may include increasing the temperature at a first predetermined rate in a first heating period, and increasing the temperature at a second predetermined rate in a second heating period. The first predetermined rate may be for example at least 5 degrees C per minute, for example at least 7 degrees C per minute. The second

predetermined rate may be for example not more than 5 degrees C per minute, for example less than 2 degrees C per minute, for example 1 degree C per minute or less.

In a third stage, the temperature may be held at a predetermined elevated temperature.

The method may further include a pre-treatment of the component. This pre- treatment may effect opening or partial opening of one or more confinement spaces in the component. This pre-treatment may for example facilitate subsequent introduction of the particles into the confinement spaces. For example, where the component comprises carbon nanotubes, the pretreatment may effect removal or partial removal of end caps on some or all of the carbon nanotubes to open ends of the nanotubes. This treatment may not be necessary in some cases. The invention extends to compositions, products, methods, processes and/or apparatus substantially as herein described optionally with reference to one or more of the accompanying drawings.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, features of product or

composition aspects may be applied to method or process aspects, and vice versa.

Preferred features of aspects of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: Figures 1(a), 1(b), 1(c) and 1(d) show TEM images of (a) Fe_xN-in-450; (b) Fe_xN-in-500; (c) Fe_xN-out-400; (d) Fe_xN-out-500, respectively. The insets in the TEM images show graphically the particle size distribution of Fe_xN measured from over 260 particles;

Figures 2(a), 2(b), 2(c), and 2(d) show TEM images of CNT-supported iron nitride prepared at different nitridation temperatures, (a) Fe_xN-in-350; (b) Fe_xN-in-400; (c) Fe_xN- out-350; (d) Fe_xN-out-450;

Figures 3(a) and 3(b) show room temperature 57Fe Mossbauer spectra of (a) FexN-in-450 (top panel) and (b) FexN-out-400 (lower panel) with the dotted line representing the measured data while solid lines donate the fitted data. Line Sum (S) is the sum of fitted lines;

Figure 4(a) shows XRD patterns of fresh FexN-in-500, used FexN-in-500 and used Fe-in catalysts (from bottom to top);

Figure 4(b) shows room temperature 57Fe Mossbauer spectra of used FexN-in-450 (top panel) and used FexN-out-400 (lower panel);

Figures 5a to 5f show TEM images of the Fe₂0₃-m catalyst taken at different tilt angles: (a) a = -10°, β = 0°; (b) a = 0°, β = 0°; (c) a = 10°, β = 0°; (d) a = 15°, β = 0°; (e) a = 0°, β = - 5°; (f) a = 0°, β = -10°; Figures 6a to 6f show TEM images of the Fe₂0₃-ow/ catalyst taken at different tilt angles: (a) a = -10°, β = 0°; (b) a - 0°, β = 0°; (c) a = 10°, β = 0°; (d) a = 20°, β = 0°; (e) a = 0°, β =5°; (f) a = 0°, β = -10°; Figures 7 a and 7b show XRD patterns of CNT supported iron nitride prepared at different nitridation temperatures (a) Fe_xN-w and (b) Fe_xN-owt, respectively; and

Figure 8 shows the room temperature ⁵⁷Fe Mossbauer spectra of 10% Fe_xN/Si0₂. The following example describes the preparation of CNTs including iron nitride nanoparticles in the CNT channels (FeN-in). Then, an example of the use of FeN-in as a catalyst is described.

Preparation of iron-nitride nanoparticles in CNTs

Preparation of CNTs

Multi-walled carbon nanotubes having an internal diameter of 4-8nm and external diameter of 10-20nm were obtained from Chengdu Organic Chemicals Co., Ltd China (MFG code M12020702R). For these nanotubes, it was found that there was a relatively high proportion of closed tipped CNTs (c-CNTs). Therefore, a treatment was carried out to open the tips in order to facilitate filling of nanoparticles inside.

Preparation of open tipped CNTs (o-CNTs)

3g CNT as received were suspended in 150ml concentrated nitric acid (68wt%) and refluxed at 130°C in an oil bath for at least 12h. After the mixture was cooled to room temperature, it was filtered through a PTFE film with a pore diameter of 0.2μπι and washed with deionized water until the pH value of the filtrate was approximately 7. Then the product was dried at 60°C for 12 hours. This treatment removed the end caps of the nanotubes as well as amorphous carbon. At the same time, surface oxygen groups (SOGs) such as carbonyl, carboxylic and ether were introduced onto the surfaces of the CNTs. The relative presence of o-CNT and c-CNT was determined by scanning a specimen using TEM.

Insertion of iron oxide into CNTs

The o-CNTs were first mixed with water. A weighted amount of Fe(N0₃)₃.9H₂0 was added to the mixture under stirring followed by ultrasonic treatment and simultaneous stirring for 4 h. Then the solvent was evaporated slowly under ambient conditions. Then the sample was heated at ramp of 0.2-4 °C to 140 °C and kept at this temperature for 6 h.

In this example, it was seen using TEM that more than 80% by number of Fe₂0₃ nanoparticles present were located inside the CNT channels. The internally-deposited Fe₂0₃ is referred to in these examples as Fe₂0₃-in.

Preparation of iron nitride in CNTs (Fe_xN- i)

The Fe₂0₃-m was nitrided using a temperature-programmed reaction method in ammonia atmosphere.

A sample of the Fe₂0₃-in was heated in a stream of ammonia NH₃ (9.99%,

GHSV=8000h^"1) at atmospheric pressure and a three-stage heating ramp was used:

First stage: temperature raised from room temperature to 300°C at 7°C/min

Second stage: temperature raised from to 300°C to the final temperature T at a rate of rC/min

Third stage: temperature maintained at the final temperature T for 2 hours.

The prepared nitride sample was then passivated at room temperature for 12 hours in a mixture of 1% 0₂/N₂ (v/v).

The resulting product was labeled as Fe_xN-z ?-T, where T represents the final nitridation temperature.

Preparation of iron nitride outside of CNTs (Fe_xN-o«i)

Samples of Fe_xN-owt were prepared for comparison with Fe_xN-m. Fe_xN-owt included

Fe_xN particles dispersed on the exterior walls of CNTs.

The o-CNTs were mixed with water and sonicated for 90 min. An ammonia water solution (1.7 wt% of NH₃) was added to the mixture under stirring. Then the aqueous solution of Fe(N0₃)₃.9H₂0 was slowly introduced to the mixture under vigorous stirring, followed by sonication for 30 min. Afterwards, it was heated in a water bath at 70 °C until dried. It was then heat treated at 140 °C for 6 hours.

The resulting Fe₂0₃-owt was nitrided using a temperature-programmed reaction (TPR) method in ammonia atmosphere.

A sample of the Fe₂0₃-owt was heated in a stream of ammonia NH₃ (9.99%,

GHSV=8000h^"1) at atmospheric pressure and a three-stage heating ramp was used:

First stage: temperature raised from room temperature to 300°C at 7°C/min Second stage: temperature raised from to 300°C to the final temperature T at a rate of l°C/min

Third stage: temperature maintained at the final temperature T for 2 hours.

The prepared nitride sample was then passivated at room temperature for 12 hours in a mixture of 1 % 0₂/N₂ (v/v).

The resulting product was labeled as Fe_xN-owt-T, where T represents the final nitridation temperature.

Preparation of Fe-in

Additionally, for comparison, samples of CNTs including metallic Fe in the channels (Fern) were prepared by reducing the CNT-confmed Fe₂0₃ directly in H₂ for 6 h at 350 °C, by a method for example as described in Chen W et al, J. Am. Chem. Soc. 2008, 130, 9414- 9419. Analysis of Samples Produced

TEM Analysis

The locations of the Fe₂0₃ particles in Fe₂0₃-w and Fe₂0₃-owt catalysts were confirmed with Transmission electron microscopy (TEM) by tilting the samples to different angles. The TEM was carried out on an FEI Tecnai G microscope at an accelerating voltage of 120 kV. Figure 5 shows TEM images of the Fe₂0₃-m catalyst taken at different tilt angles: (a) a = -10°, β = 0°; (b) a = 0°, β = 0°; (c) a = 10°, β - 0°; (d) a = 15°, β = 0°; (e) a = 0°, β = -5°; (f) a = 0°, β = -10°. The images taken at different tilt angles indicate that most of the Fe₂0₃ nanoparticles of Fe₂0₃-w catalyst are located inside the channel of CNTs. TEM images of the Fe₂0₃-owt catalyst taken at different tilt angles: (a) a = -10°, β = 0°; (b) a = 0°, β = 0°; (c) a = 10°, β = 0°; (d) a = 20°, β = 0°; (e) a = 0°, β =5°; (f) a = 0°, β = -10° are shown in Figure 6. From the images taken at different tilt angles Fe₂0₃ nanoparticles of Fe₂0₃-owt inside the CNT channels were not observed.

Inductively Coupled Plasma Atomic Emission Spectrometry Analysis and TEM Analysis

Inductively coupled plasma atomic emission spectrometry (ICP-AES, SHIMADZU

ICPS-8100) analysis showed that Fe₂0₃-w had an iron loading of 5.61 wt% and Fe₂0₃-owi of 5.24 wt%. With TEM it was observed (Figure 1 and Figure 2) that iron nitride particles started to aggregate along the axial direction of the nanotube above 500 °C during nitridation. However, the size in radial direction was limited by the spatial restriction. The particle size estimated according to the axial direction was seen to increase gradually with increasing nitridation temperature. This is shown Figure 1. For example Fe_xN-m-350 was seen to have a size of about 4-6 nm. The size for Fe_xN-m-450 was seen to have increased to about 4-10 nm (as shown in Figure 1(a) and further to 6-12 nm for Fe_xN-m-500 (as shown in Figure 1(b). In comparison, the outside particles were seen to aggregate more severely due to absence of the space restriction provided by the CNT channels for the Fe_xN-in samples. However, the Fe_xN-out particles were seen to retain a relatively spherical shape. For instance, and as shown in Figures lc and Id, the particle size of Fe_xN-owt-400 was seen to be about 8-15 nm and was seen to have increased to about 10-20 nm upon nitridation at 500 °C (Fe_xN-out-500).

XRD Analysis

X-ray diffraction (XRD) was measured on a Rigaku D/Max 2500 diffractometer with a Cu Ka (λ=1.541 A) monochromatic radiation source. Some samples were also measured at the BL14B1 beamline (λ=1.2398 A) of the Shanghai Synchrotron Radiation Facility (SSRF). XRD analysis Figure 7a showed that Fe₂0₃ has been partially reduced to Fe₃0₄ (PDF 68-3107) upon nitridation at 350 °C. The characteristic diffraction peaks of Fe₃0₄ were seen gradually to have diminished with increasing treatment temperature and to disappear at 450 °C.

Mossbauer spectroscopy

A new and broad peak around 20=36° was identified for samples subject to nitridation at or above 450°C from XRD. This peak may also be present at lower temperature, but it was difficult to distinguish from the peak for Fe₃0₄ in this example. To analyse the phase composition associated with this peak, Mossbauer spectroscopy was carried out. ⁵⁷Fe Mossbauer spectroscopy analysis was conducted on a Topologic 500A spectrometer with a proportional counter. ⁵⁷Co(Rh) was used as the radioactive source and the Doppler velocity of the spectrometer was calibrated with an a-Fe foil. The spectra were fitted with appropriate superpositions of Lorentzian lines using the Moss Winn 3.0i program. The spectrum for Fe_xN-w-450 is shown in Figure 3. The spectrum of Figure 3 can be fitted with a singlet line (Fe-I) and a quadrupole doublet (Fe-II), similar to thin iron nitride films. The Fe-I line is attributed to the γ''-FeN phase with a ZnS-type structure and the Fe-II doublet to γ''-FeN or γ'''-FeN with vacancies. γ'''-FeN has a NaCl-type structure. Since no other phases are detected in Fe_xN-/«-450, the nitridation has been completed at 450 °C and the broadened diffraction around 20=36° of Fe_xN-w-450 in Figure 7a is therefore attributed to the cubic FeN phase.

Nitridation at 500 °C leads to a new reflection at 20=41 ° (Figure 7a), which can be attributed to ζ-Ρε₂Ν (200) (PDF 50-0958) with an orthorhombic structure. Without wishing to be bound by any particular theory, this is thought to result from the diffusion of nitrogen atoms out of the cubic FeN lattice at a high temperature, and to imply a higher stability of cubic FeN confined in CNTs.

For Fe_xN-owt, the characteristic diffraction peaks of Fe₃04 were seen to disappear while those of ζ-FeiN emerge at 400 °C (Figure 7b) and another reflection of ζ-Ρ6₂Ν (023) was observed at 20=67° at 500 °C. The Mossbauer spectrum in Figure 3 shows the presence of cubic γ''-FeN and γ'''-FeN (the corresponding line Fe-I and Fe-II). The second quadrupole doublet (Fe-III) corresponds to the characteristic ζ-Ρε₂Ν phase.

According to the peak areas it was estimated that 10% iron species exists as ζ-Ρε₂Ν phase on Fe_xN-0wt-4OO (Table SI). Thus, Fe_xN-owt was considered to contain more ζ-Ρε₂Ν but less FeN species than for Fe_xN- «.

Table S 1. Fitting parameters for the Fe Mossbauer spectra in Figure 3.

Sample Subspectra IS^a QS^b H^c %Area^d Assignment

(mm/s) (mm/s) (T)

Fe_xN-m- Fe-I 0.08 0 0 14 γ''-FeN

450

Fe-II 0.32 0.92 0 86 γ''- or y'"-FeN

with vacancies

Fe_xN-ow/- Fe-I 0.06 0 0 21 γ''-FeN

400

Fe-II 0.32 0.82 0 69 Y"- or Y'"-FeN

with vacancies

Fe-III 0.41 0.27 0 10 -Fe₂N ^aIsomer shift relative to a-Fe. ^bElectric quadrupole splitting. ^cMagnetic field. Uncertainty is ±5%. Catalytic Reaction Tests

The catalytic activities of Fe_xN-z^'« and Fe_xN-owt were investigated in CO hydrogenation reaction via Fischer-Tropsch synthesis (FTS).

CO hydrogenation was carried out in a fixed bed microreactor at 300 °C, 5 bar and a gas hourly space velocity (GHSV) of 15000 h^"1 (based on the volume of syngas passed through per volume of the catalyst per hour). A H₂/CO/Ar mixture (47.5/47.4/5.1 vol.%, purity of 99.99%) was taken as the feeding gas with Ar as an internal standard. lOOmg catalyst was loaded into the reactor and pre-treated in-situ for 2 h in syngas (1 bar) at 260 °C. All gas lines after the reactor were kept at 150 °C. The effluents were analyzed by an online GC (Agilent 7890A), which was equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Four chromatography columns were installed: Porapak Q and 5 A molecular sieves packed columns, and modified A1₂0₃ and FFAP capillary columns.

The reaction was allowed to proceed for more than 24 h on stream to reach a steady state before analysis of the effluents. The CO conversion and product selectivities were determined. Selectivity for particular products was reported as the percentage of CO converted into a certain product expressed in C atoms. C₂ - C₄ refers to hydrocarbons containing 2-4 carbon atoms and C₅₊ refers to hydrocarbons containing 5 or more carbon atoms.

The activity of the catalyst was expressed as Iron-Time Yield (ITY, μ^ηιο1(:ο·8~¹ ^^{' 1} ρ6)· The ITY for Fe_xN-w-500 in this case reached 927.8 μπιο1<:ο·3~¹ ^^{" 1}ρβ, which is 1.4 times higher than for the outside catalyst Fe_xN-o«t-500 (Table 1).

The activities were compared with the samples for which nitridation was completed at 450 °C for Fe_xN-w and at 400 °C for Fe_xN-owt. Fe_xN-m-450 is again more active than Fe_xN-0wt-4OO. For the CNT confined iron nitride catalysts (Fe_xN-w-450 and Fe_xN-/^'«-500), TOF (turnover frequency) increases with the increasing particle size. Fe_xN-owt catalysts also show a similar trend. However, TOF does not seem to depend solely on the particle size since Fe_xN-w-500 exhibits a much higher TOF (0.410 s^"1) than Fe_xN-owt-450 (0.312 s^" ') although these two catalysts possess a similar average particle size. This may imply that the confinement effect of CNTs improves the reactivity of iron nitride. In comparison, iron nitride (10wt% iron loading) supported on Si0₂ (nitrided from Fe₂0₃/Si0₂ at 500 °C) yields an activity of 75^mol ys^'1-g^{' 1}Fe (Table S3), which is significantly lower than that on Fe_xN-m-500 and Fe_xN-o«t-500. Figure 8 shows the room temperature ⁵⁷Fe Mossbauer spectra of 10% Fe_xN/Si0₂. Nitridation was carried out in ammonia at 500 °C via the same temperature-programmed reaction method as CNTs supported iron nitride catalysts. The isomer shift (IS) is 0.33 mm/s, the quadrupole splitting (QS) is 0.44 mm/s and the iron species exist as £-Fe₂ iN in Fe_xN/Si0₂.

The Mossbauer spectrum in Figure 8 suggests that iron species exist as e-Fe₂.iN, which is in agreement with an early study on a Si0₂ supported iron nitride catalyst with a similar iron loading (9.8wt%) (see E B Yeh et al J Catal, 1985, 91 241-253). These results indicate beneficial effects by confining iron nitride nanoparticles inside CNTs for CO hydrogenation. Table S3. Catalytic performance of 10%Fe₂N/SiO₂ catalyst in CO hydrogenation⁰

CO C0₂ CH Product distribution (mol%) conv. (%) sel. sel. CH₄ C2 -C4 (C2 -C4 )/( C2°-C4") C5

2.6 40.5 58.3 30.5 42.4 2.8 1 1.7 "Reaction condition: 300°C, 5bar , H₂/CO/Ar (47.5/47.4/5.1), GHSV=150001^

Table 1 Catalytic performance of CNT-supported iron and iron nitride catalysts in CO hydrogenation."

Catalysts Fe-m Fe_xN-w- Fe_xN-w- Fe_xN-owt- Fe_xN-owt-

450 500 400 500

Average particle size 4.9 7.6 10.6 1 1.6 16.9

(nm)*

Activity 126.5 960.6 927.8 612.0 667.2

(μ^η οΐοο s^"1 g^"'Fe)^c

C0₂ selectivity (%) 22.2 38.0 39.4 34.5 36.5 CH selectivity (%) 75.0 61 .3 59.7 64.1 62.2

CH distribution (%)

CH₄ 30.5 27.2 26.6 31.8 29.9

C₂ -C₄ 39.6 35.2 36.5 37.9 38.3

(C₂ ⁼-C₄ ⁼)/(C₂°-C₄°) 3.3 2.6 2.6 3.2 3.4

C₅ ⁺ 18.0 22.5 22.9 18.2 20.9

Reaction condition: lOOmg catalyst, 300 °C, P = 5 bar, GHSV = 15000 h^"1 (H₂: CO: Ar = 47.5:47.4:5.1). ^Surface-weighted average particle sizes. ^cIron-Time Yield (ITY, molco-s^ - ^ Fe)-

Table 1 also shows that the nitride catalysts are significantly more active than Fe-m. The activities of both Fe_xN-/^'« and Fe_xN-owt are 5-7 times higher in this example. Even upon promotion with 0.1 wt% K, a 5wt% CNT-confined reduced iron catalyst gave an activity equivalent to 741

under conditions similar to the current example (300 °C, 5 bar, CO/H₂=l/l and GHSV=14600 h^'1), which is lower than the CNT-confined nitride catalyst here. In addition, Fe_xN catalysts exihibit higher C0₂ selectivities indicating higher activities of the nitrides than Fe-w but significantly lower than the K promoted iron catalyst for water gas shift reaction. Without wishing to be bound by any particular theory, this may be attributed to the electron donor role of N in a nitrided iron catalyst. Fe_xN catalysts also were seen to exihibit lower C₅ ⁺ selectivity (around 22%) than K promoted iron catalyst (32%). However, the hydrocarbon distribution does not differ significantly on Fe_xN and reduced Fe-w catalyst. The XRD of used Fe-z« catalyst (Figure 4a) shows the presence of £-Fe₂C and Fe₃0₄ with rather intense diffraction. This is consistent with observations during in-situ XRD study of the CNT-confined Fe catalyst under FTS reaction condition.

Iron carbides are generally accepted being a catalytically active phase for FTS.

However, Fe₃0₄ which usually results from oxidation of iron and iron carbide by the product H₂0, has frequently been blamed of for such catalyst deactivation. In comparison, XRD and Mossbauer spectroscopy in the present examples (see for example Figure 4) reveal that Fe_xN catalysts do not exhibit significant oxide phase after reaction. Without wishing to be bound by any particular theory, the lower activity of Fe-in could be attributed to the instability of iron and iron carbide under CO hydrogenation conditions. Without wishing to be bound by any particular theory, the Mossbauer spectra shown in Figure 4b and the corresponding fitting parameters in Table S2 suggest the presence of iron carbonitride (Fe₂C_xNi_-x) (Fe-I), FeC_xNi_-x (Fe-II) and γ''-FeN (Fe-III) on both Fe_xN-w and Fe_xN-owt catalysts during CO hydrogenation. This could be attributed to surface carbon forming from dissociative adsorbed CO which diffuses into the lattice of iron nitride and replaces some nitrogen atoms. At the same time, nitrogen atoms can also be removed by H₂, which has been reported to proceed faster than the replacement by carbon atoms during the initial reaction stage. As a result, iron can be enriched relative to nitrogen in the lattice and thus Fe₂C_xNi_-x forms. This appears to be corroborated by the XRD results shown in Figure 4a.

Table S2. Fitting parameters for the Fe Mossbauer spectra in Figure 4b.

Sample Subspectra IS^a QS H^c %Area^d Assignment

(mm/s) (mm/s) (T)

Fe_xN-m- Fe-I 0.22 0.05 15.9 22 Fe₂C_xN_1-x

450

Fe-II 0.34 0.99 0 74 FeC_xN_1-x

Fe-III 0.09 0 0 4 γ''-FeN

Fe_xN-owt- Fe-I 0.23 0.02 15.9 44 Fe₂C_xN_1-x

400

Fe-II 0.31 1.08 0 49 FeC_xN_1-x

Fe-III 0.09 0 0 7 γ''-FeN ^aIsomer shift relative to a-Fe. ^bElectric quadrupole splitting. ^cMagnetic field. ^dUncertainty is ±5%.

Different iron carbide species result in different textural properties which may play an important role in determining final catalyst performance. For example, x-Fe₅C₂, 0-Fe₃C and s-Fe₂C have been reported to be present for reduced iron catalyst. x-Fe₅C₂, which was the main carbidic species under typical FTS condition was highly susceptible to oxidation during reaction. 0-Fe₃C which usually formed at high reaction temperature, showed a low activity and selectivity due to the carbonaceous deposits on the catalyst surface. e-Fe₂C was enthalpically most stable under typical FTS conditions, however kinetic and entropic factors may inhibit their formation in large amounts. In comparison, iron carbonitride appears to be more resistant to oxidation under reaction conditions since no oxide phase was observed here. Without wishing to be bound by any particular theory, this might be a reason for a higher activity of iron nitride than reduced iron catalyst. Furthermore, Figure 4b shows a higher concentration of FeC_xNi_-x and lower content of Fe₂C_xN_1-x on Fe_xN- «- 450 catalyst than those on Fe_xN-owt-400. This implies that the confined catalyst has a stronger retention of nitrogen in the lattice than the outside nitride. Without wishing to be bound by any particular theory, this may be related to the higher FTS and WGS activity of Fe_xN-/^'« than Fe_xN-ow/.

The CNT-confined FeN catalyst of the examples above exhibited an activity 1.4 times higher than the nitride particles located on the CNT exterior walls in CO

hydrogenation. Without wishing to be bound by any particular theory, this might be related with the stronger retention of nitrogen atoms in the cubic nitride lattice, resulting in formation of more FeC_xNi_-x on Fe_xN-z« than that on Fe_xN-owt during reactioa Both nitride catalysts are more active than the reduced iron. This is likely owing to the better stability of iron nitride under CO hydrogenation conditions in contrast to iron and iron carbides, which tend to be oxidized by the produced H₂0 leading to deactivation.

Cubic FeN particles of a few nanometer size have been synthesized by encapsulation inside carbon nanotube (CNT) channels. Such an FeN catalyst exhibits in some examples a 5-7 times higher activity than a reduced Fe catalyst and a Si0₂ supported iron nitride in CO hydrogenation. The confined FeN catalyst is also more active than iron nitride particles dispersed on the CNT exterior walls in some examples.

It will be understood that the present invention has been described above purely by way of example, and modification of detail can be made within the scope of the invention. For example, instead of metal nitride, the particle may comprise metal carbide and/or metal phosphide. This feature may be applied to any aspect of the invention as appropriate. Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

Claims

1. A composition comprising a component including at least one nano-sized confinement space, the composition further comprising at least one particle confined within the confinement space, the at least one particle comprising metal nitride.

2. A composition according to claim 1 , wherein the at least one particle comprises iron nitride.

3. A composition according to claim 1 or claim 2, wherein the confinement space has at least one dimension 100 nm or less, preferably between from 0.1 and 100 nm.

4. A composition according to claim 3, wherein the confinement space has at least two dimensions each being 100 nm or less, preferably between from 0.1 and 100 nm.

5. A composition according to any preceding claim wherein the component includes a nanostructure having the confinement space.

6. A composition according to any preceding claim, wherein the component comprises a fullerene.

7. A composition according to any preceding claim, wherein the component comprises a carbon nanotube, wherein the at least one particle is confined in the nanotube.

8. A composition according to any preceding claim wherein the at least one or more particle confined in the space has average particle size not more than 50 nm, preferably not more than 20 nm.

9. A composition according to any preceding claim, further including particles outside the confinement space, wherein the majority of particles of the composition are confined inside confinement spaces.

10. A composition comprising a component including at least one nano-sized

confinement space, the composition further comprising at least one particle confined within the confinement space, the at least one particle comprising a metal nitride, metal carbide or metal phosphide.

1 1. A composition according to claim 10, wherein the component includes carbon nanotubes.

12. Use of a composition according to any of claims 1 to 11 in a chemical process.

13. A catalyst composition comprising a component including at least one nano-sized confinement space, the composition further comprising at least one particle confined within the confinement space, the at least one particle comprising a metal nitride.

14. A catalyst composition according to claim 13 wherein the at least one particle comprises an iron nitride.

15. Process for the hydrogenation of CO, comprising the steps of passing a feed stream over a catalyst, the catalyst comprising a composition according to any of claims 1 to 14.

16. Process for the hydrogenation of CO, the process including the step of passing a feed stream over a catalyst, the catalyst including a composition comprising a component including at least one nano-sized confinement space, the composition further comprising at least one particle confined within the confinement space, the at least one particle

comprising a metal nitride.

17. Process according to claim 15 or claim 16, wherein the component comprises carbon nanotubes.

18. Process according to any of claims 15 to 17 wherein the particle comprises iron nitride.

19. Process according to claim 18 wherein the particle comprises cubic- iron nitride.

20. Process according to any of claims 15 to 18, wherein at least 50% by number of particles of metal nitride are confined in the component.

21. A method of producing a confined metal nitride, the method including the steps of introducing a composition comprising the metal to a component including at least one nano-sized confinement space such that at least one particle including the metal is confined within the confinement space.

22. A method according to claim 21, wherein the introduced composition includes a metal-containing compound, and the method further includes the step of carrying out a nitriding treatment to form a metal nitride in the confinement space.

23. A method according to claim 22, wherein cubic-iron nitride is formed in the

confinement space.

24. A method according to any one of claims 21 to 23, wherein the confinement space has at least one dimension which is 100 nm or less, preferably between from 0.1 to 100 nm.

25. A method according to any one of claims 21 to 24, wherein the component includes a nanostructure including the confinement space.

26. A method according to any one of claims 21 to 25, wherein the introduction of the metal in to the component is carried out at elevated temperature.

27. A method according to any one claims 21 to 26 wherein the nitriding treatment includes treatment using ammonia or nitrogen at elevated temperature.

28. A method according to claim 26 or claim 27 wherein the elevated temperature is at least 400 degrees C, for example at least 450 degrees C, for example 500 degrees C or more.

29. A method according to any of claims 21 to 28 wherein the nitriding step includes increasing the temperature at a first predetermined rate in a first heating period, and increasing the temperature at a second predetermined rate in a second heating period.

30. A method according to claim 29 wherein the first predetermined rate is at least 5 degrees C per minute, for example at least 7 degrees C per minute.

31. A method according to claim 29 or claim 30, wherein the second predetermined rate is not more than 5 degrees C per minute, for example less than 2 degrees C per minute, for example 1 degree C per minute or less.

32. A method according to any of claims 21 to 31 including carrying out a pre-treatment of the component to open the confinement space.