CN113332992B - Perovskite catalyst and preparation method thereof - Google Patents
Perovskite catalyst and preparation method thereof Download PDFInfo
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- CN113332992B CN113332992B CN202110604503.XA CN202110604503A CN113332992B CN 113332992 B CN113332992 B CN 113332992B CN 202110604503 A CN202110604503 A CN 202110604503A CN 113332992 B CN113332992 B CN 113332992B
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- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
- B01J23/70—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper
- B01J23/76—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
- B01J23/84—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36 with arsenic, antimony, bismuth, vanadium, niobium, tantalum, polonium, chromium, molybdenum, tungsten, manganese, technetium or rhenium
- B01J23/85—Chromium, molybdenum or tungsten
- B01J23/86—Chromium
- B01J23/866—Nickel and chromium
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- B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
- B01J23/002—Mixed oxides other than spinels, e.g. perovskite
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- B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
- B01J23/16—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of arsenic, antimony, bismuth, vanadium, niobium, tantalum, polonium, chromium, molybdenum, tungsten, manganese, technetium or rhenium
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- B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
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- B01J37/036—Precipitation; Co-precipitation to form a gel or a cogel
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- B01J37/16—Reducing
- B01J37/18—Reducing with gases containing free hydrogen
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
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Abstract
The invention discloses a perovskite catalyst and a preparation method thereof, wherein the perovskite catalyst comprises a perovskite matrix, metal oxide particles and metal particles, wherein the A site and the B site of the perovskite matrix are doped with different metals, the metal oxide particles are dispersedly distributed on the surface of the perovskite matrix, the metal particles are precipitated on the surface of the perovskite matrix in situ, the metal oxide particles have water absorption and oxygen migration activity, the metal oxide particles are obtained by doping metals on the A site of the perovskite matrix and growing the metal-doped oxide particles on the surface of the perovskite matrix in situ in an in-situ synthesis mode; the metal particles are obtained by doping metal at the B site of the perovskite substrate and precipitating the metal in situ. The method aims to further improve the water absorption performance, the oxygen migration performance and the carbon deposition resistance of the catalyst by introducing a second nano phase, and improve the performance of the perovskite-based catalyst, thereby solving the technical problem of poor chemical stability when the existing catalyst has good water absorption performance.
Description
Technical Field
The invention belongs to the technical field of perovskite-based catalysts, and particularly relates to a perovskite catalyst and a preparation method thereof.
Background
A Solid Oxide Fuel Cell (SOFC) is a power generation device that directly converts hydrogen and hydrocarbon fuel into electrical energy and thermal energy through an electrochemical reaction. At present, the anode of the SOFC is mainly Ni-based cermet, carbon deposition of the anode of the cell is easily inactivated by directly utilizing the biological alcohol, and the cost and the process problems are caused by searching for a new anode material. Therefore, the technical route of power generation based on the biological alcohol reforming-SOFC is a more suitable power generation mode at present. According to the technical route, a proper catalytic material is selected to convert the ethanol-extracted biological alcohol into hydrogen-rich reformed gas, so that the poisoning effect of the biological alcohol on an electrode material is relieved, the biological alcohol is favorably combined with the existing mature cell system, such as a Ni-YSZ system, a Ni-GDC system and the like, and the stable and efficient power generation of the biological alcohol fuel on the SOFC is realized.
In order to realize high efficiency and stability of the catalyst, the reaction active area of the catalyst and the carbon deposition resistance of the material need to be improved. In addition to traditional simple oxide supported catalysts, perovskite-based catalysts are also one of the notable research directions. Perovskite is a mixed conductor of electrons and oxygen ions and has good resistance to carbon deposition. In recent years, in-situ precipitation processes based on perovskite materials have been extensively studied. The principle is that transition metal elements are doped into perovskite, and the metal elements are desolventized from crystal lattices after high-temperature reduction, and grow in situ in the form of metal nanoparticles. The in-situ precipitation has better interaction with the matrix and stronger combination, thereby being beneficial to improving the anti-sintering property of the catalytic active metal; in addition, the enhanced interaction can change the growth mode of the carbon deposit on the active metal, thereby reducing the possibility of the active metal being jacked up, coated and inactivated by the carbon deposit, and the in-situ precipitation process is a perovskite-based catalyst and a traditional supported catalyst (Ni/Al) 2 O 3 And Ni/SiO 2 ) In this way, novel supported catalysts with enhanced interaction of the metal particles with the oxide support can be prepared.
In addition, some perovskite materials also have excellent water absorption properties. Water preferentially adsorbs at sites near the oxygen vacancies to form hydroxyl radicals and subsequently reacts to release protons, thereby forming hydrogen gas. It is contemplated that sufficient water will participate in the reaction in steam reforming. Therefore, the proper water-absorbing perovskite material is selected, more water can participate in the reforming reaction, and the catalytic performance of the catalyst is improved. In addition, protons generated by water absorption have been confirmed to play a role in eliminating carbon deposition so that the carbon deposition resistance of the catalyst can be improved. The excellent water absorption performance is another major difference between perovskite-based catalysts and conventional supported catalysts.
However, prior studies have shown that the prior art methods of increasing water absorption by doping the perovskite a site with an alkaline earth metal result in gradual decomposition of the perovskite material in the catalytic atmosphere, thereby affecting the long term operating stability of the catalyst. Therefore, there is a need to find a suitable way to increase the water absorption of perovskite materials while ensuring the chemical stability of perovskite-based catalysts.
Disclosure of Invention
Aiming at the defects or improvement requirements in the prior art, the invention provides a perovskite catalyst and a preparation method thereof, aiming at further improving the water absorption performance, oxygen migration performance and carbon deposition resistance of the catalyst and improving the performance of the perovskite-based catalyst by introducing a second nano phase, thereby solving the technical problem of poor chemical stability when the existing catalyst has good water absorption performance.
In order to achieve the above object, according to one aspect of the present invention, there is provided a perovskite catalyst comprising a perovskite substrate doped with different metals at a-site and B-site, metal oxide particles dispersed on the surface of the perovskite substrate, and metal particles in-situ precipitated on the surface of the perovskite substrate, wherein the metal oxide particles have water absorption and oxygen migration activity, the metal oxide particles are obtained by doping a metal at the a-site of the perovskite substrate and in-situ growing the metal-doped oxide particles on the surface of the perovskite substrate by in-situ synthesis, and the metal particles are obtained by doping a metal at the B-site of the perovskite substrate and in-situ precipitating the metal.
Preferably, the metal oxide particles are MgO and CeO 2 And ZrO 2 One or more of them.
Preferably, the metal particles are one or more of Ni, co, fe, cu, ni-Cu alloy, ni-Co alloy and Ni-Fe alloy.
Preferably, the particle size of the metal oxide particles is 30 to 40nm, and the particle size of the metal particles is 20 to 30nm.
Preferably, the perovskite matrix in which the a-site and the B-site are doped with different metals is represented by the following formula: a. The 1-x A’ x B 1-y B’ y O 3-δ (ii) a Wherein the perovskite matrix is formed of ABO 3 Mainly, the ABO 3 Comprises Cr-based material or Ti-based material, wherein A-bit element and B-bit element are simultaneously +3 valence, or A-bit element and B-bit element are respectively +2 valence and +4 valence, A' is ABO 3 The A site is doped with metal, A' is one or more of Mg, ce and Zr, and x =0-0.2; b' is towards ABO 3 The middle B site is doped with metal, B' is one or more of Ni, co, fe and Cu, y =0-0.15, and delta is less than 0.5.
According to another aspect of the present invention there is provided a process for the preparation of a perovskite catalyst as hereinbefore described comprising the steps of:
(1) Respectively adding metal salts containing an A element, a B element, an A 'element and a B' element into deionized water according to the required stoichiometric ratio of the perovskite substrate to dissolve the metal salts to obtain a mixed solution, and preparing perovskite precursors with different metals doped at the A site and the B site by a sol-gel method; wherein the addition amount of the metal salt containing the element A 'is determined by the relationship between the atomic radius and the stable valence state between the element A and the element A';
(2) Grinding the perovskite precursor, calcining in the air, and obtaining the perovskite material with metal oxide particles dispersed and distributed on the surface by an in-situ synthesis method; (3) And (3) carrying out in-situ reduction on the perovskite material obtained in the step (2) to obtain the perovskite catalyst with metal oxide particles dispersed and distributed on the surface and metal particles precipitated on the surface in situ.
Preferably, the addition amount of the metal salt containing the a 'element is determined by the relationship between the atomic radius and the stable valence state between the a element and the a' element, and specifically is as follows:
when the relative difference value of the atomic radiuses of the element A and the element A ' is less than 15 percent and the stable valence states of the element A and the element A ' are the same, the addition amount of the metal salt containing the element A ' meets the condition that the doping amount of the element A ' is larger than the solubility of the element A ' in perovskite, so that A-site doped metal is precipitated on the surface of a perovskite precursor; when the relative difference of the atomic radii of the A element and the A 'element is larger than 15%, the addition amount of the metal salt of the A' element is the required stoichiometric ratio in the perovskite matrix; when the relative difference of the atomic radius of the A element and the atomic radius of the A 'element is less than 15 percent, but the stable valence states of the A element and the A' element are different, the addition amount of the metal salt of the A 'element is the required stoichiometric ratio in the perovskite matrix, wherein the relative difference is (the atomic radius of the A element-the atomic radius of the A' element)/the atomic radius of the A element.
Preferably, the calcination is specifically calcination at 700-800 ℃ for 4-6 hours.
Preferably, the in-situ reduction is carried out on the perovskite material obtained in the step (2) at 5%H 2 -N 2 Carrying out temperature programming reduction test or thermogravimetric experiment under the atmosphere, and determining the temperature interval corresponding to the fastest consumption rate of the perovskite material; and (3) placing the perovskite material in a pure hydrogen atmosphere, and reducing for 4-6 hours in the temperature range of the test, wherein the precipitation amount of metal particles precipitated in situ on the surface is less than 5wt%.
Preferably, the sol-gel method in the step (1) is specifically as follows: sequentially adding citric acid monohydrate and glycol into the mixed solution, and stirring to form sol; dropwise adding ammonia water into the sol to adjust the pH value to 8-9, stirring to form a gel, and preserving the temperature at a preset temperature to obtain a perovskite precursor; wherein, the molar weight of the total metal ions: molar amount of citric acid monohydrate: molar weight of ethylene glycol = 1.5, the total metal ions consisting of a ions, B ions and B' ions.
In general, at least the following advantageous effects can be obtained by the above technical solution conceived by the present invention compared to the prior art.
(1) The invention prepares the metal oxide nano particles in dispersion distribution by doping metal elements at the A site and adopting an in-situ precipitation synthesis process, and the metal oxide nano particles have water absorption. Therefore, on the basis that the perovskite-based catalyst precipitates metal particles doped at the B site as a first nano phase, the water absorption performance, the oxygen migration performance and the carbon deposition resistance of the catalyst are further improved by introducing a second nano phase (the second nano phase is obtained by in-situ synthesis of metal doped at the A site of the perovskite-based catalyst and in-situ growth of the metal-doped oxide particles on the surface of the perovskite substrate), and the performance of the perovskite-based catalyst is improved. The problem that the chemical stability of the existing catalyst is poor when the existing catalyst has good water absorption performance is effectively solved, so that the water-absorbing perovskite-based catalyst can stably run for a long time in biomass steam reforming, the preparation process is low in cost, and the catalyst is suitable for large-scale production and manufacture and has wide application prospects.
(2) The preparation method provided by the invention adopts the steps of oxidizing and then reducing, and calcining at high temperature and air atmosphere to precipitate one or more water-absorbing oxide nano particles in situ, and the metal oxide particles do not generate obvious grain growth at the calcining temperature. And then in high temperature and reducing atmosphere, the perovskite doped at the B site can be subjected to in-situ precipitation to obtain the doped metal simple substance nano particles. The preparation method realizes the interaction among the active metal oxide particles, the active metal particles and the perovskite substrate, provides richer load types for the active metal, and enhances the interaction force between the perovskite substrate and the active metal. The existence of the metal oxide nano particles reduces the sintering condition of the active metal, and is beneficial to the long-term stable operation of the catalyst.
(4) The common interaction of the active metal oxide particles, the active metal particles and the perovskite matrix in the present invention exists at the three phase interface of the metal oxide phase, the active metal phase and the perovskite phase; the perovskite provides strong interaction for the active metal particles, and the metal oxide particles provide water absorption and oxygen migration activity for the catalyst for the active metal particles, thereby being beneficial to improving the carbon deposition resistance and the catalytic activity of the catalyst.
(3) In the preparation method provided by the invention, in order to ensure that the A ' element is doped in a sufficient amount, the addition amount of the metal salt containing the A ' element is determined through the atomic radius and the stable valence relation between the A element and the A ' element. When the relative difference value of the atomic radii of the element A and the element A 'is less than 15 percent, and the stable valence states of the element A and the element A' are the same, the addition amount of the metal salt containing the element A 'is enough to ensure that the doping amount of the element A' is more than the solubility of the element A in the perovskite, so that the A-site doped metal is precipitated on the surface of the perovskite precursor. When the relative difference of the atomic radii of the element A and the element A 'is more than 15 percent, the element A' is not easy to enterThe perovskite lattice, therefore, exists primarily as an oxide, with doping levels approaching those of oxide formation; the metal salt of the a' element is added in the desired stoichiometric ratio in the perovskite matrix. When the relative difference value of the atomic radiuses of the element A and the element A 'is less than 15 percent, but the stable valence states of the element A and the element A' are different, such as Ce doped into LaCrO 3 In the perovskite substrate, the stable valence state of Ce is +4, the stable valence state of La is +3, so that Ce is not easily doped into crystal lattices, and the amount of the in-situ synthesized oxide of the A' element is approximate to the designed stoichiometric ratio in the perovskite substrate. By the optimized proportion, the active metal oxide particles and the active metal particles are simultaneously formed on the perovskite substrate.
Drawings
FIG. 1 (a) shows LaCrO provided in examples 1 to 6 of the present invention 3 Perovskite-based doping with Ce at the A-site (examples 1-6) and undoped Ce (LaCr) 0.85 Ni 0.15 O 3-δ ) XRD of (b) in FIG. 1 is undoped Ce (LaCr) 0.85 Ni 0.15 O 3-δ ) FIG. 1 (c) is a SEM image of example 4 of the present invention before reduction, and FIG. 1 (d) is a SEM image of example 1 of the present invention before reduction;
FIG. 2 (a) is an XRD pattern before and after reduction for example 1 of the present invention; FIG. 2 (b) shows the result of the reduction at 5%H before reduction in example 1 of the present invention 2 -N 2 Thermogravimetric test patterns in atmosphere; FIG. 2 (c) is an SEM image of example 1 of the present invention after reduction, FIG. 2 (d) is a TEM image of example 1 of the present invention after reduction, and FIG. 2 (e) is a partial enlarged view of the TEM image of example 1 of the present invention after reduction;
FIG. 3 shows examples 1 and 4 of the present invention and undoped Ce (LaCr) 0.85 Ni 0.15 O 3-δ ) Dry and wet (pH) 2 O = 5%) under a nitrogen atmosphere;
FIG. 4 (a) is a schematic representation of inventive example 1 under dry nitrogen and 5%H 2 O-N 2 Thermogravimetric testing under atmosphere; FIG. 4 (b) is a graph showing the water absorption equilibrium constant and enthalpy change versus temperature in the thermogravimetric test of example 1 of the present invention;
FIG. 5 (a) shows an embodiment of the present invention1 in N 2 XRD peak change condition of Ni before and after treatment; FIG. 5 (b) shows the result of the reaction of example 1 of the present invention at N 2 TEM images of the catalyst after treatment; FIG. 5 (c) shows the results of example 1 of the present invention at N 2 XPS chart before processing, FIG. 5 (d) is N in example 1 of the present invention 2 XPS plots after processing;
FIG. 6 shows Ce-free doped LaCr according to the present invention 0.85 Ni 0.15 O 3-δ SEM images of the perovskite catalyst after 4h of catalysis;
fig. 7 (a) is the conversion rate of the steam reforming test of example 1 of the present invention, fig. 7 (b) is the hydrogen production of example 1 of the present invention, fig. 7 (c) is the product selectivity of example 1 of the present invention, and fig. 7 (d) is the chemical stability of the catalyst of example 1;
FIG. 8 (a) shows the methanol conversion (a) of example 1 of the present invention, and FIG. 8 (b) shows the H content of the methanol product of example 1 of the present invention 2 And CO selectivity, FIG. 8 (c) is ethanol conversion of inventive example 1, and FIG. 8 (d) is H in ethanol product of inventive example 1 2 And CO selectivity, FIG. 8 (e) shows the conversion of glycerol of example 1 of the present invention, FIG. 8 (f) shows H in the glycerol product 2 And CO selectivity, wherein the reforming time periods of methanol, ethanol and glycerol are 100h, 100h and 40h respectively.
FIG. 9 is a schematic diagram of a catalyst structure according to a preferred embodiment of the present invention;
FIG. 10 is an SEM image of a catalyst provided in accordance with a preferred embodiment of the present invention.
The same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein:
1-oxygen migration process of metal oxides to metals; 2-metal oxide particles; 3-metal particles; 4-another metal oxide particle; 5-another metal particle; 6-oxide adsorbed water; a 7-perovskite matrix; interaction between the 8-metal oxide particles and the metal particles with the perovskite matrix.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the respective embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
Example 1
(1) 3.46 g La (NO) was weighed out in the stoichiometric ratio of the perovskite substrate doped with different metals at the A-site and the B-site 3 ) 3 ·6H 2 O, 0.868 g Ce (NO) 3 ) 3 ·6H 2 O, 3.40 g Cr (NO) 3 ) 3 ·9H 2 O and 0.44 g Ni (NO) 3 ) 3 ·6H 2 O in a beaker.
And dissolving the mixed nitrate into a proper amount of deionized water to form a nitrate solution. According to the ratio of metal ion citric acid monohydrate to ethylene glycol = 1.5, here, the metal ion is La 3+ 、Ce 3+ 、Cr 3+ And Ni 3+ The sum of the moles in the ion of (a); after the metal salt is dissolved, adding citric acid monohydrate and glycol in sequence, and stirring the solution in a constant-temperature magnetic stirrer at 80 ℃ to obtain transparent sol. After stirring under heating for 8 hours, a gel was obtained. Putting the obtained gel into an oven for pretreatment at 350 ℃ for 4 hours, grinding, putting the powder into a medium-temperature sintering furnace for calcination at 800 ℃ for 4 hours to obtain CeO 2 Uniformly loaded La 0.988 Ce 0.012 Cr 0.85 Ni 0.15 O 3-δ Powder of (CeO) 2 LCCrN, before reduction in example 4).
(2) Adding CeO 2 LCCrN catalyst precursor 5%H 2 -N 2 Thermogravimetric tests were carried out at 0-800 ℃ under an atmosphere. The results show that the catalyst precursor has the fastest hydrogen consumption rate around 600 c, and therefore the reduction temperature of the catalyst is set to 600 c.
(3) LCN and CeO prepared by sol-gel method 2 LCCrN in a hydrogen atmosphere at 5 ℃ for min -1 Raising the temperature from room temperature to 600 ℃, and preserving the heat for 4 hours to obtain CeO in which Ni nano particles are precipitated in situ 2 -La 0.988 Ce 0.012 Cr 0.85-y Ni y O 3-δ (Ni-CeO 2 LCCrN, after reduction of example 4) catalyst, the specific structure of which is shown in fig. 9 and 10.
Examples 2 to 6
Examples 2 to 6 were prepared according to the preparation method of example 1, and examples 2 to 6 were different from example 1 in that the compounding ratio of the metal ions was different, so that (La) was obtained in different compounding ratios 1-x Ce x ) y Cr 0.85 Ni 0.15 O 3-δ Wherein x =0, 0.1 and 0.2, y =1.0, 0.9 and 0.8. The catalyst precursor expression and the metal ratio of the series of catalysts are shown in table 1.
TABLE 1 catalyst precursor expressions and metal ratios for examples 2-6
| La(NO 3 ) 3 ·6H 2 O | Ce(NO 3 ) 3 ·6H 2 O | Cr(NO 3 ) 3 ·9H 2 O | Ni(NO 3 ) 3 ·6H 2 O | Catalyst and process for preparing same | |
| Example 2 | 3.12g | 0.78g | 3.40g | 0.44g | (La 0.8 Ce 0.2 ) 0.9 Cr 0.85 Ni 0.15 O 3-δ |
| Example 3 | 2.77g | 0.69g | 3.40g | 0.44g | (La 0.8 Ce 0.2 ) 0.8 Cr 0.85 Ni 0.15 O 3-δ |
| Example 4 | 3.90g | 0.43g | 3.40g | 0.44g | La 0.9 Ce 0.1 Cr 0.85 Ni 0.15 O 3-δ |
| Example 5 | 3.51g | 0.39g | 3.40g | 0.44g | (La 0.9 Ce 0.1 ) 0.9 Cr 0.85 Ni 0.15 O 3-δ |
| Example 6 | 3.11g | 0.35g | 3.40g | 0.44g | (La 0.9 Ce 0.1 ) 0.8 Cr 0.85 Ni 0.15 O 3-δ |
Results and characterization:
1. basic characterization of in situ Synthesis of Metal oxides
See FIG. 1, examples 1-6 for LaCrO 3 After doping the perovskite with Ce, laCrO is observed by XRD 3 (PDF # 74-1961) and CeO 2 XRD peaks of (PDF # 34-0394), whereas the sample without Ce doping contained only LaCrO 3 XRD peak of (a); respectively aligning LaCr by SEM 0。85 Ni 0.15 O 3-δ (sample not doped with Ce), ceO 2 -La 0.991 Ce 0.009 Cr 0.85 Ni 0.15 O 3-δ (example 4) and CeO 2 -La 0.988 Ce 0.012 Cr 0.85 Ni 0.15 O 3-δ (example 1) by carrying out the characterization, ceO was observed 2 The nano particles are uniformly loaded on the surface of the perovskite, and the surface of a sample not doped with Ce is smooth. This shows that CeO is realized by the preparation method provided by the invention 2 The metal oxide of (a) is synthesized in situ on the surface of the perovskite.
On the other hand, referring to Table 2, the XRD in FIG. 1 was refined, and perovskite and CeO were added to the series of catalysts 2 The ratio of the contents of (a) and (b) is close to the ratio of Ce when it is completely insoluble in perovskite; the doping amount of Ce in the perovskite is less than 5 percent by calculation; this indicates that the catalyst prepared by this process can achieve CeO 2 In-situ synthesis of nanoparticles on the surface of perovskite. Since CeO was synthesized in example 1 2 The largest amount, and the largest amount, thus the catalyst provided in example 1 has the largest specific surface area.
TABLE 2 specific surface area and LaCrO of examples 1-6 3 /CeO 2 Ratio of two phases of
| Abbreviations | BET(m 2 g -1 ) | LaCrO 3 /CeO 2 | |
| Example 1 | La8Ce2-10 | 14.76 | 0.81:0.19 |
| Example 2 | La8Ce2-9 | 11.68 | 0.83:0.17 |
| Example 3 | La8Ce2-8 | 9.42 | 0.82:0.18 |
| Example 4 | La9Ce1-10 | 6.51 | 0.91:0.09 |
| Example 5 | La9Ce1-9 | 6.12 | 0.93:0.07 |
| Example 6 | La9Ce1-8 | 5.82 | 0.94:0.06 |
2. Characterization of active metals, metal oxides and perovskite interactions by in situ precipitation
The catalyst of example 1 was further characterized, as shown in fig. 2, by thermogravimetric testing to determine the optimum reduction temperature range of the catalyst to be 500-600 ℃; by combining XRD test, SEM test and TEM test, the precipitation of Ni particles after reduction and the existence of Ni and CeO can be determined 2 And perovskite. Specifically, (a) in fig. 2 is an XRD characterization before and after reduction, and the occurrence of Ni peak can be observed; FIG. 2 (b) catalyst before reduction is 5%H 2 -N 2 Performing thermogravimetric test in the atmosphere, and determining that the reduction interval is 500-600 ℃; FIG. 2 (c) is an SEM image of the catalyst after reduction, which is Ni and CeO 2 The condition of dispersion distribution on the surface of the catalyst; TEM characterization of the catalyst after reduction is shown in FIG. 2 (d) and FIG. 2 (e), which can be determined as CeO by interplanar spacing 2 And Ni, and determined as Ni and CeO 2 Has good interaction with the perovskite carrier.
3. Change in Water absorption Properties
Mixing LaCr 0.85 Ni 0.15 O 3-δ (sample not doped with Ce), la 0.991 Ce 0.009 Cr 0.85 Ni 0.15 O 3-δ (example 4) and La 0.988 Ce 0.012 Cr 0.85 Ni 0.15 O 3-δ (example 1) drying and wetting (pH) 2 O = 5%) under a nitrogen atmosphere, a difference in the water absorption properties of the catalysts can be observed, as shown in fig. 3. By comparing Ce-free doped LaCr 0.85 Ni 0.15 O 3-δ The thermogravimetric curve under the dry and wet nitrogen atmosphere shows that the difference of the mass at 0-800 ℃ is not obvious, which indicates that the water absorption performance of the catalyst is poor; while the change in mass due to water absorption was clearly observed comparing example 4 with example 1, in which example 1 contained the most CeO 2 Particles, thus catalysingThe water absorption performance of the agent is best. By subjecting the thermogravimetric curve of example 1 to further thermogravimetric treatment, the water absorption equilibrium constant K can be obtained hydrate The relationship with temperature is shown in FIG. 4. FIG. 4 (a) Ni-CeO 2 LCCrN catalyst in dry nitrogen and 5%H 2 O-N 2 Thermogravimetric testing under atmosphere; water absorption equilibrium constant K in FIG. 4 (b) hydrate And enthalpy change versus temperature. It can be seen from fig. 4 (a) to fig. 4 (b) that the temperature increase results in a decrease in the water absorption performance of the catalyst, and the activation energy of the water absorption reaction is increased; however, at high temperature, the catalyst absorbs water and emits higher heat, which indicates that more chemical adsorption is performed at high temperature to generate hydroxyl.
4. Modification of oxygen transport Properties
The valence change of Ce in the catalyst provided by the invention causes oxygen to be separated from CeO 2 The migration process to Ni is shown in FIG. 5. N in FIG. 5 2 XRD Peak Change of Ni, N, before and after treatment 2 The peak of Ni disappears after the treatment; FIG. 5 (b) N 2 TEM characterization of the catalyst after treatment, with Ce 2 O 3 Forming; n in FIG. 5 (c) and N in FIG. 5 (d), respectively 2 XPS characterization before and after treatment, at N 2 After the treatment, ce 3+ The content of (A) is obviously improved, which indicates that valence variation behavior of Ce exists; it can be seen that the catalyst reduced in example 1 was subjected to N at 600 2 Treatment with Ce is found 4+ To Ce 3+ The valence change phenomenon of (a), which releases oxygen and gives adjacent Ni particles, causing them to be oxidized; the oxygen migration process is beneficial to improving the catalytic performance of the catalyst and improving the anti-carbon deposition performance.
5. Modification of catalytic properties
For Ce-free doped LaCr 0.85 Ni 0.15 O 3-δ Perovskite catalyst material, after reduction at 600 ℃ for 4 hours, was subjected to the ethanol steam reforming catalysis test, and the results are shown in table 3. It can be seen that the perovskite catalyst performed poorly, with a maximum ethanol conversion of 38.4%, and a CH 4 And C 2 H 4 The selectivity of (A) is higher, which indicates that the catalysis is not thorough. SEM characterization after catalysis of the catalystAs shown in fig. 6, it can be seen that a large amount of carbon deposit is formed on the surface of the catalyst, indicating that the carbon deposition resistance of the catalyst is poor.
TABLE 3La x Cr 0.85 Ni 0.15 O 3-δ 4h ethanol steam reforming test at 600 ℃ after reduction of a series of perovskite catalyst materials
In contrast to the Ce doped catalyst, the present invention compares the catalytic activity of examples 1-6 in the steam reforming of ethanol, with the results shown in FIG. 7. It can be seen that the catalysts provided in examples 1-6 achieved 90% ethanol conversion above 550 c and 100% at 600 c, see fig. 7 (a), where hydrogen production was up to 5.2 moles per mole of ethanol, see fig. 7 (b); above 450 ℃, see fig. 7 (c), the hydrogen selectivity of the series of catalysts reaches 70%, while the CH is 4 And C 2 The selectivity is lower, which indicates that the ethanol is converted by the catalyst completely, and also indicates that CeO 2 The presence of the metal oxide improves the catalytic performance of the catalyst. By XRD characterization of example 1 after catalysis, no decomposition of the catalyst occurred, which indicates that the catalysts provided in examples 1-6 maintain a certain chemical structure stability while improving the water absorption performance of the perovskite-based material.
With reference to example 1, the present invention performed long-term steam reforming tests for 100h, and 40h on methanol, ethanol, and glycerol, respectively, using catalysts, as shown in fig. 8, and fig. 8 the long-term steam reforming tests for 100h, and 40h on methanol, ethanol, and glycerol, respectively, using catalysts of example 1, including characterizing alcohol conversion, product selectivity, and yield; the specific catalytic conditions are as follows: 1) Catalytic reforming of methanol: S/C =1 and 450 ℃, 2) ethanol catalytic reforming, S/C =2.5, 650 ℃ and 3) glycerol catalytic reforming S/C =4, 700 ℃. The volume space velocity of all reforming tests was 3000h -1 . It can be seen that the results show that the catalyst has good long-term stability, the conversion rate of the alcohols is maintained above 90%, and hydrogen gasThe selectivity was maintained at 70%, 73% and 65%, respectively.
It will be understood by those skilled in the art that the foregoing is only an exemplary embodiment of the present invention, and is not intended to limit the invention to the particular forms disclosed, since various modifications, substitutions and improvements within the spirit and scope of the invention are possible and within the scope of the appended claims.
Claims (6)
1. The perovskite catalyst is characterized by comprising a perovskite matrix, metal oxide particles and metal particles, wherein the A site and the B site of the perovskite matrix are doped with different metals, the metal oxide particles are dispersedly distributed on the surface of the perovskite matrix, the metal particles are precipitated on the surface of the perovskite matrix in situ, the metal oxide particles have water absorption and oxygen migration activity, the metal oxide particles are obtained by doping metals on the A site of the perovskite matrix and growing the metal-doped oxide particles on the surface of the perovskite matrix in situ in an in situ synthesis mode, and the metal particles are obtained by doping metals on the B site of the perovskite matrix and precipitating the metals in situ;
the perovskite catalyst is prepared by the following method: (1) Respectively adding metal salts containing an A element, a B element, an A 'element and a B' element into deionized water according to the required stoichiometric ratio of the perovskite substrate to dissolve the metal salts to obtain a mixed solution, and preparing perovskite precursors with different metals doped at the A site and the B site by a sol-gel method; wherein, the addition amount of the metal salt containing the A 'element is determined by the relationship between the atomic radius and the stable valence state between the A element and the A' element;
(2) Grinding the perovskite precursor, calcining in the air, and obtaining the perovskite material with metal oxide particles dispersed and distributed on the surface by an in-situ synthesis method;
(3) Carrying out in-situ reduction on the perovskite material obtained in the step (2) to obtain a perovskite catalyst with metal oxide particles dispersed and distributed on the surface and metal particles precipitated on the surface in situ;
the addition amount of the metal salt containing the element A 'is determined by the relationship between the atomic radius and the stable valence state between the element A and the element A', and specifically comprises the following steps:
when the relative difference value of the atomic radiuses of the element A and the element A ' is less than 15 percent and the stable valence states of the element A and the element A ' are the same, the addition amount of the metal salt containing the element A ' meets the condition that the doping amount of the element A ' is larger than the solubility of the element A ' in perovskite, so that A-site doped metal is precipitated on the surface of a perovskite precursor; when the relative difference of the atomic radii of the element A and the element A 'is more than 15%, the addition amount of the metal salt of the element A' is the required stoichiometric ratio in the perovskite matrix; when the relative difference value of the atomic radius of the element A and the atomic radius of the element A ' is less than 15 percent and the stable valence states of the element A and the element A ' are different, the addition amount of the metal salt of the element A ' is the required stoichiometric ratio in the perovskite matrix, wherein the relative difference value is (the atomic radius of the element A-the atomic radius of the element A)/the atomic radius of the element A;
the metal oxide particles are CeO 2 The metal particles are one or more of Ni, co, fe, cu, ni-Cu alloy, ni-Co alloy and Ni-Fe alloy;
the calcination is calcination at 700-800 ℃ for 4-6 hours.
2. The perovskite catalyst of claim 1, wherein the metal oxide particles have a particle size of 30 to 40nm and the metal particles have a particle size of 20 to 30nm.
3. The perovskite catalyst of claim 1, wherein the perovskite matrix in which the a-site and the B-site are doped with different metals is represented by the formula: a. The 1-x A’ x B 1-y B’ y O 3-δ ；
Wherein the perovskite matrix is formed of ABO 3 Being a main body, the ABO 3 Comprises Cr-based material or Ti-based material, wherein A-bit element and B-bit element are +3 valence at the same time, or A-bit element and B-bit element are +2 valence and +4 valence respectively,
a' is to ABO 3 A metal doped at the A site, A' is Ce, and x =0-0.2; b' is towards ABO 3 Metal doped at B site, B' is one or more of Ni, co, fe and Cu, and y =0-0.15, delta less than 0.5.
4. A process for the preparation of a perovskite catalyst as claimed in any one of claims 1 to 3, characterized in that it comprises the steps of:
(1) Respectively adding metal salts containing an A element, a B element, an A 'element and a B' element into deionized water according to the required stoichiometric ratio of the perovskite substrate to dissolve the metal salts to obtain a mixed solution, and preparing perovskite precursors with different metals doped at the A site and the B site by a sol-gel method; wherein the addition amount of the metal salt containing the element A 'is determined by the relationship between the atomic radius and the stable valence state between the element A and the element A';
(2) Grinding the perovskite precursor, calcining in the air, and obtaining the perovskite material with metal oxide particles dispersed and distributed on the surface by an in-situ synthesis method;
(3) Carrying out in-situ reduction on the perovskite material obtained in the step (2) to obtain a perovskite catalyst with metal oxide particles dispersed and distributed on the surface and metal particles precipitated on the surface in situ;
the addition amount of the metal salt containing the element A 'is determined by the relationship between the atomic radius and the stable valence state between the element A and the element A', and specifically comprises the following steps:
when the relative difference value of the atomic radii of the element A and the element A 'is less than 15 percent, and the stable valence states of the element A and the element A' are the same, the addition amount of the metal salt containing the element A 'meets the condition that the doping amount of the element A' is larger than the solubility of the element A in the perovskite, so that A-site doped metal is precipitated on the surface of the perovskite precursor; when the relative difference of the atomic radii of the element A and the element A 'is more than 15%, the addition amount of the metal salt of the element A' is the required stoichiometric ratio in the perovskite matrix; when the relative difference of the atomic radius of the element A and the atomic radius of the element A ' is less than 15 percent and the stable valence states of the element A and the element A ' are different, the addition amount of the metal salt of the element A ' is the required stoichiometric ratio in the perovskite matrix, wherein the relative difference is (the atomic radius of the element A-the atomic radius of the element A)/the atomic radius of the element A.
5. The preparation method according to claim 4, wherein the in-situ reduction is specifically to perform the perovskite material obtained in the step (2) at 5%H 2 -N 2 Carrying out temperature programming reduction test or thermogravimetric experiment under the atmosphere, and determining the temperature interval corresponding to the fastest consumption rate of the perovskite material; and (3) placing the perovskite material in a pure hydrogen atmosphere, reducing for 4-6 hours within the temperature range of the test, wherein the precipitation amount of the metal particles precipitated in situ on the surface is less than 5wt%.
6. The preparation method according to any one of claims 4 to 5, wherein the sol-gel method in the step (1) is specifically:
sequentially adding citric acid monohydrate and glycol into the mixed solution, and stirring to form sol; dropwise adding ammonia water into the sol to adjust the pH value to 8-9, stirring to form a gel, and preserving the temperature at a preset temperature to obtain a perovskite precursor; wherein, the molar weight of the total metal ions: molar amount of citric acid monohydrate: molar weight of ethylene glycol = 1.5, the total metal ions consisting of a ions, B ions and B' ions.
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