Crystallite Migration Research Articles

The interparticle oxygen vacancies enrichment in the matrix of fibrous silica ceria supported nickel for CO methanation

Aspherical nickel-loaded fibrous silica mesoporous ceria (Ni/FSCe) with a cockscomb-like structure has been synthesized using a microemulsion technique integrated with a crystal-seed crystallization process and unprecedently utilized for CO methanation. Numerous systems were used to characterize the catalysts systematically. The Ni/FSCe showed greatly enhanced catalytic capability with the CO conversion of 95.67 % and the CH4 yield of 91.20 % at 400 °C, which is higher than that of the nickel-ceria loaded onto conventional KCC-1 (Ni-Ce/KCC-1). Furthermore, it demonstrated excellent stability, with no indication of deactivation for 75 h. Numerous factors, including the synergistic effect of high amount of inter-nanoparticle oxygen vacancies which lead towards high basicity and well-dispersed Ni, which act as both carbon monoxide and hydrogen adsorption sites, are considered to be responsible for the significant improvement in CO methanation. The superiority in the activity and stability of Ni/FSCe over Ni-Ce/KCC-1 also could be attributed towards better metal-support interactivity between the Ni nanoparticles and CeO2 support, which helps to inhibit crystallite migration and provides protection against carbon deposition. Core-shell FSCe, with their high catalytic characteristics, imply significant application potential in basic-catalyzed reactions, particularly methane generation.

The state of BEA zeolite supported nickel catalysts in CO2 methanation reaction

Two model nickel catalysts for CO2 methanation reaction were prepared by the impregnation method using unmodified and dealuminated BEA-type zeolite supports, denoted NiHAlBEA and NiSiBEA. Transmission electron microscopy (TEM) and X-ray diffraction studies evidenced formation of metallic nickel crystallites of different size, partially located on the external surface of zeolite grains in of NiHAlBEA catalyst and small nickel nanoparticles uniformly distributed within the zeolite grains in the NiSiBEA catalyst. Higher CO2 conversion and selectivity towards methane at low reaction temperatures were observed for NiSiBEA catalyst. This catalyst showed an improved resistance towards sintering at high reaction temperatures. The decrease of CO2 conversion with the time on stream, accompanied by the drop in methane selectivity in NiHAlBEA catalyst was attributed to the increase in Ni crystallite size, resulted from the migration of small crystallites from the inner to external surface of the zeolite grains and subsequent agglomeration. Diffuse reflectance infrared Fourier transform, and quasi in-situ X-ray photoelectron spectroscopy studies evidenced the presence of metallic nickel crystallites in the catalysts and their partial reoxidation under CO2 methanation reaction conditions. High nickel dispersion and sintering resistance of NiSiBEA led to the retardation of the deactivation of catalyst by H2S.

Chemical and Thermal Sintering of Supported Metals with Emphasis on Cobalt Catalysts During Fischer-Tropsch Synthesis.

This comprehensive critical review combines, for the first time, recent advances in nanoscale surface chemistry, surface science, DFT, adsorption calorimetry, and in situ XRD and TEM to provide new insights into catalyst sintering. This work provides qualitative and quantitative estimates of the extent and rate of sintering as functions of nanocrystal (NC) size, temperature, and atmosphere. This review is unique in that besides summarizing important, useful data from previous studies, it also advances the field through addition of (i) improved or new models, (ii) new data summarized in original tables and figures, and (iii) new fundamental perspectives into sintering of supported metals and particularly of chemical sintering of supported Co during Fischer-Tropsch synthesis. We demonstrate how the two widely accepted sintering mechanisms are largely sequential with some overlap and highly NC-size dependent, i.e., generally, small NCs sinter rapidly by Ostwald ripening, while larger NCs sinter slowly by crystallite migration and coalescence. In addition, we demonstrate how accumulated knowledge, principles, and recent advances, discussed in this review, can be utilized in the design of supported metal NCs highly resistant to sintering. Recommendations for improving the design of sintering experiments and for new research are addressed.

Removal of methyl mercaptan with highly-mobile silver on graphitic carbon-nitride (g-C3N4) photocatalyst

Modification of graphitic carbon nitride (g-C3N4) with silver metal significantly improved the adsorption capacity and the photocatalytic degradation activity for methyl mercaptan, which is a typical sulfurous compound, under visible light. These improvements were easily obtained by the shear mixing method. The ultrafine crystallites of metallic silver were formed on surface of carbon nitride by shear mixing of HT-g-C3N4 (Hydrothermal treated g-C3N4) in aqueous solution of silver acetate. One atom of the silver captured ca. 0.6 molecule of methyl mercaptan on average, while a silver atom deposited on TiO2 captured only 0.03 molecule. The migration of silver crystallites over the g-C3N4 surface and the formation of silver thin-layer during the adsorption of methyl mercaptan were observed by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The silver crystallite on g-C3N4 seemed to have higher mobility than that on TiO2 to form the coordination suitable for the adsorption of methyl mercaptan. This coordination of Ag also enhanced the photocatalytic degradation of methyl mercaptan to dimethyl disulfide.

X-ray absorption, X-ray diffraction and electron microscopy study of spent cobalt based catalyst in semi-commercial scale Fischer–Tropsch synthesis

Calcined, reduced and spent Co-Re/γ-Al2O3 catalysts for Fischer–Tropsch synthesis (FTS) were characterized and compared. Co-K-edge and Re-LIII-edge X-ray absorption near edge structure (XANES), X-ray powder diffraction (XRPD), transmission electron microscopy (TEM) and H2 chemisorption were used to provide insight into structural transformations that the catalyst experiences during a month of operation in a semi-commercial FTS plant. Results from the core techniques for nanoparticle size determination suggest that sintering of the cobalt crystallites is an important deactivation mechanism in FTS performed in slurry reactors. In addition, a higher degree of reduction is observed for the spent catalytic material, while Re appears in a partially reduced state before and after reaction. The particle size distribution together with the spread of Co nanoparticles on the γ-Al2O3 surface indicates crystallite migration as the prevailing mechanism.

A review of PEM fuel cell durability: materials degradation, local heterogeneities of aging and possible mitigation strategies

Through a tight collaboration between chemical engineers, polymer scientists, and electrochemists, we address the degradation mechanisms of membrane electrode assemblies (MEAs) during proton exchange membrane fuel cell (PEMFC) operation in real life (industrial stacks). A special attention is paid to the heterogeneous nature of the aging and performances degradation in view of the hardware geometry of the stack and MEA. Macroscopically, the MEA is not fuelled evenly by the bipolar plates and severe degradations occur during start‐up and shut‐down events in the region that remains/becomes transiently starved in hydrogen. Such transients are dramatic to the cathode catalyst layer, especially for the carbon substrate supporting the Pt‐based nanoparticles. Another level of heterogeneity is observed between the channel and land areas of the cathode catalyst layer. The degradation of Pt3Co/C nanocrystallites employed at the cathode cannot be avoided in stationary operation either. In addition to the electrochemical Ostwald ripening and to crystallite migration, these nanomaterials undergo severe corrosion of their high surface area carbon support. The mother Pt3Co/C nanocrystallites are continuously depleted in Co, generating Co2+ cations that pollute the ionomer and depreciate the performance of the cathode. Such cationic pollution has also a negative effect on the physicochemical properties of the proton‐exchange membrane (proton conductivity and resistance to fracture), eventually leading to hole formation. These defects were localized with the help of an infrared camera. The mechanical fracture‐resistance of various perfluorosulfonated membranes further demonstrated that polytetrafluoroethylene‐reinforced membranes better resist hole formation, due to their high resistance to crack initiation and propagation. WIREs Energy Environ 2014, 3:540–560. doi: 10.1002/wene.113This article is categorized under: Fuel Cells and Hydrogen > Science and Materials Fuel Cells and Hydrogen > Systems and Infrastructure Energy Research & Innovation > Science and Materials

Probing the structure, the composition and the ORR activity of Pt3Co/C nanocrystallites during a 3422 h PEMFC ageing test

Long-term (3422h) operation of proton exchange membrane fuel cell in stationary conditions causes two regimes of degradation of the cathode catalytic layer. Firstly, from the beginning of life until the first membrane electrode assembly sampling, at t=1163h, fast degradation of the fresh Pt3Co/C nanoparticles is monitored; classical degradation mechanisms of Pt-based electrocatalysts occur, such as carbon corrosion, crystallite migration, dissolution of the less noble metal (Co), and 3D Ostwald ripening. A second degradation regime sets up from 1163h to 3422h, during which the changes in composition and morphology are slower. At the end of the ageing test, three distinct populations of Pt–Co/C nanoparticles coexist: (i) Pt–Co/C core–shell particles characterized by an alloyed (but depleted, compared to the fresh material) core surrounded by a 3–5 monolayer thick Pt-rich shell, (ii) Pt–Co/C “hollow” particles containing a central cavity surrounded by a Pt–Co shell containing limited amount of Co atoms distributed at the atomic scale and (iii) pure Pt/C “hollow” particles, from which Co dissolution has been completed. Experimental evidences are provided that the Pt-rich phase remains stable, and maintains constant ORR activity over more than 2000h of operation in real PEMFC conditions.

Catalytic decomposition of N2O over Zn-Fe spinel oxides

Zn-Fe spinel oxides were prepared by co-precipitation method and used as the catalysts in N2O decomposition in the presence of oxygen; the effects of spinel oxide composition, calcination temperature, and K doping on their catalytic activity were investigated. In addition, the Zn-Fe spinel oxides were characterized by N2 physisorption, X-ray diffraction and H2-TPR techniques. The results indicated that the Zn-Fe spinel oxides are active in N2O decomposition in the presence of oxygen; over the Zn0.8Fe0.2Fe2O4-400 catalyst with the optimized composition and calcined at 400°C, the conversions of N2O in the absence and in the presence of steam achieve 63.5% and 22.2%, respectively, after reaction at 500°C for 10 h, which are much higher than those over Fe3O4. However, the K-doped Zn-Fe spinel oxides exhibited lower activity than the bare Zn-Fe oxide, as K doping may lead to a substantial decrease of surface area and the migration of potassium crystallites to FeOx surface that will inhibit the ferric species from reduction and active oxygen species from removal from the catalyst surface.

Evidences of the migration of Pt crystallites on high surface area carbon supports in the presence of reducing molecules

In this study, we made use of structural markers to gain more insights into the structural stability of commercial carbon-supported Pt electrocatalysts under oxidizing, inert and reducing atmospheres. The materials were characterized by electron microscopy and electrochemical techniques in the fresh state and after various aging conditions. The results show that Pt nanoparticles supported on Vulcan XC72 are not immobile but prone to agglomerate during potential sweeping in the presence of hydrogen (H2), methanol (CH3OH), and carbon monoxide (CO). The migration rate of the Pt crystallites is the largest in CO-containing solution and decreases in the order CO > CH3OH > H2. We postulate that the morphological changes of the Pt/C nanoparticles are related to the reduction of the oxygen-bearing carbon surface groups strongly interacting with the Pt metal phase.

Determination of Aging Markers and their Use as a Tool to Characterize Pt/C Nanoparticles Degradation Mechanism in Model PEMFC Cathode Environment

Identical Location transmission electron microscopy (IL-TEM) was coupled to CO-stripping voltammetry to characterize the morphology of a benchmark Pt/C 50 wt% electrocatalyst (TEC10 - E50E, by Tanaka TKK). From the thorough analysis of the experimental data, 19 relevant structural markers were determined. These were subsequently used to quantify the structural degradations occurring during a simple electrochemical aging procedure (square-wave voltammetry from 0.1 to 0.9 V vs. RHE) performed in model PEMFC cathode environment (1 M H2SO4). Thanks to these markers, the mechanisms at stake during the Pt/C degradation were determined: Pt crystallite migration and coalescence, carbon corrosion and platinum dissolution/re-precipitation are at stake, the latter mechanism not being predominant.

Investigation of Catalyst Degradation After Single Cell Life Tests

To investigate the effect of fuel cell operating conditions on catalyst agglomeration of a platinum catalyst, several single cell life tests were carried out under different conditions. It was found that catalyst agglomeration was accelerated by higher temperature operation, while it was not accelerated by higher current density (∼1.5 A/cm2) nor by higher potential (∼0.85 V) when the operating temperature was 100°C. Catalyst agglomeration was much more severe in the cathode, and the surface area of the platinum catalyst was decreased from 58.9 m2/g to 30.6 m2/g after a life test for 4000 hours. Because there was no sign of platinum dissolution/precipitation in the ionomer phase and there was a peak tail in the direction of larger particles in the particle distribution after 4000 hours, this catalyst agglomeration may be caused by a crystallite migration mechanism and a coalescence growth mechanism. In addition, to investigate the degradation of the CO-tolerance performance of a platinum-ruthenium catalyst, 5000-hour life tests and several life tests for shorter periods were carried out. It was found that the CO-tolerance performance was gradually decreased with time and that one of the reasons for the degradation was considered to be decomposition of the platinum-ruthenium alloy. It was also found that an air-bleeding technique could reduce the loss of cell performance due to the degradation of the CO-tolerance performance but may slightly accelerate the catalyst material degradation.

Properties of Tin Oxide Films Prepared by MOCVD

Tin oxide thin films were deposited at 375 °C on α-alumina substrate by metal-organic chemical vapor deposition (MOCVD) process. A number of hillocks on the film were formed after air annealing at 500 °C for 30 min and few things in N2 annealing. The oxygen content and the binding energy after air annealing came to close the stoichiometric SnO2. The cauliflower hillocks of the film seem to be formed by the continuous migration of crystallites from a cauliflower grain on the substrate to release the stress due to the increase of oxygen content and volume.

Sintering of nickel steam-reforming catalysts

Sintering is an important deactivation mechanism for nickel-based steam-reforming catalysts. In this work, the effects of nickel loading, carrier surface area, and temperature on the nickel surface area are studied. The experiments are performed under simulated industrial conditions, i.e., in a mixture of steam and hydrogen (10:1) in the temperature range of 500–682 °C and at 30 bar total pressure. The data are analyzed using a simple model assuming spherical particles and a lognormal metal particle-size distribution with constant standard deviation. It is also assumed that the sintering proceeds via crystallite migration and coalescence. The model predicts well the experimental nickel surface area as a function of nickel loading, carrier surface area, temperature, and time. The model is also used to analyze data from the literature. The model is discussed in the context of predicting metal surface areas of catalysts consisting of small metal particles on ceramic supports.

Sintering of Nickel Steam-Reforming Catalysts on MgAl2O4 Spinel Supports

Sintering significantly contributes to the deactivation of Ni-based steam-reforming catalysts. We have investigated the effect of Ni loading and surface area of the support on sintering of Ni particles on MgAl2O4 spinel supports. The experiments were performed under simulated industrial pre-reforming conditions, i.e., in a 10:1 mixture of steam and hydrogen at 500°C and 30 bar total pressure. The Ni particle size of fresh and sintered catalysts was determined from sulfur chemisorption capacity, X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). It was found that most of the sintering occurs in the first 200 h; after that period the Ni particle size changes only slightly. A remarkable result is that the average Ni particle size after sintering reaches a limiting value that depends only weakly on the Ni loading and surface area of the support. Sintering of the catalysts with a lower Ni loading is slower and they exhibit a greater loss in Ni surface area compared to those with a higher Ni loading. Because the sintered particles are polycrystalline agglomerates, the XRD estimates of particle size are lower than those obtained by the other techniques such as TEM or chemisorption. The particle size distributions derived from TEM and STEM follow a log normal distribution, suggesting that sintering occurs through crystallite migration and coalescence. The limiting size of the Ni particles after sintering, and the low sintering rate after 200 h, can both be related to the lower mobility of the larger Ni particles on the support.

Monopropellant decomposition catalysts: II. Sintering studies on Ir/Al 2O 3 catalysts, influence of chloride anions

Supported Ir/Al 2O 3 catalysts with low (1 wt.% Ir) and high load contents (29 wt.% Ir) have been subjected to sintering through water vapor treatment at different temperatures. They were characterized by iridium dispersion measurements, X-rays diffraction and transmission electron microscopy. The sintering process takes place at temperatures higher than 450°C and the surface chloride ions play an important role by accelerating the growing rate of the iridium particles. The chloride ions allow the formation of iridium oxychloride or hydroxychloride species in presence of steam, leading to a particle growing through migration of molecular species. For dechlorinated or chlorideless samples, the rate of this process is lowered and the sintering can be due to migration of iridium crystallites. The final dispersion and particle distribution size depend only on the treatment temperature, whatever the type of alumina, the initial dispersion, the content of chloride or the loading level of iridium. Two ways permit to prevent the sintering: (i) the presence of hydrogen in gas flow which stabilizes the metal particles and (ii) the presence of hydrogen chloride which blocks the growing process. A way is proposed to stabilize the catalyst against ageing effects : removal of the chloride ions by treatment under a gas flow containing water vapor and hydrogen.

Deactivation of steam reforming catalysts by sintering: experiments and simulation

Sintering of coprecipitated Ni/Al 2O 3 catalysts was studied in a microreactor at 500 and 800°C under steam reforming reaction. Changes in microstructure (crystalline phases, average metal crystallite sizes and size distributions) were investigated by temperature programmed reduction (TPR), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Differences between particle size distributions determined by XRD and TEM are discussed in view of their specific limitations. Low temperature calcination of unpromoted catalysts may have a beneficial stabilizing effect upon reduction. The rate of sintering of small nickel crystallites increases as a result of the combined action of steam and high temperature, particularly for narrow initial crystallite size distributions and high initial dispersions. This behavior suggests a shift, from crystallite migration in the initial stages of heat treatment, to atomic migration in the latter stages. The lanthanum promoter seems to inhibit metal crystallite growth under reduction and reaction conditions. Predictions of a developed Monte Carlo based model are in good qualitative agreement with experimental observations.

Effects of addition of Pt to PdO/Al 2O 3 catalyst on catalytic activity for methane combustion and TEM observations of supported particles

Effects of addition of Pt to PdO/α-Al 2O 3 catalyst on the catalytic activity for methane combustion have been investigated. The size distributions and the shapes of supported particles in catalysts and their behavior at high temperature were measured. The results were discussed in relation with the heat resistance of supported particles. Supported particles were characterized by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). The addition of Pt to PdO/α-Al 2O 3 catalyst did not only bring about the increase of catalytic activity, but it was also effective for preventing the catalyst deactivation in methane combustion. These effects were attributed to the higher dispersion of the supported particles and the suppression of the particle growth. From the in situ observation of the PdO/α-Al 2O 3 catalyst, it was clarified that the supported particles migrated and then coalesced with each other like dewdrops on the surface of α-Al 2O 3 support at the temperature range from 843 to 863 K. The particle growth of supported particles proceeded through the migration and coalescence of crystallites. On the other hand, with the Pt-added catalyst, the migration and coalescence of supported particles were not observed. Its EDS analysis revealed that the every supported PdO particle contained Pt. The supported PdO particles containing Pt have smaller contact angles and are flatter than PdO particles free from Pt.

Kinetics of initial catalyst sintering in supported catalysts

Sintering kinetics are developed for the initial stage of catalyst sintering in porous, supported catalysts, as opposed to sintering in model (flat-surface) catalysts, for both mechanisms of atom and crystallite migration. The theoretical results show that the effect of pore size on sintering is contained only in the sintering rate constant and that the rate constant decreases in an exponential manner with increasing pore size.

Surface Area Loss of Supported Platinum in Polymer Electrolyte Fuel Cells

Life tests of polymer‐electrolyte fuel cells using supported Pt catalyst in thin film catalyst layers are run for up to 4000 h at maximum power. Particle ripening is readily evident using these types of electrodes in which the high catalyst utilization efficiency apparently subjects the majority of the platinum to conditions that sustain particle growth. X‐ray diffraction analyses indicate that the initial platinum specific surface areas of 100 m2/g Pt eventually stabilize to about 40 to 50 m2/g in the cathode and 60 to 70 m2/g in the anode. Interestingly, this loss in surface area does not affect the apparent catalytic activity of these fuel cell electrodes. A crystallite migration particle growth mechanism is suggested by the shape of the particle size distribution curves. Since the presence of liquids is known to lower the activation energy for particle growth, the particle size difference between the two electrodes may possibly be attributable to the different hydration levels at the anode and the cathode in operating polymer electrolyte fuel cells.

The effects of the environment on the growth of nickel metal particles supported on a range of Y zeolites prepared by ion exchange and impregnation

The nature of the nickel metal phase supported on a range of alkali metal cation-exchanged Y zeolite catalysts was characterized by X-ray diffraction line broadening. The sintering process is shown to occur via a crystallite migration mechanism. Nickel crystallite size was measured and correlated with such diverse factors as reduction time, reduction temperature, heating rate, sample precalcination, and ammonia adsorption. The nature of the alkali metal co-cation (Li+, Na+, K+, Rb+, or Cs+) is shown to strongly influence the dimensions of the supported metal, yielding the following order of decreasing crystallite size: NiLiY > NiNaY > NiKY > NiRbNaY > NiCsNaY. In the case of the CeNiNaY and CeNiKY samples, the introduction of an additional level of acidity, due to the presence of the highly polarizing Ce3+ ions, during thermal activation, serves to suppress crystallite growth. For comparative purposes, data on the sizes of nickel metal crystallites formed on the reduction of nickel-impregnated Y zeolites and silica and alumina supports are also presented; the nature of the alkali metal co-cation has a much greater influence on the dispersion of nickel supported on the impregnated zeolites. Under identical reduction conditions, the amorphous alumina and silica carriers exhibit the smallest supported crystallite sizes.