Surface Composition Of Catalysts Research Articles

ChemInform Abstract: Surface Composition and Structural Stability of Bimetallic Catalysts

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Variations in the surface structure and composition of tungsten oxynitride catalyst caused by exposure to air

A tungsten oxynitride (WN00.59 O0:24 ) catalyst with a specific surface area of 63.7 m2g-1has been prepared by the temperature-programmedreaction of WO3 with NH3 and characterized by elemental analysis, X-ray powder diffraction, N2 sorption, and X-ray photoelectron spectroscopy (XPS) in order to extensively investigate the textural variation of this material caused by exposure to ambient air. No noticeable changes in the bulk structure of tungsten oxynitride prepared here are caused by the sample storage in ambient conditions for a long period of time up to 80 days. However, its N2 BET surface area and total pore volume were found to severely decrease with increasing the period of time of exposure to air. The XPS depth profile measurements reveal that the amorphous tungsten trioxide (WO3 ) species is formed on the surface of tungsten oxynitride upon exposure to air, which results in the significant modification of the surface structure and composition of the oxynitride material. In contrast, no indications of other surface tungsten phases such as dioxidic, monooxidic, and oxynitridic species are detected. The overall results of this study strongly suggest that oxygen is mobileenough to diffuse into the surface or near-surface of the oxynitride lattice even in ambient conditions, leading to the surface WO3 formation.

Study of the surface composition of vanadyl pyrophosphate catalysts by XPS and ISS - Influence of Cs + and water vapor on the surface P/V ratio of (VO) 2 P 2 O 7 catalysts

The investigation of the surface composition of vanadyl pyrophosphate catalysts and the influence of Cs ions and of water vapor on the surface P/V ratio were described and discussed. The characterization was carried out by XPS (X-ray Photoelectron Spectroscopy) and ISS (Ion Scattering Spectroscopy). It was concluded that the P/V ratio on the surface of equilibrated and non equilibrated (VO) 2 P 2 O 7 catalysts prepared in aqueous solution was between 1.3 and 1.4 (XPS) based on calibrations with different VPO glasses as standard materials having uniform composition of surface and bulk. In the outermost layer of the catalyst the P/V ratio was much higher and laid between 2 and 3 as verified by ISS. Different influences, i.e. addition of alkali metall ions (Cs + ) or a treatment with water vapor of the catalyst, caused a further increase in the concentration of phosphorus on the surface.

Characterization of Model Three-Way Catalysts: III. Infrared Study of the Surface Composition of Platinum–Rhodium Ceria–Alumina Catalysts

The surface composition of model and commercial bimetallic PtRh catalysts supported on ceria–alumina was tentatively determined by FTIR spectroscopy using the successive adsorption of NO at 473 K and CO at 298 K method which was previously applied to the case of PtRh/Al2O3 catalysts. The study of monometallic model catalysts shows that the adsorption of NO at 473 K on rhodium gives an intense band at 1912 cm−1 which is not modified by the presence of ceria and therefore can be used to quantify the number of surface rhodium atoms. However, the chemisorption properties of platinum toward the subsequent adsorption of CO at 298 K are strongly modified and the quantification of the surface platinum atoms could not be directly measured. However, their number could be indirectly obtained by combining the NO adsorption for rhodium and hydrogen volumetric adsorption for the total number of surface metal atoms, assuming that NO adsorption does not induce an important segregation of rhodium for bimetallic particles. Thus, for the fresh bimetallic model catalyst, the same composition was obtained at the surface and in the bulk. After aging at 1273 K, no rhodium was detected at the surface. This absence of surface rhodium atoms was confirmed by the data obtained from the direct adsorption of CO on platinum at room temperature, which gave the same number of surface atoms as hydrogen chemisorption. Similar results have been obtained with two commercial three-way catalysts. In particular, after aging at 1273 K, rhodium was practically not detected on the surface.

Investigation of the surface of vanadyl pyrophosphate catalysts

The surface composition of (VO)2P2O7 catalysts was investigated by x-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS). The P/V ratio on the surface of (VO)2P2O7 catalysts from aqueously prepared precursors lies between 1.3 and 1.4, based on XPS calibrations with different VPO glasses as standard materials. With ISS we obtained a higher P/V ratio of (VO)2P2O7 catalysts in the outermost layer of ∽2–3. The addition of caesium (Cs+) caused a further increase in the P/V ratio of the surface of (VO)2P2O7 with increasing Cs+ concentration. Ion scattering spectroscopy indicated that Cs+ was mainly confined to the uppermost layers, and at high doping concentrations a total surface coverage with Cs+ was achieved. The influence of water vapour on the surface composition of the catalysts was also investigated. A migration and a loss of phosphorus was found after and during water vapour treatment using XPS, ISS and thermogravimetric measurements. The surface of (VO)2P2O7 is assumed to be a dynamic system influenced by the partial pressure of water vapour in the gas phase. The rate of bulk oxidation of (VO)2P2O7 was dependent on the water vapour content in the gas phase. © 1998 John Wiley & Sons, Ltd.

VOC deep oxidation over Pt catalysts using hydrophobic supports

The active hydrophobic-supported Pt catalysts were synthesized for VOC deep oxidation at low temperature (less than 200°C). The destruction of VOC could cost less at lower temperature due to less energy consumption. The advantage using hydrophobic support was that moistures from atmosphere and oxidation would not be adsorbed on the surface. Thus the active sites would not be cloaked and catalyst activity could be maintained, especially at low temperature. The hydrophobicity of supports was characterized by wetting angles. Porous SDB (styrene divinylbenzene copolymer) was found near 113°, indicating high hydrophobicity. Three Pt catalysts were prepared on SDB and activated carbons by incipient wetness method. Specific surface areas were measured by nitrogen adsorption. The thermal stability of SDB catalyst was examined by TGA, and found no degradation below 200°C in air. The surface compositions of catalysts were analyzed by EDS. XRD showed that Pt was well dispersed on supports after hydrogen reduction at 160°C. The chemical states of Pt were investigated by XPS, and suggested that the oxidized Pt IV might be the active sites in the reaction. The deep oxidation of toluene/air mixture was carried out to test the activity of catalysts. Pt/SDB showed the highest activity among the catalysts and could completely oxidize 90 ppm toluene/air at VHSV=21 000 h −1, 150°C. Redox mechanism was proposed to reveal the enhanced kinetic rates. The results suggested that the rate of toluene oxidation might be enhanced due to the fact that water, one of the products, was expelled from the hydrophobic surface.

Catalytic Combustion of Ethane over Palladium Foil in the 300–450°C Range: Kinetics and Surface Composition Studies

Catalytic oxidation of ethane over palladium foils was studied and the kinetic parameters were determined for the reaction. The studies were carried out at 800 Torr total pressure over a wide range of reaction conditions in the temperature range of 300–425°C. The catalyst surface composition was characterized before and after reaction by Auger electron spectroscopy (AES). While we detected only metallic palladium before reaction, the postreaction analysis showed significant formation of PdOx, a surface oxide, at all temperatures and under all ethane/oxygen ratios. The catalyst that exhibits optimum combustion activity is the one covered with 0.3 to 0.5 monolayer of oxygen. Carbon monoxide temperature-programmed desorption (CO-TPD) was used to evaluate the surface area of the metallic and oxidized palladium in order to normalize the measured catalytic reaction rates. The turnover rate for ethane combustion is in the range of 1–260 s−1(molecule(s) of CO2produced per surface palladium atom per second) in the studied temperature range. Under stoichiometric reaction conditions at 325°C the rate for ethylene combustion is 100 times faster than that of ethane and the turnover rate for ethylene formation is calculated to be 0.6 s−1(molecule(s) of C2H4produced per surface palladium atom per second). At 325°C the kinetic orders are 1, 0, and −1 for ethane, carbon dioxide, and water, respectively. The order in oxygen is −1/2 under stoichiometric and fuel-rich conditions and 0 under oxidizing conditions. Apparent activation energies of 104, 116, and 189 kJ/mol under fuel lean, stoichiometric, and fuel-rich conditions, respectively, were observed. While CO2is the main carbonaceous product, unoxidized and partially oxidized species, methane and ethylene, are produced in small amounts. The product distribution is typically 98.6 mol% CO2, 0.9 mol% C2H4, and 0.5 mol% CH4at 10% conversion during a reaction at 350°C under stoichiometric conditions.

98/00280 An extensive study on Raney CuCo catalysts for synthesis of higher alcohols from coal based syngas

Raney Cu-Co catalysts of different Cu-Co (atomic ratio = 0.2-0.3) have been prepared and tested for the synthesis of higher alcohols (at 563 K, 6 MPa, and H{sub 2}/CO=2) and characterized by XRD, XPS, and H{sub 2}-TPD. Raney Cu-Co catalysts prepared from Cu-Co-Al alloys are composed of two kinds of Cu-Co solid solution. One is enriched in Cu, while another is enriched in Co. The surface composition of the Raney Cu-Co catalysts is considerably different from the bulk composition. Enrichment of Cu on the surface is intensive, and can be weakened by the ambient gases of CO, CO{sub 2}, and syngas. For CO hydrogenation, the main products on Raney Cu-Co catalysts are C{sub 2}-C{sub 6} normal alcohols and C{sub 2}-C{sub 6} normal aliphatic hydrocarbons. The distributions of the products follows the Schulz-Florry equation, but the profitability of carbon chain growth for alcohols (0.3-0.55) is lower than that for hydrocarbons (0.6-0.75). The stable alcohol yield of the Raney Cu-Co catalysts with Cu/Co=0.3-1.5 (atomic ratio) are higher than those of the coprecipitated Cu-Co catalysts. At Cu/Co=0.8 (atomic ratio), the stable alcohol yield of the catalyst reaches 0.57 g/g/h. In the H{sub 2}-TPD spectra on Raney Cu-Co catalysts, four peaks (A, B, C,more » and D) are observed when hydrogen adsorption temperature is high (563->303K), while only three peaks (A, B, and D) are observed when H{sub 2} adsorption temperature is low (298K). Peak C results from the activatedly adsorbed H{sub 2}, and is attributed to low-coordinated Co sites. The area fraction of peak C changes with catalysts in the same order as that of alcohol selectivity. Based on the results obtained, the nature of CO insertion sites has been discussed.« less

The influence of reaction temperature on the cracking mechanism of n-hexane over H-ZSM-48

The cracking of n-hexane at different temperatures over H-ZSM-48 has been analyzed using an established kinetic model. The model includes a monomolecular cracking path based on Langmuir adsorption isotherm as well as a bimolecular path following Rideal kinetics which accounts for the possibility of a chain mechanism being involved. Catalyst decay is accounted by using a time on stream decay function. The scrutiny of the optimal parameters for the model describing n-hexane conversion suggests that the catalyst surface composition is sensitive to temperature owing to the difference in the enthalpy of adsorption between the reactant and the average product of the cracking. As the temperature increases, the reactant competes more successfully for the active sites. The average activation energy for protolytic cracking of n-hexane on H-ZSM-48 was found to be 20.2 kcal mol −1. Steric inhibition during hydrogen transfer between n-hexane and C 4 and C 3 olefins is also observed.

Effects of Chemisorption on the Surface Composition of Bimetallic Catalysts

We have investigated the effect of hydrogen chemisorption on the surface composition of bimetallic RhPt catalytic clusters using the bond order metal simulation model. The differences in adsorption energies of hydrogen on Rh and on Pt for one monolayer of hydrogen are obtained by scaling the values for single-hydrogen adsorption, predicted by non-self-consistent electron density functional theory, until the results for surface segregation data from nuclear magnetic resonance are reproduced. We find the best fit with energy differences of −8.19 and −12.75 kJ/mol for three- and fourfold fcc hollow sites, respectively, in excellent agreement with the recent calorimetric measurements of −6 kJ/mol for the average heat of adsorption. In the presence of hydrogen, our cluster surfaces are Rh-rich, especially for the fcc(100) faces, completely reversing the segregation of Pt to the surface of the bare cluster.

SIMS Investigation of Fresh and Aged Automotive Exhaust Catalysts

Secondary ion mass spectrometry (SIMS) has been employed to examine the surface composition of fresh and aged three-way automotive exhaust catalysts. The washcoat components and poison species could be monitored due to the unsurpassed sensitivity of this technique. The outermost 1-2 nm was found to be covered with species containing the elements P, Ca, Zn, Pb and S. These species are known to originate from fuel and lubricating oil additives and the amounts on the surface of catalysts increased with increasing distance travelled. The presence of calcium phosphate and lead sulphate was confirmed, and these species are known surface fouling agents. Evidence of Ce sintering on each of the aged catalysts was also found using this technique. Depth profile experiments showed that the poison species penetrated the washcoat layer between 0.5 and 4 μm, depending on the distance travelled. Imaging SIMS displayed the lateral position of the washcoat components and poison species.

Surface composition, structural and chemisorptive properties of Raney nickel catalysts

The modification of the hydrogen chemisorption properties of Raney nickel by the presence of additives has been studied. It has been found that the addition of sodium hydroxide induces an increase in the amount of reversibly adsorbed hydrogen, responsible for the catalytic activity. XPS and EXAFS show that the additives remove aluminium oxides from the metallic surface, forming large particles of a stable basic aluminate and making a larger area available for chemisorption. The amount of secondary reactions, which occur on acid sites, is also reduced.

Direct differentiation of surface and bulk compositions of powder catalysts: application of electron-yield and fluorescence-yield NEXAFS to LixNi1−xO

We have applied near-edge X-ray absorption fine structure (NEXAFS) to characterize the surface and bulk properties of LixNi1−xO catalysts. In our experimental set-up, NEXAFS spectra of powder materials could be obtained by measuring the intensity of either electron-yield or fluorescence-yield. While the electron-yield method is sensitive only to the top few atomic layers, the fluorescence-yield method can detect species up to a few thousands angstroms deep into the bulk structure. The ability to distinguish surface and bulk compositions is demonstrated in studies of a number of Li0.5Ni0.5O samples, of which the surface compositions vary as a function of preparation procedures. In addition, NEXAFS investigations following the reaction of LixNi1−xO with CH4 have also been carried out and the results indicate that the initial surface reaction intermediates are Li2CO3.

Surface composition of iron oxide catalysts used for styrene production: an Auger electron spectroscopy/scanning electron microscopy study

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSurface composition of iron oxide catalysts used for styrene production: an Auger electron spectroscopy/scanning electron microscopy studyJoergen Lundin, Leif Holmlid, P. Govind Menon, and Lars NyborgCite this: Ind. Eng. Chem. Res. 1993, 32, 11, 2500–2505Publication Date (Print):November 1, 1993Publication History Published online1 May 2002Published inissue 1 November 1993https://doi.org/10.1021/ie00023a010RIGHTS & PERMISSIONSArticle Views272Altmetric-Citations14LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

The Kinetics of Hydrocarbon Cracking

A general kinetic model which describes the catalytic cracking of pure hydrocarbons is presented. The model includes a monomolecular cracking path based on the Langmuir adsorption isotherm as well as a bimolecular path, following Rideal kinetics, which accounts for the possibility of a chain cracking mechanism being involved. Catalyst decay is accounted for using the time-on-stream decay function. Fitting of experimental data from n-nonane cracking on USHY at 673 K, combined with Monte Carlo simulations indicates that, in that case, the total catalytic activity could include between 0 and 90% of activity due to chain processes. This large margin of error stems from the combined effects of a large decay rate, forcing the experimenter to use average conversion data, and of experimental error. Fitting of the model to previously published cracking data for 2-methylpentane on USHY showed that the model lacks a suitable parameter to account for thermal reactions which were not accounted for in the original data set. This observation supports the impression that the model is sensitive to departures from the postulated mechanism. The above kinetic model has also been fitted to the results of n-nonane cracking at three temperatures as well as to previously published data for various other linear paraffins. In all these systems the parameter A2, which is a function of the bimolecular cracking rate constants, is found to be statistically insignificant, in spite of other experimental evidence which supports the existence of this route of cracking. Parametric analysis of the n-nonane conversion results suggests that catalyst surface composition is very sensitive to temperature due to a large difference in the enthalpy of adsorption between the reactant and the average product of cracking. As temperature is increased, the reactant competes more successfully for active sites, with the result that the relative importance of monomolecular cracking processes increases with temperature at the expense of bimolecular reactions. The rate of cracking per crackable bond was also considered. We present arguments that previous reports of increased cracking rate per bond as chain length of the feed molecule is increased are due to the use of inadequate models of the kinetics involved, rather than constituting a real phenomenon.

Catalysis by the Rh/B system: Part 2. Highly regioselective vapour phase hydroformylation of propene at atmospheric pressure on Rh/B on silica and silica-alumina

A Rh/B catalyst prepared by reducing rhodium trichloride supported on silica and silica-alumina with NaBH 4 at low temperature is used after thermal treatments in the vapour phase, propene hydroformylation in a flow reactor at atmospheric pressure. A good regioselectivity toward normal isomers, that remains constant at increasing temperatures, is detected ( S L=0.88). The surface composition of silica-alumina catalysts is apparently not affected by thermal treatments whether in argon or in CO/H 2 or in hydroformylation mixture flow. The geometrical arrangements of the active sites appear to play an important role in controlling the regioselectivity of the catalytic system.

The nature of the iron oxide-based catalyst for dehydrogenation of ethylbenzene to styrene 2. Surface chemistry of the active phase

The combination of an XPS/UPS surface analysis instrument with a microreactor allowed the investigation of the surface composition of catalysts characterized by varying activities and selectivities. The active surface is a potassium iron oxide with a 1 : I atomic ratio of K : Fe, whereby iron is only in its trivalent state. Conversion of oxidic oxygen to OH groups is detrimental to the activity. No significant amount of promotor additives is present in the active surface. The process of regeneration with steam removes carbonaceous deposits but cannot reoxidize iron from Fe 2+ to Fe 3+. A constant but small amount of potassium carbonate that cannot be increased by addition of CO 2 to the feed of the working catalyst is present at the surface. Catalysts are precursors, active materials, and irreversibly deactivated samples were studied by SEM and TEM. The surface morphology as well as the microstructure clearly indicates a solid as the active phase. This phase is generated and maintained through solid-state reactions during operation. A potassium-rich liquid film with a thickness exceeding one monolayer can be ruled out for the catalyst performance. Formation of droplets of KOH in certain regions of the catalyst signals bulk structural desintegration of the active material.

High-resolution Auger Electron Microscopy and spectroscopy of supported metal particles

A detailed understanding of catalytic processes requires structural knowledge of the catalyst system. The surface properties of small metal particles which are highly dispersed on insulating supports must play a dominant role in determining this system's catalytic behavior. Therefore characterization of surface topography and composition of heterogeneous catalysts is important. Surface topography of the carrier materials can be obtained by high resolution secondary electron imaging (SEI) in a STEM. The small metal particles can also be located and their topographic relationship with the support morphology determined. However, an elementally specific signal such as the Auger electrons must be used to extract chemical information about the surface species. High resolution Auger electron spectroscopy (AES) and scanning Auger microscopy (SAM) have recently been incorporated into a UHV STEM. In this paper we report some results from the application of this instrumentation to the study of supported metal clusters.

XPS Determination of the Surface Composition of Vanadyl Pyrophosphate Catalysts

Abstract The surface P/V ratios of vanadyl pyrophosphates ((VO)2P2O7), which were selective catalysts for the oxidation of butane to maleic anhydride, were determined to be close to that of the bulk (P/V = 1) by XPS using V–P–O glasses as standard materials.

Changes in iron oxidation state and surface composition of iron-chromium catalyst reduced by hydrogen

X-ray photoelectron spectroscopic studies of the reduction of iron-chromium catalysts have been carried out to determine changes in the relative surface concentrations of iron(II) and (III) oxides and metallic iron and in its surface composition at 100–500°C.