On the surface sites Of MOP/SiO2 catalyst under sulfiding conditions: IR spectroscopy and catalytic reactivity studies

The surface sites of MoP/SiO2 catalysts and their evolution under sulfiding conditions were characterized by IR spectroscopy using CO as the probe molecule. The HDS activities of thiophene were measured on the MoP/SiO2 catalyst that was subjected to different sulfidation and reactivation pretreatments. Cus Modelta+ (0 < delta less than or equal to 2) sites are probed on the surface of fresh MoP/SiO2 by molecularly adsorbed CO, exhibiting a characteristic IR band at 2045 cm(-1). The surface of MoP/SiO2 is gradually sulfided in HDS reactions, as revealed by the shift of the IR band at 2045 to ca. 2100 cm(-1). Although the surface of a MoP/SiO2 catalyst becomes partially sulfided, the HDS activity tests show that MoP/SiO2 is fairly stable in the initial stage of the HDS reaction, providing further evidence that molybdenum phosphide is a promising catalytic material for industrial HDS reactions. Two kinds of surface sulfur species are formed on the sulfided catalyst: reversibly and irreversibly bonded sulfur species. The MoP/SiO2 catalyst remains stable in the HDS of thiophene because most sulfur species formed under HDS conditions are reversibly bonded on the catalyst surface. A detrimental effect of presulfidation on the HDS activity is observed for the MoP/SiO2 catalyst treated by H2S/H-2 at temperatures higher than 623 K, which is ascribed to the formation of a large amount of the irreversibly bonded sulfur species. The irreversibly sulfided catalyst can be completely regenerated by an oxidation and a subsequent reduction under mild conditions. (C) 2003 Elsevier Inc. All rights reserved.

In situ IR spectroscopic studies on molybdenum nitride catalysts: active sites and surface reactions

Recent IR spectroscopic studies on the surface properties of fresh Mo2N/gamma-Al2O3 catalyst are presented in this paper. The surface sites of fresh Mo2N/gamma-Al2O3, both Modelta+ (0<δ<2) and N sites, are probed by CO adsorption. Two characteristic IR bands were observed at 2045 and 2200 cm(-1), due to linearly adsorbed CO on Mo and N sites, respectively. The surface N sites are highly reactive and can react with adsorbed CO to form NCO species. Unlike adsorbed CO on reduced passivated one, the adsorbed CO on fresh Mo2N/gamma-Al2O3 behaves similarly to that of group VIII metals, suggesting that fresh nitride resembles noble metals. It is found that the surface of Mo nitrides slowly transformed into sulfide under hydrotreating conditions, which could be the main reason for the activity drop of molybdenum nitride catalysts in the presence of sulfur-containing species. Some surface reactions, such as selective hydrogenation of 1,3-butadiene, isomerization of 1-butene, and hydrodesulfurization of thiophene, were studied on both fresh and reduced passivated Mo2N/gammaAl(2)O(3) catalysts using IR spectroscopy. The mechanisms of these reactions are proposed. The adsorption and reaction behaviors of these molecules on fresh molybdenum nitride also resemble those on noble metals, manifesting the unique properties of fresh molybdenum nitride catalysts. Mo and N sites are found to play different roles in the adsorption and catalytic reactions on the fresh Mo2N/gammaAl(2)O(3) catalyst. Generally, Mo sites are the main active sites for the adsorption and reactions of adsorbates; N sites are not directly involved in catalytic reactions but they modify the electronic properties of Mo sites.

Carbon monoxide adsorption on molybdenum phosphides: Fourier transform infrared spectroscopic and density functional theory studies

Molybdenum phosphide (MoP) and supported molybdenum phosphide (MoP/gamma-Al2O3) have been prepared by the temperature-programmed reduction method. The surface sites of the MoP/gamma-Al2O3 catalyst were characterized by carbon monoxide (CO) adsorption with in situ Fourier transform infrared (FT-IR) spectroscopy. A characteristic IR band at 2037 cm(-1) was observed on the MoP/gamma-Al2O3 that was reduced at 973 K. This band is attributed to linearly adsorbed CO on Mo atoms of the MoP surface and is similar to IR bands at 2040-2060 cm(-1), which correspond to CO that has been adsorbed on some noble metals, such as platinum, palladium, and rhodium. Density functional calculations of the structure of molybdenum phosphides, as well as CO chemisorption on the MoP(001) surface, have also been studied on periodic surface models, using the generalized gradient approximation (GGA) for the exchange-correlation functional. The results show that the chemisorption of CO on MoP occurred mainly on top of molybdenum, because the bonding of CO requires a localized mininum potential energy. The adsorption energy obtained is DeltaH(ads) approximate to -2.18 eV, and the vibrational frequency of CO is 2047 cm-1, which is in good agreement with the IR result of CO chernisorption on MoP/gamma-Al2O3.

Studies on electrocatalytic performance of titanium oxide electrode modified with Pt toward the oxidation of methanol

Electrocatalytic performance of the Pr-TiOx/Ti electrode prepared with electrochemical reduction-oxidation method toward the oxidation of methanol has been studied, The experimental results showed that the Pt-TiOx/Ti electrode has a high electrocatalytic activity and good stability for the electrocatalytic oxidation of methanol, By means of electrochemical, XPS, STM and in-situ FTIR techniques, it was found that one reason for the electrode to exhibit an excellent performance is attributed to the high dispersion between nanosized Pt and TiOx particles, The low adsorption ability of the intermediate derived from methanol, such as linearly adsorbed CO species on the electrode surface due to the interaction between Pt and TiOx, also results in the excellent performance.

Electrocatalytic oxidation of methanol on polypyrrole film modified with platinum microparticles

The electrocatalytic oxidation of methanol on polypyrrole (PPy) film modified with platinum microparticles has been studied by means of electrochemical and in situ Fourier transform infrared techniques. The Pt microparticles, which were incorporated in the PPy film by the technique of cyclic voltammetry, were uniformly dispersed. The modified electrode exhibits significant electrocatalytic activity for the oxidation of methanol. The catalytic activities were found to be dependent on Pt loading and the thickness of the PPy film. The linearly adsorbed CO species is the only intermediate of electrochemical oxidation of methanol and can be readily oxidized at the modified electrodes. The enhanced electrocatalytic activities may be due to the uniform dispersion of Pt microparticles in the PPy film and the synergistic effects of the highly dispersed Pt microparticles and the PPy film. Finally, a reaction mechanism is suggested.

Electrocatalytic Oxidation of Methanol on Polythionine Film Modified with Pt Microparticles

The electrocatalytic oxidation of methanol on polythionine(PTn) film modified with Pt microparticles has been studied by means of cyclic voltammetry and in-situ FTIR spectroscopy. The Pt microparticles produced by cyclic voltammetry were highly dispersed in and on the PTn film. The modified electrodes exhibit significant electrocatalytic activity for the oxidation of methano and the catalytic activity was found in dependence on the Pt loading. The linearly adsorbed CO species is the only intermediate in the oxidation of methanol and the abnormal IR spectra for adsorbed CO were observed. On such modified electrodes, adsorbed CO species derived from methanol can be readily oxidized. The enhanced electrocatalytic activity may be ascribed to the high dispersion of Pt microparticles in and on the PTn film and the synergestic effect between Pt microparticles and the polymer. From the above results, a possible reaction mechanism was proposed.

The effect of chromium on sulfur resistance of Pd/HY-Al2O3 catalysts for aromatic hydrogenation

Sulfur is a major poison to noble metal catalysts for deep aromatic hydrogenation in the petroleum refining industry. In order to study the sulfur resistance of Pd-based catalysts, a series of Pd, Cr, and PdCr catalysts supported on HY-Al2O3 were studied by NH3-TPD, pyridine-adsorption IR, TPR, IR spectra of adsorbed CO, and toluene hydrogenation in the presence of 3000 ppm sulfur as thiophene under the following conditions: 533-573 K, 4.2 MPa, and WHSV 4.0 h(-1). Cr has no influence on the acidity of the catalysts. TPR patterns and in situ IR spectra of adsorbed CO revealed a strong interaction between Cr and Pd, and the frequency shift of linear bonded CO on Pd indicates that the electron density of Pd decreases with the increase of the Cr/Pd atomic ratio. The catalytic performance of Pd, Cr, and PdCr catalysts shows that the sulfur resistance of Pd is strongly enhanced by Cr, and the activity reaches its maximum when the Cr/Pd atomic ratio equals 8. The active phase model "Pd particles decorated by Cr2O3" is postulated to explain the behavior of PdCr catalysts. (C) 2001 Academic Press.

Different mechanisms for the formation of acetaldehyde and ethanol on the Rh-based catalysts

Different mechanisms for the formation of acetaldehyde and ethanol on the Rh-based catalysts were investigated by the TPR (temperature programmed reaction) method, and the active sites were studied by CO-TPD, TPSR (temperature programmed surface reaction of preadsorbed CO by H-2) and XPS techniques. The TPR results indicated that ethanol and acetaldehyde might be formed through different intermediates, whereas ethanol and methanol might result from the same intermediate. Results of CO-TPD, TPSR, and XPS showed that on the Rh-based catalyst, the structure of the active sites for the formation of C-2-oxygenates is ((RhxRhy+)-Rh-0)-O-Mn+ (M=Mn or Zr, x>>y, 2 less than or equal ton less than or equal to4). The tilt-adsorbed CO species is the main precursor for CO dissociation and the precursor for the formation of ethanol and methanol. Most of the linear and geminal adsorbed CO species desorbed below 500 K. Based on the suggested model of the active sites, detailed mechanisms for the formation of acetaldehyde and ethanol are proposed. Ethanol is formed by direct hydrogenation of the tilt-adsorbed CO molecules, followed by CH2 insertion into the surface CH2-O species and the succeeding hydrogenation step. Acetaldehyde is formed through CO insertion into the surface CH3-Rh species followed by hydrogenation, and the role of the promoters was to stabilize the intermediate of the surface acetyl species. (C) 2000 Academic Press.

In-situ FT-IR spectroscopic studies of fuel cell electro-catalysis: From single-crystal to nanoparticle surfaces

The work presented in this article shows the power of the variable temperature, in-situ FT-IR spectroscopy system developed in Newcastle with respect to the investigation of fuel cell electro-catalysis. On the Ru(0001) electrode surface, CO co-adsorbs with the oxygen-containing adlayers to form mixed [CO+(2x2)-O(H)] domains. The electro-oxidation of the Ru(0001) surface leads to the formation of active (1x1)-O(H) domains, and the oxidation of adsorbed CO then takes place at the perimeter of these domains. At 20 degrees C, the adsorbed CO is present as rather compact islands. In contrast, at 60 degrees C, the COads is present as a relatively looser and weaker adlayer. Higher temperature was also found to facilitate the surface diffusion and oxidation of COads. No dissociation or electro-oxidation of methanol was observed at potentials below approximately 950mV; however, the Ru(0001) surface at high anodic potentials was observed to be very active. On both Pt and PtRu nanoparticle surfaces, only one linear bond CO adsorbate was formed from methanol adsorption, and the PtRu surface significantly promoted both methanol dissociative adsorption to CO and its further oxidation to CO2. Increasing temperature from 20 to 60 degrees C significantly facilitates the methanol turnover to CO2.

New insight into mechanisms in water-gas-shift reaction on Au/CeO2(111): A density functional theory and kinetic study

Using density functional theory (DFT) and kinetic analyses, a new carboxyl mechanism for the water-gas-shift reaction (WGSR) on Au/CeO2(111) is proposed. Many elementary steps in the WGSR are studied using an Au cluster supported on CeO2(111). It is found that (i) water can readily dissociate at the interface between Au and CeO2; (ii) CO2 can be produced via two steps: adsorbed CO on the Au cluster reacts with active OH on ceria to form the carboxyl (COOH) species and then COOH reacts with OH to release CO2; and (iii) two adsorbed H atoms recombine to form molecular H-2 on the Au cluster. Our kinetic analyses show that the turnover frequency of the carboxyl mechanism is consistent with the experimental one while the rates of redox and formate mechanisms are much slower than that of carboxyl mechanism. It is suggested that the carboxyl pathway is likely to be responsible for WGSR on Au/CeO2.

Tetrahexahedral Pt Nanocrystal Catalysts Decorated with Ru Adatoms and Their Enhanced Activity in Methanol Electrooxidation

Tetrahexahedral Pt nanocrystals (THH Pt NCs) bound by well-defined high index crystal planes offer exceptional electrocatalytic activity, owing to a high density of low-coordination surface Pt sites. We report, herein, on methanol electrooxidation at THH Pt NC electrodes studied by a combination of electrochemical techniques and in situ FTIR spectroscopy. Pure THH Pt NC surfaces readily facilitate the dissociative chemisorption of methanol leading to poisoning by strongly adsorbed CO. Decoration of the stepped surfaces by Ru adatoms increases the tolerance to poisoning and thereby reduces the onset potential for methanol oxidation by over 100 mV. The Ru modified THH Pt NCs exhibit greatly superior catalytic currents and CO2 yields in the low potential range, when compared with a commercial PtRu alloy nanoparticle catalyst. These results are of fundamental importance in terms of model nanoparticle electrocatalytic systems of stepped surfaces and also have practical significance in the development of surface tailored, direct methanol fuel cell catalysts.

Enhancing the Activity and Tuning the Mechanism of Formic acid Oxidation at Tetrahexahedral Pt Nanocrystals by Au Decoration

Tetrahexahedral Pt nanocrystals (THH Pt NCs), bound by high index facets, belong to an emerging class of nanomaterials that promise to bridge the gap between model and practical electrocatalysts. The atomically stepped surfaces of THH Pt NCs are extremely active for the electrooxidation of small organic molecules but they also readily accommodate the dissociative chemisorption of such species, resulting in poisoning by strongly adsorbed CO. Formic acid oxidation is an ideal reaction for studying the balance between these competing catalyst characteristics, since it can proceed by either a direct or a CO mediated pathway. Herein, we describe electrochemical and in situ FTIR spectroscopic investigations of formic acid electrooxidation at both clean and Au adatom modified THH Pt NC surfaces. The Au decoration leads to higher catalytic currents and enhanced CO2 production in the low potential range. As the CO oxidation behaviour of the catalyst is not changed by the presence of the Au, it is likely that the role of the Au is to promote the direct pathway. Beyond their fundamental importance, these results are significant in the development of stable, poison resistant anodic electrocatalysts for direct formic acid fuel cells.

Acidity Characterization of Amorphous Silica-Alumina

Surface characterization of amorphous silica-alumina (ASA) by COads IR, pyridine(ads) IR, alkylamine temperature-programmed desorption (TPD), Cs+ and Cu(EDA)(2)(2+) exchange, H-1 NMR, and m-xylene isomerization points to the presence of a broad range of Bronsted and Lewis acid sites. Careful interpretation of IR spectra of adsorbed CO or pyridine confirms the presence of a few very strong Bronsted acid sites (BAS), typically at concentrations lower than 10 mu mol/g. The general procedure for alkylamine TPD, which probes both Bronsted and Lewis acidity, is modified to increase the selectivity to strong Bronsted acid sites. Poisoning of the m-xylene isomerization reaction by a base is presented as a novel method to quantify strong BAS. The surface also contains a weaker form of BAS, in concentrations between 50 and 150 mu mol/g, which can be quantified by COads IR Cu(EDA)(2)(2+) exchange also probes these sites. The structure of these sites remains unclear, but they might arise from the interaction of silanol groups with strong Lewis acid Al3+ sites. The surface also contains nonacidic aluminol and silanol sites (200-400 mu mol/g) and two forms of Lewis acid sites: (i) a weaker form associated with segregated alumina domains containing five-coordinated Al, which make up the interface between these domains and the ASA phase and (ii) a stronger form, which are undercoordinated Al sites grafted onto the silica surface. The acid catalytic activity in bifunctional n-heptane hydroconversion correlates with the concentration of strong BAS. The influence of the support electronegativity on the neopentane hydrogenolysis activity of supported Pt catalysts is considerably larger than that of the support Bronsted acidity. It is argued that strong Lewis acid sites, which are present in ASA but not in gamma-alumina, are essential to transmit the Sanderson electronegativity of the oxide support to the active Pt phase.

Structure and reactivity of the Ru(0 0 0 1) electrode towards fuel cell electrocatalysis

The reactivity of the Ru(0 0 0 1) electrode towards the adsorption and electrooxidation of CO and methanol has been studied by variable-temperature in situ FTIR spectroscopy in both perchloric acid and sodium hydroxide solution, and the results interpreted in terms of the surface chemistry of the Ru(0 0 0 1) electrode. Both linear (CO) and threefold hollow (CO) binding CO adsorbates (bands at 1970-2040 and 1770-1820 cm, respectively) were observed on the Ru(0 0 0 1) electrode in both 0.1 M HClO and 0.1 M NaOH solutions from the CO adsorption. In the acid solution, CO was detected as the main adsorbed species on Ru(0 0 0 1) surface over all the potential region studied. In contrast, in the alkaline solution, more CO than CO was detected at lower potentials, whilst increasing the potential resulted in the transformation of CO to CO. At higher potentials, the oxidation of the adsorbed CO took place via reaction with the active (1 × 1)-O oxide/hydroxide. It was found that no dissociative adsorption or electrooxidation of methanol took place at the Ru(0 0 0 1) at potentials below 900 mV vs Ag/AgCl in perchloric acid solution at both 20 and 55°C. However, in the alkaline solution, methanol did undergo dissociative adsorption, to form linearly adsorbed CO (CO) with little or no CO adsorbed at threefold hollow sites (CO) at both 20 and 55°C. Increasing the temperature from 20 to 55°C clearly facilitated the methanol dissociative adsorption to CO and also enhanced the electrooxidation of the CO. At the higher potentials, significant oxidation of methanol to CO and methyl formate in acid solution and to bicarbonate and formate in alkaline solution, was observed, which was attributed to the formation of an active RuO phase on the Ru(0 0 0 1) surface, in agreement with our previous studies. © 2003 Elsevier Ltd. All right reserved.

Methanol oxidation on PtRu electrodes. Influence of surface structure and Pt-Ru atom distribution

The activities of different types of PtRu catalysts for methanol oxidation are compared. Materials used were: UHV-cleaned PtRu alloys, UHV-evaporated Ru onto Pt(111) as well as adsorbed Ru on Pt(111) prepared with and without additional reduction by hydrogen. Differences in the catalytic activity are observed to depend on the preparation procedure of the catalysts. The dependence of the respective catalytic activities upon the surface composition is reported. UHV-STM data for Pt(111)/Ru show the formation of two- and three-dimensional structures depending on surface coverage. A molecular insight on the electrochemical reaction is given via in situ infrared spectroscopy. Analysis of the data indicates that the most probable rate-determining step is the reaction of adsorbed CO with Ru oxide.