Showing papers in "Journal of Catalysis in 2004"

The Brønsted–Evans–Polanyi relation and the volcano curve in heterogeneous catalysis

Abstract: A number of elementary reactions at metal surfaces show a linear Bronsted–Evans–Polanyi relation between the activation energy and the reaction energy, and reactions belonging to the same class even follow the same relation. We investigate the implications of this finding on the kinetics of surface-catalyzed chemical processes. We focus in particular on the variation in the activity from one metal to the next. By analyzing a number of simple microkinetic models we show that the reaction rate under given reaction conditions shows a maximum as a function of the dissociative adsorption energy of the key reactant, and that for most conditions this maximum is in the same range of reaction energies. We also provide a database of chemisorption energies calculated using density-functional theory for a number of simple gas molecules on 13 different transition metals. An important part of the analysis consists of developing a general framework for analyzing the maximum rate. We use these concepts to rationalize trends in the catalytic activity of a number of metals for the methanation process.

On the origin of the catalytic activity of gold nanoparticles for low-temperature CO oxidation

Abstract: It is suggested that there may be several effects contributing to the special catalytic properties of supported nanosized gold particles, and that it is useful to order them in a hierarchy. The most important effect is related to the availability of many low-coordinated gold atoms on the small particles. Effects related to the interaction with the support may also contribute, but to a considerably smaller extent. We base the analysis on a new set of experimental results comparing the CO oxidation rates over gold supported on different reducible and nonreducible oxides, on an analysis of a large number of published activity data, and on an analysis of density-functional calculations of the effect of metal coordination numbers in comparison to the role of charge transfer, layer thickness, and interactions with the support.

Use of DFT to achieve a rational understanding of acid–basic properties of γ-alumina surfaces

Abstract: In a recent priority communication [M. Digne et al., J. Catal. 211 (2002) 1], we proposed the first ab initio constructed models of γ -alumina surfaces. Using the same density-functional approach, we investigate in further detail the acid–basic properties of the three relevant γ -alumina (100), (110), and (111) surfaces, taking into account the temperature-dependent hydroxyl surface coverages. The simulations, compared fruitfully with many available experimental data, enable us to solve the challenging assignment of the OH-stretching frequencies, as obtained from infrared (IR) spectroscopy. The precise nature of the acid surface sites (concentrations and strengths) is also determined. The acid strengths are quantified by simulating the adsorption of relevant probe molecules such as CO and pyridine in correlation with surface electronic properties. These results seriously challenge the historical model of a defective spinel for γ -alumina and establish the basis for a more rigorous description of the acid–basic properties of γ -alumina.

Isotopic and kinetic assessment of the mechanism of reactions of CH4 with CO2 or H2O to form synthesis gas and carbon on nickel catalysts

Abstract: Kinetic and isotopic measurements for catalysts and conditions that rigorously excluded transport and thermodynamic artifacts led to a common sequence of elementary steps for reactions of CH 4 with CO 2 or H 2 O and for its stoichiometric decomposition on Ni/MgO catalysts. Turnover rates for forward reactions of CH 4 /CO 2 and CH 4 /H 2 O mixtures were proportional to CH 4 pressure (5–450 kPa) and independent of the partial pressure of the CO 2 or H 2 O coreactants (5–450 kPa). These turnover rates and their first-order rate constants and activation energies are also similar to those measured for CH 4 decomposition, indicating that these reactions are mechanistically equivalent and that CH bond activation is the sole kinetically relevant step in all three reactions. These conclusions were confirmed by identical CH 4 /CD 4 kinetic isotope effects ( k H / k D =1.62–1.71) for reforming and decomposition reactions and by undetectable H 2 O/D 2 O isotopic effects. The kinetic relevance of CH bond activation is consistent with the relative rates of chemical conversion and isotopic mixing in a CH 4 /CD 4 /CO 2 mixture and with the isotopic evidence for the quasi-equilibrated nature of coreactant activation and H 2 and H 2 O desorption obtained from reactions of CH 4 /CO 2 /D 2 and 12 CH 4 / 12 CO 2 / 13 CO mixtures. These quasi-equilibrated steps lead to equilibrated water–gas-shift reactions during CH 4 reforming, a finding confirmed by measurements of the effluent composition. These elementary steps provide also a predictive model for carbon filament growth and identify a rigorous dependence of the carbon thermodynamic activity on various kinetic and thermodynamic properties of elementary steps and on the prevalent concentrations of reactants and products, specifically given by P CH 4 P CO / P CO 2 (or P CH 4 P H 2 / P H 2 O ) ratios. These mechanistic features on Ni surfaces resemble those previously established for supported noble metal catalysts (Rh, Pt, Ir, Ru). These direct measurements of CH bond activation turnover rates allowed the first direct and rigorous comparison of the reactivity of Ni and noble metal catalysts for CH 4 -reforming reactions, under conditions of strict kinetic control and relevant commercial practice and over a wide range of compositions and metal dispersions.

Reaction network of steam reforming of ethanol over Ni-based catalysts

Abstract: The reaction of steam reforming of ethanol over nickel catalysts supported on γ -Al 2 O 3 , La 2 O 3 , and La 2 O 3 / γ -Al 2 O 3 is investigated employing transient and steady-state techniques. It is found that ethanol interacts strongly with alumina on the surface of which it is dehydrated at low temperatures, and less strongly with lanthana on the surface of which it is both dehydrogenated and dehydrated. Cracking reactions are also observed on the carriers at intermediate temperatures. In the presence of Ni, catalytic activity is shifted toward lower temperatures. In addition to the above reactions, reforming, water–gas shift, and methanation contribute significantly to product distribution. Carbon deposition is also a significant route. It is found that the rate of carbon deposition is a strong function of the carrier, the steam-to-ethanol ratio, and reaction temperature. The presence of lanthana on the catalyst, high steam-to-ethanol ratio, and high temperature offer enhanced resistance toward carbon deposition.

TiO2-supported metal oxide catalysts for low-temperature selective catalytic reduction of NO with NH3. I. Evaluation and characterization of first row transition metals

Abstract: Low-temperature SCR of NO with NH3 in the presence of excess oxygen on the oxides of V, Cr, Mn, Fe, Co, Ni, and Cu supported on anatase TiO2 has been studied. Among the catalysts tested, Mn/TiO2 supported on Hombikat TiO2 provided the best performance with 100% N2 selectivity and complete NO conversion at temperatures as low as 393 K under numerous conditions. The catalytic performance for various transition metal oxides supported on TiO2 decreased in the following order: Mn > Cu ⩾ Cr ⪢ Co > Fe ⪢ V ⋙ Ni. For Mn-based catalysts the activity increases with an increase in Mn loading and the reaction temperature. TiO2 alone did not give any NO conversion at ⩽573 K, and calcination at low temperature (⩽673 K) is preferable. XRD coupled with XPS confirmed the presence of MnO2 as a major phase (peak at 642.2 eV) with Mn2O3, and partially undecomposed Mn-nitrate as the minor phases for supported manganese catalysts. It is proposed that MnO2 contributes to the high activity of Mn/TiO2. XPS results also confirmed a higher concentration of active metal oxides on the surface of Mn/TiO2 compared to the other catalysts. The NH3 FT-IR study showed the presence of Lewis acid sites for the most active catalysts, while the peak corresponding to Bronsted acid sites was weak or absent. This strongly suggests that Bronsted acid sites are not necessary for the reaction to occur at low temperatures. The H2 TPR study indicated the difficulty of reducing Mn oxide when the metal loading is low and/or the catalysts are calcined at temperatures higher than 773 K. It is concluded that lower catalyst calcination temperatures, Lewis acidity, the redox properties of metal oxides and their higher surface concentration are important for very high SCR activity at low temperatures. Mn/TiO2 provided the best performance at 50,000 h−1 when the catalysts were tested in the presence of 11 vol% H2O. Under these conditions, the catalytic activity of the transition metal oxides decreases in the following order: Mn > V ⪢ Co > Cu > Cr > Fe ⪢ Ni.

Pd/CeO2–TiO2 catalyst for CO oxidation at low temperature: a TPR study with H2 and CO as reducing agents

Abstract: The catalysts Pd/TiO 2 , Pd/CeO 2 , and Pd/CeO 2 –TiO 2 for CO oxidation at low temperature together with their corresponding supports TiO 2 , CeO 2 , and CeO 2 –TiO 2 prepared by sol–gel precipitation followed by supercritical fluid drying were characterized by means of N 2 adsorption, XRD, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO adsorption and temperature-programmed reduction (TPR) with H 2 and CO as reducing agents The results showed that PdO is finely dispersed on the supports with high surface area DRIFTS of CO adsorption further indicated that both Pd 2+ and Pd 0 species coexist in Pd/CeO 2 , while only Pd 0 is detected in Pd/TiO 2 and Pd/CeO 2 –TiO 2 H 2 -TPR showed that no bulk CeO 2 exists in CeO 2 –TiO 2 and the reduction of CeO 2 –TiO 2 is more difficult than that of the surface oxygen in the individual CeO 2 probably due to the formation of CeO 2 –TiO 2 solid solution PdO in Pd/TiO 2 can be fully reduced by H 2 at ambient temperature, whereas PdO in Pd/CeO 2 and Pd/CeO 2 –TiO 2 is reduced respectively at 162 and 80 °C, accompanied by the reduction of surface CeO 2 CO-TPR showed that the reduction of PdO in PdO/TiO 2 is limited within the outermost layer at ambient temperature and the core PdO can only be reduced with a further increase of temperature PdO/CeO 2 is also reduced both at ambient temperature and with the increase of temperature during the CO-TPR process, and the reduction temperature of CeO 2 decreased significantly in the presence of PdO, which may be due to the different degrees of interaction between PdO and CeO 2 In contrast, a complete reduction of PdO in Pd/CeO 2 –TiO 2 by CO is observed at ambient temperature, accompanied by the partial reduction of CeO 2 , which may indicate that the special Pd–Ce–Ti interaction in Pd/CeO 2 –TiO 2 is favorable for the reduction of PdO and interfacial CeO 2 species The water–gas shift (WGS) between CO and the hydroxyl groups in the catalysts is detected at a temperature higher than 210 °C on all of the Pd-supported catalysts as well as the individual CeO 2 and TiO 2 supports

Characterization and reactivity in CO oxidation of gold nanoparticles supported on TiO2 prepared by deposition-precipitation with NaOH and urea

Abstract: Au/TiO 2 catalysts were prepared by deposition-precipitation with NaOH (DP NaOH) (∼3 wt% Au) and urea (DP Urea) (∼8 wt% Au), and calcined at various temperatures between 100 and 400 °C. They were characterized by UV-visible absorption, XANES, EXAFS, and TEM. Their activity was tested in the reaction of CO oxidation at 5 °C. After preparation, the gold species are in the oxidic state III. They begin to transform into metallic gold under air at ∼100 °C for DP NaOH and ∼150 °C for DP Urea. At 200 °C, all the gold is metallic for the two preparation methods. The particle size increases from 1.5 to 3.5 nm with the calcination temperature. The catalytic activity for CO oxidation increases with the percentage of metallic gold, and it is maximum after calcination at 200 °C for both types of samples. The activities (per mole of Au) and TOF are the same for the two types of catalysts. After higher calcination temperatures, the catalytic activity drops. The decrease of activity is mainly due to a change of particle shape with the calcination temperature rather than to the increase of the particle size. At a pretreatment temperature of 200 °C, the particles present small facets with rounded parts, and their outer surface contains a large proportion of low-coordinated sites (steps and corners) while at higher temperatures of pretreatment, they are truncated octahedra with smooth facets.

Effects of sol–gel procedures on the photocatalysis of Cu/TiO2 in CO2 photoreduction

Abstract: Copper-loaded titania (Cu/TiO2) was synthesized via an improved modified sol–gel process. Photocatalysts were applied to the CO 2 photocatalytic reduction and the yield of the major product, methanol, was used to evaluate the photocatalytic performance. Copper precursors and the adding time with sol as well as posttreatments were studied to explore the relationships between the characteristics and the activity of the photocatalysts. The results revealed that Cu/TiO 2 prepared from copper chloride and added in the early sol–gel stage was more photoactive than that from copper acetate. Additional H2 reduction of calcined catalysts before the photoreduction CO2 decreased the yield of methanol due to the change of copper dispersion and oxidation state. TPR, XPS, and XAS measurements verified the oxidation state of Cu on Cu/TiO2 catalysts. The results indicated that the primary Cu(I) served as an active site. The zeta potentials of catalysts were measured and compared, showing that a higher positive zeta potential at pH 7 would lead to higher activity. Under 30-h UVC (254 nm) irradiation, the best catalyst gave a methanol yield above 600 µmol/gcat. Switching to UVA (365 nm) resulted in a significant decrease of methanol yield in the range of 10 µmol/gcat.

On the nature of different iron sites and their catalytic role in Fe-ZSM-5 DeNOx catalysts: new insights by a combined EPR and UV/VIS spectroscopic approach

Abstract: Fe-ZSM-5 DeNO x catalysts prepared from H-ZSM-5 by different ion-exchange procedures have been analyzed by EPR and UV/VIS diffuse reflectance spectroscopy (DRS) ex situ after synthesis, calcination, and use in catalysis as well as in situ during calcination. The results have been correlated with the catalytic behavior of these materials in the selective catalytic reduction (SCR) of NO by isobutane or ammonia. In comparison to previous studies of the same samples by XAFS, XRD, XPS, TPR, and Mossbauer spectroscopy, the combination of EPR and UV/VIS-DRS was more sensitive for distinguishing between different types of isolated Fe species as well as Fe x O y aggregates of different size (oligonuclear clusters or large particles). It was found that aggregated species are formed at the expense of mononuclear Fe sites upon calcination at 873 K, and that aggregate formation is slightly favored by calcination with higher heating rates as well as by high Si/Al ratios of the parent H-ZSM-5. Use in SCR of NO leads to further growth and restructuring of Fe x O y clusters. From the comparison of structural and catalytic properties of different Fe-ZSM-5 catalysts it can be concluded that the SCR of NO by NH 3 is catalyzed by different entities (mononuclear Fe sites, Fe x O y oligomers, surface of iron oxide particles). The results suggest that mononuclear Fe sites are also involved in the SCR with isobutane. Clustered sites, which may contribute to SCR with isobutane as well, appear to cause nonselective oxidation of the reductant (isobutane or ammonia) at higher temperatures.

Atomic-scale insight into structure and morphology changes of MoS2 nanoclusters in hydrotreating catalysts

Abstract: High-resolution scanning tunneling microscopy (STM) is used in combination with density-functional theory (DFT) to provide new insight into the morphology and atomic-scale structure of MoS2 nanoclusters in hydrodesulfurization (HDS) catalysts. Atom-resolved STM images of gold-supported single-layer MoS2 nanoclusters reveal the first direct evidence that both the detailed atomic-scale structure of the catalytically important edges and the overall morphology of the nanoparticles are sensitive to sulfiding and reaction conditions. Specifically, it is shown that synthesis in H2S:H2=500 results in MoS2 nanoclusters with a triangular morphology, whereas sulfiding in H2S:H2=0.07 leads to hexagonally truncated nanoclusters. For both morphologies we identify the exact geometric edge structure of the MoS2 nanoclusters by comparing the atom-resolved STM images with STM simulations. Whereas the MoS2 triangles are terminated by dimer-saturated Mo edges, the hexagonal MoS2 structures exhibit completely different edge structures with a lower sulfur coverage on the Mo edges and S edges with adsorbed SH groups. A thermodynamic model based on DFT is employed to construct phase diagrams which can predict the stability of different MoS2 edge structures under different conditions. The present results thus provide new insight into the atomic structure of the HDS catalysts and how it may change with reaction conditions.

Selective oxidation of glycerol to sodium glycerate with gold-on-carbon catalyst: an insight into reaction selectivity

Abstract: Glycerol was oxidised with oxygen in the presence of gold on carbon as the catalyst. Two types of catalysts were identified: one, characterised by well-dispersed nanoparticles with a mean diameter centred at 6 nm, did not maintain the initial selectivity of the oxidation at full conversion; the other, characterised by larger particles (>20 nm), showed constant selectivity from the beginning to the end of the reaction. The experimental conditions were studied to optimise glycerate production with selected catalysts, with particular regard to the effect of the NaOH/glycerol ratio, glycerol concentration, temperature, and glycerol/catalyst ratio. The best result was 92% selectivity to glycerate at full conversion, obtained by oxidising glycerol at 30 °C, with a NaOH/glycerol ratio of 4, a glycerol/Au = 500, and 0.3 M concentration.

Aqueous-phase reforming of oxygenated hydrocarbons over Sn-modified Ni catalysts

Abstract: Reaction kinetics measurements were conducted for aqueous-phase reforming of oxygenated hydrocarbons over Pt/Al 2 O 3 , Ni/Al 2 O 3 , NiSn/Al 2 O 3 , Raney-Ni, and Raney-NiSn catalysts at temperatures of 498 and 538 K. Raney-Ni, Raney-NiSn, and Pt/Al 2 O 3 catalysts display good stability with time on stream during aqueous-phase reforming, whereas Al 2 O 3 -supported Ni and NiSn catalysts exhibit deactivation caused by sintering. Aqueous-phase reforming of sorbitol, glycerol, and ethylene glycol solutions produces an effluent gas stream composed of 50–70 mol% H 2 , 30–40 mol% CO 2 , and 2–11 mol% alkanes (dry basis) at high conversion. Addition of Sn to Ni improves the selectivity for production of H 2 by ethylene glycol reforming from 35 to 51% at a Ni:Sn ratio of 270:1, while the alkane selectivity is reduced from 44 to 33%. At a Ni:Sn ratio of 14:1, the hydrogen selectivity increases to 90%, while alkane production is nearly eliminated. As the system pressure decreases to the bubble point of the feed (25.1 bar at 498 K), production of alkanes decreases and the hydrogen selectivity increases accordingly. Hydrogen selectivity is also maximized by operation at higher reactor space velocities. The addition of Sn to Ni significantly decreases the rate of methane formation from CO bond cleavage, while maintaining sufficiently high rates of CC bond cleavage required for hydrogen formation. Turnover frequencies for hydrogen production at 498 K over Raney-Ni-based catalysts are several times lower than that over 3 wt% Pt/Al 2 O 3 based on CO chemisorption. However, the high CO uptakes and high densities of Raney-Ni-based catalysts lead to comparable rates of hydrogen production per unit reactor volume as 3 wt% Pt/Al 2 O 3 at 498 K. Results from XRD, SEM, and 119 Sn Mossbauer spectroscopy suggest that Raney-NiSn catalysts comprise alumina and nickel particles surrounded by a Ni–Sn alloy. After exposure to reaction conditions, Sn is present primarily as Ni 3 Sn alloy with small amounts of Sn(IV) probably associated with alumina.

Structural requirements and reaction pathways in methane activation and chemical conversion catalyzed by rhodium

Abstract: Kinetic and isotopic tracer methods led to a simple and unifying mechanistic proposal for reactions of CH 4 with CO 2 and H 2 O, for its decomposition on Rh clusters, and for water–gas shift reactions. Kinetic rates for forward reactions were measured by correcting net rates for approach to equilibrium and by eliminating transport artifacts. These rates were proportional to CH 4 pressure (5–450 kPa) and independent of CO 2 or H 2 O pressures (5–450 kPa) on all supported Rh catalysts; the resulting first-order rate constants were identical for H 2 O and CO 2 reforming and for CH 4 decomposition. Kinetic isotope effects ( k CH 4 / k CD 4 =1.54–1.60) were also independent of the concentration or identity of the co-reactant, consistent with the sole kinetic relevance of CH bond activation steps. These data indicate that co-reactant activation and its kinetic coupling with CH 4 activation via scavenging of chemisorbed carbon intermediates are fast steps and lead to Rh surfaces essentially uncovered by reactive intermediates during H 2 O and CO 2 reforming. CO oxidation rates before and after reforming reactions showed that Rh surfaces remain uncovered by unreactive species during reforming catalysis under conditions relevant to industrial practice. CH 4 conversion rates for CH 4 /CD 4 /CO 2 reactant mixtures are much faster than CH 4− x D x formation rates, indicating that CH bond activation elementary steps are irreversible. CH 4 /CO 2 /D 2 reactant mixtures led to binomial isotopomer distributions in dihydrogen and water at all reactant conversions. Their D contents were identical and corresponded to equilibration between all H atoms in reacted CH 4 and all D 2 in the inlet stream. Thus, recombinative desorption steps of H atoms and OH groups to form H 2 or H 2 O are quasi-equilibrated during CH 4 reforming. 12 CH 4 / 12 CO 2 / 13 CO mixtures led to identical 13 C contents in CO and CO 2 , as expected from quasi-equilibrated CO 2 activation steps. The quasi-equilibrated nature of all these steps requires that water–gas shift reactions also be at equilibrium during CH 4 reforming, as found experimentally. CH 4 reforming turnover rates increased as the size of Rh clusters supported on Al 2 O 3 or ZrO 2 decreased, suggesting that coordinatively unsaturated Rh surface atoms prevalent in smaller clusters activate CH bonds more effectively than atoms on lower-index surfaces, as also found on single-crystal surfaces. Turnover rates do not depend on the identity of the support; any involvement of the support in the activation of co-reactants is not kinetically relevant.

Investigation on the catalysis of COx-free hydrogen generation from ammonia

Abstract: The effects of active component (Ru, Rh, Pt, Pd, Ni, Fe) and support (CNTs, AC, Al2O3, MgO, ZrO2, TiO2) on the catalysis of ammonia decomposition were studied for the generation of COx-free hydrogen. It was shown that the Ru catalyst using CNTs as support exhibits the highest conversion of NH3. The performance can be further improved by modifying CNTs with KOH. According to the results of XRD, TEM, and chemisorption (CO and H2 as adsorbates) investigations, Ru dispersion is the highest on CNTs. In the range of 2–5 nm, the particle size of Ru on CNTs is the smallest among the supported Ru catalysts; the Ru particles on the metal oxides are in the 3–16 nm range. It seems that larger Ru particles are more active for NH3 decomposition in terms of TOF. Further investigation on the relationship between support basicity and catalytic activity disclosed that a support material of strong basicity is essential for high catalytic performance. In the N2-TPD studies of supported Ru catalysts, desorption was promoted over catalysts of strong basicity, suggesting that N2 desorption is the rate-determining step in ammonia decomposition. These results implied that it is possible to develop a highly efficient Ru catalyst for NH3 decomposition by using electron-conductive materials of strong basicity as support.

Manganese–lanthanum oxides modified with silver for the catalytic combustion of methane

Abstract: The characterization of manganese–lanthanum oxides modified with silver has been performed in order to identify factors responsible for the variation of their activity in the oxidation of methane. A significant increase in the activity per unit surface area in silver-containing catalysts occurred above 800 K, where a new source of surface oxygen appeared. It is probably oxygen released from filled oxygen vacancies, more weakly bound in the oxides structure in comparison with lattice oxide ions, more mobile, and therefore easily accessible to methane oxidation. Such oxygen is probably neighboring with silver ions. The remaining part of the catalyst may constitute a reservoir of oxygen ions with which the vacancies are filled and which is supplemented with the gaseous oxygen. A consequence of filling up oxygen vacancies is the appearance of a larger number of manganese ions in the unstable oxidation state Mn 4+ . The rate of methane oxidation is a function of the Mn 4+ /Mn 3+ surface ratio which is a parameter characterizing the intrinsic properties of the manganese–lanthanum oxides, influencing their activity.

Hydrodesulfurization reaction pathways on MoS2 nanoclusters revealed by scanning tunneling microscopy

Abstract: Single-layer MoS2 nanoclusters were synthesized on a Au substrate as a model system for the hydrotreating catalyst and studied by atomically resolved scanning tunneling microscopy (STM) in order to achieve atomic-scale insight into the interactions with hydrogen and thiophene (C4H4S). Surprisingly, STM images show that thiophene molecules can adsorb and react on the fully sulfided edges of triangular single-layer MoS2 nanoclusters. We associate this unusual behavior with the presence of special brim sites exhibiting a metallic character. The STM images reveal that these sites exist only at the regions immediately adjacent to the edges of the MoS2 nanoclusters, and from density-functional theory such sites are found to be associated with one-dimensional electronic edge states. The fully sulfur-saturated sites are from STM images found to be capable of adsorbing thiophene, and when thiophene and hydrogen reactants are coadsorbed here, a reaction path is revealed which leads to partial hydrogenation of the thiophene followed by CS bond activation and ring opening of thiophene molecules. This may be regarded as important first steps in the hydrodesulfurization of thiophene. The metallic brim sites are suggested to be important for other hydrotreating reactions over MoS2-based catalysts, and the properties of the brim sites directly explain why hydrogenation reactions of aromatics are not severely inhibited by H2S. The presence of brim sites in MoS2 nanoclusters also explains previous structure–activity relations and observations regarding steric effects and the influence of stacking of MoS2 on the reactivity and selectivity.

Vanadium oxide supported on mesoporous SBA-15 as highly selective catalysts in the oxidative dehydrogenation of propane

Abstract: The combination of different techniques (DR UV–vis, 51 V NMR, UV-Raman, FTIR, and H 2 -TPR) in the characterization of vanadia supported on mesoporous SBA-15 catalysts shows that the dispersity and the nature of the vanadium species depend strongly on the V loading. In dehydrated catalysts with V contents lower than 2.8 wt%, vanadium is mainly in a tetrahedral environment. Higher V contents in the catalyst lead to the formation of polymeric V 2 O 5 -like species. Textural, SEM/TEM, and XRD results indicate that the ordered hexagonal mesoporous structure with large pore diameters of the support is retained upon the vanadium incorporation, and therefore high surface areas were obtained on the final catalysts. Vanadium species anchored to the surface show structural properties similar to those on mesoporous V-MCM and conventional VSiO 2 catalysts, but a higher surface concentration of isolated or low polymeric VO x species could be achieved on the V-SBA samples. The number and nature of the acid sites also change with the vanadium loading. The superior performance of the present mesoporous SBA-15 catalysts in the oxidative dehydrogenation of propane has been attributed to a higher dispersion of V species achieved on the SBA-15 support with large pore diameters as well as the low surface acidity of the catalyst.

Autothermal reforming of CH4 over supported Ni catalysts prepared from Mg–Al hydrotalcite-like anionic clay

Abstract: spc-Ni/MgAl (spc: solid-phase crystallization method) catalysts have been prepared from Mg–Al hydrotalcite-like compounds containing Ni at the Mg site as the precursors and tested for partial oxidation of CH 4 into synthesis gas. The precursors based on [Mg 2+ 1− x Al 3+ x (OH) 2 ] x + (CO 3 − x ) mH 2 O, in which a ratio of Mg/Al varied and a part of the Mg 2+ ions were replaced by Ni 2+ ions, were prepared by a coprecipitation method, thermally decomposed, and reduced to form spc-Ni/MgAl catalyst. Surface areas of spc-Ni/MgAl catalysts were around 150 m 2 g cat −1 . Ni 2+ ions first substituted a part of the Mg 2+ sites in the Mg–Al hydrotalcite-like compounds and then incorporated in the rock-salt-type Mg–Ni–O solid solutions in the mixed oxide after the decomposition. The dispersion of Ni was thus repeatedly enhanced during the spc preparation, resulting in the highly dispersed Ni metal particles after the reduction. The activity of the spc-Ni/MgAl catalyst was the highest at the ratio of Mg/Al of 1/3. When the catalysts were tested in the partial oxidation of CH 4 , spc-Ni 0.5 /Mg 2.5 Al afforded enough high CH 4 conversion even at the high space velocity (9×10 5 ml h −1 g cat −1 ), exceeding the value obtained over 1 wt% Rh/MgO. Ni species on spc-Ni 0.5 /Mg 2.5 Al catalysts were stable even under the presence of O 2 , while Ni catalysts prepared by the conventional impregnation quickly lost activity due to the surface oxidation of Ni particles. Moreover, the total heat produced by the reaction was the lowest over the spc-Ni 0.5 /Mg 2.5 Al catalyst among the catalysts tested. This strongly suggests that the heat of exothermic CH 4 combustion to H 2 O and CO 2 could be quickly consumed by the following endothermic CH 4 reforming by H 2 O and CO 2 over spc-Ni 0.5 /Mg 2.5 Al. Thus, the spc-Ni 0.5 /Mg 2.5 Al catalyst is a hopeful candidate for the autothermal reforming of CH 4 which can be carried out under the copresence of both H 2 O and O 2 to feed H 2 to the fuel cell economically. Actually the autothermal reforming of CH 4 has been successfully carried out over spc-Ni 0.5 /Mg 2.5 Al catalysts.

Hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene over sulfided NiMo/γ-Al2O3, CoMo/γ-Al2O3, and Mo/γ-Al2O3 catalysts

Abstract: The hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) was studied over sulfided NiMo/ γ -Al 2 O 3 , CoMo/ γ -Al 2 O 3 , and Mo/ γ -Al 2 O 3 catalysts The Ni and Co promoters strongly enhanced the activity of the Mo catalyst in the direct desulfurization pathway of the HDS of DBT and 4,6-DMDBT and in the final sulfur-removal step in the hydrogenation pathway, while the hydrogenation was moderately promoted H 2 S had a negative effect on the HDS of DBT and 4,6-DMDBT, which was strongest for the NiMo catalyst and stronger for the direct desulfurization pathway than for the hydrogenation pathway Because the direct desulfurization pathway is less important for the HDS of 4,6-DMDBT than the hydrogenation pathway, the conversion of 4,6-DMDBT was less affected by H 2 S than the conversion of DBT The sulfur removal via the direct desulfurization pathway and the ultimate sulfur removal in the hydrogenation pathway were affected by H 2 S to the same extent over all the catalysts This suggests that the removal of sulfur from tetrahydrodibenzothiophenes takes place by hydrogenolysis, like the direct desulfurization of DBT to biphenyl The CoMo catalyst performed better than the NiMo catalyst in the final desulfurization via the hydrogenation pathway in the HDS of 4,6-DMDBT at all partial pressures of H 2 S

Effects of morphology on surface hydroxyl concentration: a DFT comparison of anatase–TiO2 and γ-alumina catalytic supports

Abstract: A comparative investigation of surface hydroxylation states for anatase–TiO 2 and γ -alumina is crucial for a better understanding of how these materials behave as catalytic supports under working conditions. Our approach combines density functional simulations and thermodynamic analysis, to determine the types of hydroxyls existing on the (100), (101), (001), and (110) surfaces of anatase–TiO 2 , as a function of temperature and water pressure. The vibrational analysis of surface OH groups allows for the assignment of experimental infrared bands as a function of the surface orientation. A consistent and quantitative comparison with recent DFT simulations on γ -alumina highlights the different acidic–basic properties of the two supports. Finally, we suggest directions for increasing the density of basic and exchangeable hydroxyls which is governed by morphology effects.

Increasing the basicity and catalytic activity of hydrotalcites by different synthesis procedures

Abstract: Hydrotalcites have been synthesized by three different procedures: conventional precipitation-aging, aging under microwave irradiation, and sonication during the coprecipitation step. The synthesis procedure has an effect on the crystal size and textural properties of the hydrotalcite (HT) and the Al 2 O 3 –MgO mixed oxides formed upon calcination. HT samples prepared under sonication at 298 K are formed by dispersed and homogenous particles of 80-nm average particle size. They also produce upon calcinations the mixed oxides with the largest surface area (∼300 m 2 g −1 ). This method of preparation increases not only the surface area but also the number of defects in the solid, leading to sites of higher basicity. This was determined by means of catalytic reactions such as Knoevenagel and aldol condensations which demand basic sites of different strengths. Hydrotalcites were regenerated from mixed oxides by hydration while giving Bronsted basic sites. Samples originally prepared by sonication present smaller crystallite size and have a larger number of accessible active sites. With these samples acetone/citral condensations with 96 and 99% conversion and selectivity, respectively, are achieved in a 15-min reaction time.

Catalytic oxidation of chlorinated benzenes over V2O5/TiO2 catalysts

Abstract: Kinetic and in situ infrared spectroscopic studies were conducted to investigate the oxidation of different chlorinated benzenes (i.e., chlorobenzene, 1,2- 1,3-, and 1,4-dichlorobenzene) over V2O5/TiO2 catalysts. The oxidation of cyclohexyl chloride and that of benzene were also examined for comparison. Observed differences in the reaction rates and activation energies can be correlated to the structural differences of these compounds, in light of a common reaction mechanism. This mechanism is supported by the results of in situ FTIR studies, which indicate the presence of similar surface intermediates on the catalyst surface under reaction conditions for all the aromatic compounds examined. The results further suggest that the following two are the important mechanistic steps in the oxidation of chlorinated benzenes: (1) the adsorption of the aromatic compound on the catalyst via a nucleophilic attack on the chlorine position in the aromatic ring and (2) the subsequent oxidation of the remaining aromatic ring.

Quantitative analysis of superoxide ion and hydrogen peroxide produced from molecular oxygen on photoirradiated TiO2 particles

Abstract: The reduction of molecular oxygen is the counter reaction of most photocatalytic reactions proceeding oxidatively on titanium dioxide particles. We have quantitatively analyzed the reductive production of hydrogen peroxide and superoxide ion from oxygen in an aqueous solution containing 2-propanol as the scavenger of positive holes. The rates for the production of hydrogen peroxide and superoxide ion were determined by colorimetry using iodide and nitroblue tetrazolium, respectively. In addition, the oxidation of 2-propanol to acetone was monitored. Based on a comparison of these production rates, it was concluded that the main product from oxygen is hydrogen peroxide when TiO2 powder consisting mainly of anatase-form particles is used, whereas the main product is superoxide ion when TiO2 powder consisting mainly of rutile-form particles is used. The difference in the photocatalytic activity between these powders can be attributed to the difference between the reduction paths of oxygen on these powders. It was also found that the superoxide ion generated from molecular oxygen spontaneously reacts with 2-propanol to produce acetone and hydrogen peroxide.

Sustainable hydrogen from bio-oil - Steam reforming of acetic acid as a model oxygenate

Abstract: Steam reforming of acetic acid over Pt/ZrO2 catalysts has been investigated. Pt/ZrO2 catalysts are very active, completely converting acetic acid, and give a hydrogen yield close to thermodynamic equilibrium. The catalyst deactivated by formation of oligomers which block the active sites. The mechanism of both the reaction and the deactivation was studied with kinetic and spectroscopic measurements on Pt/ZrO2, ZrO2, and Pt black. It is shown that Pt is essential for steam reforming to proceed. ZrO2 is needed to activate steam. ZrO2, on the other hand, is also active for oligomer?precursor formation under the conditions investigated. Results obtained in the study show that steam reforming takes place at the PtZrO2 boundary and that deactivation occurs when this boundary is blocked by oligomers.

NOx adsorption study over Pt–Ba/alumina catalysts: FT-IR and pulse experiments

Abstract: Adsorption of NO, NO/O 2 , and NO 2 on Pt–Ba–Al–O system is investigated at 350 °C by the transient response method (TRM) and FT-IR spectroscopy. The data suggest that in the presence of oxygen, NO is effectively adsorbed (at a Ba site in proximity of a Pt site) through stepwise oxidation to form at first nitrites that are progressively transformed into nitrates. NO is also oxidized to NO 2 over Pt in the presence of oxygen. NO 2 is directly adsorbed to form Ba nitrates according to a disproportionation reaction, which occurs with the evolution of NO. The stepwise oxidation route seems to play a major role in NO x storage from NO/O 2 mixtures.

Sintering of nickel steam-reforming catalysts: effects of temperature and steam and hydrogen pressures

Abstract: Steam reforming over nickel catalysts is widely used for industrial-scale production of hydrogen and synthesis gas. This work is a study of the effects of atmosphere and temperature on the rate of sintering of nickel steam-reforming catalysts. The relative nickel areas of Ni/MgAl2O4 and Ni/Al2O3 catalysts after sintering in H 2O:H2 atmospheres at high (40 bar) and low (1 bar) pressures are reported. The data are discussed in terms of the recently proposed model for the sintering rate of supported nickel catalysts [J. Sehested, J. Catal. 217 (2003) 417] and density functional theory (DFT) calculations of the stability and diffusivity of transport species at the surface of nickel particles. OH-bonded nickel dimers are found to have a much lower energy of formation than nickel adatoms (�E = 58 kJ mol −1 ). It is therefore concluded that in steam/hydrogen mixtures, OH-bonded nickel dimers are dominating the surface transport on nickel particles and consequently sintering via particle migration and coalescence. Expressions connecting the diffusion constant for nickel particles to the diffusion constant and energy of formation of nickel adatoms and of OH-bonded nickel dimers are given. These equations are used in a sintering model [J. Sehested, J. Catal. 217 (2003) 417] and good agreement between the model and the experimental data is obtained at moderate temperatures. Above temperatures of ca. 600 ◦ C at 40 bar and approximately 700 ◦ C at 1 bar total pressure, the rate and the activation energy of sintering increase considerably. The reason for this observation may be that sintering via Ostwald ripening dominates the sintering rate under these conditions.  2004 Published by Elsevier Inc.

Structure sensitivity and kinetics of d-glucose oxidation to d-gluconic acid over carbon-supported gold catalysts

Abstract: The heterogeneously catalyzed oxidation of d -glucose to d -gluconic acid over Au/C catalysts has been studied. A series of Au/C catalysts were prepared by the gold sol method with different reducing agents and different kinds of carbon support providing Au mean particle diameters in the range 3–6 nm. The activities of these catalysts with respect to d -glucose oxidation were compared, and several aspects influencing activity, especially Au particle size, were discussed. The influence of reaction conditions ( T =30–90 °C, pH 7.0–9.5) on the kinetics of the d -glucose oxidation has been examined using the most active Au/C catalyst. By a detailed analysis of all reaction products under different reaction conditions, a reaction network of the d -glucose oxidation is presented, and a reaction mechanism for d -glucose oxidation that explains the influence of pH on reaction rate is proposed. Ensuring that d -glucose oxidation takes place in the kinetic regime (sufficient stirring rate and airflow rate), a semiempirical model based on a Langmuir–Hinshelwood-type reaction pathway is assumed. At 50 °C and pH 9.5 kinetic parameters were calculated by an optimization routine. The resulting concentration courses of d -glucose and d -gluconic acid were in good agreement with the experimental data. All experiments were carried out in a semibatch reactor under pH control at atmospheric pressure.

Crotonaldehyde hydrogenation by gold supported on TiO2: structure sensitivity and mechanism

Abstract: The catalytic properties of two series of Au/TiO 2 catalysts prepared by deposition-precipitation with NaOH (DP NaOH) (∼3 wt%) and by deposition-precipitation with urea (DP Urea) (∼8 wt%) were evaluated for the reaction of crotonaldehyde hydrogenation at atmospheric pressure. There is no difference in activity (mol g Au −1 s −1 ) and selectivity between the DP Urea and DP NaOH samples for a given activation treatment. This is due to the fact that the DP Urea and DP NaOH samples exhibit a similar gold particle size distribution, although the gold loading in DP Urea catalysts is much higher than in DP NaOH. The DP Urea samples were reduced under H 2 at different temperatures (120–500 °C) or treated in air at 300 °C with various flow rates, to vary the average particle size within a large range, 1.7 to 8.7 nm. The selectivity to crotyl alcohol (selective hydrogenation of the carbonyl bond), in the 5–50% conversion range, is high, 60–70 %, and is independent of the reduction temperature, and almost constant as a function of the particle size. In contrast, the TOF depends on the gold particle size, drastically increasing when the gold particle size is ∼2 nm. These characteristic features of Au/TiO 2 catalysts in this reaction are compared with those of Pt/TiO 2 . The possible adsorption modes of crotonaldehyde are discussed. Hydrogen dissociation is proposed to be the rate-determining step, and to take place on the low-coordinated atoms of the gold particles.

Production of hydrogen from oxidative steam reforming of methanol: I. Preparation and characterization of Cu/ZnO/Al2O3 catalysts from a hydrotalcite-like LDH precursor

Abstract: Catalysts for oxidative methanol reforming were prepared by thermal and H 2 -reduction treatments of a precursor consisting mainly of a hydrotalcite-like Cu–Zn–Al hydroxycarbonate and a Zn-rich paratacamite. The precursor was obtained by homogeneous precipitation of metal cations with a properly modified urea method. XRPD and quantitative Rietveld analysis of the precursor material revealed the presence of hydrotalcite and paratacamite phases in a weight ratio 3:1. Treatment in situ of the precursor produced Cu/ZnO/Al 2 O 3 (Cu = 18%, Zn = 33%, Al = 49% mol) catalysts. Chemical properties of the precursor and of the catalysts were studied by TPR, TPO, NH 3 -TPD, N 2 O chemisorption, FTIR, and UV–vis techniques. Physical characterization was carried out by the SEM, EDS, XRPD, TG/DTA, and N 2 adsorption techniques. The influence of heating rate of the precursor was investigated. Heat-treated samples contained CuO, ZnO, and amorphous Al 2 O 3 , and probably ZnAl 2 O 4 , as suggested by XRPD and FTIR measurements. The crystallinity of oxide phases was higher for the sample treated with a lower heating rate that also showed lower surface area and lower Cu dispersion. The presence of Cu 2+ in octahedral sites of alumina was suggested by UV–vis spectra. CO adsorption gave evidence of easy reduction of Cu(II) to Cu(I) and Cu(0) and the formation of stable complexes with Cu(I). NH 3 -TPD and FTIR measurements showed the presence of surface acid sites of the Lewis type with wide strength distributions, mainly due to Zn 2+ and Al 3+ cations. Cu(II) was able to oxidize NH 3 , while Cu(0) activated NH 3 decomposition. TPR and TPO measurements indicated that Cu species are easily reduced and reoxidized and Cu(I) species are intermediate for both processes. The redox properties appeared to be influenced by the rate of the previous heat treatment.