Hydrogen Formation in the Reactions of Methanol on Supported Au Catalysts

The adsorption and reactions of methanol have been investigated on Au metal supported by various oxides and carbon Norit of high surface area. Infrared spectroscopic studies revealed the dissociation of methanol at 300 K, which mainly occurs on the oxide-supports yielding methoxy species. The presence of Au already appeared in the increased amounts of desorbed products in the TPD spectra. The reaction pathway of the decomposition and the activity of the catalyst sensitively depend on the nature of the support. As regards the production of hydrogen the most effective catalyst is Au/CeO₂ followed by Au/MgO, Au/TiO₂ and Au/Norit. In contrast, on Au/Al₂O₃ the main process is the dehydration reaction yielding dimethyl ether. On Au/CeO₂ the decomposition of methanol starts above ~500 K and approaches total conversion at 723–773 K. The products are H₂ (~68%) and CO (~27%) with very small amounts of methane and CO₂. The decomposition of methanol follows the first order kinetics. The activation energy of this process is 87.0 kJ/mol. The selectivity of H₂ formation at 573–773 K was ~90%, this value increased to 97% using CH₃OH:H₂O (1:1) reacting mixture indicating the involvement of water in the reaction. No deactivation of Au catalysts was experienced at 773 K in ~10 h. It is assumed that the interface between Au and partially reduced ceria is responsible for the high activity of Au/CeO₂ catalyst.     

Mechanistic evidences of the synthesis of methyl formate from methane and air on oxide catalysts

The direct conversion of methane into methyl formate with high molar yields (>12%) by a two reactor system has been attained. The reaction sequence consists of the partial oxidation of methane to HCHO at 600–700°C (MPO) on a “precipitated” SiO₂ catalyst and the subsequent dimerization of HCHO to methyl formate (FD) at 130–170°C on various oxide systems (TiO₂, SiO₂, ZrO₂, SO₄²⁻/ZrO₂). FTIR characterization of the oxide catalysts allowed to highlight the superior performance of amphoteric oxides like TiO₂ and ZrO₂ in the FD reaction arising from the presence of surface acid and base sites playing a synergetic role in the reaction pathway.     

Enhancement of catalytic activity of Au/TiO2 by thermal and plasma treatment

A significant enhancement in the catalytic activity of Au/TiO₂ in CO oxidation and preferential oxidation reaction by creating the active sites on the catalyst surface by thermal treatment as well as by producing small gold particles by plasma treatment has been studied. Au/TiO₂ catalyst (Au (1 wt%) supported on TiO₂) was prepared by conventional deposition-precipitation method with NaOH (DP NaOH) followed by washing, drying and calcination in air at 400 °C for 4 h. Thermal treatment of Au/TiO₂ was carried out at 550 °C under 0.05 mTorr. A small amount of Au/TiO₂ catalyst was taken from the untreated and thermally treated Au/TiO₂ and both kinds of catalysts were treated with plasma sputtering at room temperature. The activity of the catalysts has been examined in the reaction of CO oxidation and preferential oxidation (PROX) at 25–250 °C. Thermally treated Au/TiO₂ showed better catalytic activity as compared to the untreated catalyst. There is also an additional enhancement in the catalytic activity due to plasma sputtering on the both kinds of catalysts. Thermally treated Au/TiO₂ followed by plasma sputtering Au/TiO₂ showed higher conversion rates for CO oxidation reaction compared with untreated, thermally treated and plasma sputtered Au/TiO₂ catalysts. It may be concluded that the enhancement of catalytic activity of thermally treated Au/TiO₂ followed by plasma sputtering is owing to the generation of active sites such as oxygen vacancies/defects in TiO₂ support using thermal treatment as well as by producing small gold particles using plasma treatment.     

Synthesis and characterization of methanol tolerant Pt/TiOx/C nanocomposites for oxygen reduction in direct methanol fuel Cells

Pt/TiO_x/C nanocomposites have been synthesized by depositing hydrated titanium oxide on carbon-supported Pt (Pt/C), reducing H₂PtCl₆ with sodium formate on carbon-supported hydrated titanium oxide (TiO₂/C), and simultaneously depositing hydrated titanium oxide and reducing H₂PtCl₆ with formate on carbon support, followed by heat treatment at 500 and 900 °C in 90% Ar-10% H₂ mixture. The catalytic activity for oxygen reduction was evaluated in half cells with sulfuric acid electrolyte and in single direct methanol fuel cells (DMFC). Tolerance to methanol was studied with half cells containing sulfuric acid mixed with methanol. Charge transfer resistance and electrochemical active surface area of the Pt/TiO_x/C catalysts were studied with impedance and cyclic voltammetry measurements. Both the synthesis methods and heat treatments influence the catalytic activity, and some of the Pt/TiO_x/C composites exhibit higher catalytic activity than Pt/C. The Pt/TiO_x/C catalysts also exhibit better methanol tolerance than Pt/C. The mechanism for the enhanced catalytic activity of Pt/TiO_x/C is discussed.     

Simultaneous MS-IR Studies of Surface Formate Reactivity Under Methanol Synthesis Conditions on Cu/SiO<Subscript>2</Subscript>

The coverages and surface lifetimes of copper-bound formates on Cu/SiO₂ catalysts, and the steady-state rates of reverse water-gas shift and methanol synthesis have been measured simultaneously by mass (MS) and infrared (IR) spectroscopies under a variety of elevated pressure conditions at temperatures between 140 and 160 °C. DCOO lifetimes under steady state catalytic conditions in CO₂:D₂ atmospheres were measured by ¹²C–¹³C isotope transients (SSITKA). The values range from 220 s at 160 °C to 660 s at 140 °C. The catalytic rates of both reverse water gas shift (RWGS) and methanol synthesis are ~100-fold slower than this formate removal rate back to CO₂ + 1/2 H₂, and thus they do not significantly influence the formate lifetime or coverage at steady state. The formate coverage is instead determined by formate’s rapid production/decomposition equilibrium with gas phase CO₂ + H₂. The results are consistent with formate being an intermediate in methanol synthesis, but with the rate-controlling step being after formate production (for example, its further hydrogenation to methoxy). A 2–3 fold shorter life time (faster decomposition rate) was observed for formate under reactions conditions, with both D₂ and CO₂ present, than in pure Ar or D₂ + Ar alone. This effect, due in part to the effects of the coadsorbates produced under reaction conditions, illustrates the importance of using in situ techniques in the study of catalytic mechanisms. The carbon which appears in the methanol product spends a longer time on the surface than the formate species, 1.8 times as long at 140 °C. The additional delay on the surface is attributed in part to readsorption of methanol on the catalyst, thus obscuring the mechanistic link between formate and methanol.     

Methanol synthesis and reverse water-gas shift kinetics over clean polycrystalline copper

The kinetics of simultaneous methanol synthesis and reverse water-gas shift from CO₂/H₂ mixtures have been measured at low conversions over a clean polycrystalline Cu foil at pressures of 5 bar. An absolute rate of 1.2 × 10^–3 methanol molecules produced per second per Cu surface atom was observed at 510 K, with an activation energy of 77 ± 10 kJ/mol. The rate of CO production was 0.12 molecules per second per Cu surface atom at this temperature, with an activation energy of 135 ± 5 kJ/mol. The rates, normalized to the metallic Cu surface area, are equal to those measured over real, high-area Cu/ZnO catalysts. The surface after reaction was examined by XPS and TPD. It was covered by almost a full monolayer of adsorbed formate, but no other species like carbon or oxygen in measurable amounts. These results prove that a highly active site for methanol synthesis on real Cu/ZnO catalysts is metallic Cu, and suggest that the rate-determining step in methanol synthesis is one of the several steps in the further hydrogenation of adsorbed formate to methanol.     

The Role of Nanosize Particles of Uranium Oxide in the Adsorption/Reaction of Methanol Over U3O8/MCM-48: FTIR Study

The surface species formed over MCM-48, U₃O₈ and U₃O₈/ MCM48 catalysts during the adsorption/reaction of methanol were monitored using FTIR spectroscopy, in order to get an insight into the high catalytic activity exhibited by the nanosize crystallites of uranium oxide dispersed in MCM-48. The results of this in situ study revealed that the title catalysts exhibited a distinct behavior for adsorption and subsequent reaction of methanol. Thus, while the room temperature adsorption over bulk U₃O₈ resulted in the formation of formate complex and oxymethylene species, the interaction over MCM-48 resulted in simultaneous and instant formation of surface methoxy groups and dimethyl ether. On the other hand, the exposure of methanol over U₃O₈/MCM-48 under similar conditions resulted in the appearance of intense IR bands due to surface-adsorbed (–OCH₂)_n species, where n1, in addition to those of formate complexes, oxymethylene and methoxy groups. The role of the above-mentioned intermediate species in the formation of different reaction products is discussed in brief.     

Surface Properties and Catalytic Activity of TiO2–ZrO2 Mixed Oxides in Dehydration of Methanol to Dimethyl Ether

A series of TiO₂–ZrO₂ mixed oxides with varying molar ratio of TiO₂ to ZrO₂ were prepared by the co-precipitation method. The crystalline phases of the oxides were characterized by XRD and their acid–base properties by TPD of NH₃ and CO₂ and IR of adsorbed pyridine. The catalytic activities were investigated for the vapor phase dehydration of methanol to dimethyl ether (DME) in a fixed-bed reactor under atmospheric pressure. The mixed oxides are highly amorphous in nature. The acid–base properties and CH₃OH conversion activity are increasing with TiO₂ content and an optimum value is achieved for a molar ratio of Ti/Zr in the vicinity of 1/1. At lower reaction temperature (<300 °C), the selectivity for DME is nearly 100%. A good correlation is observed between dehydration activity and the acid–base properties of the TiO₂–ZrO₂ catalysts. It is significant to note that TiO₂–ZrO₂ catalysts show high stability against water during dehydration reaction. Based on our results, a surface mechanism involving both acid–base sites has been proposed for DME formation.     

TiO2 Supported Nano—Au Catalysts Prepared via Solvated Metal Atom Impregnation for Low-Temperature CO Oxidation

TiO₂ supported nano-Au catalysts were prepared by solvated metal atom impregnation (SMAI) method. The catalysts were characterized by means of AAS, TPD, H₂ reduction desorption (H₂-RD), XRD, TEM, XPS and tested for low-temperature CO oxidation. XRD and TEM results showed that the pretreatment temperature had an influence on the particle size of Au/TiO₂catalysts. The average particle size increased with the increase in pretreatment temperature. XPS indicated that gold in the catalysts was presented in the form of metallic state clusters. Catalytic studies showed these catalysts were very active and stable in low-temperature CO oxidation. The CO oxidation activity of the catalysts increased as the Au particle size decreased. The measurement results of AAS, TPD and H₂-RD revealed that there were some organic fragments on the surface of Au particles which might be responsible for the high stability of the Au/TiO₂ catalysts.     

Promotional SMSI Effect on Supported Palladium Catalysts for Methanol Synthesis

High-temperature reduction (HTR) of palladium catalysts supported on some reducible oxides, such as Pd/CeO₂, and Pd/TiO₂ catalysts, led to a strong metal-support interaction (SMSI), which was found to be the main reason for their high and stable activity for methanol synthesis from hydrogenation of carbon dioxide. But low-temperature-reduced (LTR) catalysts exhibited high methane selectivity and were oxidized to PdO quickly in the same reaction. Besides palladium, platinum exhibited similar behavior for this reaction when supported on these reducible oxides. Mechanistic studies of the Pd/CeO₂ catalyst clarified the promotional role of the SMSI effect, and the spillover effect on the HTR Pd/CeO₂ catalyst. Carbon dioxide was decomposed on Ce₂O₃, which was attached to Pd, to form CO and surface oxygen species. The carbon monoxide formed was hydrogenated to methanol successively on the palladium surface while the surface oxygen species was hydrogenated to water by spillover hydrogen from the gas phase. A reaction model for the hydrogenation of carbon dioxide was suggested for both HTR and LTR Pd/CeO₂ catalysts. Methanol synthesis from syngas on the LTR or HTR Pd/CeO₂ catalysts was also conducted. Both alcohol and hydrocarbons were formed significantly on the HTR catalyst from syngas while methanol formed predominantly on the LTR catalyst. Characterization of these two catalysts elucidated the reaction performances.     

Methanol Adsorption on V2O3(0001)

Well ordered V₂O₃(0001) layers may be grown on Au(111) surfaces. These films are terminated by a layer of vanadyl groups which may be removed by irradiation with electrons, leading to a surface terminated by vanadium atoms. We present a study of methanol adsorption on vanadyl terminated and vanadium terminated surfaces as well as on weakly reduced surfaces with a limited density of vanadyl oxygen vacancies produced by electron irradiation. Different experimental methods and density functional theory are employed. For vanadyl terminated V₂O₃(0001) only molecular methanol adsorption was found to occur whereas methanol reacts to form formaldehyde, methane, and water on vanadium terminated and on weakly reduced V₂O₃(0001). In both cases a methoxy intermediate was detected on the surface. For weakly reduced surfaces it could be shown that the density of methoxy groups formed after methanol adsorption at low temperature is twice as high as the density of electron induced vanadyl oxygen vacancies on the surface which we attribute to the formation of additional vacancies via the reaction of hydroxy groups to form water which desorbs below room temperature. Density functional theory confirms this picture and identifies a methanol mediated hydrogen transfer path as being responsible for the formation of surface hydroxy groups and water. At higher temperature the methoxy groups react to form methane, formaldehyde, and some more water. The methane formation reaction consumes hydrogen atoms split off from methoxy groups in the course of the formaldehyde production process as well as hydrogen atoms still being on the surface after being produced at low temperature in the course of the methanol ?? methoxy + H reaction.     

Effect of H2S on the hydrogenation of carbon dioxide over supported Rh catalysts

The hydrogenation of CO₂ was studied on supported noble metal catalysts in the presence of H₂S. In the reaction gas mixture containing 22 ppm H₂S the reaction rate increased on TiO₂ and on CeO₂ supported metals (Ru, Rh, Pd), but on all other supported catalysts or when the H₂S content was higher (116 ppm) the reaction was poisoned. FTIR measurements revealed that in the surface interaction of H₂ + CO₂ on Rh/TiO₂ Rh carbonyl hydride, surface formate, carbonates and surface formyl were formed. On the H₂S pretreated catalyst surface formyl species were missing. TPD measurements showed that adsorbed H₂S desorbed as SO₂, both from TiO₂-supported metals and from the support. IR, XP spectroscopy and TPD measurements demonstrated that the metal became apparently more positive when the catalysts were treated with H₂S and when the sulfur was built into the support. The promotion effect of H₂S was explained by the formation of new centers at the metal/support interface.     

An in situ high pressure FT-IR study of CO2/H2 interactions with model ZnO/SiO2, Cu/SiO2 and Cu/ZnO/SiO2 methanol synthesis catalysts

In situ FT-IR spectroscopy allows the methanol synthesis reaction to be investigated under actual industrial conditions of 503 K and 10 MPa. On Cu/SiO₂ catalyst formate species were initially formed which were subsequently hydrogenated to methanol. During the reaction a steady state concentration of formate species persisted on the copper. Additionally, a small quantity of gaseous methane was produced. In contrast, the reaction of CO₂ and H₂ on ZnO/SiO₂ catalyst only resulted in the formation of zinc formate species: no methanol was detected. The interaction of CO₂ and H₂ with Cu/ZnO/SiO₂ catalyst gave formate species on both copper and zinc oxide. Methanol was again formed by the hydrogenation of copper formate species. Steady-state concentrations of copper formate existed under actual industrial reaction conditions, and copper formate is the pivotal intermediate for methanol synthesis. Collation of these results with previous data on copper-based methanol synthesis catalysts allowed the formulation of a reaction mechanism.     

The Selective Oxidation of Methanol to Formaldehyde on Iron Molybdate Catalysts and on Component Oxides

The reaction of methanol with an industrial iron molybdate catalyst, and with Fe₂O₃ and with MoO₃, has been investigated with a pulsed flow reactor and temperature-programmed desorption (TPD). The molybdena-based samples show only formaldehyde in TPD as the carbon-containing product, arising from the decomposition of a surface methoxy species. In contrast, haematite yields no formaldehyde, only CO₂ and H₂, which evolve coincidently at 290 °C, and indicates the presence of a formate intermediate on the surface. In turn, the reactor work shows high selectivity to formaldehyde for the molybdate materials and zero for haematite. The iron molybdate sample is more active than the molybdena, conversion beginning at 150 °C for the former and 270 °C for the latter. These data are discussed in terms of a global mechanism for the reaction and a tentative reaction enthalpy profile is proposed. The main differences between the iron and molybdenum samples arise from the stronger binding of oxygen in the former and the higher concentration of cation sites.     

Nanogold-Containing Catalysts for Low-Temperature Removal of S-VOC from Air

Activity and selectivity of mono- and bimetallic catalysts containing nano-particles of gold stabilized by different supports are compared in dimethyldisulfide removal from air at 150–320 °C. TiO₂-supported Au and Au–Pd samples demonstrate stable and efficient DMDS removal at temperature as low as 155 °C, with formation of the two products: SO₂ and elemental S. On the contrary, no formation of elemental S is detected in the case of Au, Au–Rh, and Au–Pd catalysts supported on HZSM-5, H-beta, or MCM-41. The most active Au–Rh/HZSM-5 catalyst demonstrates an efficient DMDS removal at 290 °C, with quantitative DMDS-to-SO₂ oxidation. Characterization of catalysts with TPR, XRD, and (XANES + EXAFS) confirms a high dispersion of the metallic phases in all catalysts under study. Specific interaction between nano-particles of gold and titanium dioxide surface could be responsible for the unusual catalytic behavior of Au/TiO₂ samples, as distinct from Au/zeolitic systems.     

Mesoporous silica supports for improved thermal stability in supported Au catalysts

In this work, we present a comprehensive review of our research on the role of mesoporous silica pore architecture, composition of the pore walls (addition of Co or Al), and silica surface chemistry (surface modification by TiO₂) to improve the hydrothermal stability of Au particles. We have found that mesoporous silica architecture plays an important role in improving Au stability, with three dimensional mesoporous architectures being less effective than one dimensional (1-D) pores. The tortuous 1-D pores in aerosol silica were found to be most effective at controlling Au particle size. Since Au particles continue to grow larger than the pore diameter, we conclude that Ostwald ripening must be the dominant sintering pathway for these Au catalysts. These catalysts are active for CO oxidation even after the Au particles have grown large enough to block the pores, suggesting that the thin walls of mesoporous silica provide easy access to gas phase molecules. Further improvements in Au stability and reactivity were obtained by surface modification of the aerosol and MCM-41 silica with TiO₂. After TiO₂ modification of the silica, the Au particles remained smaller than the pore size (< 3 nm) even after three cycles of CO oxidation at temperatures up to 400 °C.     

Conversion of Synthesis Gas to Dimethylether Over Gold-based Catalysts

It is shown that Au?Czinc oxide?Calumina catalysts are suitable for the water?Cgas shift reaction and for methanol (MeOH) and DME synthesis, indicating their use in a direct single-stage process for converting syngas to a DME?+?methanol mixture. Temperatures above 340?°C were required in order to obtain reasonable catalytic activity. A 67?% DME selectivity was achieved at 380?°C with a low space velocity 0.75?dm³?h^?1?g^?1 and 50?bar. The lower CO conversions at the higher temperature of 460?°C was probably due to the MeOH equilibrium limitation in the range of temperatures 340 to 460?°C, but deactivation is observed as well, above 460?°C. Au/ZnO/??-Al₂O₃ is more stable than traditional copper-based catalysts, which are stable below about 300?°C, and then only in the absence of water. The gold composite catalyst was mainly selective toward DME, MeOH and CH₄, and to C₂ to C₅ hydrocarbons. An analysis of the main reactions involved indicates that only the methanol synthesis reaction reaches a near-equilibrium situation, with the other reactions being under kinetic control.     

Liquid phase selective hydrogenation of phthalic anhydride to phthalide over titania supported gold catalysts

Au/TiO₂ catalysts with different gold loadings were prepared by deposition–precipitation method and used for the liquid phase hydrogenation of phthalic anhydride. All the studied Au/TiO₂ catalysts exhibited excellent activity with high selectivity (>92%) to phthalide under mild reaction conditions (180 °C and 3.0 MPa H₂). Specially, catalysts with 2–3 wt.% gold loading were highly active and selective for the formation of phthalide. When reused, the catalyst showed a certain deactivation, but still was highly selective to phthalide. The deactivation was attributed to the leaching of gold, collapse of the pore structure and accumulation of organic species on the surface.     

Remarks on the assignments of temperature programmed desorption peaks for the surface species formed on Cu/ZnO and ZnO in the methanol synthesis from CO

Temperature programmed desorption (TPD), IR spectroscopy and chemical trapping of the surface species with H₂O revealed that the TPD peak of CO frequently assigned to zinc formate species, which were formed in the course of the methanol synthesis from CO-H₂, arose from zinc methoxide species.