Direct Synthesis of Hydrogen Peroxide from Hydrogen and Oxygen over Palladium Catalyst Supported on SO3H-Functionalized SBA-15

Palladium catalyst supported on SO₃H-functionalized SBA-15 (denoted as Pd/SO₃H-SBA-15) was applied to the direct synthesis of hydrogen peroxide from hydrogen and oxygen. For comparison, palladium catalyst supported on SBA-15 (denoted as Pd/SBA-15) was also employed for the direct synthesis of hydrogen peroxide. Selectivity for hydrogen peroxide, yield for hydrogen peroxide, and final concentration of hydrogen peroxide over Pd/SO₃H-SBA-15 catalyst were much higher than those over Pd/SBA-15 catalyst. The high catalytic performance of Pd/SO₃H-SBA-15 catalyst was attributed to the enhanced acid amount of SO₃H-SBA-15 support, which served as an alternate acid source in the direct synthesis of hydrogen peroxide from hydrogen and oxygen.     

Direct synthesis of hydrogen peroxide from hydrogen and oxygen over palladium catalyst supported on SO<Subscript>3</Subscript>H-functionalized MCF silica: Effect of calcination temperature of mesostructured cellular foam silica

Palladium catalysts supported on SO₃H-functionalized MCF silica (Pd/SO₃H-MCF-T (T=450, 550, 650, 750, 850, and 950)) were prepared with a variation of calcination temperature (T, °C) of MCF silica. They were then applied to the direct synthesis of hydrogen peroxide from hydrogen and oxygen. Conversion of hydrogen, selectivity for hydrogen peroxide, and yield for hydrogen peroxide showed volcano-shaped curves with respect to calcination temperature of MCF silica. Yield for hydrogen peroxide increased with increasing acid density of Pd/SO₃H-MCF-T catalysts. Thus, acid density of Pd/SO₃H-MCF-T catalysts played an important role in determining the catalytic performance in the direct synthesis of hydrogen peroxide. Pd/SO₃H-MCF-T catalysts efficiently served as an acid source and as an active metal catalyst in the direct synthesis of hydrogen peroxide.     

Direct synthesis of hydrogen peroxide from H2/O2 and oxidation of thiophene over supported gold catalysts

Direct synthesis of hydrogen peroxide from H₂ and O₂ was performed over supported gold catalysts. The catalysts were characterized by means of UV–vis, H₂-TPR, TEM and XPS. Based on the results we conclude that metallic Au is the active species in the direct synthesis of hydrogen peroxide from H₂ and O₂. During preparation process of catalyst by deposition–precipitation with urea, the pH value increased and the gold particle size decreased with increasing the urea concentration. The catalyst prepared with higher urea concentration showed a higher activity and its stability also was efficiently improved. Gold nanoparticles, supported on TiO₂ or Ti contained supports, gave a higher catalytic activity. Thiophene can be efficiently oxidized by hydrogen peroxide synthesized in situ from H₂ and O₂ over Au/TS-1.     

Hydrogen Production by Methanol Steam Reforming Over Pd/ZrO2�CTiO2 Catalysts

Pd/ZrO₂?CTiO₂ catalysts were synthesized by sol?Cgel method and studied on the steam reforming of methanol reaction for hydrogen production. X-ray diffraction patterns of the Pd supported on single oxides showed crystalline structures associated with the zirconia or titania respectively. However, the XRD pattern of the mixed ZrO₂?CTiO₂ oxide showed broad diffraction pattern consistent with an amorphous material. The reducibility of the PdO supported on single and mixed oxides showed a negative peak associated with the desorption of H₂ due to the decomposition of Pd-hydride (PdH); as well as, positive peaks related with the hydrogen consumption on the reduction of the PdO supported. Catalytic activity on the palladium supported on the mixed ZrO₂?CTiO₂ oxide showed higher catalytic activity than the Pd supported on the single TiO₂ or ZrO₂ oxides. This finding was associated at the higher Pd species present in the Pd/ZrO₂?CTiO₂ than on the Pd/ZrO₂ or Pd/TiO₂ catalysts how was observed by TPR.     

Characterization study of CeO2 supported Pd catalyst for low-temperature carbon monoxide oxidation

Catalysts consisting of palladium supported on cerium dioxide (Pd/CeO₂) were prepared and used for carbon monoxide oxidation in a stoichiometric mixture of carbon monoxide and oxygen. Pd/CeO₂ exhibits high catalytic activity for the oxidation of CO, showing markedly enhanced catalytic activities due to the combined effect of palladium and cerium dioxide. The Pd/CeO₂ catalyst is superior to Pd/ZrO₂, Pd/Al₂O₃, Pd/TiO₂, Pd/ZSM-5 and Pd/SiO₂ catalysts with regard to the activity under the conditions examined. The catalysts were characterized by means of XRD and TPR. The position of the H₂-TPR peak shifts to lower temperature with increasing Pd loading from 0.25 to 2.0%. CeO₂ inhibits the hydrogen reduction of PdO. CO-TPR measurements have shown the existence of three peaks. The low-temperature peak (α) is due to the Pd hydroxide species. The β peak has been attributed to finely dispersed PdO. The high-temperature peak (γ) has been attributed to crystal phase PdO. Crystal phase PdO is more difficult to reduce by CO than finely dispersed PdO. On the basis of the catalytic activity and CO-TPR results, we conclude α species (Pd hydroxide) mainly contribute to the catalytic activity for low-temperature CO oxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

由氢、氧直接合成过氧化氢研发新进展

介绍了氢、氧直接合成过氧化氢过程中催化剂、反应介质、反应器(微反应器、膜反应器等)、操作方式等方面近2～3年来的研发新进展。重点介绍了Au-Pd负载催化剂研究应用情况,涉及不同载体、不同Au与Pd质量比等对过氧化氢生成速率、选择性及浓度的影响。还较详细地介绍了无机酸和卤离子在不同条件下对合成反应结果(包括氢转化率、生成过氧化氢选择率及浓度等)的提高所起到的促进作用及其机理。     

吡啶改性Pd/SiO2催化剂用于H2和O2直接合成H2O2

引言过氧化氢(H2O2)是一种理想的绿色氧化剂,广泛应用于化学品合成、纺织、造纸、环保、食品、医药、冶金和农业等领域[1]。目前,蒽醌法[2-5]是工业上生产H2O2的主要方法。20世纪40年代,德国I.G.Farbenindustrie首先采用蒽醌法(又称Riedl-Pfleiderer法)工业化生产过氧化氢。该方法首先将2-烷基蒽醌(通常是2-乙基蒽醌)溶解于合适的有机溶剂中,溶液中的2-烷基蒽醌经催化剂催化加氢,被还原成蒽氢醌或5,6,7,8-四氢蒽氢醌,再经空气氧化得到蒽醌或四氢蒽醌和     

Direct synthesis of hydrogen peroxide from H2 and O2 and in situ oxidation using zeolite-supported catalysts

Four microporous materials, zeolites HZSM-5, Y, Beta and TS-1, were used as the supports to prepare supported gold catalysts using impregnation or deposition precipitation. The gold catalysts were tested in the direct synthesis of hydrogen peroxide from H₂ and O₂ and for CO oxidation. The effect on the catalytic activity of different metal (e.g., Pd, Pt, Cu, Ag, Rh or Ru) on the synthesis of hydrogen peroxide was also tested. Organic substrates, such as cyclohexane or cyclooctene, were introduced to investigate the possibility of in situ H₂O₂ oxidation with these catalysts.     

Mechanism of sulfur poisoning on supported noble metal catalyst — The adsorption and transformation of sulfur on palladium catalysts with different supports

A series of supported palladium catalysts (Pd/Al₂O₃, Pd/MgO and Pd/TiO₂) were prepared by the impregnating method and treated with H₂S, H₂ +O₂ or O₂, among which H₂S is used as a poison and H₂ +O₂ or O₂ are as purging atmospheres. The S^2– species in the supports was introduced by means of mechanically mixing Na₂S with the supports or catalysts. X-ray photoelectron spectroscopy (XPS) was employed to determine the changes in the chemical states of oxygen, palladium and sulfur in the catalysts before and after the treatment, while infrared (IR) spectroscopy was used to measure the SO^2– ₄ group produced in the catalysts and supports. The results show that on MgO and TiO₂ carriers whose acidities are weak, there exist two kinds of oxygen species, one is the lattice oxygen, the other one is the active species of oxygen. The latter can oxidize the S^2– into SO^2– ₄ even at room temperature in air. Because of the weak acidities and smaller specific surface area of MgO and TiO₂, the S^2– is liable to adsorb on the catalysts and to transform into SO^2– ₄. But for the case of Al₂O₃ support its acidity is rather strong, and its surface oxygen species under the experimental conditions is not so active as that in MgO and TiO₂ carries. The poison H₂S on the Al₂O₃ support only experiences a process of physical adsorption-desorption. In Pd/Al₂O₃ catalyst, the negatively charged sulfur ions are not so easily adsorbed and transformed as those in Pd/MgO and Pd/TiO₂. It is also implied that the properties of the carriers are related to the ability of self-regeneration of the corresponding catalysts. Pd/Al₂O₃ catalyst is more able to self-regenerate than Pd/MgO and Pd/TiO₂ catalyst.     

Effective modification of Pd surfaces with TiO2 promoters using selective chemical vapor deposition and the effect on catalytic performance improvement

In this paper, we describe a novel method for selectively attaching TiO₂ promoters on Pd surfaces in Pd/SiO₂ catalysts using selective chemical vapor deposition (CVD). Ti was selectively deposited on Pd active sites over a SiO₂ support under a hydrogen atmosphere when titanium tetraisopropoxide was used as a Ti precursor. The Pd–Ti/SiO₂ catalyst modified by CVD exhibits approximately 1.8 times higher ethylene selectivity than the un-modified Pd/SiO₂ catalyst in the selective hydrogenation of acetylene due to the effective geometric modification of the Pd surface, which is beneficial to ethylene selectivity, by the selectively deposited Ti species.     

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.     

Activation of Supported Pd Metal Catalysts for Selective Oxidation of Hydrogen to Hydrogen Peroxide

Catalytic activity of supported Pd metal catalysts (Pd metal deposited on carbon, alumina, gallia, ceria or thoria) showing almost no activity in the liquid-phase direct oxidation of H₂ to H₂O₂ (at 295 K) in acidic medium (0.02 M H₂SO₄) can be increased drastically by oxidizing them using different oxidizing agents, such as perchloric acid, H₂O₂, N₂O and air. In the case of the Pd/carbon (or alumina) catalyst, perchloric acid was found to be the most effective oxidizing agent. The order of the H₂-to-H₂O₂ conversion activity for the perchloric-acid-oxidized Pd/carbon (or alumina) and air-oxidized other metal oxide supported Pd catalysts is as follows: Pd/alumina < Pd/carbon < Pd/CeO₂ < Pd/ThO₂ < Pd/Ga₂O₃. The H₂ oxidation involves lattice oxygen from the oxidized catalysts. The catalyst activation results mostly from the oxidation of Pd metal from the catalyst producing bulk or sub-surface PdO. It also caused a drastic reduction in the H₂O₂ decomposition activity of the catalysts. There exists a close relationship between the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity in the oxidation process and the H₂O₂ decomposition activity of the catalysts; the higher the H₂O₂ decomposition activity, the lower the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity.     

Highly dispersed and active Pd nanoparticles over titania support through engineering oxygen vacancies and their anchoring effect

Rational synthesis of highly dispersed and active metal nanoparticles (NP) for application in catalysis has been a hot but challenging research topic. Herein, we report a facile strategy for dispersing and stabilizing noble metal NP in a reducible metal oxide support (e.g., TiO₂) through engineering oxygen vacancies and their anchoring effect. The pre-reduction of TiO₂ support produces oxygen vacancies, and Pd NP prefer to occupy these vacancies with a strong meal-support interaction and near metallic state. A near monatomic Pd NP distribution over anatase TiO₂ support is obtained with this strategy. Significant activity enhancement is demonstrated in both oxygen activation and formaldehyde oxidation reactions by this catalyst design method. H₂, NaBH₄ and HCHO are suitable reducing agent for TiO₂, the pre-reduction catalysts by H₂, NaBH₄ and HCHO shows highly activity for formaldehyde oxidation. Such a work demonstrates support pre-reduction strategy is a facile way to prepare powerful supported noble metal catalysts.     

Comparison of supports for the direct synthesis of hydrogen peroxide from H2 and O2 using Au–Pd catalysts

The direct synthesis of hydrogen peroxide from H₂ and O₂ using a range of supported Au–Pd alloy catalysts is compared for different supports using conditions previously identified as being optimal for hydrogen peroxide synthesis, i.e. low temperature (2 °C) using a water–methanol solvent mixture and short reaction time. Five supports are compared and contrasted, namely Al₂O₃, -Fe₂O₃, TiO₂, SiO₂ and carbon. For all catalysts the addition of Pd to the Au only catalyst increases the rate of hydrogen peroxide synthesis as well as the concentration of hydrogen peroxide formed. Of the materials evaluated, the carbon-supported Au–Pd alloy catalysts give the highest reactivity. The results show that the support can have an important influence on the synthesis of hydrogen peroxide from the direct reaction. The effect of the methanol–water solvent is studied in detail for the 2.5 wt% Au–2.5 wt% Pd/TiO₂ catalyst and the ratio of methanol to water is found to have a major effect on the rate of hydrogen peroxide synthesis. The optimum mixture for this solvent system is 80 vol.% methanol with 20 vol.% water. However, the use of water alone is still effective albeit at a decreased rate. The effect of catalyst mass was therefore also investigated for the water and water–methanol solvents and the observed effect on the hydrogen peroxide productivity using water as a solvent is not considered to be due to mass transfer limitations. These results are of importance with respect to the industrial application of these Au–Pd catalysts.     

Production of Synthesis Gas via Dry Reforming of Methane over Co‐Based Catalysts: Effect on H2/CO Ratio and Carbon Deposition

The effect of support type on synthesis gas production using Co‐based catalysts supported over TiO₂‐P25, Al₂O₃, SiO₂, and CeO₂ was investigated. The catalysts were prepared by the incipient wet impregnation method and characterized by various techniques for comparison. Experiments were performed in a micro tubular reactor. The results revealed that all Co‐supported catalysts produced synthesis gas ratios of 1 and below and, thus, proved to be well‐suited for methanol and Fischer‐Tropsch syntheses. Co catalysts supported over TiO₂‐P25 and Al₂O₃ provided better synthesis gas ratios and stability performances. The promotion of a Co/TiO₂‐P25 catalyst with Ce had a substantial influence on its catalytic activity and the amount of carbon deposit. A Ce‐promoted catalyst diminished markedly the extent of carbon deposition and thus boosted the performance towards better activity and stability.     

Hydroxylation of benzene with oxygen and hydrogen over catalysts containing Group VIII metals and heteropoly compounds

The vapor phase hydroxylation of benzene to phenol with a mixture of oxygen and hydrogen over silica supported bi-component catalysts containing Group VIII metals (M) and heteropoly compounds (HPC) was investigated. The productivity of the catalysts was ascertained for various metal and HPC combinations and a range of reaction conditions. The Pt–PMo₁₂/SiO₂ and Pd–PMo₁₂/SiO₂ catalysts of optimal composition provide up to 380 mol phenol/g-atom Pt or Pd/h. The observed catalysis appears to be associated with an interface between metal particles and those of the heteropoly compound, as illustrated by an HREM image of a Pt–PMo₁₂/SiO₂ sample.     

Chlorobenzene total oxidation over palladium supported on ZrO2, TiO2 nanostructured supports

Two series of supported Pd catalysts were synthesized on new mesoporous–macroporous supports (ZrO₂, TiO₂) labelled M (Zr and Ti). The deposition of palladium was carried out by wet impregnation on the calcined TiO₂ and ZrO₂ supports at 400 °C (Pd/Zr₄, Pd/Ti₄) and 600 °C (Pd/Zr₆, Pd/Ti₆) and followed by a calcination at 400 °C for 4 h. The pre-reduced Pd/MX catalysts were investigated for the chlorobenzene total oxidation and their catalytic properties where compared to those of a reference catalyst Pd/Ti-Ref (TiO₂ from Huntsman Tioxide recalcined at 500 °C) and of a palladium supported on the fresh mesoporous–macroporous TiO₂ (Pd/Ti). Based on the activity determined by T₅₀, the Pd/Ti and Pd/Ti₄ catalysts have been found to be more active than the reference one. Moreover activity decreased owing to the sequence: Pd/TiX  Pd/ZrX and in each series when the temperature of calcination of the support was raised. The overall results clearly showed that the activity was dependant on the nature of the support. The better activity of Pd/TiX compared to Pd/ZrX was likely due to a better reducibility of the TiO₂ support (Ti⁴⁺ into Ti³⁺) leading to an enhancement of the oxygen mobility. Production of polychlorinated benzenes PhCl_x (x = 2–6) and of Cl₂ was also observed. Nevertheless at 500 °C the selectivity in HCl was higher than 90% for the best catalysts.     

Pd–Ru/C as the electrocatalyst for hydrogen peroxide reduction

Pd–Ru, Pd and Ru nanoparticles supported on Vulcan XC-72 carbon were prepared by chemical reduction of PdCl₂ and/or RuCl₃ in aqueous solution using NaBH₄ as the reducing agent. Transmission electron microscopy measurements showed that Pd–Ru particles were uniformly dispersed on carbon. The particle size of Pd–Ru is around 5–9 nm. X-ray diffraction analysis indicated that Ru formed alloy with Pd in Pd–Ru/C catalyst. The electroreduction of hydrogen peroxide on Pd–Ru/C, Pd/C and Ru/C in H₂SO₄ solution was examined by linear sweep voltammetry and chronoamperometry measurements. Results revealed that Pd–Ru/C catalyst exhibited higher electrocatalytic activity for hydrogen peroxide reduction than Pd/C and Ru/C. All the catalysts showed good stability for hydrogen peroxide electroreduction in H₂SO₄ electrolyte.     

Direct synthesis of acetic acid from CH4 and CO2 by a step-wise route over Pd/SiO2 and Rh/SiO2 catalysts

The theoretical and experimental feasibility of direct conversion of CH₄ and CO₂ to acetic acid by an isothermal step-wise route over Pd/SiO₂ and Rh/SiO₂ catalysts was investigated. The methyl radical formation from CH₄ dissociation and CO₂ inserting into the intermediate are regarded as two limiting steps. Preliminary experimental results have shown that the following step-wise route can circumvent the thermodynamic limitation of this direct synthesis at low temperatures. Pd catalysts are more active than Rh catalysts at 170 °C and 200 °C, while formic acid is only produced on Pd catalysts. The optimum contact time of CH₄ and CO₂ with catalysts is 1 min under the experimental conditions. And there is no apparent deactivation resulting from carbon deposition for catalysts during the successive reaction cycles.     

Pd/Pt on Ti-containing Mixed Oxides as Dearomatization Catalysts: Physico-chemical Characterization and Activity

Pd/Pt supported on pure and doped TiO₂ (TiO₂–WO₃ and TiO₂–WO₃–SiO₂) were prepared and characterized by different techniques (XPS, TEM, XRD, H₂-TPR and TPD of ammonia). These catalysts were investigated in the hydrogenation of tetralin at 6.0 MPa, checking also their thio-tolerance by feeding increasing amounts of dibenzothiophene (DBT, 300 and 1000 wt ppm). The catalytic activity followed the order: Pd/Pt–TiO₂ > Pd/Pt–TiO₂–WO₃–SiO₂ > Pd/Pt–TiO₂–WO₃, evidencing a negative role of a second oxide inside TiO₂. The Pd/Pt–TiO₂ catalyst showed high activity regardless of reaction conditions (temperature, contact time, H₂/tetralin ratio) together with a good thio-tolerance up to 300 wt ppm of DBT.