Formation and catalytic activity of partially oxidized Pd nanoparticles

Combining multi molecular beam (MB) experiments and in-situ time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we have studied the formation and catalytic activity of Pd oxide species on a well-defined Fe₃O₄ supported Pd model catalyst. It was found that for oxidation temperatures up to 450 K oxygen predominantly chemisorbs on metallic Pd whereas at 500 K and above (~10⁻⁶ mbar effective oxygen pressure) large amounts of Pd oxide are formed. These Pd oxide species preferentially form a thin layer at the particle/support interface. Their formation and reduction is fully reversible. As a consequence, the Pd interface oxide layer acts as an oxygen reservoir providing oxygen for catalytic surface reactions. In addition to the Pd interface oxide, the formation of surface oxides was also observed for temperatures above 500 K. The extent of surface oxide formation critically depends on the oxidation temperature resulting in partially oxidized Pd particles between 500 and 600 K. It is shown that the catalytic activity of the model catalyst for CO oxidation decreases significantly with increasing surface oxide coverage independent of the composition of the reactants. We address this deactivation of the catalyst to the weak CO adsorption on Pd surface oxides, leading to a very low reaction probability.     

The catalytic oxidation of ammonia: influence of water and sulfur on selectivity to nitrogen over promoted copper oxide/alumina catalysts

The CuO/Al₂O₃ system is active for ammonia oxidation to nitrogen and water. The principal by-products are nitrous oxide and nitric oxide. Nitrous oxide levels increase with the addition of various metal oxides to the basic copper oxide/alumina system. Addition of sulfur dioxide to the reaction stream sharply reduces the level of ammonia conversion, but has a beneficial effect on selectivity to nitrogen. Added water vapour has a lesser effect on activity but is equally beneficial in terms of selectivity to nitrogen. The CuO/Al₂O₃ is also active for the selective catalytic reduction of nitric oxide by ammonia, but this reaction is not effected by sulfur dioxide addition. A mechanism for ammonia oxidation to nitrogen is proposed wherein part of the ammonia fed to the catalyst is converted into nitric oxide. A pool of monoatomic surface nitrogen species of varying oxidation states is established. N₂ or N₂O are formed depending upon the average oxidation state of this pool. An abundance of labile lattice oxygen species on the catalyst surface leads to overoxidation and to N₂O formation. On the other hand, reduced lability of surface lattice oxygen species favours a lower average oxidation state for the monoatomic surface nitrogen pool and leads to N₂ formation.     

MoVW mixed metal oxides catalysts for acrylic acid production: from industrial catalysts to model studies

(MoVW)₅O₁₄-type oxides were identified as the active and selective components in industrial acrylic acid catalysts. Tungsten is suggested to play an important role as a structural promoter in the formation and stabilization of this oxide. Vanadium is responsible for high catalytic activities but is detrimental for the stability of this oxide at the necessary high concentrations for optimum catalytic performance. The activity of mixed MoVW oxide catalysts for methanol, propene, and acrolein partial oxidation could be considerably improved, when the amount of the (MoVW)₅O₁₄-type oxide was increased by thermal annealing. A model is proposed on the basis of the correlation between Raman wavenumber and bond order and degree of reduction, which explains the observed different selectivities of MoO_3−x and the (MoVW)₅O₁₄-type oxides in terms of metal–oxygen bond strengths, i.e. oxygen basicity and oxygen lability, respectively. According to this model, the (MoVW)₅O₁₄ mixed oxide catalyses partial oxidation because of its intermediate C–H activation and oxygen releasing oxygen functionalities. However, these (MoVW)₅O₁₄-type industrial oxidation catalysts are heterogeneous and highly complex systems. Their physicochemical characterization also revealed that their chemical bulk and surface compositions vary with thermal activation and oxygen potential. A core-shell model is suggested to describe the active catalyst state, the shell providing a high number of active centers, the core high electronic conductivity and ion mobility. The fact that the surface composition of such catalysts is considerably different from their bulk compositions, most probably implies that the “molecular structure” at their surface differs too considerably from their bulk crystal structure. Hence, the posed question about the active catalyst structure and its relation to its catalytic performance cannot unambiguously be explained by the crystallographic structure, but still remains unsolved.     

Microwave-accelerated solvent-free aerobic oxidation of benzyl alcohol over efficient and reusable manganese oxides

A new facile and cost-effective process involving the solvent-free oxidation of benzyl alcohol using molecular oxygen as oxidant under controlled microwave irradiation has been developed for the production of chlorine-free benzaldehyde. Influence of different catalyst parameters (different manganese oxides and other kinds of transition metal oxides) and reaction conditions (reaction period and temperature) on the process performance has been studied. Under optimized reaction conditions, the MnO₂ catalyst showed a superior catalytic performance in the highly selective oxidation of benzyl alcohol as compared to other manganese oxide materials such as MnO, Mn₂O₃ and Mn₃O₄. Moreover, a very stable catalytic activity as a function of cycling test was observed for the MnO₂ catalyst.     

Porous Clays and Pillared Clays-Based Catalysts. Part 2: A Review of the Catalytic and Molecular Sieve Applications

Metal oxide pillared clay (PILC) possesses several interesting properties, such as large surface area, high pore volume and tunable pore size (from micropore to mesopore), high thermal stability, strong surface acidity and catalytic active substrates/metal oxide pillars. These unique characteristics make PILC an attractive material in catalytic reactions. It can be made either as catalyst support or directly used as catalyst. This paper is a continuous work from Kloprogge's review (J.T. Kloprogge, J. Porous Mater. 5, 5 1998) on the synthesis and properties of smectites and related PILCs and will focus on the diverse applications of clay pillared with different types of metal oxides in the heterogeneous catalysis area and adsorption area. The relation between the performance of the PILC and its physico-chemical features will be addressed.     

Molecular structure and reactivity of the group V metal oxides

The molecular structures and reactivity of the group V metal oxides (V₂O₅, Nb₂O₅ and Ta₂O₅) were compared. Their solid state structural chemistry, physical and electronic properties, number of active surface sites and their chemical reactivity properties were examined. For the bulk oxides, the solid state structural chemistry and the physical and electronic properties are well established. The number of active surface sites and the distribution of surface redox/acid sites were determined with methanol chemisorption and methanol oxidation, respectively. These studies revealed that the active surface sites present in pure V₂O₅ are primarily redox sites and the active surface sites in pure Nb₂O₅ are essentially acidic in nature. Furthermore, the surface redox sites present in pure V₂O₅ are orders of magnitude more active than the surface acid sites in pure Nb₂O₅. Consequently, the catalytic properties of bulk V₂O₅–Nb₂O₅ mixed oxides are dominated by the vanadia component. For the supported metal oxides, where the group V metal oxides are present as two-dimensional metal oxide overlayers, the structural and electronic properties are not well established in the literature. From a combination of molecular spectroscopic characterization methods (e.g., XANES, Raman, IR and UV–Vis DRS), it was possible to obtain this fundamental information. Methanol chemisorption studies demonstrated that a similar number of active surface sites are present in the supported vanadia and niobia catalyst systems. Similar to their bulk oxides, the surface vanadia species possess redox characteristics and the surface niobia species primarily possess acidic characteristics (Lewis acidity). The surface niobia species was a very sluggish redox site during oxidation reactions (e.g., methanol oxidation to formaldehyde and SO₂ oxidation to SO₃), but significantly promoted the surface vanadia redox sites for oxidation reactions that required dual surface redox and acid sites (e.g., butane oxidation to maleic anhydride and selective catalytic reduction of NO_x by NH₃ to produce N₂). These new fundamental insights are allowing for the molecular engineering of group V metal oxide catalysts (especially vanadia and niobia). In contrast, the molecular structure and reactivity properties of Ta₂O₅ catalysts are not yet established and will require significant research efforts.     

Higher Oxidation State Responsible for Ozone Decomposition at Room Temperature over Manganese and Cobalt Oxides: Effect of Calcination Temperature

The heterogeneous catalytic decomposition of ozone was investigated over unsupported manganese and cobalt oxide at room temperature. All catalysts were characterized by X-ray diffraction (XRD), N₂ adsorption–desorption (Brunauer–Emmet–Teller method), H₂-temperature programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS). The catalytic activity test indicated that these oxides had a good activity on ozone conversion meanwhile the catalysts remained highly active over time under reaction conditions. The treated temperature of the catalyst had a significant impact on the performance of ozone abatement and the samples treated at lower temperature showed higher activity. The surface area decreased obviously when developing the calcination temperature and H₂-TPR results demonstrated that much higher oxidation state of metal ions and active oxygen species were maintained on the surface under low treated temperature. XPS analysis showed that there were higher oxidation states of metal ions (Mn⁴⁺ and Co³⁺) and adsorbed oxygen species on the surface of catalysts treated at lower temperature, both of which play a significant role in ozone decomposition. However, the activity of manganese oxide was higher than that of cobalt oxide and the possible reason for this phenomenon was discussed.     

用于环戊烯氧化合成戊二醛反应的钨基催化剂的表征

Tungsten-containing hexagonal mesoporous silica （W-HMS） supported tungsten oxide catalysts （WOx/W-HMS） was prepared for the selective oxidation of cyclopentene with aqueous hydrogen peroxide to glutaraldehyde. X-ray diffraction （XRD） results indicated that the crystal form of the active phase （tungsten oxide） of the WOx/W-HMS catalysts was dependent on the W loading and calcination temperature. X-ray photoelectron spec- troscopy （XPS） analysis revealed that the dispersed tungsten oxides on the surface of W-HMS support consisted of a mixture of W（V） and W（VI）. It was found that a high content of amorphous W species in （5＋） oxidation state resuited in the high catalytic activity. When the W loading was up to 12% （by mass） or the catalyst precursor was treated at temperature of 623 K, the catalytic activity decreased due to the presence of WO3 crystallites and the oxidation of W（V） to W（VI） on the catalyst surface. Furthermore, NH3-temperature-programmed-desorption （NH3-TPD） analysis showed that the effects of W loading and calcination temperature on the acidity of the catalysts were related to the catalytic activity. A high selectivity of 80.2% for glutaraldehyde with a complete conversion of cyclopentene was obtained over 8%WOx/W-HMS catalyst calcined at 573 K after 14 h of reaction.     

Investigation of the preparation and catalytic activity of supported Mo,W, and Re oxides as heterogeneous catalysts in olefin metathesis

For several decades olefin metathesis reactions over supported Mo, W, and Re oxides catalysts have attracted remarkable interest owning to their growing industrial applications. Therefore, particular attention was devoted to improve the catalytic activity of these catalysts. In the way of exploration of the catalytic performances of these heterogeneous Mo, W, and Re oxides systems, it was found that high dispersion of metal oxide on the support surface and the oxidation state of the metal oxide on the surface of catalyst play crucial factors on the catalysts efficiency. Importantly, these factors have an origin in the preparation methods and the properties of the used supports. In this regard, we have tried to address the various preparation methods of immobilized Mo, W, and Re oxide catalysts as well as properties of supporting material to better understand their impacts on the catalytic performances of these catalysts.     

The role of Fe on PtPd oxidation catalyst prepared by simultaneous and sequential incipient wetnesses

The roles and effects of Fe on the catalytic performance and physicochemical properties of a PtPd diesel oxidation catalyst prepared by three different methods were investigated by CO oxidation reaction, X-ray diffraction, temperature-programmed reduction (TPR), temperature-programmed oxidation, and BET surface area. It was found that the roles of Fe depended strongly on the sequential order of Fe introduction during the preparation of the PtPd catalyst. The Fe/PtPd/Al₂O₃ catalyst was prepared by introducing Fe onto the PtPd/Al₂O₃, and the PtPd/Fe/Al₂O₃ catalyst was obtained by loading the PtPd onto the Fe/Al₂O₃. The former had a superior activity. From the TPR results, the catalytic activity of CO oxidation was correlated with the oxygen mobility of the iron oxides. For PtPd/Fe/Al₂O₃, the iron interacted preferentially with the alumina support forming FeAlO₃, which resulted in the stabilization of the support and a reduction in the surface area. The major role of Fe was to promote the enhancement of the catalytic activity of PtPd through an intimate interaction between the PtPd and iron oxides, which had lattice oxygens to generate oxygen with oxidation abilities.     

Oxidation of SO2over Supported Metal Oxide Catalysts

A systematic catalytic investigation of the sulfur dioxide oxidation reactivity of several binary (M_xO_y/TiO₂) and ternary (V₂O₅/M_xO_y/TiO₂) supported metal oxide catalysts was conducted. Raman spectroscopy characterization of the supported metal oxide catalysts revealed that the metal oxide components were essentially 100% dispersed as surface metal oxide species. Isolated fourfold coordinated metal oxide surface species are present for most oxides tested at low coverages, whereas at surface coverages approaching monolayer polymerized surface metal oxide species with sixfold coordination are present for some of the oxides. The sulfur dioxide oxidation turnover frequencies (SO₂molecules converted per surface redox site per second) of the binary catalysts were all within an order of magnitude (V₂O₅/TiO₂>Fe₂O₃/TiO₂>Re₂O₇/TiO₂∼CrO₃/TiO₂∼Nb₂O₅/TiO₂>MoO₃/TiO₂∼WO₃/TiO₂). An exception was the K₂O/TiO₂catalyst system, which is inactive for sulfur dioxide oxidation under the chosen reaction conditions. With the exception of K₂O, all of the surface metal oxide species present in the ternary catalysts (i.e., oxides of V, Fe, Re, Cr, Nb, Mo, and W) can undergo redox cycles and oxidize sulfur dioxide to sulfur trioxide. The turnover frequency for SO₂oxidation over all of these catalysts is approximately the same at both low and high surface coverages, despite structural differences in the surface metal oxide overlayers. This indicates that the mechanism of sulfur dioxide oxidation is not sensitive to the coordination of the surface metal oxide species. A comparison of the activities of the ternary catalysts with the corresponding binary catalysts suggests that the surface vanadium oxide and the additive surface oxide redox sites act independently without synergistic interactions: the sum of the individual activities of the binary catalysts quantitatively correspond to the activity of the corresponding ternary catalyst. The V₂O₅/K₂O/TiO₂catalyst showed a dramatic reduction in catalytic activity in comparison to the unpromoted V₂O₅/TiO₂catalyst. The ability of potassium oxide to significantly retard the redox potential of the surface vanadia species is primarily responsible for the lower catalytic reactivity.     

Synthesis and characterization of nanocrystalline Mo–V–W–Fe–O mixed oxide catalyst and its performance in selective methanol oxidation

A mixed oxide catalyst containing Mo, V, W and Fe with the composition of 63, 23, 09 and 06 wt% respectively for the selective oxidation of the methanol to formaldehyde is in reported in this paper for the first time. The characterization of the catalyst was done using BET surface analysis, X-ray diffraction (XRD), Infrared spectroscopy (FTIR), Scanning electron microscopy (SEM) and Energy dispersive X-ray (EDX). The mixed oxide after calcination at 673 K in N₂ which was subjected for the thermal activation in N₂flow at 813 K was used for the methanol selective oxidation. The thermal treatment shows enhanced catalytic performance. Thermal activation of the nanocrystalline Mo_0.63V₂₃W_0.09Fe₀.₀₆O _x precursor oxide in nitrogen atmospheres induces partial crystallization of a Mo₅O₁₄-type oxide only in a narrow temperature range up to 813 K. XRD showed that the thermally activated mixed oxide consists of a mixture of a majority of crystalline Mo₅O₁₄-type oxide and of small amounts of crystalline MoO₃-type and MoO₂-type oxides. The structural analysis suggests that the improvement of the catalytic performance of the MoVWFe oxide catalyst in the selective oxidation of methanol is related to the formation of the catalytic active site such as Mo₅O₁₄-type mixed oxide.     

Total oxidation of propane over Cu-Mn mixed oxide catalysts prepared by co-precipitation method

The catalytic activity of Cu-Mn mixed oxides with varying Cu/Mn ratios prepared by co-precipitation method was examined for the total oxidation of propane. The nature and phase of the metal oxide species formed were characterized by various methods such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H₂ temperature-programmed reduction (TPR) as well as BET surface area measurement. The co-precipitation method provides highly interdispersed copper and manganese metallic elements forming Cu-Mn mixed oxide of spinel structure (Cu_1.5 Mn_1.5O₄). Besides the spinel-type Cu-Mn mixed oxide, CuO or Mn₂O₃ phases could be formed depending on the Cu/Mn molar ratio of their precursors. The catalytic activity of Cu-Mn mixed oxide catalyst for propane oxidation was much higher than those of single metal oxides of CuO and Mn₂O₃. The higher catalytic activity likely originates from a synergic effect of spinel-type Cu-Mn mixed oxide and CuO. The easier reducibility and BET surface area seems to be partially responsible for the high activity of Cu-Mn mixed oxide for total oxidation of propane.     

Recent Advances on TiO2‐ZrO2 Mixed Oxides as Catalysts and Catalyst Supports

This is the first review of titanium dioxide‐zirconium dioxide (TiO₂‐ZrO₂) mixed oxides, which are frequently employed as catalysts and catalyst supports. In this review many details pertaining to the synthesis of these mixed oxides by various conventional and nonconventional methods and their characterization by several techniques, as reported in the literature, are assessed. These mixed oxides have been synthesized by different preparative analogies and were extensively characterized by employing various spectroscopic and nonspectroscopic techniques. The TiO₂‐ZrO₂ mixed oxides are also extensively used as supports with metals, nonmetals, and metal oxides for various catalytic applications. These supported catalysts have also been thoroughly investigated by different techniques. The influence of TiO₂‐ZrO₂ on the dispersion and surface structure of the supported active components as examined by various techniques in the literature has been contemplated. A variety of reactions catalyzed by TiO₂‐ZrO₂ and supported titania‐zirconia mixed oxides, namely; dehydrogenation, decomposition of chlorofluoro carbons (CFCs), alcohols from epoxides, synthesis of ?‐caprolactam, partial oxidation, deep oxidation, hydrogenation, hydroprocessing, organic transformations, NO_x abatement, and photo catalytic VOC oxidations that have been pursued in the literature are presented with relevant references.     

纳米复合氧化物催化剂研究进展

由于纳米材料催化剂具有独特的结构及表面特性，特别是纳米复合氧化物催化剂具有氧化物催化剂的复合效果及性能，因而其催化活性和选择性大大高于传统催化剂。综述了纳米复合氧化物催化剂的特性及制备方法，介绍了国内外纳米复合氧化物催化剂的应用研究状况，如在催化加氢、催化氧化、聚合反应和环境保护等方面的应用实例及研究成果。     

Wet oxidation of wastewater containing hydrocarbons by novel supported Pd catalysts

Pd catalysts supported on TiO₂, ZrO₂, ZSM-5, MCM-41 and activated carbon were used in catalytic wet oxidation of hydrocarbons such as phenol, m-cresol and m-xylene. It was found that the Pd/TiO₂ catalyst was highly effective in the wet oxidation of hydrocarbon. The activities of catalysts with various hydrocarbon species, catalyst support, oxidation state of catalyst performed in a 3-phase slurry reactor show that reaction on Pd surface is more favorable than that in aqueous phase and that the active site is oxidized Pd in catalytic wet air oxidation of hydrocarbons. Based on the experimental results, a plausible reaction mechanism of wet oxidation of hydrocarbons catalyzed over Pd/TiO₂ catalyst was proposed. This catalyst is superior to other oxide catalysts because it suppressed the formation of hardly-degradable organic intermediates and polymer.     

Selective Reduction of NO with CO Over Titania Supported Transition Metal Oxide Catalysts

A series of transition metal oxides promoted titania catalysts (MO _x /TiO₂; M = Cr, Mn, Fe, Ni, Cu) were prepared by wet impregnation method using dilute solutions of metal nitrate precursors. The catalytic activity of these materials was evaluated for the selective catalytic reduction (SCR) of NO with CO as reductant in the presence of excess oxygen (2 vol.%). Among various promoted oxides, the MnO _x /TiO₂ system showed very promising catalytic activity for NO + CO reaction, giving higher than 90% NO conversion over a wide temperature window and at high space velocity (GHSV) of 50,000 h⁻¹. It is remarkable to note that the catalytic activity increased with oxygen, up to 4 vol.%, under these conditions leading primarily to nitrogen. Our TPR studies revealed the presence of mixed oxidation states of manganese on the catalyst surface. Characterization results indicated that the surface manganese oxide phase and the redox properties of the catalyst play an important role in final catalytic activity.     

Chemistry, spectroscopy and the role of supported vanadium oxides in heterogeneous catalysis

Supported vanadium oxide catalysts are active in a wide range of applications. In this review, an overview is given of the current knowledge available about vanadium oxide-based catalysts. The review starts with the importance of vanadium in heterogeneous catalysis, a discussion of the molecular structure of vanadium in water and in the solid state and an overview of the spectroscopic techniques enabling to study the chemistry of supported vanadium oxides. In the second part, it will be shown that advanced spectroscopic tools can be used to obtain detailed information about the coordination environment and oxidation state of vanadium oxides during each stage of the life-span of a heterogeneous catalyst. Three topics will be discussed: (1) the molecular structure of supported vanadium oxide catalysts under hydrated, dehydrated and reduced conditions, including the parameters, which influence the molecular structures formed at the surface of the support oxide; (2) elucidation of the active surface vanadium oxide during the oxidation of methanol to formaldehyde, the reaction mechanism and the vanadium oxide---support effect; and (3) deactivation of fluid catalytic cracking (FCC) catalysts by migration of vanadium oxides and the development of a method preventing the structural breakdown of zeolites by trapping the mobile vanadium oxides in an aluminum oxide coating.     

Preparation and evaluation of electrocatalytic oxide coatings on conductive carbon-polymer composite substrates for use as dimensionally stable anodes

This work investigates the feasibility of developing a lower cost dimensionally stable anode based on a polymer substrate by examining possible methods of applying coatings, such as MnO₂, to catalyse the oxygen evolution reaction. The conductive polymer is based on a blend of polypropylene and a rubber, to provide the mechanical strength and acid resistance of the electrode, with graphite fibre and carbon black dispersed through the composite to ensure a reasonable conductivity. Of the different compositions analysed, the best incorporated Degussa XE-2 carbon black. This material not only displays an overpotential at its bare surface which was comparable to the lead electrode, but also it provides excellent contact with the catalyst coatings. Two methods of applying a catalytic coating, thermal decomposition and pressed oxide coating, are evaluated. Coatings prepared by thermal decomposition displays an overpotential greater than the standard lead electrode, although the poor performance of the catalysts is found to be due to poor contact with the conductive sites of the substrate and to incomplete thermal decomposition of the metal chlorides to the oxide. The pressed oxide coating method is the technique developed during this work to take advantage of the properties of the polymer composites. Polymer substrates are heated until soft and the MnO₂ catalyst (in its active form) is pushed into the surface. Electrodes coated in this manner with MnO₂ all display excellent overpotentials which, on average, are 0.2 V less than the lead electrodes. Extension of this technique to the coating of polymeric substrates with other catalytic oxides shows great potential.     

Vapor phase hydrogen transfer reaction between methacrolein and ethanol over MgO based catalysts

Magnesium oxide was modified by addition of several metal oxides to prepare effective catalysts for vapor phase hydrogen transfer between methacrolein and ethanol to form methallyl alcohol and acetaldehyde. The catalytic performances vary with the acidic and basic properties of the catalyst. An effective catalyst was prepared by controlling the acidic and basic properties to be moderately weak by addition of SiO₂ and K₂O to magnesium oxide in proper amounts.