Complete benzene oxidation over Pt-Pd bimetal catalyst supported on γ-alumina: influence of Pt-Pd ratio on the catalytic activity

Pt-Pd bimetal catalysts were prepared in order to develop and investigate catalysts with excellent activity and stability for benzene destruction. In the reaction results, the addition of Pt to Pd/γ-Al₂O₃ catalyst brought about the increase of catalytic activity. Moreover, it was effective in preventing the deactivation of the catalysts in benzene combustion. The addition of some amount of Pt made Pd particles available for better benzene combustion. On the contrary, the addition of Pt beyond a certain amount decreases activity because of the Pd active sites overlapped with the Pt active sites. The activity of the catalysts is related to oxidation state of metal, Pd/Al ratio and particle size on γ-Al₂O₃. These effects of Pt addition to Pd catalysts were studied by XPS, XRD, and TEM analyses.     

Stability of supported metal and supported metal oxide combustion catalysts

Catalysts used for high-temperature combustion of light hydrocarbons must maintain high activity over long time intervals by avoiding excessive sintering and deactivation in the hot and corrosive combustion environment. The sintering resistance and chemical stability of catalytically active phases is a key technical problem that must be solved for the development of commercially viable combustion catalysts. All noble metals and transition metal oxides that are catalytically active rapidly sinter at temperatures required for high combustion rates. Advanced materials used in the development of stable catalysts include highly sintering-resistant hexaaluminate supports for dispersion of noble metals, chemically and thermally stable supporting oxides for active transition metal oxides, and single-phase, substitutionally activated, sintering-resistant complex metal oxides. This paper will review deactivating phenomena, such as sintering and vapor transport and assess recent progress in the development of durable combustion catalysts.     

Catalytic combustion of benzene over supported metal oxides catalysts

Catalytic combustion of benzene over supported metal oxides has been investigated. The catalysts have been prepared by incipient wetness method and characterized by XRD, FT-Raman, ESR and TPR. Among supported metal oxides, CuO_x, supported on TiO₂ is found to have the highest activity for benzene oxidation. In addition, among the catalysts of copper oxide supported on TiO₂, A1₂O₃ and SiO₂, titania-supported catalyst (CuO_x/TiO₂) gives the highest catalytic activity. CuO_x/TiO₂ (Cu loading 5.5 wt%) shows the total oxidation of benzene at about 250 °C. From the ESR and FT-Raman results, the CuO dispersed on the TiO₂ surface acts as an active site of CuO_x/TiO₂ catalysts on the oxidative decomposition of benzene. The catalytic activity gradually increases with an increase of Cu loading on TiO₂. When Cu loading reaches 5.5 wt%, the total conversion temperature is lowered to 300 °C. However, the catalytic activity considerably decreases at 7 wt% Cu loading. The catalytic activity increased with an increase of oxygen concentration but the concentration of benzene showed no difference in the benzene conversion. This result suggests that the rate determining step is the adsorption of oxygen.     

The effect of water on the activity of supported palladium catalysts in the catalytic combustion of methane

Supported palladium catalysts are very active in the combustion of methane, but still little is known about the kinetic parameters. In this paper a rate expression is presented for an alumina-supported palladium oxide catalyst in the temperature range 180–515°C. Special care was taken to ensure differential conditions during the experiments. In this way, an apparent activation energy of 151±15 kJ/mol was found. The orders in methane, oxygen and water were 1.0±0.1, 0.1±0.1 and −0.8±0.2, respectively. For carbon dioxide a zero order was observed under all conditions. Inhibition by water produced during the reaction was demonstrated to cause non-differential conditions, when a dry feed was used. The rate constant that corrects for this effect could be derived.     

Methane combustion over supported palladium catalysts I. Reactivity and active phase

The performance of Al₂O₃, ZrO₂ and ZrO₂ stabilized with SiO₂ (ZrO₂-s) supported palladium catalysts for the methane combustion was studied between 473 and 873 K. The nature of the surface species of palladium catalysts under reaction conditions were detected by FT-IR and microcalorimetry of CO adsorbed. The different behavior of palladium catalysts under reaction conditions is attributed to support effects associated to differences in thermal conductivity and oxygen mobility of supports. Prereduction of the catalysts enhances their activity. Under reaction conditions, the prereduced sample becomes partially oxidized by preferential adsorption/reaction of oxygen both on Pd (1 1 1) planes and on the sites that can multibondedly adsorb CO. The reconstruction of the metallic particles and the formation of PdO_x (0<x≤1) phase were directly observed by FT-IR and microcalorimetry of adsorbed CO. Combination of different characterization techniques with reaction results suggests that a mixed phase, Pd⁰/PdO_x, is the most active phase for methane combustion, and that a redox mechanism may occur on this phase.     

Oxidative dimerisation of methane on supported palladium oxide catalysts

Supported palladium oxide catalysts are able to convert CH₄ to C₂H₆, CO, CO₂, H₂ and H₂O at temperatures 315 °C. Catalysts did not show any support effect when TiO₂, Al₂O₃, ZrO₂, La₂O₃ and MgO were used as supports. With sequential O₂ pulsing the catalyst showed long term activity when used at temperatures below 400 °C. Addition of Pt increased selectivity whereas with Ga it decreased. Results indicate participation of lattice O₂ from catalyst in the reaction pathway.     

Total combustion of m-xylene over palladium catalysts supported by stainless steel flakes

Total combustion of m-xylene over Pd/stainless steel flakes has been investigated. Ignition temperature of the catalytic combustion of m-xylene mixtures over palladium catalysts is lowered by about 330°C compared to that of thermal oxidation. Hysteresis in the conversion-temperature curves recorded in temperature programmed modes is controlled not only by the operating conditions (the temperature ramp, concentration of the reactants, contact time) but also by catalyst pretreatments. High and low activity states have been observed with typical ignition temperatures of 200–250°C and about 400°C, respectively. In high activity state the combustion is likely initiated by the catalyst followed by a chain reaction which propagates into the gas phase. This is strongly supported by the abrupt increase in the conversion profile. During the induction period preceding the combustion, active species are accumulated on the surface which are responsible for this process.     

Structured reactors for kinetic measurements under severe conditions in catalytic combustion over palladium supported systems

The collection of chemical kinetics data in catalytic combustion over very active palladium catalysts under conditions relevant to practical applications (e.g. gas turbine combustors) is extremely difficult, mainly due to strong exothermicity and very fast rate of combustion reactions. Within this purpose in this paper two types of laboratory structured reactors, which closely resemble industrial monolith catalysts, are investigated: (a) the annular reactor, consisting of a catalyst coated ceramic tube, co-axially placed in a quartz tube; (b) the metallic plate-type reactor, consisting of an assembled packet of metallic slabs coated with a ceramic catalytic layer.

The design of the annular reactor configurations for kinetic investigations is first addressed by mathematical modeling. The resulting advantages, including: (i) negligible pressure drops; (ii) minimal impact of diffusional limitations in high temperature–high GHSV experiments; (iii) effective dissipation of reaction heat are then experimentally demonstrated for the case of CH₄ combustion over a PdO/γ-Al₂O₃ catalyst with high noble metal loading (10% (w/w) of Pd).

The feasibility of a near-isothermal operation with the metallic plate-type reactor by an extremely effective dissipation of reaction heat through proper selection of highly conductive support material and of the geometry of the metallic slabs is finally discussed and experimentally demonstrated for the case of combustion of CO at high concentrations over a PdO/γ-Al₂O₃ (3% (w/w) of Pd) catalyst.     

The preparation of palladium metal catalysts supported on carbon part II: Deposition of palladium and metal area measurements

Four techniques for Pd deposition were compared and various carbons from Part I were used. Increasing burnoff produced higher area of the support and higher available metal area. However, excessive removal of carbon resulted in a decrease in Pd area for a given loading. Although HNO₃ treatment did not produce an increase in carbon area, there were significant increases in catalyst areas for some deposition techniques. Carbon deposition appeared to block micropores to some extent, but was disappointing in the areas of Pd found on these samples. A favorable technique would seem to be to use 15 to 20% burnoff followed by HNO₃ acid washing and the impregnation adsorption deposition method.     

Carbon-based monolithic supports for palladium catalysts: The role of the porosity in the gas-phase total combustion of m-xylene

Three different carbon-based monoliths have been studied in their performance as Pd catalyst supports in the total gas-phase combustion of m-xylene at low temperatures. The first monolithic support (HPM) was a classical square channel cordierite modified with -Al₂O₃, blocking the macroporosity of the cordierite and rounding the channel cross-section, on which a carbon layer was applied by carbonization of a polyfurfuryl alcohol coating obtained by dipcoating. The other two monolithic supports were composite carbon/ceramic monoliths (MeadWestvaco Corporation, USA), microporous (WA) and a mesoporous (WB) sample.

All the catalysts have a comparable total Pd loading and very similar Pd particle size (around 5–6 nm). In sample Pd/WA the Pd is situated only in the macropores, while in the case of Pd/WB the Pd is distributed throughout the mesoporous texture. In the case of Pd/HPM, Pd particles are clearly situated at the external surface of the carbon layer.

The catalytic activities of the samples were very different, decreasing in the order: Pd/WB > Pd/WA > Pd/HPM. These results show that the carbon external surface area, the macropores and mainly mesopores, play an important role in this kind of gas-phase reactions, improving the contact between the Pd particles and the m-xylene molecules. The catalytic activity of the Pd supported on carbon-based monoliths correlates with the surface area developed in macro- and mesopores of the monolithic support.     

Methanol decomposition over supported palladium and platinum

Methanol decomposition over supported Pt and Pd catalysts was investigated in the temperature range from 453 to 573 K with the partial pressure of methanol up to 0.8 arm. The specific activity of Pd was higher than that of Pt, and alumina-supported Pd was more active than silicasupported Pd, although aluminasupported catalysts produced dimethyl ether as a by-product. A silica-supported Pd catalyst prepared from a tetraamine salt solution with a proper pH exhibited higher activity than the other silica-supported Pd catalysts. Dependence of the decomposition rate on the partial pressure of methanol was similar regardless of the metal, the support or the preparation method. The apparent reaction orders were near 0.3 at low pressures below about 0.4 atm and became near zero or slightly negative at higher pressures. The apparent activation energies were about the same for most of the catalysts and were in the range from 66 to 77 kJ/g-mol.     

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.     

Palladium-hydrogen interaction on supported palladium catalysts of different metal dispersions

Palladium-hydrogen interaction on supported Pd catalysts of different metal dispersions has been studied by hydrogen chemisorption and back-sorption, and by temperature-programmed desorption (TPD) of either deuterium or hydrogen from the Pd surface after different initial gas dosages. Pd interacts with three hydrogen species whose amounts vary with the metal dispersion and with the dosage pressure. The amount of hydrogen strongly adsorbed on the Pd surface increases with the metal dispersion but is unaffected by the hydrogen pressure. The amount of hydrogen absorbed in the bulk of Pd is significant with a poorly dispersed catalyst, particularly when the hydrogen pressure is higher than about 10 Torr, but decreases with the metal dispersion. Hydrogen weakly bound with Pd shows two characteristic TPD peaks, whose intensity depends on the metal dispersion and the initial gas dosage. The peak at 270-330 K, assigned to the recombination of absorbed hydrogen with surface hydrogen, is large with a poorly dispersed catalyst and grows with the initial hydrogen dosage. The intensity of the peak at 215-226 K is not affected by the metal dispersion or by the hydrogen dosage. It has been concluded that an analysis of the intrinsic properties of Pd catalysts should be based on an understanding of the hydrogen species associated with the catalysts.     

NOx reduction with methane over mordenite supported palladium catalyst

Catalytic reduction of NO_x with small amounts of hydrocarbons in the presence of excess oxygen and water vapor have been studied over mordenite supported metal catalysts. Pd/mordenite catalyst was found to be very active for the reduction of NO_x with methane.     

VOC deep oxidation over Pt catalysts using hydrophobic supports

The active hydrophobic-supported Pt catalysts were synthesized for VOC deep oxidation at low temperature (less than 200°C). The destruction of VOC could cost less at lower temperature due to less energy consumption. The advantage using hydrophobic support was that moistures from atmosphere and oxidation would not be adsorbed on the surface. Thus the active sites would not be cloaked and catalyst activity could be maintained, especially at low temperature. The hydrophobicity of supports was characterized by wetting angles. Porous SDB (styrene divinylbenzene copolymer) was found near 113°, indicating high hydrophobicity. Three Pt catalysts were prepared on SDB and activated carbons by incipient wetness method. Specific surface areas were measured by nitrogen adsorption. The thermal stability of SDB catalyst was examined by TGA, and found no degradation below 200°C in air. The surface compositions of catalysts were analyzed by EDS. XRD showed that Pt was well dispersed on supports after hydrogen reduction at 160°C. The chemical states of Pt were investigated by XPS, and suggested that the oxidized Pt^IV might be the active sites in the reaction. The deep oxidation of toluene/air mixture was carried out to test the activity of catalysts. Pt/SDB showed the highest activity among the catalysts and could completely oxidize 90 ppm toluene/air at VHSV=21 000 h⁻¹, 150°C. Redox mechanism was proposed to reveal the enhanced kinetic rates. The results suggested that the rate of toluene oxidation might be enhanced due to the fact that water, one of the products, was expelled from the hydrophobic surface.     

The effect of mass transfer on the catalytic combustion of benzene and methane over palladium catalysts supported on porous materials

Catalytic combustion of benzene and methane over palladium catalysts supported on FAU and MOR zeolites and MCM-41 and KIT-1 mesoporous materials were studied to illustrate the effect of pore size and shape of supports on their catalytic activities. The palladium catalysts supported on mesoporous materials showed high activity and a steep increase in the conversion of benzene with rising temperature. The low activity of palladium catalysts supported on FAU zeolite was ascribed to mass transfer limitation. However, conversion profiles of methane on palladium catalysts were similar, although their supports were different as zeolites and mesoporous materials. The catalytic behavior of palladium catalysts in the combustion of benzene and methane was explained by the diffusion properties of fuels in the pores of zeolites and mesoporous materials.     

Promotional effect of gold added to palladium supported on a new mesoporous TiO2 for total oxidation of volatile organic compounds

Gold and palladium were supported on a mesoporous TiO₂ for total oxidation of volatile organic compounds (VOCs). Mesoporous high surface area titania support was synthesised using of Ti(OC₂H₅)₂ in the presence of CTMABr surfactant. After removing the surfactant molecules, 0.5 or 1.5 wt% of palladium and 1 wt% of gold were precipitated on the support by, respectively, wet impregnation and deposition–precipitation methods. The activity for toluene and propene total oxidation of the prereduced samples follows the same order: 0.5%Pd-1%Au/TiO₂ > 1.5%Pd/TiO₂ > 0.5%Pd/TiO₂ > 1%Au-0.5%Pd/TiO₂ > 1%Au/TiO₂ > TiO₂. Moreover, a catalytic comparison with samples based on a conventional TiO₂, shows the catalytic advantage of the mesoporous TiO₂ support. The promotional effect of gold added to palladium could be partly explained by small metallic particles (TEM), but meanly by metallic particles made up of Au-rich core with a Pd-rich shell. Moreover, the hydrogen TPR profile of 0.5%Pd-1%Au/TiO₂ shows only the signal attributed to small PdO particles. Gold also implies a protecting effect of the support under reduction atmosphere. Operando diffuse reflectance infrared fourier transform (DRIFT) spectroscopy was carried on and allowed to follow the VOCs oxidation and the formation of coke molecules, but also a metallic electrodonor effect to the adsorbed molecule which increases in the same order as the activity for oxidation reaction. The presence of coke after test was also shown by DTA–TGA by exothermic signals between 300 and 500 °C and by EPR (g = 2.003).     

Oscillatory behaviour during the oxidation of methane over palladium metal catalysts

Oscillatory reactions over palladium foil and wire catalysts during the oxidation of methane have been investigated over a wide range of reaction temperatures and argon/methane/oxygen feed gas compositions. Characterisation of the catalyst has also been carried out using scanning electron microscopy (SEM) techniques, which revealed the presence of a porous surface. This suggested that the metal surface has undergone a change since the reaction commenced, and using X-ray powder diffraction (XRD) techniques the palladium phase was shown to be the dominant phase present. Hysteresis phenomena were observed in the activity of the reaction as the temperature was cycled up and down, showing that the metal surface was continually changing throughout the reaction. The activation energies of the reaction during the high reactivity mode, PdO, and low reactivity mode, Pd, were also calculated. Oscillation rates were observed to depend on the dominant surface. Oscillations were frequent when the high reactivity mode was dominant while the activation energy of this mode was found to be low. When the low reactivity mode was dominant, the oscillations were slower and the activation energy was three times larger. The results obtained imply that the behaviour of the palladium surface, switching back and forth from the reduced state to the oxidised state, is responsible for the oscillatory behaviour seen in this system.     

Macroporous metal oxide foams through self-sustained combustion reactions

A number of macroporous metal oxide foams were prepared through self-sustained combustion reactions starting from dough made of the corresponding metal nitrate, urea and starch. The nitrate ion acts as an oxidizing agent, urea as fuel and starch as an organic binder. The metal oxide foams are characterized by scanning electron microscopy and powder X-ray diffraction.     

Evaluation and characterization of Mn-Cu mixed oxide catalysts supported on TiO2 and ZrO2 for ethanol total oxidation

MnCu/ZrO₂T and MnCu/TiO₂T (T = calcination temperature) catalysts were prepared by the sol-gel method with a Mn:Cu = 5:1 ratio and calcined at different temperatures, T = 673, 873 and 1073 K. The samples were characterized by X-ray diffraction, measurement of specific surface area, temperature programmed reduction, XPS and FT-IR spectroscopy. For both groups of catalysts, the increase of calcination temperature produced three effects: the segregation of phases, an increase of the crystallinity, and a phase transformation of the support. The catalytic activity was evaluated in total oxidation of ethanol, considered as model molecule of VOC. The MnCu/TiO₂ 673 and MnCu/ZrO₂ 873 catalysts showed the best catalytic performance, which was associated with the high dispersion of the MnO_x and CuO_x active phases. The catalytic activity of MnCu/TiO₂ 673 catalyst was also favored by its high surface area.