Methanol decomposition to synthesis gas at low temperature over palladium supported on ceria–zirconia solid solutions

The Ce_xZr_1−xO₂ solid solution was used as a support of a palladium catalyst for methanol decomposition to synthesis gas at low temperature. All Pd-containing catalysts tested in this study showed high selectivity to synthesis gas (over 96%). The Pd supported on the composite oxide with a Ce/Zr molar ratio of 4/1 exhibited the highest activity. Pd/Ce_0.8Zr_0.2O₂ (17 wt.%) (cop) (prepared by coprecipitation method) showed a conversion of 51.2% for the methanol decomposition at 473 K, which was higher than those over 17 wt.% Pd/CeO₂ (cop) (40.7%) and 17 wt.% Pd/ZrO₂ (cop) (24.3%) at 473 K. The 17 wt.% Pd/Ce_0.8Zr_0.2O₂ (cop) catalyst showed a higher BET surface area and smaller Pd particles than those of 17 wt.% Pd/CeO₂ (cop). Moreover, a more active Pd^σ+ state could be maintained by Zr⁴⁺ ion modification due to promotion of the oxygen mobility and enhancement of the reductibility and increase in the acid sites of the CeO₂ support. The 17 wt.% Pd/Ce_0.8Zr_0.2O₂ (cop) catalyst showed a much higher conversion (51.2%) than that over 17 wt.% Pd/Ce_0.8Zr_0.2O₂ (imp) (prepared by impregnation method) (17.2%) at 473 K. This is due to the 17 wt.% Pd/Ce_0.8Zr_0.2O₂ (cop) possessing many small Pd particles. The 17 wt.% Pd/Ce_0.8Zr_0.2O₂ (cop) catalyst showed an initial conversion of 51.2% at 473 K but the conversion decreased to 43.1% after 24 h on stream. This deactivation was attributed to carbonaceous deposit on the catalyst surface. The amounts of coke on the 17 wt.% Pd/Ce_0.8Zr_0.2O₂ (cop) catalyst were 0.9 wt.% after 24 h on stream at 473 K and 2.1 wt.% after 1 h on stream at 523 K.     

Activity and product distribution of alumina supported platinum and palladium catalysts in the gas-phase oxidative decomposition of chlorinated hydrocarbons

The complete catalytic oxidation of 1,2-dichloroethane (DCE) and trichloroethylene (TCE) over alumina supported noble metal catalysts (Pt and Pd) was evaluated. Experiments were performed at conditions of lean hydrocarbon concentration (around 1000 ppm) in air, between 250°C and 550°C in a conventional fixed bed reactor. The catalysts were prepared in a range of metal contents from 0.1 to 1 wt%. Palladium catalysts resulted to be more active than platinum catalysts in the oxidation of both chlorinated volatile organic compounds. DCE was completely destructed at 375°C, whereas TCE required 550°C. HCl was the only chlorine-containing product in the oxidation of DCE in the range of 250–400°C. Tetrachloroethylene was observed as an intermediate in the oxidation of TCE, being formed to a significant extent between 400°C and 525°C. CO was also detected in the oxidation of both DCE and TCE over Pd catalysts, though at temperatures of complete destruction, CO₂ was the only carbon-containing product. The Pt catalysts were selective to CO₂ at the studied conditions.     

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.     

Ignition of methanol partial oxidation over supported platinum catalyst

Supported platinum catalysts were prepared by precipitation of H₂PtCl₆ on powders of different metal oxides. Catalytic activity of the prepared catalysts was tested with reaction of partial oxidation of methanol (POM) for hydrogen production. Most of the prepared catalysts can ignite POM at the ambient temperature. The conversion of methanol and the selectivity of hydrogen and carbon monoxide, however, increased with the reaction temperature and varied with the kind of support and platinum loading. A 1 wt% Pt/ZnO catalyst exhibited optimized methanol conversion and selectivity at a low reaction temperature of 150 °C. The reactor may reach this temperature within 2 min after a start of the exothermic reaction.     

Methanol oxidation at platinized lead coatings prepared by a two-step electrodeposition-electroless deposition process on glassy carbon and platinum substrates

Platinized lead deposits, Pt(Pb), have been formed on glassy carbon (GC) and platinum electrodes by a two-step process, whereby a controlled amount of Pb was electrodeposited onto the substrates and was subsequently coated with a thin Pt layer upon immersion of the Pb/GC or Pb/Pt electrodes into a chloroplatinic acid solution. The spontaneous surface replacement of Pb by Pt resulted in Pt(Pb)/GC or Pt(Pb)/Pt electrodes which consisted of dispersed Pt(Pb) particles and displayed typical Pt surface electrochemistry in deaerated acid solutions. When tested as methanol oxidation anodes, these electrodes exhibited enhanced electrocatalytic activity both during voltammetric and constant potential experiments. This behaviour is attributed to an electronic effect of the underlying Pb onto the Pt surface layer.     

Toluene combustion over palladium supported on various metal oxide supports

Metal–support interaction in the catalytic combustion of toluene was studied using metal oxides with different acid–base properties as supports for Pd. The catalytic performance was correlated with XPS data and the reaction order for oxygen. These studies revealed that the affinity for oxygen of Pd surface changed according to the acid–base character of metal oxide over MgO, Al₂O₃, SiO₂, SnO₂, Nb₂O₅, and WO₃. However, ZrO₂ exhibited exceptional character in that metal Pd was unusually stabilized, which was derived from the weak interaction between Pd and support surface. The reason for high toluene combustion activity of Pd/ZrO₂ was ascribed to the stabilization of metal Pd on ZrO₂.     

A study of nitrous oxide decomposition over calcium oxide in combustion condition

A study of N₂O decomposition reaction over a bed of CaO particles in a fixed bed reactor has been conducted. Effects of parameters such as concentration of inlet N₂O, reaction temperature and effects of CO, CO₂, O₂, and NO gas presented in the combustion gas environment have been investigated. The experiment showed that the N₂O decomposition reaction was accelerated by the increase of reaction temperature, and the existence of CO, while the reaction was hindered by the existence of CO₂, NO. O₂ also affected the N₂O decomposition. Heterogeneous gas-solid reaction kinetics were proposed for the reaction conditions and compared with homogeneous reaction kinetics.     

Chemical modelling and measurements of the catalytic combustion of CH4/air mixtures on platinum and palladium catalysts

This work reports experimental measurements and a modelling study carried out on palladium and platinum based catalytic monoliths used as methane combustors for heating purposes. It concentrates on the effects of operating conditions on combustion, heat transfer efficiency and pollutant formation. The development of a detailed homogeneous/heterogeneous chemical kinetics model for methane–air combustion over palladium using literature data was undertaken to model the behaviour of one of the experimental catalytic heaters. In addition, a published detailed chemical mechanism for methane combustion over platinum was used in the platinum catalyst model. The fuel–air equivalence ratios ranged from 0.3 to 0.6 and the space velocities used were between 24 000 and 72 000 h⁻¹. Although the model assumed perfectly stirred reactor (PSR) conditions and was applied to localised regions of the monoliths where little radial gradients of temperature and concentrations were measured, it predicted the surface temperature, methane slippage, CO and NO_x at the downstream face of the monolith with reasonable accuracy in some cases, but also highlighted the shortcomings of the PSR assumption in other cases.     

Palladium and platinum catalysts supported on carbon nanofiber coated monoliths for low-temperature combustion of BTX

In this work carbon nanofiber (CNF)-coated monoliths with a very thin, homogeneous, consistent and good adhered CNF layer were obtained by means of catalytic decomposition of ethylene on Ni particles.The catalytic behaviour of Pt and Pd supported on the CNF-coated monoliths was studied in the low-temperature catalytic combustion of benzene, toluene and m-xylene (BTX) and compared with the performance of Pt and Pd supported on γ-Al₂O₃ coated monoliths.The catalysts supported on CNF-coated monoliths were the most active, independent of the metal catalyst or the type of the tested aromatic compound. TPD experiments showed that the γ-Al₂O₃ phase retained important amounts of the water molecules produced during the reaction. When water vapour was supplied to the reactant flow, the activity of Pd catalysts decreased much stronger than the Pt ones, and the activity of the Pt catalysts supported on the γ-Al₂O₃ was more affected than that of the catalysts supported on CNF.BTX combustion reactions seem to be catalyzed by Pt and Pd through different kinetic mechanisms, explaining why Pt catalysts always were more active than the Pd ones deposited on the same type of support. Pd catalyzed combustion of benzene is strongly inhibited by oxygen and by water.Catalysts supported on CNF-coated monoliths showed a selectivity to burn benzene better than toluene or m-xylene, attributed to a better aromatic-CNF surface interaction.     

Low-temperature mild partial oxidation of ethanol over supported platinum catalysts

Supported platinum catalysts containing 1.2% Pt loaded on Al₂O₃ (1.2% Pt/Al₂O₃) and 1.9% Pt loaded on ZrO₂ (1.9% Pt/ZrO₂) were prepared by incipient wetness impregnation and sol–gel method, respectively. The activity of these catalysts in the partial oxidation of ethanol (POE) was examined in a fixed-bed reactor in a temperature range between 373 and 473 K. The results indicated that significant ethanol conversion (C_EtOH > 50%) was found at the low reaction temperature with a feed ratio of O₂/EtOH ratio >0.75. Oxygen molecules introduced in reactant were completely consumed in POE reactions performed. H₂, H₂O, CO and CO₂ were the major products detected. The selectivity of hydrogen (S_H2) and CO (S_CO) varied significantly with reaction conditions. High selectivity of hydrogen (S_H2 > 95%) and low selectivity of CO (S_CO  0%) were found from a mild oxidation at T_R = 373 K over Pt/ZrO₂. However, these two selectivities were drastically deteriorated through oxidation at high T_R, high O₂/EtOH ratio or over Pt/Al₂O₃ catalyst.     

Ultrahigh Durable PtPd/C Nanowire Networks Catalyst Synthesized by Modified Phase Transfer Method for Methanol Oxidation

A novel PtPd/C nanowire catalyst with interconnected network and fewer great grain boundaries has been successfully prepared by templateless and modified phase‐transfer method using cetyltrimethylammonium bromide as a capping in ethylene glycol solution by microwave‐assisted process. Its structure, composition, and morphology are characterized by X‐ray diffraction, energy dispersive analysis of X‐ray, and transmission electron microscopy, respectively. The electrochemical measurements demonstrate that the highly dispersed and uniform PtPd/C nanowire networks catalyst has a significantly higher electrocatalytic activity and durability for the methanol oxidation as compared to solid solution PtPd/C. The greatly improved durability of PtPd/C nanowire networks catalyst is mainly a consequence of the unique interconnected network structure with fewer grain boundaries, which provide more facile pathway for the electron transfer, and inhibit the particle growth and agglomeration, as well as prevent the particles embedded in the microporous of carbon support to enhance the Pt utilization.     

Catalysts for Direct Methanol Fuel Cells

The activity of in house prepared carbon-supported Pt-Ru catalysts for methanol oxidation and carbon-supported RuSe for the oxygen reduction reaction in direct methanol fuel cells (DMFCs) was investigated. The composition of Pt-Ru/C was varied both in terms of weight loading (ratio of total metal content to carbon) as well as the ratio of Pt to Ru. The measurements were carried out in a half cell arrangement in sulphuric acid at various temperatures. The weight loading and ratio of Pt to Ru were varied in order to find out the optimum weight loading of precious metal and the temperature dependence of Pt to Ru ratio on methanol oxidation reaction. It has been found that there exists an optimum in the weight loading at 60 wt.% for carbon-supported Pt-Ru catalyst towards its maximum mass activity. While 1:1 Pt to Ru ratio exhibits a higher activity than 3:2 Pt:Ru above 60 °C, 3:2 ratio exhibits a higher activity at lower temperature. It has been observed that RuSe is inactive towards methanol and it is realised that RuSe is a potential candidate as methanol tolerant oxygen reduction catalyst. The activity of carbon supported RuSe for oxygen reduction reaction (ORR) was tested in sulphuric acid in the presence of methanol. Even though the mass specific activity of the RuSe catalyst is somewhat lower than that of Pt/C, the surface activity of carbon-supported RuSe is superior than that of carbon supported Pt which indicate the unfavourable size distribution of RuSe/C catalyst.     

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.     

Total oxidation catalysts based on manganese or copper oxides and platinum or palladium I: Characterisation

Temperature-programmed reduction (TPR), oxidation (TPO), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were used to characterise catalysts based on manganese oxides, copper oxides or one of them mixed with platinum or palladium-supported on γ-alumina. The catalysts were characterised before and after they had been exposed either to high temperature in the presence of steam or to sulphur dioxide. Raman spectroscopy, XRD, XPS and TPR performed on the fresh samples of MnO_x, mixed MnO_x–Pt and MnO_x–Pd revealed the presence of a mixture of manganese oxides, particularly Mn₂O₃. In the fresh mixed MnO_x–Pd and CuO_x–Pd samples, Pd catalysed the reduction of both MnO_x and CuO_x, whereas Pt only catalysed the reduction of MnO_x. After hydrothermal treatment at 900°C of the MnO_x, mixed MnO_x–Pt and MnO_x–Pd samples, there was a formation of new manganese oxide phase, Mn₃O₄ detected by Raman spectroscopy. TPR revealed increasing interaction between the metal oxides and the noble metals in the hydrothermally treated mixed MnO_x–Pd and CuO_x–Pd samples, and also the appearance of interaction in the treated mixed CuO_x–Pt sample. The sulphur adsorbed in all the MnO_x samples formed sulphate, which was more difficult to reduce than the oxides. Also, the reduction temperature of sulphates was lowered when noble metals are present.     

A study of nitrous oxide decomposition over calcium oxide

A study of N₂O decomposition reaction in a fixed bed a reactor over bed of CaO particles has been conducted. Effects of parameters such as concentration of inlet N₂O gas, reacting temperature and content of CO₂/ CO gas present in the reacting materials on the decomposition reaction have been investigated. The results showed that the conversion of N₂O decomposition was accelerated by the increase of reaction temperature, and the existence of CO, while the rate was hindered by the existence of CO₂. Heterogeneous gas solid reaction kinetics was proposed for N₂O decomposition and compared with homogeneous reaction kinetics. Presented at the Int’l Symp. on Chem. Eng. (Cheju, Feb. 8–10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

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.     

The influence of sulfur dioxide on propane oxidation activity over supported platinum and palladium

Earlier studies have shown that sulfur dioxide and metal-support interaction can strongly influence propane oxidation over platinum. In particular, oxidation activity is enhanced when platinum is supported on sulfated -alumina or zirconia compared to -alumina. Therefore, it is of interest to compare the performance of palladium under the same experimental conditions. Four model catalysts were examined: Pt/-alumina, Pt/zirconia, Pd/-alumina and Pd/zirconia. The metal loading was kept at or below 0.05 wt% to emphasize changes in activity attributable to metal-support interaction. Reaction rates were measured with and without sulfur dioxide. Surface sulfation was analyzed by measuring acid strength and evaluating spectra obtained by Fourier-transform infrared spectroscopy. In contrast to platinum, sulfation does not promote propane oxidation on Pd/-alumina, and Pd/zirconia is less active than Pd/-alumina.     

Carbon oxidation with platinum supported catalysts

The effect of the support oxide, Pt precursor and reactant gas composition on the catalysis of soot oxidation was investigated using carbon black as a model soot and simulated exhaust gases. The Pt precursors used were Pt(NH₃)₄(OH)₂, H₂PtCl₆·6H₂O, Pt(NH₃)₄(NO₃)₂, and Pt(NH₃)₄Cl₂. The support metal oxides used were SiO₂, Al₂O₃, and ZrO₂. Pt/SiO₂ prepared from Pt(NH₃)₄(OH)₂ showed the highest carbon oxidation activity. It had much higher activity in the condition of N₂+O₂+H₂O+NO+SO₂ than without NO and SO₂.     

Methanol electrooxidation on platinum microparticles electrodeposited on poly (o-methoxyaniline) films

In this paper we have studied the electrooxidation of methanol on electrodes obtained through the electrodeposition of platinum microparticles on poly (o-methoxyaniline) films. The dependence of the electroactivity on the electrodes preparation parameters has been detailed. It has been shown that the concentration of the monomer during the formation of the polymeric films influences the rate of polymer growth and the electrocatalytic activity. A maximum methanol electrooxidation peak current has been observed for polymers grown to an anodic voltammetric charge of approximately 75 mC cm⁻². It has been observed that the electroactivity increases when the platinum deposition is carried out in several steps instead of a single potential electrolysis. The conditions that favour a slower deposition process, i.e., smaller H₂PtCl₆ concentrations and smaller electrodeposition overpotential, lead to an enhancement of the methanol electrooxidation currents. This enhancement is attributed to an increase in the platinum surface area as a consequence of a decrease in the platinum particle size.     

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.