Performance and characteristics of platinum, palladium/rhodium and palladium automotive catalysts when subjected to ethanol, acetaldehyde and a synthetic E85 exhaust gas mixture

The performance optimisation of automotive catalysts has been the focus of a great deal of research for many years as the automotive industry has endeavored to reduce the emission of toxic and pollutant gases generated from internal combustion engines. Just as the emissions from diesel and gasoline combustion vary so do the emissions from combustion of alternative fuels such as ethanol; the variation is in both quantity and chemical composition. In particular, when ethanol is contained in the fuel, ethanol and acetaldehyde are present in the exhaust gas stream and these are two compounds which the catalytic converter has not traditionally been designed to manage. The aim of the study outlined in this paper was to assess the performance of various catalyst formulations when subjected to a representative ethanol exhaust gas mixture. Three automotive catalytic converter formulations were tested including a fully Pt sample, a PdRh three-way catalyst sample and a fully Pd sample. Initially the samples were tested using single component hydrocarbon light-off tests followed by a set of tests with carbon monoxide included as an inlet gas to observe its effect on each individual hydrocarbon oxidation. Finally, each formulation was tested using a full E85 exhaust gas mixture. The study was carried out using a synthetic gas reactor along with FTIR and FID exhaust gas analysers. All formulations showed selectivity toward acetaldehyde formation from ethanol dehydrogenation which resulted in negative acetaldehyde conversion across each of the samples during the mixture tests. The fully Pt sample was the most detrimentally affected by the introduction of carbon monoxide into the gas feed. The Pd and PdRh samples exhibited a tendency toward acetaldehyde decomposition resulting in methane and carbon monoxide formation. The Pt sample did not form methane but did form ethylene as a result of ethanol dehydration.

Development of the kinetic model of platinum catalyzed ammonia oxidation in a microreactor

The ammonia oxidation reaction on supported polycrystalline platinum catalyst was investigated in an aluminum-based microreactor. An extensive set of reactions was included in the chemical reactor modeling to facilitate the construction of a kinetic model capable of satisfactory predictions for a wide range of conditions (NH3 partial pressure, 0.01-0.12 atm; O-2 partial pressure, 0.10-0.88 atm; temperature, 523-673 K; contact time, 0.3-0.7 ms). The elementary surface reactions used in developing the mechanism were chosen based on the literature data concerning ammonia oxidation on a Pt catalyst. Parameter estimates for the kinetic model were obtained using multi-response least squares regression analysis using the isothermal plug-flow reactor approximation. To evaluate the model, the behavior of a microstructured reactor was simulated by means of a complete Navier-Stokes model accounting for the reactions on the catalyst surface and the effect of temperature on the physico-chemical properties of the reacting mixture. In this way, the effect of the catalytic wall temperature non-uniformity and the effect of a boundary layer on the ammonia conversion and selectivity were examined. After further optimization of appropriate kinetic parameters, the calculated selectivities and product yields agree very well with the values actually measured in the microreactor. (C) 2002 Elsevier Science B.V. All rights reserved.

A kinetic study of ammonia oxidation on a Pt catalyst in the explosive region in a microstructured reactor/heat-exchanger

The application of an aluminum-based microstructured reactor/heat-exchanger for measuring reaction kinetics in the explosive region is presented. Platinum-catalyzed ammonia oxidation was chosen as a test reaction to demonstrate the feasibility of the method. The reaction kinetics was investigated in a wide range of conditions [NH3 partial pressure: 0.03-0.20 atm, O-2 partial pressure: 0.10-0.88atm; reactant flow 2000-3000 cm(3) min(-1) (STP); temperature 240-360degreesC] over a supported Pt/Al2O3 catalyst (mass of Al2O3 layer in the reactor, 1.95 mg; Pt/Al molar ratio, 0.71; Pt dispersion, 20%). The maximum temperature non-uniformity in the microstructured reactor was ca. 5degreesC, even at conditions corresponding to an adiabatic temperature rise of 1400degreesC. Based on the data obtained, a previous kinetic model for ammonia oxidation was extended. The modified 13-step model describes the data in a considerably wider range of conditions including those with high ammonia loadings and high reaction temperatures. The results indicate the large potential of microstructured devices as reliable tools for kinetic research of highly exothermic reactions.

OXIDATION OF CHLORIDE TO CHLORINE BY CERIC IONS MEDIATED BY DIFFERENT CARBON-BLACKS

A number of different carbon blacks are tested for activity as chlorine catalysts in the oxidation of chloride (2 mol dm-3 in 0.5 mol dm-3 H2SO4) to chlorine by Ce(IV) ions, that is,

Tetrahexahedral Pt Nanocrystal Catalysts Decorated with Ru Adatoms and Their Enhanced Activity in Methanol Electrooxidation

Tetrahexahedral Pt nanocrystals (THH Pt NCs) bound by well-defined high index crystal planes offer exceptional electrocatalytic activity, owing to a high density of low-coordination surface Pt sites. We report, herein, on methanol electrooxidation at THH Pt NC electrodes studied by a combination of electrochemical techniques and in situ FTIR spectroscopy. Pure THH Pt NC surfaces readily facilitate the dissociative chemisorption of methanol leading to poisoning by strongly adsorbed CO. Decoration of the stepped surfaces by Ru adatoms increases the tolerance to poisoning and thereby reduces the onset potential for methanol oxidation by over 100 mV. The Ru modified THH Pt NCs exhibit greatly superior catalytic currents and CO2 yields in the low potential range, when compared with a commercial PtRu alloy nanoparticle catalyst. These results are of fundamental importance in terms of model nanoparticle electrocatalytic systems of stepped surfaces and also have practical significance in the development of surface tailored, direct methanol fuel cell catalysts.

Selective Hydrogenation of alpha,beta-Unsaturated Aldehydes and Ketones using Novel Manganese Oxide and Platinum Supported on Manganese Oxide Octahedral Molecular Sieves as Catalysts

The selective hydrogenation of ,-unsaturated aldehydes and ketones has been studied using ketoisophorone and cinnamaldehyde as model substrates using manganese oxide octahedral molecular sieve (OMS-2) based catalysts. For the first time, OMS-2 has been shown to be an efficient and selective hydrogenation catalyst. High selectivities for either the CC or CO double bond at approximate to 100% conversion were achieved by using OMS-2 and platinum supported on OMS-2 catalysts. Density functional theory (DFT) calculations showed that the dissociation of H2 on OMS-2 was water assisted and occurred on the surface Mn of OMS-2(001) that had been modified by an adsorbed H2O molecule. The theoretically calculated activation barrier was in good agreement with the experimentally determined value for the hydrogenation reactions, indicating that H2 dissociation on OMS-2 is likely to be the rate-determining step. A significant increase in the rate of reaction was observed in the presence of Pt as a result of the enhancement of H2 dissociative adsorption and subsequent reaction on the Pt or spillover of the hydrogen to the OMS-2 support. The relative adsorption strengths of ketoisophorone and cinnamaldehyde on the OMS-2 support compared with the Pt were found to determine the product selectivity.

A more direct way to make catalysts: one-pot ligand-assisted aerobic stripping and electrodeposition of copper on graphite

Electrodeposition of metals onto conductive supports such as graphite potentially provides a lower-waste method to form heterogeneous catalysts than the standard methods such as wet impregnation. Copper electrodeposition onto pressed graphite disc electrodes was investigated from aqueous CuSO4-ethylenediamine solutions by chronoamperometry with scanning electron microscopy used to ascertain the particle sizes obtained by this method. The particle size was studied as a function of pH, CuSO4-ethylenediamine concentration, and electrodeposition time. It was observed that decreasing the pH, copper-ethylenediamine concentration and time each decreased the size of the copper particles observed, with the smallest obtained being around 5-20 nm. Furthermore, electroless aerobic oxidation of copper metal in the presence of ethylenediamine was successfully coupled with the electrodeposition in the same vessel. In this way, deposition was achieved sequentially on up to twenty different graphite discs using the same ethylenediamine solution, demonstrating the recyclability of the ligand. The materials thus prepared were shown to be catalytically active for the mineralisation of phenol by hydrogen peroxide. Overall, the results provide a proof-of-principle that by making use of aerobic oxidation coupled with electrochemical deposition, elemental base metals can be used directly as starting materials to form heterogeneous catalysts without the need to use metal salts as catalyst precursors.

Low-temperature oxidation reactions of ethane over a Pt/Al2O3 catalyst

Oxidative dehydrogenation of ethane was performed under conventional microreactor and TAP reactor conditions over a Pt/Al2O3 catalyst between 100 and 600 degreesC. During TAP studies, no ethene was produced whereas under flow conditions small but significant ethene formation was observed. This is consistent with a mechanism involving the gas-phase production of ethene rather than via a surface reaction. In comparison, both hydrogen and methane formation were found under TAP conditions and the trends with temperature and surface oxide composition are interpreted in terms of successive dehydrogenation steps on the catalyst surface. It is further observed that periodic introduction of the reactants can minimize deactivation processes. (C) 2003 Elsevier Inc. All rights reserved.

Methanol oxidation on PtRu electrodes. Influence of surface structure and Pt-Ru atom distribution

The activities of different types of PtRu catalysts for methanol oxidation are compared. Materials used were: UHV-cleaned PtRu alloys, UHV-evaporated Ru onto Pt(111) as well as adsorbed Ru on Pt(111) prepared with and without additional reduction by hydrogen. Differences in the catalytic activity are observed to depend on the preparation procedure of the catalysts. The dependence of the respective catalytic activities upon the surface composition is reported. UHV-STM data for Pt(111)/Ru show the formation of two- and three-dimensional structures depending on surface coverage. A molecular insight on the electrochemical reaction is given via in situ infrared spectroscopy. Analysis of the data indicates that the most probable rate-determining step is the reaction of adsorbed CO with Ru oxide.

Critical role of water in the direct oxidation of CO and hydrocarbons in diesel exhaust after treatment catalysis

CO and C3H6 oxidation have been carried out in the absence and presence of water over a Pd/Al2O3catalyst. It is clear that water promotes CO and, as a consequence, C3H6oxidation takes place at muchlower temperatures compared with the dry feed. The significant increase in the catalyst’s activity withrespect to CO oxidation is not simply associated with changes in surface concentration as a result ofcompetitive adsorption effects. Utilising18O2as the reactant allows the pathways whereby the oxidationdue to gaseous dioxygen and where the water activates the CO and C3H6to be distinguished. In thepresence of water, the predominant pathway is via water activation with C16O2and C16O18O being themajor species formed and oxidation with dioxygen plays a secondary role. The importance of wateractivation is further supported by the significant decrease in its effect when using D2O versus H2O.

Application of heterogeneous catalysts prepared by mechanochemical synthesis

Mechanochemical synthesis has the potential to provide more sustainable preparative routes to catalysts than the current multistep solvent-based routes. In this review, the mechanochemical synthesis of catalysts is discussed, with emphasis placed on catalysts for environmental, energy and chemical synthesis applications. This includes the formation of mixed-metal oxides as well as the process of dispersing metals onto solid supports. In most cases the process involves no solvent. Encouragingly, there are several examples where the process is advantageous compared with the more normal solvent-based methods. This can be because of process cost or simplicity, or, notably, where it provides more active/selective catalysts than those made by conventional wet chemical methods. The need for greater, and more systematic, exploration of this currently unconventional approach to catalyst synthesis is highlighted.

Low loading platinum nanoparticles on reduced graphene oxide-supported tungsten carbide crystallites as a highly active electrocatalyst for methanol oxidation

In this study, low loading platinum nanoparticles (Pt NPs) have been highly dispersed on reduced graphene oxide-supported WC nanocrystallites (Pt-WC/RGO) via program-controlled reduction-carburization technique and microwave-assisted method. The scanning electron microscopy and transmission electron microscopy results show that WC nanocrystallites are homogeneously decorated on RGO, and Pt NPs with a size of ca. 3 nm are dispersed on both RGO and WC. The prepared Pt-WC/RGO is used as an electrocatalyst for methanol oxidation reaction (MOR). Compared with the Pt/RGO, commercial carbon-supported Pt (Pt/C) and PtRu alloy (PtRu/C) electrocatalysts, the Pt-WC/RGO composites demonstrate higher electrochemical active surface area and excellent electrocatalytic activity toward the methanol oxidation, such as better tolerance toward CO, higher peak current density, lower onset potential and long-term stability, which could be attributed to the characterized RGO support, highly dispersed Pt NPs and WC nanocrystallites and the valid synergistic effect resulted from the increased interface between WC and Pt. The present work proves that Pt-WC/RGO composites could be a promising alternative catalyst for direct methanol fuel cells where WC plays the important role as a functional additive in preparing Pt-based catalysts because of its CO tolerance and lower price. 

Unraveling the synergy between gold nanoparticles and chromium- hydrotalcites in aerobic oxidation of alcohols

The combination of gold nanoparticles (AuNPs) with chromium-substituted hydrotalcite (Cr-HT) supports makes very efficient heterogeneous catalysts (Au/Cr-HT) for aerobic alcohol oxidation under soluble-base-free conditions. The Au-support synergy increases with increasing Cr content of the support and decreasing AuNP size. In situ UV-Raman, X-ray absorption and photoelectron spectroscopic studies firmly establish that the strong Au-Cr synergy is related to a Cr ↔ Cr redox cycle at the Au/Cr-HT interface, where O activation takes place accompanied by electron transfer from Cr-HT to Au. The interfacial Cr species can be reduced by surface Au-H hydride and negative-charged Au species to close the catalytic cycle. A study of kinetic isotope effect indicates that alcohol O-H cleavage is facilitated by the presence of Cr, making a-C-H bond cleavage step more rate-controlling. Accordingly, a dual synergistic effect of Au/Cr-HT catalysts on the activation of O2 and alcohol reactants is proposed.