Catalytic oxidation: I. The oxidation of ethylene over Pd and PdAu alloys

UNTIL recently, nearly all successful catalysts used for the partial heterogeneous oxidation of ethylene contained silver as the active component1. In 1962, however, Kemball and Patterson2 reported that ethylene could be oxidized to acetic anhydride and acetic acid over evaporated palladium films, and several applications have since then appeared in the patent literature3. Kemball2 suggested that the partial oxidation products (which amounted to only about 3 per cent of the total) were formed by a path parallel to that for the complete oxidation to carbon dioxide. Acetaldehyde was proposed as the intermediate in the partial oxidation reaction, but none was detected, presumably because of its rapid oxidation to acetic anhydride. The purpose of the present work was to isolate and identify the proposed intermediate, and to determine whether higher selectivities might be obtained under more favourable reaction conditions.

Process modeling and design of the ethylene oxide production plant is mainly depended on the kinetics of the ethylene oxide production reactions. In this article, the kinetics equations for partial oxidation (epoxidation) of ethylene on silver catalyst were reviewed. There are three competitive reactions in this system: ethylene partial and total oxidation and ethylene oxide total oxidation. The reaction rate equations for these three reactions were compared and advantage and the disadvantage of kinetic models were discussed. Dichloroethane (DCE) is used in these reactions to increase the ethylene oxide selectivity. Therefore, the kinetics of the reactions considering the role of DCE is also reported. Most of the kinetics models have some weaknesses. However, one of the reviewed models was the complete model because of including all three reactions of partial and total oxidation of ethylene and total oxidation of ethylene oxide. Also, this model considered the concentration of selectivity promoter (DCE) and reverse reactions.

Catalytic partial oxidation of methane in supercritical water

The present work investigates both the diffusivity and permeability of hydrogen (H) in palladium-silver (PdAg) and palladium-gold (PdAu) alloys over a 400-1200 K temperature range for Pd(100-X)M(X), M=Ag or Au and X=0%-48% using density functional theory (DFT) and kinetic Monte Carlo simulations (KMC). DFT has been employed to obtain octahedral (O)-, tetrahedral (T)-, and transition state (TS)- site energetics as a function of local alloy composition for several PdAg and PdAu alloys with compositions in supercells of X=14.18%, 25.93%, 37.07%, and 48.15% with the nearest (NNs) and next nearest neighbors (NNNs) varied over the entire range of compositions. The estimates were then used to obtain a model relating the O, T, and TS energies of a given site with NN(X), NNN(X), and the lattice constant. The first passage approach combined with KMC simulations was used for the H diffusion coefficient predictions. It was found that the diffusion coefficient of H in PdAg alloy decreases with increasing Ag and increases with increasing temperature, matching closely with the experimental results reported in the literature. The calculated permeabilities of H in these novel binary alloys obtained from both diffusivity and solubility predictions were found to have a maximum at approximately 20% Ag and approximately 12% Au, which agree well with experimental predictions. Specifically, the permeability of H in PdAg alloy with approximately 20% Ag at 456 K is three to four times that of pure Pd, while the PdAu alloy at 12% Au is four to five times that of pure Pd at 456 K.

Acetaldehyde partial oxidation on the Au(111) model catalyst surface: C–C bond activation and formation of methyl acetate as an oxidative coupling product

Reactions of saturated hydrocarbons with hydrogen and deuterium on epitaxially oriented (111) Pd and Pd-Au alloy films

Active catalysts are typically metastable, and their surface state depends on the gas-phase chemical potential and reaction kinetics. To gain relevant insights into structure-performance relationships, it is essential to investigate catalysts under their operational conditions. Here, we use operando TEM combining real-time observations with online mass spectrometry (MS) to study a Cu catalyst during ethylene oxidation. We identify three distinct regimes characterized by varying structures and states that show different selectivities with temperature, and elucidate the reaction pathways with the aid of theoretical calculations. Our findings reveal that quasi-static Cu2O at low temperatures is selective towards ethylene oxide (EO) and acetaldehyde (AcH) via an oxometallacycle (OMC) pathway. In the dynamic Cu0/Cu2O oscillation regime at medium temperatures, partially reduced and strained oxides decrease the activation energies associated with partial oxidation. At high temperatures, the catalyst is predominantly Cu0, partially covered by a monolayer Cu2O. While Cu0 is extremely efficient in dehydrogenation and eventual combustion, the monolayer oxide favors direct EO formation. These results challenge conclusions drawn from ultra-high vacuum studies that suggested metallic copper would be a selective epoxidation catalyst and highlight the need for operando study under realistic conditions.

Experiments have been carried out at temperatures of 263° C and higher between oxygen adsorbed as atoms on the silver catalyst, and ethylene, ethylene oxide and acetaldehyde. The course of reaction was followed by measuring the change in pressure, and analyses of the products were made by micro-fractionation of the gases at low temperatures. In the reaction of ethylene with an oxygen-covered catalyst, the absence of an induction period in the pressure-time curve showed that oxidation of ethylene to carbon dioxide and water by a route not through ethylene oxide is possible. The reaction of acetaldehyde with the oxygenated catalyst was too fast to measure. The reactions of ethylene oxide were found to be complex, and reaction occurred both with the oxygenated and the clean catalyst. On a clean catalyst, ethylene oxide was simultaneously isomerized to acetaldehyde and converted back to ethylene and adsorbed oxygen; the acetaldehyde and adsorbed oxygen then reacted to form carbon dioxide and water. Both ethylene oxide and acetaldehyde, but not ethylene, were adsorbed with decomposition to form a non-volatile layer on the catalyst. This was composed of carbon, hydrogen and possibly oxygen, combined in indefinite and varying proportions. The kinetics of the reaction between ethylene and the adsorbed oxygen layer were measured. Throughout the course of any one reaction, the rate of oxidation to carbon dioxide was proportional to the square of the concentration of adsorbed oxygen, but the velocity constant depended on the initial concentration. The apparent energy of activation was 10 kcal. It is thought that when ethylene reacts with a single adsorbed oxygen atom, ethylene oxide is produced, and that with a pair of adsorbed oxygen atoms, intermediates such as formaldehyde are produced which react rapidly to form carbon dioxide and water.

Apparent activation energy for hydrogen permeation and its relation to the composition of homogeneous PdAu alloy thin-film membranes

Production of hydrogen by short contact time partial oxidation and oxidative steam reforming of propane

Kinetic hydrogen isotope effects in ethylene oxidation on silver catalysts

The interaction between heterogeneous and homogeneous chemistries is a crucial issue for high-temperature catalytic processes. In particular, the assessment of the main routes that control the selectivity to the desired products is essential for the design and safe operation of reaction units. This assessment is particularly true for the ultrafast conversion of hydrocarbons in short-contact-time (SCT) reactors that play a pivotal role in the effort to cope with the worldwide growing demand for more efficient exploitation of energy andmaterial resources. Examples are the catalytically assisted combustion for gas turbines with ultralow emissions, the catalytic partial oxidation (CPO) of hydrocarbons to H2 or CO/H2 mixtures (i.e., syngas), and the oxidative dehydrogenation (ODH) of light alkanes to olefins. As a common feature, these processes operate in autothermal and compact reactors, with noble-metal catalysts (palladium, rhodium, and platinum). An enormous energy intensity is peculiar to the SCT autothermal conversion of hydrocarbons over noble metals. Strongly exothermic and endothermic reactions proceed on the catalyst surface at extremely high rates. As a consequence, sharp gradients of temperature (up to 200 8Cmm ) and concentration are established within the small reactor volumes. Temperatures ranging from 250 to 1100 8C are generally experienced within a few millimeters. Such a level of severity—in terms of power density and extent of temperature and concentration gradients—is typical of flames and gas-phase oxidation processes in general. For a catalytic process, however, these conditions represent a thoroughly unconventional kinetic regime. To best grasp the intensity and the speed of the involved phenomena, we need to think of SCT conversion of light alkanes as the catalytic equivalent of a flame. This analogy can clearly depict the complexity of the process and emphasize the related scientific issues: To what extent does a catalytic process “stick” to the catalyst surface at these very high temperatures, where the adsorption of species is thermodynamically unfavored? Can the gas-phase activation of C H bonds (e.g., the formation and propagation of radicals) cooperate or compete with the catalytic process? In this respect, it is largely accepted that in the case of CH4 CPO over rhodium the gas-phase paths are negligible at atmospheric pressure and the catalytic route dominates. Conversely, the SCT-ODH of short alkanes over platinum proceeds mainly in the gas phase, thus giving rise to the production of olefins and other hydrocarbon species. On the basis of these examples, we could conclude that either catalytic or gas-phase chemistry governs the SCT conversion of hydrocarbons, depending only on the stability of the C H bond in the gas phase. Herein, by using novel techniques for collecting spatially resolved temperature and concentration profiles, we show that the partial oxidation of short-chain alkanes over rhodium breaks the paradigmatic compartments of heterogeneous processes and gas-phase processes, revealing the real complexity of these “flame-like” processes. Specifically, we examine the reaction of C3H8 CPO (C3H8+ 3/2O2!3CO+ 4H2) as a case study, and we apply novel techniques for collecting spatially resolved gas-phase and solid temperature and concentration profiles within a rhodium-coated honeycomb monolith to monitor the evolution of a propane/air mixture fed at high flow rate. The temperature and the composition of the reacting system were monitored from the inlet reactor section where the mixture was fed to the outlet section of the catalytic unit where the syngas stream was delivered. The results are reported in Figure 1 and Figure 2 as spatially resolved profiles of temperature and molar fraction of reactants and products. In the first 5 mm of the honeycomb, a sharp drop of O2 and C3H8 concentration was observed, accompanied by the formation of total oxidation products (CO2 and H2O) and partial oxidation products (H2 and CO). Correspondingly, a hot spot formed on the catalyst surface (980 8C, measured by the pyrometer) and a steep rise was observed in the gas temperature (up to 945 8C, measured by the thermocouple). In line with the occurrence of the endothermic steam reforming reaction, the evolution of H2O showed a maximum. Moreover, the temperature of the solid surface and in the gas phase decreased toward the exit of the honeycomb. Qualitatively, this behavior is what our and other research groups have observed also in the case of a CH4CPO experiment on rhodium, and can be fully explained by the catalytic production of syngas on rhodium. Thus, integral measurements (i.e., measurements of temperature and composition collected exclusively at the reactor outlet) would have suggested the unique existence of a heterogeneous process. A purely catalytic process was, for instance, [*] Dr. A. Donazzi, D. Livio, Dr. M. Maestri, Prof. A. Beretta, Prof. G. Groppi, Prof. E. Tronconi, Prof. P. Forzatti Laboratory of Catalysis and Catalytic Processes Dipartimento di Energia, Politecnico di Milano Piazza Leonardo da Vinci 32, 20133 Milano (Italy) Fax: (+39)0223993284 E-mail: alessandra.beretta@polimi.it Homepage: http://www.lccp.polimi.it

A single crystal study of the silver-catalysed selective oxidation and total oxidation of ethylene

Modeling spatially resolved data of methane catalytic partial oxidation on Rh foam catalyst at different inlet compositions and flowrates

A series of bimetallic Ga‐containing materials using TiO2 and TiO2‐promoted SiO2 supports have been prepared. Rhodium, palladium, and platinum have been used as additional metals in this system. The materials are characterized and used as catalysts for the partial oxidation of methane into synthesis gas (H2 and CO). The presence of a low quantity of titanium in the form of anatase TiO2 was shown to improve the overall activity of catalytic methane oxidation and to strongly increase the selectivity of partial oxidation products over the total oxidation of methane to carbon dioxide and water. Particular attention is paid to the formation of gallium‐metal alloys on the surface of the catalyst supports. Rh‐Ga‐Ti‐SiO2 was found to be the most active and selective catalyst, giving 89 % conversion of methane and 99 % selectivity to synthesis gas at 750 °C, as well as exhibiting catalytic activity and preferential conversion to partial oxidation products at temperatures as low as 350 °C.