Catalytic reactions on oxides

J. E. Germain, Structure-Sensitive Catalytic Reactions on Oxide Surfaces, in Adsorption and Catalysis on Oxide Surfaces G. C. Bond M. Che, Ed., Vol. 21, Elsevier Science Publishers, Amsterdam 1985, S. 355-368. 

E. Heterogeneous-Homogeneous Catalytic Reactions on Oxide Catalysts... 

A series of examples has become known recently, and more are reported in this volume, of catalytic reactions on oxide surfaces, involving electron transfer from reactant molecules to the catalyst, or vice versa. The general electronic concept of catalytic activation, first established for metals and alloys, has thus been extended to semiconductors. It appears certain that mobile quasi-free electrons or positive holes can migrate to the surface and can there bind reactant molecules in a charged or polarized state. This presupposes the presence of electrons in the conduction band (or of holes in the valence band), which in normal oxide semiconductors contains appreciable concentration of electrons only at elevated temperatures. Hence, the examples mentioned refer to high-temperature catalysis (N2O decomposition, CO oxidation). At ordinary temperatures, only those substances capable of releasing electrons from surface atoms or surface bonds, i.e., solid Lewis bases, are suitable as catalysts. This has been shown (I) to be true for the decomposition of ozone by various metal oxides. 

The following will describe the adsorption of a number of common molecules in catalytic reactions on oxide surfaces. 

Real reasons due to (a) the occurance of very fast (and therefore in most cases diffusion controlled) catalytic reactions on the electrode surface, (b) Formation of non-conducting carbonaceous or oxidic layers on the catalyst electrode surface. 

There is a third real reason for deviations from Eq. (5.18) in the case that a non-conductive insulating product layer is built via a catalytic reaction on the catalyst electrode surface (e.g. an insulating carbonaceous or oxidic layer). This is manifest by the fact that C2H4 oxidation under fuel-rich conditions has been found to cause deviations from Eq. (5.18) while H2 oxidation does not. A non-conducting layer can store electric charge and thus the basic Eq. 5.29 (which is equivalent to Eq. (5.18)) breaks down. 

Hybrid density functional calculations have been carried out for AU-O2, Au-CO, Aui3, AU13-O2, Au -CO, AU13-H2, and AU55 clusters to discuss the catalytic behavior of Au clusters with different sizes and structures for CO oxidation [179]. From these calculations, it was found that O2 and CO could adsorb onto several Au model systems. Especially, icosahedral Aun cluster has a relatively weak interaction with O2 while both icosahedral and cubooctahedral Aui3 clusters have interactions with CO. These findings suggest that the surfaces of the Au clusters are the active sites for the catalytic reactions on the supported and unsupported Au catalysts. 

Examples of Elementary Processes in Heterogeneous Catalytic Reactions on Metal Oxides... 

The complexity and inhomogenicity of catalytic sites of metals and metal oxides make it difficult to interpret the mechanism of catalytic reactions on solid surfaces. Investigations that may lead to a better characterization of adsorbed species on catalytic sites could add much to our understanding of heterogeneous catalysis. 

Figure 1.1 Schematic representation of a well known catalytic reaction, the oxidation of carbon monoxide on noble metal catalysts CO + Vi 02 —> C02. The catalytic cycle begins with the associative adsorption of CO and the dissociative adsorption of 02 on the surface. As adsorption is always exothermic, the potential energy decreases. Next CO and O combine to form an adsorbed C02 molecule, which represents the rate-determining step in the catalytic sequence. The adsorbed C02 molecule desorbs almost instantaneously, thereby liberating adsorption sites that are available for the following reaction cycle. This regeneration of sites distinguishes catalytic from stoichiometric reactions.

In these polymer-metal complexes of the Werner type, however, organometallic compounds are formed as reaction intermediates and/or activated complexes. As a result, the properties of polymer-metal catalysts in reductive reactions are different from those of polymer-metal catalysts in oxidative reactions. In the former, the catalytic reactions are very sensitive to moisture and air, and the complex catalysts often decompose in the presence of water and oxygen. Thus, reductive catalytic reactions are carried out under artificial conditions such as organic solvent, high pressure, and high temperature. Oxidative catalytic reactions, on the other hand, proceed under mild conditions aqueous solution, oxygen atmosphere, and room temperature. Therefore, it is to be expected that the catalytic effects of a polymer ligand will differ from the latter to the former. 

When titanium oxides are irradiated with UV light that is greater than the band-gap energy of the catalyst (about X < 380 nm), electrons (e ) and holes (h+) are produced in the conduction and valence bands, respectively. These electrons and holes have a high reductive potential and oxidative potential, respectively, which, together, cause catalytic reactions on the surfaces namely photocatalytic reactions are induced. Because of its similarity with the mechanism observed with photosynthesis in green plants, photocatalysis may also be referred to as artificial photosynthesis [1-4]. As will be introduced in a later section, there are no limits to the possibilities and applications of titanium oxide photocatalysts as environmentally harmonious catalysts and/or sustainable green chemical systems. ... 

All the above species have been detected in various quantities at oxide surfaces. The discussion of this example serves mainly to show that catalytic reactions at oxide surfaces are very complex. This is a mixed blessing from the sensing point of view. It provides a broad spectrum of reactions that could be used. On the other hand, it can lead to great variation in the results obtained with only slightly different sensors. Another drawback of such a complex and diverse mechanism is the relatively slow time response which, in most cases, is limited by the rates of the chemical reactions (Fig. 8.10). Naturally, one tendency of the current research in this field is to increase the selectivity of the surface reactions by introducing additional catalytic control, for example, by incorporation of catalytic metals, metal clusters, and other surface modifiers. 

McBride and Hall (37,38) reported the first observation of a controlled catalytic reaction on alumina using IETS. They studied the catalytically induced transfer hydrogenation from water vapor to unsaturated hydrocarbon chains chemisorbed on alumina at both ends of the chain. They absorbed muconic acid ( trans-trans-1,3 butadiene 1,4 dicarboxylic acid, HOOC-CH=CH-CH=CH-COOH ) onto oxidized aluminum strips using the liquid doping technique. The samples were returned to the vacuum system, and in the presence of 0.3 torr of D2O vapor, heated to up to 400° C by passing current through a heater strip evaporated on the back of the glass slide. The films were then allowed to cool and the junctions completed by evaporation of the Pb counter electrode. 

In catalytic reactions on metals a decrease in activation energy under UV illumi-tion is also observed 97 "). In experiments on CO oxidation on evaporated Ag, Au, Pd and Pt films 98 a decrease is observed in the activation energy due to... 

Abstract. The object of this work was to investigate hydrogen oxidation and oxygen reduction from surface of modified nanodispersed diamond. The catalytic reactions on the surface of nanodispersed diamond have been studied after different treatment using heat-treated and electrochemical methods. 

We have seen in the preceding section that the strategy to investigate catalytic reactions on model catalysts is rather complex and needs to combine several surface science techniques. In practice all these techniques are not available in the same experimental set-up, then it is necessary to have well controlled preparation techniques to reproduce the same collection in different vacuum chambers. In fact this goal is difficult to reach without a complete knowledge of the nucleation and growth process of the metal particles on the considered oxide surface. Even in this case the density of particle number density (and then the particle size) can change from sample to sample due the different number of... 

Enzyme-based biosensors are very suitable for the antioxidant status evaluation, since they show excellent selectivity for biological substances and can directly determine and/or monitor antioxidant compounds in a complex media such as biological or vegetable samples without needing a prior separation step. During the course of the catalytic reaction on the electroactive substrates, the current produced at an applied potential is related to the concentration of a specific biomarker, for which the biosensor is selective. HRP-based biosensors for antioxidant status evaluation have been applied in the detection of superoxide radical [119], nitric oxide [120], glutathione [119, 121], uric acid [122, 123], and phenolic compounds [124—126],... 

When oxidizing iron and manganese using dissolved oxygen, the process is usually carried out under catalytic reaction on some contact surfaces. To accomplish... 

The further step of monitoring a dynamic catalytic reaction on a surface is barely approachable with existing surface science method by using molecular beam techniques, or by isolating a model catalysts prepared and analysed in UHV in a reactor appended to the UHV system. Although such studies apparently have not been performed for ceria supported model catalysts, it is appropriate to mention reactor studies of CO oxidation and water gas shift d reactions performed... 

The experiments of Rosser, Inami and Wise [57] were the continuation of their w ork on catalytic decomposition of ammonium nitrate [74]. They examined the action of copper chromite. They found that it acted at the early stage of the reaction and its action disappeared after copper diromiie was oxidized by the products of catalytic reaction. Cobalt oxide was found to be an exceptional catalyst it produced NOCl and NO2CI as major products and only a trace quantity of N2O3. Tlic authors came to the conclusion that copper chromite catalysed thermal decomposition of AP according to an electron transfer mechanism (4). 

Catalytic Reactions on Uranium and Titanium-Oxide Surfaces... 

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