Metal catalysts compensation effect

One of the most important parameters is the temperature, the starting temperature, and the initial reaction rate. One can determine experimentally the initial reaction rate after elimination of diffusion and mass transport effects and then determine the Arrhenius constants, which depend on the temperature. The collision factor (ko) and activation energy (E) parameters influence significantly the activity pattern and selectivity. Figure 3.1 illustrates the influence of the temperature on these parameters for different reactions and metallic catalysts. This effect is known as compensation effect, although empirically there are attempts on theoretical interpretations for different heterogeneous systems [1, 2]. 

Thus, as will be shown in this book, the effect of electrochemical promotion (EP), or NEMCA, or in situ controlled promotion (ICP), is due to an electrochemically induced and controlled migration (backspillover) of ions from the solid electrolyte onto the gas-exposed, that is, catalytically active, surface of metal electrodes. It is these ions which, accompanied by their compensating (screening) charge in the metal, form an effective electrochemical double layer on the gas-exposed catalyst surface (Fig. 1.5), change its work function and affect the catalytic phenomena taking place there in a very pronounced, reversible, and controlled manner. 

The points for Ag and Pd-Ag alloys lie on the same straight line, a compensation effect, but the pure Pd point lies above the Pd-Ag line. In fact, the point for pure Pd lies on the line for Pd-Rh alloys, whereas the other pure metal in this series, i.e., rhodium is anomalous, falling well below the Pd-Rh line. Examination of the many compensation effect plots given in Bond s Catalysis by Metals (155) shows that often one or other of the pure metals in a series of catalysts consisting of two metals and their alloys falls off the plot. Examples include CO oxidation and formic acid decomposition over Pd-Au catalysts, parahydrogen conversion (Pt-Cu) and the hydrogenation of acetylene (Cu-Ni, Co-Ni), ethylene (Pt-Cu), and benzene (Cu-Ni). In some cases, where alloy catalysts containing only a small addition of the second component have been studied, then such catalysts are also found to be anomalous, like the pure metal which they approximate in composition. 

McKee (21, 195) and McKee and Norton (219, 249, 250) have reported compensation effects in the exchange reactions of methane on several pairs of binary noble metal alloy catalysts. For each combination of elements kinetic measurements were made at a number of different compositions. Although the compensation behavior was generally very similar, there were perceptible differences in the values of B and e calculated for the various alloy combinations. The parameters found, by use of the formulas given in Appendix II, are summarized in Table IV, A-E, and are subject to the following comments. In consideration of data for the Pd-Rh alloys, the point for... 

Compensation effects have been reported for the oxidation of ethylene on Pd-Ru and on Pd-Ag alloys (207, 254, 255) discussion of the activity patterns for these catalysts includes consideration of the influence of hydrogen dissolved in the metal on the occupancy of energy bands. Arrhenius parameters reported (208) for ethylene oxidation on Pd-Au alloys were an appreciable distance from the line calculated for oxidation reactions on palladium and platinum metals (Table III, H). Oxidation of carbon monoxide on Pd-Au alloys also exhibits a compensation effect (256). 

This paper focuses on the influence of the support on the H/D exchange of CP over supported Pt catalysts. It will be shown that kinetics and selectivities are largely affected by the support material. Particle size effects are separated from support effects. The activity shows a compensation effect, and the apparent activation energy and pre-exponential factor show an isokinetic relationship . This can be explained by different adsorption modes of the CP on the metallic Pt surface. The change in adsorption modes is attributed to a change in the electronic structure of the Pt particles, which in turn is induced by changes in the acid/base properties of the support. 

The support induced changes in hydrogenolysis reactions of alkanes can be explained to a large extent by support induced changes in the Pt-H bond strength and hydrogen adsorption site on Pt. This can easily explain the well-known compensation effect found in the kinetics of the hydrogenolysis of alkanes catalyzed by supported metal catalysts. 

Summary Both in the Rochow synthesis of methylchlorosilanes and in the reaction of transition metal silicides with HCl, catalytic reactions of silicon, bound as metal silicide, with gaseous reactants are involved. With both reactions, the kinetic parameters ko and Ea exhibit consequent compensation effects, with the isokinetic temperature positioned within the range of reaction temperatures investigated. In this paper, we ply the model of selective energy transfer fiorn the catalyst to adsorbed species to the kinetic data. With Rochow synthesis Si-CHs rocking frequencies, and with hydrochlorination of silicides Si—H vibration frequencies could correspond to the isokinetic temperatures observed. An interpretation in terms of accessibility of the reactive silicon atom to reactant molecules is given. 

The basis for a common interpretation of the two compensation effects should be the control of the reactivity of silicon atoms by the nature of neighboring metal atoms as catalysts or promoters as well as by structural or morphological properties of the silicide phases involved. The reactivity of silicon atoms can vary in dependence on such influences however, the essential step of the reactions is independent of them. The variation of the reactivity of silicon atoms with their environment always results in compensation behavior. 

Galwey AK, Bettany DG, Mortimer M (2006) Kinetic compensation effects observed during oxidation of carbon monoxide on y-alumina supported palladium, platinum, and rhodium metal catalysts toward a mechanistic explanation. Int J Chem Kin 38 689... 

Figure 7.4. Compensation effect for the methanation reaction. The logarithm of the preex-pontial factor is plotted againt the apparent activation energy, A , for this reaction over several transition-metal catalysts [18].

Arrhenius parameters for nickel carbide hydrogenation 162) is close to both lines on Fig. 3. Compensation behavior for reactions on the carbide phase must include an additional feature in the postulated equilibria, to explain the removal of excess deposited carbon, if the active surface is not to be poisoned completely. The relative reduction in the effective active area of the catalyst accounts for the lower rates of reaction on nickel carbide, and the difference in the compensation line from that of the metal (Fig. 3) is identified as a consequence of the poisoning-regeneration process. After any change in reaction conditions, a period of reestablishment of surface equilibria was required before a new constant reaction rate was attained (22). 

Our results show that, as before, the attachment of the metal complex to the polymeric support has little effect on the yields of reaction compared to the homogeneous analogue, any small decrease in yield being more than compensated by the ease of removal of the catalyst from the product mixture. To show that 3 can be recycled a number of times, the oxidation of 1-phenylethanol to benzaldehyde was repeated five times using the same batch of supported catalyst. As seen in Table 2, the yields remain around 85% clearly illustrating the re-usability of the catalyst. 

A significant drop in catalytic activity for catalytic combustion of methane due to the above-mentioned PdO decomposition or the inability of metallic Pd to chemisorb oxygen above 650 C, however, can be effectively avoided by using a catalytically more active Mn-substituted hexa-aluminate (X = Mn) as a catalyst support [71]. The catalytic activity of this Mn-hexa-aluminate compensates for the drop in activity of Pd so that a stable combustion reaction can be attained in a whole temperature range. Thus, the use of catalytically active support materials is one possible solution to overcome the unstable... 

Thus, dispersion of Me/C catalysts prepared by impregnation is in many respects dependent on the probability of side processes such as the metal compounds adsorption, even though their intensity is low that makes the proportion of the adsorbed species negligible among the supported precursors. In the general case, the detrimental effect on the metal dispersion caused by deficit of the sites for crystallization of the metal precursors can be compensated by means of repeated impregnation with low-concentrated salt solutions. An example is the preparation of Pt/C from Pt(NH3)2(N02)2 [185]. 

Metal effects. The Ti- and Hf-based catalysts are remarkably less active than the corresponding Zr-based catalysts (see Figure 46). For Ti-based catalysts, the lower activity is compensated by a remarkable increase in the molecular mass. Instead, the Hf-based catalysts yield molecular masses comparable to those obtained with the corresponding Zr-based catalyst.1149... 

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