Dissociative adsorption, hydrogen

From the H/M values for the catalysts NiSn-BM (Sn/Ni = 0.29) and PtSn-BM (Sn/Pt = 0.71), and the H/M values for the corresponding monometallic ones, it can be inferred that Sn blocks about 70% of the originally accessible M atoms. For these systems, based on the dispersion measured for Pt and Ni, the atomic ratios Sn/M correspond to values higher than 1. Notably, even in these cases, an important portion of the metallic surface has sites accessible to hydrogen dissociative adsorption, which is essential for the phase to be active in hydrogenation reactions. 

In any of the schemes presented, hydrogen dissociative adsorption on Pt is possible after Sn addition, as was checked by hydrogen chemisorption. From these results, it is possible to think of a scheme to represent the main reaction pathway during the hydrogenation of a,(3-unsaturated aldehydes. Depending on the catalyst used, such a scheme is shown in Figure 6.12, which summarizes the results from the characterizations and catalytic tests performed in this work [47]. 

This temperature corresponds well with the desorption temperature observed in this study. Therefore the propene desorption peak observed here must be from the recombination of an adsorbed ir-allyl and hydrogen. Dissociative adsorption of propene requires a Zn-0 pair site which is abundant on the nonpolar surface. That the 0-polar surface, but not the Zn-polar surface adsorbs propene in this manner suggests that surface defects which expose the nonpolar surfaces (or other Zn-0 pairs) exist more abundantly on the 0-polar than on the Zn-polar surface. 

The first mechanism involved direct dehydrogenation of gas phase molecule to propylene on Pt surface, hydrogen dissociative adsorption, and propylene desorption ... 

A general conclusion regarding H2 adsorption on alkali modified metal surfaces is that alkali addition results in a pronounced decrease of the dissociation adsorption rate of hydrogen as well as of the saturation coverage. 

On K modified Ni(100) and Ni(lll)62,63 and Pt(lll)64 the dissociative adsorption of hydrogen is almost completely inhibited for potassium coverages above 0.1. This would imply that H behaves as an electron donor. On the other hand the peaks of the hydrogen TPD spectra shift to higher temperatures with increasing alkali coverage, as shown in Fig. 2.22a for K/Ni(lll), which would imply an electron acceptor behaviour for the chemisorbed H. Furthermore, as deduced from analysis of the TPD spectra, both the pre-exponential factor and the activation energy for desorption... 

The influence of electronegative additives on the CO hydrogenation reaction corresponds mainly to a reduction in the overall catalyst activity.131 This is shown for example in Fig. 2.42 which compares the steady-state methanation activities of Ni, Co, Fe and Ru catalysts relative to their fresh, unpoisoned activities as a function of gas phase H2S concentration. The distribution of the reaction products is also affected, leading to an increase in the relative amount of higher unsaturated hydrocarbons at the expense of methane formation.6 Model kinetic studies of the effect of sulfur on the methanation reaction on Ni(lOO)132,135 and Ru(OOl)133,134 at near atmospheric pressure attribute this behavior to the inhibition effect of sulfur to the dissociative adsorption rate of hydrogen but also to the drastic decrease in the... 

The volcano curve is bounded by the rate of dissociative adsorption of CO and hydrogenation of adsorbed carbon. This is illustrated in Figure 1.9. 

For example, consider the dissociative adsorption of methane on a Ni(lOO) surface. If the experiment is performed above 350 K, methane dissociates into carbon atoms and hydrogen that desorbs instantaneously. Consequently, one determines the uptake by measuring (e.g. with Auger electron spectroscopy) how much carbon is deposited after exposure of the surface to a certain amount of methane. A plot of the resulting carbon coverage against the methane exposure represents the uptake curve. 

Methane reforming with carbon dioxide proceeds in a complex sequence of reaction steps involving the dissociative adsorption/reaction of methane and COj at metal sites. Hydrogen is generated during methane dissociation In the second set of reactions CO2 dissociates into CO and adsorbed oxygen. The reaction between the surface bound carbon (from methane dissociation) and the adsorbed oxygen (from CO2 dissociation ) yields carbon monoxide. A stable catalyst can only be achieved if the two sets of reactions are balanced. 

In our previous work [11], it has been shown that the reduction of NO with CH4 on Ga and ln/H-ZSM-5 catalysts proceeds through the reactions (1) and (2), and that CH4 was hardly activated by NO in the absence of oxygen on these catalysts. Therefore, NO2 plays an important role and the formation of NO2 is a necessary step for the reduction of NO with CH4. In the works of Li and Armor [17] and Cowan et al. [18], the rate-determining step in NO reduction with CH4 on Co-ferrierite and Co-ZSM-5 catalysts is involved in the dissociative adsorption of CH4, and the adsorbed NO2 facilitates the step to break the carbon-hydrogen bond in CH4. It is suggested that NO reduction by use of CH4 needs the formation of the adsorbed NO2, which can activate CH4. 

Mills G, Jonsson H, Schenter GK. 1995. Reversible work transition state theory application to dissociative adsorption of hydrogen. Surf Sci 324 305-337. 

A discussion along this line has been made in regard to the orientation of the hydrogen molecule in the dissociative adsorption on metals 82>. Thus, the interpretation of the function of heterogeneous catalysis on a molecular basis is no longer beyond our reach. The important role of LU MO in the process of polarographic reductions has also been discussed... 

Based on the assumption that hydrogen atoms are adsorded in pairs, instead of the expression of dissociative hydrogen atom adsorption, it s adequate to use H2 adsorption as the fractional surface coverage for the rate equation. 

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