Adsorption of hydrogen

Topsoe and co-workers have reported that for two ammonia synthesis catalysts containing iron particles of 6 and 40-60 nm, an equal extent of adsorption 

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Miehelsen H A, Rettner C T and Auerbaeh D J 1993 The adsorption of hydrogen at eopper surfaees A model system for the study of aetivated adsorption Surface Reacf/onsed R J Madix (Berlin Springer) p 123... 

Conrad FI, ErtI G and Latta E E 1974 Adsorption of hydrogen on palladium single crystal surfaces Surf. Sc/41 435-46... 

Olsen R A, Philipsen P H T, Baerends E J, Kroes G J and Louvik O M 1997 Direct subsurface adsorption of hydrogen on Pd(111) quantum mechanical calculations on a new two-dimensional potential energy surfaced. Chem. Phys. 106 9286... 

Adsorption Processes. The processes based on adsorption of hydrogen sulfide onto a fixed bed of soHd material are among the oldest types of gas treating appHcations (4). Two common sorbent materials for low concentration gas streams are iron oxide and zinc oxide. 

Catalysis and the Adsorption of Hydrogen on Metal Catalysts Otto Beeck... 

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... 

In the same spirit DFT studies on peroxo-complexes in titanosilicalite-1 catalyst were performed [3]. This topic was selected since Ti-containing porous silicates exhibited excellent catalytic activities in the oxidation of various organic compounds in the presence of hydrogen peroxide under mild conditions. Catalytic reactions include epoxidation of alkenes, oxidation of alkanes, alcohols, amines, hydroxylation of aromatics, and ammoximation of ketones. The studies comprised detailed analysis of the activated adsorption of hydrogen peroxide with... 

The fits indicate that the Langmuir-Hinshelwood model describes the measurements very well. The equilibrium constants point to a relatively strong adsorption of thiophene and, in particular, H2S, while adsorption of hydrogen is weak. Hence the term K may safely be ignored in Eq. (32). The order in H2 is 0.93, i.e. close to one, which is another indication that hydrogen adsorbs only weakly. 

As the adsorption of hydrogen is rather weak, the corresponding term in the denominator may be omitted. The rate expression shows that the reaction is suppressed by H2S. Hence, the most active catalysts (which appear at the top of the volcano curve of Fig. 9.7)... 

Such information can be obtained from cyclic voltammetric measme-ments. It is possible to determine the quantity of electricity involved in the adsorption of hydrogen, or for the electrooxidation of previously adsorbed CO, and then to estimate the real surface area and the roughness factor (y) of a R-C electrode. From the real surface area and the R loading, it is possible to estimate the specific surface area, S (in m g ), as follows ... 

Pritchard, J. and Tompkins, F.C. (1960) Surface-potential measurements. Adsorption of hydrogen by Group IB metals. Transactions of the Faraday, Society, 56, 540-550. 

It is most convenient to explain catalysis using an example. We have chosen a hydrogenation catalysed by nickel in the metallic state. According to the schematic of Fig. 3.1 the first step in the actual catalysis is adsorption . It is useful to distinguish physisorption and chemisorption . In the former case weak, physical forces and in the latter case relatively strong, chemical forces play a role. When the molecules adsorb at an active site physisorption or chemisorption can occur. In catalysis often physisorption followed by chemisorption is the start of the catalytic cycle. This can be understood from Fig. 3.2, which illustrates the adsorption of hydrogen on a nickel surface. 

Figure 3.2. Potential energy diagram of chemisorption for the adsorption of hydrogen on nickel (after Le Page, 1987).

Parsons R. 1958. The rate of electrolytic hydrogen evolution and the heat of adsorption of hydrogen. Trans Farad Soc 94 1053-1063. 

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

Trasatti S. 1972. Electronegativity, work function, and heat of adsorption of hydrogen on metals. J Chem Soc Faraday Trans I 68 229-236. 

Eeulner P, Menzel D. 1985. The adsorption of hydrogen on Ru(OOOl) Adsorption states, dipole moments and kinetics of adsorption and desorption. Surf Sci 154 465. 

Shimizu H, Christmann K, Ertl G. 1980. Model studies on bimetallic Cu/Ru catalysts II. Adsorption of hydrogen. J Catal 61 412. 

On experimental level the question regarding the centers of adsorption was addressed in numerous papers. For instance, in [66] the experimental data were used to show that in case of adsorption of hydrogen atoms on the surface of zinc oxide the centers of chemisorption can be provided by regular oxygen ions of the lattice, i.e. the process of chemisorption of H-atoms can be shown as the following sequence of reactions ... 

This function becomes obvious if, as it was the case of adsorption of hydrogen atoms if we assume that the change in adsorbed metal atoms, whose concentration [Me,] is... 

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