Effect of Preadsorbed Species

The reaction between H2 and preadsorbed O is an important model for the reduction of the catalyst. For Fe(l 00) this reaction has been studied at 473 and 673 K at 2-10 torr of H2 [134]. The apparent activation energy is 59 + 4 kJ/mole [134]. The reaction proceeds via the dissociative chemisorption of H2 [134]. For this reason it is not unexpected that the reaction becomes quite inhibited by the presence of a monolayer of O [134]. 

For catalysts the results obtained when N2 is adsorbed on preadsorbed H and vice versa are not readily interpreted [380]. For single crystals the results are easier to interpret. When preadsorbed H is exposed to N2 (g) at 77 K, N2 (g) is displaced [386, 414] and physisorbed N2 may be formed on top of the H layer [386]. 

By coadsorption of N2 and H2, FeN2 is seen in secondary ion mass spectroscopy [379]. An Eley-Rideal mechanism has been suggested. 

Preadsorbed N does not inhibit dissociative adsorption of H2O [415]. Preadsorbed N has been reported to decrease [207, 416, 406, 414] or to increase [417] the amount of chemisorbed hydrogen. Preadsorbed N has been found to decrease [389], to increase [394] or to have no effect [414] on the heat of adsorption for H2 [389]. Presumably, the results of the coadsorption of N and H depends sensitively on the experimental conditions as both displacement to the gas phase and the formation of NH3 are realistic possibilities. 

The synthesis of ammonia by hydrogenation of chemisorbed N is faster than by the reaction of H2 and N2 over the same catalyst [420, 421]. This 

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The effect of preadsorbed hydrogen upon the spectra of adsorbed olefins has been extensively investigated [75,78,79]. In general, the spectra are more intense when hydrogen-precovered surfaces are used and they show bands resulting from predominantly saturated species formed by the hydrogenation of associatively adsorbed olefin. 

A few previous studies (83a, 181,183,185,187) have examined the effects of preadsorbed sulfur on the nature of adsorbed CO species or CO adsorption states on metals. Rhodin and Brucker (181) investigated variations in CO bonding on clean and partially sulfur-deactivated oc-Fe(lOO) surfaces. 

In a subsequent paper Yasumori et al. describe the effect of preadsorbed [14-C]acetylene on the hydrogenation of acetylene over Pd foil. The results suggested that four types of surface species were formed by the pre-adsorbed acetylene. Type A desorbed on evacuation, prior to the reaction. Type B was removed from the surface during the course of the reaction. Type C, which could only be removed by reduction with hydrogen at 150°C, and type D, which remained on the surface even after reduction and which were thought to be surface polymers. The removal of type B species during the reaction was postulated to be the creation of active sites for acetylene hydrogenation. 

There is no satisfactory description of the irreversible adsorption process. It was ascribed to chemical reaction with adsorbed hydrogen [53] and to a local cell mechanism through PtOH formation [56]. The interaction of cations with the surface M—OH species should also be considered as a possible mechanism, since it is known that they can strongly interact with oxidized surfaces [57, 58]. This was observed even at very positive potentials, as in the case of the oxide monolayers on Au and Pt [59]. Except for Bi, generally small catalytic effects of the preadsorbed adatoms on Pt single-crystal electrodes were reported usually at positive potentials for Sb [60], As [61], Sn [62], Se [63], and Te [64]. An analysis of metal adlayers causing the most interesting effects is presented below. 

Generally, a species of interest is preadsorbed at low temperature and the sample is then heated at a constant rate, while desorbing species are monitored using mass spectrometry. TPD can be performed in either flow reactors or under vacuum conditions. For flow-based TPD studies involving industrial catalysts, care must be taken with the experimental conditions to minimize the effects of pore diffusional and film mass transfer limitations. This can be addressed by using low heating rates... 

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