Adsorption of cyclic hydrocarbons on Pt and the interaction of the adsorbed species with hydrogen
Introduction
Although the adsorption of C6-rings has been scrutinized [1], [2], [3], their position on polycrystalline or dispersed surfaces, especially during the initial period of adsorption and interaction with hydrogen is not clear.
Former studies [4], [5], [6], [7] on the adsorption of C6 hydrocarbons on platinum and other metals have indicated both reversible and irreversible adsorption of these substances with considerably higher total and reversible adsorption of benzene. In a number of studies on benzene adsorption both surface π-complex formation and adsorption via CH bond formation have been observed, [4], [8], [9], [10] whereas the orientation of adsorbed cyclohexane was different from that of benzene [7], [11]. These observations were supported also by LEED experiments [12], [13].
Investigating the adsorption of aliphatic hydrocarbons, we found that up to a certain surface carbon concentration (C/Pt = 0.65) the work function change (Δφ) is proportional to the carbon uptake. Above this point, after a transition period, the carbon uptake proceeded without any further work function change [14], indicating the absence of further electronic transfer between the surface species and the metal. The WFC values and the corresponding electric dipole moments indicate dissociative adsorption of aliphatic hydrocarbons [15], [16].
The exposure of the adsorbed species to hydrogen removed up to 50% of the species especially under 400 K both from the chemisorbed first layer and from the carbonaceous deposit [17]. However, no significant differences were observed between cyclohexane and benzene with respect to the |Δφ| values characterizing their adsorption on Pt-foil [18]. These observations, made at the maximal |Δφ| values, i.e. at high surface coverages, suggested a similar final structure of the adspecies formed. This observation seemed to differ significantly from the results referred above. This motivated us to determine the |Δφ| and C/Pt values on the initial stage of adsorption, i.e. to study the adsorption dynamics.
We report here on the dynamics of cyclohexane, benzene, toluene and cyclopentane adsorption. The aim of the study was:
- (i)
to decide whether there are differences in the structure of adspecies formed in the adsorption of these cycles, similar to those observed in the investigations [11] referred to;
- (ii)
to distinguish the C/Pt range of direct hydrocarbon–platinum interaction from the region of the formation of three-dimensional surface adspecies and the differences between the different cyclic hydrocarbons in this respect, if there are any.
Section snippets
Experimental
The pretreatment and cleaning of the polycrystalline platinum foil was described previously [14]. The sample was stabilized by several cycles of benzene adsorption, oxidation and reduction at 473 K after the Ar+ bombardment and flash to 1200 K. No carbon impurities were revealed by AES after oxidation and reduction. After cleaning, the sample was separated from the high vacuum part of the chamber by an isolation chamber. The work function changes were followed by a Kelvin probe during and after
Results and discussion
The adsorption of C6 cycles (benzene, toluene and cyclohexane) and cyclopentane was investigated. Experiments were performed first by procedure 1. Exposure of the Pt sample to benzene, toluene and cyclohexane for 10 min changed neither the Δφ nor the C/Pt values significantly in the pressure range between 10−5 and 10−2 mbar. As a general tendency, the increase of C/Pt ratio and the decrease of the |Δφ| values were observed with increasing temperature. The only exception was the small increase of
Conclusions
The state of the adlayer is similar for the total adsorption of benzene and cyclohexane, as indicated by the nearly equal Δφ values. The carbon deposition is higher to some extent at benzene adsorption. The data characteristic for toluene adsorption are similar to those measured for benzene, indicating that the two compounds are attached to the Pt surface by identical types of interaction and by identical parts in these molecules.
The |Δφ| versus C/Pt plots, representing the initial part of the
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