Interaction of cyclohexadiene with platinum(111) studied by BPTDS and HREELS

Abstract

The interactions of 1,3-and l,4-cyclohexadiene with Pt(ll1) have been studied with bismuth postdosing thermal desorption mass spectroscopy (BPTDS) and high-resolution electron energy loss spectroscopy (HREELS). Both species adsorb molecularly on Pt( 11 1) at 100 K. Upon warming, both species dehydrogenate completely to produce adsorbed benzene. The reaction occurs at about 230-260 K with an activation energy of 14 f 2 kcalfmol. The adsorbed benzene thus produced either dehydrogenates, ultimately to graphitic carbon, or desorbs, both occurring above 450 K. When cyclohexadiene is coadsorbed with hydrogen adatoms, no significant amounts of hydrogenated products (<2%) are observed in normal TDS or in BPTDS.

STM studies of cyclohexene hydrogenation/dehydrogenation and its poisoning by carbon monoxide on Pt(111)

“…Since adsorbed benzene remains molecular in our experimental conditions, and since the periodicities are very similar to those of the structure formed by 1,3-cyclohexadiene, we conclude that this molecule dehydrogenates to benzene. This agrees with the conclusions of a thermal desorption mass spectroscopy study performed by the Campbell group [24], which showed that at low coverage 1,3-cyclohexadiene dehydrogenates to benzene at temperatures between 20 K and 30 K lower than that for the dehydrogenation of 1,4-cyclohexadiene. Bismuth post dosing thermal desorption indicates dehydrogenation temperatures of $230 K and 260 K respectively while thermal desorption spectroscopy shows H 2 evolution beginning at $280 K and 300 K. Our studies indicate temperatures higher than 230 K and 260 K. We attribute this to the fact that in our case we are close to saturation coverage, where fewer vacant sites needed for hydrogen abstraction exist.…”

Adsorption of cyclohexadiene, cyclohexene and cyclohexane on Pt(111)

“…TPD has been used to assess the adsorption energy of 1,4-cyclohexadiene (e.g. [10,12]). However, no value could be determined as 1,4-cyclohexadiene dehydrogenates rather than desorbs.…”

“…The adsorption and dehydrogenation of 1,3-cyclohexadiene on Pt(1 1 1) has been investigated by different surface science techniques [9][10][11][12][13]. No experimentally determined adsorption energy is available, since the dehydrogenation barrier is lower than the desorption energy.…”

“…No experimentally determined adsorption energy is available, since the dehydrogenation barrier is lower than the desorption energy. Based on semi-empirical valence bond theory, Hugenschmidt et al [12] estimated the adsorption energy to be between À155 and À239 kJ/mol. Based on the same theory but using more accurate parameters, Koel et al [14] proposed a value of À142 kJ/mol.…”

Density functional theory study of surface catalysis and adsorption on several elements in Group I-B and VIII

“…For the first C-H scission, the calculated energetic values are E a,1 = 1.65 eV and E r,1 = 0.50 eV. In experiments, the barrier for benzene dehydrogenation is found to be 1.21 to 1.87 eV [102,116,117]. On a cluster model, E r for the dehydrogenation of hollow-bound benzene is endothermic by 0.52 eV [67].…”