Probing ensemble effects in surface reactions. 2. Benzene adsorption on clean and bismuth-covered platinum(111)

Abstract

The interactions of benzene with the clean and Bi-dosed Pt(lll) surface have been studied between 110 and 850 K with a combination of thermal desorption mass spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). Below ~350 K, benzene adsorbs molecularly. The first monolayer saturates at a coverage of 0.16 (molecules per Pt atom). About 55% of this dehydrogenates upon heating to liberate H2 in a series of steps between 450 and 800 K. This leaves residual carbon on the surface in a graphitic overlayer. The remaining benzene desorbs molecularly at ~505 K (Eá qí 30.8 kcal/mol) and 350 K (E¿ =* 21 kcal/mol). Substantial isotopic scrambling is seen in TDS from coadsorbed mixtures of C6H6 and C6D6. The activation energies for dehydrogenation of perdeuterated benzene are ~1.2

Calorimetric Measurement of the Heat of Adsorption of Benzene on Pt(111)

“…Thus, its adsorption energy is of substantial fundamental importance. At 300 K, benzene adsorbs molecularly on Pt(111) 30,33,36,43 with its molecular plane parallel to the surface, interacting with the Pt through its aromatic π electron system. 28,29,33,37,51 Thus, its heat of adsorption is 6 times the C-Pt(111) bond energy for a carbon atom in an aromatic ring parallel to the surface.…”

“…52 Benzene adsorbs molecularly on Pt(111) at 300 K, but upon heating coverages below ∼0.6 ML (ML ) monolayer ) saturation coverage at 300 K), it dissociates completely into H 2 gas and adsorbed graphitic carbon before desorbing. 36 Therefore, its adsorption energy cannot be measured with desorption-based methods such as temperature programmed desorption (TPD), molecular beam relaxation spectroscopy (MBRS), and equilibrium adsorption isotherms, at least below 0.6 ML. Even at higher coverages where a fraction desorbs intact, the TPD and MBRS line shapes are complicated by the competition from dissociation, rendering rigorous data analysis of desorption rates to extract the desorption energy impossible.…”

“…Even at higher coverages where a fraction desorbs intact, the TPD and MBRS line shapes are complicated by the competition from dissociation, rendering rigorous data analysis of desorption rates to extract the desorption energy impossible. Even at saturation coverages, only ∼45% of the adsorbed benzene desorbs intact, whereas the remainder dehydrogenates, 36 so that the use of equilibrium adsorption isotherms is also impossible. Thus, a direct method like SCAC is required for the measurement of benzene adsorption energies, and that is what we report here.…”

“…31,33,36,43 This gave values ranging from 133 kJ/mol 36 to 121 kJ/mol. 43 It was assumed that the adsorption energy approximately equals this activation energy, giving It should be noted that these values are for ∼0.8 ML initial coverage.…”

“…[53][54][55][56] There, we used low coverages of coadsorbed bismuth atoms to block Pt sites, and quantified the resulting decrease in dehydrogenation rate as a function of Bi coverage. By fitting the data to a kinetic model, we were able to determine the number of unoccupied Pt surface atoms needed for dehydrogenation.…”