Site Selective Detection of Methane Dissociation on Stepped Pt Surfaces

Abstract

We report a combined experimental and theoretical study comparing methane dissociation on three different platinum surfaces Pt(111), Pt(211), and Pt( 110)-(1 × 2). Reflection absorption infrared spectroscopy (RAIRS) was used to detect chemisorbed methyl species formed by dissociative chemisorption of CH 4 on specific surface sites and to measure surfacesite-specific sticking coefficients of CH 4 on the terrace, step, and ridge sites as function of incident translational energy. Methane dissociation is observed to be direct on all sites and diffusion of the chemisorbed methyl species is absent for surface temperature below 150 K. The experimental data are compared with the results of density functional (DFT) calculations that give minimum energy barriers for CH 4 chemisorption that properly account for the experimental relative site-specific reactivities. Also in agreement with experiments, DFT results predict a negligible effect of co-adsorbed H and CH 3 species on the vibrational frequency of a methyl group chemisorbed on terrace and step sites of Pt(211). However, the origin of the red-shift of the RAIRS peak of CH 3 chemisorbed on terrace sites compared with that on step sites of Pt(211) remains elusive and still demands further investigation.

Infrared Activities of Adsorbed Species on Metal Surfaces: The Puzzle of Adsorbed Methyl (CH3)

“…CH 3 binds less strongly to the top site of Ni( 111) with E ad of −1.57 eV and becomes metastable on the fcc site of Pt(111) with an approximate E ad of ∼−1.33 eV (we can only find this geometry with a loose convergence criterion). The calculated adsorption energies and harmonic frequencies agree well with previous theoretical predictions, 14,16,18,26 although the predicted frequencies are systematically higher than the experimental ones, 14,22,33 due presumably to the intrinsic error in DFT as well as the harmonic approximation used in the theoretical calculations. Note that Gutieŕrez-Gonzaĺez et al 14 have applied vdW-corrected functionals to study CH 3 adsorbed on Pt surfaces and found similar site preference and site-specific vibrational frequencies as using PBE.…”

Transferability of the SRP32-vdW specific reaction parameter functional to CHD3 dissociation on Pt(110)-(2 × 1)

“…The values in brackets correspond to those calculated using the PBE functional in a previous study by Jackson and co-workers. 33 Transition 1), where dissociation is seen to occur only on the ridge atoms of the surface using site selective detection by reflection absorption infrared spectroscopy 69 and with a previous study by Bisson et al 42 Figure 7(b) presents the same analysis as Fig. 7(a), but for the trapped trajectories, which shows that as the incident energy increases the fraction of trapped molecules that first hit the ridge atom tends to increase, although the analysis becomes less reliable at higher incident energies due to the lower number of trapped trajectories.…”

Dynamical Study of the Dissociative Chemisorption of CHD3 on Pd(111)

“…38 Finally, the reaction of methane is site-specific. 2,13,15,39 For all of these reasons, we will present in this work a detailed analysis of the results from the BOMD calculations and compare them to the results obtained on Pt(111) and Ni(111).…”