A three-site Langmuir adsorption model to elucidate the temperature, pressure, and support dependence of the hydrogen coverage on supported Pt particles

Abstract

The three-site adsorption model, previously developed to describe H adsorption on small Pt particles, was used to gain insight into dependence of hydrogen coverage on temperature, pressure, and support ionicity. The three sites, in order of decreasing PtH bond strength, involve H in an atop, a threefold, and an ontop Pt site. The ontop site designates H bonded in an atop site surrounded by occupied threefold sites (hence referred to as ontop site). The model includes an emptying of the H atop sites into H threefold sites with increasing H pressure to reduce lateral interactions. Hydrogen chemisorption on an acidic Pt/H-USY and a basic Pt/NaY and TPD results on a acidic Pt/H-LTL and basic Pt/K-LTL are modeled using a Langmuir isotherm for each H site. The H/M data can be directly compared with Pt L2,3 XANES results on the same samples. A new analysis method (Delta XANES technique) using the difference in the absorption coefficient, Δμ=μ(H/Pt)−μ(Pt), allows an in situ spectroscopic determination of the type of H adsorption site and H coverage. The adsorption enthalpies (ΔH's) for the atop, threefold, and ontop sites are found to be highly dependent on the support ionicity, increasing for ionic (basic) supports consistent with previous results. The calculations using the three-site model confirm that the support-induced changes in the PtH bond strength produce dramatically different H coverages and dominant adsorption sites at catalytic reaction temperatures and pressures. Depending on uptake versus desorption of H, a hysteresis is found in the atop to threefold site rearrangement, believed to result from a requirement for collective rearrangement involving an entire domain or island of H on the surface.