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 Pt-H 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 L 2,3 XANES results on the same samples. A new analysis method (Delta XANES technique) using the difference in the absorption coefficient, μ = μ(H/Pt) − μ(Pt), allows an in situ spectroscopic determination of the type of H adsorption site and H coverage. The adsorption enthalpies ( H '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 Pt-H 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.

Nature of the Intermediate Binding Sites in Hydrogen Oxidation/Evolution over Pt in Alkaline and Acidic Media

“…Sites shaded in grey represent filling of the corner/edge sites and those in red the face sites. Adapted from Ji et al . copyright 2007 with permission from Elsevier.…”

“… Left side and center: Schematic illustration of the five or three different H‐binding sites (below the red horizontal line) that have been identified previously and how they correlate to large and medium particles . The notation 6/3,4 indicates that the H atom in site 6 exists ontop of or in the presence of sites 3 and 4.…”

“…Comparable TPD data for Pt nanoparticles (around 1.5–2 nm diameter) supported on H‐LTL (acidic) and K‐LTL (basic) zeolites have been reported by Ji et al. (Figure ) . These data have been quantitatively modelled assuming the TPD coverage follows A d θ H /d T (in which A is an arbitrary normalization constant depending on the experiment, where the total H coverage θ H = N at θ at + N nf θ nf + N ot θ ot , assuming a three‐site model as summarized in Figure .…”

“…(Figure ) . These data have been quantitatively modelled assuming the TPD coverage follows A d θ H /d T (in which A is an arbitrary normalization constant depending on the experiment, where the total H coverage θ H = N at θ at + N nf θ nf + N ot θ ot , assuming a three‐site model as summarized in Figure . N i corresponds to the number of each type of site [atop(at), n ‐fold (nf) and ontop (ot)] and θ i is the relative coverage assuming Langmuir adsorption (i.e.…”

Operando XAS Techniques: Past, Present, and Future

“…In all of these studies, unprecedented new details on the coverage and binding sites of adsorbates in situ or operando have been revealed. Further, similar XAS studies in gas-phase catalytic reactors have been reported, including H on supported Pt clusters in zeolites and various flat supports [101] as well as ethylene on Pt and its different molecular geometries (atop p-bonded, bridged di-s bonded, and a dehydrogenated vertical ethylidyne species) as the temperature was increased, revealing the ability of the Dm technique to differentiate between different bonding geometries [102] on Pt, and even ethylene-type intermediates in the hydroepoxidation of propene over Au [103]. With the complexity of the systems and reactions that can be studied with the Dm technique increasing more recently, biofuel cell investigations and biocatalysis experiments could come into focus soon, with some experiments already underway [104].…”