Dynamics of Reactive Species and Reactant-Induced Reconstruction of Pt Clusters in Pt/Al₂O₃Catalysts

Abstract

We report a complete experimental characterization of the surface Pthydride species on an industrial 5 wt % Pt/Al 2 O 3 catalyst (average particle size of 1.4 ± 0.4 nm) under different hydrogenation/dehydrogenation conditions. By combining inelastic neutron scattering, FT-IR spectroscopy, and synchronous DRIFT/XAS/MS, we identified n-fold coordinated Pt-hydrides and four different types of linear Pthydrides characterized by different adsorption strength, whose relative proportion depends on the experimental conditions. In particular, we observed that the n-fold coordinated hydrides convert into linear ones upon decreasing hydrogen coverage, and vice versa, and we traced this phenomenon to a morphological and electronic reconstruction of the Pt nanoparticles. Although only a fraction of the surface Pthydrides are directly involved in the hydrogenation of toluene, all the others play an indirect but fundamental role, maintaining the Pt nanoparticles electronically and morphologically stable during the reaction and hence avoiding the occurrence of deactivation processes. Our results, which are in good agreement with the theoretical predictions reported in the literature, offer a comprehensive picture of the dynamics of Pt nanoparticles in hydrogenation conditions. This always involves a change in the relative proportion of the Pt-hydride species, and only in some cases an electronic and morphological reconstruction of the Pt particles.

Identification of hydrogen species on Pt/Al2O3 by in situ inelastic neutron scattering and their reactivity with ethylene

“…Major reasons for the discrepancies between Fig. 4b and c 29 We presented their results in the region of 40-100 meV. They expected that these signals are assigned to n-fold (bridged, hollow and fourfold coordinated) Pt-H species.…”

“…2a), which are assigned to on-top hydride and atop H species on the Pt surface, respectively, based on the previous reports. 29,[44][45][46] We could not detect the n-fold H species because of the selection rule of IR spectroscopy. 29 The bands in the regions of 1600-1750 and 3100-3700 cm −1 , which are attributed to Al-OH species, appeared upon the introduction of H 2 .…”

“…2a and b because these species were consumed in the C 2 H 4 hydrogenation reaction, which is in good accordance with previous works. 29,44,48 Observation of adsorbed hydrogen species on Pt/Al 2 O 3 by INS…”

“…Identification of the H species that are silent for FT-IR and investigation of their reactivity are crucial to understand the ethylene hydrogenation catalysis. 29 To achieve this goal, INS measurements were performed at the BL01 beamline "4SEASONS" of J-PARC. To determine the motions and positions of atoms, INS is the unique technique, in particular for light atoms, e.g.…”

“…28 In a recent study, Pt-H species with n-fold coordination were detected on a Pt/Al 2 O 3 catalyst by INS. 29 These recent results have demonstrated that the INS technique is effective to detect active H species on supported metal catalysts. However, the individual assignment of each H species on catalysts is still a challenge in the field of catalytic science.…”