Dynamic Blocking by CO of Hydrogen Transport across Pd₇₀Au₃₀(110) Surfaces

CO adsorption affects hydrogen transport across surfaces of hydrogen-absorbing materials. On Pd₇₀Au₃₀(110), CO was found to block the desorption sites for absorbed hydrogen. However, the detailed CO adsorption site and hence the blocking mechanism have not been clarified yet. In this study, we investigated the CO adsorption structure on Pd₇₀Au₃₀(110) by using reflection–absorption infrared spectroscopy (RAIRS). We demonstrate that the CO adsorption structure depends on the CO coverage and sample temperature. We also performed thermal desorption spectroscopy (TDS) simulations on the basis of the RAIRS results and clarified the dynamical mechanism of the CO blocking where the CO site transfer from the Pd on-top to the Pd–Pd bridge sites enhances the blocking efficiency. These discoveries would lead to understanding and controlling the hydrogen transport across the Pd–Au alloy and Pd-related surfaces

Near-Surface Accumulation of Hydrogen and CO Blocking Effects on a Pd–Au Alloy

Alloying Pd with Au has remarkable features of enhancement of hydrogen solubility compared to Pd and catalytic activity for reactions such as partial hydrogenation of unsaturated hydrocarbons. A key to understanding these phenomena is clarification of hydrogen behavior in the near-surface region. In the present work, by applying nuclear reaction analysis for high-resolution depth profiling of hydrogen in combination with thermal desorption spectroscopy, we show that hydrogen substantially accumulates in the near-surface region and is absorbed in the bulk of a single-crystal Pd₇₀Au₃₀(110) alloy. We also demonstrate a molecular cap effect of CO, where a small amount of CO adsorption greatly changes the hydrogen absorption and desorption behavior by blocking the entrance/exit channel for hydrogen. These findings lead to understanding and controlling the catalytic activity of the Pd–Au alloy and Pd-related surfaces and also open up a new method to control hydrogen transport across metal surfaces

Complete H–D Exchange of Butene via D Absorbed in a Pd–Au Alloy

The H–D exchange reaction is one of the easiest ways to synthesize deuterium-labeled compounds. H–D exchange of olefins has been widely studied on metal surfaces such as Pd and Pt. However, H–D exchange of butene on these surfaces is hardly completed and always accompanied by hydrogenation or dehydrogenation, and its reaction mechanism is yet to be elucidated. In the present study, we investigated the reaction of <i>cis</i>-2- and 1-butenes with H and D atoms absorbed in the near-surface region of Pd₇₀Au₃₀(110) by using thermal desorption spectroscopy (TDS), reflection–absorption infrared spectroscopy (RAIRS), and TDS simulation. We show complete H–D exchange of both butene isomers without hydrogenation and dehydrogenation. Similar product yield distributions for both butene isomers indicate a fast di-σ bond migration on this surface, which is a key for the complete H–D exchange. Our results would lead to understanding and controlling the catalytic activity and selectivity of the Pd–Au alloy and Pd-related surfaces and would open up a new chemistry using absorbed hydrogen in combination with alloyed surfaces

Mechanism of Olefin Hydrogenation Catalysis Driven by Palladium-Dissolved Hydrogen

The Pd-catalyzed hydrogenation of CC double bonds is one of the most important synthetic routes in organic chemistry. This catalytic surface reaction is known to require hydrogen in the interior of the Pd catalyst, but the mechanistic role of the Pd-dissolved H has remained elusive. To shed new light into this fundamental problem, we visualized the H distribution near a Pd single crystal surface charged with absorbed hydrogen during a typical catalytic conversion of butene (C₄H₈) to butane (C₄H₁₀), using H depth profiling via nuclear reaction analysis. This has revealed that the catalytic butene hydrogenation (1) occurs between 160 and 250 K on a H-saturated Pd surface, (2) is triggered by the emergence of Pd bulk-dissolved hydrogen onto this surface, but (3) does not necessarily require large stationary H concentrations in subsurface sites. Even deeply bulk-absorbed hydrogen proves to be reactive, suggesting that Pd-dissolved hydrogen chiefly acts by directly providing reactive H species to the surface after bulk diffusion rather than by indirectly activating surface H through modifying the surface electronic structure. The chemisorbed surface hydrogen is found to promote hydrogenation reactivity by weakening the butene-Pd interaction and by significantly reducing the decomposition of the olefin