Wiebke Ludwig

Role of low-coordinated surface sites in olefin hydrogenation: a molecular beam study on Pd nanoparticles and Pd(111).

hydrogenation of the olefinic double bond was shown to crucially depend on the presence of hydrogen species absorbed in the subsurface region of a metal catalyst. [2–4] Particularly for Pd nanoparticles, slow replenishment of these species required for hydrogenation was identified as a rate-determining process in a broad range of reaction conditions. [2–6] In view of these results, the permeability of the metal surface for hydrogen can play a crucial role in its activity in hydrogenation catalysis. Since subsurface hydrogen diffusion is known to be a strongly structure-sensitive process on Pd surfaces, [7] the degree of coordination of the surface Pd atoms can be very important in determining the formation rate of subsurface hydrogen species, and therefore may be decisive for the hydrogenation activity. In line with this suggestion, it was recently observed experimentally that modification of the low-coordinated surface sites of Pd nanoparticles, such as edges and corners, with carbon significantly affects the hydrogenation activity and results in a sustained hydrogenation rate that is not possible on the C-free surface. [2, 5] This observation was proposed to arise from faster subsurface diffusion of hydrogen through the edge sites modified with carbonaceous deposits. Recent theoretical calculations supported the hypothesis that carbon adsorbed in the vicinity of particle edges strongly reduces or nearly eliminates the activation barrier for subsurface hydrogen diffusion on Pd particles. [8] This effect was ascribed mainly to C-induced expansion of the surface openings for penetration of H into the subsurface region. In contrast, no notable reduction of the activation barrier was found with carbon on the intrinsically rigid regular Pd(111) surface. [8] The conceptual importance of atomic flexibility of sites near particles edges in subsurface hydrogen diffusion, demonstrated by the theoretical calculations, suggests that the low-coordinated surface sites can play a crucial role in the hydrogenation process. In order to obtain more insight into the role of low-coordinated surface sites in hydrogenation, we studied the reaction of cis-2-butene with deuterium on well-defined model Pd nanoparticles supported on a thin Fe3O4/Pt(111) film. Two complementary strategies were applied herein to address the role of low-coordinated surface sites experimentally: first, we compared the hydrogenation activity of Pd particles with an extended Pd(111) single crystal surface. Secondly, we selectively modified the low-coordinated surface sites by deposition of carbon, which has previously been determined to predominantly occupy edges and corners of Pd nanoclusters, [5] and follow the reactivity changes upon such modification. Herein, we demonstrate that modification of the low-coordinated surface sites of Pd particles with carbon promotes sustained hydrogenation activity in a pronounced manner, while carbonaceous deposits adsorbed on the extended Pd(111) surface do not noticeably affect the reactivity. We ascribe this phenomenon to facilitation of hydrogen subsurface diffusion through carbon-modified low-coordinated sites of Pd nanoparticles, which are not available on Pd(111). Generally, alkene conversions promoted by transition-metal catalysts are accounted for by the Horiuti–Polanyi mechanism, [9] which proceeds through a series of successive hydrogenation-dehydrogenation steps:

Toward Low-Temperature Dehydrogenation Catalysis: Isophorone Adsorbed on Pd(111)

Adsorbate geometry and reaction dynamics play essential roles in catalytic processes at surfaces. Here we present a theoretical and experimental study for a model functional organic/metal interface: isophorone (C9H14O) adsorbed on the Pd(111) surface. Density functional theory calculations with the Perdew-Burke-Ernzerhoff (PBE) functional including van der Waals (vdW) interactions, in combination with infrared spectroscopy and temperature-programmed desorption (TPD) experiments, reveal the reaction pathway between the weakly chemisorbed reactant (C9H14O) and the strongly chemisorbed product (C9H10O), which occurs by the cleavage of four C-H bonds below 250 K. Analysis of the TPD spectrum is consistent with the relatively small magnitude of the activation barrier derived from PBE+vdW calculations, demonstrating the feasibility of low-temperature dehydrogenation.

Tuning the Surface Chemistry of Pd by Atomic C and H: A Microscopic Picture

Palladium is crucial for industry-related applications such as heterogeneous catalysis, energy production, and hydrogen technologies. In many processes, atomic H and C species are proposed to be present in the surface/near-surface area of Pd, thus noticeably affecting its chemical activity. This study provides a detail and unified view on the interactions of the H and C species with Pd nanoparticles (NPs), which is indispensable for insight into their catalytic properties. Density functional calculations of the interplay of C and H atoms at various concentrations and sites on suitable Pd NPs have been performed, accompanied by catalysis-relevant experiments on oxide-supported bare and C-modified Pd NPs. It is shown that on a Pd(79) NP a subsurface C atom destabilizes nearby atoms H at low coverage. Our experiments confirm that H atoms bind more weakly on C-containing Pd NPs than on C-free NPs. Various factors related to the presence of both H and C atoms on a Pd(79) surface, which may influence the penetration of H atoms from the surface into the subsurface area, have been investigated. Carbon atoms facilitate the subsurface penetration of atomic H both thermodynamically and kinetically when the surface is densely covered by H atoms. Moreover, subsurface H atoms are also energetically favored, even in the absence of C atoms, when several facets of the NP are covered by H atoms.

A kinetic study on the conversion of cis-2-butene with deuterium on a Pd/Fe3O4 model catalyst

The conversion of cis-2-butene with deuterium over a well-defined Pd/Fe(3)O(4) model catalyst was studied by isothermal pulsed molecular beam (MB) experiments under ultra high vacuum conditions. This study focuses on the processes related to dissociative hydrogen adsorption and diffusion into the subsurface of Pd nanoparticles and their influence on the activity and selectivity toward competing cis-trans isomerization and hydrogenation pathways. The reactivity was studied both under steady state conditions and in the transient regime, in which the reaction takes place on a D-saturated catalyst, over a large range of reactant pressures and reaction temperatures. We show that large olefin coverages negatively affect the abundance of D species, as indicated by a reduction of both reaction rates under steady state conditions as compared to the transient reactivity on the catalyst pre-saturated with D(2). Limitations in D availability during the steady state lead to a very weak dependence of both reaction rates on the olefin pressure. In contrast, when the surface is initially saturated with D, the transient reaction rates of both pathways exhibit positive kinetic orders on the butene pressure. Cis-trans isomerization and hydrogenation show kinetic orders of +0.7 and +1.0 on the D(2) pressure, respectively. Increasing availability of D noticeably shifts the selectivity toward hydrogenation. These observations together with the analysis of the transient reaction behavior suggest that the activity and selectivity of the catalyst is strongly controlled by its ability to build up and maintain a sufficiently high concentration of D species under reaction conditions. The temperature dependence of the reaction rates indicates that higher activation energies are required for the hydrogenation pathway than for the cis-trans isomerization pathway, implying that different rate limiting steps are involved in the competing reactions.

Kinetic Evidence for a Non-Langmuir-Hinshelwood Surface Reaction: H/D Exchange over Pd Nanoparticles and Pd(111)

The mechanism of hydrogen recombination on a Pd(111) single crystal and well-defined Pd nanoparticles is studied using pulsed multi-molecular beam techniques and the H2/D2 isotope exchange reaction. The focus of this study is to obtain a microscopic understanding of the role of subsurface hydrogen in enhancing the associative desorption of molecular hydrogen. HD production from H2 and D2 over Pd is investigated using pulsed molecular beams, and the temperature dependence and reaction orders are obtained for the rate of HD production under various reaction conditions designed to modulate the amount of subsurface hydrogen present. The experimental data are compared to the results of kinetic modeling based on different mechanisms for hydrogen recombination. We found that under conditions where virtually no subsurface hydrogen species are present, the HD formation rate can be described exceptionally well by a classic Langmuir-Hinshelwood model. However, this model completely fails to reproduce the experimentally observed high HD formation rates and the reaction orders under reaction conditions where subsurface hydrogen is present. To analyze this phenomenon, we develop two kinetic models that account for the role of subsurface hydrogen. First, we investigate the possibility of a change in the reaction mechanism, where recombination of one subsurface and one surface hydrogen species (known as a breakthrough mechanism) becomes dominant when subsurface hydrogen is present. Second, we investigate the possibility of the modified Langmuir-Hinshelwood mechanism with subsurface hydrogen lowering the activation energy for recombination of two hydrogen species adsorbed on the surface. We show that the experimental reaction kinetics can be well described by both kinetic models based on non-Langmuir-Hinshelwood-type mechanisms.