Dissociative hydrogen adsorption on palladium requires aggregates of three or more vacancies

Abstract

During reaction, a catalyst surface usually interacts with a constantly fluctuating mix of reactants, products, 'spectators' that do not participate in the reaction, and species that either promote or inhibit the activity of the catalyst. How molecules adsorb and dissociate under such dynamic conditions is often poorly understood. For example, the dissociative adsorption of the diatomic molecule H2--a central step in many industrially important catalytic processes--is generally assumed to require at least two adjacent and empty atomic adsorption sites (or vacancies). The creation of active sites for H2 dissociation will thus involve the formation of individual vacancies and their subsequent diffusion and aggregation, with the coupling between these events determining the activity of the catalyst surface. But even though active sites are the central component of most reaction models, the processes controlling their formation, and hence the activity of a catalyst surface, have never been captured experimentally. Here we report scanning tunnelling microscopy observations of the transient formation of active sites for the dissociative adsorption of H2 molecules on a palladium (111) surface. We find, contrary to conventional thinking, that two-vacancy sites seem inactive, and that aggregates of three or more hydrogen vacancies are required for efficient H2 dissociation.

When Langmuir Is Too Simple:H2Dissociation on Pd(111) at High Coverage

“…The experimental measurements [1] at 65 K show that on the 3V ensembles the hydrogen sticking coefficient s is more than 50 times larger than on the 2V ones. The present model gives a reactivity of H-free trimers centered on top positions to be about 10 9 times higher than that of the 2V ensembles, and 10 6 times higher than that of the 3V H ones, and thus explains the experimental observations.…”

“…In order to adsorb on such a surface the reactant molecules must find empty surface sites (active sites) created either by one missing adsorbate (vacancy) or by aggregates (ensembles) of vacancies. In the case of H 2 dissociation on Pd-a process of interest in many industrial reactions, including hydrogenation and fuel cell technologies -a recent scanning tunneling microscopy (STM) study [1] has revealed that the classically assumed mechanism where diatomic molecules require ensembles of two empty sites is too simplistic [1]. Indeed, Mitsui et al [1,2] have found that near saturation coverage the sites for the facile molecular dissociation on Pd(111) require ensembles of three or more H-free nearest neighbors fcc sites.…”

“…In the case of H 2 dissociation on Pd-a process of interest in many industrial reactions, including hydrogenation and fuel cell technologies -a recent scanning tunneling microscopy (STM) study [1] has revealed that the classically assumed mechanism where diatomic molecules require ensembles of two empty sites is too simplistic [1]. Indeed, Mitsui et al [1,2] have found that near saturation coverage the sites for the facile molecular dissociation on Pd(111) require ensembles of three or more H-free nearest neighbors fcc sites. Holloway [3] has suggested that such an unexpected result could be due to chemical selfpoisoning of the surface originated by adsorbed hydrogen, further hindering the activity by increasing the dissociation barrier.…”

Coexistence of one- and two-dimensional supramolecular assemblies of terephthalic acid on Pd(111) due to self-limiting deprotonation

“…Pd͑111͒ has been intensely investigated in the context of hydrogenation catalysis, electrolysis, and hydrogen purification. 20 This metal surface acts as a catalyst for the dissociation of H 2 molecules into H atoms 21,22 and is capable of storing hydrogen both on the surface [23][24][25] and in the subsurface region. 26,27 Thus, the affinity of the Pd͑111͒ surface towards hydrogen adsorption can be used as the regulator for the hydrogen transfer process between adsorbed building blocks and the substrate.…”

Observation and manipulation of subsurface hydride in Pd{111} and its effect on surface chemical, physical, and electronic properties

“…For this reason, the adsorption and absorption of hydrogen by Pd has been the subject of much experimental and theoretical work (6,(8)(9)(10)(11)(12)(13)(14). It is known that hydrogen can occupy subsurface sites that are stable absorption sites below the topmost surface layer of Pd atoms.…”