Theoretical Explanation of Pt Trimers Observed by Z-Contrast STEM
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ABSTRACT First-principles quantum-mechanical calculations on y-alumina have revealed a fascinating "reactive sponge" phenomenon. -,-alumina can store and release water, but in a unique, "reactive" way. This "reactive sponge process" facilitates the creation of aluminum and oxygen vacancies in the alumina surface. Earlier atomic-resolution Z-contrast STEM images of ultradispersed Pt atoms on a -y-alumina support showed the individual atoms to form dimers and trimers with preferred spacings and orientations that are apparently dictated by the underlying support[i]. In turn, the reactive sponge property of -y-alumina is the key to understanding the Pt clusters. Our calculations demonstrate that if three Pt atoms fill three vacancies created during the reactive sponge process, the resulting geometry precisely matches that of the Pt trimers observed in the Z-STEM images. Understanding the initial nucleation of small clusters on the complex gamma alumina surface is an essential first step in determining the origins of catalytic activity.
INTRODUCTION -y-alumina is technologically one of the most important catalytic materials, serving both as a catalytic support[2] and as a catalyst[3, 4]. For example, catalytic reduction and oxidation of automotive pollutants such as nitric oxide (NO.), carbon monoxide (CO), and hydrocarbons (HC) is accomplished with platinum (Pt) or rhodium (Rh) dispersed on a 7,-alumina surface[5]. In refinery catalysis, the process is predominantly olefin cracking and it is carried out on catalysts formed by dispersal of transition-metal atoms on an insulating support surface[6], frequently a zeolite[7] or alumina[8]. It has long been recognized that electron microscopy is a powerful tool for investigating supported catalysts[9]. The ORNL STEM generates the smallest electron probe in the world (1.3 A), and has revealed never-before-seen details about the distribution of the metal atoms and the structure of the metal clusters. It is capable of imaging individual catalyst atoms on the surface of 7y-alumina and has revealed that Pt nucleates in the form of trimers and dimers[1]. Very recently, through quantum mechanical calculations, we have made some interesting discoveries about 7y-alumina: It may exist over a range of hydrogen content, and this variable hydrogen content gives rise to a remarkable "reactive sponge" property[10]. In the reactive sponge process, water is absorbed and evolved much like in a kitchen sponge, but in a unique reactive way. Here we first review the reactive sponge process and then demonstrate how the filling of surface vacancies left behind by the reactive sponge process with Pt atoms leads to the formation of the Pt 3 clusters observed by Z-contrast STEM.
THEORY The calculations were carried out with density functional theory[ll] and employed the generalized gradient approximation (GGA) to the exchange-correlation energy[12], as described in the review by Payne et at.[13]. The electron-ion interactions were described with reciprocal-space pseudopotentials[14] in the Kleinma
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