Chromium and Lanthanum on Transition Alumina Surfaces: The Role of Bulk Point-Defect Distributions on Catalytic Activity
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Chromium and Lanthanum on Transition Alumina Surfaces: The Role of Bulk PointDefect Distributions on Catalytic Activity S. N. Rashkeev,1 K. Sohlberg,2 M. V. Glazoff,3 J. Novak,3 S. J. Pennycook,1,4 and S. T. Pantelides1,4 1 Dept. Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, U.S.A. 2 Dept. Chemistry, Drexel University, Philadelphia, PA 19104, U.S.A. 3 Alcoa Technical Center, Alcoa Center, PA 15069, U.S.A. 4 Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A. ABSTRACT We employ a combination of Z-contrast scanning transmission electron microscopy (ZSTEM) and first-principles density-functional calculations to investigate the interaction between metal atoms and cubic alumina catalytic supports. We show that there are two observed La sites at the porous γ- alumina surface, and that single La atoms do not exhibit a tendency to cluster. Cr behaves very differently from La - it has a tendency to cover the alumina surface in ordered, periodic raft-like patches. The degradation of the chromia/alumina catalyst is related to the possibility for the chromium to move away from the surface into the bulk, and the activation barrier for such a process is higher in η- alumina than in γ- alumina, i.e., η- alumina is a more durable support for chromium catalyst. INTRODUCTION γ-alumina is one of many polytypes of Al2O3 that is used extensively as a catalytic support material because of its high porosity and large surface area [1]. At temperatures in the range 1000-1200ºC, however, γ-alumina transforms rapidly into the thermodynamically stable αalumina phase (corundum), the pores get closed thus suppressing the catalytic activity of the system [2]. The phase transformation can be slowed down by doping γ-alumina with one of many elements such as La, Ba, P, or Si. It has been demonstrated that as little as 0.3-0.5% La can induce stabilization of γ-alumina [3]. Though the generic science of phase transformations is highly developed, the atomic-scale mechanisms and the role of dopants in phase transformations in specific systems remains wide open. A second problem is related to the interactions of metal particles with different alumina substrates in catalytic systems. These interactions determine the size and shape of metallic particles, which in turn determine their catalytic activity. Direct Z-contrast atomic resolution imaging of individual dispersed catalyst atoms (Pt and Rh) on the insulating γ-alumina surface has demonstrated that these atoms may exist on the surface as isolated atoms, in small clusters (dimers, trimers, etc.) or as more extended raft-like structures depending on the type of atoms [4]. In this paper we report the use of a combination of experimental and theoretical/computational tools to investigate the interaction between La and Cr atoms and alumina catalytic supports. This combination of tools has proven to be quite powerful in a range of other similar problems in materials science [5].
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EXPERIMENTAL AND COMPUTATIONAL DETAILS On the experimental sid
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