Insensitivity of the extent of surface reduction of ceria on termination: comparison of (001), (110), and (111) faces
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Research Letter
Insensitivity of the extent of surface reduction of ceria on termination: comparison of (001), (110), and (111) faces Weizi Yuan and Sossina M. Haile
, Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA
Address all correspondence to Sossina M. Haile at [email protected] (Received 17 July 2020; accepted 15 September 2020)
Abstract The enhanced reducibility of the surface of ceria relative to the bulk has long been established. Several studies also show that ceria nanoparticles with different facets exhibit different catalytic activities. Despite consensus that the activity is correlated with the surface Ce3+ concentration, experimental measurements of this concentration as a function of termination are lacking. Here, X-ray absorption near-edge spectroscopy (XANES) is used to quantify the Ce3+ concentration in films with (001), (110), and (111) surface terminations under reaction relevant conditions. While an enhanced Ce3+ concentration is found at the surfaces, it is surprisingly insensitive to film orientation.
Introduction Ceria is widely used as a catalyst for a range of industrially important chemical,[1] and increasingly, biomedical,[2] transformations. It is well-established that the surface of ceria, at which heterogeneous reactions are catalyzed, is substantially more reduced that the bulk.[3–5] This surface reducibility has been identified as a key factor in the catalytic activity.[6,7] In parallel, strong evidence has emerged that the activity of different facets of ceria can differ substantially, and in recent years, there has been an explosion in efforts to exploit this behavior.[7–10] Studies of termination effects on ceria catalysis rates have largely focused on the three most stable faces, which occur with stability sequence (111) > (110) > (001).[11,12] Computational studies of these surfaces suggest oxygen vacancy formation energies increase in the order (110) > (001) > (111),[12] implying a decrease in vacancy concentrations in this sequence, which notably does not correlate with the stability sequence. Experimental investigations have leveraged shape-selective synthetic approaches that yield faceted ceria nanostructures dominated by one or the other of these terminations.[8–10] In particular, conditions can be tuned to yield ceria cubes which display {001} faces; octahedra, which display {111} faces; truncated octahedra which display {111} and {001} faces; and rods, which grow along the [110] direction and are terminated either primarily by {110} faces or by a combination of {001} and {110} faces.[2,8,9,13–18] Using such materials, the rates of a broad range of reactions catalyzed have been characterized,[10] including for example, the reaction between hydrogen peroxide and tetramethylbenzidine in aqueous solution,[2] photocatalytic degradation of methylene blue,[19] and fairly commonly, gas-phase oxidation of carbon monoxide.[20–22] Similarly, surface defect concentrations have
been studied by a variety of techniques,[2
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