Abstract
The present study focuses on the modification of surface compositional profiles induced in nanoporous (NP) Au catalysts by the catalytic oxidation of carbon monoxide to carbon dioxide in the presence of oxygen. The phenomenon has deep implications concerning the catalytic behavior of NP Au foams in particular, and more in general for the design of more efficient catalysts. Aimed at gaining deeper insight into the mechanisms governing surface segregation, we exposed NP Au foams containing residual Ag to a mixture of gaseous carbon monoxide and oxygen at different temperature. Structural and surface composition analyses pointed out the concomitant occurrence of both NP Au coarsening and Ag surface segregation processes. Experimental findings suggest for Ag surface segregation a two-stage kinetics. During the initial, rapid coarsening of the NP Au structure, Ag surface segregation is mediated by surface rearrangements, which allow the Ag atoms to reach the surface at anomalously fast rate. As coarsening decelerates, the slower diffusion of buried Ag atoms towards the surface predominates, due to favorable chemical interactions with adsorbed oxygen. This novel mechanism’s understanding can benefit strategic areas of science and technology.
Highlights
Last ten years have seen a flowering of interest in the study of monolithic nanoporous (NP) Au foams[1,2,3]
Au foams[6,20,21,22,23,24,25] and that chemically active surface sites participate in surface diffusion processes mediating coarsening[6,20,21,22,23,24,25], further complicates the conceptual framework. Under these circumstances, characterizing the kinetics of Ag surface segregation in NP Au foams exposed to reactive carbon monoxide (CO)-O2 gaseous mixtures represents a crucial step to advance fundamental knowledge and enable the design of superior NP Au catalysts. Focusing exactly on this issue, we provide sound experimental evidence of the deep link between the Ag surface segregation and coarsening processes that occur simultaneously in NP Au foams throughout the catalytic CO oxidation
We exposed NP Au catalysts to reactant gas mixtures consisting of 1% CO, 10% O2, and 89% N2 under flow conditions
Summary
Last ten years have seen a flowering of interest in the study of monolithic nanoporous (NP) Au foams[1,2,3]. The evidence that catalytic transformations promote the coarsening of NP www.nature.com/scientificreports/ Under these circumstances, characterizing the kinetics of Ag surface segregation in NP Au foams exposed to reactive CO-O2 gaseous mixtures represents a crucial step to advance fundamental knowledge and enable the design of superior NP Au catalysts. Focusing exactly on this issue, we provide sound experimental evidence of the deep link between the Ag surface segregation and coarsening processes that occur simultaneously in NP Au foams throughout the catalytic CO oxidation. We exposed the NP Au foams to a reactant gas mixture consisting of 1% CO, 10% O2, and 89% nitrogen (N2) under flow conditions at temperatures between 273 and 303 K (see Supporting Information SI. 1 for details)
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