Fourth Meeting of the
Catalysis Club of Chicago 2014-2015

Monday, January 12, 2015


 

Praga/Bonton
229 W St Charles Rd
Lombard, IL 60148
(630) 495-0470

Cost $45 Professionals
$20 Students/Post Docs

Non-isothermal reaction kinetics and operando spectroscopies reveal the inherently dynamic behavior of catalysts

 

Prof. Susannah Scott
Department of Chemical Engineering
Department of Chemistry and Biochemistry
University of California, Santa Barbara
Santa Barbara, CA 93106

Web Site

Abstract

Kinetic analyses and in situ spectroscopic methods have long been used to characterize catalysts, and are considered essential to our understanding of how these materials behave under reaction conditions. Our recent explorations of simple heterogeneous catalysts (e.g., Pd/Al2O3) using a combination of unconventional kinetic methods and operando spectroscopies reveal that active sites respond to variations in the redox environment, which are a strong function of conversion for reactions like CO oxidation. Inspired by the observation that the slope of the light-off profile increases abruptly at intermediate conversion, we investigated the origin of the discontinuity by recording spectra below and above the transition to correlate activity with structural changes.

Operando IR spectroscopy of adsorbed CO and operando X-ray absorption spectroscopy at the Pd K-edge were used to obtain complementary information about surface and the sub-surface states in PdOx nanoparticles during lean CO oxidation for three distinct activity regimes: low, intermediate and high conversion. A change in the oxidation state of surface Pd atoms coincides with a first-order kinetic phase transition associated with light-off and extinction. Specifically, IR shows that the surface switches between reduced and oxidized states, while EXAFS demonstrates that the sub-surface remains reduced. The PdOx surface coexists with reduced, sub-surface Pd in the high activity, intermediate conversion regime.


Of course, these results raise a philosophical question about whether it is even meaningful to identify "the" active sites, since a complete picture of even this simple catalyst reveals it to be an inherently dynamic material. Nevertheless, we have identified the most active state to be a metallic nanoparticle with an oxidized surface. While this is a meta-stable form of Pd/Al2O3, it provides us with impetus to design catalysts that have stable cationic Pd sites for permanently enhanced reactivity. A family of such catalysts based on Pd-containing complex oxides does indeed show remarkably high turnover frequencies and follows the same rate law over virtually the entire conversion range.