Atomically dispersed supported metal catalysts offer new prospects for low-cost fuels and chemicals production, as is discussed in a recent review1. In this presentation, recent advances in the design of such catalysts for the low-temperature water-gas shift (WGS) reaction will be highlighted. The WGS reaction is an integral part of fuel gas processing and hydrogen production.
For fuel cell applications, a new generation of active and robust shift catalysts is needed. The new catalysts must also be economical. If noble metals are to be used, can they be used with 100% atom efficiency? Are atomically dispersed metals supported on oxides viable (i.e. stable) catalysts for practical development? Is the support directly or indirectly involved in the reaction pathway? Answers to these fundamental questions are important to guide the rational design of catalysts for the WGS and other reaction systems. We will address these questions in the presentation drawing from our own work with Au and Pt catalysts prepared on a variety of supports in a way that achieves stable single-site metal centers bound to the support through –O ligands2-7. When the support is inert, such as silica, a shell of alkali ions are used to stabilize the active metal center though –O ligands3,5,6. Addition of alkali ions (sodium or potassium) to gold on KLTL-zeolite and mesoporous MCM-41 silica stabilizes mononuclear gold in Au-O(OH)x-(Na or K) ensembles5. This single-site gold species is active for the low-temperature (< 200 °C) water-gas shift (WGS) reaction. We have reported similar findings with Pt-O(OH)x-(Na or K)6. Gold is thus similar to platinum in creating –O linkages with more than eight sodium ions and establishing an active site on various supports. New, simpler approaches to catalyst preparation have been developed to prepare these active sites exclusively. Since the effect of support is “indirect,” these findings pave the way for the use of a variety of earth-abundant supports as carriers of atomically dispersed platinum or gold with alkali additives for the WGS and potentially other fuel processing reactions. A generalized approach to the design of WGS catalysts has emerged from these studies.
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4. M. Yang, L.F. Allard, M. Flytzani-Stephanopoulos, J. Amer. Chem. Soc. 2013, 135, 3768−3771.
5. M. Yang, S. Li, Y. Wang, J.A. Herron, Y. Xu , L. F. Allard, S. Lee, J. Huang, M. Mavrikakis, M. Flytzani-Stephanopoulos, Science 2014, 346, 1498.
6. M. Yang, J. Liu , S. Lee, B. Zugic, J. Huang, L. F. Allard, M. Flytzani-Stephanopoulos, J. Amer. Chem. Soc. 2015, 137, 3470.
7. B. Zugic, S. Zhang, D. Bell, F. Tao, M. Flytzani-Stephanopoulos, J. Am. Chem. Soc. 2014, 136, 3238.