catalysts Article Low Temperature Water-Gas Shift: Enhancing Stability through Optimizing Rb Loading on Pt/ZrO 2 Caleb Daniel Watson 1 , Michela Martinelli 2 , Donald Charles Cronauer 3 , A. Jeremy Kropf 3 and Gary Jacobs 1,4, *   Citation: Watson, C.D.; Martinelli, M.; Cronauer, D.C.; Kropf, A.J.; Jacobs, G. Low Temperature Water-Gas Shift: Enhancing Stability through Optimizing Rb Loading on Pt/ZrO 2 . Catalysts 2021, 11, 210. https://doi.org/10.3390/catal11020210 Academic Editors: Panagiotis G. Smirniotis, Devaiah Damma, Sibudjing Kawi and Minghui Zhu Received: 16 December 2020 Accepted: 2 February 2021 Published: 5 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Biomedical Engineering and Chemical Engineering, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA; caleb.watson378@gmail.com 2 University of Kentucky Center for Applied Energy Research, 2540 Research Park Drive, Lexington, KY 40511, USA; michela.martinelli@uky.edu 3 Argonne National Laboratory, Argonne, IL 60439, USA; dccronauer@anl.gov (D.C.C.); kropf@anl.gov (A.J.K.) 4 Department of Mechanical Engineering, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA * Correspondence: gary.jacobs@utsa.edu; Tel.: +1-210-458-7080 Abstract: Recent studies have shown that appropriate levels of alkali promotion can significantly improve the rate of low-temperature water gas shift (LT-WGS) on a range of catalysts. At sufficient loadings, the alkali metal can weaken the formate C–H bond and promote formate dehydrogenation, which is the proposed rate determining step in the formate associative mechanism. In a continuation of these studies, the effect of Rb promotion on Pt/ZrO 2 is examined herein. Pt/ZrO 2 catalysts were prepared with several different Rb loadings and characterized using temperature programmed reduc- tion mass spectrometry (TPR-MS), temperature programmed desorption (TPD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), an X-ray absorption near edge spectroscopy (XANES) difference procedure, extended X-ray absorption fine structure spectroscopy (EXAFS) fit- ting, TPR-EXAFS/XANES, and reactor testing. At loadings of 2.79% Rb or higher, a significant shift was seen in the formate ν(CH) band. The results showed that a Rb loading of 4.65%, significantly improves the rate of formate decomposition in the presence of steam via weakening the formate C–H bond. However, excessive rubidium loading led to the increase in stability of a second intermediate, carbonate and inhibited hydrogen transfer reactions on Pt through surface blocking and accelerated agglomeration during catalyst activation. Optimal catalytic performance was achieved with loadings in the range of 0.55–0.93% Rb, where the catalyst maintained high activity and exhibited higher stability in comparison with the unpromoted catalyst. Keywords: rubidium (Rb); platinum (Pt); zirconia (ZrO 2 ); low temperature water-gas shift (LT-WGS); alkali promotion; electronic effect; formate; associative mechanism; hydrogen 1. Introduction Pure hydrogen sources are necessary for many industrial processes, such as Fischer– Tropsch synthesis, hydrogenolysis, and the Haber process. Among the methods used to produce hydrogen, water-gas shift (WGS) is very attractive, as it simultaneously reacts with excess or unwanted CO. The reaction is reversible and mildly exothermic, so it is typically employed in two stages. First, WGS is performed at high temperatures to take advantage of kinetically superior rates; however, at these high temperatures, the conversion is equi- librium limited. Thus, the second stage is carried out at low temperatures (LT-WGS) with a highly active catalyst to achieve higher CO conversion. Polymer electrolyte membrane fuel cells (PEMFCs) are a heavily researched topic and an important technology for the future of renewable energy and portable power, as they have the potential to cleanly and efficiently provide electrical energy from hydrogen [1–6]. However, these PEMFCs are very susceptible to poisoning by CO, which is a product or byproduct of many hydrogen production reactions (e.g., steam reforming of hydrocarbons or alcohols). Thus, water-gas Catalysts 2021, 11, 210. https://doi.org/10.3390/catal11020210 https://www.mdpi.com/journal/catalysts