catalysts Article Shape Effects of Ceria Nanoparticles on the Water-Gas Shift Performance of CuO x /CeO 2 Catalysts Maria Lykaki 1 , Sofia Stefa 1 ,Sónia A. C. Carabineiro 2,3 , Miguel A. Soria 4 , Luís M. Madeira 4 and Michalis Konsolakis 1, *   Citation: Lykaki, M.; Stefa, S.; Carabineiro, S.A.C.; Soria, M.A.; Madeira, L.M.; Konsolakis, M. Shape Effects of Ceria Nanoparticles on the Water-Gas Shift Performance of CuO x /CeO 2 Catalysts. Catalysts 2021, 11, 753. https://doi.org/10.3390/ catal11060753 Academic Editor: Leonarda Francesca Liotta Received: 25 May 2021 Accepted: 18 June 2021 Published: 21 June 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 School of Production Engineering and Management, Technical University of Crete, GR-73100 Chania, Greece; mlykaki@isc.tuc.gr (M.L.); sstefa@isc.tuc.gr (S.S.) 2 LAQV-REQUIMTE, Departmentof Chemistry, NOVA School of Science and Technology, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal 3 Laboratory of Catalysis and Materials (LCM), Associate Laboratory LSRE-LCM, Faculty of Engineering, University of Porto, 4200-465 Porto, Portugal; sonia.carabineiro@fct.unl.pt 4 LEPABE–Laboratory for Process Engineering, Environment, Biotechnology and Energy, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal; masoria@fe.up.pt (M.A.S.); mmadeira@fe.up.pt (L.M.M.) * Correspondence: mkonsol@pem.tuc.gr; Tel.: +30-28210-37682 Abstract: The copper–ceria (CuO x /CeO 2 ) system has been extensively investigated in several cat- alytic processes, given its distinctive properties and considerable low cost compared to noble metal- based catalysts. The fine-tuning of key parameters, e.g., the particle size and shape of individual counterparts, can significantly affect the physicochemical properties and subsequently the catalytic performance of the binary oxide. To this end, the present work focuses on the morphology effects of ceria nanoparticles, i.e., nanopolyhedra (P), nanocubes (C), and nanorods (R), on the water–gas shift (WGS) performance of CuO x /CeO 2 catalysts. Various characterization techniques were employed to unveil the effect of shape on the structural, redox and surface properties. According to the acquired results, the support morphology affects to a different extent the reducibility and mobility of oxygen species, following the trend: R > P > C. This consequently influences copper–ceria interactions and the stabilization of partially reduced copper species (Cu + ) through the Cu 2+ /Cu + and Ce 4+ /Ce 3+ redox cycles. Regarding the WGS performance, bare ceria supports exhibit no activity, while the addition of copper to the different ceria nanostructures alters significantly this behaviour. The CuO x /CeO 2 sample of rod-like morphology demonstrates the best catalytic activity and stability, approaching the thermodynamic equilibrium conversion at 350 ◦ C. The greater abundance in loosely bound oxygen species, oxygen vacancies and highly dispersed Cu + species can be mainly accounted for its superior catalytic performance. Keywords: ceria nanoparticles; CuO x /CeO 2 mixed oxides; shape effects; water–gas shift reaction (WGSR) 1. Introduction Hydrogen production is considered an interesting alternative in the overall energy demand scheme. Concerning the hydrogen economy, polymer electrolyte membrane fuel cells (PEMFCs) ought to play a major role in future years [1–3]. In this regard, it is important that CO impurities are eliminated in order for the anode electrodes to be protected. Hence, the water–gas shift (WGS: CO + H 2 O ↔ CO 2 +H 2 ) is a reaction of particular importance in various reforming processes towards the production of hydrogen-rich gas streams for numerous applications [4]. Catalysts 2021, 11, 753. https://doi.org/10.3390/catal11060753 https://www.mdpi.com/journal/catalysts