Design Aspects of Doped CeO2 for Low-Temperature Catalytic CO Oxidation: Transient Kinetics and DFT Approach

Kyriaki Polychronopoulou; Ayesha A AlKhoori; Angelos M Efstathiou; Maguy Abi Jaoude; C M Damaskinos; Mark A Baker; Alia Almutawa; Dalaver H Anjum; Michalis A Vasiliades; Abderrezak Belabbes; Lourdes F Vega; Abdallah Fathy Zedan; Steven J Hinder

doi:10.1021/acsami.1c02934

Design Aspects of Doped CeO₂ for Low-Temperature Catalytic CO Oxidation: Transient Kinetics and DFT Approach

ACS Appl Mater Interfaces. 2021 May 19;13(19):22391-22415. doi: 10.1021/acsami.1c02934. Epub 2021 Apr 9.

Authors

Kyriaki Polychronopoulou^{1

2}, Ayesha A AlKhoori^{1

2}, Angelos M Efstathiou³, Maguy Abi Jaoude^{2

4}, C M Damaskinos³, Mark A Baker⁵, Alia Almutawa¹, Dalaver H Anjum^{2

6}, Michalis A Vasiliades³, Abderrezak Belabbes², Lourdes F Vega^{2

7}, Abdallah Fathy Zedan⁸, Steven J Hinder⁵

Affiliations

¹ Department of Mechanical Engineering, Khalifa University of Science and Technology, Main Campus, Abu Dhabi 127788, UAE.
² Center for Catalysis and Separations, Khalifa University of Science and Technology, Main Campus, Abu Dhabi 127788, UAE.
³ Department of Chemistry, Heterogeneous Catalysis Lab, University of Cyprus, 1 University Avenue, University Campus, 2109 Nicosia, Cyprus.
⁴ Department of Chemistry, Khalifa University of Science and Technology, Main Campus, Abu Dhabi 127788, UAE.
⁵ The Surface Analysis Laboratory, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford GU2 4DL, U.K.
⁶ Department of Physics, Khalifa University of Science and Technology, Main Campus, Abu Dhabi 127788, UAE.
⁷ Research and Innovation Center on CO2 and H2 (RICH), and Chemical Engineering Department, Khalifa University, Abu Dhabi 127788, UAE.
⁸ National Institute of Laser Enhanced Science, Cairo University, Giza 12613, Egypt.

Abstract

CO elimination through oxidation over highly active and cost-effective catalysts is a way forward for many processes of industrial and environmental importance. In this study, doped CeO₂ with transition metals (TM = Cu, Co, Mn, Fe, Ni, Zr, and Zn) at a level of 20 at. % was tested for CO oxidation. The oxides were prepared using microwave-assisted sol-gel synthesis to improve catalyst's performance for the reaction of interest. The effect of heteroatoms on the physicochemical properties (structure, morphology, porosity, and reducibility) of the binary oxides M-Ce-O was meticulously investigated and correlated to their CO oxidation activity. It was found that the catalytic activity (per gram basis or TOF, s^-1) follows the order Cu-Ce-O > Ce-Co-O > Ni-Ce-O > Mn-Ce-O > Fe-Ce-O > Ce-Zn-O > CeO₂. Participation of mobile lattice oxygen species in the CO/O₂ reaction does occur, the extent of which is heteroatom-dependent. For that, state-of-the-art transient isotopic ¹⁸O-labeled experiments involving ¹⁶O/¹⁸O exchange followed by step-gas CO/Ar or CO/O₂/Ar switches were used to quantify the contribution of lattice oxygen to the reaction. SSITKA-DRIFTS studies probed the formation of carbonates while validating the Mars-van Krevelen (MvK) mechanism. Scanning transmission electron microscopy-high-angle annular dark field imaging coupled with energy-dispersive spectroscopy proved that the elemental composition of dopants in the individual nanoparticle of ceria is less than their composition at a larger scale, allowing the assessment of the doping efficacy. Despite the similar structural features of the catalysts, a clear difference in the O_lattice mobility was also found as well as its participation (as expressed with the α descriptor) in the reaction, following the order α_Cu > α_Co> α_Mn > α_Zn. Kinetic studies showed that it is rather the pre-exponential (entropic) factor and not the lowering of activation energy that justifies the order of activity of the solids. DFT calculations showed that the adsorption of CO on the Cu-doped CeO₂ surface is more favorable (-16.63 eV), followed by Co, Mn, Zn (-14.46, -4.90, and -4.24 eV, respectively), and pure CeO₂ (-0.63 eV). Also, copper compensates almost three times more charge (0.37e^-) compared to Co and Mn, ca. 0.13e^- and 0.10e^-, respectively, corroborating for its tendency to be reduced. Surface analysis (X-ray photoelectron spectroscopy), apart from the oxidation state of the elements, revealed a heteroatom-ceria surface interaction (O_a species) of different extents and of different populations of O_a species.

Keywords: 18O isotopic labeling; CO oxidation; DFT; SSITKA-DRIFTS; binary metal oxides; ceria; microwave; oxygen mobility; transition metal.