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(Ce-La-xCu)O2 catalysts with low (3 at.%) and high (10 at.%) Cu content were prepared by conventional microwave (MW) and enhanced microwave methods where air cooling (AC), while heating, was applied. The catalysts were tested for the CO oxidation reaction in the 25–500 °C range using 4%CO/20%O2/He feed gas. Varying spectroscopic, microscopic and catalytic studies were used to probe the effect of synthesis on the nanostructure and the CO oxidation performance. It was found that the synthesis method adopted impacts on the extent of the Cu doping into the (Ce-La)O2 fluorite lattice, hence leading to one and two phases system in the case of catalyst prepared through enhanced (AC) and conventional (MW) microwave methods, respectively. Furthermore, only Ce4+ species were found on the surface of the (Ce-La-10Cu)O2 catalysts synthesized using MW and AC (XPS studies), whereas oxygen vacant sites which are associated with Ce3+ ions were indicated in the sub-surface/bulk (Raman studies). Ultimately, the catalysts with the low and high Cu loading, prepared under the AC-promoted microwave method, presented a superior performance against CO oxidation, exhibiting an overall improvement of the catalytic activity by 16% and 32%, respectively. KP wouldlike to acknowledge the financial support from Abu Dhabi Educational Council (ADEC) through the ADEC 2015 Award for Research Excellence (ADEC B3111 ). KP thanks Khalifa University Core Nanocharacterization Facilities and acknowledges KU and CNCF staff support. AFZ and ASA acknowledges the support of the grant number NPRP 8-1912-1-354 from the Qatar National Research Fund (a member of Qatar Foundation). The microscopy works have been conducted in the �Laboratorio de Microscopias Avanzadas� at �Instituto de Nanociencia de Aragon - Universidad de Zaragoza�. VS acknowledges the LMA-INA for offering access to their instruments and expertise. CIBER-BBN is an initiative funded by the VI National R&D&i Plan 2008-2011 financed by the Instituto de Salud Carlos III with assistance from the European Regional Development Fund . Scopus

CeO2 and CexSm1-xO2 nanoparticle mixed oxides have been synthesized by microwave assisted sol-gel (MW sol-gel) and conventional sol-gel synthesis carried out at 60°C (typical sol-gel) and 100°C (approaching the MW temperature). Different characterization techniques, namely, XRD, BET, Raman, SEM, FTIR, TEM, XPS, H2-TPR, CO2-TPD, and XPS have been employed to understand the process-structure-properties relationship of the catalysts. The CO oxidation performance has been determined both in the absence and in the presence of H2 in the feed gas stream. Microwave heating yields a more thermally stable precursor material, which preserves 75% of its mass up to 600°C, attributable to the different chemical nature of the precursor, compared to the typical sol-gel material with the same composition. Varying the synthesis method has no profound effect on the surface area of the materials, which is in the range 4–35m2/g. Conventional sol-gel synthesis performed at 60 and 100°C yields CeO2 particles with a crystallite size of 29nm and 24nm compared to 21–27nm for MW sol-gel synthesis (at different power values). The MW sol-gel CexSm1-xO2 catalysts exhibit a smaller crystallite size (12–18nm). The pure ceria nanoparticles were shown to have a stoichiometry of approximately CeO1.95. The presence of Ce3+ and Sm3+ in the mixed oxide particles facilitates the presence of oxygen vacant sites, confirmed by Raman. Oxygen mobile species have been traced using H2-TPR studies and a compressive lattice strain in the 0.45–1.9% range of the cubic CexSm1-xO2 lattice were found to be strongly correlated with the CO oxidation performance in the presence and absence of H2 in the oxidation feed stream. MW sol-gel synthesis led to more active CeO2 and Ce0.5Sm0.5O2 catalysts, demonstrated by T50 (temperature where 50% CO conversion is achieved), being reduced by 131°C and 47°C, respectively, compared to typical sol-gel catalysts. Conventional synthesis performed at 100°C leads to a CeO2 catalyst of initially higher activity at a certain temperature window (220–420°C), though with a slower increase of XCO as a function of temperature compared to the MW synthesized catalyst. MW sol-gel synthesized Ce0.8Sm0.2O2 exhibited a high performance (∼90%) for CO oxidation over a period of more than 20h in stream. In addition the effect of reaction temperature and contact time (W/F) on the activity of the CeO2-based materials for CO oxidation kinetics were investigated. The activation energy of the reaction was found to be in the range 36–43kJ/mole depending on the catalyst composition. Abu Dhabi Educational Council (ADEC B3111) and Khalifa University Internal Research Fund (L1 KUIRF-210103) for supporting this research. Work by AFZ and SYA was made possible by the grant number NPRP 6-351-1-072 from the Qatar National Research Fund (a member of Qatar Foundation).

The present work reports on the investigation of the catalytic performance for the methanation of CO2 over Ni catalysts based on CeO2, and for the first time, of Ni catalysts supported on binary CeO2-based oxides, namely, Sm2O3-CeO2, Pr2O3-CeO2 and MgO-CeO2. The supports were obtained using the microwave assisted sol-gel method under reflux, while the catalysts were prepared by the wet impregnation method. For the investigation of the morphological, textural, structural and other intrinsic properties of the catalytic materials a variety of characterization techniques were used, i.e., Raman spectroscopy, XRD, N2 physisorption-desorption, CO2-TPD, H2-TPR, H2-TPD, XPS and TEM. Carbon deposition and sintering were investigated using TEM. It was shown that the addition of Sm3+ or Pr3+, incorporated into the lattice of CeO2, generated oxygen vacancies, but the Ni/Pr-Ce catalyst was found to possess more surface oxygen vacancies (e.g. Ce4+-Ov-Pr3+ entities). Moreover, modification of CeO2 using Sm3+ or Pr3+ restricted the agglomeration of nickel active sites and led to the genesis of Lewis basic positions. These characteristics improved the hydrogenation reaction at lower temperature. On the other hand, the addition of Mg2+ resulted at strong metal support interactions reinforcing the resistance of the Ni/Mg-Ce catalyst against sintering. Furthermore, the addition of Sm3+, Pr3+ and Mg2+ cations increased the overall basicity and the moderate adsorption sites and led to the formation of smaller Ni nano particles; these physico-chemical properties enhanced the CO2 methanation reaction. Finally, the activity experiments (WGHSV = 25,000 mL g−1 h−1, H2/CO2 = 4:1, T =350 °C) showed that at lower reaction temperature the Ni/Pr-Ce had the highest catalytic performance in terms of CO2 conversion (54.5%) and CH4 yield (54.5%) and selectivity (100%). The TOF values were found to follow the order Ni/Pr-Ce >> Ni/Mg-Ce > Ni/Sm-Ce > Ni/Ce., GIS is grateful for financial support by Greece and ESF through the Operational Programme «Human Resources Development, Education and Lifelong Learning» in the context of the project “Strengthening Human Resources Research Potential via Doctorate Research” (MIS-5000432), implemented by the State Scholarships Foundation (IKY). NDC is grateful to the Research Committee of UOWM for financial support through grant no. 80304. IVY and MAG are grateful for financial support from the European Union and Greek national funds through the operational program Competitiveness, Entrepreneurship and Innovation, under the call Research-Create-Innovate (Project code: T1EDK-00782). SAK, AAK and KP acknowledge the financial support from the Abu Dhabi Department of Education and Knowledge (ADEK) under the AARE 2019-233 grant and support by the Khalifa University of Science and Technology under Award No. RC2-2018-024.