Uncovering the nature of nanocrystalline Ce-Ni oxide catalysts in heterogenous CO2 reduction with interface effect and reaction mechanism studies
Ákos Szamosvölgyi (1), Rajkumar Thangavel(1), András Sápi(1,2), Imre Szenti(1), Marietta Ábel(1), Juan Fernando Gómez-Pérez(1), Kornélia Baán(1), Zsolt Fogarassy(3), Erzsébet Dodony(3), János Kiss(1,4), Ákos Kukovecz(1), Zoltán Kónya(1,4)
(1) University of Szeged, Interdisciplinary Excellence Centre, Department of Applied and Environmental Chemistry
(2) University of Szeged, Institute of Environmental and Technological Sciences
(3) Centre for Energy Research, Institute of Technical Physics and Materials Sicence
(4) University of Szeged, MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group
Catalytic CO2 conversion is an important subject, since producing chemical feedstock from the anthropogenic CO2 is benefitial both for the environment and the chemical industry. The research for noble metal free catalytic solutions is of great interest.
In this study the effect of interface and phase state are investigated in the case of CeO2 based catalysts enhanced with NiO, which is added in the form of nanoparticles (interface) or lattice dopant (solid solutions state). These prepared catalysts were also compared to the pure Ce or Ni oxide samples. The synthesised materials were characterised with HRTEM (HAADF), EDS, XRD, Raman spectroscopy, BET and H2-TPR techniques. Reactions were run in a continuous flow test reactor attached to a GC equipped with TCD and FID detectors. In situ DRIFTS and ex situ XPS results were evaluated to comprehend reaction mechanisms.
During CO2 hydrogenation test reaction we compared the catalytic behaviour of the interfacial mixed oxide and the solid solution state material. We found the behaviour was significantly different in terms of product selectivity and conversion. The interfacial catalytic system converts 79% of the CO2 feed, with a product selectivity of 97% favouring CH4, opposed to the solid solution sample, which converts only 2% and promotes CO with 94% selectivity.
With in situ DRIFTS we confirmed that both catalytic systems promote the formate pathway to form the products. During ex situ XPS the Ce 3d, Ni 2p and O 1s spectrum regions were analysed before and after reaction. Ce3+ and Ce4+ species were identified; the interfacial sample has a higher ratio of Ce3+/Ce4+ indicating increased oxygen storage capacity thus it is more capable of chemisorbing and activating CO2 before reaction and it is capable of re-oxidizing the reduced NiO nanoparticles, creating a mixed oxide state by the end of the reaction cycle. However, the solid solution sample has less oxygen storage capacity, due to fewer lattice defects and less Ce3+/Ce4+ ratio. The HAADF and EDS images reveal that the solid solution material has homogenous Ni distribution, while the NiO nanoparticles form coherent inclusions in the material.