XANES STUDY OF CERIUM OXIDATION STATE OF CE-FE-O CATALYSTS IN CATALYTIC OPERATING CONDITIONS

L. M. TOSCANI; M. MAZAN; S.A. LARRONDO.

Campinas, Brasil

Encuentro; 29th. RAU Annual Users Meeting LNLS; 2019

Laboratorio Nacional de Luz Sincrotrón

Ceria-based mixed oxides have been widely studied in the past years due to their redox properties. The presence of the Ce4+/Ce3+ couple provides high oxygen storage capacity to these materials which can be further improved by the adition of aliovalent dopants to the CeO2 fluorite structure. These properties are ofgreat interest for catalytic and electrocatalytic applications. Therefore, the main objective of this work is to study the effect of temperature and fuel composition (H2 or CH4/CO2) in the Ce3+/Ce4+ ratio in Ce-Fe-O mixed oxides, which is strongly related with redox and catalytic performance of the samples. Ce-Fe-O mixed oxides were synthesized by cation complexation with citric acid, liquid mix and freeze-drying methods and fired at 500°C. The solids studied have the following compositions: Ce1-xFexOδ with x = 0, 0.05; 0.10; 0.15, 0.20and 1. Furthermore, some selected samples with superior catalytic and electrocatalytic performance at conventional laboratory experiments were fired at 800°C and impregnated with NiO (5%at. Ni). In-situ DXAS experiments were performed at the D06A-DXAS dispersive beam line of the LNLS in the energy regioncorresponding to the Ce LIII-edge. Temperature programmed reduction (TPR) experiments were carried out by heating the samples in a flow of diluted hydrogen, from room temperature to 800°C. Synthetic biogas conversion to syngas was studied in samples impregnated with NiO. From in-situ DXAS experiments weconcluded that product distribution observed in mass spectrometer data was significantly correlated to Ce4+/Ce3+ ratio in the samples during catalytic tests with CH4/CO2 feed flow. Sample 5Ni/Ce0.9Fe0.1Oδ exhibited very intense H2 and CO signals when a steep peak of Ce reduction was observed at 750°C.Afterwards Ce re-oxidation was observed followed by H2O and CO2 production. Sample 5Ni/Ce0.9Fe0.15Oδ exhibited a less intense peak in Ce reduction and very small production of CO and H2 whereas unsupported Ce0.9Fe0.1Oδ exhibited a continuous increase in Ce reduction and virtually no CO and H2 production, reaching ca. 20% Ce reduction values in contrast to the 7.5% obtained for sample5Ni/Ce0.9Fe0.1Oδ. This results can be correlated to our previous conventional laboratory tests where Sample 5Ni/Ce0.9Fe0.1Oδ exhibited superior catalytic activity and stability in time-on-stream experiments. Therefore, samples that are more prone to exchange oxygen from the lattice during experiments with hydrocarbon feed appear to be more tolerant to carbon formation and subsequent deactivation.