Unraveling the chemical state of Cobalt in Co-based catalysts during Ethanol Steam Reforming: an in situ study by Near Ambient Pressure XPS and XANES

HUCK-IRIART, CRISTIAN; SOLER, LLUÍS; CASANOVAS, ALBERT; MARINI, CARLO; PRAT, JORDI; LLORCA, JORDI; ESCUDERO, CARLOS

ACS Catalysis

American Chemical Society

Año: 2018 vol. 8 p. 9625 - 9636

2155-5435

The steam reforming of ethanol (ESR) has been studied by Near Ambient Pressure XPS (NAP-XPS), Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) under in situ conditions in the ALBA synchrotron facility at 200-580ºC and S/C=3 over different cobalt-based catalysts that showed different catalytic performance: Co3[Si2O5]2(OH)2 (Co-talc), [Co2Mg4Al2(OH)16]CO3·4H2O (Co-hydrotalcite shortened as Co-HT) calcined at 550ºC, and Co3O4 (Co-spinel). Both, Co-spinel and Co-talc yield to a greater or lesser degree metallic cobalt under ESR conditions. While the Co-spinel shows a complete reduction to metallic cobalt under the conditions used for the XANES measurements, more bulk-sensitive, the Co-talc sample exhibits only a partial reduction. On the other hand under the ESR conditions used with the NAP-XPS, a more surface sensitive technique, the results indicate a higher reduction degree for the Co-talc sample as compared to the Co-spinel. In contrast, the catalyst prepared from the Co-HT does not show metallic cobalt traces under the experimental conditions used with both techniques. Comparing these three cobalt-based catalysts, the stable operation exhibited by Co-HT under ESR reaction conditions is justified by the absence of metallic cobalt formation under in situ conditions, which is identified as responsible for the carbon deposition phenomena that triggers the deactivation suffered by most cobalt-based catalysts during ESR.