Insights into the state of ceria during ethanol steam reforming over Ir/CeO2

Highly active Ir/CeO2 is investigated following 240 h membrane-assisted ethanol steam reforming at 873 K to elucidate the effect of H-2 removal on catalyst activity. Sub-nm Ir species aggregated into particles with 1.4 nm mean diameter retaining ca. 78% Ir dispersion. Surface area declined by 83% due to ceria particle growth which were extensively reduced and difficult to re-oxidize at 673 K. This distinct CeO2-x state includes 60-65% Ce-III near its surface. Correlations with ethanol and acetaldehyde steam reforming kinetic analyses of Ir/CeO2, Ir/CeO2-x, and a partially re-oxidized catalyst suggest that the initiating ethanol dehydrogenation involves only Ir sites and possibly CeO2 at their perimeter. In contrast, ceria controls the subsequent acetaldehyde conversion via activation of steam and probably also directly as oxidant. Product distributions reveal that acetaldehyde reforming via acetic acid is favored on Ir/CeO2 at lower temperatures while its decomposition into CH4 and CO prevails on Ir/CeO2-x at elevated temperatures.