Isotopic transient analysis of steady state of CO hydrogenation catalyzed by alumina-supported MO2C nanoclusters was performed at 573 K, 1.2 bar syngas with H-2/CO = 1. An isotope switch from (CO)-C-12 to (CO)-C-13 during the steady-state reaction produced a rapid transient associated with CO hydrogenation and a very slow transient associated with turnover of ineffective catalytic sites or exchange of carbidic carbon with reaction intermediates. Although the intrinsic turnover frequency for hydrocarbon formation was similar to that observed on late transition metals, the coverage of reaction intermediates on the MO2C clusters was very low (<< 1%), which accounts for the overall low activity of MO2C. Promotion of MO2C with Rb2CO3 lowered the coverage of intermediates even further, but increased the selectivity toward alcohols. Very strong interaction of the alcohols with the promoter suggests that Rb may form alkoxides at reaction conditions. (C) 2013 Elsevier Inc. All rights reserved.