The mechanism of CO2 reduction by H-2 at atmospheric pressure was investigated on Ru(0001) by coupling density functional theory (DFT) calculations with mean-field microkinetic modeling. The initial CO2 hydrogenation step leading to CH4 production was shown to occur through CO2 dissociation and subsequent hydrogenation of CO* to CHO*. The dissociation of CHO* to form CH* and O* was identified as the rate limiting step for CH4 formation, while the rate limiting step for CO production through the reverse water gas shift reaction was identified as CO* desorption. Based on a scaling relations analysis of competing CHO* dissociation and CO* desorption, O* adsorption energy was found to be an effective descriptor of differences in selectivity between CO and CH4 production previously observed on late-transition metal catalysts. These mechanistic insights provide critical information to guide the design of catalysts with tunable selectivity for CO2 reduction by H-2 at atmospheric pressure. (C) 2016 Elsevier Inc. All rights reserved.