The mechanism of water gas shift reaction (WGSR; CO + H2O -> CO2 + H-2) catalyzed by nanosized single-atom core-shell clusters have been studied using density-functional theory (DFT) calculations. To investigate single atom effects on catalytic activation, Cu-12-based nanoclusters with nine different transition-metal (TM) elements have been designed into core shell TM@Cu-12 (TM = Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au) bimetallic nanoclusters. Three WGSR mechanisms including the redox, carboxyl, and formate mechanisms, which are equal to CO* + O* -> CO2 (g), CO* + OH* -> COOH* -> CO2(g) + H*, and CO* + H* + O* -> CHO* + O* -> HCOO** CO2(g) + H*, respectively, have been investigated. And the WGSR prefer to follow the carboxyl mechanism on the entire TM@Cu-12 surfaces. Free energy profiles are calculated in order to find the turnover-determining transition state (TDTS) and the turnover-determining intermediate (TDI) that govern turnover frequency (TOF) and selectivity. The TOF of the group 9 (Co, Rh, and Ir) are all higher than group 10 and 11. They are thus the promising candidates for improved WGSR catalysts. Our results will be important for predicting the energetic trends of other catalytic reactions and for designing a better catalyst for the industrially important reaction. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.