Periodic DFT calculations have been performed to systematically investigate the mechanisms of methanol decomposition and oxidation on the PtRu/Pt(111) surface. Geometries and energies for the primary species involved are analyzed and the reaction network has been mapped out. The calculation shows that among three initial C-H, O-H, and C-O bond scissions of methanol, the O-H bond scission is found to be the most favorable and bears a lower energy barrier than the desorption of methanol. The decomposition of CH3O occurs via the path CH3O -> CH2O -> CHO -> CO with the limiting step of the first dehydrogenation. Although the oxidation of CO is hindered by a high barrier, the CHO oxidation to CHOOH could occur facilely. Further decomposition of formic acid to CO2 and/or CO could occur via four possible pathways, that is, initial C-H, O-H, and C-O bond activations as well as simultaneous activation of C-H and C-O bonds, where the first pathway, HCOOH -> COOH -> CO2, is the most favorable from a kinetic point of view. Compared to that on Pt(111), methanol on PtRu/Pt(111) prefers to decomposition rather than desorption and then oxidation via the favorable non-CO path with a lower rate-determining energy barrier of CH3O -> CH2O for the whole reaction, which indicates that PtRu alloy can improved tolerance toward CO poisoning compared with pure Pt. (C) 2015 Elsevier B.V. All rights reserved.