The　dissociation　and　hydrogenation　of　CO　2　on　Cu(100)　surfaces　that　are　modified　by　introducing　Co　nanoclusters　with　different　size　into　the　top　layer　have　been　investigated　using　density　functional　theory　method.　Our　results　show　that　on　all　surfaces　the　Co　atoms　are　the　sites　for　the　adsorption　of　CO　2　,　and　in　the　early　stage　of　introducing　Co　dopant,　the　chemisorption　behavior　of　CO　2　is　sensitive　to　the　amount　of　Co　atom.　According　to　the　predicted　pathways　for　the　dissociation　of　CO　2　to　CO,　it　is　interesting　that　the　energy　barrier　decreases　first　and　then　increases　as　more　Co　atoms　are　dispersed　on　the　surface,　forming　a　“V”　shape.　The　minimum　energy　barrier　of　CO　2　decomposition　is　predicted　on　the　Cu(100)　surface　that　contains　four　Co　atoms　aggregated　together　on　the　top　layer,　namely　Co　4　/Cu(100)　bimetallic　surface.　The　most　favorable　reaction　pathway　for　the　hydrogenation　of　CO　to　methanol　on　such　surface　is　further　determined,　which　follows　the　sequence　of　CO*　→　HCO*　→　H　2　CO*　→　H　3　CO*　→　H　3　COH*,　and　the　rate-limiting　step　is　the　hydrogenation　of　H　3　CO　species　with　an　activation　barrier　of　106.4　kJ/mol.　It　is　noted　that　with　respect　to　the　pure　Cu(100),　since　more　stronger　Co–O　adsorption　bonds　are　formed　on　the　Co-modified　surface,　the　stability　of　formaldehyde　intermediate　is　significantly　enhanced.　Correspondingly,　the　introducing　of　Co　4　cluster　tends　to　improve　the　productivity　and　selectivity　towards　methanol　synthesis　on　Cu(100)　surface.　©　2017　Elsevier　B.V.