Despite　extensive　studies　of　hydrogen　spillover　on　single-atom　alloy　surfaces,　a　thorough　understanding　of　the　structure-activity　relationship　is　still　lacking.　Here,　we　investigate　H-2　dissociation　and　diffusion　of　the　dissociated　H　species　on　the　near-surface　alloys　embedded　with　single　Pt　atoms　using　density　functional　theory　(DFT)　calculations　and　ab　initio　molecular　dynamics　(AIMD)　simulations.　The　DFT　results　indicate　that　subsurface　alloying　with　early　transition　metals　(X)　(Pt1-X/Cu(111))　can　generally　promote　the　initial　hydrogen　spillover　but　suppress　the　H-2　dissociation　process,　showing　an　intractable　trade-off　effect.　While　the　DFT-calculated　H-2　dissociation　barrier　on　Pt-1-Co/Cu(111)　is　higher　than　that　on　Pt-1-Ni/Cu(111),　the　AIMD　results　show　that　the　H-2　dissociation　probability　on　the　Pt-1-Co/Cu(111)　surface　is　much　higher　than　that　on　Pt1-Ni/Cu(111).　The　trajectory　analysis　shows　that　H-2　molecules　on　Pt-1-Co/Cu(111)　can　adopt　a　more　convenient　conformation　for　dissociation　when　approaching　the　so-called　close-range　physisorption　zone　(CPZ)　due　to　the　relatively　flat　topography　of　the　potential　energy　surface,　thus　increasing　the　H-2　dissociation　probability　compared　to　the　case　on　Pt-1-Ni/Cu(111).　This　work　provides　a　clear　picture　for　understanding　the　structure-activity　relationships　of　H-2　activation　and　hydrogen　spillover　over　single-atom　catalysts.　More　importantly,　it　highlights　an　overlooked　but　essential　role　of　the　dynamic　orientation　of　the　reactant　in　heterogeneous　catalysis.