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We have investigated how the reaction yield of methane photochemistry on Pt(111) is correlated with the work function change of the substrate. We used Cu, oxygen, Cs as a modifier of the substrate. It was found that the photoyield decreases with decreasing the work function. In addition, the photochemical cross section strongly depends on the number of Cu layers in the case of Cu/Pt(111) bimetallic surface. Possible origins of these effects will be discussed.