Low-temperature complete oxidation of ethylene by the mesoporous silica-supported Pt catalyst is a forefront technology for food preservation. Public implementations of the Pt catalyst have already begun, and spectroscopy analyses on the catalytic mechanism have been reported. In this study, density functional theory calculations were conducted to clarify the potential energy profile and electronic mechanism of the catalytic reaction. Based on the experimental findings, a reaction pathway was proposed for ethylene oxidation up to CO2 formation via the HCHO intermediate. Among several possibilities, a reaction pathway via ethylene dioxide species is energetically plausible for the C-C bond cleavage to generate HCHO. Particular focus was given to the electronic effect of the silica support in the ethylene dioxide route. The reservoir effect, in which the siloxide groups take electrons from the Pt moiety, reduces the activation energy of the C-C bond cleavage step by taking electrons from the σ(C-C) orbital.