A silica-supported zirconium hydride catalyst for depolymerization of polyethylene is studied using density functional theory (DFT) together with a generalized gradient approximation (GGA) for the exchange and correlation energy. The (100) and (111) surfaces of beta-cristobalite are used as two possible models of a silica surface. Based on the experimental surface structure determined by J. Corker et al. (Science 1996, 271, 966), we propose a detailed atomic model of the zirconium monohydride that is believed to be the active site for depolymerization of polyolefins. Our model of the zirconium monohydride on the (100) surface is found to be very stable and the structure is in good agreement with extended X-ray absorption fine structure (EXAFS) measurements. We have carried out depolymerization of a small polyolefin chain (C3H8) to give CH4 and C2H6 by addition of H-2 The rate-limiting step is a beta-methyl transfer to the zirconium atom, and the activation energy is 29 kcal/mol on the (100) surface.