Current chemical recycling of bulk synthetic plastic, polyethylene (PE), operates at high temperature/pressure and yields a complex mixture of products. PE conversion under mild conditions and with good selectivity toward value-added chemicals remains a practical challenge. Here, we demonstrate an atomic engineering strategy to modify a TiO ₂ photocatalyst with reversible Pd species for the selective conversion of PE to ethylene (C ₂ H ₄ ) and propionic acid via dicarboxylic acid intermediates under moderate conditions. TiO ₂ -supported atomically dispersed Pd species exhibits C ₂ H ₄ evolution of 531.2 μmol g _cat ⁻¹ hour ⁻¹ , 408 times that of pristine TiO ₂ . The liquid product is a valuable chemical propanoic acid with 98.8% selectivity. Plastic conversion with a C ₂ hydrocarbon yield of 0.9% and a propionic acid yield of 6.3% was achieved in oxidation coupled with 3 hours of photoreaction. In situ spectroscopic studies confirm a dual role of atomic Pd species: an electron acceptor to boost charge separation/transfer for efficient photoredox, and a mediator to stabilize reaction intermediates for selective decarboxylation.