A general strategy and a consolidated mechanism for low-methane hydrogenolysis of polyethylene over ruthenium

Polyethylene (PE) is one of the most environment-threatening plastic waste. Its Ru-catalyzed hydrogenolysis is rapid but produces too much methane. Here, low-density PE hydrogenolysis is performed at mild conditions and short times over Ru-doped zirconia catalysts (Ru-XZr, X = Ti, Nb, Ce, W, V, Mo, Fe) to determine low-methane catalysts. Methane is produced via direct terminal C-C scission and surface cascade of consecutive C-C scissions, with the latter sensitive to hydrogen availability and dominant in hydrogen-lean conditions. Reactivity studies, characterization and theory reveal that the most effective dopant oxides (W, V, and Mo) are intermediately reducible, as they store and supply extra hydrogen to Ru via reverse hydrogen spillover. This hydrogen readily hydrogenates and desorbs long alkyl surface intermediates that would otherwise produce cascade methane. Our proposed mechanism reconciles the low methane production achieved by increasing the hydrogen pressure or reducing the particle size and exposes the key selectivity descriptors. Ru-catalyzed LDPE hydrogenolysis is rapid but produces too much methane. Ru-doped zirconia catalysts with intermediately reducible dopant oxides suppress methane because they store and supply extra hydrogen to Ru via reverse hydrogen spillover. This hydrogen readily hydrogenates and desorbs long alkyl surface intermediates that would otherwise produce cascade methane. [Display omitted] •LDPE hydrogenolysis is performed over Ru-XZr to determine low-methane catalysts.•Two CH4-producing pathways are identified and investigated using surrogate reaction.•Intermediately reducible dopants suppress methane based on a volcano correlation.•A C-C scission cascade dominates CH4 production, and is sensitive to H/Ru coverage.•Promotional dopants store and supply extra hydrogen to Ru via reverse spillover.