SURFACE COVERAGE EFFECTS IN THE HYDROGENOLYSIS OF LINEAR HYDROCARBON OVER RUTHENIUM CATALYST

Abstract

The catalytic hydrogenolysis of hydrocarbons represents a promising strategy for the chemical upcycling of polyolefin waste into valuable liquid products under mild conditions. However, the fundamental factors governing reaction rates and product selectivity remain incompletely understood, particularly with respect to the role of surface coverage and competitive adsorption.In this work, the hydrogenolysis of linear hydrocarbons (n-hexane, n-octane, and n-decane) was systematically investigated over Ru-based catalysts under controlled reaction conditions to elucidate the effects of hydrogen partial pressure, hydrocarbon chain length, and the presence of unsaturated species. Experiments were conducted in a continuous-flow fixed-bed reactor under differential conditions, enabling the measurement of intrinsic turnover frequencies and carbon-based product selectivities. Hydrogenolysis rates exhibited a strong dependence on hydrogen partial pressure, with shorter hydrocarbons displaying a volcano-type behavior characterized by positive reaction orders at low hydrogen pressure and negative reaction orders at high hydrogen pressure. In contrast, longer hydrocarbons showed reduced sensitivity to hydrogen pressure, approaching zero-order behavior due to stronger adsorption and increased surface coverage of hydrocarbon-derived intermediates. Product selectivity was also strongly influenced by hydrogen pressure, with low hydrogen coverage favoring extensive C–C bond scission and methane formation, while higher hydrogen coverage suppressed over-cracking and promoted the formation of larger hydrocarbon fragments. The effect of hydrocarbon partial pressure demonstrated that reaction rates increase with increasing hydrocarbon concentration, while product selectivity remains largely invariant, indicating that hydrocarbon adsorption influences the formation of reactive intermediates without altering the dominant reaction pathway. Residence-time studies confirmed that hydrogenolysis proceeds under intrinsic kinetic control through a single adsorption–desorption cycle, with negligible contribution from secondary reactions. The role of unsaturated hydrocarbons was further investigated through co-feeding studies using corresponding alkenes. Alkenes exhibited lower intrinsic hydrogenolysis activity than alkanes but significantly modified catalytic behavior when present in the feed. Even small concentrations of alkenes led to substantial changes in both reaction rates and product selectivity, consistent with their strong adsorption on the catalyst surface. Depending on reaction conditions, alkenes were found to either inhibit hydrogenolysis through site blocking or promote C–C bond cleavage by increasing the population of reactive surface intermediates. Overall, the results demonstrate that hydrogenolysis over Ru catalysts is governed by a surface-coverage-controlled mechanism involving competitive adsorption between hydrogen, alkanes, and alkenes. The balance between these species determines both catalytic activity and product distribution, providing a unified framework for understanding and controlling hydrogenolysis reactions relevant to plastic waste valorization.