Aquathermolysis of heavy oil using a mixed nickel-oxide/iron-oxide catalyst: Effects induced by crystal phase of iron oxide

•Physically mixed NiO–Fe2O3 catalysts were used in aquathermolysis of heavy oil.•Their coke-restraining ability was probed using an asphaltene upgrading index.•The crystal phase of Fe2O3 in the NiO–Fe2O3 catalysts affected their performance.•NiO/α-Fe2O3 with highly crystalline α-Fe2O3 exhibited optimal performance.•Deuterium tracing clarified the hydrogen transfer during catalytic aquathermolysis.