The mechanism of photothermal catalytic reforming of biomass tar: An exploratory study based on density functional theory

•The mechanism of photothermal catalytic reforming of biomass tar was revealed by DFT.•The reaction pathways under photothermal and conventional conditions were compared.•The oxygen vacancy and hydroxyl radical under photothermal conditions facilitate the ring-open reaction. Tar removal is a hot topic in the biomass gasification field. Photothermal catalytic reforming (PCR) is an emerging approach that shows high efficiency, but the mechanism and tar cracking reaction pathway under PCR are still not clear. In this paper, as a widely recognized catalyst that has both thermal and photo-catalysis effects, Ni/TiO2 was investigated and the model was calculated by density functional theory (DFT). Toluene was selected as the model tar compound, and the cracking performances were analyzed in both PCR and conventional heating conditions. It was found that H2O and toluene can be stably adsorbed on the catalyst surface but the adsorption sites are different, which provides support for the comparisons of toluene cracking pathways. The energy barriers of CC bond breaking reactions were calculated and the toluene cracking pathway showed significant changes under PCR, due to the formation of oxygen vacancy and hydroxyl radical. Toluene mainly occurs in the dissociation reaction of methyl group under conventional conditions, but toluene tends to open aromatic ring to form chain hydrocarbons under PCR conditions. It indicates that PCR shows greater activation on the aromatic ring which is crucial for tar cracking. This study confirms the change of the toluene cracking pathway under photothermal conditions by the DFT calculation method and also provides guidance for the optimization of the tar removal process and photothermal catalyst preparation.