CO2 electro-reduction CO2 hydrogenation CO2 methanation CO2 reduction H2 dropout Titania alkali oxide alkali promoter carbon dioxide carbon nanofibers carbon-based electrodes catalysis catalyst preparation dehydration diatomite dimethyl carbonate dynamic reaction conditions high pressure photocatalysis mechanochemistry metal-carbon-CNF composites microwaves operando XAS photocatalysis photoreduction plastic waste quick-EXAFS surface oxidation-reduction ultrasound water diffusion water sorption