Enhancement of Ru on furoic acid synthesis from furfural oxidation over Rux/δ-MnO2 catalysts

•Reveals that small-sized, high-valence Ru has the highest TOF in the furfural oxidation reaction.•Methanol was used as a probe to verify the evolution of intermediates in furfural oxidation processes.•The Ru's valence state is positively correlated with Ru's TOF.•Using the least amount of Ru (0.25 wt%) and maximizing its gain, the reaction rate was increased by 18 times.