Fig. 4: Characterizations of catalysts with various techniques.

a XPS spectra of the Co 2p orbital and b X-band EPR spectra of unreduced Co3O4, Co3O4-200, Co3O4-250 and Co3O4-300 at 77 K. c Comparison of the difference INS spectra of the Co3O4-250 and Co catalysts upon hydrogenation showing unique signals of hydrogenous species on Co3O4-250. The difference spectra are obtained by subtracting the INS spectrum of fresh catalyst from that of the hydrogenated catalyst.