Promoting Effect of the Residual Silver on the Electrocatalytic Oxidation of Methanol and Its Intermediates on Nanoporous Gold

Abstract

Nanoporous gold (NPG) obtained by dealloying Ag75Au25 with an overall residual Ag content of less than 1% was investigated as an electrocatalyst for the oxidation of methanol, formaldehyde, and formate in aqueous 0.1 M NaOH solution. The NPG was used to fill cavity microelectrodes, which allowed the recording of well-resolved voltammetry from the porous material. NPG differs from polycrystalline Au (Au­(poly)) by its microstructure and its residual Ag content and also behaves distinctly different than Au­(poly). The residual Ag content is higher at the surface of the ligaments than in the bulk. By cycling the NPG electrodes in 0.1 M H2SO4, the surface concentration of Ag could be decreased. It could then be set to a defined value by underpotential deposition (UPD) of Ag. The surface structure, and specifically its evolution upon the removal of Ag from the surface, was analyzed by the characteristic voltammetric features of Pb UPD. The effect of Ag on the electrocatalytic methanol oxidation reaction (MOR) is different in different potential regions. Ag coverage shifts the onset of the methanol oxidation current to less positive potentials. In the range of the peak current density, only a defined low Ag concentration enhanced the MOR current density compared to the Ag-free NPG. The {1 0 0} and {1 1 1} facets contributed the largest current, as concluded from selective poisoning experiments. At a potential of 1.63 V vs RHE, Ag2O at the surface is oxidized to AgO. Those layers can oxidize methanol and formate to CO2. The oxidation of formaldehyde proceeds at a much higher reaction rate than the MOR and formate oxidation; the reaction leads to CO and CO2 depending on the applied potential. Given the high oxidation rate of formaldehyde, it would be immediately further oxidized should it be formed as an intermediate of MOR. This is an important difference to the methanol oxidation at Pt. The water oxidation that occurs at the same potential range in the methanol-free solution was suppressed during CO2 formation.

Highly Efficient Methanol Oxidation Reaction Achieved by Cobalt Doping in Delafossite AgNi1–xCoxO2 Solid Solution

“…This proves that there is also a synergistic effect between Ag and Ni/Co atoms on the surface. OH ads plays a very important role in the methanol oxidation process, which can promote the adsorption of methanol, and accelerate the removal of intermediates. ,− Previous studies have demonstrated that Ag 2 O can efficiently adsorb OH – and form Ag 2 O–OH ads . , We speculate that the reason for the high activity of ANCO-2 is also the adsorption of OH – species by Ag oxides.…”

“…However, it can be seen from Figure b that the anode peak appears at 1.5 V, and Ag 0 cannot exist at 1.5 V where the oxidation peak appears, so the catalytic performance of ACO on formaldehyde is mainly due to the presence of Co. The high initial current at 1.2 V when the solution contains formaldehyde (Figure b,c) can be attributed to the presence of Ag nanoparticles . The MOR performance of ANCO-2 and its precursor LiNi 0.8 Co 0.2 O 2 is compared and shown in Figure d, and ANCO-2 significantly exhibits a higher oxidation current and much lower onset potential than LiNi 0.8 Co 0.2 O 2 .…”

“…The introduced oxygen vacancies accelerate charge transfer and (2) In the previous characterization, we can see that for ANCO-2, there are trace amounts of nanosilver particles on its surface. The residual silver particles will be oxidized at 1.5 V and adsorb more OH – , thereby accelerating the release of active sites and improving its durability. , The result of the long-term cyclic stability test by CV analysis for 500 cycles is shown in Figure b. The remaining current density is 41.4, 81.4, and 95.6% for ANCO-1, ANCO-2, and ANCO-3, respectively.…”

“…54,64−66 Previous studies have demonstrated that Ag 2 O can efficiently adsorb OH − and form Ag 2 O−OH ads . 63,67 We speculate that the reason for the high activity of ANCO-2 is also the adsorption of OH − species by Ag oxides.…”

“…The high initial current at 1.2 V when the solution contains formaldehyde (Figure 6b,c) can be attributed to the presence of Ag nanoparticles. 63 The MOR performance of ANCO-2 and its precursor LiNi 0.8 Co 0.2 O 2 is compared and shown in Figure 6d, and ANCO-2 significantly exhibits a higher oxidation current and much lower onset potential than LiNi 0.8 Co 0.2 O 2 . This proves that there is also a synergistic effect between Ag and Ni/Co atoms on the surface.…”