| The dramatic increase in atmospheric CO2 level has caused severe global warming. In this paper, graphite oxides (GO) were synthesized from graphite powder according to the modified Hummers' method. Metal (Pd, Cu, Fe, In, Ru, Pd-In)/RGO nanostructure catalysts for electrocatalytic reduction of CO2 has been prepared by reducing agent NaBH4 and some small molecule acids such as formic acid have been obtained thereby. The nanostructure catalysts were characterized and tested by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry (CV) technologies. Moreover, metal/graphene/polyurethane sponge (M/RGO/PS) for electrocatalytic reduction of CO2 via a facile and inexpensive'dipping and drying'method was studied and characterized by SEM and linear sweep voltammetry scans. Electro-catalytic reduction product of CO2 by M/RGO/PS electrode was measured with ion chromatography.Metal nano-particles (Pd, Cu, Fe, In, Ru, Pd-In) with average size of 5.9,5.4,4.3,4.2,6.2,7.3 nm were highly dispersed in the graphene sheets with amorphous structure. Reduction degree of all metal salts have reached more than 75%, indicating a successful preparation of the catalyst. A small amount of residual oxygen-containing functional groups on the graphene surface provides sites for metal nanoparticles combined with graphene. When the voltage reached -2V, the increased multiples for reduction current of CO2 relative to blank value according to the following CV curve was sorted: 1%Pd-1%In/RGO (2.93)> 1%Cu/RGO (1.87)> 1%Pd/RGO (1.73)> 1%In/RGO (1.62)> 1%Ru/RGO (1.39)> 1%Fe/RGO (1.10). And the peak potentials of M/RGO catalysts can be sorted:1%Ru/RGO (-1.38 V)< 1%Fe/RGO (-1.36 V)< 1%Cu/RGO (-1.25 V)< 1%Pd/RGO (-1.21 V)< 1%In/RGO (-1.10 V)< 1%Pd-1%In/RGO (-0.98 V). This provided obvious evidence for catalytic reduction of CO2 by M/RGO catalysts.1%Pd/RGO and 1%In/RGO have best electrocatalytic performance in all the single metal catalysts, but the electrocatalytic performance of 1%Pd-1%In/RGO is better than all single metal catalysts.M/RGO nanostructure catalysts have been well covered on polyurethane sponge. However, the peak potentials of all M/RGO/PS electrodes were also sorted:Fe/RGO/PS (-1.10 V)< Ru/RGO/PS (-0.80 V) < Cu/RGO/PS (-0.77 V)< Pd/RGO/PS (-0.75 V)< In/RGO/PS (-0.60 V)< Pd-In/RGO/PS (-0.50 V). The increased multiples for reduction current of CO2 relative to blank value was sorted:Pd-In/RGO/PS (0.69)> Pd/RGO/PS (0.65)> Fe/RGO/PS (0.57)> Ru/RGO/PS (0.54)> In/RGO/PS (0.28)> Cu/RGO/PS (0.26). Accordingly, the comprehensive ability of electro-catalytic reduction of CO2 was obtained:Pd-In/RGO/PS> Pd/RGO/PS> In/RGO/PS>Ru/RGO/PS> Fe/RGO/PS> Cu/RGO/PS. Formic acid, acetic acid and oxalic acid were the reduction products of CO2 by Pd-In/RGO/PS, Pd/RGO/PS and In/RGO/PS catalytic electrodes. Formic acid is the major product. Faraday efficiency of formic acid was sorted:Pd/RGO/PS (81.3%)> In/RGO/PS (71.9%)> Pd/RGO/PS (65.2%). Most adsorbed CO2(ad) on the surface of Pd-In/RGO/PS electrode did not form stable adsorbed radical anion ·CO2(ad)_-, and formic acid was formed directly. However, a small part of adsorbed CO2(ad) was converted to stable ·CO2(ad)-.Acetic acid and oxalic acid were produced after catalytic hydrogenated by · CO2(ad)-. So M/RGO/PS electrode, as supporting skeleton of metal/graphene catalysts, not only has similar electrochemical properties of metal/graphene catalysts, but also inherited the nature of polyurethane sponge as a support material. Hence, M/RGO/PS electrode made by polyurethane sponge and M/RGO catalysts has high electrocatalytic effect for reduction of CO2. |
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