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• Higher activity of smaller gold nanoparticles (AuNPs) towards glucose and xylose hydrogenation and oxidation was observed. • Higher extent of HER observed with xylose solution compared to glucose when moving towards more negative potentials (in-situ FTIR-ATR). Electrocatalytic conversion of hemicellulose-derived glucose and xylose presents a sustainable approach to utilize renewable energy (e.g. solar, wind) to produce value-added chemicals. In this work, the electrochemical hydrogenation and oxidation of glucose and xylose at ambient conditions were studied using dispersed, mesoporous Sibunit Carbon (SC)-supported Au nanocatalysts (SC/AuNPs) with different cluster sizes (4.4 nm, 5.9 nm, 10-30 nm), providing novel results on the cluster size - activity relation. For the electrocatalytic hydrogenation (ECH) of glucose and xylose into sorbitol and xylitol, respectively, higher conversion rates were obtained when more negative potentials were applied. This indicates that the hydrogenation reaction proceeds concurrently with hydrogen evolution reaction (HER). SC/AuNPs with smaller Au clusters were more active towards glucose or xylose ECH at all potentials applied. The selectivity (Faradaic efficiency) increased towards more negative potentials for glucose ECH but followed an opposite trend for xylose ECH. Analysis using in-situ FTIR-ATR spectroscopy showed that water adsorption which leads to HER, was more extensive in xylose solution than in glucose solution at lower potentials. This would indicate greater inhibition of xylose ECH than glucose ECH at more negative potentials. The electrocatalytic oxidation (ECO) of glucose and xylose to gluconic acid and xylonic acid, respectively, was observed to occur at -0.05, +0.3, and +0.4 V (vs Ag/AgCl) using the present electrocatalytic set up. Constant-potential electrolysis at these potentials showed differences in ECO rate depending on the applied potential and average Au cluster size. The highest glucose and xylose ECO rate was obtained for the SC/AuNPs with the smallest average Au cluster size (4.4 nm) at +0.3 V. Constant-potential electrolysis of the sugars at +0.3 V using this catalyst resulted in 42 % yield of gluconic acid and 32 % yield of xylonic acid in 6 hours, both with a very low Au loading of 0.08 % wt.. These results show the strong influence of Au cluster size on the catalytic activities of SC/AuNPs toward sugar ECH or ECO. [Display omitted] [ABSTRACT FROM AUTHOR]

• Different AuNPs size distribution among CF/AuNPs catalysts resulting from different pre-treatment methods of the CF support (chemical and thermal pre-treatment) prior to Au deposition. • Higher reaction rates of glucose and xylose toward ECH over a CF/AuNPs catalyst than polycrystalline Au. • Variation of ECSA-normalized ECH activity and Faradaic Efficiency (FE) of CF/AuNPs catalysts depending on the catalyst Au cluster size. Recent evidences have shown catalyst structure sensitivity of electrocatalytic hydrogenation (ECH) of organic molecules in aqueous phase. In this study, ECH of glucose and xylose was investigated using gold nanoparticles (AuNPs) on activated carbon fiber (CF) support as catalyst. CF supported AuNPs catalysts (CF/AuNPs) were synthesized by deposition-precipitation method and were characterized by physicochemical (SEM, TEM, XRD, N 2 physisorption, ICP-OES) and electrochemical methods. Chemical or thermal pre-treatment of CF prior to Au deposition resulted in different AuNPs size distributions. The activity towards glucose or xylose ECH were observed to be dependent on the particle size of AuNPs and the applied potential. Compared to a polycrystalline Au catalyst, higher reaction rates for glucose and xylose ECH were observed for a CF/AuNPs catalyst with a higher surface area. This indicates that the activity of Au towards glucose and xylose ECH is dependent on the surface area and particle size, and particle size control can be achieved by simple chemical or thermal pretreatment of the catalyst support. [Display omitted] [ABSTRACT FROM AUTHOR]