Electrocatalytic Oxidation of Glucose over Carbon Nanofiber-Supported Platinum Catalyst: Effect of Oxygen-Containing Surface Groups

Abstract

With the transition towards the use of renewable feedstocks and renewable energy the use of bio-based molecules as feedstock and electro(catalytic) conversions gain in interest. Our target is to electrocatalytically convert starch to oxidized starch, which form a replacement for the currently used fossil based poly-acrylates. To develop efficient electrocatalysts for the oxidation process fundamental insights in catalyst property-performance relationships are essential. Since starch is a complex molecule we aim at getting these insights by using gradually more complex feedstocks, going from monomers to oligomers to polymers (e.g. from glucose to cellobiose to starch). Here we will show that for the electrocatalytic oxidation of glucose over Pt on carbon nanofibers (Pt/CNF), carboxylic acid groups on the carbon support have a positive influence on the catalytic activity, while hydroxyl groups have a negative influence. This is, also based on the influence of bulk pH effects, tentatively explained by local pH effects induced by surface oxygen groups.CNF supports were synthesized by chemical vapor deposition using CO/H2 and a Ni/SiO2 growth catalyst. After synthesis they were treated in sequential steps with 1M NaOH and 68% HNO3 to remove the growth catalyst and to introduce carboxylic acid and hydroxyl groups on the support (CNF-CA).1 Incipient wetness impregnation of Pt ammonium nitrate on CNF-CA followed by calcination was used to make the catalyst (5.0 wt.% Pt/CNF-CA). Pt/CNF-CA was heat treated at 300, 500 and 700 ˚C to gradually remove the support oxygen groups.1 The prepared electrocatalysts were drop-casted on a glassy carbon electrode to evaluate their performance for glucose oxidation by linear sweep voltammetry (LSV). The properties of the electrocatalysts were thoroughly analyzed by N2-physisorption, TEM, CO-stripping, TPD-MS, TGA, XPS, CV and titration.N2-physisorption, CO-stripping and TEM analysis showed that the physical properties of all electrocatalyst were similar: porosity (0.33 cm3.g-1), BET-surface area (182 m2.g-1), electrochemical surface area (120 m2.g-1) and the average Pt particle size (2.1 nm). The type and content of support oxygen groups of the electrocatalysts were assessed by CV, TPD-MS and TGA and revealed that the Pt/CNF-CA contains both carboxylic acid and hydroxyl groups on the carbon support surface. After treating the sample at 300 ˚C these carboxylic acid groups were removed and only hydroxyl groups were retained. After further increasing the treatment temperature the hydroxyl content decreased gradually. Therefore 4 different catalysts were prepared, i.e. with: carboxylic acid and high contents of hydroxyl groups (Pt/CNF-CA), and without carboxylic acid groups but with high, medium and low contents of hydroxyl groups (Pt/CNF-HO, Pt/CNF-MO and Pt/CNF-LO).The LSV measurements at pH 1 (Fig. 1A) show that Pt/CNF-CA has two oxidation peaks i.e., at 0.62 and 0.78V. This indicates that two oxidation processes occur, most likely the oxidative adsorption of glucose at 0.62 V and the oxidation of adsorbed glucose to δ-gluconolactone at 0.78 V.2 As the carboxylic acid groups were removed, going from Pt/CNF-CA to Pt/CNF-HO, the current densities decrease at 0.62 and 0.78 V. This decrease in current density indicates that the removal of carboxyl acid groups from the support results in a loss in catalytic activity. When the hydroxyl groups were removed, i.e., going from Pt/CNF-HO to Pt/CNF-MO to Pt/CNF-LO, the current density at 0.52 V increases, while the current density at 0.8 V remains unaffected. This increase in current density at 0.52 V represents an increase in catalytic activity for the oxidative adsorption of glucose for electrocatalysts with fewer hydroxyl groups on the support and might therefore affect the product distribution (currently under investigation). When the bulk pH of the electrolyte was increased from 1 to 3, it was found that Pt/CNF-CA is not affected by the pH, while at pH 3 the performance of all electrocatalysts that differ in hydroxyl content on the support (Pt/CNF-LO, P/CNF-MO and Pt/CNF-HO) resemble the performance of Pt/CNF-LO at pH 1. This is explained by the oxidation of the hydroxyl groups to carbonyl groups on the support resulting in the release of protons at pH 13 and thus a decrease in concentration of local hydroxyls in solution, which in turn hampers the electrocatalytic oxidation of glucose.The relation between the type and quantity of oxygen functionalities and current densities, shows that we can tune these oxygen functionalities to increase the catalytic activity, and simultaneously reduce the energy input (W=I*V) for the catalytic conversion. Our results also demonstrate how the type and presence or absence of support oxygen groups are related to the bulk pH, which in turn influences the catalytic performance of Pt towards the electrocatalytic oxidation of glucose.1)https://doi.org/10.1016/j.jcat.2004.05.0262)https://doi.org/10.1016/0022-0728(92)80454-C3)https://doi.org/10.1016/j.carbon.2013.07.082 Figure 1