Selective photoelectrochemical oxidation of glycerol to high value-added dihydroxyacetone

It is highly profitable to transform glycerol – the main by-product from biodiesel production to high value-added chemicals. In this work, we develop a photoelectrochemical system based on nanoporous BiVO4 for selective oxidation of glycerol to 1,3-dihydroxyacetone – one of the most valuable derivatives of glycerol. Under AM 1.5G front illumination (100 mW cm−2) in an acidic medium (pH = 2) without adscititious oxidant, the nanoporous BiVO4 photoanode achieves a glycerol oxidation photocurrent density of 3.7 mA cm−2 at a potential of 1.2 V versus RHE with 51% 1,3-dihydroxyacetone selectivity, equivalent to a production rate of 200 mmol of 1,3-dihydroxyacetone per m2 of illumination area in one hour.


Supplementary Table 1 | Production and carbon balance of glycerol oxidation reaction.
Consumption rate of glycerol, production rate of glycerol derivatives, carbon balance and charge-to-chemical balance at pH = 2 and 1. 2

Supplementary Note 1 | Mott-Schottky plots of the BiVO 4 photoanode.
The Mott-Schottky plot measured without glycerol under AM 1.5G illumination in 0.5 M Na2SO4 at pH = 2 in Supplementary Fig. 6 shows a clear shift towards lower potentials as compared to that measured in dark, indicating that surface states do play a role in water oxidation via a surface-state charging process. S1 After adding glycerol in the PEC system, surface-state charging, i.e. hole accumulation, is largely suppressed. Supplementary Fig. 6b shows that Mott-Schottky plots do not change with and without light illumination in the presence of glycerol.

Supplementary Note 3 | Electrochemical impedance of BiVO 4 photoanode.
As shown in Supplementary Fig. 8c, enhanced adsorption of glycerol on BiVO4 at lower pH together with higher productivity but poor desorption of oxidation products would lead to increased Rtotal with increasing potential or decreasing pH.

Supplementary Note 4 | Comparison of BiVO 4 porous nanoarrays and film photoanodes.
BiVO4 film photoanode was prepared by the reported method. S9 As shown in Fig. 1a and Supplementary Fig. 15, photocurrent density of BiVO4 film photoanode is higher than that of porous BiVO4 nanoarray photoanode for water oxidation. After adding glycerol, the photocurrent density of film photoanode is lower than that of porous nanoarray photoanode.

Supplementary Note 5 | Photoelectrochemical characterization of TiO 2 and Fe 2 O 3 photoanode.
TiO2 nanowires and Fe2O3 nanoporous photoanode were fabricated by hydrothermal methods. S10, S11 TiO2 nanowires shows excellent stability in pH = 2 electrolyte, and the photocurrent density increases a little after adding glycerol. As shown in Supplementary Fig. 16, no DHA is detected after PEC glycerol oxidation by TiO2 nanowires. The liquid products efficiency is quite low for TiO2 nanowires photoanode, which may because of competitive water oxidation and further oxidation of glycerol to CO2. Fe2O3 nanoporous photoanode shows poor stability in pH = 2 electrolyte, and the photocurrent density increases a little after adding glycerol. Little DHA and other liquid products are detected after PEC glycerol oxidation by Fe2O3 nanoporous photoanode. The production rate, selectivity and efficiency of DHA and high-value added products obtained by Fe2O3 nanoporous photoanode are much less than that of BiVO4 nanoporous photoanode.
Supplementary Note 6 | Liquid chromatography-mass spectrum results. Supplementary Fig. 17 and Table 2 indicate that 18 O originated from water in the electrolyte could be detected in the product -DHA after PEC glycerol oxidation reaction. This qualitative analysis suggests that water participates in the oxidation of hydroxyl group in glycerol.

Supplementary Note 7 | Photocatalytic performance of glycerol oxidation by BiVO 4 photocatalysts.
BiVO4 scraped from the BiVO4 nanoarrays were used as the photocatalysts. Hole scavenger (ammonium formate) and hydroxyl radical scavenger (t-butanol) were added in the photocatalysis system. The catalytic performance (Supplementary Table 3) shows that after adding hole scavenger, DHA could not be produced, and meanwhile, after adding hydroxyl radical scavenger, the production of DHA was not affected too much. It indicates that holes should directly participate in the glycerol oxidation reaction.

Supplementary Note 8 | Catalytic performance of glycerol oxidation by hydroxyl radicals.
Hydroxyl radicals were introduced in this glycerol photocatalysis system by Fenton reaction. S12 0.3 M H2O2 and 3 mM Fe2SO4 were added into 0.5 M Na2SO4 at pH = 2 with 0.1 M glycerol under stirring in a single cell. As shown in Supplementary Table 4, the selectivity of DHA by oxidation of OH radicals is as low as 13~14 %. The results indicate that this homogeneous glycerol oxidation process driven by hydroxyl radicals could only produce DHA with low selectivity. On the other hand, it also shows that it is harder for middle hydroxyls of glycerol to be oxidized than terminal