Aqueous TiO2 Nanoparticles React by Proton-Coupled Electron Transfer.

Redox reactions of aqueous colloidal TiO2 4 nm nanoparticles (NPs) have been examined, including both citrate-capped and uncapped NPs (c-TiO2 and uc-TiO2). Photoreduction gave stable blue colloidal c-TiO2R NPs with 10-60 electrons per particle. Equilibration of these reduced NPs with soluble redox reagents such as methylviologen (MV2+) provided measurements of the colloid reduction potential as a function of pH. The potentials of c-TiO2 from pH 2-9 varied linearly with pH, with a slope of -60 ± 5 mV/pH. Estimates of the potential at pH 12 were consistent with extrapolating that line to high pH. The reduction potentials did not correlate with the zeta potentials (ζ) or the surface charge of the NPs across this pH range. Similar reduction potentials were observed for c- and uc-TiO2 at low pH even though they have quite different ζ potentials. These results show that the common surface-charging explanation of the pH dependence is not tenable in these systems. Oxidation of reduced c-TiO2R with the electron-transfer oxidant potassium triiodide (KI3) occurred with a significant drop in pH, showing that protons were released when the electrons were removed from the NPs. Smaller pH drops were observed for the proton-coupled electron transfer (PCET) reagents O2 (air) and 4-MeO-TEMPO (4-methoxy-2,2,6,6-tetramethylpiperine-1-oxy radical). The difference in the number of protons released with KI3 vs O2 and 4-MeO-TEMPO was roughly one proton per electron removed. Thus, the thermodynamically preferred reactivity of these colloidal TiO2 NPs is PCET over the pH 2-13 range studied. The measured redox potentials refer to the chemical process TiO2 + H+ + e- → TiO2·e-,H+; and therefore they do not correspond with an electronic energy such as a conduction band edge or flat band potential. The 1e-/1H+ stoichiometry means that the TiO2 reduction potentials correspond to a TiO2-H bond dissociation free energy (BDFE), determined to be 49 ± 2 kcal mol-1. The PCET description is consistent with the pH dependence of E(TiO2/TiO2·e-,H+), the release of protons upon oxidation, the lack of correlation with ζ potentials, the similarity of capped and uncapped NPs, and the small change in the potential and BDFE from the first to the last electron/proton pair (H atom) removed. This behavior is suggested to be the norm for redox-active oxide/water interfaces.

Iron Porphyrin Allows Fast and Selective Electrocatalytic Conversion of CO2 to CO in a Flow Cell.

Molecular catalysts have been shown to have high selectivity for CO2 electrochemical reduction to CO, but with current densities significantly below those obtained with solid-state materials. By depositing a simple Fe porphyrin mixed with carbon black onto a carbon paper support, it was possible to obtain a catalytic material that could be used in a flow cell for fast and selective conversion of CO2 to CO. At neutral pH (7.3) a current density as high as 83.7 mA cm-2 was obtained with a CO selectivity close to 98 %. In basic solution (pH 14), a current density of 27 mA cm-2 was maintained for 24 h with 99.7 % selectivity for CO at only 50 mV overpotential, leading to a record energy efficiency of 71 %. In addition, a current density for CO production as high as 152 mA cm-2 (>98 % selectivity) was obtained at a low overpotential of 470 mV, outperforming state-of-the-art noble metal based catalysts.