Materials. Oleic acid (90%, Fisher Chemical), titanium(IV) bis(ammonium lactate)dihydroxide (TiALH, 50% wt in water * , Alfa Aesar), Sr(OH) 2 •8H 2 O (99%, Alfa Aesar), Cr(NO 3 ) 3 •9H 2 O (Crystalline Certified, Fisher Chemical), tetramethylammonium hydroxide (10 M NMe 4 OH, Acros Organics), hydrazine hydrate (N 2 H 4 •H 2 O, 99%, Acros Organics), ethanol (200 proof, PHARMCO-AAPER) and hexanes (optima, Fisher Chemical) were used as received. Synthesis of colloidal SrTiO 3−δ and Cr 3+ -doped SrTiO 3−δ nanocrystals. Colloidal SrTiO 3 NCs were prepared by a modified hydrothermal method. 1-3 In a typical synthesis, 1.25 mmol of titanium(IV) bis(ammonium lactate)dihydroxide and Sr(OH) 2 •8H 2 O were dissolved in 30 mL of distilled H 2 O. The pH was adjusted to 12.1 with NMe 4 OH (10 M). The solution was transferred to a 45-mL Teflon-lined autoclave and N 2 H 4 •H 2 O (5 mmol) and oleic acid (2.5 mmol) were added. 2 The autoclaves were sealed and heated to 200 °C for 24 h. The resulting oleic acid functionalized NCs were washed with ethanol and suspended in hexanes. The synthetic * The batch of TiALH received from Alfa Aesar contains common impurities of TiO 2 at ~14% and pH of the solution ~8.4-8.5. The presence of insoluble TiO 2 indicates that slow hydrolysis is taking place in the TiALH bottle and definitely overestimates [Ti] in the calculation of x nom =[Cr]/([Cr]+[Ti]).

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The electrochemical reduction of CO 2 and H 2 O to solar fuels remains a promising strategy for storing intermittent energy sources in the form of chemical bonds. These electrochemical reductions occurring at the cathode typically are coupled to the oxygen evolution reaction at the anode. The electrochemical oxidation of organic alcohols in the alcohol oxidation reaction is a promising alternative anode reaction that occurs at decreased operating potentials compared to the oxygen evolution reaction and that produces more valuable products than O 2 . Co 2 NiO 4 is a particularly promising catalyst for the oxidation of alcohols, able to promote alcohol oxidation at current densities of 10 mA cm −2 at potentials of only 1.42 V vs reversible hydrogen electrode (RHE) in alkaline aqueous conditions, significantly less positive than typical potentials required for the oxygen evolution reaction. In this work, we study the alcohol oxidation reaction by Co 2 NiO 4 for a series of straight-chain primary alcohols of increasing chain length from ethanol to n-pentanol. We show that the product distribution for alcohol oxidation depends on the alcohol chain length, changing from primarily aldehyde products for shorter-chain alcohols to primarily carboxylic acid products for longer-chain alcohols. These results suggest that alcohols are oxidized sequentially to first aldehydes and then carboxylic acids at Co 2 NiO 4 . During the oxidation of longer-chain alcohols, the aldehyde intermediates are retained at the catalyst surface for longer times, facilitating further oxidation to terminal carboxylic acid products. We also explored the potential-dependent activities and product distributions for nbutanol oxidation at Co 2 NiO 4 , and showed that alcohol oxidation is able to outcompete chloride oxidation in aqueous solutions containing Cl − at seawater concentrations. These studies provide further insight into the alcohol oxidation reaction at Co 2 NiO 4 and highlight its promise as an alternative anode reaction for the production solar fuels.

Postmortem analysis of a CoV2O4 catalyst for oxygen evolution reveals the active species is likely a vanadium-free amorphous cobalt oxide.