The catalytic activities of sulfur sites in amorphous MoS x for the electrochemical hydrogen evolution reaction (HER) was investigated in aqueous 0.5 M H2SO4 electrolyte. Using X-ray photoelectron spectroscopy and linear sweep voltammetry, we found the turnover frequency for H2 production to increase linearly with the percentage of S atoms with higher electron binding energies. These S atoms could be apical S2– and/or bridging S2 2–. To distinguish the catalytic performances of these two types of atoms, we turn to quantum chemical simulations using density functional theory. The apical S2– atoms were found to adsorb H weakly with a Gibbs free energy for atomic H adsorption (ΔG H) in excess of +1 eV, and were thus ruled out as reaction sites for HER. In situ Raman spectroscopy of the model [Mo3S13]2– cluster further demonstrate the higher catalytic reactivity of the bridging S2 2– over terminal S2 2– (which have lower electron binding energy) for proton reduction.

Amorphous molybdenum sulfide (MoS x ) is currently being developed as an economically viable and efficient catalyst for the electrochemical hydrogen evolution reaction (HER). An important yet unsolved problem in this ongoing effort is the identification of its catalytically active sites for proton reduction. In this work, cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy were used to investigate the catalytically active sites and structural evolution of MoS x films during HER in 1 M HClO4 electrolyte. Transformation of anodically deposited MoS x (x ≈ 3) to a structure with MoS2 composition during the cathodic sweep of a CV was demonstrated using XPS and operando Raman spectroscopy. Interestingly, a Raman peak at 2530 cm–1 was recorded at potentials relevant to H2 evolution, which we ascribed to the S–H stretching vibration of MoS x –H moieties. This assignment was corroborated by H/D isotope exchange experiments. Mo–H (or Mo–D) stretching vibrations were not observed, which thus allowed us to rule out Mo centers as catalytic sites for proton reduction to H2. Density functional theory (DFT) calculations were performed on a variety of MoS x structures to capture the heterogeneous nature of amorphous materials and corroborated the assignments of the observed vibrational frequencies. On the basis of these experimental measurements and quantum chemical simulations, we have for the first time directly pinpointed the sulfur atoms in amorphous MoS x to be the catalytically active sites for evolving H2.

Hydrogen evolution was performed in KOH and H2SO4 electrolytes using α-Mo2C and γ-Mo2N synthesized by the ‘urea glass’ route. α-Mo2C shows excellent performance especially in KOH.

A fundamental question in the electrochemical CO2 reduction reaction (CO2RR) is how to rationally control the catalytic selectivity. For instance, adding a CO-selective cocatalyst like Ag to Cu shifts the latter’s CO2RR selectivity toward C2 products, but the underlying cause of the change is unclear. Herein, we show that, during CO2RR, the abundant CO availability at Cu−Ag boundaries facilitates C-C coupling on Cu to selectively generate ethanol through an otherwise closed pathway. Oxide-derived Cu nanowires mixed with 20 nm Ag particles (Cu:Ag mole ratio of 1:20) catalyzed CO2 reduction to ethanol with a maximum current density of −4.1 mA/cm2 and ethanol/ethylene Faradaic efficiency ratio of 1.1 at −1.1 V vs RHE. These figures of merit are, respectively, 5 and 3 times higher than those for pure oxide-derived Cu nanowires. CO2RR on CuAg composite catalysts with different Ag:Cu ratios and Ag particle sizes reveals that ethanol production scales with the amount of CO evolved from Ag sites and the abundance of Cu–Ag boundaries, and, very interestingly, without significant modifications to ethylene formation. Computational modeling shows selective ethanol generation via Langmuir–Hinshelwood *CO + *CH x (x = 1, 2) coupling at Cu–Ag boundaries and that the formation of energy-intensive CO dimers is circumvented.

The electroreduction of carbon dioxide using renewable electricity is an appealing strategy for the sustainable synthesis of chemicals and fuels. Extensive research has focused on the production of ethylene, ethanol and n‐propanol, but more complex C4 molecules have been scarcely reported. Herein, we report the first direct electroreduction of CO2 to 1‐butanol in alkaline electrolyte on Cu gas diffusion electrodes (Faradaic efficiency=0.056 %, j1‐Butanol=−0.080 mA cm−2 at −0.48 V vs. RHE) and elucidate its formation mechanism. Electrolysis of possible molecular intermediates, coupled with density functional theory, led us to propose that CO2 first electroreduces to acetaldehyde‐a key C2 intermediate to 1‐butanol. Acetaldehyde then undergoes a base‐catalyzed aldol condensation to give crotonaldehyde via electrochemical promotion by the catalyst surface. Crotonaldehyde is subsequently electroreduced to butanal, and then to 1‐butanol. In a broad context, our results point to the relevance of coupling chemical and electrochemical processes for the synthesis of higher molecular weight products from CO2.