We report selective electrocatalytic reduction of carbon dioxide to carbon monoxide on gold nanoparticles (NPs) in 0.5 M KHCO3 at 25 °C. Among monodisperse 4, 6, 8, and 10 nm NPs tested, the 8 nm Au NPs show the maximum Faradaic efficiency (FE) (up to 90% at -0.67 V vs reversible hydrogen electrode, RHE). Density functional theory calculations suggest that more edge sites (active for CO evolution) than corner sites (active for the competitive H2 evolution reaction) on the Au NP surface facilitates the stabilization of the reduction intermediates, such as COOH*, and the formation of CO. This mechanism is further supported by the fact that Au NPs embedded in a matrix of butyl-3-methylimidazolium hexafluorophosphate for more efficient COOH* stabilization exhibit even higher reaction activity (3 A/g mass activity) and selectivity (97% FE) at -0.52 V (vs RHE). The work demonstrates the great potentials of using monodisperse Au NPs to optimize the available reaction intermediate binding sites for efficient and selective electrocatalytic reduction of CO2 to CO.

In this communication, we show that ultrathin Au nanowires (NWs) with dominant edge sites on their surface are active and selective for electrochemical reduction of CO2 to CO. We first develop a facile seed-mediated growth method to synthesize these ultrathin (2 nm wide) Au NWs in high yield (95%) by reducing HAuCl4 in the presence of 2 nm Au nanoparticles (NPs). These NWs catalyze CO2 reduction to CO in aqueous 0.5 M KHCO3 at an onset potential of -0.2 V (vs reversible hydrogen electrode). At -0.35 V, the reduction Faradaic efficiency (FE) reaches 94% (mass activity 1.84 A/g Au) and stays at this level for 6 h without any noticeable activity change. Density functional theory (DFT) calculations suggest that the excellent catalytic performance of these Au NWs is attributed both to their high mass density of reactive edge sites (≥16%) and to the weak CO binding on these sites. These ultrathin Au NWs are the most efficient nanocatalyst ever reported for electrochemical reduction of CO2 to CO.

The well-known hydrogen evolution reaction (HER) volcano plot describes the relationship between H binding energy and the corresponding hydrogen evolution catalytic activity, which depends on the species of metal. Under CO 2 /CO reduction conditions or in cases where CO impurities enter electrodes, the catalyst may exist under a high coverage of coadsorbed CO. We present DFT calculations that suggest that coadsorbed CO during hydrogen evolution will weaken the binding strength between H and the catalyst surface. For metals on the right-hand side (too weak of hydrogen binding) this should lead to a suppression of the HER, as has been reported for metals such as Cu and Pt. However, for metals on the left-hand side of the volcano (too strong of hydrogen binding), this may actually enhance the kinetics of the hydrogen evolution reaction, although this effect will be countered by a decreased availability of sites for HER, which are blocked by CO. We performed experiments in Ar and CO 2 environments of two representative metals that bind CO on the far right-and left-hand side of the volcano, namely, Cu and Mo (respectively). On Cu, we find that the CO 2 environment suppresses HER, which is consistent with previous findings. However, on Mo we find that the CO 2 environment enhances HER in the kinetically active region. This helps to explain the outstanding performance of copper in CO 2 reduction and suggests that searches for high-selectivity CO 2 /CO reduction catalysts may benefit from focusing on the right-hand side of the HER volcano. This also suggests principles for assessing the activity of catalysts for fuel cell and electrolysis reactions in which impurities such as CO may be present.

Metal carbide catalysts are alternative nonprecious electrode materials for electrochemical energy conversion devices, such as for H 2 fuel cells or electrolyzers. In this article, we report the experimental exchange current densities for the hydrogen evolution reaction (HER) on eight mono-and bimetallic carbide electrocatalysts and correlate the current densities to hydrogen binding energies that we have calculated via electronic structure computations. We find these materials to have activities higher than those of their parent metals and intermediate between the catalytic activities of the Pt group and early transition-metal surfaces. Increased HER activities on metal carbides relative to their parent metals can be understood with a 3-fold higher sensitivity of metal carbides to the coverage-induced weakening of hydrogen adsorption relative to metal surfaces. The trends presented here can be useful for the design of bimetallic carbide electrocatalysts.

Design principles for reducible metal nitride catalysts are developed and demonstrated for ambient-pressure solar-driven N2 reduction into NH3.

The activity of heterogeneous catalysts is often limited by a strong correlation between the chemisorption energies of reaction intermediates described by the “scaling relations” among the transition metals. We present electronic structure calculations that suggest that metal carbides do not in general follow the transition-metal scaling relations and tend to exhibit a carbophobic departure relative to the transition metals, meaning they tend to bind carbon-bound species weakly compared to oxygen-bound species. This contrasts with the oxophobic departure exhibited by Pt and Pd. Relative to the parent metals, carbides tend to bind carbon and oxygen more weakly and hydrogen more strongly. The departures are rationalized with the adsorbate–surface valence configuration and the energy of the metal sp-states. We employ these general trends to aid in the understanding of various catalytic properties such as the high activity of iron carbides for Fischer–Tropsch synthesis and Pt-group catalysts for partial oxidation of methane. These conclusions are shown to extend beyond atomic probe adsorbates to molecular fragments of relevance to catalysis, making these concepts generally useful for the theory-based design of catalytic materials.

SummarySplitting CO2 with a thermochemical redox cycle utilizes the entire solar spectrum and provides a favorable path to the synthesis of solar fuels at high rates and efficiencies. However, the temperature/pressure swing commonly applied between reduction and oxidation steps incurs irreversible energy losses and severe material stresses. Here, we experimentally demonstrate for the first time the single-step continuous splitting of CO2 into separate streams of CO and O2 under steady-state isothermal/isobaric conditions. This is accomplished using a solar-driven ceria membrane reactor conducting oxygen ions, electrons, and vacancies induced by the oxygen chemical potential gradient. Guided by the limitations imposed by thermodynamic equilibrium of CO2 thermolysis, we operated the solar reactor at 1,600°C, 3·10−6 bar pO2 and 3,500 suns radiation, yielding total selectivity of CO2 to CO + ½O2 with a conversion rate of 0.024 μmol·s−1 per cm2 membrane. The dynamics of the oxygen vacancy exchange, tracked by GC and XPS, further validated stable fuel production.

Ammonia is an important input into agriculture and is used widely as base chemical for the chemical industry. It has recently been proposed as a sustainable transportation fuel and convenient one-way hydrogen carrier. Employing typical meteorological data for Palmdale, CA, solar energy is considered here as an inexpensive and renewable energy alternative in the synthesis of NH 3 at ambient pressure and without natural gas. Thermodynamic process analysis shows that a molybdenum-based solar thermochemical NH 3 production cycle, conducted at or below 1500 K, combined with solar thermochemical H 2 production from water may operate at a net-efficiency ranging from 23-30% (lower heating value of NH 3 relative to the total energy input). Net present value optimization indicates ecologically and economically sustainable NH 3 synthesis at above about 160 tons NH 3 per day, dependent primarily on heliostat costs (varied between 90 and 164 dollars/m 2 ), NH 3 yields (ranging from 13.9 mol% to stoichiometric conversion of fixed and reduced nitrogen to NH 3 ), and the NH 3 sales price. Economically feasible production at an optimum plant capacity near 900 tons NH 3 per day is shown at relative conservative technical assumptions and at a reasonable NH 3 sales price of about 534 ± 28 dollars per ton NH 3 .