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CO2 electroreduction powered by renewable energy is an attractive strategy to close the carbon cycle. Among the possible reduction products, CO is of particular interest due to its large industrial applications. Transition metals in the Pt group are able to electrochemically reduce CO2 to CO, but suffer from CO surface poisoning, which causes a quick deactivation and overall sluggish kinetics. Here, we show that by introducing In to Pd-rich bimetallic particles we can tune the selectivity and limit the surface poisoning of these catalysts. The addition of large amounts of In blocks CO2 reduction activity and leads to a material selective for hydrogen evolution and insensitive to CO poisoning. This study provides insights into the dependence of CO2 reduction selectivity on the composition of Pd-In nanoparticles, revealing the effect that different phases have on catalytic activity. The application of similar screenings to other bimetallic systems can potentially yield cheap, selective, and poison-resistant catalysts for electrochemical applications.

Methanol decomposes on oxides, in particular CeO2, producing either formaldehyde or CO as main products. This reaction presents structure sensitivity to the point that the major product obtained depends on the facet exposed in the ceria nanostructures. Our density functional theory (DFT) calculations illustrate how the control of the surface facet and its inherent stoichiometry determine the sole formation of formaldehyde on the closed surfaces or the more degraded by-products on the open facets (CO and hydrogen). In addition, we found that the regular (1 0 0) termination is the only one that allows hydrogen evolution via a hydride–hydroxyl precursor. The fundamental insights presented for the differential catalytic reactivity of the different facets agree with the structure sensitivity found for ceria catalysts in several reactions and provide a better understanding on the need of shape control in selective processes.

Temporary supply of CO during propyne hydrogenation over a copper-based catalyst derived from a Cu–Al hydrotalcite permanently increased the alkene selectivity from 60% to 92% at 100% alkyne conversion (473 K). The superior performance of the CO-modified copper catalyst (higher alkene selectivity and resistance against deactivation by fouling) was further confirmed at lower temperature (373 K). Our catalytic results suggested that carbon monoxide in the presence of propyne and hydrogen irreversibly restructures the catalyst surface leading to a smaller copper ensemble that minimizes C–C coupling. Consequently, the enhanced alkene production in the CO-modified catalyst was coupled to a decreased oligomer production and very little propane production. The pretreatment mixture, feed CO concentration, and type of alkyne are important aspects to attain a selective copper catalyst. The CO effect in alkyne hydrogenation over copper radically differs from that over palladium or nickel. On the latter two metals, the increased alkene selectivity in the presence of CO is mostly due to a reduction in the over-hydrogenation channel. In addition, the effect is fully reversible, that is, it vanishes on removing CO from the feed. The permanent modification of the copper ensemble by CO might be advantageous for (chemo) selective hydrogenations of high acetylenic substrates.

Alkyne hydrogenation, a widely used process in industry to purify olefin streams, comprises a prototype reaction to understand selectivity in heterogeneously catalyzed reactions. The selectivity of the reaction on palladium catalysts to the alkene, alkane, or oligomers strongly depends on the state of the (sub)surface; i.e. , the occurrence of complex carbide/hydride phases. In practice, hydrogenation reactors in C2 and C3 cuts of steam crackers require continuous CO feeding in order to enhance the alkene selectivity of palladium-supported catalysts. In the present work, we have studied the impact of carbon monoxide on the formation of carbide and hydride phases as a standpoint to derive structure–performance relationships under realistic process conditions. For this purpose, catalytic tests on a standard 1 wt.% Pd/Al 2 O 3 and Density Functional Theory on Pd(1 1 1) were combined. The influence of: (i) the alkyne (ethyne and propyne), (ii) the hydrogen:alkyne ratio (1–10), (iii) the carbon monoxide:hydrogen ratio (0–0.2), and (iv) the catalyst pretreatment on the product distribution was assessed in a continuous flow fixed-bed reactor at ambient pressure. In absence of CO, subtle changes in the hydrogen:alkyne ratio generate undesired products. Carbon monoxide enables the external control of the catalyst state by suppressing the formation of subsurface hydride and carbide phases, thereby stabilizing a high alkene yield in a broad range of feed hydrogen:alkyne ratios. This scenario contrasts with the more fragile regime of the hydride–carbide phases under CO-free conditions. DFT calculations obtained a single Bronsted–Evans–Polanyi relationship independently of the state of the catalyst (carbide, hydride, CO-covered) and the alkyne–alkene–alkane set (C2, C3).

The partial hydrogenation of propyne was studied over copper-based catalysts derived from Cu–Al hydrotalcite and malachite precursors and compared with supported systems (Cu/Al 2 O 3 and Cu/SiO 2 ). The as-synthesized samples and the materials derived from calcination and reduction were characterized by XRF, XRD, TGA, TEM, N 2 adsorption, H 2 -TPR, XPS, and N 2 O pulse chemisorption. Catalytic tests were carried out in a continuous flow-reactor at ambient pressure and 423–523 K using H 2 :C 3 H 4 ratios of 1–12 and were complemented by operando DRIFTS experiments. The propyne conversion and propene selectivity correlated with the copper dispersion, which varied with the type of precursor or support and the calcination and reduction temperatures. The highest exposed copper surface was attained on hydrotalcite-derived catalysts, which displayed C 3 H 6 selectivity up to 80% at full C 3 H 4 conversion and stable performance in long-run tests at T ⩾ 473 K. Both activated Cu–Al hydrotalcites (this work) and Ni–Al hydrotalcites [S. Abello, D. Verboekend, B. Bridier, J. Perez-Ramirez, J. Catal. 259 (2008) 85] exhibited a relatively high alkene selectivity under optimal operation conditions, but they present a markedly distinctive catalytic behavior with respect to temperature and hydrogen-to-alkyne ratio. The product distribution was assigned through Density Functional Theory (DFT) simulations to the different stability of subsurface phases (carbides, hydrides) and the energies and barriers for the competing reaction mechanisms. The behavior of Cu in partial alkyne hydrogenation resembles that of Au nanoparticles, while Ni is closer to Pd.

The catalytic oxidation of hydrogen chloride to chlorine (Deacon reaction) is an attractive means to recover Cl 2 from HCl-containing waste streams in the chemical industry. Recently, RuO 2 supported on TiO 2 rutile has been industrially implemented for large-scale chlorine manufacture. However, detailed understanding of the reaction mechanism over claimed catalysts has not been acquired. Our tests in a fixed-bed reactor at ambient pressure concluded that RuO 2 powder is highly active for Cl 2 production. Characterization of the fresh and used RuO 2 samples by ex situ XRD and XPS revealed no appreciable alteration of the bulk structure and limited chlorination of the catalyst. Ab initio thermodynamics predicted that the initial state of the RuO 2 (110) surface is partially over-oxidized, while the surface after reaction contains both oxygen and chlorine. DFT described the Deacon reaction with a Mars–van Krevelen type mechanism consisting of five steps: hydrogen abstraction from HCl, recombination of atomic chlorine, hydroxyl recombination, water desorption, and dissociative oxygen adsorption. An increased chlorine production was observed experimentally when increasing the feed O 2 -to-HCl ratio. This can be ascribed both to the lower recombination energy of chlorine atoms to gas-phase Cl 2 at high coverages and the faster surface reoxidation due to higher partial oxygen pressures.

DFT simulations have uncovered the origin of the highly selective character of gold nanoparticles in hydrogenation of triple bonds. This is ascribed to the better adsorption of CC at the edges of Au nanoparticles compared with CC. Since the barriers for hydrogenation of triple and double bonds on gold are comparable, selectivity is determined by the binding energy of the reactants. The situation is less favorable over palladium (commercial hydrorefining catalyst), because both CC and CC bonds are adsorbed on the Pd surface. The outcome of the simulations was demonstrated experimentally in mixtures with both propyne and propylene over a Au/CeO2 catalyst, where C3H6 selectivities up to 95% were attained. Our finding opens a new path for chemoselective hydrogenation of molecules containing -yne and -ene groups on gold, with prospective application in petrochemical operations and in the fine chemical industry.

The effect of oxygen coverage on the molecular mechanism of ethylene epoxidation on Cu(111) under ultra-high-vacuum conditions was studied by means of periodic density functional calculations. Oxygen coverages of 1/16 and 1/4 monolayer were considered. The results indicate that the barriers leading to ethylene oxide and acetaldehyde from a common oxametallacycle intermediate depend on the oxygen coverage. In particular, the decrease in the energy barrier leading to the epoxide with oxygen coverage can be correlated with a concomitant change in the formation energy of the desired product. It is also suggested that the optimum oxygen coverage for the epoxidation of a given olefin on Cu surfaces likely vary from one molecule to the other.

The adhesion, shape, and electronic structure of gold particles supported on TiO 2 (110) have been determined by density-functional theory calculations, to gain more insight into the catalytic activity of small supported gold particles. These calculations show that gold particles do not bind to a perfect TiO 2 surface, but have a binding energy of about 1.6 eV/defect on an oxygen vacancy in TiO 2 . This results in an indirect effect of the support material on the catalytic activity: The distribution and dynamics of the oxygen vacancies determine the dispersion and shape of the gold particles, which in turn affect the catalytic activity. A theoretical analysis of the gold particle shape reveals that it is flat, about 3–4 layers for a gold particle of 3 nm diameter. The calculated flat geometry is in good agreement with electron microscopy and EXAFS measurements of a high surface area Au/TiO 2 catalyst containing 2.4 wt% gold.

It is suggested that there may be several effects contributing to the special catalytic properties of supported nanosized gold particles, and that it is useful to order them in a hierarchy. The most important effect is related to the availability of many low-coordinated gold atoms on the small particles. Effects related to the interaction with the support may also contribute, but to a considerably smaller extent. We base the analysis on a new set of experimental results comparing the CO oxidation rates over gold supported on different reducible and nonreducible oxides, on an analysis of a large number of published activity data, and on an analysis of density-functional calculations of the effect of metal coordination numbers in comparison to the role of charge transfer, layer thickness, and interactions with the support.