Electrochemical reduction of CO2 to produce useful fuels and chemicals is one of the attractive means to reuse CO2. Herein, we constructed a (TiO2)3/Ag(110) model electrocatalyst and examined CO2 reduction pathways. Our results show that the interface between oxide and supporting Ag provides the active sites for CO2 adsorption and activation. These active sites enable the electron transfer to the adsorbed CO2. In this setup, Ag acts as an electron donor, partially reducing the supported (TiO2)3 and supplies the needed electrons to the adsorbed CO2. Once CO2* is formed at the interface, the subsequent hydrogenation steps take place sequentially. Our results further indicate that the dominating pathway to produce CH3OH is via the H2COOH* intermediate following the formation of HCOO*. The formation of H2COOH* with a free energy of 0.47 eV is the potential-limiting step. Furthermore, protonating H2COOH* followed by dehydration to CH3O* and hydrogenation of CH3O* leads to CH4 formation. The COOH* pathway may converge to the same H2COOH* intermediate instead of forming CO*. These results demonstrated the benefit of metal supported metal oxides as electrocatalysts to produce CH3OH or CH4 from electrochemical reduction of CO2.

•Co/NaY was prepared by impregnation of NaY with ethanol-cobalt nitrate solution.•Co/NaY was applied in the ethyl lactate hydrogenation.•100% conversion and 96% selectivity to 1,2-PDO was achieved.•The structural characteristics were affected by the condition of preparation.•Conversion of ethyl lactate correlates linearly with number of surface metallic Co.Co/NaY catalysts prepared by impregnation of NaY with a cobalt nitrate-ethanol solution were used to catalyze liquid-phase hydrogenation of ethyl lactate to 1,2-propanediol (1,2-PDO). To elucidate the effect of calcination conditions and cobalt loading, the catalysts were characterized using XRD, N2 physical adsorption, H2-TPR, XPS, H2-TPD, and TEM. The results show that both high calcination temperature and high amount of Co loading result in relatively large Co3O4 particles. The reduced cobalt catalysts consist of three cobalt species, including metallic Co, cobalt oxides and cobalt species interacting with zeolite (cobalt aluminate/silicate), due to incomplete reduction of cobalt species. The absence of the characteristic peak of metallic Co in the XRD pattern confirms its high dispersion on NaY, in agreement with the TEM results. H2-TPD effectively quantifies the number of surface metallic Co sites, which correlates linearly with the ethyl lactate conversion. This correlation indicates that metallic Co is the active site for ethyl lactate hydrogenation. A complete conversion of ethyl lactate with a selectivity of 96% to 1,2-PDO has been achieved over the 6Co/NaY-673-5 catalyst.

•Oxygen doping site in graphene forms an anchor for supported metal clusters.•O2 adsorbs as a di-oxygen species on supported Pt4 and Pt3Fe clusters.•O2 dissociates spontaneously on supported Pt3V.•On supported Pt4 and Pt3Fe, both HO* + HO* and H2O* + O* routes are possible.•On supported Pt3V, the HO* + HO* route dominates.Density functional theory calculations were performed to investigate the pathways of the oxygen reduction reaction over the Pt4 and Pt3M (M = Fe, V) clusters supported on the O-doped graphene substrate. The results show that the defect sites resulting from oxygen doping become the anchor sites for metal clusters. Our results also showed that O2 adsorbs as a di-oxygen species on the supported Pt4 and Pt3Fe clusters, but dissociates spontaneously on supported Pt3V. The di-oxygen species dissociates into co-adsorbed HO* and O* upon reduction on both supported Pt4 and Pt3Fe and no stable HOO* intermediates were isolated. On supported Pt4, further reduction via both HO* + HO* and H2O* + O* routes is possible, with reaction favoring the HO* + HO* route energetically. On supported Pt3Fe, the HO* + HO* and H2O* + O* routes are competitive. On supported Pt3V, the reduction reaction is likely to proceed exclusively through the HO* + HO* route as no stable co-adsorbed H2O and O* state was isolated. The results were discussed in the context of the experimentally observed enhancement of ORR reactivity on the graphene supported Pt3Cr and Pt3Co nanocatalysts.

Electrochemical reduction of CO2 has the benefit of turning greenhouse gas emissions into useful resources. We performed a comparative study of the electrochemical reduction of CO2 on stepped Pb(211) and Sn(112) surfaces based on the results of density functional theory slab calculations. We mapped out the potential energy profiles for electrochemical reduction of CO2 to formate and other possible products on both surfaces. Our results show that the first step is the formation of the adsorbed formate (HCOO*) species through an Eley-Rideal mechanism. The formate species can be reduced to HCOO− through a one-electron reduction in basic solution, which produces formic acid as the predominant product. The respective potentials of forming HCOO* are predicted to be −0.72 and −0.58 V on Pb and Sn. Higher overpotentials make other reaction pathways accessible, leading to different products. On Sn(112), CO and CH4 can be generated at −0.65 V following formate formation. In contrast, the limiting potential to access alternative reaction channels on Pb(211) is −1.33 V, significantly higher than that of Sn.

Tungsten was incorporated into mesoporous alumina via a one pot sol–gel route. The structure and properties of the as-synthesized materials were well characterized by N2 sorption, XRD, 27Al NMR, XPS, DR–UV–Vis, Py-FTIR, HR-TEM and EDS. These samples possess high surface area, uniform pore size and large pore volume. Tungsten was evidently introduced into the framework of all samples, while extra-framework WO3 crystals were detected when W/Al mole ratio increased to 0.12 . The Py-FTIR results indicate that the Lewis acidity sites are predominant in W(x)-MA samples, and Brønsted acid sites are detected in W(0.24)-MA. The catalytic performance of the mixed W(x)-MA + Ru/C catalyst was evaluated in the hydrogenolysis conversion of cellulose. The notably enhanced selectivity of ethylene glycol was obtained when using W(0.24)-MA catalyst.

Alkaline pretreatment was applied to enhance hydrogenolysis conversion of cellulose to C2–C3 polyols. The alkali cellulose was obtained by treating cellulose with different concentration of NaOH solution. The scanning electron microscopy (SEM) and X-ray diffraction (XRD) results indicate that the cleavage of cellulose chains occurs and the amorphous part is increased after alkaline treatment, which means the alkali cellulose has more accessible structure. Moreover, the absorbed NaOH crystal in alkali cellulose could make the further reaction perform in weak basic condition. When hydrogenolysis of alkali cellulose over Ru/C was conducted at 433 K, 59.23% of the substrate was converted with 1,2-propanediol and ethylene glycol as main products, whereas the corresponding conversion rate of untreated cellulose was 25.05% and no C2–C3 polyols were detected. These preliminary results suggested the advantages of activating the cellulose by alkaline pretreatment and potentials for efficient conversion of cellulose. Finally the plausible mechanism was also discussed.Highlights► Alkaline treatment causes the cleavage of H-bond and more amorphous part. ► Alkali cellulose has more accessible surface area. ► Some of NaOH molecule and H2O molecule penetrate into the alkali cellulose. ► Hydrogenolysis of cellulose to C2–C3 polyols is enhanced via alkaline pretreatment.

A Pb loaded gas diffusion electrode was fabricated and used for the electroreduction of CO2 to formic acid. The Pb/C catalyst was prepared by isometric impregnation. The crystal structure and morphology of the Pb/C catalyst were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM). The preparation conditions of the gas diffusion electrode were optimized by adjusting the amounts of polytetrafluoroethylene (PTFE) in the gas diffusion layer and acetylene black in the catalytic layer. The electrochemical performance of the as-prepared gas diffusion electrode was studied by chronoamperometry and cyclic voltammetry. The optimized gas diffusion electrode showed good catalytic performance for the electroreduction of CO2. The current efficiency of formic acid after 1 h of operation reached a maximum of 22% at −2.0 V versus saturated calomel electrode (SCE).

•A porous Ag foam electrode with average pore size of 10–25 μm and porous wall composed of 40–100 nm Ag nanoparticles.•The competing hydrogen evolution reaction remarkably slow down and the onset potential of CO2 reduction occurs at 0.15 V less than that on Ag foil electrode.•High current density and CO faradaic efficiency of over 90% were stable over the course of several hours.Nanostructured electrocatalysts for CO2 reduction have attracted much attention due to their unique properties compared to their bulk counterparts. Here we report the synthesis of porous Ag foams on a polished Ag substrate via electrodeposition using the hydrogen bubble dynamic template. The as-prepared porous Ag foams has an average pore size of 10–25 μm with the porous wall composed of 40–100 nm Ag nanoparticles. CV tests indicate that the competing hydrogen evolution reaction remarkably slow down on porous Ag electrode and the onset potential of CO2 reduction occurs at 0.15 V less than that on the Ag foil electrode. Moreover, the high current density and CO faradaic efficiency of over 90% were stable over the course of several hours, whereas Ag foil electrode exhibited the drop of CO faradaic efficiency from 74.4% to 58.6% under identical conditions. We found that the enhanced activity and stability are the result of a large electrochemical surface area (approximately 120 times larger) which can provide more active sites. The noteworthy difference between the two electrodes suggests that the nanostructured surface of porous Ag foams is likely to not only favor the formation of CO2− intermediate but also suppress the competitive reaction of hydrogen evolution.Download high-res image (137KB)Download full-size image

•The presence of SnOx on tin electrode enhances the catalytic activity for CO2 electrochemical reduction.•Formation of surface hydroxyls plays an important role for CO2 reduction.•Bicarbonate is the common intermediate to produce both formate and CO.•From bicarbonate to COOH∗ opens the channel for CO formation.Tin oxide (SnOx) formation on tin-based electrode surfaces during CO2 electrochemical reduction can have a significant impact on the activity and selectivity of the reaction. In the present study, density functional theory (DFT) calculations have been performed to understand the role of SnOx in CO2 reduction using a SnO monolayer on the Sn(1 1 2) surface as a model for SnOx. Water molecules have been treated explicitly and considered actively participating in the reaction. The results showed that H2O dissociates on the perfect SnO monolayer into two hydroxyl groups symmetrically on the surface. CO2 energetically prefers to react with the hydroxyl, forming a bicarbonate (HCO3(t)∗) intermediate, which can then be reduced to either formate (HCOO∗) by hydrogenating the carbon atom or carboxyl (COOH∗) by protonating the oxygen atom. Both steps involve a simultaneous CO bond breaking. Further reduction of HCOO∗ species leads to the formation of formic acid in the acidic solution at pH < 4, while the COOH∗ will decompose to CO and H2O via protonation. Whereas the oxygen vacancy (VO) in the oxide monolayer maybe formed by the reduction, it can be recovered by H2O dissociation, resulting in two embedded hydroxyl groups. The results show that the hydroxylated surface with two symmetric hydroxyls is energetically more favorable for CO2 reduction than the hydroxylated VO surface with two embedded hydroxyls. The reduction potential for the former has a limiting-potential of −0.20 V (RHE), lower than that for the latter (−0.74 V (RHE)). Compared to the pure Sn electrode, the formation of SnOx monolayer on the electrode under the operating conditions promotes CO2 reduction more effectively by forming surface hydroxyls, thereby providing a new channel via COOH∗ to the CO formation, although formic acid is still the major reduction product.Download high-res image (81KB)Download full-size image