Selective CO Production Research Articles

Dumbbell to Core–Shell Structure Transformation of Ni–Au Nanoparticle Driven by External Stimuli

AbstractConversion of CO2 gas to CO fuels is one of the most promising solutions for the increasing threat of global warming and energy crisis. The efficient catalyst Ni–Au dumbbell converting CO2 into CO at elevated temperatures has high CO product selectivity; however, the accompanied atomic diffusion and subsequent surface reconstruction affect the catalytic efficiency of chemical reaction. Atomic scale characterization of structural evolution of the catalyst, which is essential to correlate the functional mechanism to active catalyst surfaces, is yet to be studied. Here, in situ transmission electron microscopy experiments and atomistic simulations are performed to characterize the structural evolution of Ni–Au dumbbell nanoparticles under two different external stimuli. In the condition of high temperature and vacuum, the Ni–Au nanostructure reveals a clear shape reconstruction from the initial dumbbell to core–shell‐like, which is induced by capillary force to minimize free surface energy of the system. The shape transformation involves two stages of processes, initial fast Au diffusion followed by slow source‐controlled diffusion. At ambient temperature, the combination of CO2 and electron flux surprisingly induces analogous structural transformation of Ni–Au nanostructure, where the associated chemical reaction and CO absorption stimulate the Au migration on Ni surface. Such surface reconstruction can be widely present in catalytic reactions in different environmental conditions, and the results herein demonstrate the detailed processes of Ni–Au structure evolution, which provide important insights for understanding the catalyst performance.

Ultrathin Conductor Enabling Efficient IR Light CO2 Reduction.

The concurrent transformation of carbon dioxide and water into hydrocarbons and oxygen by low-photonic-energy IR light still represents a huge challenge. Here, we design an ultrathin conductor system, in which the special partially occupied band serves as the mediator to simultaneously guarantee IR light harvesting and satisfy band-edge positions, while the ultrathin configuration improves charge separation rates and surface redox kinetics. Taking the low cost and earth-abundant CuS as an example, we first fabricate ultrathin CuS layers, where temperature-dependent resistivities, valence-band spectra, and theoretical calculations affirm their metallic nature. Synchrotron-radiation photoelectron and ultraviolet-visible-near-infrared spectra show that metallic CuS atomic layers could realize a new cooperative intraband-interband transition under IR light irradiation, where the generated electrons and holes could simultaneously involve the carbon dioxide reduction and water oxidation reactions. As a result, CuS atomic layers exhibit nearly 100% selective CO production with an evolution rate of 14.5 μmol g-1 h-1 under IR light irradiation, while the catalytic performance shows no obvious decay after a 96 h test. Briefly, benefiting from ultrahigh conductivity and a unique partially occupied band, abundant conductor materials such as conducting metal sulfides and metal nitrides hold great promise for applications as effective IR light responsive photocatalysts.

Tunable copper-hydrotalcite derived mixed oxides for sustainable ethanol condensation to n-butanol in liquid phase

Improvement of processes starting from renewable feedstocks to produce chemicals and biofuels up to now derived from fossil carbon feedstocks is paramount to creating environmentally sustainable industrial processes. Guerbet condensation of ethanol to give n-butanol represents a turning point not only respect to the traditional petrochemical “oxo” process, but also to the bio-chemical path (Acetone Butanol Ethanol “ABE” process). In this work, different Cu/Mg/Al hydrotalcite-derived mixed oxides catalysts, with Cu atomic content in the range 7.6–1.0%, were synthetised, characterized, and tested for the Guerbet reaction. The adopted catalysts are easy to produce, and based on available, cheap and non-toxic metals. Tailoring Cu-content in hydrotalcite-derived Cu/Mg/Al mixed oxides allowed us to selectively address this reaction towards n-butanol formation, highlighting how a low Cu-content favours the condensation to n-butanol and higher alcohols, in particular n-hexanol, decreasing the ester formation. The Guerbet selective co-production of [n-butanol + n-hexanol] is economically even more attractive than that of the sole n-butanol, being both saleable products. Good catalytic performances to n-butanol and n-hexanol were achieved, in a batch process carried out under subcritical conditions. These catalysts have required a simple procedure of recovery, showing good recyclability, in term of catalytic performances to give the target products, thus opening the way to applicative perspectives.

Electrochemical training of nanoporous Cu-In catalysts for efficient CO2-to-CO conversion and high durability

Electrochemical carbon dioxide (CO2) reduction into valuable chemicals is usually implemented at high overpotential, and most bimetallic catalysts suffer from the dramatic evolution of surface composition especially in case of long-term operation. In this work, as a proof-of-concept experiment, one effective protocol is first identified to drive the positive evolution of surface contents on nanoporous Cu-In by electrochemical training in KHCO3 electrolyte. Interestingly, superior performances including high selectivity for CO production and robust reliability are realized. Thus, Faradaic efficiency can approach the maximum of 91% at −0.95 V (vs. RHE), and an excellent durability can be simultaneously testified by no detectable performance decay for 7 h in operation. More importantly, current investigation paves a new road for designing bimetallic electrocatalysts with high selectivity and long-term durability.

Selective production of anhydrosugars and furfural from fast pyrolysis of corncobs using sulfuric acid as an inhibitor and catalyst

The objective of this study was to selectively coproduce anhydrosugars and furfural from the fast pyrolysis of biomass by H2SO4 impregnation. The pyrolysis behaviors of raw and H2SO4-impregnated corncobs, cellulose and xylan were systematically studied by a thermogravimetric analyzer (TGA) and commercial pyroprobe reactor. The results demonstrate that H2SO4 impregnation can reduce the formation of char and drastically improve the yield of anhydrosugars and furfural. The maximum yields of levoglucosan (38.45 wt% based on cellulose), furfural (19.18 wt% based on hemicellulose) and xylosan (9.49 wt% based on hemicellulose) were obtained by fast pyrolysis of corncobs impregnated with 2.75 wt% H2SO4. By comparing the product distributions from fast pyrolysis of H2SO4-impregnated cellulose, xylan, and raw and demineralized corncobs, it is concluded that H2SO4 can act as an inhibitor to suppress the catalytic functions of structural alkali and alkaline earth metals (AAEM) to improve the yield of anhydrosugars, and H2SO4 can also act as a catalyst to accelerate the dehydration of hemicellulose to form more furfural. It is speculated that H2SO4 could first react with structural AAEM in lignin to form lignosulfonates (e.g., potassium lignosulfonate), thus reducing the catalytic functions of structural AAEM during fast pyrolysis of corncobs. These findings provide a simple and efficient method for the selective coproduction of anhydrosugars and furfural from waste biomass.

Molecular Electrochemical Catalysis of the CO2-to-CO Conversion with a Co Complex: A Cyclic Voltammetry Mechanistic Investigation

The electrochemical catalytic reduction of CO2 into CO could be achieved with excellent selectivity and rate in acetonitrile in the presence of phenol with cobalt 2,2′:6′,2″:6″,2‴-quaterpyridine complex [CoII(qpy)(H2O)2]2+ (Co) acting as a molecular catalyst. Upon using cyclic voltammetry at low and high scan rate (up to 500 V/s) two catalytic pathways have been identified. At a low concentration of phenol (<1 M), catalysis mainly occurs after the reduction of Co with three electrons. In that case, the selectivity for CO production is ca. 80% with 20% of H2 as by product, along with a turnover frequency of 1.2 × 104 s–1 for CO production at an overpotential η of ca. 0.6 V. The triply reduced active species binds to CO2 and the C–O bond is cleaved thanks to the acid. At very large concentration of phenol (3 M), another pathway becomes predominant: the doubly reduced species binds to CO2, while its reductive protonation leads to CO formation. As already shown, this later process is endowed with fast rate at...

Visualization of catalytic edge reactivity in electrochemical CO2 reduction on porous Zn electrode

In the study, the catalytic edge reactivity on porous Zn electrode has successfully visualized through the electrochemical CO2 reduction to CO. It is well known that the activity of a CO2 reduction reaction catalyst depend on the type of material and surface nano-structure. Consequently, numerous researchers are interested in the relation between the catalyst activity and surface conditions such as morphology, oxidation state, and crystal orientation. However, it is difficult to explain the mechanisms of catalytic CO2 reduction and visualize the catalytic activity. Our results demonstrate, that this strategy not only improved the selective CO production, but also helped visualize the catalytic reactivity on the edge site via open-loop electric potential microscopy (OL-EPM). The obtained OL-EPM image strongly suggests that the edge site of porous Zn acts as an efficient reactive site in the CO2 electrochemical reduction reaction.

Study on carbon dioxide reduction with water over metal oxide photocatalysts

Various metal oxides with 0.1 wt% Ag loaded as a cocatalyst were prepared by an impregnation method and examined their photocatalytic activity for CO2 reduction with water. Among all the prepared Ag‐loaded metal oxides, Ga2O3, ZrO2, Y2O3, MgO, and La2O3 showed activities for CO and H2 productions under ultraviolet light irradiation. Thus, metal oxides involving metal cations with closed shell electronic structures such as d0, d10, and s0 had the potential for CO2 reduction with water. In situ Fourier transform infrared measurement revealed that the photocatalytic activity and selectivity for CO production are controlled by the amount and chemical states of CO2 adsorbed on the catalyst surface and by the surface basicity, as summarized as follows: Ag/ZrO2 enhanced H2 production rather than CO production due to very little CO2 adsorption. Ag/Ga2O3 exhibited the highest activity for CO production, because adsorbed monodentate bicarbonate was effectively converted to bidentate formate being the reaction intermediates for CO production owing to its weak surface basicity. Ag/La2O3, Ag/Y2O3, and Ag/MgO having both weak and strong basic sites adsorbed larger amount of carbonate species including their ions and suppressed H2 production. However, the adsorbed carbonate species were hardly converted to the bidentate formate.

Understanding Selective Reduction of CO2 to CO on Modified Carbon Electrocatalysts

AbstractModified carbon powders are attractive electrocatalysts for chemical conversion due to their low cost and high activity. Here, we demonstrate that electrochemical conversion activities of CO2 to CO in aqueous media are enhanced by the incorporation of transition metals into nitrogen‐doped carbon (M−N/C; M=Fe, Co, Cu). The type of the metals is thereby critical to achieve selective CO production over H2. Among M−N/C and N/C catalysts, Fe−N/C exhibited the highest faradaic efficiency for CO conversion, significantly enhanced compared with the metal‐free N‐doped carbon electrocatalyst. In addition, the catalysts containing higher pyridinic N and graphitic N but lower pyrrolic N contents favor CO production at lower bias potentials. Meanwhile, the total amount of N content in the carbon material is less important. Our CO2 reduction reaction results combined with X‐ray photoelectron spectroscopy analysis propose that the modification of nitrogen states in the carbon structure is crucial in CO2RR activity.

Selective CO Production by Photoelectrochemical Methane Oxidation on TiO2.

The inertness of the C–H bond in CH4 poses significant challenges to selective CH4 oxidation, which often proceeds all the way to CO2 once activated. Selective oxidation of CH4 to high-value industrial chemicals such as CO or CH3OH remains a challenge. Presently, the main methods to activate CH4 oxidation include thermochemical, electrochemical, and photocatalytic reactions. Of them, photocatalytic reactions hold great promise for practical applications but have been poorly studied. Existing demonstrations of photocatalytic CH4 oxidation exhibit limited control over the product selectivity, with CO2 as the most common product. The yield of CO or other hydrocarbons is too low to be of any practical value. In this work, we show that highly selective production of CO by CH4 oxidation can be achieved by a photoelectrochemical (PEC) approach. Under our experimental conditions, the highest yield for CO production was 81.9%. The substrate we used was TiO2 grown by atomic layer deposition (ALD), which features high concentrations of Ti3+ species. The selectivity toward CO was found to be highly sensitive to the substrate types, with significantly lower yield on P25 or commercial anatase TiO2 substrates. Moreover, our results revealed that the selectivity toward CO also depends on the applied potentials. Based on the experimental results, we proposed a reaction mechanism that involves synergistic effects by adjacent Ti sites on TiO2. Spectroscopic characterization and computational studies provide critical evidence to support the mechanism. Furthermore, the synergistic effect was found to parallel heterogeneous CO2 reduction mechanisms. Our results not only present a new route to selective CH4 oxidation, but also highlight the importance of mechanistic understandings in advancing heterogeneous catalysis.

Toward an Effective Control of the H2 to CO Ratio of Syngas through CO2 Electroreduction over Immobilized Gold Nanoparticles on Layered Titanate Nanosheets

In recent years, the electroreduction of CO2 to valuable products has emerged as a rational answer to rising CO2 emissions and a strategic approach to incorporate renewable electricity from intermittent sources (e.g., wind and solar) into the global energy supply. The reduction of CO2 to CO has been highlighted in the widely explored industrial conversion of syngas (CO and H2) to fuels. Herein, we report a promising electrocatalyst incorporating well-dispersed gold nanoparticles (Au NPs) on ultrathin titanate nanosheets (TiNS). By tuning the contents of Au (in the ranges of 0 to 93 wt % Au) in the hybrid Au/TiNS architecture, CO product selectivity was effectively controlled (in the range of CO Faradaic efficiency from 3 to over 80%) with the sole additional formation of H2, which is of pronounced industrial interest. Most importantly, a control of both component amounts was suggested to result in a variation of corresponding electronic properties based on the interaction between Au NP and TiNS substrate, dictating the stabilization of formed reaction intermediates and resulting product selectivity. In addition, our Au/TiNS achieved optimally high CO and H2 production current densities, with 73 wt % Au at the low cathodic potential region (−0.6 to −0.9 VRHE). The suggested synergetic effect between both catalytic components underlines the promising character of this hybrid system and is expected to significantly add to the strategic production of syngas for subsequent applications.

Highly Selective Molecular Catalysts for the CO2-to-CO Electrochemical Conversion at Very Low Overpotential. Contrasting Fe vs Co Quaterpyridine Complexes upon Mechanistic Studies

[MII(qpy)(H2O)2]2+ (M = Fe, Co; qpy: 2,2′:6′,2″:6″,2‴-quaterpyridine) complexes efficiently catalyze the electrochemical CO2-to-CO conversion in acetonitrile solution in the presence of weak Bronsted acids. Upon performing cyclic voltammetry studies, controlled-potential electrolysis, and spectroelectrochemistry (UV–visible and infrared) experiments together with DFT calculations, catalytic mechanisms were deciphered. Catalysis is characterized by high selectivity for CO production (selectivity >95%) in the presence of phenol as proton source. Overpotentials as low as 240 and 140 mV for the Fe and Co complexes, respectively, led to large CO production for several hours. In the former case, the one-electron-reduced species binds to CO2, and CO evolution is observed after further reduction of the intermediate adduct. A deactivation pathway has been identified, which is due to the formation of a Fe0qpyCO species. With the Co catalyst, no such deactivation occurs, and the doubly reduced complex activates CO2....

Hot Hole Collection and Photoelectrochemical CO2 Reduction with Plasmonic Au/p-GaN Photocathodes.

Harvesting nonequilibrium hot carriers from plasmonic-metal nanostructures offers unique opportunities for driving photochemical reactions at the nanoscale. Despite numerous examples of hot electron-driven processes, the realization of plasmonic systems capable of harvesting hot holes from metal nanostructures has eluded the nascent field of plasmonic photocatalysis. Here, we fabricate gold/p-type gallium nitride (Au/p-GaN) Schottky junctions tailored for photoelectrochemical studies of plasmon-induced hot-hole capture and conversion. Despite the presence of an interfacial Schottky barrier to hot-hole injection of more than 1 eV across the Au/p-GaN heterojunction, plasmonic Au/p-GaN photocathodes exhibit photoelectrochemical properties consistent with the injection of hot holes from Au nanoparticles into p-GaN upon plasmon excitation. The photocurrent action spectrum of the plasmonic photocathodes faithfully follows the surface plasmon resonance absorption spectrum of the Au nanoparticles and open-circuit voltage studies demonstrate a sustained photovoltage during plasmon excitation. Comparison with Ohmic Au/p-NiO heterojunctions confirms that the vast majority of hot holes generated via interband transitions in Au are sufficiently hot to inject above the 1.1 eV interfacial Schottky barrier at the Au/p-GaN heterojunction. We further investigated plasmon-driven photoelectrochemical CO2 reduction with the Au/p-GaN photocathodes and observed improved selectivity for CO production over H2 evolution in aqueous electrolytes. Taken together, our results offer experimental validation of photoexcited hot holes more than 1 eV below the Au Fermi level and demonstrate a photoelectrochemical platform for harvesting hot carriers to drive solar-to-fuel energy conversion.

A Monodispersed Spherical Zr-Based Metal-Organic Framework Catalyst, Pt/Au@Pd@UIO-66, Comprising an Au@Pd Core-Shell Encapsulated in a UIO-66 Center and Its Highly Selective CO2 Hydrogenation to Produce CO.

A Zr-based metal-organic framework (MOF) catalyst, Pt/Au@Pd@UIO-66, is assembled, where UIO-66 is Zr6 O4 (OH)4 (BDC)6 (BDC = 1,4-benzenedicarboxylate). The gold nanoparticles (NPs) act as the core for the epitaxial growth of Pd shells, and the core-shell monodispersed nanosphere Au@Pd is encapsulated into UIO-66 to control its morphology and impart nanoparticle functionality. The microporous nature of UIO-66 assists the adsorption of Pt NPs, which in turn enhances the interaction between NPs and UIO-66, favoring the formation of isolated and well-dispersed Pt NP active sites. This MOF exhibits high catalytic activity and CO product selectivity for the reverse-water-gas-shift reaction in a fixed-bed flow reactor.

Selective CO2 Reduction on Zinc Electrocatalyst: The Effect of Zinc Oxidation State Induced by Pretreatment Environment

Here, we have developed porous nanostructured Zn electrocatalysts for CO2 reduction reaction (CO2RR), fabricated by reducing electrodeposited ZnO (RE-Zn) to activate the CO2RR electrocatalytic performance. We discovered that the electrochemical activation environment using CO2-bubbled electrolyte during reducing ZnO in a pretreatment step is important for highly selective CO production over H2 production, while using Ar gas bubbling instead can lead to less CO product of the Zn-based catalyst in CO2RR later. The RE-Zn activated in CO2-bubbled electrolyte condition achieves a Faradaic efficiency of CO production (FECO) of 78.5%, which is about 10% higher than that of RE-Zn activated in Ar-bubbled electrolyte. The partial current density of CO product had more 10-fold increase with RE-Zn electrodes than that of bulk Zn foil at −0.95 V vs RHE in KHCO3. In addition, a very high FECO of 95.3% can be reached using the CO2-pretreated catalyst in KCl electrolyte. The higher amount of oxidized zinc states has been...

Tuning Ni-catalyzed CO2 hydrogenation selectivity via Ni-ceria support interactions and Ni-Fe bimetallic formation

CO2 hydrogenation over Fe-modified Ni/CeO2 catalysts was investigated in a batch reactor using time-resolved in situ FTIR spectroscopy. Low loading of Ni/CeO2 was associated with high selectivity to CO over CH4, while higher Ni loading improved CO2 hydrogenation activity with a reduced CO selectivity. X-ray absorption near-edge structure (XANES) analysis revealed Ni to be metallic for all catalysts including the CO-selective low loading 0.5% Ni catalyst, suggesting that the selectivity trend is due to structural rather than oxidation state effects. The loading amount of 1.5% Ni was selected for co-impregnation with Fe, based on the significant shift in product selectivity towards CH4 for that loading amount, in order to shift the selectivity towards CO while maintaining high activity. Temperature programmed reduction (TPR) results indicated bimetallic interactions between Ni and Fe, and XANES analysis showed that about 70% of Fe in the bimetallic catalysts was oxidized. The Ni-Fe catalysts demonstrated improved selectivity towards CO without significantly compromising activity, coupling the high activity of Ni catalysts and the high CO selectivity of Fe. The general trends in Ni loading and bimetallic modification should guide efforts to develop non-precious metal catalysts for the selective production of CO by CO2 hydrogenation.

Co-Production of Ethanol and 1,2-Propanediol via Glycerol Hydrogenolysis Using Ni/Ce–Mg Catalysts: Effects of Catalyst Preparation and Reaction Conditions

Crude glycerol from biodiesel production is a biobased material capable of co-producing biofuels and chemicals. This study aimed to develop a line of Ni catalysts supported on cerium–magnesium (Ce–Mg) to improve the process efficiency of glycerol hydrogenolysis for ethanol and 1,2-propanediol (1,2-PDO). Results showed that catalytic activity was greatly improved by changing the preparation method from impregnation to deposition precipitation (DP), and by adjusting calcination temperatures. Prepared via DP, the catalysts of 25 wt % Ni supported on Ce–Mg (9:1 mol/mol) greatly improved the effectiveness in glycerol conversion while maintaining the selectivities to ethanol and 1,2-PDO. Calcination at 350 °C provided the catalysts better selectivities of 15.61% to ethanol and 67.93% to 1,2-PDO. Increases in reaction temperature and time improved the conversion of glycerol and the selectivity to ethanol, but reduced the selectivity to 1,2-PDO. A lower initial water content led to a higher conversion of glycerol, but lower selectivities to ethanol and 1,2-PDO. Higher hydrogen application affected the glycerol conversion rate positively, but the selectivities to ethanol and 1,2-PDO negatively. A comparison to the commercial Raney® Ni catalyst showed that the Ni/Ce–Mg catalyst developed in this study showed a better potential for the selective co-production of ethanol and 1,2-PDO from glycerol hydrogenolysis.

(Invited) Molecular Electrochemical Catalysis of CO2 Reduction with Fe and Co Complexes. from Mechanistic Studies to Highly Selective CO Production and to More Reduced Products

Recent attention aroused by the reduction of carbon dioxide has as main objective the production of useful products - the “solar fuels” - in which solar energy would be stored. One route to this goal consists in first converting sunlight energy into electricity, then used to reduce CO2 electrochemically. Conversion of carbon dioxide into carbon monoxide is thus a key-step through the classical dihydrogen-reductive Fischer-Tropsch chemistry. We will describe our work in this field using variously substituted iron tetraphenylporphyrin derivatives, that prove to be the most efficient molecular catalysts of the reduction of CO2 to CO when generated electrochemical at the Fe(0) oxidation state, both in terms of selectivity, durability, overpotential and turnover frequency. These catalysts are remarkably active not only in organic solvent (e.g. DMF) but also in pure water at pH 7. Benchmarking of the catalytic performances, through catalytic Tafel plots, leads to a fair comparison between various reported catalysts and detailed mechanisms were obtained from electrochemical studies. Going further, a complete CO2 conversion electrochemical cell functioning in pure water was also designed. Finally, our recent work and strategy to reduce CO2 beyond CO (for generating hydrocarbons) will be briefly presented.

A spongy nickel-organic CO2 reduction photocatalyst for nearly 100% selective CO production.

Solar-driven photocatalytic conversion of CO2 into fuels has attracted a lot of interest; however, developing active catalysts that can selectively convert CO2 to fuels with desirable reaction products remains a grand challenge. For instance, complete suppression of the competing H2 evolution during photocatalytic CO2-to-CO conversion has not been achieved before. We design and synthesize a spongy nickel-organic heterogeneous photocatalyst via a photochemical route. The catalyst has a crystalline network architecture with a high concentration of defects. It is highly active in converting CO2 to CO, with a production rate of ~1.6 × 104 μmol hour-1 g-1. No measurable H2 is generated during the reaction, leading to nearly 100% selective CO production over H2 evolution. When the spongy Ni-organic catalyst is enriched with Rh or Ag nanocrystals, the controlled photocatalytic CO2 reduction reactions generate formic acid and acetic acid. Achieving such a spongy nickel-organic photocatalyst is a critical step toward practical production of high-value multicarbon fuels using solar energy.

Tunable-High Performance Sustainion™ Anion Exchange Membranes for Electrochemical Applications

This paper will consider the properties of anionic-conductive SustainionTM membranes to highlight its potential for different electrochemical activities. Specifically, the latest CO2 electrolysis results will be unveiled in which we have managed to maintain a CO product selectivity of over ~ 98%. This was accomplished using a steady state current of 200 mA/cm2 at under 3 V for 3000 hours with a very modest 6 uV/hr voltage increase. This translates to a degradation of only ~250 mV in five years which underlines the stability of the SustainionTM membrane that lies at the heart of these CO2 electrolyzers.