The utilization of fossil fuels has enabled an unprecedented era of prosperity and advancement of well-being for human society. However, the associated increase in anthropogenic carbon dioxide (CO2) ...emissions can negatively affect global temperatures and ocean acidity. Moreover, fossil fuels are a limited resource and their depletion will ultimately force one to seek alternative carbon sources to maintain a sustainable economy. Converting CO2 into value-added chemicals and fuels, using renewable energy, is one of the promising approaches in this regard. Major advances in energy-efficient CO2 conversion can potentially alleviate CO2 emissions, reduce the dependence on nonrenewable resources, and minimize the environmental impacts from the portions of fossil fuels displaced. Methanol (CH3OH) is an important chemical feedstock and can be used as a fuel for internal combustion engines and fuel cells, as well as a platform molecule for the production of chemicals and fuels. As one of the promising approaches, thermocatalytic CO2 hydrogenation to CH3OH via heterogeneous catalysis has attracted great attention in the past decades. Major progress has been made in the development of various catalysts including metals, metal oxides, and intermetallic compounds. In addition, efforts are also put forth to define catalyst structures in nanoscale by taking advantage of nanostructured materials, which enables the tuning of the catalyst composition and modulation of surface structures and potentially endows more promising catalytic performance in comparison to the bulk materials prepared by traditional methods. Despite these achievements, significant challenges still exist in developing robust catalysts with good catalytic performance and long-term stability. In this review, we will provide a comprehensive overview of the recent advances in this area, especially focusing on structure–activity relationship, as well as the importance of combining catalytic measurements, in situ characterization, and theoretical studies in understanding reaction mechanisms and identifying key descriptors for designing improved catalysts.
Abstract
Efficient electroreduction of CO
2
to multi-carbon products is a challenging reaction because of the high energy barriers for CO
2
activation and C–C coupling, which can be tuned by ...designing the metal centers and coordination environments of catalysts. Here, we design single atom copper encapsulated on N-doped porous carbon (Cu-SA/NPC) catalysts for reducing CO
2
to multi-carbon products. Acetone is identified as the major product with a Faradaic efficiency of 36.7% and a production rate of 336.1 μg h
−1
. Density functional theory (DFT) calculations reveal that the coordination of Cu with four pyrrole-N atoms is the main active site and reduces the reaction free energies required for CO
2
activation and C–C coupling. The energetically favorable pathways for CH
3
COCH
3
production from CO
2
reduction are proposed and the origin of selective acetone formation on Cu-SA/NPC is clarified. This work provides insight into the rational design of efficient electrocatalysts for reducing CO
2
to multi-carbon products.
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•CO2 electroreduction mechanisms are determined with density functional theory (DFT).•DFT calculations incorporate the role of kinetic barriers and water solvation.•COH* is the key ...intermediate to produce methane and ethylene on Cu(111).•Impact of extended water solvation on reaction energies and barriers is reported.•COH* versus CHO* preferences are dependent on Cu facet.
Density functional theory (DFT) was used to determine the potential-dependent reaction free energies and activation barriers for several reaction paths of carbon dioxide (CO2) electrochemical reduction on the Cu(111) surface. The role of water solvation on CO2 reduction paths was explored by evaluating water-assisted surface hydrogenation and proton (H) shuttling with various solvation models. Electrochemical OH bond formation reactions occur through water-assisted H-shuttling, whereas CH bond formation occurs with negligible H2O involvement via direct reaction with adsorbed H* on the Cu(111) surface. The DFT-computed kinetic path shows that the experimentally observed production of methane and ethylene on Cu(111) catalysts occurs through the reduction of carbon monoxide (CO*) to a hydroxymethylidyne (COH*) intermediate. Methane is produced from the reduction of the COH* to C* and then sequential hydrogenation. Ethylene production shares the COH* path with methane production, where the methane to ethylene selectivity depends on CH2∗ and H* coverages. The reported potential-dependent activation barriers provide kinetics consistent with observed experimental reduction overpotentials and selectivity to methane and ethylene over methanol for the electroreduction of CO2 on Cu catalysts.
On the right path: Based on DFT calculations (incorporating the role of water solvation) of the activation barriers of elementary steps, a new path that leads to methane and ethylene for CO2 ...electroreduction on Cu(111) was identified. Methane formation proceeds through reduction of CO to COH (path II, see picture), which leads to CHx species that can produce both methane and ethylene, as observed experimentally.
Direct hydrogen peroxide (H
O
) electrosynthesis via the two-electron oxygen reduction reaction is a sustainable alternative to the traditional energy-intensive anthraquinone technology. However, ...high-performance and scalable electrocatalysts with industrial-relevant production rates remain to be challenging, partially due to insufficient atomic level understanding in catalyst design. Here we utilize theoretical approaches to identify transition-metal single-site catalysts for two-electron oxygen reduction using the *OOH binding energy as a descriptor. The theoretical predictions are then used as guidance to synthesize the desired cobalt single-site catalyst with a O-modified Co-(pyrrolic N)
configuration that can achieve industrial-relevant current densities up to 300 mA cm
with 96-100% Faradaic efficiencies for H
O
production at a record rate of 11,527 mmol h
g
. Here, we show the feasibility and versatility of metal single-site catalyst design using various commercial carbon and cobalt phthalocyanine as starting materials and the high applicability for H
O
electrosynthesis in acidic, neutral and alkaline electrolytes.
Density functional theory (DFT) calculations on Pd-Cu bimetallic catalysts reveal that the stepped PdCu(111) surface with coordinatively unsaturated Pd atoms exposed on the top is superior for CO2 ...and H2 activation and for CO2 hydrogenation to methanol in comparison to the flat Cu-rich PdCu3(111) surface. The energetically preferred path for CO2 to CH3OH over PdCu(111) proceeds through CO2* → HCOO* → HCOOH* → H2COOH* → CH2O* → CH3O* → CH3OH*. CO formation from CO2 via a reverse water-gas shift (RWGS) proceeds more quickly than CH3OH formation in terms of kinetic calculations, in line with experimental observation. A small amount of water, which is produced in situ from both RWGS and CH3OH formation, can accelerate CO2 conversion to methanol by reducing the kinetic barriers for O–H bond formation steps and enhancing the TOF. Water participation in the reaction alters the rate-limiting step according to the degree of rate control (DRC) analysis. In comparison to CO2, CO hydrogenation to methanol on PdCu(111) encounters higher barriers and thus is slower in kinetics. Complementary to the DFT results, CO2 hydrogenation experiments over SiO2-supported bimetallic catalysts show that the Pd-Cu(0.50) that is rich in a PdCu alloy phase is more selective to methanol than the PdCu3-rich Pd-Cu(0.25). Moreover, advanced CH3OH selectivity is also evidenced on Pd-Cu(0.50) at a specific water vapor concentration (0.03 mol %), whereas that of Pd-Cu(0.25) is not comparable. The present work clearly shows that the PdCu alloy surface structure has a major effect on the reaction pathway, and the presence of water can substantially influence the kinetics in CO2 hydrogenation to methanol.
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•The selective promotion of CH3OH in CO2 hydrogenation correlates to Pd-Cu alloy content.•Pd-Cu combination shifts adsorption towards weakly-bonded H2 and CO2.•Weakly-bonded H2 and ...CO2 species over Pd-Cu appear to correlate with CH3OH promotion.•DFT study rationalizes CO2 adsorption and initial hydrogenation on Pd-Cu bimetallic surface.
A strong synergetic effect was observed in our previous work on Pd-Cu bimetallic catalysts for CH3OH formation from CO2 hydrogenation when the Pd/(Pd + Cu) atomic ratio lied within 0.25–0.34. In the present study, the importance of Pd-Cu alloy in selective CH3OH promotion was evidenced and correlated with alloy contents quantitatively through X-ray diffraction (XRD), scanning transmission electron spectroscopy with energy-dispersive X-ray spectroscopy (STEM/EDS), and H2-O2 titration and N2O titration. The surface chemical properties of Pd-Cu combinations were characterized by H2-/CO2-temperature-programmed desorption (TPD), diffuse reflectance infrared FT spectroscopy (DRIFTS), and density functional theory (DFT), and experimentally evaluated along with monometallic counterparts. Detailed characterization results reveal a unique shift in adsorption towards weakly-bonded H2 and CO2 on Pd-Cu bimetallic surface which appear to correlate to the CH3OH promotion. DFT calculations on adsorption properties of H2 and CO2 show good agreement with the observation from TPD experiments. DFT study also provides insights into the impact of Pd-Cu combination on the activation and initial hydrogenation of CO2 to formate (HCOO∗) and hydrocarboxyl (COOH∗) intermediates. HCOO∗ formation was found to be kinetically more favored than COOH∗ on monometallic Cu and Pd-Cu surfaces. The lowest barrier for HCOO∗ formation was observed at Pd/(Pd + Cu) atomic ratio of 0.33, around which a good CO2 conversion and high methanol selectivity were achieved experimentally.
Developing robust water splitting photocatalyst remains a pivot challenge for solar-to-fuel conversion. Herein, two-dimensional (2D) Janus bilayer heterostructures are reported by ...sulfur-vacancy-confined-in ZnIn2S4 (Vs-ZnIn2S4) and WO3 nanosheets as an all-solid-state Z-scheme prototype. First-principle calculations and experimental observations clearly confirm the spontaneous formation of this redox-mediator-free Z-scheme van der Waals heterostructure at atomic level, not only facilitating the space separation of photoexcited carriers with high charge density, enhancing charge dynamics and optimizing charge lifetime, but also accumulating electrons in conduction band of Vs-ZnIn2S4 and holes in valence band of WO3 by internal electric field through W–S bonds. After integrated by NiS quantum dots, novel 2D/2D NiS/Vs-ZnIn2S4/WO3 heterostructures with high stability exhibited an outstanding visible-light hydrogen evolution rate of 11.09 mmol g−1 h−1 and an apparent quantum efficiency about 72% at 420 nm, the highest value so far reported among the family of ZnIn2S4 photocatalysts. This work not only presents novel Janus heterostructures but also paves the atomic-level structural and interfacial design and the construction of 2D Janus bilayer Z-scheme heterojunctions for solar energy conversion applications.
The redox-mediator-free Z-scheme Janus bilayer Vs-ZnIn2S4/WO3 heterostructures have been achieved for excellent visible-light-driven hydrogen evolution. Display omitted
•2D Janus bilayer Vs-ZnIn2S4/WO3 heterostructures were achieved.•Z-scheme heterostructures promote atomic-level charge transport and separation.•NiS/Vs-ZnIn2S4/WO3 heterostructures achieve highest hydrogen evolution rate.•Z-scheme mechanism via DTF calculations and experimental observations is proposed.
Aromatization of light alkanes is of great interest because this can expand the raw materials used to produce aromatics to include fractions of natural gas that are readily available and inexpensive. ...Combining CO2 reduction with ethane dehydrogenation and aromatization can also mitigate CO2 emissions. A one-step process that can produce liquid aromatics from the reactions of CO2 and ethane using phosphorus (P)- and gallium (Ga)-modified ZSM-5 has been evaluated at 873 K and atmospheric pressure. The addition of P improves the hydrothermal stability of Ga/ZSM-5, reduces coke formation on the catalyst surface, and allows the formation of more liquid aromatics through the tandem reactions of CO2-assisted oxidative dehydrogenation of ethane and subsequent aromatization. Density functional theory calculations provide insights into the effect of Ga- and P- modification on ethane dehydrogenation to ethylene as well as the role of CO2 on the production of aromatics.
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•Adding 0.1 mol% H2O in the feed increases CH3OH formation rate by 20% over In2O3/ZrO2.•H2O-induced oxygen vacancies improve CO2 adsorption capacity.•DFT reveals the correlation of ...InOOH and CH3OH formation with H2O addition.•Excess H2O leads to aggregation of catalyst and negatively affects H2 dissociation.•Excess H2O causes surface variations of InOOH species and oxygen vacancies.
CO2 hydrogenation with renewable energy is one of the promising approaches to mitigate CO2 emissions and produce sustainable chemicals and fuels. The effect of adding H2O in the feed gas on the activity and selectivity of In2O3/ZrO2 catalysts for CO2 hydrogenation to methanol was studied using combined experimentatal and computational efforts. Notably, adding an appropriate amount of H2O (0.1 mol%) in the feed gas significantly enhanced the CH3OH formation (ca. 20%) with improved selectivity. Characterization with STEM/EDS and CO2-TPD confirmed the preservation of In-Zr strong interaction in the presence of additional H2O and H2O-induced oxygen vacancies, which significantly improved CO2 adsorption capacity. XPS analysis revealed the formation of InOOH species due to H2O addition, which appeared to correlate to H2O-dependant enhancement of CH3OH formation. Density functional theory calculations rationalized the effect of surface H2O on InOOH formation and its correlation to CH3OH synthesis activity. Adding H2O was found to facilitate surface InOOH formation, suppress CO formation through COOH* intermediate, and promote CH3OH formation via HCOO* intermediate. However, excess H2O addition resulted in aggregation of In species and reduction of surface In0 for H2 dissociation.
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