CO2 Photoreduction Performance Research Articles

Visible-Light-Driven Reduction of CO2 to CO with Highly Active and Selective Earth-Abundant Metal Porphyrin-Conjugated Organic Polymers.

The field of artificial photosynthesis, which focuses on harnessing solar light for the conversion of CO2 to economically valuable chemical products, remains a captivating area of research. In this study, we developed a series of photocatalysts based on Earth abundant elements (Fe, Co, Ni, Cu, and Zn) incorporated into 2D metalloporphyrin-conjugated organic polymers known as MTBPP-BEPA-COPs. These photocatalysts were utilized for the photoreduction of CO2 employing only H2O as the electron donor, without the need for any sacrificial agents or precious-metal cocatalysts. Remarkably, all of the synthesized MTBPP-BEPA-COPs exhibited an exceptional CO2 photoreduction performance only irradiated by visible light. Particularly, upon optimizing the metal ion coordinated with porphyrin units, ZnTBPP-BEPA-COP outperformed the other MTBPP-BEPA-COPs in terms of photocatalytic activity, achieving an impressive CO reduction yield of 152.18 μmol g-1 after just 4 h of irradiation. The electrostatic potential surfaces calculated by density functional theory suggest the potential involvement of metal centers as binding and catalytic sites for the binding of CO2. The calculated adsorption energy of CO2 with ZnTBPP-BEPA-COP exhibited one of the two smallest values. This may be the reason for the excellent catalytic effect of ZnTBPP-BEPA-COP. Thus, the present study not only demonstrates the potential of porphyrin-based conjugated polymers as highly efficient photocatalysts for CO2 reduction but also offers valuable insights into the rational design of such materials in the future.

Local microenvironment modulation of zirconium-porphyrinic frameworks for CO2 reduction

Solar-powered photocatalysis offers a sustainable strategy to convert carbon dioxide into valuable fuels, which mitigates the energy crisis and the greenhouse effect. Nevertheless, exploring efficient, selective, stable, and environmentally friendly photocatalysts for CO2 reduction continues to meet a significant challenge. Herein, two iron-porphyrinic zirconium-based metal–organic frameworks, Zr6O4(OH)4(FeTCBPP)3 (MOF-526-H) and Zr6O4(OH)4(FeTCBPP-NH2)3 (MOF-526-NH2) were achieved by the integration of the light-harvesting sites and catalytic centers into one phase (FeTCBPP = iron 5,10,15,20-tetra[4-(4′-carboxyphenyl)phenyl]-porphyrin, FeTCBPP-NH2 = iron 5,10,15,20-tetra[4-(4′-carboxyphenyl)-2-aminophenyl]-porphyrin), which were employed as a couple of models for CO2 photoreduction. MOF-526-NH2 exhibited excellent CO2 reduction with a CO yield of 21.2 mmol·g−1 in the absence of any photosensitizer under visible light, which was 4-fold higher than that of MOF-526-H. Additionally, MOF-526-NH2 showed no significant reduction in CO production during the four rounds, indicating that MOF-526-NH2 possessed good stability. The presence of amino groups in MOF-526-NH2 played an important role in adjusting the micro-environment and electronic structure, which was beneficial to CO2 enrichment and light absorption, and thus improved CO2 photoreduction performance. Based on the density functional theory calculations, a possible mechanism of photocatalytic CO2 reduction over porphyrinic MOFs was proposed.

In situ synthesis of g-C3N4/Ag@Ag3PO4 S-scheme heterostructure with tandem ohmic- and d-p bond-junction for photocatalytic reduction of CO2

Low-mass interfacial contacts and poor charge transfer efficiency severely limit the CO2 photoreduction performance of semiconductor heterojunctions. In this work, g-C3N4/Ag@Ag3PO4 S-scheme heterojunction is prepared by an in-situ growth method. The main product for CO2 photoreduction is CO with the selectivity of 97 %. The evolution rate of CO for optimal g-C3N4/Ag@Ag3PO4 heterojunction is 123.8 μmol g−1 h−1, which are 8.3-and 3.4-fold higher than those of pristine Ag3PO4 and g-C3N4, respectively. It is found that Ag3PO4, Ag, and g-C3N4 form strong interacting interfacial structure, in which Ag3PO4 links with Ag nanoparticles through tandem Ohmic contact. More importantly, σ bonds are formed via hybridization of px orbital for C and N with dx2-y2 orbitals of Ag in g-C3N4/Ag@Ag3PO4 S-scheme heterojunction, establishing an atomic-level dx2-y2(Ag)-px(C,N) charge flow highway. This work provides new viewpoints into the design of heterojunction photocatalysts with high-quality interfaces and highlights the insights of charge transfer behavior in regulating the catalytic activity.

Surface-Integrating Oxygen Vacancy and CuxO Nanodots Enabling Synergistic Electric Field and Dual Catalytic Sites Boosting CO2 Photoreduction.

High carrier separation efficiency and rapid surface catalytic reaction are crucial for enhancing catalytic CO2 photoreduction reaction. Herein, integrated surface decoration strategy with oxygen vacancies (Ov) and anchoring CuxO (1 < x < 2) nanodots below 10nm is realized on Bi2MoO6 for promoting CO2 photoreduction performance. The charge interaction between Ov and anchored CuxO enables the formation of enhanced internal electric field, which provides a strong driving force for accelerating the separation of photocharge carriers on the surface of Bi2MoO6 (ηsurf ≈71%). They can also cooperatively reduce the surface work function of Bi2MoO6, facilitating the migration of carrier to the surface. Meanwhile, surface-integrated Ov and CuxO nanodots allowing dual catalytic sites strengthens the adsorption and activation CO2 into *CO2 over Bi2MoO6, considerably boosting the progression of CO2 conversion process. In the absence of co-catalyst or sacrificial agent, Bi2MoO6 with Ov and CuxO nanodots achieves a photocatalytic CO generation rate of 12.75µmol g-1 h-1, a remarkable increase of over ≈15 times that of the original counterpart. This work provides a new idea for governing charge movement behaviors and catalytic reaction thermodynamics on the basis of synergistic improvement of electric field and active sites by coupling of the internal defects and external species.

Perovskite Nanocrystals In Situ Encapsulated in TiO2 Microspheres for Stable CO2 Photoreduction in Water.

Photoreduction of carbon dioxide (CO2) into fuels presents a promising approach to mitigate global warming and energy crises. Halide perovskite nanocrystals (NCs) with prominent optoelectronic properties have triggered substantial attention as photocatalysts but are limited by the charge recombination and instability. Here, we develop stable CsPbBr3/titania microspheres (TMs) by in situ growth of CsPbBr3 NCs inside mesoporous TMs through solid-state sintering, which significantly improves the stability of perovskite NCs, making them applicable in water with efficient CO2 photoreduction performance. Notably, the CsPbBr3/TMs demonstrates a 6.73- and 9.23-fold increase in the rate of CH4 production compared to TMs and CsPbBr3, respectively. The internal electric field facilitates S-scheme charge transfer, enhancing the separation of electron-hole pairs, as evidenced by X-ray photoelectron spectroscopy and electron paramagnetic resonance analysis, which is pivotal for the selective photoreduction of CO2. These insights pave the way for the design of CsPbBr3-based photocatalysts with superior efficiency and stability.

Hollow heterojunction with dual-vacancy engineering to boost photocatalytic performance for photoreduction of CO2 coupled with selective oxidation of benzyl alcohol to benzaldehyde

CO2 photoreduction into chemical fuels is hopeful and sustainable for a carbon–neutral future. Herein, a bifunctional dual-vacancy-modified hollow heterojunction photocatalyst is ingeniously designed for CO2 photoreduction to CO coupling with selective oxidation of benzyl alcohol to benzaldehyde. Synergistic catalysis effect resulted from hollow heterostructures, TiO2 with O vacancies (TiO2-x) and Zn0.3-xCd0.7S with Zn vacancies (ZCS) leads to excellent photoreduction and photoxidation performances. The optimized sample (TiO2-x/ZCS hollow sphere) exhibits highest photocatalytic activity of all the obtained samples. The yields of CO and benzaldehyde are 105 and 323.5 μmol g–1h−1, respectively. The construction of Z-scheme heterojunction realizes the spatial separation of redox reaction sites, and the dual-vacancy distributed on the heterojunction enhance the interfacial reactivity. Additionally, density functional theory calculation and in-situ technology reveal that Zn vacancy in ZCS and O vacancy in TiO2-x function as active sites for the binding and activation of *COOH and *PhCH2O-, respectively, thereby stabilizing the crucial step in the formation of intermediates such as CO and benzaldehyde. This work enhances the utilization of photogenerated carrier, and affords a new opportunity to design hollow heterojunction with dual-vacancy engineering to boost photocatalytic performance for coupling reaction system.

Design of atomically dispersed N-Bi(3+x)+--OV sites in ultrathin Bi2O2CO3 nanosheets for efficient and durable visible-light-driven CO2 reduction

The introduction of oxygen vacancies (OVs) into photocatalysts has proven to be a successful tactic to boost CO2 reduction. However, the challenge lies in acquiring OV sites that are stable in the long term, highly dispersed, and tunable in concentration. Herein, an innovative configuration, referred to as N-Bi(3+x)+--OV, was developed for the model semiconductor Bi2O2CO3 via an in situ anion doping approach. The structure enables the synthetic photocatalyst to exhibit superb CO2 photoreduction performance, with approximately 100% CO selectivity and remarkable long-term stability. Experimental studies and density functional theory (DFT) calculations show that replacing O2- with N3- uniformly in the [Bi2O2]2+ structural unit increases the chemical valence of Bi, elongates nearby Bi─O bonds, releases lattice O, improves CO2 absorption, and decreases the energy barrier for the formation of the critical intermediate *COOH. This study offers new insights and potential opportunities for the development of reliable defect-type semiconductors and their catalytic applications.

Enhanced charge separation of Cu-BTC@CuSe@TiO2 hollow octahedrons for efficient CO2 photoreduction with superior CO selectivity

Solar-driven photocatalytic conversion of CO2 into high-value-added fuels has attracted widespread attention. However, the relatively low conversion efficiency and product yield severely limited photocatalytic applications. In this work, binary Cu-BTC@TiO2 catalysts with adjustable inner cavity were prepared using copper-based metal organic framework (Cu-BTC) octahedrons as substrate via a solvothermal reaction. In this process, the inner Cu-BTC octahedrons can be controllably etched into a hollow octahedral structure in the presence of HF. Subsequently, the Cu-BTC@CuSe@TiO2 hollow octahedrons (HOs) were fabricated by selenization reaction and exhibited various properties such as abundant active sites for CO2 adsorption and reduction reactions, shortened charge transfer distance to prevent electron-hole recombination, and internal reflection/scattering effects to improve solar light utilization. Moreover, the formed dual p-n heterostructures between p-type CuSe and n-type semiconductors (Cu-BTC and TiO2) effectively promote spatial separation and migration of charge carriers. The synergistic effect of these advantages makes the optimized Cu-BTC@CuSe@TiO2 HO catalyst exhibit remarkable CO2 photoreduction performance with a CO production rate of 72.3 μmol h−1 g−1 and near 100 % selectivity. This work opens a new pathway for designing highly active photocatalysts with excellent product selectivity.

Hydrogen‐Bond‐Network Breakdown Boosts Selective CO2 Photoreduction by Suppressing H2 Evolution

AbstractConventional strategies for highly efficient and selective CO2 photoreduction focus on the design of catalysts and cocatalysts. In this study, we discover that hydrogen bond network breakdown in reaction system can suppress H2 evolution, thereby improving CO2 photoreduction performance. Photosensitive poly(ionic liquid)s are designed as photocatalysts owing to their strong hydrogen bonding with solvents. The hydrogen bond strength is tuned by solvent composition, thereby effectively regulating H2 evolution (from 0 to 12.6 mmol g−1 h−1). No H2 is detected after hydrogen bond network breakdown with trichloromethane or tetrachloromethane as additives. CO production rate and selectivity increase to 35.4 mmol g−1 h−1 and 98.9 % with trichloromethane, compared with 0.6 mmol g−1 h−1 and 26.2 %, respectively, without trichloromethane. Raman spectroscopy and theoretical calculations confirm that trichloromethane broke the systemic hydrogen bond network and subsequently suppressed H2 evolution. This hydrogen bond network breakdown strategy may be extended to other catalytic reactions involving H2 evolution.

Pd nanocubes/In2O3/hollow carbon microspheres with ultra-low interface resistance of Ohmic contact for efficient CO2 photoreduction

The utilization of CO2 photoreduction into valuable hydrocarbon fuels presents a promising avenue for mitigating climate change and providing a sustainable energy source. However, photogenerated carriers are highly prone to recombination, which adversely affects photocatalytic activity. In our study, we introduce a novel photocatalyst synthesis method, featuring an Ohmic contact (OC) structure that incorporates biomass-derived hollow carbon microspheres (HCM), Pd nanocubes (NC), and Indium Oxide (In2O3) through a simple yet effective thermal reduction method. This modification significantly enhances the photocatalytic efficiency of the Pd NC/In2O3/HCM (donated as PIC) system, achieving an optimal value of 496.89 μmol∙g−1 over 5 h without the need for additional additives, 8.7-fold higher than that of the pristine In2O3. Experimental and theoretical results reveal that the substantial enhancement in CO2 photoreduction performance can be attributed to the ultra-low resistance (122 Ω) interfaces within the PIC system, facilitating accelerated transfer of photogenerated electrons. This work provides a strategy for boosting CO2 photoreduction efficiency by the reduction of interface resistance to promote rapid photogenerated electron migration.

Oxygen vacancy-enrich Ag/brookite TiO2 Schottky junction for enhanced photocatalytic CO2 reduction

Insufficient surface catalytic sites and charge kinetics constrains the progress of CO2 photoreduction reaction. In this work, oxygen vacancy-enriched Ag/brookite TiO2 hybrid by anchoring Ag nanoparticles on brookite TiO2 has been synthesized through the transformation of Ti-glycerol precursor with simultaneous reduction of Ag+ in the diethylene glycol (DEG)-H2O solution. In particular, the involving of oxygen vacancy and Ag species could extend light absorption range, facilitate charge separation and transfer, and accumulate the electrons on Ag center. As a result, the Ag/brookite TiO2 with formed Schottky junction reveals enhanced CO2 photoreduction performance compare with pristine TiO2 and solvothermal treated TiO2 (TiO2-sol). Particularly, the oxygen vacancy-enriched Ag/TiO2 hybrid shows the highest catalytic activity with CH4 and CO yields. This work offers an approach to modify brookite phase TiO2 with metallic sites and oxygen vacancy for improved CO2 photoreduction.

Dipole increase synergistic frustrated Lewis pairs induced by Sn dopant boosts ultrathin layered bismuth oxychloride for CO2 photoreduction

Photocatalytic conversion of CO2 into valued chemicals is limited by the CO2 adsorption and activation capability on catalyst surface. In this work, Sn element was successfully doped in ultrathin BiOCl with oxygen vacancies. The dipole moment of catalyst efficient increased due to the Sn doping, leading to a stronger internal electric field. In addition, the surface frustrated Lewis pairs were simultaneously constructed near the oxygen vacancies due to Sn dopant. The formed region of frustrated Lewis pairs-oxygen vacancies as active sites enable an improved CO2 adsorption, and the enhanced internal electric field intensity promoted the charge carrier separation and transfer, leading to the enhanced CO2 reduction performances. This work provides light on enhancing internal electric field by increasing dipole moment and presents feasible tunability of catalytic active sites at atom-scale, providing fundamental insights for understanding of mechanism of element doping in catalyst for improving CO2 photoreduction performance.

Bi2O2S/CAU-17 bifunctional heterojunction for boosting CO2 photoreduction performance origin from the simultaneous photoxidation of tetracycline

To address the issues of energy scarcity and environmental pollution, a photocatalytic coupling system is employed to simultaneously harness electrons and holes in a single photocatalytic system. The current study presents a novel approach to construct a dual reaction pathway system for tetracycline hydrochloride (TC) oxidation and CO2 reduction. Herein, Bi2O2S/CAU-17 composite was designed and fabricated through a facile solvothermal method. The hybrid photocatalyst exhibited an excellent performance for simultaneous TC degradation and CO2 reduction. The CO-production rate can be reached 22.0 μmol g-1h−1 through the optimized 20% Bi2O2S/CAU-17 sample. Notably, the source of CO2 in the reaction system is solely derived from the photoxidation of TC. The theoretical calculations and experimental findings have shown that the existed Bi2O2S can enrich electron and separation of photo-generated electrons from CAU-17, thus, the holes in Bi2O2S and CAU-17 can be involved in the photodegradation of TC. Afterwards, the CO2 generated by TC oxidation moves to Bi2O2S via diffusion of molecules and could be efficiently reduced by electrons to produce CO. The development of this novel bifunctional Bi2O2S/CAU-17 hybrid photocatalytic system could provide valuable insights for the advancement of photocatalytic coupling reaction systems.

Construction of carbonized polymer dots/potassium doped carbon nitride nanosheets Van der Waals heterojunction by ball milling method for facilitating photocatalytic CO2 reduction performance in pure water

When the advancement of conventional heterojunctions is restricted by lattice matching of materials, Van der Waals heterojunctions formed by interlayer Van der Waals force present enormous development prospects with the revolution of graphene and graphene-like materials. Ascribed to similar graphene-like structures, carbonized polymer dots (CPDs) and potassium-doped carbon nitride nanosheets (KCNNS) were stacked by the ball milling method to form a novel Van der Waals heterojunction. Experimental analysis and density functional theory (DFT) calculations demonstrate that the constructed CPDs/KCNNS heterojunctions can effectively suppress sluggish reaction dynamics. CPDs/KCNNS-2 composites demonstrate enhanced CO2 photoreduction performance (78.98 μmol g−1∙h−1) with 100% CO selectivity in pure water, which is 13.71 times higher than that of KCNNS. In-situ FT-IR spectra were employed to identify the intermediates formed in the CO evolution process of CPDs/KCNNS composites. The manuscript provides a scientific reference for the stacking of Van der Waals heterojunction based on C3N4 in the artificial photosynthesis field.

Asymmetric Electron Redistribution in Niobic-Oxygen Vacancy Associates to Tune Noncovalent Interaction in CO2 Photoreduction.

The role of vacancy associates in photocatalytic CO2 reduction is an open question. Herein, the Nb─O vacancy associates (VNb─O) are engineered into niobic acid (NA) atomic layers to tailor the CO2 photoreduction performance. The intrinsic charge compensation from O to Nb around Nb─O vacancy associates can manipulate the active electronic states, leading to the asymmetric electron redistribution. These local symmetry breaking sites show a charge density gradient, forming a localized polarization field to polarize nonpolar CO2 molecules and tune the noncovalent interaction of reaction intermediates. This unique configuration contributes to the 9.3 times increased activity for photocatalytic CO2 reduction. Meantime, this VNb─O NA also shows excellent photocatalytic activity for NO3 --NH4 + synthesis, with NH4 + formation rate up to 3442µmol g-1 h-1. This work supplies fresh insights into the vacancy associate design for electron redistribution and noncovalent interaction tuning in photocatalysis.

Study on synergistic effects of 4f levels of erbium and black phosphorus/SnNb2O6 heterostructure catalysts by multiple spectroscopic analysis techniques.

Lanthanide single atom modified catalysts are rarely reported because the roles of lanthanide in photocatalysis are difficult to explain clearly. Based on the construction of Er single atom modified black phosphorus/SnNb2O6 (BP/SNO) heterojunctions, the synergistic effect of 4f levels of Er and heterostructures was studied by combining steady-state, transient, and ultrafast spectral analysis techniques with DFT theoretical calculations. According to the Judd-Ofelt theory of lanthanide ions, the CO2 photoreduction test under single wavelength excitation verifies that the 4F7/2/2H11/2 → 4I15/2 emissions of Er in BPEr/SNOEr can be more easily absorbed by SNO and BP, further proving the role of the 4f levels. As a result, the CO and CH4 yields of BPEr/SNOEr-10 under visible light irradiation are 10.7 and 10.1 times higher than those of pure BP, respectively, and 3.4 and 1.5 times higher than those of SNO. The results of DFT calculations show that the Er single atoms can cause surface reconstruction, regulate the active sites of BP, and reduce the energy change value in the key steps (CO2* + H+ + e- → COOH* and COOH* → CO* + H2O). This work provides novel insights into the design of lanthanide single atom photocatalysts for CO2 reduction.

Synergy between metal sites and oxygen vacancy for CO2 photoreduction: Activity and stability enhancement

In this work, TiO2 NPs (NPs) with abundant defects were successfully prepared as a substrate by photochemical method to load highly dispersed Pt NPs, focusing on the synergy between metal sites and oxygen vacancies for the enhancement of CO2 photoreduction performance and stability. As expected, H2-Pt/TiVo-UV exhibited excellent photocatalytic efficiency, with a photoreduction selectivity of up to 92.9 % for CO2 and a total electron yield of 382.4 μmol g−1 h−1. More importantly, H2-Pt/TiVo-UV activity decreased by only 6.8 % after a continuous reaction of up to 24 h. TiO2 NPs with abundant defects promote the migration of photogenerated electrons. The highly dispersed Pt NPs acted as electron traps, promoting photogenerated carrier separation. Meanwhile, the synergy between metal sites and oxygen vacancies enhances the adsorption capacity to the reactants. The combined effect of these three factors provides the driving force for the improved photocatalytic performance. This work provides insight into the efficient use of noble metal catalysts.

Construction of local coordination environment of iron sites over g-C3N4/PCN-222(Fe) composite with high CO2 photoreduction performance

Photocatalytic conversion of CO2 to usable fuels provides a new strategy for solving energy problems and the greenhouse effect. Porphyrin-based metal-organic frameworks (MOFs) are promising in photocatalysis due to their remarkable light harvesting ability, and the metal active site and its coordination environment can be further adjusted to achieve the high activity. In this work, g-C3N4/PCN-222 composites with different porphyrin metal centers were fabricated and the g-C3N4/PCN-222(FeIII) catalyst showed the highest photocatalytic CO2 reduction performance than that of g-C3N4/PCN-222(FeII) and other g-C3N4/PCN-222(M, M = Co, Ni, Cu, Zn) composites. Experimental studies and theoretical calculations showed that the axial coordination in g-C3N4/PCN-222(FeIII) distorts the square planar field (Fe-N4) to quasi-octahedral coordination, which led to the moderate adsorption ability of CO2 reduction intermediate (*CO), resulting in the high photocatalytic activity.

Mechanism-Guided Design of Photocatalysts for CO2 Reduction toward Multicarbon Products.

Photocatalytic reduction of CO2 to high value-added multicarbon (C2+) products is an important way to achieve sustainable production of green energy but limited by the low efficiency of catalysts. One fundamental issue lies in the high complexity of catalyst structure and reaction process, making the rational catalyst design and targeted performance optimization a grand challenge. Herein, we performed a mechanism-guided design of photocatalysts for CO2 reduction by using the experimentally reported Cu doped TiO2 (Cu-TiO2) with high C3H8 selectivity and well-defined structure as the prototype. Our mechanistic study highlights three key factors for C3H8 formation, i.e., formation of double O vacancies (Vdi-O) for selectivity, C-C coupling for activity, and Vdi-O recovery for durability. More importantly, Vdi-O formation/recovery and C-C coupling are negatively correlated, indicating that ideal candidates should achieve a balance between oxygen vacancy (VO) formation and C-C coupling. On this basis, TiO2 with the doping of two adjacent Cu atoms (Cu-Cu-TiO2) was designed with enhanced performance for CO2 photoreduction toward C3H8. Furthermore, a simple descriptor (Nμ, "effective d electron number") based on inherent atomic properties was constructed to uncover the underlying causes of the performance variation of different systems. These results provide new insights into the "structure-performance" relation of metal oxide-based photocatalysts, thus offering useful strategies for the rational design of excellent catalysts for CO2 photoreduction.

Nanostructurally Engineering Covalent Organic Frameworks for Boosting CO2 Photoreduction.

Herein, a series of imine-linked covalent organic frameworks (COFs) are developed with advanced ordered mesoporous hollow spherical nanomorphology and ultra-large mesopores (4.6nm in size), named OMHS-COF-M (M = H, Co, and Ni). The ordered mesoporous hollow spherical nanomorphology is revealed to be formed via an Ostwald ripening mechanism based on a one-step self-templated strategy. Encouraged by its unique structural features and outstanding photoelectrical property, the OMHS-COF-Co material is applied as the photocatalyst for CO2-to-CO reduction. Remarkably, it delivers an impressive CO production rate as high as 15 874µmol g-1 h-1, a large selectivity of 92.4%, and a preeminent cycling stability. From in/ex situ experiments and density functional theory (DFT) calculations, the excellent CO2 photoreduction performance is ascribed to the desirable cooperation of unique ordered mesoporous hollow spherical host and abundant isolated Co active sites, enhancing CO2 activation, and improving electron transfer kinetics as well as reducing the energy barriers for intermediates *COOH generation and CO desorption.