Theoretical Study on the Electro-Reduction of Carbon Dioxide to Methanol Catalyzed by Cobalt Phthalocyanine

Tuning the product selectivity of single-atom catalysts for CO2 reduction beyond CO formation by orbital engineering

Electrochemical CO2 reduction (CO2R) is one of the promising strategies for developing sustainable energy resources. Single-atom catalysts (SACs) have emerged as efficient catalysts for CO2R. However, the efficiency of SACs for the formation of reduction products beyond two-step CO formation is low due to the lower binding strength of the CO intermediate. In this study, we present an orbital engineering strategy based on density functional theory calculations and the fragment molecular orbital approach to tune product selectivity for the CO2R reaction on macrocycle based molecular catalysts (porphyrin and phthalocyanine) and extended SACs (graphene and covalent organic frameworks) with Fe, Co, and Ni dopants. The introduction of neutral axial ligands such as imidazole, pyridine, and trimethyl phosphine to the metal dopants enhances the binding affinity of the CO intermediate. The stability of the catalysts is investigated through the thermodynamic binding energy of the axial ligands and ab initio molecular dynamics simulations (AIMD). The grand canonical potential method is used to determine the reaction free energy values. Using a unified activity volcano plot based on the reaction free energy values, we investigated the catalytic activity and product selectivity at an applied potential of -0.8 V vs. SHE and a pH of 6.8. We found that with the imidazole and pyridine axial ligands, the selectivity of Fe-doped SACs towards the formation of the methanol product is improved. The activity volcano plot for these SACs shows a similar activity to that of the Cu (211) surface. The catalytic activity is found to be directly proportional to the sigma-donating ability of the axial ligands.

Cobalt Tetracationic 3,4-Pyridinoporphyrazine for Direct CO2 to Methanol Conversion Escaping the CO Intermediate Pathway

Molecular catalysts offer a unique opportunity to implement different chemical functionalities to steer the efficiency and selectivity for the CO2 reduction for instance. Metalloporphyrins and metallophthalocyanines are under high scrutiny since their most classic derivatives the tetraphenylporphyrin (TPP) and parent phthalocyanine (Pc), have been used as the molecular platform to install, hydrogen bonds donors, proton relays, cationic fragments, incorporation in MOFs and COFs, to enhance the catalytic power of these catalysts. Herein, we examine the electrocatalytic properties of the tetramethyl cobalt (II) tetrapyridinoporphyrazine (CoTmTPyPz) for the reduction of CO2 in heterogeneous medium when adsorbed on carbon nanotubes (CNT) at a carbon paper (CP) electrode. Unlike reported electrocatalysis with cobalt based phthalocyanine where CO was advocated as the two electron and two protons reduced intermediate on the way to the formation of methanol, we found here that CoTmTPyPz does not reduce CO to methanol. Henceforth, ruling out a mechanistic pathway where CO is a reaction intermediate.

Proton-Coupled Electron Transfer Mechanisms for CO2 Reduction to Methanol Catalyzed by Surface-Immobilized Cobalt Phthalocyanine

Immobilized cobalt phthalocyanine (CoPc) is a highly promising architecture for the six-proton, six-electron reduction of CO2 to methanol. This electroreduction process relies on proton-coupled electron transfer (PCET) reactions that can occur by sequential or concerted mechanisms. Immobilization on a conductive support such as carbon nanotubes or graphitic flakes can fundamentally alter the PCET mechanisms. We use density functional theory (DFT) calculations of CoPc adsorbed on an explicit graphitic surface model to investigate intermediates in the electroreduction of CO2 to methanol. Our calculations show that the alignment of the CoPc and graphitic electronic states influences the reductive chemistry. These calculations also distinguish between charging the graphitic surface and reducing the CoPc and adsorbed intermediates as electrons are added to the system. This analysis allows us to identify the chemical transformations that are likely to be concerted PCET, defined for these systems as the mechanism in which protonation of a CO2 reduction intermediate is accompanied by electron abstraction from the graphitic surface to the adsorbate without thermodynamically stable intermediates. This work establishes a mechanistic pathway for methanol production that is consistent with experimental observations and provides fundamental insight into how immobilization of the CoPc impacts its CO2 reduction chemistry.

Role of Mass Transport in Electrochemical CO2 Reduction to Methanol Using Immobilized Cobalt Phthalocyanine

Electrochemical CO2 reduction (CO2R) using heterogenized molecular catalysts usually yields 2-electron reduction products (CO, formate). Recently, it has been reported that certain preparations of immobilized cobalt phthalocyanine (CoPc) produce methanol (MeOH), a 6-electron reduction product. Here, we demonstrate the significant role of intermediate mass transport in CoPc selectivity to methanol. We first developed a simple, physically mixed, polymer (and polyfluoroalkyl, PFAS)-free preparation of CoPc on multiwalled carbon nanotubes (MWCNTs) which can be integrated onto Au electrodes using a poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) adhesion layer. After optimization of catalyst preparation and loading, methanol Faradaic efficiencies and partial current densities of 36% (±3%) and 3.8 (±0.5) mA cm-2, respectively, are achieved in the CO2-saturated aqueous electrolyte. The electrolyte flow rate has a large effect. A linear flow velocity of 8.5 cm/min produces the highest MeOH selectivity, with higher flow rates increasing CO selectivity and lower flow rates increasing the hydrogen evolution reaction, suggesting that CO is an unbound intermediate. Using a continuum multiphysics model assuming CO is the intermediate, we show qualitative agreement with the optimal inlet flow rate. Polymer binders were not required to achieve a high Faradaic efficiency for methanol using CoPc and MWCNTs. We also investigated the role of formaldehyde as an intermediate and the role of strain, but definitive conclusions could not be established.

C2 Product Formation over the C1 Product and HER on the 111 Plane of Specific Cu Alloy Nanoparticles Identified through Multiparameter Optimization

C2 products are more desirable than C1 products during CO2 electroreduction (CO2ER) because the former possess higher energy density and greater industrial value. For CO2ER, Cu is a well-known catalyst, but the selectivity toward C2 products is still a big challenge for researchers due to complex intermediates, different final products, and large space of the catalyst due to its morphology, plane, size, host surface etc. Using density functional theory (DFT) calculations, we find that alloying of Cu nanoparticles can help to enhance the selectivity toward C2 products during CO2ER with a low overpotential. By a systematic investigation of 111 planes (which prefer the C1 product in the case of bulk Cu), the alloys show the generation of C2 products via *CO-*CO dimerization (* indicates adsorbed state). It also suppresses the counter-pathway of hydrogenation of *CO to *CHO, which leads to C1 products. Further, we find that *CH2CHO is the bifurcating intermediate to distinguish between ethanol and ethylene as the final product. We have used simple graphical construction to identify the catalyst for CO2ER over HER, and vice versa. We have also defined the case of hydrogen poisoning and projected a parity plot to recognize the catalyst for C2 product evolution over the C1 product. Our study reveals that Cu-Ag and Cu-Zn catalysts selectively promote ethanol production on 111 planes. Moreover, an edge-doped 2SO2 graphene nanoribbon as the host layer further lowers the barrier and selectively promotes ethanol on Cu38- and Cu79-based alloys. This work provides new theoretical insights into designing Cu-based nanoalloy catalysts for C2 product formation on the 111 plane.

Active Sites of Cobalt Phthalocyanine in Electrocatalytic CO2 Reduction to Methanol

Many metal coordination compounds catalyze CO2 electroreduction to CO, but cobalt phthalocyanine hybridized with conductive carbon such as carbon nanotubes is currently the only one that can generate methanol. The underlying structure-reactivity correlation and reaction mechanism desperately demand elucidation. Here we report the first in situ X-ray absorption spectroscopy characterization, combined with ex situ spectroscopic and electrocatalytic measurements, to study CoPc-catalyzed CO2 reduction to methanol. Molecular dispersion of CoPc on CNT surfaces, as evidenced by the observed electronic interaction between the two, is crucial to fast electron transfer to the active sites and multi-electron CO2 reduction. CO, the key intermediate in the CO2 -to-methanol pathway, is found to be labile on the active site, which necessitates a high local concentration in the microenvironment to compete with CO2 for active sites and promote methanol production. A comparison of the electrocatalytic performance of structurally related porphyrins indicates that the bridging aza-N atoms of the Pc macrocycle are critical components of the CoPc active site that produces methanol. In situ X-ray absorption spectroscopy identifies the active site as Co(I) and supports an increasingly non-centrosymmetric Co coordination environment at negative applied potential, likely due to the formation of a Co-CO adduct during the catalysis.

Mechanistic Insights into Photochemical CO2 Reduction to CH4 by a Molecular Iron-Porphyrin Catalyst

Iron tetraphenylporphyrin complex modified with four trimethylammonium groups (Fe-p-TMA) is found to be capable of catalyzing the eight-electron eight-proton reduction of CO₂ to CH₄ photochemically in acetonitrile. In the present work, density functional theory (DFT) calculations have been performed to investigate the reaction mechanism and to rationalize the product selectivity. Our results revealed that the initial catalyst Fe-p-TMA ([Cl-Fe(III)-LR₄]⁴⁺, where L = tetraphenylporphyrin ligand with a total charge of -2, and R₄ = four trimethylammonium groups with a total charge of +4) undergoes three reduction steps, accompanied by the dissociation of the chloride ion to form [Fe(II)-L^••2-R₄]²⁺. [Fe(II)-L^••2-R₄]²⁺, bearing a Fe(II) center ferromagnetically coupled with a tetraphenylporphyrin diradical, performs a nucleophilic attack on CO₂ to produce the ¹η-CO₂ adduct [CO₂^•--Fe(II)-L^•-R₄]²⁺. Two intermolecular proton transfer steps then take place at the CO₂ moiety of [CO₂^•--Fe(II)-L^•-R₄]²⁺, resulting in the cleavage of the C-O bond and the formation of the critical intermediate [Fe(II)-CO]⁴⁺ after releasing a water molecule. Subsequently, [Fe(II)-CO]⁴⁺ accepts three electrons and one proton to generate [CHO-Fe(II)-L^•-R₄]²⁺, which finally undergoes a successive four-electron-five-proton reduction to produce methane without forming formaldehyde, methanol, or formate. Notably, the redox non-innocent tetraphenylporphyrin ligand was found to play an important role in CO₂ reduction since it could accept and transfer electron(s) during catalysis, thus keeping the ferrous ion at a relatively high oxidation state. Hydrogen evolution reaction via the formation of Fe-hydride ([Fe(II)-H]³⁺) turns out to endure a higher total barrier than the CO₂ reduction reaction, therefore providing a reasonable explanation for the origin of the product selectivity.

Aqueous Photoelectrochemical CO2 Reduction to CO and Methanol over a Silicon Photocathode Functionalized with a Cobalt Phthalocyanine Molecular Catalyst

We report a precious-metal-free molecular catalyst-based photocathode that is active for aqueous CO₂ reduction to CO and methanol. The photoelectrode is composed of cobalt phthalocyanine molecules anchored on graphene oxide which is integrated via a (3-aminopropyl)triethoxysilane linker to p-type silicon protected by a thin film of titanium dioxide. The photocathode reduces CO₂ to CO with high selectivity at potentials as mild as 0 V versus the reversible hydrogen electrode (vs RHE). Methanol production is observed at an onset potential of -0.36 V vs RHE, and reaches a peak turnover frequency of 0.18 s^-1 . To date, this is the only molecular catalyst-based photoelectrode that is active for the six-electron reduction of CO₂ to methanol. This work puts forth a strategy for interfacing molecular catalysts to p-type semiconductors and demonstrates state-of-the-art performance for photoelectrochemical CO₂ reduction to CO and methanol.