Porphyrin-silver acetylide cluster catalysts with dual active sites for the electrochemical reduction of CO₂†

Leonard Curet ^a, Dominique Foix ^a, Emilio Palomares ^bc, Laurent Billon ^a and Aurelien Viterisi *^a
aUniversite de Pau et Pays de l’Adour, E2S UPPA, CNRS, IPREM UMR 5254 Technopole Hélioparc 2 avenue du Président Pierre Angot, 64053 PAU CEDEX 09, France. E-mail: aurelien.viterisi@univ-pau.fr
^bInstitute of chemical research of Catalonia (ICIQ) Avda, Països Catalans, 16 43007 Tarragona, Spain
^cICREA. Passeig Lluís Companys, 28, E-08010 Barcelona, Spain

First published on 22nd August 2024

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The electrochemical reduction of CO₂ into feedstocks, such as permanent gases or alcohols, is a promising alternative for offsetting greenhouse gas emissions.^1–3 The transformation is generally mediated by a catalyst of metallic nature under electrochemical conditions. Since the pioneering work of Hori on CO₂ electroreduction, the properties of copper, gold or silver have been well studied.^4,5 However, catalysts with high selectivity for C₂ products are still scarce.

Molecular catalysts have seen a surge of interest in recent years. Indeed, high faradaic efficiencies for CO or methane have been described under high electrolysis currents. They benefit from a high structural versatility with respect to elemental metals and can generally be deposited from a liquid slurry or colloidal suspension directly onto carbon or PTFE-based gas diffusion layers (GDLs). However, molecular catalysts suffer from a main drawback: their catalytic selectivity is, to date, limited to C₁ products. Their chemical structure, typically a metal bound by coordinating ligands, holds the former catalytic centres further apart, impeding the formation of the necessary dimerised intermediate leading to C₂ adducts. As such, CO has remained the primary CO₂ reduction adduct from molecular catalysts in the CO₂RR. However, hybrid systems have recently emerged in which a molecular catalyst is coupled to a metallic catalyst with complementary selectivity. For example, CO-selective catalysts such as Cobalt-phthalocyanine or porphyrin have been adsorbed on the surface of copper nanoparticles^6,7 or sputtered copper layers,⁸ boosting the overall selectivity for ethylene and ethanol. Although very elegant, this strategy lacks the versatility attributes of molecular catalysts since their deposition of the copper surfaces remains somewhat challenging.

To remedy this setback, herein we propose an approach in which both the molecular and metallic counterparts are synthesised in one step from wet chemical methods. This is achieved through the controlled synthesis of metallic clusters, whose organic part is composed of catalytically active porphyrins, and the cluster's core is made from a CO₂RR-active metal (Fig. 1). The material is generally isolated as an organo-metallic discrete molecule, which can be further deposited on an electrode or GDL. Clusters have recently been proven catalytically active for the CO₂RR, with impressive selectivity and unique crystalline properties.^9–11 Silver acetylide clusters have particularly shown outstanding selectivity for CO and ease of synthesis. Copper acetylides are equally attractive in that C₂ selectivity could be expected since the metal cluster would allow the formation of dimeric intermediates.¹² A synergetic effect would be expected by combining the molecular and metallic species in a single entity. Therefore, this study is intended as a proof of concept for the further development of molecular catalyst hybrid structures with complementary catalytic properties.

Silver acetylides are formed upon adding an alkyne-containing mixture to a solution of silver salts. The extremely high selectivity to alkyne groups results from the favourable formation of a π-bond between the silver(I) atoms and the triple bond. Thus, the activation provided renders the alkynyl proton labile, which is readily abstracted by any base, however weak, leading to the formation of a silver-acetylene bond (Scheme 1). The favourable formation of metal–metal bonds dictates the formation of large clusters of silver atoms with alkynyl groups at the outer edge. Consequently, the near-perfect ordering of the silver atoms is most often transferred over long distances, leading to highly crystalline materials with discrete molecular packing. Although generally limited to hundreds of nanometers in size, large single crystals of silver acetylides have been isolated, and a plethora of X-ray crystal structures are available to date,¹³ hence the recent coining of “atomically precise” materials.⁹

Contrary to other metal–carbon bonds, however, silver acetylides have no significant nucleophilic character, conferring them high stability at neutral and basic pH. As a consequence of both above-mentioned features, silver acetylides have been found to have a perfect application in the field of CO₂ reduction.^14–17 The extremely high level of structural control and extended stability confer silver acetylides remarkable catalytic properties. We, therefore, used the silver acetylide reaction to design two different types of catalyst structures with hypothetical complementary activity by forming silver acetylides from tetra-functionalised porphyrins. The aim was to demonstrate the compatibility of a previously described silver acetylide synthesis method to the synthesis of complex architectures. Cobalt and iron porphyrin were chosen as the organic ligands for their known catalytic efficiency towards the CO₂RR.

For that matter, alkyne of tetra-functionalised porphyrins were synthesised using reported procedures. In brief, a typical Lindsey synthetic route from 4-((trimethylsilyl)ethynyl) benzaldehyde and freshly distilled pyrrole yielded the desired tetra-substituted porphyrin in respectable yield (see ESI† for details).¹⁸ The respective adduct was metalated using 10 molar equivalents of a cobalt precursor and H₂TPP derivatives in DMF at room temperature. The iron metalation proved challenging, and long reaction times in refluxing DMF with lutidine were required to obtain the Iron-metalated porphyrin in high yields (83%).^19,20 Corresponding acetylides were synthesised using an adapted version of Sheiber's original route to silver acetylides.²¹

The procedure involved, first, forming an aqueous silver ammoniacal complex by adding concentrated ammonia to a solution of silver nitrate in a mixture of water and methanol. A solution of the alkyne-functionalised porphyrin dissolved in a mixture of DMF, methanol and dichloromethane was subsequently added to the latter silver precursor. The silver acetylide formed upon stirring overnight at room temperature. Simple filtration followed by washing with copious amounts of dichloromethane, methanol and water afforded the final product in quantitative yields. The insoluble nature of the latter confirmed the poly-substitution, forming a presumable pseudo-2D material. An acetylide of an un-metalated porphyrin was synthesised as a control compound, [TaTPPAg]_x, to assess the extent to which further catalytic activity could be attributed to one or the other metallic species of the cluster. Despite the insolubility of the isolated solids, solid-state ATR FT-IR spectroscopy first confirmed the formation of the silver-carbon bond. Both the disappearance of the characteristic CC–H stretching band and the appearance of a characteristic CC stretching band confirm the formation of the desired adduct from a qualitative perspective (Fig. S7, S14 and S21, ESI†). Encouraged by the above features, we recorded powder XRD diffractograms on the tetrasubstituted porphyrins samples. Fig. 2 shows that, in agreement with our expectations, the diffractogram of all three acetylides shows unresolved peaks archetypical of polymeric 2D materials.

The large values of full-with-at-half maximum (FWHM) advocate for very short-range order corresponding to diffraction planes with d-spacing in the order of 1 nm and 0.5 nm. Additionally, XPS data allowed for further structure elucidation. The presence of emission peaks arising from chlorine, iron, nitrogen and silver atoms provided qualitative information. Further comparison of peak positions with those recorded from reference compounds (Fig. S29 and S30, ESI†) allowed for a precise assignment and quantitative analysis. Consequently, the position of the silver peak (Ag 3d_3/2 = 3680 eV), together with the presence of the carbon peak at 284.0 eV, confirms the nature of the Ag–C acetylide bonds (Fig. 3a and b).

Further details on the stoichiometry of the elemental constituents of the acetylides (Fe to Ag ratio and N to Ag ratio) allowed drawing an experimental Ag/acetylene ratio, shown to be in the range of 1:1 (Table S1, ESI†). A Comparison of the N 1s emission spectrum (Fig. 3c) with that of a reference non-metalated porphyrin pointed to a singular environment around the nitrogen atoms. This confirms the nearly exclusive presence of C–N–Fe bonds and attests to the porphyrinic origin of the Fe atoms. Finally, a weak peak with a characteristic binding energy corresponding to a Fe–O bond observed on the O 1s spectrum (530 eV) initially raised our suspicion about the potential presence of residual iron oxide (Fig. 3d).

However, a close inspection of the sample's powder revealed the presence of minute amounts of monocrystalline fragments, morphologically distinct from the bulk of the sample. A single crystal XRD structure was obtained from isolated crystals and revealed that the subproduct corresponded to a porphyrin dimer, in which two porphyrin units are linked by a bridging oxygen atom (Fig. S31, ESI†), hence the Fe–O peak observed in the XPS spectrum. The formation was readily rationalised by the fact that the labile chlorine atom bore by the iron centre is readily pulled out of solution by the halophilic silver reagent, thus favouring the formation of the dimer. This is, fortunately, only limited to a small fraction of the sample. Overall, the synthesis conditions showed to be mild enough to leave the porphyrin's metals largely unaffected.

To assess the catalytic activity of our catalysts, electrochemical CO₂RR experiments were carried out. The silver clusters were deposited on carbon-based gas diffusion layers typically used for CO₂ reduction. Hydrophobised (Sigracet 39BB) GDLs incorporating a top microporous carbon nanoparticle-based layer were utilised. The catalyst was immobilised on the GDL using a procedure reported by Robert and co-workers.²² The GDL was coated with a carbon nanoparticles-Nafion-catalyst slurry by successive sonication-deposition-evaporation sequences until the appropriate weight-amount of catalyst was deposited. Field emission SEM images of the resulting electrode and electron diffraction (EDX) (Fig. S16 and S17, ESI†) show segregation between the catalyst and the former two compounds. This was to be expected due to the insoluble nature of the catalysts, however, this was not expected to have a significant impact on the catalyst selectivity. The catalyst loading was further optimised by varying the weight-amount of catalyst with respect to the carbon nanoparticles and Nafion to maximise the catalytic characteristics, particularly the faradaic efficiency towards the major adduct.

The catalytic activity of the above GDEs was tested with a homemade 3-electrode cell in a CO₂-saturated 0.5 M potassium bicarbonate electrolyte. First, Linear sweep voltammetry (LSV) experiments carried out in identical conditions showed a clear catalytic wave with varying catalytic thresholds corresponding to overpotentials in the range of −0.40 to −0.6 V vs. RHE. Interestingly, a very significant drop of overpotential is seen in the Co and Fe-metalated porphyrin versions of the clusters with respect to the un-metalated control compound, the effect of which is attributed to the lower overpotential of Co and Fe-metalated porphyrins.²³

The selectivity of our catalyst was assessed in a series of CO₂RR carried out with 0.1 V increments from −0.55 to −1.15 V applied potentials (vs. RHE) in chronoamperometry (CA) experiments, and the produced gaseous adducts were sampled and quantified from the cell's headspace after a 15 min (see ESI† for details). Interestingly, all samples showed a rather high faradaic efficiency towards CO at somewhat low overpotentials, with FE over 70% at electrolysis potentials as low as 0.65 Volts (vs. RHE) for [CoTaTPPAg]_x (Fig. 4). Apart from the latter catalyst, most acetylides show a modest total FE under applied potentials below 0.55 V (see ESI†). The rationale lies in that a significant portion of the electric charge is consumed in a pre-activation step, forming a reduced metallic species, both from the Ag and Fe or Co centres. The selectivity of the CO₂RR remains reasonably high over a wide range of voltages, reaching electrolysis currents in the range of 5 mA cm⁻² (Fig. S28, ESI†). Both [FeTaTPPAg]_x and [CoTaTPPAg]_x show an elevated propensity to suppress the competing hydrogen evolution reaction (HER), however, HER tends to dominate at high potentials. Surprisingly, the un-metalated porphyrin showed somewhat good selectivity for CO, being only slightly lower than its metalated counterparts. This advocates for a predominant role of the silver on the catalytic properties of the acetylides, however, the slightly higher selectivity of the latter two catalysts toward CO is evidence of a co-activity between the silver and iron or cobalt centres. Finally, the stability of the clusters was assessed under prolonged electrolytic conditions, carrying out a chronoamperometry experiment at 0.75 V (vs. RHE) for a duration of 6 h (Fig. 5). The cobalt-metalated porphyrin cluster showed a virtually constant selectivity and constant electrolysis current for over 6 hours. The selectivity towards CO tends to decrease slightly over time, however, this effect is likely due to a change in hydrophobicity of the substrate. Overall, these results demonstrate the stability of the catalytic species.

To conclude, to the best of our knowledge, this study is the first example of a one-pot synthesis of catalysts composed of an inorganic core involving metal–metal bonds and a molecular macrocyclic catalyst. The methodology relies on a mild synthetic procedure compatible with organometallic complexes and yields products with high added value without requiring a purification step. Most importantly, the catalytic performance of silver acetylides synthesised from tetrasubstituted porphyrins indicate that a prior low-yielding porphyrin-de-symmetrisation step is not required. The disordered nature of the tetrasubstituted acetylides was not shown to be linked with any significant decrease in catalytic activity. Additionally, the highly insoluble nature of the materials synthesised from the alkyne-tetrasubstituted porphyrins constitutes a further positive point for the immobilisation of such catalysts. The methodology described herein demonstrates that such materials can be synthesised in large quantities from less synthetically demanding precursors. Although only intended as a proof of concept, the metalated porphyrins show a subtle catalytic additivity feature with respect to the un-metalated control acetylide, which augurs positively for future porphyrin-functionalised copper acetylides, which are currently being investigated in our laboratory.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data (AgPh)_n has been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition number 2374521.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, materials characterisation, X-Ray crystal structure and electrochemical characterisation. CCDC 2374521. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc03836c

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