The effect of the nature of supports on the selective reduction of CO₂ to CO catalysed by a supported single-site heterobimetallic iron–potassium complex

Abstract

The utilization of CO₂ for the production of value-added chemicals has attracted significant interest. Among various strategies, the selective hydrogenation to liquid hydrocarbons via a two-step process, first the reduction of CO₂ to CO through the reverse water–gas shift (RWGS) reaction, followed by CO conversion, is particularly promising. In this work, [{(THF)₂KFe(OtBu)₃}₂] is supported on various oxides in order to investigate the influence of the support's properties (neutral, basic, acidic, and redox) on the catalytic performance in the RWGS reaction. A series of catalysts with 1 wt% Fe are synthesized and characterized. Remarkably, catalysts supported on neutral and basic oxides maintain the close Fe–K proximity and exhibit high catalytic activity (conversion around 20%) and full selectivity toward CO (100%). Among these, the FeK/ZrO_2-250 catalyst appears to be stable after 40 h on stream, which is supported by XAFS and TEM analyses of the spent catalyst. K⁺ serves as an electronic promoter, as well as a structural stabilizer of the active Fe site, preventing agglomeration. In contrast, the use of acidic or redox-active supports results in loss of the activity or the selectivity in CO. The formation of methane is attributed to the dissociation of FeK species into Fe nanoparticles and the incorporation of K into the CeO₂ lattice or exchange with acid sites of ZSM-5, Nb₂O₅ or SiO₂–Al₂O₃, thereby disrupting the critical Fe–K synergy. These findings underscore the pivotal role of support selection in preserving the structural and electronic integrity of Fe–K active sites. Supports that inhibit sintering and maintain the FeK single-site configuration are essential for achieving high activity, full CO selectivity, and long-term stability in the RWGS reaction.

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1. Introduction

Recently, the catalytic reduction of CO₂ to CO via the reverse water gas shift (RWGS) reaction has gained attention from industry and academia due to its increasing importance in the CO₂ conversion technology and the interest of using CO as feedstock in the Fischer–Tropsch process, a mature technology for converting this intermediate into valuable chemicals and fuels.^1–7 However, due to the high thermodynamic stability of CO₂, the RWGS reaction is an endothermic process (ΔH° = 42.1 kJ mol⁻¹) that requires higher temperatures and high H₂/CO₂ ratios to be optimal.^8–11 Although pressure does not influence its equilibrium, the reaction faces competition from undesired methanation and the exothermic Sabatier reaction, which are favored at lower temperatures and pressures (eqn (1)–(3)).^12–15 In general, for temperatures below 600 °C, methane formation dominates, whereas higher temperatures (>600 °C) enhance CO selectivity.^11,16 In order to reduce energy and capital costs, the reaction temperature shall be kept as low as possible and simultaneously optimize the CO production with minimum CH₄ as the undesired byproduct. Therefore, understanding the mechanisms, the structure of the active species and kinetics of the RWGS reaction is crucial for the catalyst design for CO production. These strategies include varying the support, tuning metal–support interactions, adding reducible transition metal oxide promoters, forming bimetallic alloys, adding alkali metals, and enveloping metal particles.^17–20 Selecting an appropriate support is important to tune the activity and selectivity of this reaction. Various oxides such as SiO₂, ZrO₂, TiO₂, mullite (aluminum–silicate), Al₂O₃ and CeO₂ have been reported as catalyst supports in the RWGS reaction.^11,16,21,22 The authors described that the redox and acid–base properties of the supports can greatly influence CO₂ adsorption/activation, as well as the nature of the reaction intermediates and the mechanisms (redox or associative). SiO₂ and Al₂O₃ are neutral and irreducible supports,¹⁷ and many studies indicate that these supports indirectly participate in the RWGS reaction by enhancing metal dispersion and CO₂ adsorption. For example, when Pt single site catalysts are supported on silica, it is proposed that CO₂ adsorbs at the interface between Pt and SiO₂via hydrogen bonding with silanol groups.¹⁶ This CO₂ adsorption strength is enhanced by 0.21 eV compared to unsupported Pt nanoparticles, thus allowing the formation of formate species, an intermediate for CO production.¹⁶ Similarly, the use of Al₂O₃ as an irreducible and amphoteric oxide for the dispersion of Pd single site catalysts provides high selectivity to CO compared to Pd supported on inert multiwall carbon nanotubes (MWCNTs).²³ In order to explain this effect of the support, Bobadilla et al.²⁴ proposed that CO₂ reacts with the basic hydroxyl groups of Al₂O₃ to form bicarbonate species which, via hydrogen spillover from Pd, lead to the formation of aluminum formate intermediates that further decompose into CO.

CO₂ + H₂ ⇄ CO + H₂O_(g) ΔH^θ = +41.3 kJ mol⁻¹ (1)

CO₂ + 4H₂ ⇄ CH₄ + 2H₂O_(g) ΔH^θ = −164.7 kJ mol⁻¹ (2)

CO + 3H₂ ⇄ CH₄ + H₂O_(g) ΔH^θ = −206.2 kJ mol⁻¹ (3)

The effect of an acidic support such as a zeolite (H-ZSM-5) is also reported for the conversion of CO₂ to CO.^25–27 For example, Wang et al.²⁷ compared CO₂ hydrogenation on Rh supported on zeolite such as H-ZSM-5 and pure silica MFI, revealing that neutral silica MFI modified by rhodium exhibits high CO selectivity with high CO₂ conversions. However, Rh supported on acidic H-ZSM-5 shows moderate selectivity to CO with the formation of methane under the same conditions. Strong correlations are observed between the nanoporous environment and catalytic selectivity, indicating that the neutral silica MFI minimizes hydrogen spillover and favors the fast desorption of CO to limit total hydrogenation to methane.²⁷ Furthermore, the use of basic support such as zirconia is reported and shows that the selectivity to CO depends on the nature of the metal and metal loading.^28–30 Based on literature studies, the nature and properties of the supports thus have a strong effect on the catalytic performance.

Redox supports such as TiO₂, CeO₂–ZrO₂ and CeO₂ are known to generate oxygen vacancies, particularly in the presence of an immobilized metal during the activation of H₂ and CO₂.³¹ In the case of ceria-based catalysts, CO₂ hydrogenation is generally attributed to oxygen storage capacity, redox properties, and the ability to enhance the reaction rates and selectivity.³² CeO₂ is reported to exhibit unique synergistic effects when doped with other metals that improve their catalytic activity.³³ Using DFT calculations, ceria supported copper and iron catalysts^34,35 are both reported and exhibit moderate activity in the RWGS reaction.³⁶ Conversely, ceria supported Ni and Co favor CO₂ methanation.³⁷ From these studies, the high activity of metal doped CeO₂ is attributed to the increase of the reducibility and consequently the number of oxygen vacancies.³⁸ However, the selectivity in CO is favored when the dopants are well dispersed on the ceria surface. On the other hand, the presence of metal nanoparticles on the ceria surface increases the formation of undesired products such as methane.^39,40 Recently, the inherent relationships between the performance of transition metal (Fe, Co, Ni and Cu) doped CeO₂ catalysts, the concentration of transition metals doped onto CeO₂ surfaces, CO₂ binding strength, and the oxygen vacancy formation energy have been explored for the RWGS reaction. At low loading, Fe/CeO₂ showed low conversion (10%) and high selectivity in CO (100%) at high temperature (600 °C). However, at temperatures lower than 500 °C, no activity in the RWGS reaction was reported.³⁹ In addition to the effect of support, the presence of alkali metal ions is reported to enhance the metal–support interactions for transition metal-based catalysts, leading to increased stability and thereby increased activity and CO selectivity.^36,41,42 The incorporation of potassium into Fe/Al₂O₃ with a K/Fe ratio of 1 results in a significant increase in CO formation rates compared to unpromoted Fe/Al₂O₃.⁴³ The increased basicity using K improves the adsorption of CO₂ by forming Lewis acid–base pairs that enhance CO₂ binding by chemisorption on the surface through electron transfer.³⁶ Thus, the potassium promoter activates a secondary pathway for CO formation, which may be the so-called associative pathway.⁴³ Based on this finding, we recently reported a well-defined supported single-site FeK RWGS catalyst with low Fe loading that essentially provided 100% selectivity to CO at 400 °C. The heterobimetallic FeK complex, [{(THF)₂KFe(OtBu)₃}]₂, containing K as the metal cation, is grafted by surface organometallic chemistry (SOMC)⁴⁴ onto partially dehydroxylated Al₂O₃ at 500 °C and characterized by ICP, IR, XPS, EDX, XAFS, and EPR. The material consists of an isolated anionic Fe(II) site coordinated by two tert-butoxide ligands and one anionic surface oxygen, associated with K⁺. The postulated mechanism (Scheme 1) proceeds through H₂ activation to form an iron hydride, which evolves to an iron hydroxycarbonyl intermediate upon insertion of CO₂ that by further decomposition leads to CO.⁴⁵ Interestingly, the Na and Li counterparts exhibit substantially lower activity in the RWGS reaction. Taking into account that the potassium in the proximity of Fe has a significant effect on the activity and selectivity during the adsorption and the activation of CO₂, the modification of the nature and the properties (acidity, basicity and redox) of the supports may also affect the activity and the selectivity during CO₂ conversion to CO.

Scheme 1 Proposed mechanism for selective conversion of CO₂ to CO on supported anionic heterobimetallic Fe and K complexes independent of the nature of support (neutral/basic/acidic or redox).

Here, this approach has been extended to study the effect of the supports (neutral, basic, acidic and redox) on activity and selectivity in the RWGS reaction. This methodology allows us to study the effect of the combination of the two parameters (K and support properties) on the activity and selectivity to CO, as well as the validation of the proposed mechanism on supported anionic iron single active species. To ensure the aforementioned criteria, neutral (SiO₂), basic (MgO–Al₂O₃ (Pural) and ZrO₂), acidic (Nb₂O₅, SiO₂–Al₂O₃ and μ-H-ZSM-5) and redox (CeO₂ and CeZrO_x) supports have been selected to immobilize the isolated iron species. The resulting materials have been characterized using ICP, DRIFT, HRTEM, BET, EDX, EPR, XRD and XAFS for the catalyst based on zirconia before and after catalysis. The catalysts have been tested for the reduction of CO₂ to CO.

2. Experimental methods

2.1. Chemicals and materials

All experiments have been carried out under a controlled atmosphere using Schlenk and glovebox techniques for organometallic synthesis. For the synthesis and treatment of supported species, reactions have been carried out using high-vacuum lines (ca. 1 mPa) and gloveboxes. Pentane (99%) and THF (99.9%) were purchased from MilliporeSigma, distilled from NaK and degassed using freeze–pump–thaw cycles. FeBr₂ (98%) and tBuOK (98%) were purchased from MilliporeSigma and used as received.

2.2. Characterization of materials

All characterization studies have been performed under an inert atmosphere, except for EDX/STEM/HRTEM for which sample preparation is performed in air. Elemental analysis has been carried out at the Mikroanalytisches Labor Pascher, Remagen, Germany, and CREALINS, Villeurbanne, France, using an iCAP 6500 Duo inductively coupled plasma atomic emission spectrometer (Thermo Scientific). All samples sent for elemental analysis have been sealed under high vacuum (10⁻⁵ mbar). Infrared spectra have been recorded in diffuse reflectance mode on a Nicolet 6700 FT-IR spectrometer, in an air-tight cell with CaF₂ windows under an atmosphere of Ar at room temperature. Each spectrum is composed for 64 scans recorded at a resolution of 4 cm⁻¹. A Micromeritics ASAP 2020 (surface area and porosity analyzer) has been used for the determination of textural properties (e.g., Brunauer–Emmett–Teller, BET surface area). Off-line gas chromatographic analyses have been performed on an HP 5890 series II GC, equipped with an HP5 GC column and an FID detector. High resolution transmission electron microcopy (HRTEM) and energy dispersive X-ray (EDX) spectroscopy have been performed at the Centre Technologique des Microstructures, Université Lyon 1. Prior to EDX/STEM/HRTEM analysis (JEOL 2100F, 200 kV), samples are prepared by placing a suspension spot containing the test material on an ultrathin Ni or Cu grid. Powder X-ray diffraction (XRD) has been recorded at the Institut des Sciences Analytiques Lyon 1. Electron paramagnetic resonance (EPR) spectroscopy has been carried out with a Bruker spectrometer Elexsys E500 using X Band (9.4 GHz) radiation at T = 110–120 K, in the Laboratoire de Chimie, ENS Lyon. Samples for EPR have been prepared in air-tight quartz tubes loaded inside a glove box. For quantitative studies of the paramagnetic phase, double integration of the EPR signal is performed and compared to that of a reference composed of a known amount of vanadyl(IV) sulfate. XAS spectra have been acquired at ESRF, Grenoble, France, using the BM23 beamline at the iron K-edge in the transmission mode between 7.0 and 8.2 keV. Four scans are recorded at room temperature for each sample. Each data set is collected simultaneously with a Fe metal foil and is later aligned according to that reference (first inflection point set at 7112.0 eV). The samples have been packed in an argon-filled glovebox within double air-tight sample holders. The data analyses have been carried out using the program “Athena”⁴⁶ and the EXAFS fitting program “RoundMidnight”, from the “MAX” package,⁴⁷ using spherical waves. For each neighboring atom or path i, the refined parameters are N_i, the number of neighbors; R_i (Å), the distance of the Fe center to that neighbor; σ_i² (Ų), the Debye–Waller parameter; and ΔE₀ (eV) designing the global energy shift. The program FEFF8 has been used to calculate theoretical files for phases and amplitudes based on model clusters of atoms.⁴⁸

2.3. Catalyst preparation

2.3.1. The molecular iron complexes. [{(THF)₂KFe(OtBu)₃}₂] has been synthesized as reported in a recent work.⁴⁵

2.3.2. Preparation of the supports. All supports (5 g) have been calcined at 500 °C (heating rate: 4 °C min⁻¹) under continuous flow of dry air for 6 hours. Then, 2 g each of SiO₂, SiO₂–Al₂O₃ and Al₂O₃–MgO has been dehydroxylated for 12 h at 700 °C, 500 °C and 500 °C, respectively, which involves a heat treatment under high vacuum. Nb₂O₅, μ-ZSM-5, CeO₂, CeZrO_x and ZrO₂ have been hydrated (exposed to O₂-free water vapor) at 100 °C prior to the dehydroxylation process at 250 °C (12 h) in a glass reactor (500 mL).

2.3.3. Grafting of molecular complexes onto supports. As described most recently in the case of catalysts, FeK/Al₂O_3-500,⁴⁵ a solution of [{(THF)₂KFe(OtBu)₃}₂] (0.25 g) in n-pentane (15 mL) has been added to 3 g of ZrO_2-250 to produce a solid containing ca. 1 wt% Fe. The brownish suspension has been stirred for 12 h at room temperature and then filtered through a glass frit. The solid has been dried under high vacuum (10⁻⁵ mbar), while the filtrate has been analyzed by GC. The same procedure is used to graft the complex [{(THF)₂KFe(OtBu)₃}₂] onto SiO_2-700, Al₂O₃–MgO_-500, Nb₂O_5-250, μ-ZSM-5₂₅₀, SiO₂–Al₂O_3-500, CeO_2-250 and CeZrO_x-250 (all ca. 1 wt% Fe).

2.4. Catalytic testing

Catalysts have been evaluated in a ½″ stainless-steel tubular continuous flow reactor whose outlet is connected to an online GC (Agilent Technologies 7890A). The dead volume is minimized using stainless steel inserts. The GC is equipped with two columns in series: a carbon PLOT column to separate CO₂, CO, and CH₄ and a PLOT Q column to separate higher hydrocarbons. The gas effluent from the reactor passes through both columns before reaching the detectors. Separated products are detected using a thermal conductivity detector (TCD) and a flame ionization detector (FID), connected in series. The FID is equipped with a methanizer (Jetanizer™). The reactor is charged (250 mg catalyst) in the glovebox and equipped with a 4-way valve, allowing extensive purging of the gas lines with the reactant gas once connected to the reactor host (PID, Micrometrics). The gas flow is controlled using Bronkhorst mass flow controllers. Catalytic tests are performed using a certified gas mixture with a H₂/CO₂ ratio of 3 (Mélange Crystal, Air Liquide), at a constant flow rate of 3 mL min⁻¹ (measured at room temperature). Prior to starting the reaction, the catalyst has been activated in situ in flowing Ar (Alphagaz 1, Air Liquide) for 2 h at 300 °C. CO₂ conversion and product selectivity are calculated using eqn (4)–(6):

3. Results and discussion

3.1. Supported [{(THF)₂KFe(OtBu)₃}₂] single site on ZrO_2-250 through surface organometallic chemistry

3.1.1. Preparation and characterization of ZrO_2-250. Commercial ZrO₂ is obtained from MilliporeSigma as white powder with a surface area of 80 m² g⁻¹ determined by BET using N₂ adsorption at 77 K (Table S1). After calcination at 500 °C for 24 hours and hydration at 100 °C with water vapor, ZrO_2-250 is obtained by dehydroxylation at 250 °C for 12 hours. The surface area and pore volume of ZrO_2-250 support by N₂ adsorption at 77 K are found to be 72 m² g⁻¹ and 0.10 cm³ g⁻¹, respectively, and these values are similar to the one reported in the literature.^49–51 Furthermore, the adsorption–desorption isotherm plot (Fig. S1 and Table S2) of the zirconia dehydroxylated at 250 °C follows the type IV adsorption–desorption isotherm which is similar to those already reported in the literature.^51–53 X-ray diffraction pattern of ZrO_2-250 is presented in Fig. S2 and shows diffraction peaks from 18 to 65° (2θ scale) corresponding to the presence of mainly monoclinic (baddeleyite), mixed with the tetragonal phase based on the match search with the data of the Joint Committee on Power Diffraction Standards (JCPDS). This is similar to the diffraction patterns of ZrO₂ reported in the literature.^54–56 Peaks at 2θ = 30.32°, 35.20°, 42.84°, 50.56°, 60.04°, 63.00 and 74.52° correspond to the XRD pattern of the tetragonal phase, and peaks at 2θ = 24.5°, 28.20°, 31.5°, 34.56°, 42.10°, 46.5°, 50.50, and 66.04° correspond to the baddeleyite phase.^57,58 The surface properties of the commercial ZrO₂ remain the same after thermal treatment. The IR spectrum of ZrO_2-250 recorded in diffuse reflectance mode (DRIFT) shows hydroxyl groups attributed to a mixture of tetragonal and monoclinic phases. In fact, different zirconia polymorphs possess various surface hydroxyl groups and acid–base properties.⁵⁹ Bi-bridged hydroxyl only exists on tetragonal zirconia and amorphous zirconia shows obvious H-band hydroxyl, while two types of tri-bridged hydroxyl appear on monoclinic zirconia.^59,60 These differences affect surface acidity and basicity: amorphous zirconia has strong Lewis acid sites and monoclinic zirconia has strong Brønsted acid sites, but for tetragonal zirconia, the basicity is predominant.^59,61,62 The DRIFT spectrum of ZrO_2-250 shows three main bands at 3774, 3738, and 3675 cm⁻¹ assigned to the OH stretches of terminal, 2-fold coordinated (bi-bridged), and 3-fold coordinated (tri-bridged) hydroxyl groups, respectively (Fig. 1a).^62–65 This spectrum is similar in situ DRIFT of ZrO₂ heated at different temperatures reported in the literature.^65,66 The current spectrum is also in good agreement with the theoretical calculations in which terminal hydroxyls are found to vibrate between 3822 and 3743 cm⁻¹, bi-bridged hydroxyls in the range of 3755–3568 cm⁻¹, and tri-bridged hydroxyls between 3647 and 3498 cm⁻¹.^65–67 To achieve the grafting and functionalization of surface hydroxides under optimum conditions, their quantification is required. Among the different quantification methods, a chemical titration using Al(iBu)₃, a surface reaction with a highly reactive organometallic complex, has been shown to be reliable.^68,69 This complex quantitatively reacts in pentane with surface hydroxyl groups of ZrO_2-500 by releasing one equivalent of isobutane per OH. The quantification of isobutane by GC shows that ZrO_2-250 contains 0.25 mmol OH g⁻¹ that corresponds to 2.1 OH per nm².

Fig. 1 DRIFT spectra of (a) ZrO_2-250 and (b) ZrO_2-250 after reaction with [{(THF)₂KFe(OtBu)₃}₂], FeK/ZrO_2-250.

3.1.2. Preparation and characterization of the catalyst, FeK/ZrO_2-250. The supported catalyst is prepared by grafting [{(THF)₂KFe(OtBu)₃}₂] (Fe/OH = 0.7, 1 wt Fe%) from a pentane solution onto zirconia partially dehydroxylated at 250 °C (ZrO_2-250). The reaction is proceeded at room temperature for 12 h. The solid is washed with pentane and dried under vacuum, affording a yellowish material. Upon grafting, tBuOH and THF are released and confirmed by GC. The resulting material is characterized by DRIFT, elemental analysis, STEM/EDX, BET, EPR, XRD and XAFS. The DRIFT spectrum reveals that the surface OH groups of ZrO_2-250 (ν(O–H): 3795–3660 cm⁻¹) are largely consumed by the grafting reaction (Fig. 1b). Simultaneously, bands corresponding to the organic ligands appear at 3000–2800 cm⁻¹ (ν(C–H)), at 1466 and 1455 cm⁻¹ (δ(CH_x)) and at 1380 and 1356 cm⁻¹ (δ(CH₃)). The FeK/ZrO_2-250 material is further characterized for the determination of textural properties using N₂ adsorption at 77 K. The isotherm plot in Fig. S1 and Table S2 show a decrease in both the pore volume and the surface area (0.07 cm³ g⁻¹ and 55.26 m² g⁻¹, respectively) compared to those of ZrO_2-250 (0.11 cm³ g⁻¹ and 71.90 m² g⁻¹). The decrease in pore volume, pore size and surface area after grafting of the complex onto ZrO_2-250 is as expected and previously reported.^70–77 The adsorption–desorption plot (type IV) remains the same after grafting of the complex onto ZrO_2-250. The X-ray diffraction pattern of FeK/ZrO_2-250 (Fig. S2) shows characteristic phases corresponding to monoclinic and tetragonal zirconia. No additional crystalline phases are observed after introducing 1 wt% iron, such as iron oxides or iron particles (iron oxides XRD main peaks, 2θ, Cu λ_Kα: FeO at 36.1, 41.9 and 60.8°; Fe₃O₄ at 30.1, 35.5, 57.1 and 62.7°; Fe₂O₃ at 24.1, 33.1, 35.6, 49.5, 54.0, 62.4 and 64.0°). There is no visible difference between the diffraction patterns of the ZrO_2-250 support (Fig. S2a) and the grafted material, FeK/ZrO_2-250 (Fig. S2b). This result is similar to what is observed in the case of several metal supported catalysts with low metal loading.^78–81 The result indicates that the anionic iron complex is highly dispersed on the surface of ZrO_2-250 in agreement with HRTEM. This reveals that with very low average density of Fe on the surface of ZrO_2-250 (1.6 Fe per nm²), the formation of iron clusters during the grafting at low temperature (25 °C) is limited.

Elemental analysis of the resulting material, FeK/ZrO_2-250, reveals Fe, K and C contents of 1.16 wt%, 0.86 wt% and 2.98 wt%, respectively, representing a Fe/K ratio of 1. The amount of grafted Fe, 0.208 mmol g⁻¹ ZrO_2-250, represents a grafting stoichiometry (mol Fe per mol OH) of 0.75 in agreement with the DRIFT spectrum of FeK/ZrO_2-250 where most OH groups are consumed. Furthermore, the C/Fe ratio of 11.96 is consistent with the loss of one HOtBu ligand per Fe and retention of one THF solvating K⁺ (expected C/Fe: 12). Based on mass balance analysis, the nature of the support appears to have no effect on the reactivity of the FeK precursor and the resulting structure on the ZrO_2-250 surface, FeK/ZrO_2-250, since a similar surface complex is also obtained on Al₂O_3-500, FeK/Al₂O_3-500.⁴⁵ The Fe/K ratio of 1 is also confirmed by EDX analysis. Moreover, the EDX mapping and spectrum (Fig. S3) indicate that Fe and K are uniformly dispersed on the surface. No agglomerated phases of Fe or K are observed by HRTEM (Fig. S4). Importantly, the oxidation state in FeK/ZrO_2-250 is predominantly Fe(II), based on the EPR analysis (Fig. S5). In fact, EPR spectroscopy of FeK/ZrO_2-250 shows evidence of only a small amount of Fe(III) (<0.02%). This small amount is probably due to traces of Fe(III) in the molecular complex [{(THF)₂KFe(OtBu)₃}₂].

The structure of FeK/ZrO_2-250 is further studied by X-ray absorption fine structure (XAFS) spectroscopy at the Fe K-edge. The XANES of FeK/ZrO_2-250 (Fig. S6) shows a pre-edge at +1.2 (±0.2) eV from the Fe(0) edge with a maximum normalized intensity of 0.10. This suggests that the Fe sites in the supported complex are in a distorted tetrahedral coordination with an iron oxidation state of +2,^82,83 as also revealed by EPR spectroscopy (Fig. S5). The EXAFS curve fit for FeK/ZrO_2-250 is displayed in Fig. 2, with the resulting parameters in Table 1 (left part, fresh sample). The results are consistent with a Fe center in a distorted tetrahedral geometry, with ca. three oxygen atoms at 2.00(2) Å and one more at 2.35(3) Å. The shorter distance is in the range of Fe–OR bond lengths observed for the [{(THF)₂KFe(OtBu)₃}₂] molecular complex by XRD (average: 1.995 ± 0.09 Å),⁴⁵ with the EXAFS distance resolution being ca. 0.13 Å (π/2k_max). The longer distance is attributed to a surface oxygen either from a surface hydroxyl group (see Scheme 1), or bridged between two surface Zr atoms. The fit could be further improved by including additional shells of back-scatterers, in particular, two C atoms at 3.04(4) Å, one K atom at 2.87(4) Å and ca. one Zr atom at 3.55(4) Å. However, there is no evidence for a well-defined Fe–Fe path (expected at ca. 3.3 Å in a dimer structure), despite attempts to include it in the model. Based on this fit, a proposed monomeric structure for zirconia-supported [{(THF)₂KFe(OtBu)₃}₂] is displayed in Scheme 2.

Fig. 2 k ²-Weighted Fe K-edge EXAFS of FeK/ZrO_2-250. (a) k-space; (b) R-space (FT modulus and imaginary part). Solid red lines: experimental data; dashed blue lines: curve fit obtained using spherical wave theory.

Table 1 EXAFS curve fit parameters for FeK/ZrO_2-250 before and after the catalytic reaction^a

Path	Fresh sample			After the catalytic reaction
	N	R (Å)	σ ^2 (Å^2)	N	R (Å)	σ ^2 (Å^2)
a Errors generated using the EXAFS fitting program “RoundMidnight” are indicated in parentheses. Global fit parameter: S0^2 = 0.93. Fresh sample: 1.7 ≤ k ≤ 12.2 Å^−1; 0.3 ≤ R ≤ 3.6 Å. Used sample: 1.7 ≤ k ≤ 12.1 Å^−1; 0.2 ≤ R ≤ 3.4 Å. Fit residual: ρ; quality factor: (Δχ)^2/ν. b Short. c Long. d Shell constrained to a parameter above.
Fe–O1^b	2.7(5)	2.00(2)	0.0022(9)	2.9(5)	2.04(2)	0.0027(12)
Fe–O2^c	1.1(3)	2.35(3)	0.0022^d	0.9(4)	2.34(3)	0.0027^d
Fe–K	1.0	2.87(4)	0.0057(21)	1.0	2.90(4)	0.0084(20)
Fe–C/O	2.0	3.04(4)	0.0022(12)	1.9(6)	3.07(4)	0.0028(15)
Fe–ZrS	0.9(3)	3.55(4)	0.0057^d	1.8(7)	3.39(4)	0.0084^d
	ΔE0 = 6.4 ± 1.0 eV; ρ = 8.2%			ΔE0 = 3.5 ± 0.9 eV; ρ = 6.4%
	(Δχ)^2/ν = 2.47 (ν = 12/24)			(Δχ)^2/ν = 2.38 (ν = 10/23)

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Scheme 2 Proposed monomeric structure of FeK/ZrO_2-250, prepared by grafting dimeric [{(THF)₂KFe(OtBu)₃}₂], 1, onto ZrO_2-250. The structure is consistent with the XAFS analysis (the K coordination is proposed analogous to that of molecular complex 1).

Overall, the evidence suggests that, in accordance with elemental analysis, the dimeric complex, [{(THF)₂KFe(OtBu)₃}₂], reacts readily with OH groups from zirconia by the protonolysis reaction, affording isolated, bipodal [(THF)K(Zr_sO)Fe(OtBu)₂] species (Scheme 3). In this species, Fe is close to a K⁺ counter-cation, and with a labile proton, that may be located on a coordinated oxygen from the zirconia surface, as already reported on alumina.⁴⁵

Scheme 3 Reaction of the heterobimetallic [{(THF)₂KFe(OtBu)₃}₂] complex with ZrO_2-250 to produce supported isolated single site [(THF)K(Zr_sO)Fe(OtBu)₂] species.

3.1.3. Catalytic activity in CO₂ hydrogenation over FeK/ZrO_2-250. The RWGS activity and CO selectivity of the catalyst, FeK/ZrO_2-250, are evaluated in a continuous flow reactor at 30 bar and 400 °C. The catalyst provides high CO₂ conversion (22%, well below the equilibrium value of ca. 45% to ensure meaningful kinetic measurements) and selectivity towards CO (100%). This catalyst thus exhibits similar catalytic performance in terms of CO₂ conversion (22.5%) and CO selectivity (100%) than its FeK/Al₂O_3-500 counterpart recently reported (Fig. 3b).⁴⁵

Fig. 3 CO₂ conversion (open circles) and CO selectivity (filled circles) in the RWGS reaction catalyzed by (a) FeK/ZrO_2-250 and (b) FeK/Al₂O_3-500 (feed composition: CO₂/H₂ = 1/3, volumetric flow rate = 3 mL min⁻¹, 400 °C, 30 bar).

No significant deactivation is observed over the course of 40 h (Fig. 3a). This high stability is likely due to the presence of well-dispersed isolated single sites, compared with a reported catalyst with a higher Fe loading that evolves to a material with a mixture of single sites and nanoparticles.⁸⁴ The presence of K⁺ in proximity to Fe appears to stabilize Fe(II) and provides a favorable site for CO₂ adsorption. The EXAFS spectrum at the Fe K-edge of FeK/ZrO_2-250 after 40 h on stream is presented in Fig. 4, with the fit resulting parameters shown in Table 1 (right part, after the catalytic reaction). The coordination sphere of iron in sample FeK/ZrO_2-250 after the catalytic reaction is very similar to that in the fresh sample with iron single sites firmly grafted to the surface by two Fe–OZr_s bonds and in a pseudo-tetrahedral environment (ca. three oxygen atoms at 2.04(2) Å and one at 2.34(3) Å), with a close K atom (Scheme 4). This result suggests that no sintering or oxide phase occurred during the catalytic process.

Fig. 4 k ²-Weighted Fe K-edge EXAFS of FeK/ZrO_2-250 after the catalytic reaction. (a) k-space; (b) R-space (FT modulus and imaginary part). Solid red lines: experimental data; dashed blue lines: curve fit obtained using spherical wave theory.

Scheme 4 Proposed monomeric structure for the iron site after the catalytic reaction, consistent with the XAFS analysis.

In addition, HRTEM of FeK/ZrO_2-250 performed after catalyst testing (Fig. S7) supports the XAFS result showing that the active single site is robust in a range of reaction conditions. The presence of K⁺ in proximity to Fe seems to stabilize Fe(II) and to provide a favorable site for CO₂ adsorption, leading only to CO formation.

3.2. Preparation and characterization of catalysts, [{(THF)₂KFe(OtBu)₃}₂], supported on different oxide supports

The current approach is extended to other oxide supports in order to study the effect of the acidity and redox properties on the catalytic performance of single site FeK catalysts. All the supports used in this study are pretreated at different temperatures to afford the given surface OH group concentration required for the grafting reaction of [{(THF)₂KFe(OtBu)₃}₂]. Table S1 shows the treatment conditions for each support. The supports are calcined in air at 500 °C followed by a dehydroxylation treatment and are denoted as SiO_2-700, Al₂O₃–MgO_-500, Nb₂O_5-250, μ-ZSM-5₂₅₀, SiO₂–Al₂O_3-500, CeO_2-250 and CeZrO_x-250 based on the dehydroxylation temperature used. The supported catalysts are all prepared by grafting [{(THF)₂KFe(OtBu)₃}₂] in pentane solution for 12 h with the same loading (1 wt% Fe).^85,86 The resulting materials have been washed with pentane and dried in a high vacuum (10⁻⁵ mbar), affording yellowish powder (dark yellow for CeO_2-250), as reported in the case of [{(THF)₂KFe(OtBu)₃}₂] supported alumina.⁴⁵ Upon grafting, tBuOH and THF are released as confirmed by GC analysis. The resulting supported materials, FeK/SiO_2-700, FeK/Al₂O₃–MgO_-500, FeK/Nb₂O_5-250, FeK/μ-ZSM-5_-250, FeK/SiO₂–Al₂O_3-500, FeK/CeO_2-250 and FeK/CeZrO_x-250, have been characterized by BET (Table S2), elemental analysis (Table S3), DRIFT, XRD, STEM/EDX, and EPR (see the SI, Fig. S8–S18). The results confirm the successful grafting of the heterobimetallic [{(THF)₂KFe(OtBu)₃}₂] complex via protonolysis of Fe–OtBu bonds by surface OH groups, as revealed by DRIFT and mass balance analysis. This suggests that well dispersed Fe(II) mononuclear anionic surface species have been obtained on various supports with a similar structure to what was observed in the case of Al₂O_3-500⁴⁵ and ZrO_2-250 (Scheme 3), with only a difference in the second neighbor of Fe (Si, Al, Zr or Ce).

3.3. Catalytic activities of FeK on various supports in the RWGS reaction

The catalytic activities and CO selectivity of all [{(THF)₂KFe(OtBu)₃}₂] supported materials are evaluated using a continuous-flow reactor at 30 bar and 400 °C. During each catalytic test, 250 mg of catalyst with a same 1 wt% Fe loading is introduced into the flow reactor. Prior to the reaction, the catalysts have been treated under an argon atmosphere at 300 °C for 120 min. The flow rate of the CO₂ and H₂ mixture (1 : 3) is set to 3 mL min⁻¹, and the temperature and pressure are ramped to the desired conditions (400 °C and 30 bar). Table 2 summarizes and compares the catalytic performances after 20 h on stream for all catalysts during CO₂ hydrogenation. The catalytic performances as a function of time on stream are presented in Fig. S19 and S20. No significant deactivation is observed for most catalysts (FeK/SiO_2-700, FeK/Al₂O_3-500, FeK/ZrO_2-250, FeK/Al₂O₃–MgO_-500 and FeK/Nb₂O_5-250) over the course of 40 h (Fig. S19 and S20). The high stability of these catalysts is likely due to the presence of well-dispersed isolated single sites and the presence of K⁺ in proximity to Fe which stabilize Fe(II) and provide a favorable site for CO₂ adsorption.^43,45,84 However, the catalyst FeK/CeO_2-250 showed an evolution of the stability as a function of time on stream.

Table 2 Catalytic performance of the different metal oxide supported materials during CO₂ hydrogenation under identical conditions

Catalyst		Conversion (%)	Selectivity (%)
			CO	CH4	C2^+
Feed composition: CO2/H2: 1/3, flow = 3 mL min^−1, 400 °C, 30 bars, amount = 250 mg.
Neutral supports	FeK/Al2O3-500 ^45	22.5	100
	FeK/SiO2-700	18	100	—	—
Basic supports	FeK/Al2O3–MgO-500	17	100	—	—
	FeK/ZrO2-250	22	100	—	—
Acidic supports	FeK/SiO2–Al2O3-50000	7.8	97	3	—
	FeK/μ-ZSM-5-250	10	90	10	—
	FeK/Nb2O5-250	14	100	—	—
Redox supports	FeK/CeO2-250	34	100–40	0–55	0–10
	FeK/CeZrOx-250	26	100		—

---

For acidic supports, the catalysts FeK/SiO₂–Al₂O_3-500, FeK/μ-ZSM-5_-250 and FeK/Nb₂O_5-250 show a lower CO₂ conversion compared to other supports (Table 2). The activity of these catalysts decreases when the acidic strength of the supports increases (μ-ZSM-5_-250 > SiO₂–Al₂O_3-500 > Nb₂O_5-250).^87–93 For the zeolite support, an ionic exchange process between a Brønsted proton and K⁺ may have occurred during the catalyst preparation, as already reported for Fe/Y-zeolite and Co/ZSM-5 promoted by K.^94–96 This leads to a decrease of the amount of FeK active sites and a loss of the important K/Fe synergy for the RWGS reaction.^43,45 However, for neutral or basic supports (Al₂O_3-500, Al₂O₃–MgO_-500, SiO_2-700 or ZrO_2-250), no substantial support effect is observed in the RWGS reaction, with similar stable CO₂ conversion (with just minor variations) and high CO selectivity (100%). Therefore, the use of basic and neutral supports for single-site catalysts containing tailored potassium promoters remains a promising strategy to tune the activity, selectivity and stability for the RWGS reaction. Interestingly, supports based on distinct elements such as Mg, Al, Si and Zr show virtually the same activity and selectivity in the RWGS reaction. The present data indicate that these elements contribute only to the dispersion and the stability of the active sites by keeping K⁺ in the proximity of Fe. Such proximity may limit the sintering of Fe single site to Fe nanoparticles, generally known to generate methane.^97,98 This finding confirms the critical role of K/Fe proximity in CO₂ adsorption and H₂ dissociation, as described in the previously proposed mechanism (Scheme 1) and supported by DFT calculations.⁴⁵ The latter is further supported by the use of ceria as a support. Indeed, when compared to acidic, neutral and basic supports, single-site FeK supported on redox oxide CeO_2-250 shows a different behavior and undergoes several evolutions during the time on stream. At initial time (Fig. 5a), the conversion and the selectivity are similar to those obtained on neutral and basic supports (Conv. ca. 20% and Sel. 100%), indicating that Fe and K are in proximity and the selective conversion of CO₂ to CO occurs in accordance with the postulated mechanism without any contribution of ceria. After 200 min, a gradual decrease in CO selectivity by formation of methane accompanied by an increase in CO₂ conversion is observed (Fig. 5a). This feature can be associated with an evolution of the RWGS active site (isolated anionic FeK) upon the feed exposure, in particular, H₂. This is consistent with partial Fe agglomeration under the reaction conditions, resulting in small Fe nanoparticles responsible for the methanation and light hydrocarbon production (C₂H₆ and C₃H₈).⁹⁹ Indeed, it is known that small Fe particles are more selective towards methane during CO₂ reduction, while larger Fe particles lead generally to light olefins and alkanes.^97,98 The agglomeration of Fe nanoparticles is assumed to pursue until a steady state (after 1300 min), where the conversion (34%) and selectivities remain constant (CO = 38%; CH₄ = 54%; C₂–C₃ = 9%). The evolution of the selectivity from CO towards methane during CO₂ conversion has already been reported for Ru/Al₂O₃ where the authors observe the evolution of the active phase from Ru single site to Ru nanoparticles using HRTEM and DRIFT during the RWGS reaction.⁹⁹ Another important example is reported in the literature describing the RWGS reaction over Ni supported on ceria with 1% loading, modified or unmodified by K⁺.¹⁰⁰ This study suggests a K-guided selectivity control method based on the regulation of key intermediates HCO*/H₃CO* for Ni/CeO₂ catalysts. By incorporating K into Ni/CeO₂, the CO selectivity of CO₂ hydrogenation at 400 °C shifts from around 9% for Ni/CeO₂ to approximately 85% for Ni/CeO₂ promoted by K. The high stability of the K–Ni/CeO₂ catalyst during the selective conversion of CO₂ to CO is attributed to the presence of excess of K (K/Ni ratio of 6) which limits the formation of nickel nanoparticles while always keeping a K in the proximity of each Ni. Importantly, Ang et al. reported that at low loadings of Na (0.5 to 2 wt%), Na⁺ can readily incorporate into the defective CeO₂ lattice,¹⁰¹ generating a lattice strain and activating the lattice oxygen, thereby increasing the reducibility of the catalyst. However, beyond the solubility limit of 2 wt%, Na is deposited on the CeO₂ surface, thus retarding the reducibility and decreasing the activity in WGS.¹⁰¹ Based on these findings, we propose a similar pathway in the case of FeK/CeO_2-250 where the evolution of the stability and selectivity during the RWGS reaction may be explained by the incorporation of K⁺ into the CeO_2-250 lattice, leading to the partial loss of K in the proximity of Fe. The naked Fe single site without K in proximity can agglomerate over time in the presence of H₂ at 400 °C to form Fe nanoparticles responsible for the formation of methane. The evolution of the selectivity to CO and methane during the RWGS reaction can thus be correlated with the ratio of FeK single sites and Fe nanoparticles. The local structure of FeK/CeO_2-250 before and after catalysis has been investigated using a combination of XRD and HRTEM analyses (Fig. S21 and S22). However, due to the low loading of Fe, no Fe particles on ceria can be detected. Such limitations in XRD and HRTEM analyses have previously been reported for Fe and Cu supported on CeO₂, where no sign of Fe and Cu is observed until the loadings exceeded 10 wt%.^102–104 In addition, Fe K-edge X-ray spectroscopy of the catalyst FeK/CeO_2-250 before and after the catalytic tests cannot lead to any significant changes due to the heavy absorption of Ce.¹⁰⁵ Furthermore, when the catalyst, FeK/CeO_2-250, is reduced under hydrogen at a higher temperature (500 °C), the amount of the promotor, K, in the lattice of CeO₂ may increase and leads to an increase of Fe nanoparticles on the surface and consequently a drop of the selectivity in CO from 45% to 15% due to the methanation reaction (Fig. S23). For comparison, FeK/CeZrO_x-250 is tested under the same conditions as FeK/CeO_2-250 in the RWGS reaction. The catalyst FeK/CeZrO_x-250 exhibits high activity and stability over time on stream with full selectivity in CO (100%) compared to FeK/CeO_2-250. Thus, the presence of zirconia on the mixed CeZrO_x support seems to inhibit the diffusion phenomenon of K in the case of the FeK/CeZrO_x-250 catalyst, resulting in high selectivity and activity in the RWGS reaction due to the synergy between Fe and K. This result reveals that the active site FeK is selectively immobilised on the ZrO₂ phase rather than CeO₂ in CeZrO_x mixed oxide since we demonstrate (vide supra) that FeK/ZrO_2-250 is more selective in the RWGS reaction than FeK/CeO_2-250. Indeed, previous studies report that under reductive conditions, the structure of ceria–zirconia support is more stable than that of pure CeO₂.^106–108

Fig. 5 CO₂ conversion and product selectivity over (a) FeK/CeO_2-250 and (b) FeK/CeZrO_x-250 (feed composition: CO₂/H₂: 1/3, flow = 3 mL min⁻¹, 400 °C, 30 bar).

Therefore, all the results obtained support the hypothesis that the reduction of CO₂ to CO is directed by the presence of K and Fe in proximity and that the supports contribute to the dispersion of the active phase. However, while neutral and basic supports have little influence on performance, ZrO₂ provided the best results, maintaining high activity, selectivity, and long-term stability by preserving the Fe–K single site without agglomeration. In contrast, acidic supports degrade the performance due to ion exchange, disrupting Fe–K synergy, and CeO₂ causes Fe agglomeration and methane formation, though CeZrO₂ mitigates these issues. Mechanistic studies confirm that the cooperative action between Fe(II) and nearby K⁺ cations is key to the reaction, with the support's role being primarily to stabilize this active site. This work highlights the importance of support-controlled dispersion and stabilization of Fe–K sites for efficient CO₂-to-CO conversion.

4. Conclusions

The heterobimetallic complex, [{(THF)₂KFe(OtBu)₃}₂], is successfully grafted with 1 wt% Fe loading onto a variety of solid supports, encompassing a wide spectrum of surface chemistries: neutral (SiO₂ and Al₂O₃), basic (Al₂O₃–MgO and ZrO₂), acidic (SiO₂–Al₂O₃, μ-H-ZSM-5, and Nb₂O₅), and redox (CeO₂ and CeZrO_x) oxides. This is achieved via a synthetic strategy based on SOMC for the grafting of reactive species in a stable and well dispersed fashion. Regardless of the support used, the grafted complexes consistently retain a K/Fe ratio of 1, with Fe remaining in its +2 oxidation state. The potassium cations play a stabilizing role, ensuring that anionic Fe–K single sites remains atomically dispersed across the support surfaces. Comprehensive characterization by DRIFT, EPR, ICP, EDX, HRTEM, BET, XRD, and Fe K-edge XAFS (especially for the ZrO₂ supported sample) confirm these structural features.

Catalytic tests in the RWGS reaction reveal the critical influence of the support in tuning activity, selectivity, and long-term performances. On neutral and basic oxides, the catalytic behavior is similar, showing minor dependence on the support's properties. However, ZrO₂ stands out: FeK/ZrO_2-250 demonstrates exceptional RWGS activity and stability. This is attributed to zirconia's unique ability to anchor and disperse Fe–K species without promoting aggregation or sintering. Post-reaction analyses confirm this stability. HRTEM images show no signs of clustering even after 40 hours on stream, while EXAFS spectra reveal no Fe–Fe interactions, indicating that the local environment, defined by Fe–O, Fe–K, and Fe–Zr distances, remains similar throughout the reaction.

In contrast, catalysts supported on acidic oxides (μ-ZSM-5, SiO₂–Al₂O₃, and Nb₂O₅) are far less effective. These materials likely suffer from ion exchange between K⁺ and Brønsted acid sites, disrupting the subtle Fe–K synergy essential for catalysis. Redox-active CeO₂ initially exhibits promising CO selectivity, but this quickly declines due to structural changes by diffusion of K⁺ into the ceria lattice and formation of Fe nanoparticles, as previously reported in the literature, shifting the selectivity toward methane. Interestingly, due to the presence of the ZrO₂ phase, the mixed oxide, CeZrO_x, overcomes this issue by offering a more stable framework, preserving both catalytic performance and selectivity. Mechanistically, the RWGS reaction is found to rely on a cooperative interaction between Fe(II) and K⁺: Fe(II) facilitates H₂ and CO₂ activation, forming hydroxycarbonyl intermediates, while nearby K⁺ promoted CO₂ adsorption and transition state stabilization. Across all supports, this Fe–K cooperation, not the inherent properties of the oxide supports, is the principal driver of activity and CO selectivity. Thus, this study highlights the importance of pairing Fe(II) with K⁺ at the atomic level and choosing the right precursor and support to preserve this synergy. This work offers valuable insight into designing robust, selective RWGS catalysts for CO₂ conversion under mild conditions. Importantly, such materials may easily be transformed into bifunctional catalysts by adding a Fischer–Tropsch (FT) active phase (for example, well-defined metallic nanoparticles based on Fe, Co or Ru), yielding hydrocarbons directly from CO₂ through consecutive RWGS and FT reactions.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Abdulrahman Adamu Isah and Yahaya Nasiru performed the synthesis and characterization of catalysts and testing of the catalysts. Fadila Hamachi and Jie Pan performed the synthesis and characterization of catalysts and testing of the catalysts based on silica and ZSM-5. Pierre-Yves Dugas conducted the electron microscopy studies. All these tasks were carried out under the supervision of Kai C. Szeto for testing of the catalysts and Aimery De Mallmann for the characterization of materials; Mostafa Taoufik and Cyril Godard conceived the initial idea and designed the project.

Conflicts of interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this study.

Data availability

The data supporting this article have been included as part of the supplementary information (SI): Supplementary information is available. See DOI: https://doi.org/10.1039/d5nj03345d.

Acknowledgements

A. A. I. and N. Y. thank the Petroleum Technology Development Fund (PTDF). A. D. M. thank Olivier Mathon and Kiril Lomachenko for their help during the recording of the X-ray absorption spectra at ESRF, on beamline BM23 (proposal IN-1134). J. P. acknowledges grant PID2021-128128NB-I00 funded by MICIU/AEI/10.13039/501100011033 and by “FEDER/UE” and the Generalitat de Catalunya (2021SGR00110).

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