Improved CO selectivity during CO2 hydrogenation by bimetallic copper-cobalt supported SBA-15

Improved CO selectivity during CO 2 hydrogenation by bimetallic copper-cobalt supported SBA-15 María Escamilla , Alfonso Caballero , Gerardo Col´ on * Instituto de Ciencia de Materiales de Sevilla, Centro Mixto Universidad de Sevilla-CSIC, Am´ erico Vespucio, 49, Sevilla 41092, Spain ARTICLE INFO Keywords: CO 2 valorization Carbon monoxide Cobalt Copper SBA-15 ABSTRACT Co-Cu/SBA-15 systems have been studied for the reverse water gas shift (rWGS) reaction within mild temperature range of 250–650 ºC. CuOx and CoOx species at different weight ratios were deposited from wet impregnation method over SBA-15 support. We have stated that pre-reduction treatment before reaction leads to the complete Co and Cu reduction for Co-Cu/SBA systems. A low copper content on Co/SBA system leads to similar CO 2 conversion than monometallic Co-catalyst. However, competitive Sabatier side reaction appears almost suppressed. So, upon Cu incorporation a significant improvement on CO yield has been attained with respect to Co/SBA catalyst. Thus, a good compromise is attained at 450 ºC using Co5Cu5/SBA catalyst for which a stable CO 2 conversion of 22 % and 97 % CO selectivity has been obtained under lowest H 2 :CO 2 ratio. 1. Introduction It is widely accepted that the anthropogenic activities during the last century resulted in a significant increase in the atmospheric carbon dioxide (CO 2 ) levels, disrupting the natural carbon cycle. The chemical industry’s overall CO 2 consumption is a relatively minor factor in emissions. An attractive solution for using and valorising carbon dioxide is to use it as raw material for the synthesis of fuels and chemicals [1–3]. Among the possible reactions, the main products that can be obtained are CO via rWGS (Reverse Water Gas Shift) reaction, methanol, and hydrocarbons [4,5]. The first process results in the production of carbon monoxide that can be used as a feedstock in the production of methanol or adopted for the production of synthetic hydrocarbons through Fischer-Tropsch (FT) synthesis [6,7]. After these considerations, we could confirm that the production of CO via rWGS is a flexible choice [8]. As above discussed, rWGS reaction has been the subject of much interest in recent years, however, this reaction is limited by thermodynamics, and challenges remain in the implementing it on a larger scale. To enhance the process, the development of an efficient and effective catalyst is crucial [9]. Supported noble-metal catalysts (such as Au, Pt, and Pd) have demonstrated remarkable catalytic performance in rWGS reaction. However, the constraints of scarcity and elevated costs associated with these catalysts hinders their large-scale implementation. In contrast, non-noble metal catalysts (e.g., Cu, Ni, Fe, Co, Mo, etc.) are relatively inexpensive. Nevertheless, the CO₂ conversion at low temperatures remains a significant challenge, since the active species tend to aggregate at high temperatures [10–12]. It is very contradictory that at a relatively low reaction temperature, the main product attained is methane, which seriously reduces the selectivity of the pursued product, CO [13]. In this sense, Cu-based catalysts show interesting advantages regarding to the low cost and high CO selectivity [14]. However, they have revealed lower catalytic activity, low thermal stability, and poor water resistance, leading to active site agglomeration and oxidation. Thus, different approaches have been proposed to enhance the catalytic performance of Cu-based catalysts which includes tuning the interaction with the support [15] or the formation of bimetallic catalysts [16]. On the other side, Co-based catalysts demonstrate higher activity than other non-noble metals (Fe, Ni, or Cu), indicative of their great potential in CO 2 hydrogenation [17,18]. Unfortunately, catalysts based on cobalt are widely reported for methanation during CO 2 hydrogenation, showing low selectivity toward CO [19]. The main drawback observed for these systems regards to the low selectivity and stability. This is mainly due to the coexistence and easy transformation of different Co species during the reaction. Indeed, at least five cobalt phases (metallic Co, CoO, Co 3 O 4 , and different Co 2 SiO 4 forms) were found coexisting on Co/SiO 2 catalyst [20]. Formulation of bimetallic catalysts exhibit unique properties by modifying the surface properties of the former monometallic catalyst. Thus, Cu-based bimetallic compounds containing transition metals (e.g., * Corresponding author. E-mail address: [email protected] (G. Col´ on). Contents lists available at ScienceDirect Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou https://doi.org/10.1016/j.jcou.2025.103032 Received 29 November 2024; Received in revised form 21 January 2025; Accepted 27 January 2025 Journal of CO2 Utilization 92 (2025) 103032 Available online 30 January 2025 2212-9820/© 2025 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/bync-nd/4.0/ ). Fe, Co, and Ni) have been widely investigated as CO 2 hydrogenation catalysts [21,22]. Cobalt containing systems Co have attracted particular attention owing to their excellent CO 2 hydrogenation activity [23]. However, Cu–Co bimetallic catalysts in the form of atomically mixed metal alloy are difficult to prepare because of the miscibility gap between Cu and Co. It has been widely reported that the ability to form surface vacancies during the oxidation and reduction step is crucial for high performance rWGS. In this sense, lanthanides-based perovskites with an ABO 3 structure have been widely investigated for this reaction [24]. In particular, they have shown a very stable structure at high temperatures. However, perovskites typically have very low specific surface areas (< 20 m 2 /g), and so they might show mass transfer restrictions [25]. It has been reported several methods for dispersing perovskite particles on high surface areas supports such as SiO 2 , TiO 2 , Al 2 O 3 . In the present paper we use SBA-15 as active site support. Indeed, the use of mesoporous silica has been previously proposed as a good candidate due to the high surface area values and the structural and surface stability at high temperature [26]. 2. Experimental section 2.1. Catalysts preparation The mesoporous silica SBA-15 used as support was prepared according to a method previously described in the literature [27]. Briefly, 18 g of P123 was dissolved in 270 mL of distilled water, then a solution of 675 mL of 2.5 M HCl was added and heated up to 50 ◦C. An amount of TEOS was then added and kept at 50 ◦C for 18 h. The product obtained was filtered, washed with boiling distilled water, dried under vacuum at 70 ◦C and calcined on for 3 h at 550 ◦C. The cobalt and copper metal phases were supported on SBA-15 by impregnation through incipient wetness method. Bimetallic supported catalysts were prepared with a total nominal loading of 10 wt%, using Co(NO 3 ) 2 ⋅6 H 2 O (Sigma-Aldrich) and Cu(NO 3 ) 2 ⋅3 H 2 O (Sigma-Aldrich) as the metal precursors. Then, catalytic systems were dried at 100 ◦C for 2 h and calcined in flowing air at 500 ◦C for 2 h. The resulting products Fig. 1. a) X-ray diffraction patterns for CoCu/SBA catalysts; b) Small-angle X-ray diffraction patterns of CoCu/SBA catalysts; c) N 2 adsorption-desorption isotherms for CoCu/SBA catalysts; d) Pore size distribution for CoCu/SBA catalysts. Table 1 Surface features, chemical composition and H 2 consumption for CoCu/SBA catalysts. Samples S BET m 2 /g V pore cm 3 /g D pore nm ICP H 2 -TPR Co wt% Cu wt% H 2 consumption *mmol/mg SBA−15 777 0.64 3.8 — — — Co10/SBA 463 0.44 3.9 9.8 — 2.03⋅10 −3 / 2.75⋅10 −3 Co8Cu2/ SBA 410 0.43 3.9 7.4 1.9 2.21⋅10 −3 / 2.07⋅10 −3 Co5Cu5/ SBA 409 0.39 4.1 4.9 5.0 2.19⋅10 −3 / 2.05⋅10 −3 Co2Cu8/ SBA 392 0.40 3.9 1.9 8.1 2.05⋅10 −3 / 1.85⋅10 −3 Cu10/SBA 397 0.39 5.1 — 9.8 1.54⋅10 −3 / 1.60⋅10 −3 * Measured consumption/theoretical consumption M. Escamilla et al. Journal of CO2 Utilization 92 (2025) 103032 2 were labelled as Co10/SBA, CoxCuy/SBA and Cu10/SBA, being x and y the weight percent of each metal in the bimetallic systems. 2.2. Catalysts structural and surface characterization BET surface area measurements were carried out by N 2 adsorption using a Micromeritics 2000 instrument. SAXS measurements were carried out using a Panalytical X ′ PERT PRO instrument. attached to an ID 3003 laboratory X-ray generator (General Electric). Sealed X-ray tubes (PANalytical, λCu (K α ) =0.1542 nm) operating at 40 kV and 50 mA were used. A translucent beam-stop allowed the measurement of an attenuated primary beam at q =0. X-ray scattered beams were recorded by a CCD detector placed 309 mm from the sample holder in the q range from 0.09 to 5 nm –1 . Wide angle X-ray diffraction (XRD) Fig. 2. HAADF-STEM images and element distribution images of Co10/SBA (upper panel); Co5Cu5/SBA (mid panel); and Cu10/SBA (lower panel). M. Escamilla et al. Journal of CO2 Utilization 92 (2025) 103032 3 patterns were obtained using a Panalytical X ′ PERT PRO diffractometer with Ni filter and graphite monochromator. The X-ray source was Cu K α radiation. The Co and Cu contents in the catalysts were determined by inductively coupled plasma atomic emission spectroscopy (ICP-OES) using an iCAP 7200 Duo spectrometer. Solids were previously dissolved by using a microwave digester Ethos Easy. Temperature-programmed reduction experiments were performed on a home-made setup using a thermal conductivity detector. The H 2 consumption was determined by using commercial CuO as reference. Conventional experiments were carried out from room temperature up to 800 ºC with a heating ramp of 10 ºC/min, under 5 % H 2 /Ar flow of 10 mL/min. The transmission electron microscopy (TEM) images, high angle annular dark field (HAADF) and element mapping analysis images were obtained by using a Thalos microscope in STEM mode operated at 300 kV equipped with a Gatan GIF Quantum 963 energy filter. The samples were directly dropped on a nickel grid. XPS analyses were carried out by using a SPECS Phoibos 150 instrument using the Al K α X-ray source (20 mA, 14 kV). The charge corrections of spectra were performed using the main signal of the Si2p 3/ 2 at ca. 103.5 eV, with double checking with this shift value adventitious carbon C 1 s spectrum peaks at 284.6 +/- 0.1 eV. The in-situ treatments were performed on a high-temperature commercial cell (SPECS HPC) coupled to the main chamber of the spectrometer. The high-temperature cell design allows sample heating up to 800 ◦C, under flow or static conditions. This arrangement enabled a transfer of treated samples from the reaction chamber to the spectrometer under UHV conditions, avoiding exposure to the laboratory atmosphere. In a typical experiment, the sample was initially placed in the sample holder and transferred to the spectrometer chamber, where XPS spectra were acquired. The sample was then transferred under vacuum to the reaction chamber where it was exposed to the reactive gases and heated to the appropriate temperature to perform the reduction pre-treatment. After the treatment, the sample was cooled to room temperature under the same reaction atmosphere, evacuated down to 10 −7 mbar in less than two minutes, and then transferred back to the spectrometer chamber for analysis, avoiding ambient exposure. This allows us to analyse the surface and chemical state of the elements exposed at the surface of the catalysts after reduction conditions. 2.3. CO 2 hydrogenation catalytic runs The experimental measurements were carried out with 0.25 g of catalyst diluted in 0.25 g of SiC, using a stainless-steel fixed-bed tubular reactor (300 mm length and 8 mm internal diameter) equipped with a temperature controller and three mass flow controllers. The catalysts were previously reduced in-situ by flowing H 2 (50 %) in N 2 at 500 ◦C for 1 h. The temperature was then lowered to 350 ºC and CO 2 was introduced. The reaction mixture consists of a CO 2 /H 2 /N 2 flow with volume ratio of 3:3:1 (N 2 was used as an internal standard for gas chromatography analysis) and a flow rate of 35 mL/min through the reactor giving a constant gas hourly space velocity (WHSV) of 8400 mL⋅g −1 ⋅h −1 . The Fig. 3. H 2 -TPR profiles of different CoCu/SBA catalysts. Fig. 4. XPS analysis for Co-Cu/SBA catalysts. M. Escamilla et al. Journal of CO2 Utilization 92 (2025) 103032 4 reaction is monitored at different temperatures between 350 ºC to 650 ºC. The reaction products were analysed by means of a previously calibrated GC (Agilent 7820) equipped with two columns and TCD and FID detectors. All lines connecting the reactor to the GC were heated to prevent condensation of the obtained products (CO and CH 4 ; negligible traces of ethane were not quantified). Conversion and selectivity values were calculated using the following equations: Conversion(%) = [CO2]t [CO2]i ×100 (1) where [CO2]t represents the moles of reacted carbon dioxide and [CO]i the initial carbon monoxide amount. [CO2]t and [CO2]iare calculated by taking into account the CO 2 /N 2 area ratio during the reaction and that obtained under reaction conditions without catalyst, respectively. Selectivity to CO(%) = [CO]t [CO]t+ [CH4]t ×100 (2) where [CO]t and [CH4]t represent the moles of the specific product in the reaction. CO yield has been quantified by considering the production rate at each time. 3. Results and discussion 3.1. Structural and surface features Fig. 1.a shows the wide angle XRD patterns for CoCu/SBA systems. The presence of peaks indicating the formation of Co 3 O 4 species in the Co/SBA-15 catalyst has been identified. The presence of peaks at 31.3◦, 36.9◦, 45.1◦, 59.4◦and 65.4◦(2θ angles) indicates that the cobalt species (after calcination at 500 ºC) were mainly in the crystalline Co 3 O 4 spinel form. Similarly, for Cu/SBA, the strong diffraction peaks at 32◦, 35.4◦, 38.6◦, 48.7◦and 61.3◦indicated the presence of CuO structure (JCPDS48–1548). The shape of the peaks, which are very intense and narrow, would suggest the growth of the crystalline phases mainly outside the mesopores, though confined metals inside the pores could not be discarded. On the contrary, the broader and less intense peaks associated to CuO in Co2Cu8/SBA and Co5Cu5/SBA catalysts would point out that Cu-species would be deposited inside the pores [28]. The small-angle X-ray diffraction patterns for SBA-15 (Fig. 1.b) show a well resolved peak at about 1º which could be indexed as (100) diffraction of highly ordered two-dimensional hexagonal mesoporous structure (p6 mm), suggesting a good long-range order within this material [29]. Other peaks corresponding to (200) and (211) reflections are not as prominent as reported. The ordered mesopore structure of SBA-15 was verified by transmission electron microscopy (Figure S1). A significant low-angle shift of the (100) reflection can be observed for the Cu10_SBA sample, indicating an enlargement of the unit cell parameter with respect to CoCu/SBA and Co/SBA samples [30]. The pore enlargement expected from the increase in unit cell size is confirmed by the adsorption/desorption of nitrogen. The N 2 physisorption isotherm of CoCu/SBA series (Fig. 1.c) corresponds to type IVa according to the IUPAC classification, while the hysteresis loop (H1) is characteristic of the capillary condensation that takes place in the mesopores. The adsorption branch shows a very steep Fig. 5. Co2p and Cu2p fitted XPS spectra for Co10/SBA and Co5Cu5/SBA catalysts during reduction and reaction at 450 ºC. M. Escamilla et al. Journal of CO2 Utilization 92 (2025) 103032 5 increase at 0.7–0.8 relative pressure, evidencing a very uniform channel diameter, consistent with the narrow pore size distribution displayed in Fig. 1.d. The BET surface area values summarized in Table 1 indicate a notable diminution in the surface when Co and Cu are deposited. It is also noteworthy to mention that Cu deposition has a greater effect on the surface feature of the catalysts. Thus, for the Cu10/SBA catalyst we have obtained the lowest surface area, 397 m 2 /g against 463 m 2 /g for Co10/ SBA. This fact, in correlation with the small angle XRD, would indicate a better impregnation of Cu species inside the pore structure. The Co and Cu contents measured by ICP-OES are also shown in Table 1. In all cases, the cobalt and copper concentrations appear quite similar to the nominal values. These findings align with above previous results, which indicate that copper has a pronounced impact on the surface characteristics of SBA15. Noteworthy, for the bimetallic catalyst it is possible to see aggregates formed by Co and Cu. So, the simultaneous deposition of Co and Cu lead to intimate formation of particles containing both metals. Fig. 2 shows the high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images of Co10/SBA and Co5Cu5/ SBA catalysts, which evidences that the resulting SBA-15 had a perfect hexagonal porous structure. Moreover, Co appears deposited on SBA-15 particles forming large aggregates of around 30–40 nm. In contrast, monometallic Cu10/SBA showed a good dispersion of copper over the SBA-15 surface. (Fig. 2). The reduction behaviour of the studied catalysts was determined by H 2 -TPR (Fig. 3). In all cases the reduction process takes place at temperatures below 500 ºC. For Co10/SBA the reduction profile shows two broad H 2 -consumption peaks between 300 ºC and 500 ºC. These peaks in the 300–400 ◦C range can be attributed to the two-step reduction of the oxide Co 3 O 4 , as suggested by many works on materials containing this compound in a dispersed form [31]. In addition, a tiny signal at high temperature can be observed at around 700 ◦C. This high temperature reduction peak could be related to the reduction of Co ions bonded to the siloxane framework [32]. These species can be regarded as amorphous Co silicate with similar redox properties, although the crystalline phase Co 2 SiO 4 is only formed at much higher temperatures, as previously observed [33]. The amounts of H 2 corresponding to the signals at 300–500 ◦C (low T) and 700 ◦C (high T) are related to the reduction of Co 3 O 4 and Co in amorphous silicate [34]. Similarly, Cu10/SBA catalyst underwent a two-step reduction process that has been described in the literature as the isolated Cu 2+ ions on SiO 2 reduction [35]. Thus, the lower temperature peak was ascribed to Cu 2+ to Cu + , and the higher temperature peak to Cu + to Cu 0 . According to the literature [18,19], in the low temperature reduction stage, the peak at 260 ºC is ascribed to the presence of highly dispersed CuO species. The bulk CuO species with different particle sizes may be at different positions on the mesoporous materials with different interactions. The different reduction peaks may be caused by these bulk CuO species. The reduction peak at ca 300 ºC is ascribed to the presence of some larger particles which shows relatively weakly interaction with the external surface of the SBA-15. For bimetallic Co-Cu/SBA catalyst the reduction profile changes and only a wide reduction process at ca 300 ºC can be noticed. This would imply that the presence of both ions at the surface induces their Fig. 6. a) CO 2 conversion; b) CO selectivity; and c) CO yield for CoCu/SBA systems. Reaction conditions: H 2 /CO 2 =1:1, WHSV =8400 mL g –1 h –1 . Temperatureprogrammed reaction from 250 ºC to 650 ºC. M. Escamilla et al. Journal of CO2 Utilization 92 (2025) 103032 6 simultaneous reduction at this temperature of both metal ions. In Table 1 we also include the calculated H 2 consumption for all catalyst in comparison with the corresponding theoretical values (in parenthesis). It can be observed that copper catalysts showed an almost complete reduction while for monometallic cobalt catalyst it is possible to assess a lower reduction degree at the end of the TPR experiment. This could be associated to the formation of above-mentioned cobalt silicate which would show a difficult reduction even at higher temperatures. On the contrary, when copper is present it seems that cobalt silicate is not formed and reduction is favoured at lower temperature. The Co2p 3/2 XPS spectrum of each sample can be assigned to a main component at binding energy of ca 780 eV, corresponding to surface Co 3+ (Fig. 4). By peak deconvolution we could state also the presence of Co 2+ showing a doublet peak at 781.6 and 797.3 eV. This would be in accordance to the presence of Co 3 O 4 spinel structure noticed by XRD. The spectra of Cu2p peaks in Cu10/SBA and CoCu/ SBA systems revealed the similar shapes with two main peaks centered at ca. 933.6 and 953.4 eV, along with two satellite peaks at ca. 943 and 963 eV, which can be assigned to Cu 2+ 2p 3/2 and 2p 1/2 , respectively (Figure S2). It is worthy to note that in the case of the lower Cu content catalyst, Co8Cu2/SBA, certain asymmetry in the Cu2p 3/2 peak and the lower satellite contribution would indicate the presence of Cu + species. This could be associated to the higher dispersion and lower dimensionality of Cu clusters due to the low copper content in this sample [36]. In Fig. 5, we have depicted the evolution of Cu2p and Co2p signals from XPS analysis for Co10/SBA and Co5Cu5/SBA catalysts during in situ reduction treatment and reaction conditions at 450 ºC. The reduction at 500 ºC completely reduces copper (the Cu LMM peaks shape and positions mainly correspond to pure metallic Cu, supporting the interpretation of the Cu2p spectra, Figure S2) while for cobalt a mixture of Co 2+ and Co 3+ still appear together with the main Co 0 signal. Under reaction conditions at 450 ºC the Co2p and Cu2p signals do not show any changes. However, by observing the evolution of the Co/Si and Cu/Si ratio we may infer that the behaviour of cobalt and copper sites at the surface are rather different (Fig. 5). While Cu/Si atomic ratio remains almost constant along reduction and long-time reaction period, the Co/Si atomic ratio seems to decrease after reduction remaining constant during reaction. The Co/Si ratios are slightly lower when copper is present denoting the lower Co content in the bimetallic catalyst. This lower Co/Si would indicate that cobalt suffers a certain aggregation during reduction. Fig. 6 shows the CO 2 conversion and CO selectivity trends over the CoCu/SBA catalysts as a function of temperature. We firstly have to highlight that all catalysts are active for rWGS in the temperature range 250–650 ◦C. Regarding the selectivity, the incorporation of copper is crucial for achieving higher CO selectivity and suppress the Sabatier reaction. In terms of CO 2 conversion Co10/SBA shows better values at low temperature, however as we see in the selectivity plot, most of the product is CH 4 . As copper is included, the conversion slightly lower, being Cu10/SBA the system showing the lower CO 2 conversion. Summarizing, for temperatures higher than 450 ºC, Co8Cu2/SBA and Co5Cu5/SBA catalysts show similar CO 2 conversions as Co10/SBA, however CO selectivity for these CoCu systems arises (upto 90 % and 97 % for Co8Cu2/SBA and Co5Cu5/SBA respectively). Indeed, the CO yield is always higher for bimetallic catalyst than for Co10/SBA in the whole range of temperature (Fig. 6.c). From these results, Co5Cu5/SBA appears to be a highly selective catalyst (about 97 % selectivity to CO) with high conversion value (from Fig. 7. a) CO 2 conversion; b) CO selectivity; and c) CO yield for Co10/SBA and CoCu/SBA selected systems at 450ºC during 20 hours reaction. Reaction conditions: H 2 /CO 2 =1:1, WHSV =8400 mL g –1 h –1 . Constant reaction temperature of 450ºC for 20 hours. M. Escamilla et al. Journal of CO2 Utilization 92 (2025) 103032 7 25 % to 40 % of CO 2 conversion) in the temperature range of 450–650 ◦C. Specifically at 450 ºC, Co5Cu5/SBA shows a CO yield of ca 2⋅10 −4 mol/s⋅g (Fig. 6.c). In order to assure the stability of the catalytic systems, we have performed the catalytic experiments during long reaction times at 450 ºC (Fig. 7). From long-time catalytic run, all catalysts exhibit stable activity performance, showing a CO 2 conversion around 22 % at 450 ºC (Fig. 7. a). While Co10/SBA and Co8Cu2/SBA present similar CO 2 conversion, Co5Cu5/SBA shows a slightly lower value. The attained catalytic performance denotes the stability of catalyst during long-term reaction. Indeed, morphological features do not experiment significant changes after 20 h of reaction and surface features (Figure S3). Copper and cobalt appear forming similar aggregates, perhaps slightly larger than those observed in fresh samples. This fact was already noticed from Co/Si values in Fig. 5. Concerning the CO selectivity, the three studied catalyst present different behaviour. The CO selectivity for copper containing catalysts clearly keep a stable value for the whole reaction time (Fig. 7.b). For these catalysts, Sabatier reaction is almost completely suppressed and CO selectivity reaches values around 95–97 %. On the contrary, Co10/ SBA catalyst initially starts with a low CO selectivity that progressively increases, reaching a final steady stable selectivity of 70 %. Thus, for this catalyst, methanation is initially favoured at low temperature. Moreover, this result clearly reveals that the catalyst evolves over the reaction time. In Fig. 7.c we show the different CO yields for these three catalysts. Thus, copper based bimetallic catalysts present similar CO production of about 100 mmol at the end of the reaction period. This value is far from the value attained for Co10/SBA, 60 mmol. Our results clearly show that supported CoCu/SBA catalysts are robust and stable materials that can run satisfactorily for continuous operations displaying complete rWGS selectivity. It is well known that, within this process, the suppression of the Sabatier reaction is particularly significant for the efficient use of hydrogen. Thus, at mild temperature and low H 2 /CO 2 ratio we reach a good conversion value of more than 20 %. The complete rWGS selectivity across the full range of temperatures showed by Co5Cu5/SBA and conversions studied herein make it suitable for exploring tandem catalysis schemes where this system could be coupled with CO consuming Fischer–Tropsch active catalysts. This area of tandem catalysis for CO 2 utilization has gathered considerable interest and requires the development of fully selective rWGS catalysts [37,38]. The comparison of these results with previously reported values reveals that our system can be considered a promising catalyst for the rWGS reaction [39]. Moreover, the catalytic performance achieved here with CO 2 /H 2 ratio of 1:1 can be comparable to most of the reported one. This clearly implies a significant efficient use of hydrogen, considering that currently even green H 2 is still expensive. Knowing that the equilibrium conversion is strongly affected by inlet composition, different inlet ratios of CO 2 /H 2 were tested at every temperature. So, in order to find the better catalytic conditions for this reaction, we have studied the influence of the CO 2 :H 2 ratio in the steam flow (Fig. 8). In general, an increase in conversion with increasing the amount of fed H 2 is noticed, being more evident for Co10/SBA. Moreover, for all ratios a conversion enhancement is apparent with Fig. 8. a) CO 2 conversion for Co10/SBA; b) CO 2 conversion for Co5Cu5/SBA; c) CO selectivity for Co10/SBA; d) CO selectivity for Co5Cu5/SBA. Reaction conditions: H 2 /CO 2 =1:1–1:4, WHSV =8400 mL g –1 h –1 . Temperature-programmed reaction from 250 ºC to 650 ºC. M. Escamilla et al. Journal of CO2 Utilization 92 (2025) 103032 8 temperature, thus it follows the trend of equilibrium conditions. Thus, at our previous reference temperature 450 ºC, the conversion shifted from 22 % to 57 % for Co10/SBA (Fig. 8.a and 8.c). It is worthy to mention that CO selectivities show a contrary effect and Sabatier reaction is favoured with increasing the selectivity to CH 4 as H 2 ratio increases. For Co5Cu5/SBA this increase is not so notable and reaches 30 % conversion for CO 2 /H 2 ratio of 1:4 with no any modification on the CO selectivity (Fig. 8.b and 8.d). Therefore, since hydrogen is one of the main issues for the economic and sustainable feasibility of the process, on the basis of these results we may propose that the better reaction conditions for rWGS reaction would consider lower H 2 ratio in the reaction fed. Using Co5Cu5/SBA as catalyst and moderate temperature as 450 ºC leads to a good compromise of CO 2 conversion and CO selectivity. 4. Conclusions A bimetallic Co-Cu/SBA-15 series of catalysts has been prepared by wet impregnation method over SBA support. The CoOx and CuOx species were observed to be dispersed on the SBA-15 surface affecting to its surface area and pore structure. The reduction pretreatment applied to the catalysts resulted in the complete reduction of both metals. The catalytic activity runs demonstrate that the Cu incorporation affects to the CO 2 conversion specially at lower temperatures. However, in all bimetallic systems Sabatier reaction appears to be effectively suppressed. The optimal Cu content was observed to be 5 wt%, which resulted in a comparable conversion value and the highest CO selectivity. It can be reasonably concluded that the optimal compromise is attained at 450 ºC using the Co5Cu5/SBA catalyst. This catalyst has demonstrated a stable CO 2 conversion of 22 % and a CO selectivity of 97 % after 20 hours reaction period, under the lowest H 2 :CO 2 ratio, which fulfils the economic and sustainable feasibility of the process. CRediT authorship contribution statement Caballero Alfonso: Supervision, Project administration, Funding acquisition. Col´ on Gerardo: Writing – original draft, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. Escamilla María: Methodology, Investigation, Formal analysis, Data curation. Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests. Gerardo Colon reports financial support was provided by Consejo Superior de Investigaciones Científicas. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments We acknowledge the financial support from Ministerio de Ciencia, Innovaci´ on y Universidades/FEDER through PID2020–119946RB-I00 projects funded by MCIN/AEI/ 10.13039/501100011033 and, as appropriate, by “ERDF A way of making Europe”, by the “European Union” or by the “European Union NextGenerationEU/PRTR”. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jcou.2025.103032. Data availability Data will be made available on request. References [1] J. Podder, B. Patra, F. Pattnaik, S. Nanda, A.K. Dalai, A review of carbon capture and valorization technologies, Energies 16 (2023) 2589. [2] L.Q. 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