Totally-green Fuels via CO2 Hydrogenation

Hydrogen is the cleanest energy vector among any fuels, nevertheless, many aspects related to its distribution and storage still raise serious questions concerning costs, infrastructure and safety. On this account, the chemical storage of renewable-hydrogen by conversion into green-fuels, such as: methanol, via CO2 hydrogenation assumes a role of primary importance, also in the light of a cost-tobenefit analysis. Therefore, this paper investigates the effects of chemical composition on the structural properties, surface reactivity and catalytic pathway of ternary CuO-ZnO-CeO2 systems, shedding light on the structure-activity relationships. Thus, a series of CuZnCeO2 catalysts, at different CuO/CeO2 ratio (i.e. 0.2-1.2) were performed in the CO2 hydrogenation reactions at 20 bar and 200-300 °C, (GHSV of 4800 STP L∙kg∙cat-1∙h-1). Catalysts were characterized by several techniques including X-ray Diffraction (XRD), N2-physisorption, single-pulse N2O titrations, X-ray Photoelectron Spectroscopy (XPS), and Temperature-programmed Reduction with H2 (H2-TPR). Depending on preparation method, the results clearly diagnostics the occurrence of synergistic structural-electronic effects of cerium oxide on copper activity, with an optimal 0.5 copper-to-cerium content. The rise of CuO loading up to 30% drives to a considerable increase of hydrogenation activity: C2Z1-C catalyst obtains the best catalytic performance, reaching methanol yield value of 12% at 300 °C. Catalyst activity proceeds according to volcano-shaped relationships, in agreement with a dual sites mechanism. Copyright © 2020 BCREC Group. All rights reserved


Introduction
Although electric vehicles are the best environmental choice, the high costs and the limited availability of several materials do not allow the total replacement of thermal engines worldwide. most a quarter of Europe's energy needs and allow to maximize business opportunities towards a future of zero emissions. Indeed, the European Union has identified hydrogen as one of the six strategic areas where major interventions and investments are needed, according to joint funding and assistance from the Horizon Europe program [4]. Despite the potential, the building of the infrastructure, for a hydrogenbased economy, negatively affects this technological choice, as demonstrated by the FCH-JU European industrial partnership (Fuel Cells and Hydrogen Joint Undertaking) which is working on it since 2008, managing more than 250 projects financed with 2 billion euro by public-private funds. In particular, many aspects related to distribution and storage of hydrogen (i.e. H2 highly compressed; 300 bar) still raise serious issues concerning infrastructure and safety. Therefore, to achieve the climate objectives, green-hydrogen from renewable energy sources (i.e. wind and sun) is now produced, obtaining new advanced synfuels at low carbon energy for powering hybrid and bi-fuel vehicles, as the most suitable route in the decarbonisation process and eco-life [5][6][7]. In fact, hydrogen has the double advantage to be a fuel, immediately available for using, and an energy vector, whose chemical accumulation can be pursued through its combination with other molecules.
Furthermore, more than 3 Gton of CO2 are currently emitted to the atmosphere every month, with tremendous impact on climate change and global warming of the Earth. For this reason, efforts must be put forth to decrease emissions, permanently sequester CO2 and convert it into valuable products [8][9][10]. In the light of a cost-to-benefit analysis, the combination of green-hydrogen with CO2 from industrial processes can assume a key role in the perspective of a more efficient and cleaner energy generation [11][12][13].
Actually, the application of green-fuels of second-generation is one of the most suitable route to reach industrial and ecological targets in the short term, because of the lower costs, easy commercial adoption and fully industrial development. Differently from fossil, a totallygreen fuel does not give rise to extra CO2 emission in atmosphere, because its combustion, in the well-to-wheel cycle, generates a CO2 release equal to that necessary for its manufacturing, closing the CO2 balance, on the other words a fuel at zero-charge of carbon dioxide. On the other hand, besides the efforts to limit the presence of CO2 in the atmosphere by CCUS tech-nologies, CO2 is assuming an even more important and strategic play-role in the energy field, as well as in the synthesis of industrial relevant products and chemicals [8,9,[14][15][16][17][18][19][20][21]. As new raw material, the direct utilization of CO2 for industrial purposes finds commercial applications in the synthesis of methanol and other chemical compounds such as olefins and aromatics, which are a rapidly growing field, since CO2 represents an abundant and economic carbon source [10, 22,23]. As one of the most worldwide relevant topics, the work would also reflect on the practical use of the hydrogenation catalysts in combination to economically useful CCUS technologies. Indeed, the catalytic conversion of CO2 coming from the atmosphere into fuels and fine chemicals would be one of the most profitable and practical solutions to the problem of greenhouse gas emissions, provided that capture/sequestration and high pressure storage technologies are made economically available [8].
Nevertheless, in the very short period, the most prosecutable actions and promising prospective look upon the use of CO2-rich streams, coming from the industrial exhausted emissions, such as: of brick and cement work, although needs of clean-up stages, and further purification and concentration [16]. On this address, Haldor-Topsoe is at the forefront in the use of CO2 as carbon feedstock through different power-to-gas technologies, while ENI is redesigning its lines of production in a greener vision, by the development of new hydrogenation processes, called ENI Ecofining TM , for the green-fuels synthesis, strategically prefiguring the production of ultra-pure CO2 as industrial practice [13,24]. Despite considerable progresses made in the science of fuel, the industrial economic feasibility of hydrogen-to-liquid-fuels (HTL), such as: methanol, is still under debate. In this scenario, the development of more efficient catalytic materials for an effective hydrogenation of CO2 appears of great interest [25][26][27][28].
As known, CuO/ZnO-based catalysts have the advantages of lower cost and higher chemical stability compared with other catalysts, such as those based on transition metal carbides (TMCs), bimetallic catalysts or Ausupported catalysts [29,30]. Although CuO/ZnO/Al2O3 is the most intensively studied catalyst for the methanol synthesis, the use of ZrO2, CeO2 and TiO2 oxides was found to improve both the activity and selectivity of CuO/ZnO-based catalysts in the CO2 hydrogenation processes. Indeed, Si et al. recently discussed on the influence of replacing Al2O3 with CeO2 in the typical Cu-ZnO/Al2O3 catalytic composition for syngas conversion, suggesting that the use of cerium oxide led to a remarkable positive effect on the catalyst stability, as a result of the preservation the ratio of Cu 0 /Cu + on catalyst surface, as well as on the particle growth, due to the strong electron interactions of the copper/ceria phase (Cu/Cu2O/CuO, Ce 3+ /Ce 4+ ) during hydrogenation reactions [31]. Metal dispersion and CO2 adsorption capacity are two key parameters affecting CO2 hydrogenation functionality of Cubased systems [32]. Similarly, in our previous works, we displayed that the use of several oxides (i.e. as ZnO, ZrO2, CeO2, Al2O3, Gd2O3, Ce2O3, Y2O3) and alkaline metals (i.e. Li, Cs, K) could remarkably influence both catalyst structure and morphology, balancing the amount of the diverse copper species (i.e. Cu°/Cu + /Cu 2+ ) and leading to a notable improvement of the catalytic performance [14,15,17,20,33,34]. On this account, ZrO2 has been shown to positively affect morphology and texture of Cu-ZnO based catalysts, favoring also CO2 adsorption / activation and methanol selectivity, while CeO2 could act as both electronic promoter and improver of surface functionality of Cu phase. As reported in our preliminary works, we have proved a greater specific activity of CuZnO-CeO2 catalyst with respect to that of similar catalytic systems containing ZrO2 or other promoter oxides, facing several electronic and structural effects [14,15,18]. In spite of a generally higher specific activity, we have evidenced that CuZ-nO-Ceria catalyst suffers from several practical limitations, such as: a lower surface area, especially compared to that of ZrO2 and Al2O3 promoted catalysts. In particular, we have showed the effects of replacing ZrO2 carrier with CeO2 oxide, especially on the development of the surface area of CuZnO-based catalysts. Indeed, by fixing the amount of carrier phase (i.e. around 40%), the catalysts without Ceria or at the less content of Ceria obtained a larger MSA and SA exposure and, consequently, a higher CO2 conversion, despite the minor value of methanol yield per surface area [15,18].
In agreement with the formation of carbonates intermediates induced by the oxygen vacancies related to CeO2, we have observed a major selectivity to methanol on CuZnO catalysts promoted by CeO2 addition with respect to other carrier oxides such as ZrO2 and Al2O3. On this regard, our preliminary findings perfectly agree with recent studies of Wang et al. who find a higher methanol selectivity on Cu/CeO2 catalysts with respect to ZrO2 supported sys-tems, according to an improved metal dispersion and oxygen vacancies formation induced by the interaction at the Cu/CeO2 interface [35]. Furthermore, Shi et al. gave evidence of an improvement in methanol productivity by TNTs addition on CuO-ZnO-CeO2 catalysts, as a results of enhanced Cu dispersion and CO2 uptake [36].
On this account, we think that CeOx phase can act modifying the red-ox properties of the catalyst and improve the surface chemisorption-activation process of CO2, which remains, actually, one of the main factors determining the catalytic performance of copper-based catalysts [13]. In particular, the process of chemisorption of CO2 is affected by several factors such as the presence of basic sites and defects on the surface of the catalytic active elements [12,35]. In regard of this, a systematic study on the influence and effect of CeO2 content on physical-chemical property of CuZnO-based catalysts and, consequently, on the catalysts behavior into CO2 hydrogenation processes have not still reported, being almost unclear the role of CeO2 loading. Therefore, in this study a series of CuZnO-CeO2 catalysts, with different CuO/CeO2 weight ratio (i.e. 0.2-1.2) and a constant ZnO loading (ca. 15 wt.%), were prepared and tested in the CO2 hydrogenation reactions at 20 bar and 200-300 °C, aiming to go insight on the activity and selectivity pathways and to shed light on the structure-activity relationships of CuZnCeO2 catalytic formulations.

Catalysts
A series of CuZnO-CeO2 catalysts, with different CuO/CeO2 weight ratio (i.e. 0.2-1.2) and a constant ZnO loading (ca. 15 wt.%), were prepared by reverse co-precipitation under ultrasounds irradiation route, which represent a synthesis method successfully employed since 2007, even from an initial bench scale of grams up to some kilograms, always proving high reproducibility, accuracy and stability in the synthesis of different Cu-based catalytic materials, as confirmed by our previous studies [17,20,33]. Accordingly, an aqueous solution (ca. 100 mL) of Cu(NO3)2•3H2O (Sigma-Aldrich, assay 99-104%), Zn(NO3)2•6H2O (Sigma-Aldrich, assay ≥99.0%) and (NH4)2Ce(NO3)6 (Sigma-Aldrich, assay ≥98.5%) precursors was added drop-wise to a 1 M KHCO3 (Sigma-Aldrich, assay 99.7%) solution (500 mL) under vigorous stirring and ultrasound irradiation, keeping pH constant at 8.0 by addition of 1M KHCO3 solution. After precipitation, the solid was kept for 30 minutes under stirring and ultrasounds irradiation. Then, the solid was aged at 30 °C for 2 h, filtered and washed with hot distilled water. Thereafter, the catalysts were dried at 130 °C overnight and further calcined in air at 400 °C for 4 h. After preparation, all catalysts were doped with potassium solution by impregnation for achieving 1.0 wt.% of K2O. Powdered catalysts were pressed (10 ton•cm -2 ) and then crushed and sieved to the particle size fraction (16-20 mesh) used for both characterization and testing measurements. Then, in order to further assess the accuracy and preciseness of both synthesis procedure and analytical methods, each batch of prepared catalyst (ca. 500 g each) was studied in double blind tests. Table 1 shows the list of catalysts, also reporting the chemical-physical properties.

Catalyst Characterization
Surface area (SA), pore volume (PV), and average pore diameter (APD) were obtained from nitrogen adsorption / desorption isotherms (77 K), using a fully automated gas adsorption device (ASAP 2020, Micromeritics Instrument). Physical adsorption isotherms were elaborated by standard BET and BJH methods for SA and PV evaluation, respectively. Before measurements, the samples were out-gassed at 150 °C under vacuum (2 h).
Temperature Programmed Reduction (TPR) analysis was carried out to settle a proper catalyst activation protocol. The TPR measurements were performed in the range 25-700 °C by the use of a semi-automatized laboratory micro-plant equipped with a linear quartz microreactor (i.d. 4 mm) fed with a 5% H2/Ar mixture flowing at 60 stp cm 3 ·min -1 and heated at the rate of 10 °Cmin -1 . The H2 consumption was monitored through a Thermal-Conductivity Detector (TCD), calibrated with CuO standard. Under such conditions, TPR was reliable and accurate in terms of both peak position (±5 °C) and hydrogen consumption (±5 %).
Metal Surface Area (MSA) values of catalysts were obtained in the same apparatus by single-pulse N2O titrations, assuming a Cu:N2O=2 titration stoichiometry [37]. MSA values were calculated according to procedure elsewhere described [17]. Before measurements, catalysts were reduced in-situ for 1 h at 400 °C under H2 flow. After reduction, the samples were flushed in the He carrier flow at 410 °C for 15 min, and then cooled to 90 °C for the measurement.
X-ray Diffraction analysis (XRD) of powder samples in the 2θ range of 5-80° were performed by a Philips X-Pert diffractometer operating with Ni -filtered Cu-K radiation at 40 kV and 30 mA, at a scan rate of 0.5°·min -1 [14,17]. X-ray fluorescence analysis (XRF) of catalyst was performed with a S8-Tiger WDXRF spectrometer (Bruker-AXS). To calculate the chemical composition of catalysts, K1 core-emissions of all chemical elements were taken for the quantitative analysis. The X-ray Photoelectron Spectroscopy (XPS) of samples was performed to value the chemical composition of the catalysts surface. Spectra and quantitative data were obtained using a spectrometer (GMBH PHI 5800-01, Physical Electronics) operating with a monochromatized Al-K radiation with a power beam of 300 W. The pass energy for the analysis of concentration and oxidation state of the surface atoms was 58 and 11 eV, respectively. The BE regions of C1s (280-300 eV), Cu2p (920-968 eV), Zn2p (1010-1060 eV), Ce3d (870-935 eV), and O1s (525-535 eV) were investigated, taking the C1s line (284.8 eV) of adventitious carbon as reference [33,34,[38][39][40][41][42] where (Areapeakstandard,in/Areapeakstandard,out) is introduced in Equation 1 to normalize the response of the analyte (i.e. H2 or CO2) to the response of the internal standard (i.e. N2), improving the accuracy of quantitative GC-TCD analysis, as proved by good agreement between IS and MS methods (±5 %) and a reproducibility better than 95%. Meanwhile, selectivity of product i (Si, %) was calculated by GC-FID analysis, according to the following formula (Equation 3): (3) According to the use of both IS and MS calculation method, reaching the steady state condition, the reported value of selectivity agrees with that valued on the basis of the amount of formed i product with respect to the converted hydrogen (± 2%), as further proof of the accuracy of the analytical method. The absence of diffusional resistance and the kinetic regime of the catalytic measurements were proved according to the Weisz-Prater (NW-P < 1.0) criterion, verifying NW-P val-ues on the order of 10 -4 -10 -5 , (Equation 4): (4)

Structure and Physical-chemical Properties
Reflecting the synthesis of the catalysts with a different formulation, the chemical analysis of the bulk agrees with the expected composition, proving the effectiveness of the preparation method, which also allows obtaining meso-pore structures with enhanced textural properties. Indeed, the catalysts exhibit total surface area (SA) which ranges from 126 to 181 m 2 •gcat -1 with a similar average pore diameter (APD) of 16-18 nm (Table 1). Then, according to catalyst formulation, the SA has a parabolic tendency with the copper-to-cerium ratio, reaching the greatest extension at the middle CuO content (i.e. C2Z1-C catalyst), with the most favorable 0.5 copper-to-cerium ratio. Not depending on preparation method, the similar SA value of C1Z1-C (126 m 2 •gcat -1 ) and C3Z1-C (129 m 2 •gcat -1 ) catalysts clearly diagnostics the occurrence of a synergistic structural effect between cerium and copper oxides (Table 1).
Apart from minor differences, the XRD analysis reveals that all catalysts are characterized by very similar diffraction patterns, showing a prevalently crystalline catalyst architecture ( Figure 1). According to the chemical composition, the XRD patterns of all systems ( ) 100 peak standard,in peak R,in peak R,out peak standard,out   (Figure 1). Then, TPR analysis proves similar reduction behavior (Figure 2). In fact, the reduction pattern of all catalysts is characterized by two reaction zones, placed at low (LT; 100-270 °C) and high (HT; 300-670 °C) temperature, due to copper (Equations 5-8) and surface ceria (Equation 9) reduction, respectively.
Reduction of ionic dispersed Cu species and/or of coordinative-unsaturated species (CUS): Reduction of isolated copper oxides: 2CuO + H2 → Cu2O + H2O (6) Cu2O + H2 → 2Cu° + H2O Reduction of copper in strong interaction with ZnO and CeO2 phase: CuO-MO + H2 → Cu° + H2O Reduction of the ceria surface: 2CeO2 + H2 → Ce2O3 + H2O (9) In particular, the reduction of Cu oxides occurs at temperatures below 290 °C, which is typical of bulk CuO [17], agreeing with the reduction of ionic dispersed copper and CUS (trange of 150-180 °C), isolated CuO and copper clusters in strong interaction with ceria (trange of 200-220 °C) [14,17,33], as shown in Figure 2. In addition, the TPR profile of C1Z1-C catalyst in the LT range reports the presence of an asymmetric peak with a TM1 maximum at ca. 210 °C and a shoulder at 190 °C, while the reduction pattern of the high CuO loading catalysts results in two better resolved peaks. Indeed, with equivalent area, the TPR profile of C2Z1-C catalyst revels two convoluted peaks with maxima at 180 °C and 215 °C, respectively. Then, C3Z1-Csample shows a growth of the reduction component at lower temperature, with a peak maximum centered at 178 °C, mirroring an increase of isolated CuO phase, as a consequence of higher loading and minor Cu dispersion [14,17].
The chemical analysis of catalyst surface (ESCA) revels an evident enrichment of Cu and Ce atoms at the surface of the catalyst with respect to the bulk (Figure 3), highlighting the elevate grade of dispersion reached by Cu and Ce species on the catalyst surface through the preparation method [14,15]. In addition, the higher surface concentration of Cu and Ce ions compared to Zn, prefigures the encapsulation of the ZnO phase into a CuO-CeO2 matrix, also justifying a remarkable chemical affinity between copper and cerium oxide species [18,43,44].
Then, Figure 4 displays the core level spectra of O1s, Cu2p, Zn2p and Ce3d atoms. Apart from minor differences, all catalysts display similar features. Namely, Cu2p spectra display 2p1/2 (933.0-934.3eV), 2p3/2 (953.0-953.4 eV)  transitions, and shake-up lines characteristic of oxidized copper [45][46][47]. According to a higher binding energy of Cu2p3/2 (934.3 eV) and O1s (531.3 eV), Cu on the surface of C2Z1-C catalyst prefigures a higher oxidation state, as further confirmed by the evident asymmetry of O1s peak, pointing out the existence of different oxide species [45]. Indeed, the shape of O1s spectra is diagnostic of the presence of various kinds of oxygen species. In particular, the deconvolution analysis well distinguishes the presence of two O1s peaks centered at about 529.9eV (OI) and 531.6eV (OII), according to Gaussian-Lorentzian convolution shape (after Shirley-background subtraction), accounting for lattice oxygen (OI) and molecular oxygen (weakly bonded) and/or adsorbed species such as carbonate and hydroxyl groups (OII) [48][49][50][51][52][53][54], as summarized in Table 2. Furthermore, the higher energy position of O1s peaks of C2Z1-C catalyst also prefigures electronic effects of the surrounding ions affecting the electron density, perhaps due to incorporation of Cu n+ ions into ceria lattice (i.e. solid solutions). Therefore, Zn2p spectra does not reveal any significant difference in term of binding energy. In turn,  the spectra of Ce3d is rather complex due to the evident contribution of six transitions relative to Ce 4+ and four hidden components of Ce 3+ ions [40,55,56]. Taking the energy-transition at 916.3 eV as characteristic of Ce 4+ [40,57], the XPS analysis signals a rise of the Average Oxidation Number (AON) of cerium from 3.5 (i.e. C2Z1-C) to 3.8-3.9 (i.e. C1Z1-C and C3Z1-C, respectively), as proof of the greater content of Ce 3+ ions on the surface of C2Z1-C catalyst (Figure 4).

Results of the Catalytic Tests
The results of catalytic tests in the CO2 hydrogenation are summarized in Figure 5A-C and in Table 3 in terms of H2 and CO2 conversion (XH2/CO2), methanol selectivity (SCH3OH), methanol yield (YCH3OH) and normalized reaction rate (rate). Denoting a similar catalytic pathway, all systems report a progressive increase of the activity with the rise of reaction temperature from 200 °C to 300 °C. However, higher temperature of reaction significantly favors the formation of CO, leading to a notable drop in the methanol selectivity ( Figure 5A-C and Table 3).
Thus, the catalysts performance clearly demonstrates the close relationship between chemical composition and catalytic activity. Indeed, the conversion of hydrogen proceeds according to a volcano shaped trend with the growth of the CuO/CeO2 ratio, as shown in Figure 5A. Namely, regardless of reaction temperature, the increase of CuO loading in the catalyst from 15% to 30% (associated to a consequent decrement of CeO2 from ca 56% to 40%), drives to a considerable increase of hydrogenation activity at any temperature.  Table 3. Catalysts behavior at diverse temperatures: hydrogen and CO2 conversion (XH2/CO2, %), methanol selectivity (SCH3OH, %), and normalized reaction rate (rate, mol(H2)·gcat -1 .s -1 ). gen slowly goes down ( Figure 5A) due to the loss of dispersion, as proved by decreasing of MSA and SA with the Cu loading. Indeed, the MSACu-to-Cu ratio decreases from ca. 2.0 of C1Z1-C and C2Z2-C catalyst to 1.0 of C3Z1-C sample, who also shows a surface concentration of Cu comparable to that of bulk, Figure 3. Then, the selectivity to methanol follows an almost exponential decay with the rise of the CuO/CeO2 ratio, pointing out a greater promoting effect towards the Reverse Water Gas Shift (RWGS) reaction, also favored by the increase of temperature ( Figure 5B). As result of the enhanced structural and chemical properties, C2Z1-C catalyst obtains the best catalytic performance at any temperature, reaching the highest methanol yield value at 300 °C (more than 12%, Figure 5C). Despite of the lesser activity, C3Z1-C catalyst displays a catalytic behavior not so far from that of C2Z1-C sample. In particular, this system shows a progressive rise of methanol formation with temperature up to 250 °C (5.5% of yield), followed by a slight decrease in the methanol synthesis performance consequent with the increasing temperature (5.2% of yield at 300 °C), reflecting the drop in the methanol selectivity ( Figure 5C). Then, mirroring the lowest content of active phase (Table 1), methanol yield grows almost exponentially with the rise of temperature on C1Z1-C catalyst, reaching 5.6% at 300 °C, according to the higher selectivity.
Therefore, Table 4 compares the performances of C1Z1-C, C2Z1-C and C3Z1-C catalysts with those other Cu-CeO2 based catalysts reported in the literature, at similar reaction conditions [23,36,45,58,59], although, the influence of the loading of active CuO elements on the catalytic properties and structure-activity relationships of CuZnCeO2 ternary systems has not been currently systematically analyzed.
The values of methanol productivity reported are comparable, if not superior, with those referred to in the literatures [36,45,59]. In fact, only few works have discussed comparable activity and, when greater than C2Z1-C catalyst (i.e. methanol formation rate of 1.40 mol•gcat -1 ·s -1 @ 250 °C and 20 bar), the catalytic performance was almost always referred to catalytic results carried out at higher value of pressure (i.e. 30-50 bar), temperature (>250 °C) or H2-to-CO2 ratio (>3). In particular, Shi et al. [36] recently synthesized a series of CuO-ZnO-CeO2 and CuO-ZnO-CeO2/TNTs composite catalysts, attesting an improvement in methanol productivity from 1.39 to 2.64 mol•gcat -1 ·s -1 only after TNTs addition, as improver of Cu dispersion and CO2 adsorption capacity. Similarly, the lower rate of 1.24 mol•gcat -1 ·s -1 was obtained by Angelo and co-workers at 260 °C and applying very higher pressure (i.e. 50 bar) [59] on of CuO-ZnO-CeO2 based catalysts prepared by conventional co-precipitation. An improvement of productivities were reached when ZrO2 and Al2O3 were introduced in the catalytic formulation in combination with ceria, further evidencing that an optimal balance between the various oxides is a key factor for obtaining materials with enhanced chemical and catalytic properties [23,59]. Thus, the synthesis of methanol and hydrocarbons proceeds through CO2 activation, which runs with CO2 chemisorption on catalyst surface, representing a crucial issue of the process. Despite CO2 is a kinetically stable molecule, the chemisorption process can be  improved through several factors, such as: a larger exposure of the surface area, surface defects, and a great presence of basic sites. On this regards, all catalysts were promoted in the alkaline character by K2O adding (1 wt.%).

Structure-Activity Relationship of CuO-ZnO -CeO2 Systems
The results of the physical-chemical analysis prove that a high copper loading leads to the sintering of the catalytic surface, also affecting the morphology of the composite materials. Then, the synergism occurring between the oxides of copper and cerium seems to be the key-factor controlling both physical-chemical properties and catalytic behavior of catalyst. With regard to the structural properties (i.e. SA and MSA), this study stresses the existence of an optimal content of copper with respect to cerium, which also affects the catalytic performance of the catalyst. Indeed, the CuO-to-CeO2 weight ratio of about 0.5 accounts for the optimal chemical composition, according to the highest exposure of the surface of C2Z1-C catalyst (SA = 181 m 2 •gcat -1 ; MSA = 45 m 2 •gcat -1 , Table 1). Then, the quantitative results of XPS deconvolution analysis clearly give evidence of the fact that the catalysts which achieve the better catalytic performance also diagnostic the larger number of oxygen vacancies (Table 2). Indeed, the higher value of Area(OII)/Area(OI+OII) ratio reported by C2Z1-C catalyst proves the larger extension of oxygen vacancies. In particular, C2Z1-C catalyst shows the highest 0.6 value in term of Area(OII)/Area(OI+OII) ratio, also reflecting the lowest A.O.N. of cerium on the surface.
Independently to CuO content, the progressive increase of H2 conversion with the increase in temperature is associated to a significant drop in selectivity. From one side, the higher temperature favors the reaction kinetics of hydrogenation and the catalyst activity, prompting methanol synthesis and RWGS reactions. On the other side, the increase of temperature hinders catalyst selectivity to methanol. Because of the opposite effect of temperature, the catalysts report the maximum methanol productivity within 300 °C ( Figure 5C). In particular, the best catalytic performance has been achieved by C2Z1-C catalyst at 300 °C, both in term of hydrogenation rate and methanol productivity, reflecting the highest exposure of the catalytic surface and prefiguring the existence of structure-activity relationships. Then, Figure 6 shows the tendency of methanol selectivity as a function of hydrogen conversion. Irrespectively to copper content, C1Z1-C and C3Z1-C catalysts follow an identical reaction profile, as proof of same reaction mechanism occurring on the surface of the two catalysts. Diversely, the trend of C2Z1-C catalyst diverts significantly from that of the other systems, according to a more favorable catalytic pathway, which could prove a different reaction mechanism. Hence, these findings suggest the existence of several factors governing the methanol synthesis, in addition to those strictly related to the hydrogenation functionality of catalyst.
As extensively reported in literature [12,13,[60][61][62][63][64][65], the methanol synthesis via the catalytic CO2 hydrogenation occurs through different consecutive reactive stages. According to a dual site mechanism, CO2 is reactively chemisorbed on Cu + sites, while H2 is exclusively chemisorbed on the metallic surface of copper (Cu° sites) and further dissociated. Thus, the chemisorbed CO2 species and their intermediates are hydrogenated by atomic hydrogen. On this address, the stereochemistry and the chemisorption of the reagents influence the product selectivity. In particular, the CO2 chemisorbed can assume two different geometries and, among them, the bridge-formate intermediate leads more easily to methanol, also favored by the lower temperature [35,66]. In our previous works [14,15,17,18,20,33,34], we have clarified the role explicated by several promoters as the oxide carriers. In particular, we found that the use of several oxides (i.e. as ZnO, ZrO2, CeO2, Al2O3, Gd2O3, Ce2O3, Y2O3) and alkaline metals (i.e. Li, Cs, K) could remarkably influence both catalyst structure and copper AON, balancing of the amount of the di- Figure 6. Catalytic results: conversion of hydrogen (%) versus Selectivity to methanol (%). verse copper species (i.e. Cu°/Cu + /Cu 2+ ) and leading to a notable improvement of the catalytic performance. In particular, we have demonstrated that ZnO and ZrO2 act as both structural promoters (favoring the development of the metal surface area and hindering the sintering of Cu° clusters) and electronic promoters (stabilizing the unstable Cu + sites through the formation of oxygen bridges Cu + -O-M). Hence, in agreement with the dual site mechanism, we have also shed light on the nature of structureactivity relationships in terms of interfacial area [17,18,20]. The interfacial area represents the contact surface between the Cu° cluster and the oxide phase, reflecting the equilibrium of Cu° and Cu + sites involved in the methanol synthesis. Affecting both the chemisorption process and the catalyst surface coverage, the extension of interfacial area can play a fundamental role in the methanol synthesis [30]. Indeed, irrespectively to the reaction kinetics order, the methanol formation rate is directly proportional to the partial pressure of H2 and CO2. Being the concentration of CO2 and H2 chemisorbed proportional to the number of active sites, it can be assumed that the reaction rate is directly related to the value of interfacial area, as follows: Hence, an excessive presence of one reagent with respect to the other can be a limit for the reaction kinetics; for instance, a too large exposure of the metallic surface can drive to an excessive hydrogen coverage, which can negative-ly affect the CO2 chemisorption and the reaction rate. In addition, the existence of Cu° sites is in direct competition with that of Cu + centers, due to the occurring of both reduction and oxidation process during the catalytic cycle. On this address, the role played by the interfacial area appears even more clear by plotting reaction rate (molCH3OH•gcat -1 ·s -1 ) and turn-overfrequency number, esteemed on Cu sites calculated by N2O titrations (TOF; s -1 ), as a function of  parameter (i.e.  = MSA/SA), taken as an index of the extension of the interfacial area [18].
In particular, the reaction rate closely follows the profile of a typical volcano-shaped relationship, with the maximum value reached at about 0.25  (i.e. C2Z1-C catalyst), as shown in Figure 7A. On this route, the higher catalytic performance of C2Z1-C catalyst would reflect the occurring of a greater synergism between copper and cerium oxides, driving to a more favorable combination of Cu° and Cu + active sites, as well as it would mirror a better structural and electronic effects played by the combination of ZnO and CeO2 promoters. As further proof of the dual site mechanism, the Fig.  7B shows similar relationships with TOF at the different temperatures. In particular, moving from 200 °C to 300 °C, the peak maximum in TOF shift from  values of ca. 0.255 to 0.233, reflecting a different kinetic dependence of Cu° and Cu + sites on temperature. In particular, the higher temperature seems to favor catalyst with a greater number of Cu + active sites, which can be explained by the greater energy required for the formation of bidentate formate species on Cu + active sites. In addition, catalyst reduction and surface dewatering are thermodynamically favored by the higher temperature, while the oxidation processes are disadvantaged by temperature, due to the exothermic nature of the process. On the other hand, a greater reduction of cerium oxide can contribute to stabilize Cu + sites. In addition, the methanol formation might involve highly dispersed Cu ions across the surface and Cu 0 -O-Cu +1 species in interaction with CeO2 phase, which are strongly hindered by the presence of water on the catalyst surface [16].

Conclusions
The influence and effect of CeO2 content on physical-chemical and catalytic properties into CO2 hydrogenation processes of CuZnCeO2 catalysts has been assessed. The results diagnostic the occurrence of synergistic structuralelectronic effects of cerium oxide on copper activity, with an optimal 0.5 copper-to-cerium ratio. Indeed, C2Z1-C catalyst with CuO-to-CeO2 ratio of 0.5 reports the highest surface exposure (SA = 181 m 2 •gcat -1 ; MSA = 45 m 2 •gcat -1 ), even obtaining the highest number of oxygen vacancies. As result of the enhanced structural and chemical properties, C2Z1-C catalyst obtains the best catalytic performance at any temperature, reaching the highest methanol yield value of 12% at 300 °C. Therefore, all findings account for a mutual synergistic effect played by CuO-CeO2 oxides. In particular, the chemical affinity is at the basis of the synergic activity of copper and cerium, reflecting some electronic effects possibly due to incorporation of Cu ions into ceria. The copper-to-cerium ratio is the key-factor controlling physical-chemical properties and catalytic behaviour of catalyst. The conversion of hydrogen proceeds according to a volcano shaped trend with the CuO-to-CeO2 ratio. Cerium oxide stabilizes Cu + sites, in particular when highly dispersed Cu 0 -O-Cu +1 species interact with CeO2 phase across interface. As proof of the dual site mechanism of methanol synthesis, the interfacial area extension remarkably affects reaction rates and methanol turn-over-frequency.
Properly, the results obtained in this study assess the practical employment of CuZnO-CeO2 catalytic compositions in the direct utilization of industrial streams containing CO2 and in the storage of the surplus of electrical energy through the conversion of H2 used as chemical-energy vector. This would allow to achieve both the full exploitation of renewable energy resources (e.g. wind and solar photovoltaic), through the development of integrated micro-generation energy systems, and the improvement of smart grid systems.