nanomaterials Article Conversion of Carbon Dioxide into Methanol Using Cu–Zn Nanostructured Materials as Catalysts Anna Carrasco García 1 , Javier Moral-Vico 1, * , Ahmad Abo Markeb 1,2 and Antoni Sánchez 1 1 Departament of Chemical, Biological and Environmental Engineering, Escola d’Enginyeria, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Barcelona, Spain; anna.carrasco@uab.cat (A.C.G.); a_markeb@aun.edu.eg (A.A.M.); antoni.sanchez@uab.cat (A.S.) 2 Departament of Chemistry, Faculty of Science, Assiut University, Assiut 71516, Egypt * Correspondence: antoniojavier.moral@uab.cat Abstract: Nowadays, there is a growing awareness of the great environmental impact caused by the enormous amounts of carbon dioxide emitted. Several alternatives exist to solve this problem, and one of them is the hydrogenation of carbon dioxide into methanol by using nanomaterials as catalysts. The aim of this alternative is to produce a value-added chemical, such as methanol, which is a cheaply available feedstock. The development of improved materials for this conversion reaction and a deeper study of the existing ones are important for obtaining higher efficiencies in terms of yield, conversion, and methanol selectivity, in addition to allowing milder reaction conditions in terms of pressure and temperature. In this work, the performance of copper, zinc, and zinc oxide nanoparticles in supported and unsupported bimetallic systems is evaluated in order to establish a comparison among the different materials according to their efficiency. For that, a packed bed reactor operating with a continuous gas flow is used. The obtained results indicate that the use of bimetallic   systems combined with porous supports, such as zeolite and activated carbon, is beneficial, thus Citation: García, A.C.; Moral-Vico, J.; improving the performance of unsupported materials by four times. Abo Markeb, A.; Sánchez, A. Conversion of Carbon Dioxide into Keywords: carbon dioxide hydrogenation; methanol; nanomaterials; heterogeneous catalysis Methanol Using Cu–Zn Nanostructured Materials as Catalysts. Nanomaterials 2022, 12, 999. https://doi.org/10.3390/ 1. Introduction nano12060999 Climate change stands as one of the greatest issues and challenges that humanity Academic Editors: Oana Gherasim faces, and emissions of greenhouse gases produced by the use of fossil fuels for energy and Alexandru Mihai Grumezescu production are identified as the main cause of global warming. Gases such as carbon Received: 26 February 2022 dioxide and methane are the most important representatives. The alarming increase in Accepted: 16 March 2022 atmospheric carbon dioxide concentration caused by anthropogenic emissions has become Published: 18 March 2022 a serious threat to the planet, causing global warming [1]. During 2018, 33 gigatonnes of carbon dioxide emitted into the atmosphere resulted in an increase in its concentration Publisher’s Note: MDPI stays neutral from 280 to 410 ppm [2]. To tackle this issue, two strategies have been proposed: carbon with regard to jurisdictional claims in capture and storage (CCS), whose main drawback is economical justifiability, since it is published maps and institutional affil- a non-profitable option [3], and carbon capture and utilisation (CCU) [2], which intends iations. to transform waste carbon dioxide into useful products, such as chemical feedstocks and renewable fuels. There currently exist several technologies that achieve the conversion of carbon dioxide into other value-added products, such as carbon dioxide hydrogenation, Copyright: © 2022 by the authors. electrochemical reduction, photochemical reduction, and photoelectrochemical reduction. Licensee MDPI, Basel, Switzerland. Specifically, this project focuses on the catalytic hydrogenation of carbon dioxide, since it is This article is an open access article considered as the most commonly used and simplest process [4]. Several chemical products distributed under the terms and can be obtained from carbon dioxide hydrogenation, among which methanol stands out conditions of the Creative Commons for two main reasons: its use as a renewable energy source and its utility as a feedstock for Attribution (CC BY) license (https:// obtaining other chemical products. As an energy source, methanol presents the advantage creativecommons.org/licenses/by/ of being liquid at environmental temperature, which makes it easier to transport and 4.0/). store. Moreover, 85% of methanol is used in the chemical industry as a solvent or as a Nanomaterials 2022, 12, 999. https://doi.org/10.3390/nano12060999 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2022, 12, 999 2 of 16 reagent for the synthesis of formaldehyde and acetic acid [4], which are valuable products in industrial terms. In the early 1920s, BASF successfully produced methanol from syngas during the ammonia synthesis process using the ZnO-Cr2 O3 catalyst at a temperature of 320–450 ◦ C and a pressure of 250–350 bar [5]. In the 1960s, ICI (Imperial Chemical Industries) developed a route for methanol synthesis in which syngas containing a high proportion of carbon dioxide reacted with highly selective copper and zinc oxide catalysts, resulting in the first commercially developed Cu/ZnO/Al2 O3 catalyst [6]. This material is currently the most commonly used for the conversion of carbon dioxide into methanol, which means that heterogeneous catalysts are preferable to homogeneous systems due to their high stability and easy use. The hydrogenation of carbon dioxide into methanol consists of two main competing reactions. The first is the synthesis of methanol from carbon dioxide and hydrogen [3]. CO2 + 3H2 → CH3 OH + H2 O ∆H0 298 K = −49.5 kJ/mol (1) The second reaction is the reverse water–gas shift reaction (RWGS), which produces carbon monoxide: CO2 + H2 → CO + H2 O ∆H0 298 K = 41.2 kJ/mol (2) Furthermore, catalytic hydrogenation of carbon dioxide into methanol can also occur indirectly from carbon monoxide formed through the RWGS reaction: CO + 2H2 → CH3 OH ∆H0 298 K = −90.6 kJ/mol (3) It is observed in Equations (1) and (3) that, according to Le Chatelier’s principle, the increase in the pressures and the decrease in the temperatures will shift the reaction towards the products to produce more methanol and not towards the production of carbon monoxide. Therefore, catalytic hydrogenation of carbon dioxide into methanol can occur directly or indirectly. Among the 92 natural elements present on Earth, copper is 25th most abundant ele- ment, which makes the use of copper in catalysis sustainable and economically favourable. For the synthesis of methanol through the hydrogenation of carbon monoxide and carbon dioxide, special emphasis is placed on copper metal acting as the active phase and zinc oxide as the active promoter. The high metal dispersion and stability of the commercial Cu/ZnO/Al2 O3 catalyst is due to the unique microstructure achieved after reduction, where the ZnO nanoparticles act as spacers between the Cu0 particles, and Al2 O3 enhances the thermal and chemical stability [6]. Thanks to the development of Cu-Zn-based active catalysts, the reaction pressure of methanol synthesis has been substantially reduced to 50–100 bar, while a satisfactory methanol selectivity of 50% can be achieved. Bimetallic catalysts have emerged as an important class of catalyst due to their unique properties that combine those of the constituent metals, permitting a change in the adsorption properties of metal surfaces, leading to improved catalytic yield [3]. Apart from the Cu-Zn-based system, other bimetallic systems, such as Pd-Zn, Pd-Ga [7], Cu-Ni, Ga-Ni [8], and other ones, have also been investigated. It was recently reported that methanol production is more favourable when using a Pd-Zn based catalyst. It was found that the Pd-Zn surface could act as an unusually high kinetic barrier (activation) for the RWGS reaction, thus effectively enhancing methanol production while reducing carbon monoxide levels under low-pressure conditions (20 bar). Furthermore, it was discovered that Ni-Ga system has a better catalytic activity and lower carbon monoxide production compared to the commer- cial Cu/ZnO/Al2 O3 catalyst [8]. Hence, it was observed how bimetallic catalytic systems could mitigate the issue of working at high pressures and the low selectivity towards methanol with respect to carbon monoxide, and among, them the Cu-Zn bimetallic system presents certain advantages in economic terms and is by far the most used. In addition to high pressure and low selectivity, another issue arises: the low catalytic yield. In order to Nanomaterials 2022, 12, 999 3 of 16 overcome this issue, several types of supports are used to increase the adsorption capacity of the active surfaces and to increase the surface area, with the final aim of increasing the catalytic activity. Capping agents are of utmost importance as stabilisers that inhibit the overgrowth of nanoparticles and prevent their aggregation [9]. The aim of using a capping agent is to obtain the smallest nanoparticles possible, which would lead to the highest area-per- volume rate. In this work, the capping agent used is polyvinylpyrrolidone (PVP) and sodium citrate. Among all the supports that have been investigated, this work is focused on zeolite, activated carbon, and polypyrrole nanotubes. Zeolite is a kind of microporous aluminosilicate with a three-dimensional structure based on SiO4 and AlO4 , and it is widely known for its catalytic and adsorption properties. Activated carbon is a highly porous material with a carbonaceous structure that is micro- and mesoporous, and it is widely used for adsorption purposes. Both materials have a high adsorption capacity for a wide variety of gas molecules, including carbon dioxide [10,11]. Polypyrrole nanotubes consist of carbon nanotubes containing nitrogen at elevated temperatures. The presence of nitrogen atoms in polypyrrole and its carbonised analogues positively affects their catalytic properties when they are used alone or as supports for noble-metal nanoparticles. The electron pair in nitrogen atoms participates in the adsorption/desorption of reactants/products that is needed for an efficient catalytic performance [12]. Although the main characteristic of polypyrrole nanotubes is their high conductive capacity, we intended to study this material because the direct application of bimetallic nanoparticles is difficult due to their high agglomeration tendency because of their nanometric size and the Van der Waals interactions between metals [13]. This work will focus on studying the catalytic yield of hydrogenation of carbon dioxide into methanol using a Cu-Zn-based catalyst with and without support. The aim of this work is to find the bimetallic catalyst with the best catalytic yield and selectivity under the mildest possible operating conditions of temperature and pressure. 2. Materials and Methods 2.1. Materials Zinc sulfate heptahydrate (ZnSO4 ·7H2 O) (99.0%), sodium hydroxide (NaOH) (98.0%), acetic acid (99.7%), copper nitrate trihydrate (Cu (NO3 )3 ·3H2 O) (99.0%), zinc nitrate hex- ahydrate (Zn (NO3 )2 ·6H2 O) (98.0%), trisodium citrate (99.0%), copper sulfate pentahydrate (CuSO4 ·5H2 O) (98.9%), ammonium hydroxide (NH4 OH) (28.0%), pyrrole (99.0%), iron (III) chloride hexahydrate (FeCl3 ·6H2 O) (98.0%), methyl orange (85.0%), ethanol (96.0%), sodium borohydride (NaBH4 ) (99.0%), potassium hydroxide (85.0%), 4-Bromo butyric acid (98.0%), dimethylformamide (99.8%), zeolite, activated carbon, and polyvinylpyrrolidone (PVP) were all purchased from Sigma-Aldrich (Barcelona, Spain). A mixed-gas bottle of carbon dioxide and hydrogen with a molar ratio of 1:3, respectively, was provided by Carburos Metálicos, S.A. (Barcelona, Spain). 2.2. Synthesis of Nanocomposites 2.2.1. ZnO Nanoparticle Synthesis ZnO nanoparticles were synthesised using the co-precipitation method [14]. The co-precipitation of the salts occurs by the addition of a precipitating agent, such as sodium hydroxide [15]. PVP is used to control the size of the nanoparticles and to prevent their agglomeration [9]. Briefly, in 120 mL of deionised water, the precursor salt, 7 g of zinc sulfate heptahydrate at a concentration of 0.2 M, was dissolved, and in another 120 mL of deionised water, the precipitating agent, NaOH, was dissolved at a concentration of 4 M. The synthesis was performed in a 500 mL Scharlau Minireactor HME-R/500 with mechanical stirring and heating. The 240 mL zinc sulfate heptahydrate solution in deionised water was added to the reactor. Then, the NaOH solution was added at a flow rate of 5 mL/min using a peristaltic pump (Watson Marlow SCI 400), and the reaction was carried out at 60 ◦ C for 2 h with constant stirring at 120 rpm [16]. The product was centrifuged (Beckman Avanti Nanomaterials 2022, 12, 999 4 of 16 J-20 Centrifuge) three times for 15 min at 5000 rpm to remove any impurities and dried in an oven at 105 ◦ C overnight. The synthesis of ZnO with PVP was performed with the same method, but 2 g of PVP was added to the precursor salt solution. 2.2.2. Cu Nanoparticle Synthesis Cu nanoparticles were synthesised using the chemical reduction method [17]. A total of 10 g of copper sulfate pentahydrate was dissolved in an Erlenmeyer flask with 100 mL of deionised water with magnetic stirring and nitrogen bubbling to avoid the oxidation of copper. After 30 min stirring, the reducing agent, sodium borohydride, was added. A total of 7.5 g of sodium borohydride was dissolved in 50 mL of deionised water and then added with a Pasteur pipette. Once all of the reducing agent was added, the solution was left to stir overnight to ensure that all of the copper had been reduced. Once the reaction was complete, it was centrifuged 3 times at 5000 rpm for 15 min. Finally, it was dried at 105 ◦ C overnight. 2.2.3. CuO/ZnO Bimetallic Nanoparticle Synthesis CuO/ZnO nanoparticles were synthesised using the co-precipitation method. In 100 mL of deionised water containing 1% v/v of acetic acid at room temperature, 6 g of copper nitrate trihydrate and 3 g of zinc nitrate hexahydrate were added, and then the pH was adjusted to a value of 7 by the addition of aqueous 1.0 M NH4 OH. Afterwards, the synthesis of these nanoparticles was carried out in a Scharlau Minireactor HME-R/500 with mechanical stirring, and the temperature was controlled at 60 ◦ C. The product was obtained after centrifugation and drying [18]. 2.2.4. Cu/ZnO Bimetallic Nanoparticle Synthesis Cu/ZnO nanoparticles were synthesised using the chemical reduction method. In 100 mL of deionised water, 1.4 g of previously synthesised ZnO was dispersed in an Erlenmeyer flask. Then, 0.01 g of sodium citrate was added and dissolved for 30 min with magnetic stirring and another 30 min in an ultrasonic bath (Ultrasons P. Selecta. 3000513) to complete the homogenisation of the mixture. When all of the ZnO was dissolved, nitrogen was bubbled into the solution for 10 min. In a beaker, 10 g of copper sulfate pentahydrate was dissolved in 100 mL of deionised water, and it was added to the ZnO solution. The solution was magnetically stirred for 1 h with nitrogen bubbling. After this, the solution was treated with the reducing agent sodium borohydride, of which 7.5 g was dissolved in 50 mL of deionised water that was previously cooled in a refrigerator at 4 ◦ C. Nitrogen bubbling was stopped and, using a Pasteur pipette, the sodium borohydride solution was added dropwise, and the copper was allowed to reduce overnight. The next day, it was centrifuged at 5000 rpm and dried at 105 ◦ C [19]. The synthesis of Cu/ZnO with PVP was performed by the same procedure, but instead of adding sodium citrate, 2 g of PVP was added. 2.2.5. Cu/ZnO/Zeolite Nanocomposite Synthesis The method for the Cu/ZnO/Zeolite (Cu/ZnO/Z) preparation consisted of first synthesising the nanoparticles (Cu/ZnO) and then immobilising them on zeolite. First, 6 g of zeolite was dispersed in 100 mL of deionised water and 1.4 g of previously synthesised ZnO was dispersed in 100 mL of deionised water. Both solutions were homogenised in an ultrasonic bath for 30 min. The ZnO solution was transferred to the zeolite solution and homogenised again for 30 min. Finally, the same procedure was followed for the Cu/ZnO synthesis [19]. 2.2.6. Cu/ZnO/Polypyrrole Nanotube Synthesis The synthesis of polypyrrole nanotubes with Cu/ZnO nanoparticles (Cu/ZnO/PN) was based on the previous synthesis of polypyrrole nanotubes followed by their corre- sponding surface functionalisation with carboxylic acid groups and cation exchange with Nanomaterials 2022, 12, 999 5 of 16 copper ions, followed by the reduction of the metal ions by a reducing agent. First, polypyr- role nanotubes were synthesised. This synthesis was based on the oxidation of pyrrole from a solution of iron (III) chloride in the presence of methyl orange. A total of 1.46 g of iron (III) chloride hexahydrate was dissolved in 90 mL of deionised water, and 0.29 g of methyl orange was dissolved in 90 mL of deionised water in another beaker. The iron (III) chloride solution was added dropwise to the methyl orange solution and allowed to stir at 150 rpm for 10 min. Then, 0.75 mL of pyrrole that was previously distilled using a rotary evaporator (Heidolph Hei-VAP Advantage) was added and left to stir at 150 rpm and at room tem- perature overnight. The day after, it was filtered (Albet LabScience) under vacuum using water first and then ethanol several times until the filtrate was colourless. The polypyrrole nanotubes were dried at 80 ◦ C for 24 h [13]. Then, the polypyrrole nanotubes were function- alised. A mixture containing 0.4 g of polypyrrole nanotubes, 3.15 g of potassium hydroxide, and 1.77 mL of 4-bromobutyric acid was dispersed in 400 mL of dimethylformamide and stirred at 120 rpm and at 50 ◦ C overnight. The product was washed with water and ethanol to remove potassium hydroxide and unreacted 4-bromobutyric acid, then dried at 80 ◦ C overnight. Once the polypyrrole nanotubes were functionalised, ZnO and Cu nanoparticles were added. For this purpose, ZnO nanoparticles were added first, followed by the copper and the sodium borohydride. In 250 mL of deionised water, 0.3 g of the functionalised polypyrrole nanotubes, 1.5 g of zinc nitrate hexahydrate, and 2 g of sodium hydroxide were mixed. This was stirred at room temperature for 2 h. Next, the mixture was centrifuged 3 times at 5000 rpm for 15 min and dried at 105 ◦ C overnight. Finally, the copper nanopar- ticles were added to the polypyrrole nanotubes with the ZnO nanoparticles by chemical reduction, wherein 0.3 g of polypyrrole nanotubes with ZnO, 2.5 g of PVP were dissolved in 150 mL of deionised water. Next, 6.5 g of copper nitrate trihydrate was added and stirred for 2 h. A total of 5.7 g of sodium borohydride was dissolved in 50 mL of deionised water and added afterwards to the solution dropwise with a Pasteur pipette. It was left to stir at 150 rpm and at room temperature overnight, centrifuged 4 times at 5000 rpm for 15 min, and dried in an oven at 105 ◦ C overnight [13]. 2.2.7. Cu/ZnO/Activated Carbon Synthesis The synthesis of Cu/ZnO/activated carbon (Cu/ZnO/AC) was carried out using the impregnation method, which was based on the prior synthesis of Cu/ZnO nanoparticles followed by their adsorption into the support. First, the Cu/ZnO synthesis was performed, and the nanoparticles were dispersed in water. Then, 2.9 g of activated carbon was added and stirred for 3 h. The mixture was centrifugated 5 times and dried [20]. 2.3. Characterisation of Catalysts X-ray diffraction (XRD) was used to perform a structural analysis of the nanoparticles and their crystallographic study [21]. The X-ray diffraction patterns were recorded in a diffractometer (PANalytical X’Pert) using Cu–Kα radiation. The measurements were made at room temperature at a range of 3.5◦ –85.0◦ on 2θ with a step size of 0.0167◦ . All XRD results were analysed with the simulation of the nanoparticles obtained using the Mercury software provided by the Cambridge Crystallographic Data Centre (CCDC), who developed the Mercury software, which provides the full Cambridge Structural Database (CSD). Mercury offers a comprehensive range of tools for 3D structure visualisation and the exploration of crystal packing. This software is able to load structural data from a variety of formats and provides an extensive array of options to aid the investigation and analysis of crystal structures [22]. A scanning electron microscopy (SEM) (FEI Quanta 650F ESEM) equipped with an energy-dispersive spectroscopy (EDS) source was used to determine the morphology, size distribution, and composition of the nanoparticles. Transmission electron microscopy (TEM) (FEI TECNAI G2 F20) was used to determine the microstructure of the samples and to analyse the size, morphology, and size distribution of the nanoparticles. Samples were prepared on copper and graphite grids (TED PELLA, Inc., Redding, CA, USA). Nanomaterials 2022, 12, 999 6 of 16 2.4. Catalytic Activity Test The catalytic activity of the hydrogenation of carbon dioxide into methanol was evaluated in a stainless-steel-packed bed reactor with an internal diameter of 5.25 mm, a height of 8.90 cm, and, thus, a volume of 1.92 cm3 . All samples were first calcinated at 350 ◦ C for 4 h, and those containing elemental copper were reduced by a hydrogen flow of 40 mL/min at 350 ◦ C for 2 h in order to ensure its reduction. A certain amount of catalyst was added and a small amount of glass wool was added at each end of the reactor to avoid possible leakage of the material itself. In order to investigate the impact of pressure, the work was carried out under two moderate pressure values: 10 and 15 bar. The flow rate of the stoichiometric H2 /CO2 mixture was 10 mL/min, and the tested temperatures were 180, 200, 220, 240, 260, and 280 ◦ C. The samples were collected using sampling bags (SKC FlexFoil PLUS Sample Bag), and the obtained methanol was analysed on a chromatograph (Shimadzu GC-2010) with a flame ionisation detector (FID) with helium as the carrier gas. The software used was Chromeleon, the inlet temperature was 260 ◦ C, and the flow was 50 mL/min; the detector temperature was 280 ◦ C and the flow was 35 mL/min. An Agilent 7890B GC System chromatograph served to detect carbon monoxide and dioxide by using a thermal conductivity detector (TCD) with helium as the carrier gas. The software used was OpenLab, the inlet temperature was 120 ◦ C, and the inlet flow was 20 mL/min; the detector temperature was 150 ◦ C, and the detector flow was 50 mL/min. For the study of the catalytic activity of each material, three parameters were calculated: space–time yield (STY), which indicates the mass of methanol obtained per mass of catalyst and time, conversion (%), and methanol selectivity, measured as the percentage of carbon dioxide that was converted into methanol rather than carbon monoxide. In the selectivity calculation, it was assumed that only methanol or carbon monoxide was produced. 3. Results and Discussion 3.1. Structural and Morphological Characterisation of Nanomaterials The XRD patterns obtained from monometallic nanomaterials and their corresponding simulations using the Mercury software are presented. In Figure 1, it is shown that the highest peaks of copper correspond to the diffraction angles of 43.4◦ , 50.5◦ , and 74.5◦ , which correspond to the (111), (200), and (220) planes. These highest peaks were also found in the literature [23]. Furthermore, it was observed that the obtained patterns for synthesised copper materials fit those obtained using the simulation. Regarding the zinc oxide with and without PVP, it was observed that PVP did not interfere with the crystallinity of zinc oxide, and the same peaks as in the simulation were obtained at all corresponding diffraction angles. The 2θ angles and their corresponding crystallographic planes were the following: 31.5◦ (100), 34.5◦ (002), 36.3◦ (101), 47.6◦ (102), 56.9◦ (110), 63.3◦ (103), and 68.0◦ (112). These highest peaks were also found in the literature [18]. The X-ray diffraction patterns of the bimetallic nanomaterials without support are shown in Figure 2. The diffraction peaks obtained for CuO/ZnO and their CuO simulation were at the same angles: 32.5◦ (110), 35.5◦ (002), 38.8◦ (111), 48.5◦ (202), and 61.5◦ (113). These highest peaks were also found in the literature [18]. However, it was observed that Cu/ZnO had two diffraction peaks at the angles of 35.6◦ (002) and 39.1◦ (111), which correspond to CuO. This indicates that the copper was not completely reduced. Small diffraction peaks were seen at the angles of 47.6◦ , 63.3◦ , and 68.0◦ , corresponding to zinc oxide. Hence, it can be suggested that the diffraction peaks obtained were very small due to the low amount of zinc oxide in the material. Figure 3 depicts the XRD patterns for bimetallic catalysts with support. First of all, the Cu/ZnO/Zeolite nanocomposite was analysed, observing that the diffraction peaks from 5◦ to 30◦ correspond to zeolite [24]. There were also two peaks that correspond to zinc oxide: 32.0◦ and 34.6◦ [E]. Regarding the two remaining composite catalysts, the Cu/ZnO/activated carbon and Cu/ZnO/polypyrrole nanotubes, it was observed that both materials presented the same XRD patterns with two large peaks at 35.7◦ and 38.9◦ , which correspond to copper oxide [18]. In addition, small peaks corresponding to zinc oxide Nanomaterials 2022, 12, 999 7 of 16 Nanomaterials 2022, 12, x FOR PEER REVIEW 7 of 16 were observed at 56.9◦ and 68.0◦ [18]. Polypyrrole nanotubes and activated carbon did not present XRD patterns due to their amorphous microstructures. Figure 1. XRD patterns of all catalysts: (A) Cu nanoparticle simulation; (B) Cu nanoparticles; (C) ZnO nanoparticle simulation; (D) ZnO nanoparticles with PVP; (E) ZnO nanoparticles. The X-ray diffraction patterns of the bimetallic nanomaterials without support are shown in Figure 2. The diffraction peaks obtained for CuO/ZnO and their CuO simulation were at the same angles: 32.5° (110), 35.5° (002), 38.8° (111), 48.5° (202), and 61.5° (113). These highest peaks were also found in the literature [18]. However, it was observed that Cu/ZnO had two diffraction peaks at the angles of 35.6° (002) and 39.1° (111), which cor- respond to CuO. This indicates that the copper was not completely reduced. Small dif- fraction peaks were seen at the angles of 47.6°, 63.3°, and 68.0°, corresponding to zinc ox- Figure Figure 1.1.XRD ide. Hence, XRD patterns itpatterns can be ofof allall suggested catalysts: that catalysts: (A) (A)the Cu nanoparticle Cudiffraction peaks nanoparticle simulation; obtained simulation; (B) were (B) Cu Cuvery nanoparticles; small(C) nanoparticles; due (C) ZnOto ZnO nanoparticle the low amount nanoparticle simulation; of zinc simulation; (D)oxide(D) ZnO in ZnO nanoparticles the material. nanoparticles with PVP; (E) ZnO nanoparticles. with PVP; (E) ZnO nanoparticles. The X-ray diffraction patterns of the bimetallic nanomaterials without support are shown in Figure 2. The diffraction peaks obtained for CuO/ZnO and their CuO simulation were at the same angles: 32.5° (110), 35.5° (002), 38.8° (111), 48.5° (202), and 61.5° (113). These highest peaks were also found in the literature [18]. However, it was observed that Cu/ZnO had two diffraction peaks at the angles of 35.6° (002) and 39.1° (111), which cor- respond to CuO. This indicates that the copper was not completely reduced. Small dif- fraction peaks were seen at the angles of 47.6°, 63.3°, and 68.0°, corresponding to zinc ox- ide. Hence, it can be suggested that the diffraction peaks obtained were very small due to the low amount of zinc oxide in the material. Figure2.2.XRD Figure XRDpatterns patternsofofall allcatalysts: catalysts:(A) (A)Cu Cunanoparticle nanoparticlesimulation; simulation;(B) (B)ZnO ZnO nanoparticle nanoparticle simu- simula- lation; (C) Cu/ZnO nanoparticles; (D) CuO nanoparticle simulation; (E) CuO/ZnO nanoparticles. tion; (C) Cu/ZnO nanoparticles; (D) CuO nanoparticle simulation; (E) CuO/ZnO nanoparticles. Figure The 3 depicts presence the XRD of copper patterns oxide formaterials in some bimetallic catalysts meant that with support. not all First ofwas of the copper all, the Cu/ZnO/Zeolite reduced. It is assumed nanocomposite to be for the was analysed, following two observing reasons: The thatoxygen the diffraction present inpeaks the from 5° was solution to 30° correspond not completelytodisplaced, zeolite [24]. or There it couldwere alsototwo be due thepeaks amount that ofcorrespond the reducing to zinc oxide: 32.0° agent not being enough.and 34.6° [E]. Regarding the two remaining composite catalysts, the Cu/ZnO/activated The morphology carbon of theand Cu/ZnO/polypyrrole materials was examined using nanotubes, it waselectron a scanning observed that both microscope (SEM) equipped with EDX. The morphology of the three supports used in this work is shown in Figure 4. The polypyrrole nanotubes (Figure 4A) presented an approximate Figure of length 2. XRD patterns 3–5 µm and of all catalysts: a diameter of (A) Cu nanoparticle 0.3–0.6 simulation; (B) µm [25]. Furthermore, ZnO observed it was nanoparticle simu- that all lation; of (C) Cu/ZnO the tubes nanoparticles; had similar (D) CuO nanoparticle sizes. Regarding the activatedsimulation; (E) CuO/ZnO carbon (Figure 4B), itnanoparticles. was observed that there were small and large granules, probably because the material was not ground FigureZeolite and sifted. 3 depicts had the XRD morphology a square patterns for bimetallic (Figure 4C),catalysts and the with sizes support. First of all, were homogeneous, the Cu/ZnO/Zeolite as expected [26]. nanocomposite was analysed, observing that the diffraction peaks from 5° to 30° correspond to zeolite [24]. There were also two peaks that correspond to zinc oxide: 32.0° and 34.6° [E]. Regarding the two remaining composite catalysts, the Cu/ZnO/activated carbon and Cu/ZnO/polypyrrole nanotubes, it was observed that both Nanomaterials 2022, 12, x FOR PEER REVIEW 8 of 16 materials presented the same XRD patterns with two large peaks at 35.7° and 38.9°, which Nanomaterials 2022, 12, 999 correspond to copper oxide [18]. In addition, small peaks corresponding to zinc oxide 8 of 16 were observed at 56.9° and 68.0° [18]. Polypyrrole nanotubes and activated carbon did not present XRD patterns due to their amorphous microstructures. Figure 3. XRD patterns of all catalysts: (A) zeolite simulation; (B) Cu nanoparticle simulation; (C) ZnO nanoparticle simulation; (D) Cu/ZnO/polypyrrole nanotubes; (E) Cu/ZnO/activated carbon; (F) Cu/ZnO/zeolite. The presence of copper oxide in some materials meant that not all of the copper was reduced. It is assumed to be for the following two reasons: The oxygen present in the solution was not completely displaced, or it could be due to the amount of the reducing agent not being enough. The morphology of the materials was examined using a scanning electron micro- scope (SEM) equipped with EDX. The morphology of the three supports used in this work is shown in Figure 4. The polypyrrole nanotubes (Figure 4A) presented an approximate length of 3–5 μm and a diameter of 0.3–0.6 μm [25]. Furthermore, it was observed that all of the tubes had similar sizes. Regarding the activated carbon (Figure 4B), it was observed thatFigure there3. Figure 3.were XRD small XRD patternsand patterns large ofofall all granules, catalysts: (A) catalysts: probably zeolite (A) because simulation; zeolite (B)(B) simulation; thenanoparticle Cu Cu material nanoparticle was not(C) simulation; ground simulation; and(C) sifted. ZnO Zeolite simulation; nanoparticle ZnO hadsimulation; nanoparticle a square morphology (D) (D) Cu/ZnO/polypyrrole (Figure Cu/ZnO/polypyrrole 4C), nanotubes; and the (E) sizes werecarbon; (E) Cu/ZnO/activated nanotubes; homogene- Cu/ZnO/activated (F) asCu/ZnO/zeolite. ous,carbon; expected [26]. (F) Cu/ZnO/zeolite. The presence of copper oxide in some materials meant that not all of the copper was reduced. It is assumed to be for the following two reasons: The oxygen present in the solution was not completely displaced, or it could be due to the amount of the reducing agent not being enough. The morphology of the materials was examined using a scanning electron micro- scope (SEM) equipped with EDX. The morphology of the three supports used in this work is shown in Figure 4. The polypyrrole nanotubes (Figure 4A) presented an approximate length of 3–5 μm and a diameter of 0.3–0.6 μm [25]. Furthermore, it was observed that all of the tubes had similar sizes. Regarding the activated carbon (Figure 4B), it was observed that there were small and large granules, probably because the material was not ground and sifted. Zeolite had a square morphology (Figure 4C), and the sizes were homogene- Figure ous,4.as Figure SEM images 4.expected SEM ofofthe [26]. images thesupports: (A)polypyrrole supports: (A) polypyrrole nanotubes; nanotubes; (B) activated (B) activated carbon; carbon; (C) zeolite. (C) zeolite. Figure Figure 5A5Adepicts depictsthe the granular granular morphology morphology of of thethe copper coppernanoparticles with homo- nanoparticles with homo- geneous dimensions and distribution. The diameter of the nanoparticles geneous dimensions and distribution. The diameter of the nanoparticles was between was between 0.27–0.40 µm, as in the literature [23]. Zinc oxide synthesised with and without PVP is 0.27–0.40 μm, as in the literature [23]. Zinc oxide synthesised with and without PVP is shown in Figure 5B–C, and it was observed that when the PVP was added, the nanopar- ticles obtained were smaller and had better distribution. Moreover, the geometry of the nanoparticles was more defined due to the presence of PVP, as observed in Figure 5B, in which the morphology of the nanoparticles obtained presented a needle shape with larger dimensions than those of the ZnO nanoparticles with PVP (Figure 5C). Figure 5D depicts the granular morphology of the CuO/ZnO nanoparticles with ho- mogeneous dimensions and distribution. The diameter of the nanoparticles was between Figure 4. SEM images of the supports: (A) polypyrrole nanotubes; (B) activated carbon; (C) zeolite. 0.13–0.26 µm, as in the literature [18]. As for the bimetallic nanomaterials, the Cu/ZnO nanoparticles were granular with no observed agglomeration. The comparison of the use Figure 5A depicts the granular morphology of the copper nanoparticles with homo- of PVP in the bimetallic catalyst was also performed for ZnO. In this case, the morphology geneous dimensions and distribution. The diameter of the nanoparticles was between obtained during the synthesis when using PVP was not well defined, but it presented a 0.27–0.40 μm, as in the literature [23]. Zinc oxide synthesised with and without PVP is tendency towards agglomeration instead. This may have occurred because the copper nanoparticles were wrapped in PVP, which reduced their interaction with ZnO. Neverthe- less, when PVP was not added, it was observed how nanoparticles were granular in some areas, and furthermore, the interaction between both metals was stronger. Regarding the images of the materials with supports, the Cu/ZnO nanoparticles– polypyrrole nanotubes are shown in Figure 5G, with the nanotubes indicated by an arrow. It of PVP in the bimetallic catalyst was also performed for ZnO. In this case, the morphology obtained during the synthesis when using PVP was not well defined, but it presented a tendency towards agglomeration instead. This may have occurred because the copper na- noparticles were wrapped in PVP, which reduced their interaction with ZnO. Neverthe- less, when PVP was not added, it was observed how nanoparticles were granular in some Nanomaterials 2022, 12, 999 areas, and furthermore, the interaction between both metals was stronger. 9 of 16 Regarding the images of the materials with supports, the Cu/ZnO nanoparticles– polypyrrole nanotubes are shown in Figure 5G, with the nanotubes indicated by an arrow. It was was observed observed thatthat incorporating incorporating a support a support hadhad the advantage the advantage of reducing of reducing the agglom- the agglomeration eration and and improving improving the distribution the distribution of theofnanoparticles. the nanoparticles. Figure Figure 5H5H showsthe shows thesuccessful success- ful impregnation impregnation of Cu/ZnO of Cu/ZnO nanoparticles nanoparticles on on thethe porous porous structure structure of activated of activated carbon. carbon. The The circumferences circumferences indicate indicate thethe activated activated carbonand carbon andthe thetriangle triangle indicates indicates the thebimetallic bimetallic nanoparticles. The nanoparticles. The enhancement enhancement of ofthe thenanomaterials nanomaterialsininterms termsofofnanoparticle nanoparticleaggregation aggrega- tion distribution and and distribution when when a support a support waswas was used used was confirmed finally finally confirmed by Figure by5I,Figure 5I, in in which the which the nanocomposite of Cu/ZnO/zeolite is shown. This was translated nanocomposite of Cu/ZnO/zeolite is shown. This was translated into an increased specific into an in- creased which surface, specificmeans surface, thatwhich means the active that sites of the the active catalystsites wereofmore the catalyst exposed,were more ex- increasing the posed, increasing catalytic activity. the catalytic activity. Figure 5. SEM images of the catalysts: (A) Cu nanoparticles; (B) ZnO nanoparticles; (C) ZnO nanoparti- cles with PVP; (D) CuO/ZnO bimetallic nanoparticles; (E) Cu/ZnO bimetallic nanoparticles with PVP; (F) Cu/ZnO bimetallic nanoparticles; (G) Cu/ZnO/polypyrrole nanotubes; (H) Cu/ZnO/activated carbon; (I) Cu/ZnO/zeolite. The arrow indicates the polypyrrole nanotubes, the triangle indicates the bimetallic nanoparticles, and the circumferences indicate the activated carbon. The composition of the chemical elements present in the catalysts was analysed using energy-dispersive X-ray spectroscopy (EDX). This analysis was performed on the two samples with porous supports: zeolite and activated carbon. It was determined that the catalyst with zeolite (Table 1) had the expected chemical elements, the two metals of the nanoparticles (Cu and Zn), and the two typical elements of zeolite (Al and Si). Oxygen was present in both the nanoparticles and the support. Regarding the EDX data of the activated carbon (Table 2), all present elements were those that were expected. There was a non-significant amount of sodium remaining due to the use of sodium borohydride during the synthesis of the material. Nanomaterials 2022, 12, 999 10 of 16 Table 1. Element quantification with EDX for the Cu/ZnO/zeolite material. Element Weight (%) Atomic (%) O 46.93 70.51 Al 9.67 8.61 Si 9.60 8.21 Cu 21.84 8.26 Zn 11.98 4.40 Table 2. Element quantification with EDX for the Cu/ZnO/activated carbon material. Element Weight (%) Atomic (%) O 20.90 21.05 C 53.94 72.39 Na 0.51 0.36 Cu 16.97 4.31 Zn 7.68 1.89 To clarify some of the data obtained regarding the catalytic efficiency of the synthesised materials, transmission electron microscopy (TEM) was used. The first sample analysed was Cu/ZnO with PVP due to its poor results in terms of catalysis, despite the addition of a stabilising agent. As expected, in the Figure 6A some aggregates of these nanoparticles were observed, and an unsatisfactory homogenisation of the material was seen. The Cu/ZnO with PVP nanoparticles obtained in this synthesis was bigger compared to that in the other images. As mentioned above, SEM images did not distinguish the nanoparticles and the support well in the Cu/ZnO samples with polypyrrole nanotubes and zeolite. Therefore, the TEM images will provide more information about the distribution of nanoparticles and the support. The Cu/ZnO/PN was the second catalyst to confirm whether the synthesis was successful and whether the nanotubes fulfilled their main function of avoiding the agglomeration of these nanoparticles. Figure 6B,C depict that there was no agglomeration indeed, and it was observed that they were smaller than those obtained in the previous catalysts. The last sample was Cu/ZnO/zeolite, which was the catalyst with the best performance. Nanomaterials 2022, 12, x FOR PEER REVIEW In this sample, the distribution of the chemical compounds was homogeneous, 11 of 16 and the size of these nanoparticles was smaller, giving rise to an increased specific surface, which fit the catalysis results. Figure 6. TEM Figure 6. TEMimages. images. (A) Cu/ZnO bimetallic (A) Cu/ZnO bimetallic nanoparticles nanoparticles withwith PVP;PVP; (B,C) Cu/ZnO/polypyrrole (B,C) Cu/ZnO/polypyrrole nanotubes; (D) nanotubes; Cu/ZnO/zeolite. (D) Cu/ZnO/zeolite.The Thearrows indicatethe arrows indicate thepolypyrrole polypyrrole nanotubes. nanotubes. 3.2.3.2. Catalytic Catalytic ActivityofofNanomaterials Activity Nanomaterials The space–time yield (STY) evaluated the amount of methanol produced per mass The space–time yield (STY) evaluated the amount of methanol produced per mass unit of catalyst and time. It was determined for each catalyst used at different temperatures, unitasof catalyst shown and time. in Figure It was the 7. In general, determined productionfor each catalyst of methanol usedwith increased at different tempera- temperature. tures, as shown in Figure 7. In general, the production of methanol increased with tem- perature. Nevertheless, for some materials, it was observed that the same values were obtained for the STY at 260 and at 280 °C; even in the case of Cu/ZnO/PN, a decrease in the STY was observed in this range. This could be related to the selectivity of the reaction, Nanomaterials 2022, 12, 999 11 of 16 Nanomaterials 2022, 12, x FOR PEER REVIEW 12 of 16 Nevertheless, for some materials, it was observed that the same values were obtained for the STY at 260 and at 280 ◦ C; even in the case of Cu/ZnO/PN, a decrease in the STY was observed in this range. This could be related to the selectivity of the reaction, bimetallic system is not a viable alternative. The possible advantages of the use of sup- which can favour the production of carbon monoxide compared to methanol at these high ports in the production of methanol were analysed. For this purpose, the catalyst Cu/ZnO temperatures. If we consider the results of the STY analysis when classified by types of without PVP materials, was chosen materials monometallic due to its good shouldresults, be theand three first to betypes of supports analysed. were chosen. In particular, the The first supports tested were polypyrrole nanotubes in order to solve results indicate that methanol was hardly obtained with copper—only at high temperatures the problem of nanoparticle and agglomeration with non-significant and, thus, of concentrations to methanol. increase theThe exposure of active lower activity sites. of the Figure copper was7 shows the obvious difference between Cu/ZnO and Cu/ZnO with polypyrrole suggested to result from formate poisoning of the copper surface, which is an intermediate nanotubes, confirming of the effectof the hydrogenation ofcarbon the support dioxidein into terms of methanol methanol [27]. production. Furthermore,The otherwas copper twofound sup- ports used were zeolite and activated carbon, which are both porous materials. to be ineffective as a catalyst material for its low surface area [28]. Regarding zinc oxide, The best results of conversion from carbon dioxide to methanol were achieved the methanol concentrations were slightly higher than those of copper. However, these with the use of these porous materials, concentrations werethanks smalltocompared the synergy between with thethe those of carbon otherdioxide adsorption catalysts. capacity The difference in of the supports and the active sites of the Cu/ZnO catalyst. Nevertheless, methanol concentrations between zinc oxide and copper and the role that these two metals the Cu/ZnO/ze- olite material played produced in methanol the best synthesis results, was probably previously due to in analysed zeolite’s higher carbon the literature, wheredioxide it was adsorption capacity per gram of material [10,11]. discussed that zinc oxide is fundamental for stabilising reaction intermediates and reducing energyThese three barriers forsupports were analysed the production of methanolto verify whether [28]. For theyreasons, these two catalysed methanol more methanol by themselves. was produced Therefore, with zincaoxidecatalytic thantest was with performed copper. at thesynthesised Zinc oxide same conditions of tempera- with and without ture, was PVP pressure, and flow compared as with in terms the other catalyst. of methanol STY, withThe results better indicated results for thethat casenone of them synthesised had catalytic with PVP. activity. Figure 7. Figure 7. STY STY values values for for each each catalyst catalyst at at different different temperatures temperatures and and at at aa pressure pressureof of15 15bar. bar. Theobserved As methanol inselectivity Section 3.1of all of 5B,C), (Figure the studied adding catalysts is shown PVP entailed the in Figure 8. obtention of In all of smaller nanoparticles the cases, from with 180more to 220 regular °C, themorphology, which made methanol selectivity the specific obtained surface was 100%, higher. which The means bimetallic that no carbon materials were was monoxide compared produced as well. It was at these observed that temperatures; with thus, onlyCuO/ZnO, the methanol the amount selectivityof methanol values from produced was°Clower 240 to 280 than with are plotted Cu/ZnO. in Figure It was found 8. Almost in the had all catalysts literature 100% that the active component 0 , and although Cu2+ can also act as an active component, methanol selectivity at 240is°C, Cubut as the temperature increased, in general, all materials it offers lost less catalytic selectivity. activity This was due[29,30]. to the standard enthalpy of the reaction of methanol being Moreover, negative; hence,several it is anpublications exothermicreported reaction thatthatcopper nanoparticles is favoured are an active phase at low temperatures. How- for carbon ever, dioxide in three hydrogenation, catalysts, the amountand zinc oxide of carbon helps toobtained monoxide disperse wasthesezero copper nanopar- for tempera- ticles tures onup the surface to 280 of the catalysts °C. These catalyst [31,32]. were zinc Thus, the with oxide presence and of zinc oxide without PVP increases and Cu/ZnO the exhibition of copper active sites, as it is a physical spacer between with PVP. As stated in the literature [27], the catalytic activity and methanol selectivity these nanoparticles. Nevertheless, can be correlated thewith mosttherecent literature increase in Zn0suggests that Zn concentration. atoms It was at the edges discovered that of copper a zinc en- nanoparticles are also active sites [33]. Thus, metallic Zn on the copper richment in the catalyst leads to greater activity and methanol selectivity due to two fac- surface increases the tors:binding improvingenergy theofCu oxygen-bound intermediates, and ZnO interactions that is, formate, that increase and thishydrogenation carbon dioxide facilitates the hydrogenation and inhibiting pathway the RWGS of reaction, carbon dioxide. which Therefore, produces carbonzinc oxide also catalyses monoxide [34]. Inthe thereaction, case of materials synthesised using PVP, this lack of carbon monoxide was possibly due to the poor distribution of zinc oxide and copper nanoparticles, which were also agglomerated. Nanomaterials 2022, 12, 999 12 of 16 although in smaller quantities than Cu/ZnO because the active sites for carbon dioxide hydrogenation are found in both metals. A complete study was performed on Cu/ZnO, and the effect of adding PVP was also compared. In the case of this material, the results clearly showed that adding PVP to a bimetallic system is not a viable alternative. The possible advantages of the use of supports in the production of methanol were analysed. For this purpose, the catalyst Cu/ZnO without PVP was chosen due to its good results, and three types of supports were chosen. The first supports tested were polypyrrole nanotubes in order to solve the problem of nanoparticle agglomeration and, thus, to increase the exposure of active sites. Figure 7 shows the obvious difference between Cu/ZnO and Cu/ZnO with polypyrrole nanotubes, confirming the effect of the support in terms of methanol production. The other two supports used were zeolite and activated carbon, which are both porous materials. The best results of conversion from carbon dioxide to methanol were achieved with the use of these porous materials, thanks to the synergy between the carbon dioxide adsorption capacity of the supports and the active sites of the Cu/ZnO catalyst. Nevertheless, the Cu/ZnO/zeolite material produced the best results, probably due to zeolite’s higher carbon dioxide adsorption capacity per gram of material [10,11]. These three supports were analysed to verify whether they catalysed methanol by themselves. Therefore, a catalytic test was performed at the same conditions of temperature, pressure, and flow as with the other catalyst. The results indicated that none of them had catalytic activity. The methanol selectivity of all of the studied catalysts is shown in Figure 8. In all of the cases, from 180 to 220 ◦ C, the methanol selectivity obtained was 100%, which means that no carbon monoxide was produced at these temperatures; thus, only the methanol selectivity values from 240 to 280 ◦ C are plotted in Figure 8. Almost all catalysts had 100% methanol selectivity at 240 ◦ C, but as the temperature increased, in general, all materials lost selectivity. This was due to the standard enthalpy of the reaction of methanol being negative; hence, it is an exothermic reaction that is favoured at low temperatures. However, in three catalysts, the amount of carbon monoxide obtained was zero for temperatures up to 280 ◦ C. These catalysts were zinc oxide with and without PVP and Cu/ZnO with PVP. As stated in the literature [27], the catalytic activity and methanol selectivity can be correlated with the increase in Zn0 concentration. It was discovered that a zinc enrichment in the catalyst leads to greater activity and methanol selectivity due to two factors: improving the Cu and ZnO interactions that increase carbon dioxide hydrogenation and inhibiting the RWGS reaction, which produces carbon monoxide [34]. In the case of materials synthesised using PVP, this lack of carbon monoxide was possibly due to the poor distribution of zinc oxide and copper nanoparticles, which were also agglomerated. As expected, the active sites of copper were poisoned by formate, and only the active sites of zinc oxide were available. The relationships between the methanol selectivity and the CO2 conversion for some catalysts with good STY results are shown in Figure 9. The displayed values correspond to the methanol selectivity and conversion of carbon dioxide from 240 to 280 ◦ C. As observed, the general behaviour was that as selectivity decreased, conversion and temperature increased. Figure 9 shows that the catalyst with the highest carbon dioxide conversion was Cu/ZnO/Zeolite, similarly to the STY results seen in Figure 7. The yield and conversion data were low compared with the literature. For example, using Cu/ZnO plates, the maximum CO2 conversion reported was 8% [35,36]. However, the operating conditions of pressure in these studies were higher (30 bar) than those of this work (15 bar). Even though many existing publications studied the catalytic properties of similar materials, it is very rare to find documents with studies operating at pressures lower than 20 bar [37,38]. Nanomaterials 2022, 12, x FOR PEER REVIEW 13 of 16 Nanomaterials 2022, 12, 999 13 of 16 As expected, the active sites of copper were poisoned by formate, and only the active sites of zinc oxide were available. Figure 8. Methanol selectivity (%) values of all catalysts. The relationships between the methanol selectivity and the CO2 conversion for some catalysts with good STY results are shown in Figure 9. The displayed values correspond to the methanol selectivity and conversion of carbon dioxide from 240 to 280 °C. As ob- served, the general behaviour was that as selectivity decreased, conversion and tempera- ture increased. Figure 9 shows that the catalyst with the highest carbon dioxide conversion was Cu/ZnO/Zeolite, similarly to the STY results seen in Figure 7. The yield and conver- sion data were low compared with the literature. For example, using Cu/ZnO plates, the maximum CO2 conversion reported was 8% [35,36]. However, the operating conditions of pressure in these studies were higher (30 bar) than those of this work (15 bar). Even though many existing publications studied the catalytic properties of similar materials, it is very Figure rare to8.8. Figure Methanol find selectivity documents Methanol with selectivity (%) valuesof studies (%) values ofall allcatalysts. catalysts. operating at pressures lower than 20 bar [37,38]. The relationships between the methanol selectivity and the CO2 conversion for some catalysts with good STY results are shown in Figure 9. The displayed values correspond to the methanol selectivity and conversion of carbon dioxide from 240 to 280 °C. As ob- served, the general behaviour was that as selectivity decreased, conversion and tempera- ture increased. Figure 9 shows that the catalyst with the highest carbon dioxide conversion was Cu/ZnO/Zeolite, similarly to the STY results seen in Figure 7. The yield and conver- sion data were low compared with the literature. For example, using Cu/ZnO plates, the maximum CO2 conversion reported was 8% [35,36]. However, the operating conditions of pressure in these studies were higher (30 bar) than those of this work (15 bar). Even though many existing publications studied the catalytic properties of similar materials, it is very rare to find documents with studies operating at pressures lower than 20 bar [37,38]. Figure9.9. Representation Figure Representationofofthe theselectivity selectivityvs. vs.carbon carbondioxide dioxideconversion conversionofof allall catalysts. catalysts. Finally, ◦C Finally,the theSTY STYvalues valuesand andselectivity selectivitytowards towardsmethanol methanol ofof allall the catalysts the at at catalysts 260260 °C and and the thetemperature temperaturewith withthe themaximum maximumSTY STYvalues valuesare plotted are inin plotted Figure Figure10.10. Figure 10 shows that the catalyst that produced the highest yield of methanol was Cu/ZnO/zeolite. As expected, by adding a support such as zeolite to the Cu/ZnO bimetal- lic system, the STY values obtained were 76.75% higher than without the support. It can also be stated that bimetallic systems have a better catalytic efficiency than ZnO and Cu0 by themselves, with the latter having no methanol production. Figure 9. Representation of the selectivity vs. carbon dioxide conversion of all catalysts. Finally, the STY values and selectivity towards methanol of all the catalysts at 260 °C and the temperature with the maximum STY values are plotted in Figure 10. Nanomaterials 2022,12, Nanomaterials2022, 12,999 x FOR PEER REVIEW 14 14of of16 16 Figure10. Figure 10.Representation Representationofofthe theSTY STYand andthe the methanol methanol selectivity selectivity of of allall catalysts. catalysts. TheThe arrows arrows indi- indicate cate that methanol selectivity values can be read in the right vertical axis, and STY in the left verti- that methanol selectivity values can be read in the right vertical axis, and STY in the left vertical axis. cal axis. 4. Conclusions Figure 10 Several shows types that theand of copper catalyst zinc that produced the compounds— highest yield in particular, of methanol single was metals, their Cu/ZnO/zeolite. As expected, by adding a support such as zeolite oxides, and bimetallic materials, which were embedded or not embedded in several types to the Cu/ZnO bimetal- licsupports—were of system, the STYsuccessfully values obtained were 76.75% synthesised in this higher study, and thantheir without the support. catalytic efficiencyItwas can also be stated studied. that bimetallic In general, systems the bimetallic have a showed materials better catalytic a higher efficiency efficiencythan thanZnOtheand Cu0 single- by themselves, metal materials within termsthe latter having yield. of methanol no methanol In these production. types of materials, both metals (Cu and Zn) had active sites and differentiated functions, as zinc acted physically as a spacer, 4. Conclusions exposing the active sites of both metals on the surface, which significantly helped their performance as catalysts. Several types of copper Even andthough adding PVPinasparticular, zinc compounds— a cappingsingle agentmetals, improvedtheirthe ox- catalytic ides, andcapacity bimetallic of materials, single-metal nanoparticles, which were embedded sinceor it normally not embeddedhelpsintoseveral achieve more types of homogeneous supports—were andsuccessfully smaller particles, in the in synthesised case of study, this the bimetallic and theirmaterials, catalyticitefficiency hindered wasthe correct studied. homogenisation In general, theofbimetallic both metals, causingshowed materials an evident tendency a higher to agglomerate, efficiency which than the single- clearly metal materials in terms of methanol yield. In these types of materials, both metalswas diminished their catalytic capacity. The great importance of using a support (Cu also and confirmed Zn) had active by using bothdifferentiated sites and nanotubes forfunctions, a greateras spacing of the zinc acted active sites physically as aorspacer, with porous exposing supports suchsites the active as zeolite of bothor metals activatedoncarbon, whichwhich the surface, couldsignificantly improve the helped distribution their of metals and as performance thecatalysts. adsorption Evenof though carbon adding dioxide.PVP Theseas atwo particular capping agentfeatures improved of porous the cat- supports clearlyof alytic capacity improved the catalytic single-metal efficiency nanoparticles, of these since nanocomposites it normally when compared helps to achieve more ho- with the other studied materials. mogeneous and smaller particles, in the case of the bimetallic materials, it hindered the correct homogenisation of both metals, causing an evident tendency to agglomerate, Author Contributions: Conceptualisation, A.S. and J.M.-V.; methodology, A.C.G. and A.A.M.; valida- which clearly diminished their catalytic capacity. The great importance of using a support tion, J.M.-V. and A.C.G.; formal analysis, A.C.G., J.M.-V. and A.A.M.; investigation, A.C.G. and J.M.-V.; was also confirmed by using both nanotubes for a greater spacing of the active sites or resources, A.S. and J.M.-V.; data curation, A.C.G. and A.A.M.; writing—original draft preparation, with porous A.C.G.; supports such writing—review as zeolite and editing, or activated A.C.G., J.M.-V. andcarbon, which could A.C.G.; A.S.; visualisation, improve the distri- supervision, bution of metals and the adsorption of carbon dioxide. These two particular J.M.-V. and A.S.; project administration, A.S.; funding acquisition, A.S. All authors have read and features of poroustosupports agreed the publishedclearly improved version the catalytic efficiency of these nanocomposites when of the manuscript. compared with the other studied materials. Funding: This research was funded by Fundación Ramón Areces in the hallmark of the META2NOL project granted in the XIX National Contest of Life and Matter Sciences Projects, grant number Author Contributions: Conceptualisation, A.S. and J.M.-V.; methodology, A.C.G. and A.A.M.; val- CIVP19A5952. Anna Carrasco García thanks UAB for the PIF scholarship granted for the completion idation, J.M.-V. and A.C.G.; formal analysis, A.C.G., J.M.-V., and A.A.M.; investigation, A.C.G. and of her PhD thesis. J.M.-V.; resources, A.S. and J.M.-V.; data curation, A.C.G. and A.A.M.; writing—original draft prep- aration, A.C.G.; Institutional writing—review Review and editing, Board Statement: A.C.G., J.M.-V., and A.S.; visualisation, A.C.G.; super- Not applicable. vision, J.M.-V. and A.S.; project administration, A.S.; funding acquisition, A.S. All authors have read Informed and agreed Consent Not applicable. Statement:version to the published of the manuscript. Acknowledgments: Thewas Funding: This research authors are by funded alsoFundación thankful to Institut Ramón Catalàinde Areces Nanociència the hallmark ofi the Nanotecnologia META2NOL (ICN2) for offering their technology for material characterisation. project granted in the XIX National Contest of Life and Matter Sciences Projects, grant number CIVP19A5952. Conflicts Anna The of Interest: Carrasco Garcí authors a thanks declare UAB forofthe no conflict PIF scholarship granted for the completion interest. of her PhD thesis. Nanomaterials 2022, 12, 999 15 of 16 References 1. Alotaibi, M.A.; Din, I.U.; Alharthi, A.I.; Bakht, M.A.; Centi, G.; Shaharun, M.S.; Naeem, A. Green methanol synthesis by catalytic CO2 hydrogenation, decipehring the role of metal-metal interaction. Sustain. Chem. Pharm. 2021, 21, 100420. [CrossRef] 2. Saeidi, S.; Najari, S.; Hessel, V.; Wilson, K.; Keil, F.J.; Concepción, P.; Suib, S.L.; Rodrigues, A.E. 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