Hydrogen Utilization in Green Fuel Synthesis via CO₂ Conversion to Methanol over New Cu-Based Catalysts

Abstract

The use of hydrogen as an energy vector and raw material for “very clean liquid fuels” manufacturing has been assessed by the catalytic conversion of CO₂ to methanol over copper based catalysts. A systematic evaluation of copper based catalysts, prepared varying the chemical composition, has been carried out at 0.1–5.0 MPa of total pressure and in the range of 453–513 K by using a semi-automated LAB-microplant, under CO₂/H₂ reactant mixture (1/3), fed at GHSV of 8.8 NL∙kg_cat⁻¹∙h⁻¹. Material’s properties have been investigated by the means of chemical-physical studies. The findings disclose that the addition of structure promoters (i.e., ZrO₂/CeO₂) strongly improves the textural properties of catalysts, in term of total surface area and exposure of metal surface area (MSA), also reducing the sintering phenomena. The results of the catalytic study clearly prove a structure-activity relationship at low reaction pressure (0.1 MPa), while at higher pressure (3.0–5.0 MPa) the reaction path is insensitive to structure and chemical composition.

1. Introduction

The combined use of CO₂ and renewable H₂, provided by electrolysis or water splitting, in the chemicals and fuels manufacturing, nowadays looks very attractive. In particular, CO₂ conversion into fuel products is the most promising way for a larger H₂ utilization, as an energy vector, providing for instance methanol and other light oxygenated hydrocarbons for fuel cell (FC) applications (i.e., direct methanol/alcohols FC). Indeed, the catalytic hydrogenation of CO₂ is still more effective than direct photo-conversion [1,2,3,4,5,6,7,8,9].

On the other side, the production of methanol via the hydrogenation of CO₂, using electro-catalytically generated H₂, is of particular interest also for renewable energy concepts, since it can serve as a way to convert and store the excess electrical energy into chemical energy, contributing to smooth the natural fluctuation in the supply of renewable energy [10,11]. Moreover, the CO₂ hydrogenation can contribute to curb CO₂ emissions, which are a cause of the increase in global temperatures and climate change due to the “greenhouse effect” [12].

Prompted by seeking better performing formulations, many authors have explored new catalytic compositions, active in the CO₂ hydrogenation, alternative to the copper-based, such as combinations of Pd [13,14], Au [10,15], or Mo₂C [16,17]. Nevertheless, copper-based catalysts still remain the most suitable catalysts for commercial applications [18]. Cu-based catalysts suffer from detrimental effects of the water produced during catalyst activation and reaction, which inhibits catalytically active sites through the irreversible oxidation of Cu⁺ species to Cu²⁺, leading to a loss of interfacial area and accelerating the catalyst sintering and the synthesis rate decay [18]. On this address, in the past decades, significant effort has been devoted to improve the properties of Cu-based catalysts by the use of promoters and carriers/supports [19,20,21,22,23]. Several metal oxides are reported in literature for improving the catalytic performance of copper-based catalysts, in term of activity, selectivity, and stability [24,25,26,27,28,29], also enhancing CO₂ adsorption with respect to un-promoted systems. In particular, in our previous works [30,31,32,33,34], we disclose the positive effects of ZrO₂ on the catalytic performance of Cu-ZnO catalyst with respect to that of Al₂O₃-supported, as a result of the lower hydrophilic nature of ZrO₂ than Al₂O₃, which facilitates water desorption, and its property of stabilizing Cu⁺–O species, which favor CO₂ adsorption/activation and methanol selectivity. Then, in more recent studies, we have clearly assessed that ZrO₂ affects structure/morphology and texture of Cu-ZnO based catalysts, while cerium oxides can divalently act as an electronic promoter and improver of surface functionality of Cu phase [33,34].

Therefore, the present work aims to study the combined effects of ZrO₂ and CeO₂ addition on the chemical-physical properties of copper based catalyst, in efforts to shed light on the CO₂ hydrogenation reaction pathways.

2. Materials and Methods

2.1. Synthesis of the Catalysts

Cu–ZnO based catalysts, with a Cu/Zn atomic ratio of circa 3, were prepared by reverse coprecipitation under ultrasound irradiation route, varying Zr_xCe_(1−x)O₂ carrier composition between 0.2 < x < 0.95, according to procedure elsewhere described [30].

Table 1. List of catalytic system based on Cu-Zn oxides composites.

Sample		Chemical Composition (wt%)				Physical Properties
		CuO	ZnO	ZrO2	CeO2	SA (m2·g−1)	PV (cm3·g−1)	APD (a) (nm)	MSA (c) (m2·g−1)	MSA/SAR
CuZn_CZ_100		44.2	13.4	42.1	0.0	154	0.34	9.0	-	-
	R (b)	-	-	-	-	132	0.28	8.0	73	0.55
CuZn_CZ_95		42.8	13.2	38.9	4.2	125	0.40	11.2	-	-
	R (b)	-	-	-	-	62	0.20	12.9	28	0.45
CuZn_CZ_90		42.6	12.8	34.0	8.3	114	0.60	18.7	-	-
	R (b)	-	-	-	-	56	0.21	10.8	25	0.44
CuZn_CZ_50		39.0	12.3	15.6	31.8	80	0.89	48.6	-	-
	R (b)	-	-	-	-	40	0.20	15.7	16	0.40
CuZn_CZ_20		40.1	13.0	5.8	40.3	68	0.32	20.8	-	-
	R (b)	-	-	-	-	36	0.20	21.0	14	0.38
CuZn_CZ_0		38.7	12.2	0.0	48.8	47	0.24	20.0	-	-
	R (b)	-	-	-	-	34	0.17	20.0	10	0.29

(a) Average Pore Diameter (APD, 4PV/SA); (b) sample after reduction treatment at 573 K (1 h) in flowing H₂/Ar and passivation; (c) obtained by ‘‘single-pulse’’ N₂O titration measurements.

shows the list of catalysts, also reporting the chemical-physical properties.

2.2. Characterization of the Catalysts

Surface area (SA), pore volume (PV), and average pore diameter (APD) were obtained from nitrogen adsorption/desorption isotherms at 77 K, using ASAP 2020, Micromeritics Instrument. Isotherms were elaborated by the standard BET and BJH methods for SA and PV evaluation, respectively.

X-ray diffraction (XRD) analysis of samples in the 2θ range 10–80° was performed using a Philips X-Pert diffractometer operating with Ni β-filtered Cu Kα radiation at 40 kV and 30 mA.

Metal Surface Area (MSA) values were obtained by measurements of N₂O “single-pulse” titrations, assuming a Cu:N₂O = 2 titration stoichiometry [35,36]. Before measurements, catalysts were reduced in situ at 573 K in H₂ flow (100 STP mL/min) for 1 h. After reduction, the samples were “washed” in He carrier flow at 583 K for 15 min, and then cooled to 363 K.

2.3. Catalytic Measurements

Catalytic tests were performed at 0.1–5.0 MPa and 453–513 K in a semiautomatic Laboratory microplant, equipped with a AISI 316 stain steel Plug Flow Reactor (i.d., 10 mm; e.d., 12 mm; l., 250 mm), loaded with 1.0 g of catalyst diluted (1/1, wt/wt) with quartz granules (2 mm). Before catalytic tests, catalysts were in-situ activated at 573 K in reducing atmosphere, flowing (5.5 NL·g⁻¹·h⁻¹) 5% H₂/Ar gas mixture for 1 h at a heating rate of 10 K·min⁻¹. After catalysts activation the reactor was fed with a CO₂/H₂/N₂ (3/9/1) mixture flowing at 80 STPcm³·min⁻¹ (GHSV, 8.800 NL∙kg_cat⁻¹∙h⁻¹) using nitrogen as internal standard for GC analysis. The composition of inlet and outlet streams was analyzed in real time by an inline gas chromatograph HP 6890 equipped with a TCD detector and two packed columns (Porapack Q and 5A molecular sieve, Sigma-Aldrich (Milano, Italy)) connected in series for the analysis of permanent gasses, and a FID detector coupled with a capillary column (PoraPLOT Q, i.d., 0.53 mm; l, 30 m, Agilent (Cernusco sul Naviglio (MI), Italy)) for the hydrocarbon species quantification. Methanol and CO were almost the only products formed, while methane and DME are detected only at trace levels (<0.01 mol/mol %).

3. Results

3.1. Chemical-Physical Characterization

As shown in Table 1 and Figure 1, the progressive replacement of Ce^3+/4+ ions with Zr⁴⁺ (i.e., Zr_xCe_(1-x)O₂; 0.2 < x < 0.95) strongly affects both structural and morphological properties of catalysts. Indeed, a remarkable rise of both total and metallic surface area was reported with a consequent increase of the copper–zinc oxide interfacial area, accompanied by an also improved resistance to sintering due to the activation procedure (reduction).

As reported in our previous work [30,31,32], catalyst activation consists in a controlled reduction to obtain active metallic copper. The action of temperature and H₂ for oxygen removal (MeO_x + xH₂ → xH₂O; Me = Cu and Ce) leads to the reconstruction of catalyst which can also drive to a higher crystallization with a consequent loss of total surface area (sintering). On this address, the effects of the activation procedure were fully investigated in our previous works [30,31,32].

In particular, the catalyst without ZrO₂ (CuZn_CZ_0) displays the lowest value of surface area (47 m²·g⁻¹), which further decreases by 28% after reduction (34 m²·g⁻¹). By contrast, the system with no CeO₂ (CuZn_CZ_100) shows the highest exposure of total surface (~154 m²·g⁻¹) and the lowest decay for the reduction treatment (ΔSA = 14%), confirming the strong effect of zirconia as structural promoter (Figure 1). Indeed, along the replacement of Ce ions in the lattice, the total surface area grows from 67 m²·g⁻¹ to 125 m²·g⁻¹, also enhancing MSA value (14–28 m²·g⁻¹), which mirrors an improved dispersion of active phase, confirmed by an increase of the MSA/SA_R ratio. Then, Figure 2 displays the diffraction patterns of catalysts as prepared (a) and after reduction treatment (b). Irrespective of chemical formulation, XRD analysis signals the lack of “long-range” crystalline order in all samples, due to the ultrasonic preparation method, Figure 2a. In particular, the XRD profile is characterized for all catalysts by the presence of two broad peaks, placed at ca. 31 and 56 2θ degrees, with an increasing intensity at the raise of Zr content. Therefore, catalyst reduction leads to a bulk reconstruction, as shown in Figure 2b. Namely, the catalysts at higher Zr content exhibit the signals of copper in the metallic state, whose most intense reflections appear at 43.3° (<111>) and 50.4° (<200>), respectively. In addition, the occurrence into CuZn_CZ_90 catalyst of sharp peaks, at 35.68° and 38.88°, diagnostics the presence of isolate Tenorite phase (CuO) [JCPDS 5–661], while in the XRD pattern of the reduced CuZn_CZ_95 sample are also evident the diffraction signals of cubic Zirconia [JCPDS 27-0997]. On the other hand, the reflection peaks of metallic Cu are less marked in the reduced samples at lower Zr content (i.e., CuZn_CZ_50 and CuZn_CZ_20), probing a stronger re-oxidation, due to the greater oxygen storage capacity of CeO₂. Furthermore, small intense lines of Zincite phase (ZnO) are detected in the XRD profile of reduced CuZn_CZ_50 catalyst. Therefore, the good chemical stability of catalysts was further proved by results of catalytic test and by XRD analysis on samples after catalytic tests (data not reported here for the sake of brevity).

3.2. Catalytic Activity

The results of catalytic tests in the CO₂ hydrogenation in the range of 453–513 K and at 0.1, 3.0 and 5.0 MPa are summarized in

Table 2. Results of the catalytic test in the CO₂ hydrogenation at different temperature and pressure.

0.1 MPa	${\mathrm{X}}_{{\mathrm{H}}_2}$ (%)				${\mathrm{S}}_{{\mathrm{CH}}_3}{}_{\mathrm{OH}}$ (%)				Reaction Rate (mmolH2·h−1·gcat−1)
	453 K	473 K	493 K	513 K	453 K	473 K	493 K	513 K	453 K	473 K	493 K	513 K
CUZN_CZ_100	0.5	1.4	4.9	8.9	34	26	13	3	0.46	1.37	4.85	8.69
CUZN_CZ_95	0.4	2.6	3.6	6.2	56	45	23	7	0.37	2.56	3.57	6.04
CUZN_CZ_90	0.5	2.0	4.1	6.3	71	52	30	11	0.46	1.92	4.02	6.22
CUZN_CZ_20	0.4	1.2	2.8	3.2	77	62	39	19	0.37	1.19	2.74	3.11
CUZN_CZ_0	0.7	1.0	1.9	2.3	96	92	80	61	0.73	1.01	1.83	2.29
3.0 MPa	${\mathrm{X}}_{{\mathrm{H}}_2}$ (%)				${\mathrm{S}}_{{\mathrm{CH}}_3}{}_{\mathrm{OH}}$ (%)				Reaction Rate (mmolH2·h−1·gcat−1)
	453 K	473 K	493 K	513 K	453 K	473 K	493 K	513 K	453 K	473 K	493 K	513 K
CUZN_CZ_100	4.1	7.8	13.1	17.7	87	74	57	50	4.02	7.68	12.81	17.38
CUZN_CZ_95	3.5	5.5	7.7	13.1	93	86	65	57	3.38	5.40	7.59	12.81
CUZN_CZ_90	3.5	5.5	9.1	14.9	93	87	71	50	3.38	5.40	8.87	14.63
CUZN_CZ_20	2.9	4.2	6.0	7.9	94	90	82	69	2.84	4.12	5.85	7.77
CUZN_CZ_0	1.9	3.2	4.8	6.7	97	95	90	80	1.83	3.11	4.66	6.59
5.0 MPa	${\mathrm{X}}_{{\mathrm{H}}_2}$ (%)				${\mathrm{S}}_{{\mathrm{CH}}_3}{}_{\mathrm{OH}}$ (%)				Reaction Rate (mmolH2·h−1·gcat−1)
	453 K	473 K	493 K	513 K	453 K	473 K	493 K	513 K	453 K	473 K	493 K	513 K
CUZN_CZ_100	5.3	9.3	15.9	20.5	90	78	65	64	5.21	9.15	15.55	20.12
CUZN_CZ_95	4.4	6.3	9.3	14.9	93	89	74	54	4.30	6.22	9.15	14.63
CUZN_CZ_90	4.1	6.7	10.3	15.9	94	87	76	56	4.02	6.59	10.06	15.55
CUZN_CZ_20	3.8	5.2	6.9	9.1	94	91	86	74	3.75	5.12	6.77	8.96
CUZN_CZ_0	2.1	3.7	6.1	7.9	97	95	90	81	2.01	3.66	5.95	7.77

in terms of H₂ conversion (${\mathrm{X}}_{{\mathrm{H}}_2}$%), selectivity to methanol (${\mathrm{S}}_{{\mathrm{CH}}_3}{}_{\mathrm{OH}}$%) and reaction rate (mmolH₂·h⁻¹·g_cat⁻¹).

As extensively reported in literature, CO₂ hydrogenation drives to methanol and CO over Cu-based catalyst through the reactions of methanol synthesis (Equations (1) and (2)) and Reverse Water Gas Shift (Equation (3)), while the methane formation (Equation (4)) is strongly un-favored under such reaction conditions [27,37]:

CO₂ + 3H₂ ⇆ CH₃OH + H₂O

(1)

CO + 2H₂ ⇆ CH₃OH

(2)

CO₂ + H₂ ⇆ CO + H₂O

(3)

CO₂ + 4H₂ ⇆ CH₄ + 2H₂O

(4)

In respect of this, a progressive increase of H₂ conversion, associated with a significant decrease in methanol selectivity, has been observed with the increase in temperature, independent of chemical composition and pressure. According to the greater partial pressure of reactants on the catalyst surface, the H₂ conversion increases further over all samples by upping the pressure from 0.1 MPa to 5.0 MPa. As a rule, ZrO₂ content growth leads to a higher hydrogenation of CO₂, although, the selectivity to methanol is notably reduced.

In particular, reaction rate raises from 8.7 mmol·h⁻¹·g_cat⁻¹ to the greater 20.1 mmol·h⁻¹·g_cat⁻¹ at the highest temperature (513 K), moving from 0.1 MPa to 5.0 MPa by using the most active catalyst (CuZn_CZ_100). While the reaction rate increases only in the range of 2.3–7.8 (mmol·h⁻¹·g_cat⁻¹), at the same test condition, employing the less active catalyst (CuZn_CZ_0).

In catalysis, “Activation Energy” counts for the energy that must be supplied to catalyst to observe any chemical reaction. On this account, a further evaluation of the effects of carrier composition on the catalytic functionality of Cu-ZnO systems can be done comparing the values of Apparent Activation Energy (E_app (kJ·mol⁻¹)), obtained from the Arrhenius’s plot of differential conversion data at 0.1 MPa (${\mathrm{X}}_{{\mathrm{H}}_2}$ < 10%, T = 453–513 K), reported in Figure 3 [30,31,38]. Indeed, the progressive increase of CeO₂ content leads to a decrease in E_app value from 97 kJ·mol⁻¹ of CuZn_CZ_0 sample to 26 kJ·mol⁻¹ of CuZn_CZ_100 system, as proof of the promoter effect of ceria. The catalytic data at higher pressure (i.e., 3.0 and 5.0 MPa) were not considered for the evaluation of E_app, because the hydrogen conversion (${\mathrm{X}}_{{\mathrm{H}}_2}$) overcame the value of 10%.

As well known, at the kinetic regime, the value of the Apparent Activation Energy (E_app) is directly related to the kinetic constant “K” and, consequently, to the reaction rate (rate ∝ K) through Arrhenius’s Law, also reflecting the energies needed for the chemisorption and desorption processes of the molecule on the surface of active sites [39,40]. Therefore, the decreasing in the value of E_app observed with the raise of Ceria contents in the catalyst formulation can also mirror a more favorable chemisorption or desorption steps in the hydrogenation reaction.

4. Discussion

CO₂ hydrogenation to methanol by using Cu-based catalytic systems promoted by several oxides/carriers has been widely investigated in literature [3,5,16,17,18,19,20,21,22,23,24,27,28,29,30,31,32], although very few examples of multicomponent CuO-ZnO-CeO₂ and CuO-ZnO-CeO₂-ZrO₂ systems are still present [25,26,33,34,41,42].

Cerium oxides are well recognized catalytic materials for the oxygen storage capacity of Ceria and for several features such as to be an enhancer of active phase dispersion, for weakening the C–O bond and for improving the chemisorption of H₂ [43,44,45]. As an electronic promoter, Ceria also plays a crucial role in the behavior of the catalyst, thanks to its peculiar reactivity [43,44]. In this work, the effect of catalyst formulation becomes more clear by plotting the logarithm of H₂ conversion (lg${\mathrm{X}}_{{\mathrm{H}}_2}$) against the selectivity to methanol, Figure 4. Namely, at 0.1 MPa the catalytic behavior of all catalysts can be described through three different curves of activity, which respond to different catalyst formulation. Differently, at the higher pressure (3.0 and 5.0 MPa) the samples exhibit similar catalytic paths. These findings suggest that catalyst chemisorption capacity, also with respect to intermediate products, could affect the catalytic behavior at low reactants coverage. In particular, at 0.1 MPa the catalyst with composition “Cu-ZnO-ZrO₂” (CuZn_CZ_100) promotes mainly the reaction of hydrogenation which leads to form CO. By contrast, the catalyst with the chemical formulation “Cu-ZnO-CeO₂” (CuZn_CZ_0) favors mainly the methanol synthesis, perhaps, thanks to the oxygen storage capacity of Ceria. Indeed, according to the reaction mechanism in which methanol synthesis occurs via hydrogenation of a “bridge-formate” intermediate [46,47], CuZn_CZ_0 would promote methanol because of its higher capacity to weaken and bend C–O bonds. In addition, the higher hydrogen coverage on the surface, due to the H₂-spillover phenomena, could further enhance the selectivity to methanol for CuZn_CZ_0 catalyst. Therefore, the catalytic behavior held by the catalysts having both ZrO₂ and CeO₂ promoter/carrier results in a right equilibrium of chemical-physical properties of the two different oxides, being a good compromise between the two extreme catalytic performance.

Therefore, the effects of ceria, as structural and electronic promoter, can be both at level of solid-state interactions and reactivity toward reactant gas [30,31,32,33,34,45,48], affecting the redox and electronic properties of the active phase and modelling the adsorption behavior. On the other hand, the enhanced reducibility (data not here reported) and the well stated surface affinity for H₂ can further justify the lower value of Apparent Activation Energy [30,31,32,33,34,45,48].

Then, to shed light on structure-activity relationship, the catalytic results were analyzed according to their textural properties (SA and MSA). On this account, Figure 5a,b reports reaction rate and “normalized methanol yield” at 5.0 MPa and at varying of MSA and MSA/SA_R values, respectively. In particular, the CO₂ hydrogenation rate increases almost proportionally with the increase of metal exposure, Figure 5a, according to the consistent rise and reconstruction of the active phases/sites (Cu⁰ ⇆ Cu⁺). On the other side, Figure 1 and Table 1 shown that the value of MSA increases by the growth of the atomic fraction of Zr, as proof of the greater efficacy of ZrO₂ carrier as structural promoter and improver of metal dispersion. Moreover, methanol synthesis over Cu-based catalyst is characterized by a “dual site mechanism”, which invokes a right equilibrium between Cu⁺ and Cu⁰ active sites, according to the following equation 5:

rate_methanol = K · [CO₂] · [H₂] ∝ K· [Cu^(I)] · [Cu⁽⁰⁾]

(5)

where a greater concentration of Cu⁰ sites drive to high hydrogen supply on the catalytic surface, while a major exposition of Cu⁺ active sites on SA strongly enhance CO₂ chemisorption. Therefore, the catalyst performance depends on the right “deal” between the two catalytic functionalities [3,38,45,46,49,50,51].

In other words, all “Cu-ZnO-ZrO₂-CeO₂” catalysts obey the occurrence of a similar “dual-site” reaction path that is a mechanism with two active centers involved in the catalytic process of CO₂ hydrogenation. The first one should be the oxide phase, with basic properties, which attends to adsorb and activate CO₂ molecules for running hydrogenation to methanol, through the formation of a formate as intermediate [46]. In more detail, the CO₂ chemisorption can occur both via Zirconia Lewis basic sites [47,49,50], as well as by action of metal alkaline ZnO phase which enhances the affinity of Cu⁽⁺⁾–O to CO₂ [18]. As proof, in our previous works we have attested a larger CO₂ uptake, consequently to a greater concentration of basic sites [33,34]. On the other hand, the other sites should accomplish the adsorption and dissociation of H₂ molecules for the reaction with CO₂ [24,47]. On this account, the occurring of a reaction pathway which involves the existence of two kind of reaction sites is confirmed for “Cu-ZnO-ZrO₂-CeO₂” catalytic systems by a “volcano-shape” relationship, obtained by plotting normalized methanol yield versus the ratio MSA/SA_R, here used as an indirect index of the dual-sites equilibrium (Figure 5b). Namely, the wide range of values assumed by the normalized yield at the varying of MSA/SA_R discloses the structure-activity relationship of these catalytic materials, and at the same time, reflects the effect of chemical composition. In addition, the existence of a maximum value of methanol yield, which should be achieved at 5.0 MPa at an “ideal” ratio of MSA/SA_R near to 0.3, clearly signs the need of a perfect synergism between the two catalytic functionalities, available through a proper design of the catalyst [51,52,53,54].

In particular, despite the lower activity, the use of Ceria in combination with Zirconia leads to a more favorable Activation Energy (ranging from 97 to 27 kJ·mol⁻¹), which also results in higher values of methanol yield than that of the sample with no Ceria. In other words, even the addition of small quantities of CeO₂ as promoter (i.e., 5–10 wt%) confers to catalyst a peculiar reactivity towards the methanol formation, by enhancing the surface functionality of Cu-ZnO-ZrO₂ system.

These findings concur well with our earlier results, especially in term of methanol yield and E_app [30,31,32,33,34]. In addition, the catalytic performance of CuO-ZnO-CeO₂-ZrO₂ systems represent an advancement with respect to the results recently reported in literature by Cu-based catalysts [26,41,42].

5. Conclusions

The catalytic performance in CO₂ hydrogenation process for a series of new “Cu-ZnO-ZrO₂-CeO₂” catalysts have been assessed, approaching real industrial process conditions.

The work assessed the effect of chemical composition and preparation method regarding the sintering phenomena due to the activation procedure (reduction) in which the action of temperature and H₂ in the oxygen removing and metallic copper formation (MeO_x + xH₂ = xH₂O; Me = Cu and Ce). This leads to catalyst reconstruction which can also drive to higher crystallization with a consequent loss of total surface area. The progressive replacement of Ce^3+/4+ ions with Zr⁴⁺ (i.e., Zr_xCe_(1−x)O₂; 0.2 < x < 0.95) has been proved affecting both SA and MSA exposure, improving also the resistance to sintering phenomena.

The progressive increase of CeO₂ content has been ascertained to lead to a more favorable Activation Energy values. The results obtained with CuO-ZnO-CeO₂-ZrO₂ systems represent a further advancement in the comprehension of catalytic pathway of Cu-based catalyst, The “dual-site” mechanism, as reaction pathway in methanol synthesis, has been confirmed, through a “volcano-shape” relationship, obtained between hydrogenation activity and catalytic structure of “Cu-ZnO-ZrO₂-CeO₂” systems.

The addition of even low quantities of CeO₂ (i.e., 5–10%) allows one to obtain materials with enhanced chemical properties, maintaining the striking structural promoter effect of ZrO₂, and, at the same time, introducing the functionality of CeO₂, with a satisfactory compromise between activity and selectivity.