The Activity and Stability of Promoted Cu/ZnO/Al₂O₃ Catalyst for CO₂ Hydrogenation to Methanol

Abstract

Cu/ZnO/Al₂O₃ catalyst with the addition of tri-promoters (Mn/Nb/Zr) was investigated with respect to their catalytic activity and stability in a prolonged reaction duration in methanol synthesis. Spent catalysts were characterized using N₂ adsorption-desorption, FESEM/EDX, TEM, N₂O chemisorption, and XPS for their physicochemical properties. The catalyst longevity study was evaluated at two days, seven days, and 14 days at 300 °C, 31.25 bar, 2160 mL/g.hr GHSV, and H₂:CO₂ at 10:1. The CO₂ conversion and methanol yield decreased by about 5.7% and 7.7%, respectively, when the reaction duration was prolonged to 14 days. A slight reduction in catalytic activity under prolonged reaction duration was found due to thermal degradation.

1. Introduction

Since Imperial Chemical Industries’ (ICI) discovery of a low-temperature and low-pressure Cu/Zn/Al₂O₃ catalyst in 1966, the Cu/Zn/Al₂O₃ catalyst has been widely utilized in industrial methanol synthesis [1,2]. Commercially, methanol has been synthesized from syngas, which primarily comprises CO and H₂ with a trace quantity of CO₂ [3,4]. Methanol synthesis from CO₂ hydrogenation has recently received a lot of attention as one of the most promising CO₂ mitigation strategies. Several studies on the production of methanol from CO₂ hydrogenation have been reported [5,6,7,8,9,10,11,12]. The synthesis of methanol from CO₂ hydrogenation can be seen from two angles. CO₂ is transformed directly to methanol in Equation (1), but there is also an indirect pathway via the reverse water-gas shift (RWGS) in Equation (2) and CO hydrogenation in Equation (3).

CO₂ + 3H₂ ⇋ CH₃OH + H₂     ∆H° 298K = −49.5 kJ/mol

(1)

CO₂ + H₂ ⇋ CO + H₂O     ∆H° 298K = +41 kJ/mol

(2)

CO + 2H₂ ⇋ CH₃OH     ∆H° 298K = −90.5 kJ/mol

(3)

A feasible methanol synthesis process necessitates a highly efficient catalyst that is both highly active and selective for methanol synthesis and stable over long periods of continuous operation [13]. In a typical methanol synthesis plant, the catalyst lifespan is typically in the range of several years. Supported copper catalysts, which are frequently used in the production of methanol, require an initial period of activation, during which the copper is reduced from cupric oxide to essentially metallic copper under the influence of a diluted hydrogen stream at a high temperature. The initial peak of activity is frequently followed by a gradual decline in the rate of methanol synthesis. The reactor temperature gradually rises over the course of a catalyst’s lifetime to make up for the decline in the catalytic activity [1,14].

Catalysts with high stability and a long lifespan are extremely valuable for developing industrial processes. Nevertheless, catalysts always exhibit deactivation behavior during industrial application. Identifying and investigating the causes of catalyst deactivation, followed by developing more stable catalysts, appears to be crucial. Prior investigation into possible causes of Cu/Zn/Al catalyst deactivation during the production of methanol from syngas has revealed the following: (1) the trace of the contaminant in the feed gas poisoned the catalyst [15,16]; (2) high reaction temperature led to sintering of Cu particles [6]; and (3) carbon deposition [17]. Recently, some new observations for Cu/Zn/Al catalyst deactivation in CO hydrogenation to methanol were made, owing to the advancement of modern characterization techniques. Lunkenbein et al. studied the deactivation behavior of a Cu/Zn/Al catalyst for methanol synthesis over a 148-day time-on-stream (TOS) period. It was proposed that the deactivation was caused primarily by changes in the ZnO moiety, given that ZnO was the most dynamic species and Cu nanoparticles underwent relatively minor modifications [18]. It is worth noting that there are differences between CO hydrogenation and CO₂ hydrogenation for methanol synthesis. The in-situ water formation during CO₂ hydrogenation to methanol may hasten the crystallization of Cu and ZnO components, resulting in the deactivation of Cu/ZnO-based catalysts [19]. The catalyst’s dynamic structural changes throughout the deactivation period are still not fully understood, and further research is needed to determine the true causes of deactivation [20].

In this study, the Cu/ZnO/Mn/Nb/Zr/Al₂O₃ catalyst, which was denoted as CZMNZA, was evaluated for extended reaction duration. With the aid of several characterization techniques, the detailed changes in the structural and chemical states of the spent catalysts were investigated, and the stability of the catalyst was comprehensively examined. In addition, the catalyst formulation was expected to enhance CO₂ conversion and methanol selectivity and at the same time slow the rate of deactivation. From a technoeconomic perspective, enhancement in catalytic activity can save capital expenditure (CAPEX) by reducing the number of recycling compressors to reduce the recycling volume.

2. Materials and Methods

2.1. Catalyst Preparation

CZMNZA catalyst was prepared via the wet impregnation method. Firstly, commercially purchased Al₂O₃ support from Merck was heated at 400 °C in flowing Ar for 5 h to eliminate moisture and impurities. The amount of precursor, Cu/ZnO (15 wt.%) with ratio of 70:30 and promoters (0.09 wt.%) with Mn:Nb:Zr ratio of 4:1:1, was calculated based on the prepared total mass of the catalyst. The desired amount of copper (II) nitrate trihydrate (Cu(NO₃)₂.3H₂O), zinc nitrate hexahydrate (Zn(NO₃)₂.6H₂O), manganese (II) nitrate tetrahydrate (Mn(NO₃)₂.4H₂O), ammonium niobate (V), oxalate hydrate (C₄H₄NNbO₉.xH₂O), and zirconium (IV) oxynitrate hydrate (ZrO(NO₃)₂.H₂O) were dissolved in a required amount of deionized water to produce a 0.5 M aqueous precursor solution. The solution was stirred for one hour to ensure homogeneity. The prepared aqueous precursor solution was then added into a beaker containing Al₂O₃ support in a dropwise manner. The solution’s pH was maintained at 7 with the addition of 10% ammonia solution or 10% nitric acid solution. The mixture was filtered and rinsed with deionized water after 24 h of stirring. The paste was dried for 12 h at 120 °C and calcined for 4 h at 350 °C.

2.2. Catalyst Performance Evaluation

The catalyst longevity study was carried out using a stainless steel fixed-bed reactor (Microactivity Reference, PID Eng.Tech) for two, seven, and 14 days. The catalyst powder was reduced in H₂ flow at 243 °C for 6 h and then evaluated for CO₂ hydrogenation reaction at a 300 °C reaction temperature, 31.25 bar reaction pressure, 2160 mL/g.hr GHSV, and H₂:CO₂ of 10:1. After each reaction period, spent catalyst samples were retrieved and characterized.

2.3. Catalyst Characterization

Textural analysis was performed on a Micromeritics ASAP 2020 analyzer by determining nitrogen adsorption/desorption isotherms at −196 °C. Prior to analysis, the sample was degassed at 350 °C (heating rate 10 °C/min). The Brunauer-Emmett-Teller (BET) specific surface area and pore volume were calculated using the adsorption data. The mean pore diameter of the isotherm desorption branch was calculated using the Barrett-Joyner-Halenda (BJH) model [21]. Morphologies and elemental composition of the samples were observed on the Hitachi SU8020 field emission scanning electron microscope-energy dispersive x-ray (FESEM/EDX). Samples were prepared by dispersing the powdered samples on carbon tape and aluminum tape. Particle size distribution of the spent catalysts was measured using a high-resolution transmission electron microscope (HRTEM). The powdered sample was suspended in isopropanol, followed by ultrasonication for 30 min. The grid was then placed in the sample chamber to analyze the shape of the nanoparticles, the size of metal oxide particles, and the coverage on the support. The N₂O chemisorption technique was used to determine Cu dispersion, surface area, and particle size. The analysis was performed using a Micromeritics Autochem II 2920 instrument. The surface composition of the spent catalyst was determined by x-ray photoelectron spectroscopy (XPS). The XPS spectra were obtained using an XPS Thermo-Fisher K-Alpha instrument. X-ray diffraction (XRD) patterns were carried out on a Shimadzu XRD-7000 by Shimadzu, Japan. The XRD measurements were performed at room temperature using 2θ (Bragg angle) ranging from 10° to 80°, with step size 0.024°/step and 1s/step.

3. Results and Discussion

3.1. Catalyst Performance in Extended Reaction Duration

Table 1. Catalytic performance of CZMNZA catalyst in extended reaction durations.

Reaction Duration	CO2 Conversion, XCO2 (%)	MeOH Selectivity, SMeOH (%)	MeOH Yield, YMeOH (%)
5 h	30.7	66.3	20.3
2 days	27.5	48.8	13.4
7 days	27.5	49.4	13.6
14 days	25.0	50.4	12.6

Reaction conditions: 300 °C, 31.25 bar, 2160 mL/g.hr and H₂: CO₂ at 10:1.

summarizes the results of the stability tests for the CZMNZA catalyst at a reaction duration of 5 h (reference), 2 days, 7 days, and 14 days. Comparing the 2 days reaction results to those of 7 days reaction showed that they remained stable. However, when the reaction duration was prolonged to 14 days, the CO₂ conversion and methanol yield decreased by 5.7% and 7.7%, respectively, whereas methanol selectivity remained nearly stable, as the 1.6% increase can be attributed to experimental uncertainties. Nonetheless, when comparing the methanol selectivity at 14 days to that of 5 h, a significant decrease in methanol selectivity was observed, most likely due to changes in the physicochemical properties of the catalyst. According to the literature, the deactivation rate of a Cu-based catalyst was higher in the first 350 h than it was for longer periods of time [21].

Figure 1a–c shows the time-on-stream (TOS) plots for all extended reactions in terms of CO₂ conversion and MeOH yield. These plots show that the catalytic activity remained relatively stable throughout the tested durations.

3.2. Physicochemical Properties of Spent Catalyst

3.2.1. Textural Properties (N₂ Adsorption-Desorption)

Table 2. Textural properties of catalyst samples.

Samples	BET Surface Area (m2/g)	Pore Volume (cm3/g)	Pore Size (nm)
Fresh catalyst	110.9	0.2	7.7
Spent catalyst 5 h	107.8	0.2	8.0
Spent catalyst 2 days	105.0	0.2	8.0
Spent catalyst 7 days	88.1	0.2	8.2
Spent catalyst 14 days	100.4	0.2	8.3

shows the textural properties of the synthesized catalysts in terms of the BET surface area, pore volume, and pore size. The pore volumes remained the same at 0.2 cm³/g for all samples. Since N₂ physisorption demonstrated no loss of pore volumes over prolonged exposure to the catalyst, pore clogging did not contribute to the reported catalyst deactivation [22]. The BET surface areas varied between 88.1 and 110.9 m²/g, and the pore diameters ranged between 7.7 and 8.3 nm. A slight decrease in BET surface areas was observed for all the spent catalysts relative to that of the fresh catalyst sample. On the other hand, the pore size exhibited an increasing trend upon extended reaction duration. This trend suggests that sintering occurred to a lesser extent after the reaction was carried out at 300 °C for a longer duration, possibly due to the presence of water vapor in the product mixture. Sintering is often observed at high temperatures (>500 °C); however, it can be accelerated by the presence of water vapor [23].

3.2.2. Morphology and Elemental Composition (FESEM/EDX)

The morphologies and elemental mappings of the fresh and spent catalyst samples are depicted in Figure 2a–e. These FESEM images revealed no discernible trend in the morphology of the catalyst after prolonged reaction duration. All the samples have an irregular morphology.

The elemental compositions were determined using EDX analyses on carbon tape and aluminum tape. The results are presented in

Table 3. Elemental compositions of samples using EDX analyses on carbon tape.

Samples	Element (wt.%)
	Al	O	Cu	Zn	Mn	Nb	Zr
Fresh catalyst	29.87	43.87	16.11	6.33	0.12	0.30	3.40
Spent catalyst 5 h	29.69	43.14	17.34	5.70	-	0.23	3.63
Spent catalyst 2 days	28.98	40.72	18.09	8.48	0.21	-	3.52
Spent catalyst 7 days	30.22	41.20	16.45	8.25	-	0.22	3.66
Spent catalyst 14 days	30.56	41.13	16.43	7.62	0.21	0.40	3.65

and

Table 4. Elemental compositions of samples using EDX analyses on Al tape.

Samples	Element (wt.%)
	C	O	Cu	Zn	Mn	Nb	Zr
Fresh catalyst	8.24	76.55	9.70	4.09	0.07	-	1.35
Spent catalyst 5 h	23.73	64.62	7.70	2.83	0.05	0.04	1.03
Spent catalyst 2 days	23.45	62.89	8.83	3.76	0.10	0.07	0.90
Spent catalyst 7 days	20.00	65.25	9.53	4.16	0.01	0.05	1.00
Spent catalyst 14 days	24.91	62.07	8.37	3.67	-	0.05	0.93

, respectively. The amount of Cu active metal remained in the range of 16% to 18.1%. The Zr promoter was detected in all samples between 3.4% to 3.7%, while the Mn and Nb promoters were present in some but not all tested samples. The observed variation in the bulk elemental compositions of these elements could be due to sample inhomogeneity during the impregnation process, as suggested by the EDX analysis. Nonetheless, the carbon content of the spent catalysts was higher than that of the fresh catalyst. Although carbon was found on every spent catalyst (Table 4), it did not accumulate significantly enough to clog pores since the pore volumes were constant across all samples. According to the classification of the methanol synthesis reaction as “coke insensitive” [23], carbon formed on active sites can be readily removed by hydrogen or other gasifying agents. This trend suggests that the slight decay in the catalyst performance during prolonged reaction duration may not be entirely due to carbon formation.

3.2.3. Particle Size Distribution (TEM)

Figure 3a–e show TEM images of the catalyst samples. The average particle sizes of the samples are shown in

Table 5. Average particle size of catalyst samples.

Samples	Average Particle Size (nm)	Standard Deviation (nm)
Fresh catalyst	20.9	±4.3
Spent catalyst 5 h	22.0	±5.9
Spent catalyst 2 days	11.7	±1.8
Spent catalyst 7 days	10.6	±3.6
Spent catalyst 14 days	9.4	±1.7

. It shows a general decreasing trend as reaction duration increases. After 14 days of reaction, the average particle size was reduced by about 55% compared to that of the fresh catalyst. Particle size reduction was most likely the result of thermal degradation via the redispersion phenomena [20,23]. The redispersion resulted from the formation of volatile metal oxide complexes that attached to the catalyst surface which were subsequently decomposed to smaller crystallites. The particle size distribution of fresh and spent catalysts is shown in Figure 4. Reactions prolonged for up to 14 days showed significantly smaller particle size distribution, which is in agreement with the redispersion mechanism.

3.2.4. Cu Dispersion, Surface Area, and Particle Size (N₂O Chemisorption)

The values of Cu dispersion, Cu surface area, and Cu metal particle size are listed in

Table 6. Cu dispersion, surface area, and particle size.

Samples	Cu Dispersion (%)	Cu Surface Area (m2/g)	Cu Particle Size (nm)
Fresh catalyst	10.3	6.2	11.4
Spent catalyst 5 h	9.7	5.8	12.1
Spent catalyst 2 days	14.8	8.9	8.0
Spent catalyst 7 days	13.5	8.1	8.7
Spent catalyst 14 days	16.0	9.6	7.3

. As the reaction duration increased, the Cu dispersion and Cu surface areas exhibited an upward trend, whereas the Cu particle size showed a reverse trend. These trends agree with the results of TEM analyses which suggest the occurrence of thermal degradation via the redispersion mechanism. The thermal degradation of the catalyst may result in a slight decrease in the catalytic performance over extended reaction durations.

According to the literature, the deactivation of Cu-based catalysts during methanol synthesis is primarily due to the aggregation of Cu particles [6,17]. In many cases, sintering, Ostwald ripening, or some other process caused the metal to grow in size at high temperatures. The Cu particle size of the fresh catalyst increased from 11.4 nm to 12.1 nm after 5 h of reaction, while the Cu dispersion decreased from 10.3% to 9.7%. Nevertheless, it is interesting to note that the average size of the Cu particle of the fresh catalyst significantly decreased from 11.4 nm to 8.0 nm after 2 days of reaction, while the Cu surface area significantly increased from 10.3% to 14.8%. This change may have resulted from rebuilding Cu species during the early phase of the reaction [20]. By extending the reaction time from 2 days to 14 days, the size of the Cu particle decreased to 7.3 nm, and the Cu dispersion increased from 14.8% to 16.0%. The dispersion of Cu was not significantly altered. In other words, there was no discernible aggregation of Cu particles throughout the CO₂ hydrogenation reaction phase [20]. Based on these findings, it was suggested that the sintering of Cu particles might not be the primary cause of CZMNZA catalyst deactivation for CO₂ hydrogenation to methanol. This trend in Cu surface area was different from that in BET surface areas obtained from N₂ adsorption-desorption analyses. The BET surface area of the samples decreased after an extended reaction duration, however, the Cu surface area increased. Thus, other factors that affect BET surface area values could be responsible for the variation in the results obtained. Nevertheless, the trend of catalytic activity shows a correlation to Cu surface area in which the catalytic activity remained stable under prolonged reaction durations. This catalyst formulation shows a potential to be used for methanol synthesis via CO₂ hydrogenation in which a similar trend in catalytic activity has been observed to the one reported in the literature [6].

3.2.5. Surface Composition (XPS)

Figure 5a–f show the XPS spectra of Cu 2p, Zn 2p, Mn 2p, Zr 3d, and Nb 3d, respectively.

Table 7. Binding energy for each element.

Samples	Binding Energy (eV)
	Cu 2p3/2	Zn 2p3/2	Mn 2p3/2	Nb 3d3/2	Zr 3d5/2
Fresh catalyst	932.7	1020.9	638.9	205.5	183.2
Spent catalyst 5 h	933.1	1021.1	641.1	206.9	182.3
Spent catalyst 2 days	932.3	1020.8	638.2	206.5	185.1
Spent catalyst 7 days	930.2	1020.0	638.4	205.7	182.6
Spent catalyst 14 days	931.9	1021.2	651.5	207.1	183.4

and

Table 8. Surface atomic composition.

Catalyst	Atomic Composition (%)								Atomic Ratio
	Al	O	C	Cu	Zn	Mn	Nb	Zr	Cu/ Zn	C/ Al	Cu/ Al	Mn/ Al	Nb/ Al	Zr/ Al
Fresh catalyst	36.6	49.4	0.5	5.0	7.7	1.0	0.1	0.2	0.7	0.01	0.1	0.02	0.003	0.01
Spent catalyst 5 h	23.2	63.8	2.7	5.0	4.5	0.6	0.1	0.1	1.1	0.1	0.2	0.03	0.04	0.04
Spent catalyst 2 days	28.3	57.8	3.0	4.8	5.6	0.1	0.1	0.2	0.9	0.1	0.2	0.04	0.04	0.07
Spent catalyst 7 days	27.7	59.2	4.1	3.0	5.6	0.04	0.2	0.1	0.5	0.1	0.1	0.01	0.07	0.03
Spent catalyst 14 days	27.3	62.0	3.6	2.3	4.9	0.04	0.1	0.2	0.5	0.1	0.1	0.01	0.04	0.07

show the binding energies and atomic compositions, respectively. The surface composition of Cu was detected at 5% on the fresh catalyst and a decreasing trend in Cu:Zn ratio was observed on samples used in longer reaction durations. The observed decrease in Cu:Zn ratios could be due to the enrichment of ZnO particles on the surface of the catalyst, which increased the spacing of the Cu sites. The results from XPS analyses agree with the results of N₂O chemisorption studies, which found an increase in Cu dispersion on spent catalysts as reaction duration increased. The amounts of surface carbon on the spent catalyst samples were higher than those on the fresh catalyst.

However, the C/Al ratios remained constant as reaction durations increased, suggesting that the observed catalyst decay might not be due to carbon formation on the surface of the catalyst. Changes in the oxidation state of Cu have been observed upon the extended duration of the reaction. Cu²⁺ species were found on the fresh catalyst at 932.7 eV, showing that the Cu species for fresh Cu/ZnO/Al₂O₃ catalyst primarily existed in the form of CuO [24,25]. It was clearly seen that the shakeup satellite peak at the high binding energy of 941 eV existed, which was another hallmark of the Cu²⁺ compound. This is due to the charge transfer between the 3d orbital of the transition metal and the 2p orbital of the oxygen-containing ligand [26]. There were no significant changes in the peak position of Cu²⁺ in the catalyst after a reaction of 5 h and 2 days, indicating that there is no difference in the chemical states of copper between the fresh and the spent catalyst which were exposed to reaction gas mixture (CO₂/H₂). The observation in this study is supported by the findings reported by Dai et al. [25]. Nevertheless, the loss of Cu²⁺ species from the surface catalysts can be confirmed by a decrease in shake-up satellite species [26]. Upon extended reaction duration of 7 days and 14 days, mixed oxidation states of Cu, such as 0, +1, and +2 have been detected, suggesting that Cu species can be reduced in the CO₂ hydrogenation environment. The decrease in Cu:Zn ratio is expected in the reductive atmosphere, as reported by Liang et al. [20].

The binding energies of the Zn 2p_3/2 and Zn 2p_1/2 XPS peaks of the fresh catalyst, which can be observed in Figure 5b are ~1021 eV and ~1044 eV, respectively. These Zn species peaks on the catalyst surface are attributable to Zn²⁺ species in bulk ZnO [27,28]. After 5 h and 14 days of reaction durations, the Zn 2p_3/2 peaks moved toward slightly higher BEs at 1021.1 eV and 1021.2 eV, respectively. This positive chemical shift could be due to the adsorption and heterolysis of H₂ on ZnO. It has been reported that H₂ can be adsorbed by ZnO in the presence of Cu in a reduced Cu/ZnO catalyst, which can lead to heterolysis according to the following expression:

H-H + ZnO → H^δ− Zn-OH^δ+

(4)

Thus, the adsorbed hydrogen species at the Zn site (H^δ−) may trigger a shift in the Zn 2p core-level peak toward a higher BE position [29]. When the ZnO coverage is increased, the Zn 2p_3/2 peak position decreases, but it remains near the values for Zn²⁺ [30,31,32]. Some oxygen vacancies may be created inside the oxide layer that covers Cu (111) for significant zinc oxide coverages when hydrogen and oxygen react to form water [33]. The above findings supported the observed increase in the amount of Zn, which resulted in a decreasing trend on the Cu:Zn ratio on the surface of spent catalysts.

The XPS spectra for promoters, Mn 2p, Nb 3d, and Zr 3d are shown in Figure 5c–e. Mn 2p XPS spectra for fresh catalyst were deconvoluted into a few peaks in the binding energy (BE) range of ~638.9 eV to ~651 eV, suggesting the existence of Mn in the catalyst with varied oxidation states. The Mn 2p_3/2 peak can be found to be quite broad, which may be due to the coexistence of various Mn 2p_3/2 levels, Mn²⁺, Mn³⁺, and Mn⁴⁺ species at the interfacial catalyst [34]. The BEs at about 638.9 eV, 642.5 eV, and 651.1 eV can be assigned to Mn²⁺, Mn³⁺, and Mn⁴⁺, respectively [35,36]. After a reaction of 2 days and 7 days, a slight shift appears towards lower BE for the Mn²⁺ oxidation state which suggested that Mn²⁺ species can be reduced to metallic Mn. In addition, other Mn species had disappeared after prolonged reaction duration. However, when the reaction was extended up to 14 days, only a single small peak at higher BE was observed which revealed that all Mn species were removed during the extended CO₂ hydrogenation reaction. The atomic composition of Mn also decreased after an extended reaction duration. This observation agrees with the result of EDX analysis on Al tape where no Mn was detected.

The spectrum of the core level Nb 3d is shown in Figure 5d. The appearance of the doublet peaks at 3d_3/2 and 3d_5/2 for the fresh catalyst at 205.5 eV and 204.4 eV, respectively, suggested that the binding energy for NbO/NbO₂ species is in accordance with that of the standard spectra with a slight shift observed [37]. Nevertheless, the binding energy of 205.5 eV was shifted to the right at 206.5 eV–207.1 eV after an extended reaction duration of up to 14 days (except for 7 days), which indicates the presence of Nb₂O₅ [38]. The second peak at 204.4 eV was shifted to the lower binding energy in the range between ~201 eV–203 eV, indicating the existence of metallic Nb [39] and suggesting that during the longer reaction, the metal oxides can be reduced to their metallic form.

The XPS profile for Zr 3d (Figure 5e) revealed two peaks for the fresh, 5 h, and 7 days samples, with the lower binding energy (182 eV–183 eV) indicating the Zr 3d_5/2 orbital and the higher binding energy (185 eV–187 eV) reflected the Zr 3d_3/2 orbital. These values are consistent with those reported in the literature [40]. The 3.0 eV energy difference between these two peaks implies that there are two distinct forms of zirconium ions, with the lower binding energy corresponding to Zr⁴⁺ (ZrO₂) and the higher binding energy representing Zr²⁺ species [41]. The peaks were shifted to lower and higher binding energies for spent catalysts of 2 days and 14 days, which was caused by hydrogen dissolution. The formation of a Zr-H chemical bond substantially modifies the dependency of core-level changes on the distance between hydrogen and zirconium atoms [42]. In addition, the peak shift to higher binding energy suggests that zirconium’s oxidation state has increased. As the reaction prolonged, there may have been a greater proportion of zirconia (ZrO₂) than zirconium oxides (ZrO_x,y, x,y < 2, higher and lower oxidation states). This could be explained by the fact that zirconium, with a lower oxidation state, completely oxidizes into zirconia with a +4-oxidation state. Whilst it is vice versa for the peak shifting to a lower binding energy [43,44].

3.2.6. Phase Analysis (XRD)

XRD analysis was used to determine the catalyst’s phase composition and crystallographic structure. Figure 6 shows the XRD patterns of the CZMNZA catalyst at different stages (fresh, 5 h, 2 days, 7 days, and 14 days). Two significant peaks of gamma aluminum oxide (γ-Al₂O₃) as catalyst support can be identified at 2θ of 45.90° and 67.18° [45] in fresh and all-spent catalysts, which suggested there is no phase change of gamma aluminum oxide pre and post reaction. Tenorite CuO peaks were found in fresh catalyst at 2θ 32.5°, 35.5°, 38.8°, 48.8°, 58.3°, and 61.5° [45]. Under H₂ flow and reaction conditions, the peaks decrease in intensity, with the appearance of Cu (111), Cu (200), and Cu (220) peaks at 43.1°, 50.1°, and 73.7°, respectively, in all spent catalyst. Bulk crystalline CuO in a fresh catalyst is reduced to metallic Cu, an active phase for CO₂ hydrogenation to methanol [46]. Nevertheless, after a reaction duration of 14 days, the peak intensity of metallic Cu was significantly diminished, which may have been caused by a change in the species of Cu [20]. This statement agreed with the change in catalytic performance after 14 days of reaction. On the other hand, zincite and ZnO diffraction peaks of fresh catalyst were observed at 32.5° and 48.8° [45], although the peaks may overlap with CuO peaks [47,48]. The ZnO diffraction peaks did not appear on the spent catalyst samples compared to the fresh catalyst. It was suggested that the metal species agglomeration occurred by blockage of CuO and ZnO surface sites under hydrogenation testing in time on stream, resulting in low catalytic activity [49].

4. Conclusions

Cu/ZnO-based catalyst was evaluated for its stability and activity in methanol synthesis via CO₂ hydrogenation. Catalytic activity was stable in a duration of reaction of up to seven days. However, the catalytic activity in terms of CO₂ conversion and methanol yield decreased by about 5.7% and 7.7%, respectively, upon 14 days of reaction. A slight reduction in catalytic activity during prolonged reaction duration has been suggested due to thermal degradation instead of carbon formation. There was no loss of pore volumes over prolonged exposure to the catalyst, and pore-clogging did not contribute to the catalyst deactivation. Surface composition analysis showed changes in metal species after prolonged reaction duration, which was agreed by XRD analysis. N₂O chemisorption and TEM analysis revealed that the redispersion mechanism phenomenon had occurred.

Since catalyst stability and deactivation are crucial in the performance of methanol synthesis reactions, future work should focus on catalyst performance testing using a recycling reactor setup to study the effect of the unconverted feed gases and by-product CO on the catalytic activity. The catalyst aging test with the right standard procedure should also take place to investigate the lifetime of the catalyst which contributed to the downtime of the reactor at an industrial scale.