Preparation and activity of Cu/Zn/Al/Zr catalysts via hydrotalcite-containing precursors for methanol synthesis from CO₂ hydrogenation

Abstract

A series of Cu/Zn/Al/Zr catalysts were synthesized by calcination of hydrotalcite-containing precursors with different Cu²⁺/Zn²⁺ atomic ratios (n). Two other catalysts (n = 2) were also prepared via phase-pure hydrotalcite-like and conventional rosasite precursors for comparison. XRD and UV-Vis-NIR DRS characterizations demonstrate that most Cu²⁺ of hydrotalcite-containing materials did not enter the layer structure. The Cu dispersion of the catalysts decreases with the increase of Cu content, while both the exposed Cu surface area and the Cu⁺ and Cu⁰ content on the reduced surface reach a maximum when n is 2. The catalytic performance for the methanol synthesis from CO₂ hydrogenation was also tested. The catalytic activity and selectivity of the catalysts (n = 0.5–4) via hydrotalcite-containing precursors rise first and then decrease with increasing Cu²⁺/Zn²⁺ ratios, and the optimum performance is obtained over the catalyst with Cu²⁺/Zn²⁺ = 2. Moreover, the Cu/Zn/Al/Zr catalyst (n = 2) via hydrotalcite-containing precursor exhibits the best catalytic performance, which is mainly due to the maximum content of active species compared with another two catalysts derived from different precursors.

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1. Introduction

It is well known that the increase of the CO₂ concentration in the atmosphere is considered a major anthropogenic source for global warming (“the greenhouse effect”). Pressed by the imperative to avoid further irreversible damage to the environment for the future of mankind, many countries worldwide are now pursuing a general reduction of carbon dioxide emissions. Recently, the catalytic hydrogenation of carbon dioxide into valuable chemicals and fuels has been recognized as one of the most effective and economical ways to fix and utilize a large amount of emitted CO₂.^1–3 Among them, methanol is considered the most valuable product since methanol is a starting material for several important chemicals such as formaldehyde, dimethyl ether (DME), acetic acid, and a wide variety of other products. It can be used as a fuel additive and as a clean burning fuel.³ Methanol can also be converted to high-octane gasoline (MTG process),⁴ aromatics (MTA process),⁵ ethylene and propylene in the MTO (methanol-to-olefins) process,⁶ as well as other useful petrochemicals. Therefore, methanol can reduce the dependence on fossil fuel as a source of both energy and resource in the future. Furthermore, compared with state-of-the-art syngas-based methanol synthesis process, the lower byproduct content of methanol produced from CO₂ hydrogenation may allow a simplified distillation, and is thus beneficial for further chemical conversions.⁷

Cu/Zn/Al catalysts are used predominantly in the industrial methanol synthesis process starting from synthesis gas (CO/CO₂/H₂).^8,9 Moreover, the Cu/Zn/Al catalysts have been widely investigated for methanol synthesis from CO₂ hydrogenation.^7,10,11 In the previous work, it was found that the catalytic activity of Cu/Zn/Al catalyst for methanol synthesis not only dependent on the number of active sites but also on the Cu/Zn dispersion.^12–15 The promoter, such as Zr, is known to enhance the Cu/Zn dispersion and thus improve the catalytic activity of methanol synthesis catalysts. By using a well controlled co-precipitation procedure, the Cu/Zn/Al/Zr catalyst shows excellent performance for CO₂ hydrogenation to methanol.^12,13,16

Studies on the phases in the precipitate of Cu-containing hydroxocarbonates revealed that some precipitate phases played important roles for the catalytic preformance.^9,17 The mixed metal oxides obtained by controlled thermal decomposition of hydrotalcite-like compounds (HTlcs), with general formula of [M²⁺_1−xM³⁺_x(OH)₂]^x+ (Aⁿ⁻)_x/n·mH₂O where M²⁺ and M³⁺ are the divalent cation and trivalent cation respectively,^18,19 possess homogeneous microstructure, good dispersion of M²⁺ and M³⁺ at an atomic level, enhanced metal-oxide interaction after reduction, stability against sintering and high specific surface area, as well as strong basic properties.^20–23 Therefore, HTlcs are among the most investigated catalyst precursors for the remarkable properties of the final catalysts.

However, to the best of our knowledge, there are few reports on the structure and activity of Cu/Zn/Al/Zr catalysts derived from HTlcs for methanol synthesis from CO/CO₂ hydrogenation. The copper-containing HTlcs represent a rather peculiar system, probably attributed to the coordination requirements of Cu²⁺ to form distorted octahedra, introducing the Jahn–Teller effect into the layers and, thereby, destabilizing the hydrotalcite-like (htl) structure.^21,24 According to Behrens et al., the most compounds derived from hydrotalcite-like precursors will be amorphous after mild calcination, more than one Cu species is formed, only a small fraction of Cu particles is exposed to the gas phase and Cu species, present in ex-htl areas, reduce at higher temperatures for the copper rich catalysts derived from htl materials.²¹ Therefore, it seems unlikely that the phase-pure htl phase has an activity promoting effect on the resulting copper rich catalysts.

To overcome the disadvantage of the phase-pure copper-containing HTlcs, the Cu/Zn/Al/Zr hydrotalcite-containing materials were synthesized with changing the content of Cu, which contain Zn/Al/Zr htl phase with most Cu²⁺ located on the surface of the layered structure. For comparison, two other phase-pure hydrotalcite-like and conventional rosasite materials as precursors for Cu/Zn/Al/Zr catalysts were synthesized. The catalytic properties of the catalysts derived from these precursors were examined for methanol synthesis from CO₂ hydrogenation. The effect of Cu²⁺/Zn²⁺ atomic ratios for the hydrotalcite-containing precursor and the influence of precursors on the structure and the activity of the Cu/Zn/Al/Zr catalysts were discussed in detail.

2. Experimental

2.1 Preparation of catalysts

The Cu/Zn/Al/Zr hydrotalcite-containing precursors with different Cu²⁺/Zn²⁺ atomic ratios (0–4) were prepared by co-precipitation method at room temperature (20 °C) under air, and the atomic ratios of Zn²⁺ : Al³⁺ : Zr⁴⁺ was 3 : 0.7 : 0.3, adapted from that reported by Tichit et al.²⁵ Typically, two aqueous solutions, a metal salt solution of Zn(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O and ZrO(NO₃)₂·2H₂O and a mixed solution of 2 M NaOH and 0.5 M Na₂CO₃ precipitant, were prepared and then were added dropwise to 200 mL of deionized water under vigorous stirring. After the precipitation was complete, an aqueous solution of Cu(NO₃)₂·3H₂O and the solution of precipitant were added simultaneously into the resulting suspension. The pH in the glass reactor during precipitation was kept at a constant value of 10.5 ± 0.2 by a HI 8424 pH meter. Thereafter the product was aged at 80 °C for 15 h under stirring in the mother liquor and then filtered and washed several times with deionized water to remove residual sodium. Finally, the filter cakes were dried overnight at 80 °C in an oven and further calcined in air at 450 °C for 4 h. For comparison, Cu/Zn/Al/Zr catalysts were also prepared via phase-pure htl and conventional rosasite precursors according to ref. 19 and 26, respectively. Metal compositions and nominations of the prepared materials are listed in Table 1.

Table 1 Metal compositions and nomination of the prepared materials

Precursor	Cu^2+/Zn^2+/Al^3+/Zr^4+	Cu^2+/Zn^2+	Precursor nomination	Calcined materials nomination
a The Al content should be increased to above 20 mol% to synthesize phase-pure Cu/Zn/Al/Zr HTlcs.^17 b n means the Cu^2+/Zn^2+ atomic ratio.
Hydrotalcite-containing	X : 3 : 0.7 : 0.3, X = 0, 1.5, 3, 6, 9 and 12	0, 0.5, 1, 2, 3 and 4	n-CZAZ^b	cn-CZAZ
Phase-pure htl	6 : 3 : 2.7 : 0.3^a	2	2-htlCZAZ	c2-htlCZAZ
Rosasite	6 : 3 : 0.7 : 0.3	2	2-coCZAZ	c2-coCZAZ

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2.2 Characterization of catalysts

The surface area of samples was determined by N₂ adsorption–desorption at liquid nitrogen temperature 77.35 K using a Micromeritics Tristar3000 instrument. Sample degassing was carried out at 473 K prior to acquiring the adsorption isotherm. The surface areas were calculated according to the Brunauer–Emmett–Teller (BET) method.

X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-A X-ray diffractometer using a nickel-filtered Cu-Kα (0.15418 nm) radiation source. The intensity data were collected over a 2θ range of 5–75° with a 0.05° step size and using a counting time of 1 s per point.

The exposed copper surface area (S_Cu) was determined by dissociative N₂O adsorption and carried out on Micromeritics AutoChem 2920 instrument the procedure described by Yuan et al.²⁷ The catalysts (0.1 g) were first reduced in 5% H₂/Ar mixture (30 mL min⁻¹) for 1 h at 350 °C, then cooled to 90 °C and isothermally purged with Ar for 30 min, after which the sample was exposed to N₂O (85 mL min⁻¹) for 1 h to ensure complete oxidation of Cu. The samples were then flushed with Ar to remove the N₂O and cooled to room temperature. Finally, a pulse of pure H₂ was passed over the catalyst at 300 °C, and the surface Cu⁺ were reduced in the pulse of pure H₂. By quantifying the amount of consumed H₂, the dispersion of Cu and exposed Cu surface area of the catalyst were calculated by the eqn (1) and (2).

S_Cu = (2n_H2 × N)/(1.4 × 10¹⁹ × W)(m²g⁻¹) (2)

where n_H2 was molar number of consumed H₂, D was the dispersion of Cu, M_Cu was relative atomic mass (63.546 g mol⁻¹), W was the weight of the catalyst, and X was the stoichiometric composition of Cu (wt%) calculated under assumption that only CuO has undergone the reduction to metal. S_Cu was the exposed copper surface area per gram catalyst, N is Avogadro’s constant (6.02 × 10²³ atoms mol⁻¹), and 1.4 × 10¹⁹ was the number of copper atoms per square meter.^8,28

Diffuse reflectance spectroscopy (DRS) measurements of the calcined samples were recorded by a Cary 5000 UV-Vis-NIR spectrophotometer in the range 200–1100 nm at room temperature in air, and the Schuster–Kubelka–Munk function (F(R)) was calculated.

Thermal decomposition of the catalyst precursors were studied by thermogravimetric (TG) and differential scanning calorimetry (DSC) method by using STA-409 (NETZSCH Co.). Measurements were performed in the temperature range of 30–700 °C with linear temperature program β = 10°C min⁻¹ in continuous flow of pure nitrogen (50 mL min⁻¹).

The morphology of the samples was observed by a JEM 2010 high-resolution transmission electron microscope (TEM) operated at 200.0 kV and a Hitachi S-4800 scanning electron microscope (SEM) operated at 20.00 kV.

X-ray photoelectron spectroscopy (XPS) measurements were performed over a Quantum 2000 Scanning ESCA Microprobe instrument with Al Ka radiation (15 kV, 25W, hν = 1486.6 eV) under ultrahigh vacuum (10⁻⁷ Pa), calibrated internally by the carbon deposit C (1 s) (Eb = 284.7 eV). Samples were treated under pure hydrogen at 350 °C for 2 h in the pre-treatment chamber before being transferred to the analysis chamber.

Temperature program reduction (TPR) was carried out in a U-tube quartz reactor, using hydrogen-argon mixture (containing 5 vol.% of hydrogen) as the reductive gas at a flow of 30 mL min⁻¹. The samples (50 mg) were purged with Ar (30 mL min⁻¹) at 150 °C to remove physically adsorbed water and then reduced in a flow of H₂ + Ar at a heating rate of 5 °C min⁻¹ up to 600 °C. TCD was used to monitor the consumption of H₂.

2.3 Evaluation of catalysts

Activity measurements in the hydrogenation of CO₂ were carried out in a continuous-flow, high-pressure, fixed-bed reactor. Catalyst (1.5 mL, 40–60 mesh) diluted with 1.5 mL quartz sand (60 mesh) was placed in a stainless steel tube reactor. Prior to reaction, the catalyst was reduced in pure H₂ at a flow-rate of 80 mL min⁻¹ under atmospheric pressure. The reduction temperature was programmed to increase from room temperature to 350 °C and maintained at 350 °C for 8 h. The catalyst was then cooled to room temperature. After reduction, the activities of the catalyst samples in CO₂ hydrogenation process were determined under reaction conditions of 230–290 °C, 5.0 MPa, n(H₂) : n(CO₂) = 3 : 1, GHSV = 4000 h⁻¹. The steady-state activity measurements were taken after at least 24 h on the stream. The CO₂ hydrogenation-conversion and the carbon-based selectivity values for the hydrogenation products, CH₃OH, CO and hydrocarbons were calculated by an internal normalization method. The space time yields (STYs) of CH₃OH, which gave the amounts of CH₃OH produced per gram catalyst per hour, were defined as in eqn (3):where W_T was the total weight of CH₃OH and H₂O product (g); X(CH₃OH) was the mass fraction of CH₃OH; t was the reaction time (h); V was the volume of catalyst (mL).

Each data set was obtained, with an accuracy of ±2%, from an average of three independent measurements.

3. Results and discussion

3.1 Textural properties of the prepared materials

The XRD patterns of the precursors of the Cu/Zn/Al/Zr catalysts are shown in Fig. 1. For the uncalcined n-CZAZ samples, besides the major phase of typical pattern of the hydrotalcite-like structure [Zn₆Al₂(OH)₁₆CO₃·4H₂O] (JCPDS 38-0486), the CuO phase and some extent of zincite-type ZnO phase were also observed in n = 1, 2, 3 samples, which is different from the diffraction pattern of the pure hydrotalcite-like compound. The high pH of 10.5 and ageing temperature of 80 °C play an important role in the process formation of CuO phase by oxolation of Cu hydroxide species.^17,29 Benito et al.²⁹ have reported that it was impossible to hinder the ZnO segregation in Zn/Al HTlcs when the Zn²⁺/Al³⁺ atomic ratio was higher than 2. Furthermore, when the Cu²⁺/Zn²⁺ atomic ratio increased from 0 to 3, the intensity of the diffraction peaks for HTlcs and ZnO phase decreased, while that of CuO phase became more and more prominent.

Fig. 1 XRD patterns of the precursors of the Cu/Zn/Al/Zr catalysts. (■) Hydrotalcite, Zn₆Al₂(OH)₁₆CO₃·4H₂O; (□) hydrotalcite, Cu₃Zn₃Al₂(OH)₁₆CO₃·4H₂O; (○) Cu/Zn-malachite, (Cu,Zn)₂(OH)₂CO₃; (●) CuO; (✦) ZnO.

On the other hand, the 2-htlCZAZ material only showed the diffraction pattern of pure hydrotalcite-like compound [Cu₃Zn₃Al₂(OH)₁₆CO₃·4H₂O] (JCPDS 38-0629) and the diffraction peaks were much broader than that of Zn/Al/Zr htl structure in the n-CZAZ cases. In n-CZAZ sample, most Cu²⁺ did not exist in the layer structure of Zn/Al/Zr htl but in the form of CuO, while the Cu²⁺ was located in the layered structure for 2-htlCZAZ. It seems likely that the presence of Jahn–Teller distorted Cu-centered octahedra in the hydroxylated layers introduces lattice strain and causes the peak broadening.²¹ In addition, rosasite phase [(Cu,Zn)₂(OH)₂CO₃] (JCPDS 36-1475) was detected as a single phase in the 2-coCZAZ material.

Fig. 2 shows the XRD patterns of the calcined samples. The pattern of cn-CZAZ shows that the hydrotalcite-like structure completely collapsed after calcination,^30,31 and the intensities of diffraction peaks belonging to ZnO and CuO phases increased. In addition, the intensity of the diffraction peaks of CuO increases gradually with the Cu²⁺/Zn²⁺ atomic ratio increasing. It should be noticed that only weak peaks assigned to CuO can be found, and there are no peaks that can be attributed to ZnO, Al₂O₃ and ZrO₂ for c2-htlCZAZ and c2-coCZAZ cases, which indicate that CuO was poorly crystallized, and ZnO, Al₂O₃ and ZrO₂ existed in an amorphous state.¹²

Fig. 2 XRD patterns of the calcined Cu/Zn/Al/Zr catalysts prepared with different Cu²⁺/Zn²⁺ atomic ratios and via different precursor. (●) CuO; (✦) ZnO.

The physicochemical properties of the calcined Cu/Zn/Al/Zr catalysts are summarized in Table 2. For the cn-CZAZ samples, an increase in the Cu content leads to a marked decrease in the BET surface area. And the Cu dispersion of the catalysts decreases with increasing Cu²⁺/Zn²⁺ atomic ratio, however, the maximum exposed Cu surface area (17.24 m² g⁻¹) was observed in c2-CZAZ.

Table 2 Physicochemical properties of the calcined Cu/Zn/Al/Zr catalysts

Sample	Cu^a (wt%)	BET specific surface area (m^2 g^−1)		Dispersion^c (%)	Cu surface area^c (m^2 g^−1)
		Before reduction	After reduction^b
a The stoichiometric composition of Cu calculated under assumption that only CuO has undergone the reduction. b After reduction at 350 °C. c Calculated from N2O dissociative adsorption.
c0-CZAZ	0	53.5	—	—	—
c1-CZAZ	37.57	35.0	36.8	5.30	13.16
c2-CZAZ	54.62	28.8	30.6	4.66	17.24
c3-CZAZ	64.35	21.4	24.1	3.82	16.62
c2-htlCZAZ	47.66	52.3	58.7	5.07	16.34
c2-coCZAZ	54.62	58.0	60.0	4.57	16.90

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Although the BET specific surface areas of the cn-CZAZ is lower than that of the catalysts derived from the two other precursors, the exposed copper surface area increases in the order of c2-htlCZAZ < c2-coCZAZ < c2-CZAZ. It is therefore expected that the c2-CZAZ can provide more catalytically active sites with good dispersion. Moreover, the catalyst surface area slightly increases after reduction at 350 °C, the effect being more pronounced for the c2-htlCZAZ sample.

The UV-vis-NIR diffuse reflectance spectroscopy results reveal that all the calcined materials derived from different precursors show (Fig. 3) intense bands between 240 and 290 nm, which can be assigned to oxygen to copper charge-transfer bands.³² The absorption band around 370 nm ascribed to ZnO crystals can be observed in c2-CZAZ material,^33,34 while this band is absent in the c2-htlCZAZ and c2-coCZAZ samples.

Fig. 3 UV-vis-NIR diffuse reflectance spectroscopy of the calcined materials derived from different precursors.

It can be noted that a weak band at around 450 nm, which can be attributed to the formation of Cu⁺ three-dimensional clusters in the CuO matrix, existed in all the calcined materials except c2-htlCZAZ.³⁵ In addition, a very broad and weak band of c2-htlCZAZ sample at around 700 nm can be attributed to the 2E_g → 2T_2g spin-allowed d–d transition of Cu²⁺ ions in the distorted octahedral environment of the surface alumina sites with a spinel-type structure.³² However, this band is absent in c2-CZAZ and c2-coCZAZ samples, indicating that there is no octahedrically coordinated Cu²⁺ as most Cu²⁺ does not enter the layer structure of Zn/Al/Zr htl.

In order to clarify the decomposition process during the calcination, simultaneous thermogravimetry (TG) and differential scanning calorimetry (DSC) analysis techniques were carried out. The TG curve of samples n-CZAZ (Fig. 4) shows two major weight losses. The first weight loss occurs at 100–250 °C, mainly ascribed to the elimination of interlayer and physically adsorbed water molecules. The second weight loss between 250 and 350 °C is definitely smaller and can be attributed to the dehydroxylation of hydroxyl groups in the hydrotalcite-like layers as well as the removal of CO₂ from the decomposition of interlayer carbonate, resulting in the collapse of the htl crystal structure.³⁶ The DSC curve for samples n-CZAZ (Fig. 4) shows a single endothermic peak at around 190 °C with an endothermic shoulder at 270 °C, which involved the decomposition of the initial layered compound and collapsing of the layered structure.²⁴ Furthermore, a third weight loss of ca. 2–3 wt% is also observed in the n-CZAZ samples as a broad and very weak endothermic peak between 450–600 °C in the DSC curve, which was attributed to the decomposition of additional Zn-containing carbonates.²⁵ The peak temperatures corresponding to the three stages changed little with the increase in the Cu²⁺/Zn²⁺ atomic ratio indicating that Cu content can slightly influence the thermal stability of these materials.

Fig. 4 TG-DTG-DSC of the precursors of the Cu/Zn/Al/Zr catalysts.

For 2-htlCZAZ, the peaks corresponding to the former two processes of weight loss shift to lower temperature, which indicates rather low thermal stability of 2-htlCZAZ with Cu²⁺ located in htl structure. It might because that the introduction of Cu²⁺ by the Jahn–Teller effect destabilizes the layered structure. In addition, the third weight loss (ca. 4 wt%), between 500 and 600 °C, is recorded in the TG curve of phase-pure HTlcs containing Cu²⁺ and can be assigned to decomposition of Cu oxocarbonates formed during the second step of the thermal decomposition.²⁴

The 2-coCZAZ case, prepared by the conventional co-precipitation method, exhibits three weight loss peaks. The first step, attributed to the loss of physically adsorbed water molecules, is small and occurs at about 100 °C. The second step, a considerable weight loss (ca. 20 wt%) between 150 and 400 °C, is assigned to the decomposition of the rosasite phase. In addition, the third weight loss (ca. 4 wt%), between 450 and 550 °C, should be ascribed to decomposition of stable Cu/Zn hydroxocarbonates, which is similar to the third step of 2-htlCZAZ case. Furthermore, these Cu-containing oxocarbonates species with higher thermal stability were not so reducible, leading to lower catalytic efficiency of those samples.

TEM and SEM images can give detailed information about the structures of the materials and the shape of the individual particles. The TEM and SEM micrographs of precursor material 2-CZAZ (Fig. 5a and e) show aggregated hexagonal plate-like particles with crystal size ranging from 200 to 300 nm. Some rounded and rod-like particles should also be noticed on the hexagonal lamellar particles. According to Benito et al., the rod-like particles of different sizes are nanorod ZnO particles formed under hydrothermal treatment.²⁹

Fig. 5 TEM images of (a) 2-CZAZ, (b) c1-CZAZ, (c) c2-CZAZ and (d) c3-CZAZ; SEM images of (e) 2-CZAZ and (f) c2-CZAZ.

After thermal treatment of the precursors at 450 °C, some lamellar particles were decomposed and well-crystallized particles with particle size of around 10–20 nm can be found (see Fig. 5(b–d)). A large amount of isolated and agglomerated particles located on the destroyed platelets can also be observed from SEM micrographs of the calcined c2-CZAZ sample (see Fig. 5f). However, minor amounts of hexagonal plate-like particles still existed, on which few or no particles can be found (Fig. 5(b–d)). Furthermore, dark areas can be noticed in the micrographs, which can be assigned to the agglomerated of particles. Obviously, the agglomerated particles on the destroyed plate-like particles increased with increasing copper content which might result in the decrease of the BET specific surface area as well as the copper dispersion of the catalysts with higher Cu²⁺/Zn²⁺ atomic ratio.

3.2 XPS investigations

XPS spectra of the reduced samples are presented in Fig. 6a. All of the spectra consisted of a principal peak at 932.0–932.5 eV, which was the characteristic peak of reduced Cu⁺/Cu⁰ species. The Cu²⁺ has the binding energy (BE) of the Cu 2p_3/2 band above 933.5 eV, which was absent on the reduced surface of all the samples. Since the BE of the Cu 2p_3/2 band in metal (932.67 eV) and in Cu⁺ (932.4 eV) are almost the same, they can be distinguished by different kinetic energy of the Auger Cu LMM line position in metal (918.65 eV), Cu⁺ (916.8 eV) or in Cu²⁺ (917.9 eV).³⁷ The Cu LMM Auger electron spectroscopies of reduced samples are shown in Fig. 6b. All reduced samples present a principal peak at around 917.0 eV and a smaller peak centered around 918.9 eV, which can be attributed to Cu⁺ and Cu⁰ species within the error limit, respectively. Similar results have been reported by Toyir et al. for Cu/ZnO based catalysts.^38,39 Moreover, it is a popular viewpoint that both Cu⁺ and Cu⁰ species were essential to catalyze the CO₂ hydrogenation to methanol.^38–42

Fig. 6 (a) XPS of Cu 2p_3/2 levels and (b) Cu LMM Auger electron spectroscopy of 1: c2-CZAZ, 2: c2-CZAZ, 3: c3-CZAZ, 4: c2-htlCZAZ and 5: c2-coCZAZ samples after being reduced.

The Cu⁺ and Cu⁰ content on the surface of the reduced catalysts, as determined by XPS results, are summarized in Table 3. Both the content of Cu⁺ and Cu⁰ reach a maximum when the Cu²⁺/Zn²⁺ atomic ratio is 2 for the catalysts via hydrotalcite-containing precursors. In addition, compared to the Cu⁺ and Cu⁰ content on the reduced surface of catalysts derived from different precursors, both the Cu⁺ and Cu⁰ content increases in the order of c2-htlCZAZ < c2-coCZAZ < c2-CZAZ. The maximum content of both Cu⁺ (27.6%) and Cu⁰ (9.0%) is found for c2-CZAZ, and as was indicated above, both Cu⁺ and Cu⁰ species are essentiality for CO₂ hydrogenation to methanol, thus, better activity towards the target reaction is expected for c2-CZAZ catalyst.

Table 3 TPR characteristics and the Cu⁺ content for samples

Sample	TPR peak relative area^a	TPR peak position [T/°C] and contribution (%)^b		Cu^+ content (at.%)^c	Cu^0 content (at.%)^c
		Peak 1	Peak 2
a The relative area represent the total TPR peak area ratio of each catalyst to the c1-CZAZ case. b The contribution (%) of each species peak to the total TPR peak area. c The Cu^+ and Cu^0 content on the surface of the reduced catalysts.
c1-CZAZ	1.0	252 (75.6)	272 (27.4)	14.5	5.7
c2-CZAZ	1.5	267 (80.0)	297 (20.0)	27.6	9.0
c3-CZAZ	1.9	315 (71.0)	362 (29.0)	21.3	6.9
c2-htlCZAZ	1.2	293 (62.1)	348 (37.9)	19.0	5.0
c2-coCZAZ	1.3	283	—	27.1	6.6

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3.3 The reducibility of catalyst

TPR measurements were carried out to investigate the reduction pattern of copper species for various Cu/Zn/Al/Zr catalysts (Fig. 7). The c2-coCZAZ sample presented only one peak of hydrogen consumption, indicating only one kind of Cu²⁺ species was present in the sample. However, for cn-CZAZ and c2-htlCZAZ catalysts, broad reduction profiles with shoulders were observed. The reduction peak of cn-CZAZ and c2-htlCZAZ catalysts can be divided into two peaks: the lower temperature peak (peak 1) assigned to the reduction of surface highly dispersed CuO, and the peak at higher temperature (peak 2) attributed to the reduction of bulk-like CuO phases or composite metal oxide phase.^19,43 The peak positions and their contributions were summarized in Table 3. Obviously, with the increase of Cu²⁺/Zn²⁺ atomic ratio the two reduction peaks shift towards higher temperature, but the relative contribution of peak 1 reaches a maximum of 80.0% when Cu²⁺/Zn²⁺ ratio is 2. It is proposed that the shift in reduction peaks can be ascribed to the dispersion of particle. The better the dispersion, the lower the reduction temperature.^9,19,43,44 According to the above analysis of N₂O dissociative adsorption, the Cu dispersion of the catalysts decreases with the elevation of Cu content. Moreover, the position of peak 1 for c2-htlCZAZ (293 °C) is higher than that of c2-coCZAZ (283 °C), which indicates that the Cu²⁺ located in the hydroxylated layer results in the increase of reduction temperatures of catalysts. However, the hydrotalcite-containing precursor can reduce the reduction temperature, which can be seen from the lower position of peak 1 for c2-CZAZ (267 °C) compared with c2-coCZAZ (283 °C). For the c2-htlCZAZ sample, the relative contribution of peak 1 to the TPR pattern is much lower compared with c2-CZAZ catalyst. The result indicates that the amount of easily reducible well-dispersed CuO for c2-CZAZ is higher than that for c2-htlCZAZ.

Fig. 7 TPR profiles of the Cu/Zn/Al/Zr catalysts.

The relative area of hydrogen consumption of c2-CZAZ is larger than that of c2-coCZAZ and c2-htlCZAZ, which means the amount of reducible copper is higher than that of c2-coCZAZ and c2-htlCZAZ. The result is in good agreement with the results from other characterization results: (1) the c2-CZAZ possesses the maximum exposed Cu surface area according to the N₂O dissociative adsorption analysis; (2) some stable Cu²⁺ oxocarbonates are still present for c2-coCZAZ and c2-htlCZAZ samples even after calcination at 400 or 500 °C according to the TG and DSC analysis, and the reduction of these Cu²⁺ species is difficult; (3) some octahedrically coordinated Cu²⁺ with spinel-type structure can be observed for c2-htlCZAZ material by UV-vis-NIR DRS, which would lead to reduction at higher temperature.³⁴

3.4 Catalytic performance in the CO₂ hydrogenation to methanol

The catalytic performance of Cu/Zn/Al/Zr catalysts (n = 0.5–4) via hydrotalcite-containing precursors in the methanol synthesis from CO₂ hydrogenation is summarized in Table 4. Methanol, carbon monoxide and water are the main products under the reaction conditions, and traces of methane and higher hydrocarbons can also be detected. It can be found that both the conversion of CO₂ and selectivity to CH₃OH increased with the elevation of Cu content for cn-CZAZ catalyst system until it reached a maximum for c2-CZAZ, which results in the same trend of CH₃OH yield. For c2-CZAZ catalyst, the CH₃OH yield reached maximum of 0.18 g mL⁻¹ h⁻¹ with conversion of 23.9% and selectivity of 55.0% at 250 °C.

Table 4 The activity and selectivity for methanol synthesis from CO₂ hydrogenation over Cu/Zn/Al/Zr catalysts

Sample	CO2 Conversion (%)	Selectivity (C-mol.%)			CH3OH yield (g mL^−1 h^−1)
		CO	MeOH	CH4
Reaction conditions: T = 250 °C, P = 5.0 Mpa, GHSV = 4000 h^−1, H2 : CO2 (atomic) = 3 : 1.
c0.5-CZAZ	17.3	50.0	48.8	1.2	0.12
c1-CZAZ	21.8	48.3	51.0	0.7	0.14
c2-CZAZ	23.9	44.4	55.0	0.6	0.18
c3-CZAZ	22.1	47.7	51.6	0.7	0.15
c4-CZAZ	20.9	49.6	49.6	0.8	0.13
c2-htlCZAZ	21.2	50.5	48.8	0.8	0.14
c2-coCZAZ	23.0	46.8	52.5	0.7	0.17

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It is well known that the amount of active species is responsible for the catalytic activity for copper-based catalysts. Guo et al.^43,45 reported that there was a linear relationship between CO₂ conversion and CH₃OH yield of Cu-based catalysts and their exposed copper surface areas. Similar results were also reported by other researchers.^46,47 The results of Table 2 show that the exposed copper surface area of c3-CZAZ calculated from N₂O dissociative adsorption is lower than that of c2-CZAZ. In addition, Cu species, present in c3-CZAZ, will be partially reduced at a low reduction temperature (350 °C) according to the result of TPR, and the active centers may decrease owing to being covered with the Cu²⁺-containing species which cannot be reduced. Therefore, further increase in Cu content results in a decrease in the CO₂ conversion and CH₃OH yield. Moreover, the result can also be related to variation tendency of the Cu⁺ and Cu⁰ content for reduced catalysts with the increase in Cu²⁺/Zn²⁺ ratios.

The catalytic performances of Cu/Zn/Al/Zr catalysts derived from different precursors are shown in Table 4 and Fig. 8. Obviously, the c2-CZAZ catalyst obtained from hydrotalcite-containing precursors exhibits the best catalytic performance for methanol synthesis. It can be found that the conversion of CO₂ on c2-CZAZ catalyst is slightly higher than that of c2-coCZAZ catalyst at 230 °C, and the trend became obvious with the further increasing of the temperature (250–290 °C). In addition, the conversion of CO₂ on c2-htlCZAZ catalyst is much lower than that on c2-CZAZ and c2-coCZAZ catalysts in the whole temperature range. The result can be closely related to the increase in the amount of active species, c2-htlCZAZ < c2-coCZAZ < c2-CZAZ, which had been illustrated by N₂O dissociative adsorption and XPS analysis on reduced catalysts, as well as TPR measurements. It is noteworthy that the yield of the CH₃OH increases in the series c2-htlCZAZ < c2-coCZAZ < c2-CZAZ. Consequently, the c2-CZAZ catalyst derived from hydrotalcite-containing precursors exhibits the best catalytic performance compared with the catalysts derived from the two other precursors.

Fig. 8 Comparison of the CO₂ conversion and the yield of CH₃OH for various Cu/Zn/Al/Zr catalysts derived from different precursors. Reaction conditions: T = 230–290 °C, P = 5.0 Mpa, GHSV = 4000 h⁻¹, H₂ : CO₂ (atomic) = 3 : 1.

4. Conclusions

The Cu/Zn/Al/Zr hydrotalcite-containing materials with Cu²⁺/Zn²⁺ atomic ratio from 0 to 4, as well as phase-pure htl and rosasite samples with Cu²⁺/Zn²⁺ = 2 were prepared by co-precipitation method. The Cu content and the structure of precursors have a significant influence on the property and catalytic performance of the resulting Cu/Zn/Al/Zr catalysts for CO₂ hydrogenation to methanol. There are some conclusions as follows:

(1) For hydrotalcite-containing precursors, most Cu²⁺ did not enter the layer structure and separated CuO was formed.

(2) For cn-CZAZ catalysts, the Cu dispersion decreases with increasing Cu²⁺/Zn²⁺ atomic ratio, while both the exposed copper surface area, the Cu⁺ and Cu⁰ content reach maximum when Cu²⁺/Zn²⁺ molar ratio is 2. The catalytic performance of the Cu/Zn/Al/Zr catalysts via hydrotalcite-containing precursors rise first and then decrease with the increase in Cu content, and the highest catalytic activity and selectivity is obtained over the c2-CZAZ catalyst with Cu²⁺/Zn²⁺ = 2.

(3) Both the exposed copper surface area and the Cu⁺ and Cu⁰ content on the reduced surface for Cu/Zn/Al/Zr catalysts derived from different precursors increase in the order: c2-htlCZAZ < c2-coCZAZ < c2-CZAZ.

(4) The c2-CZAZ catalyst obtained from hydrotalcite-containing precursors exhibits the best catalytic performance compared with the catalysts derived from two other precursors, e.g. the CH₃OH yield reaches maximum of 0.18 g mL⁻¹ h⁻¹ with the CO₂ conversion of 23.9% at 250 °C.

Acknowledgements

This work was supported by the Knowledge Innovation Programme of the Chinese Academy of Science (KGCX2–YW–323); the Ministry of Science and Technology of the People's Republic of China (2009BWZ003); “Strategic Priority Research Program-Climate Change: Carbon Budget and Related Issues” of the Chinese Academy of Sciences, Grant No. XDA05010109, XDA05010110, XDA05010204 and the Natural Science Foundation of China (50976116).

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