Influence of element doping on La–Mn–Cu–O based perovskite precursors for methanol synthesis from CO₂/H₂

Abstract

Doped La–M–Mn–Cu–O based (M = Ce, Mg, Y, Zn) perovskite materials were prepared by the sol–gel method and characterized by XRD, N₂-adsorption, ICP-OES, SEM, TPR, N₂O-adsorption, XPS and TPD techniques. Upon the introduction of the fourth elements, all the samples keep the stable LaMnO₃ perovskite structure, and part of the copper species is separated from the perovskite lattice. More structure defects, lower reduction temperature and better low-temperature H₂ adsorption on the unit surface area are observed. In the application for methanol synthesis from CO₂/H₂, the Zn doped catalyst showed better performance which may because the strength of the weak basic sites play a significant role on methanol selectivity and the amount of H₂ adsorbed on the unit surface area is the key for CO₂ conversion.

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1. Introduction

Perovskite-type oxides with the general formula of ABO₃ has received much attention because of their important physical properties such as ferro-, piezo-, pyroelectricity, magnetism and electro-optic effects.^1,2 Perovskites were first applied as catalysts for oxidation of carbon monoxide in 1952–1953 by Parravano.¹ For a typical ABO₃ perovskite, the A-site is a larger rare earth and/or alkaline earth cation and the B-site is a smaller transition metal cation. Fine dispersion of “metal on oxide” catalyst can be obtained by reduction of the perovskite oxides.³ The advantages of perovskite materials could originate from the ability to accommodate a variety of ions of different valence with appropriate radius that meet the tolerance factor .^3,4 The excellent tunable properties make perovskite potential candidate for catalysis.

Recently, the perovskites with AA′BB′O₃ structure have been used in catalysis since the substitution of A-site could influence the catalytic performance of B-site.⁵ The perovskites with special properties can be obtained by advisable tailoring. At present, the perovskite has been widely studied as the catalyst for NO_x, CH_x, and CO conversion.⁶

CO₂ generated from fossil fuel combustion has led to environmental problem in the form of greenhouse effect and ozone depletion. For the sake of reducing the concentration of CO₂ in atmosphere, various strategies have been implemented such as capture, storage, and utilization of carbon dioxide.^7–10 An important means of carbon dioxide utilization is the hydrogenation reaction. Among the products for carbon dioxide hydrogenation, methanol is considered as the most valuable since it can be used as solvent, alternative fuel and raw material for synthesis of olefins, aromatics or gasoline that derived from traditional petrochemical processes.^11,12

Many catalytic systems have been employed for methanol synthesis from CO₂ hydrogenation among which Cu-based catalysts have been regarded as most effective.¹³ Different promoters have also been investigated such as Zn, Al, Mg, Mn, B, Zr, Y, Ce,^14–16 which can improve the catalytic performance via controlling the reactivity of the active site Cu phase by determining texture, exposure of the active sites and interaction pattern with reagents, products and reaction intermediates.^17,18

Our previous works have found that the Cu-based catalysts from perovskite precursors exhibit better methanol selectivity compared with the other Cu-based catalysts due to the appearance of Cu^α+ in the structure.¹⁹ The study for CO hydrogenation and CO₂ hydrogenation over LaMn_1−xCu_xO₃ perovskite oxides suggested that when the copper amount that substituted manganese was less than or equal to 50%, the LaMnO₃ could keep the perovskite structure and show preferable performance for methanol synthesis because of the interaction of Cu⁺ and Mn.²⁰ However, little work had been conducted on the influence of the fourth metal elements doping for the La–Mn–Cu–O perovskite catalysts. Therefore, in the present work, the La–M–Mn–Cu–O (M = Ce, Mg, Y, Zn) perovskites were prepared and characterized to obtain a clear insight in the influences of the fourth elements doping on the structure and the catalytic performance. The methanol synthesis from CO₂/H₂ was chosen as the model reaction for the perovskite materials to investigate the influence of the structure changing on the catalytic performance.

2. Experimental

2.1. Catalysts preparation

The La–M–Mn–Cu–O (M = Ce, Mg, Y, Zn) perovskite-type oxides were prepared by sol–gel method using citric acid as complexing agent. The ratio for La, M, Mn, Cu is 0.8 : 0.2 : 0.5 : 0.5. Adequate amounts of the precursor salts (La(NO₃)₃·nH₂O; Mn(NO₃)₂, 50% solution; Y(NO₃)₃·6H₂O; Cu(NO₃)₂·3H₂O; Zn(NO₃)₂·6H₂O; Ce(NO₃)₃·6H₂O; Mg(NO₃)₂·6H₂O) along with citric acid were dissolved in deionized water at a molar ratio of 2 : 1 (metal cations: citric acid). The solution was heated to 353 K to remove the water, and then enhanced the temperature to 423 K until ignition. The resulting powder were finally calcined under air at 673 K for 2 h and then at 1073 K for 4 h. The La–Mn–Cu–O catalyst and Mg, Y, Zn, Ce doping catalysts were then denoted as P, Mg–P, Y–P, Zn–P and Ce–P, respectively.

2.2. Characterization of catalysts

X-ray diffraction (XRD) analysis of powered samples was performed using a PANalytica X'Pert Pro X-ray diffractometer with Cu Kα radiation in the range of 10° < 2θ < 90° and a scan step of 0.1° min⁻¹.

The surface area of samples was determined by N₂ adsorption–desorption at liquid nitrogen temperature 77.30 K, using a Micromeritics Tristar 3000 instrument. Sample degassing was carried out at 473 K prior to acquiring the adsorption isotherm. The special areas were calculated from the isotherm according to the Brunauer–Emmett–Teller (BET) method.

The dispersion of Cu (D_Cu) and exposed Cu surface area (S_Cu) were determined by dissociative N₂O adsorption and carried out on Micromeritics AutoChem 2920 instrument. The catalysts (0.15 g) were first reduced in 5% H₂–Ar mixture (30 mL min⁻¹) for 2 h at 603 K, and the amount of hydrogen consumption was denoted as X. Then, the reduced samples cooled to 338 K 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 all the metallic copper change into cuprous oxide (N₂O + Cu → N₂ + Cu₂O). 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 603 K. The surface Cu₂O were reduced in the pulse of pure H₂, and the amount of consumed H₂ was denoted as Y. The dispersion of Cu and exposed Cu surface area of the catalyst were calculated by the eqn (1) (ref. 21) and eqn (3) (ref. 22)

n_Cu = 2Y (2)

S_Cu = (n_Cu × N)/(1.4 × 10¹⁹ × W)(m² g⁻¹) (3)

where D_Cu is the dispersion of Cu, W is the weight of the reduced catalyst, S_Cu is the exposed copper surface area per gram catalyst, n_Cu is molar number of copper, N is the Avogadro's constant (6.02 × 10²³ atoms mol⁻¹), and 1.4 × 10¹⁹ is the number of copper atoms per square meter.^21,22

The elemental composition was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Thermo iCAP 6300), 100 mg of the each catalyst was dissolved in 5% HCl solution and diluted. The reference solutions were prepared with the metal nitrates or standard metal oxides used in the catalyst preparation.

Investigations of the sample microstructure morphology were performed on a FETXL30 S-FEG scanning electron microscope (SEM) with an accelerating voltage of 10.0 kV.

X-ray photoelectron spectroscopy (XPS) analyses were performed over a Kratos XSAM800 spectrometer equipped with Al Kα radiation (12 kV × 15 mA, hν = 1486.6 eV) under ultrahigh vacuum (10⁻⁷ Pa). The binding energies were calibrated internally by adventitious carbon deposit C(1s) with E_b = 284.6 eV (accuracy within ±0.1 eV). Samples were treated under pure hydrogen at 603 K for 2 h in the pre-treatment chamber before transferred to the analysis chamber.

Temperature program reduction (TPR) measurements were carried out in order to determine the reducible species and performed in a U-tube quartz reactor. The samples (50 mg) were purged with Ar (30 mL min⁻¹) at 423 K to remove physically adsorbed water and then reduced in the flow of 5 vol% H₂ + Ar (30 mL min⁻¹) at a heating rate of 5 K min⁻¹ up to 873 K. Thermal conductivity detector (TCD) was used to monitor the consumption of H₂.

The adsorption property of H₂ for the studied sample was measured by H₂ temperature-programmed desorption (H₂-TPD). The catalyst was first reduced at 603 K and cooled to 318 K to saturate for 60 min in H₂ flow of 30 mL min⁻¹ for 2 h. Then the catalyst was flushed with Ar flow (40 mL min⁻¹) to remove all physical adsorbed molecules. Afterward, the TPD experiment was started with a heating rate of 10 K min⁻¹ under Ar flow (40 mL min⁻¹), and the change of hydrogen signal was monitored by a TCD and quantitatively calibrated by H₂ pulses.

The basicity of the catalyst was measured by CO₂ temperature-programmed desorption (CO₂-TPD) was performed in the same way as that of H₂-TPD; the only difference was that the desorbed CO₂ was detected by a BALZER mass spectrometer. CO₂ peak area was quantitatively calibrated by injecting CO₂ pulses.

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 g, 40–60 mesh) diluted with 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 603 K at a heating rate of 1 K min⁻¹ and maintained at 603 K for 8 h. The reactor was then cooled to room temperature. After reduction, the activities of the catalyst samples in CO₂ hydrogenation process were determined under reaction conditions of 523 K, 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. Products were quantitative analyzed with gas chromatograph equipped with a TCD, and the columns were TDX-01 for gas of H₂, CO and CO₂ and the Propake-Q for liquid of water and methanol. The CO₂ conversion and the carbon-based selectivity for CH₃OH, CO and CH₄ 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 the eqn (4):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); m was the weight of catalyst (g).

3. Results and discussion

3.1. Textural and structural properties

The X-ray diffraction patterns of the fresh and reduced perovskites are presented in Fig. 1a and b respectively. It can be seen that the LaMnO₃ (JCPDS # 75-0440) are the main phase for all samples. The diffraction peak at about 2θ = 32.5° shift towards higher values with the addition of the fourth elements. According to the Bragg formula, nλ = 2d sin θ (where d denotes the crystalline plane distance for indices (hkl), λ is the X-ray wave-length and θ is the diffraction angle), the d value will decrease when the ions with large radii are substituted by those with small radii (1.36 Å for La³⁺, 1.34 Å for Ce³⁺, 1.08 Å for Y³⁺, 0.74 Å for Zn²⁺, 0.73 Å for Cu²⁺, 0.72 Å for Mg²⁺, 0.65 Å for Mn³⁺), which results in the shifts of the diffraction peaks to higher values.^23,24 The result implies that the fourth elements have strong influences on La. Since the Mg²⁺ is slightly smaller than Cu²⁺, it might also have a complex effect on perovskite structure. For P sample, only LaMnO₃ phase is observed which means that all the copper either penetrates into the perovskite structure or well dispersed on the catalyst.²⁰ With the addition of other ions, small peaks at 2θ = 35.6° and 38.9° assigned to CuO (JCPDS # 89-5899) appear and the intensity of the diffraction peak of the LaMnO₃ phase increase in the order of: Mg–P < Ce–P < Y–P < P < Zn–P. The strength of the CuO peak is in the order of: Y–P < Ce–P < Zn–P < Mg–P, which suggests the influence on the perovskite structure with the introduction of the fourth elements. When Mg is introduced, the strongest diffraction peak of CuO and the weakest diffraction peak of LaMnO₃ are found which implies the stronger impact of Mg for the penetration of copper into the catalyst lattice as well as the perfection of LaMnO₃ in comparison with other elements. As Mg has the similar radius with Cu, it could have effect on both A-site and B-site of the perovskite oxide. However, doping with Y in the sample has little influence i.e. the characters of Y³⁺ are almost the same with La³⁺ which then lead to little disturbance for the catalyst structure. With the doping of Zn, the strongest diffraction peak of LaMnO₃ is observed which suggests that the perovskite structure tends to perfection due to the similar radius, valence state and electronegativity for Zn and Cu. In all diffraction patterns, no phases that ascribe to Mg, Y or Zn is observed while a new phase ascribed to CeO₂ is found for the sample of Ce–P which demonstrates that it is difficult for all the Ce to enter the perovskite lattice which agrees with the conclusion by Weng et al.²⁵

Fig. 1 XRD patterns of the calcined (a) and reduced (b) perovskite-type catalysts: (□) LaMnO₃; (●) CuO; (♦) CeO₂; (*) Cu.

As shown in Fig. 1b, for all reduced samples, LaMnO₃ phase is still the main phase while the CuO phase disappears and the Cu phase emerges which reveals that the reduction process does not destroy the perovskite structure. However, the perovskite structure has undergone some changes, e.g. the phase symmetry of all samples has changed except for P (Table 1). It is interesting that the reduction process leads to ordered LaMnO₃ phase only for Mg–P. The transition may occur in several steps and the deviation from the cubic perovskite structure may proceed from a simple distortion of the cubic unit cell, or an enlargement of cubic unit cell, or a combination of both.³ Moreover, the less particles shrink for Mg doped sample may result from the electronic property of alkaline-earth metals.

Table 1 The phase symmetry, lattice parameters and the physiochemical properties of the calcined perovskite-type catalysts

Samples	Phase symmetry		Size of LaMnO3 crystallites (Å)		Elemental composition^a (ICP-OES)	SBET (m^2 g^−1)	Dispersion^b (%)	SCu (m^2 g^−1)
	Calcined	Reduced	Calcined	Reduced
a Subscripts came from ICP results.b Calculated from N2O dissociative adsorption.
P	Cubic	Cubic	418	290	La0.84Mn0.51Cu0.50	6.5	—	—
Mg–P	Orthorhombic	Cubic	206	332	La0.67Mg0.22Mn0.49Cu0.50	5.4	0.9	1.2
Y–P	Cubic	Hexagonal	378	316	La0.67Y0.23Mn0.47Cu0.50	11.3	3.8	4.6
Zn–P	Cubic	Orthorhombic	467	462	La0.67Zn0.18Mn0.50Cu0.50	4.1	0.7	0.9
Ce–P	Cubic	Orthorhombic	333	238	La0.68Ce0.19Mn0.49Cu0.50	7.2	—	—

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The physicochemical properties of the calcined perovskite-type catalysts are also summarized in Table 1. The Y–P and the Zn–P possesses the largest and the lowest specific surface area, respectively. Moreover, the exposed Cu surface area and the Cu dispersion are measured by N₂O adsorption technique. The largest copper surface area is observed for Y–P, nevertheless, the copper surface area cannot be measured for both P and Ce–P. Since the surface copper may have strong influence on the activity for CO₂ hydrogenation reaction,²⁶ the lower copper surface area may not favorable for the conversion of CO₂. The ICP results show that the real contents of the samples are similar to the nominal values. For all catalysts, the experimental lanthanum amount is lower than the theoretical value.

Fig. 2 shows the SEM images of the prepared catalysts. The results show that all samples present as irregular granules. Compared with other samples, the particle size is smaller for Y–P and P. Particle agglomeration is observed for Mg–P and Ce–P while not for Zn–P in spite of the large particles.

Fig. 2 SEM images of the calcined catalysts (A) P, (B) Mg–P, (C) Y–P, (D) Zn–P, (E) Ce–P.

3.2. The XPS investigations

The reduced perovskite-type catalysts are analyzed by XPS, and the binding energies (BE) of La 3d_5/2, Mn 2p and O 1s are presented in Table 2. The La 3d_5/2 peak at around 834.3 eV is close to the value of pure lanthana at 834.4 eV, indicating that lanthanum ions are present in the trivalent form.²⁷ However, with the addition of the fourth elements, the BE of La 3d_5/2 shift towards lower values which implies the increasing of the electron cloud density around La ions. It may due to the fourth elements affect the transfer of the electrons of La to O since O has the highest electronegativity value among all elements.²⁸

Table 2 The binding energy of La, Mn, O, the ratio of different oxygen species and relative surface concentration of metals measured by XPS

Binding energy (eV)					Relative surface concentration of metal^a (%)
Samples	La 3d5/2 (eV)	Mn 2p3/2 (eV)	O 1s (eV)	Oad/O^2	La (%)	Mn (%)	Cu (%)	M (%)
a Values in parentheses are nominal concentration normalized to the total metal content.
P	835.1	642.2	529.2 (50.5%)	1.02	46.7 (50)	22.2 (25)	31.1 (25)	—
			531.7 (49.5%)
Mg–P	834.0	642.0	529.1 (51.5%)	1.06	25.3 (40)	28.7 (25)	25.6 (25)	Mg: 20.4 (10)
			531.5 (48.5%)
Y–P	834.4	641.3	529.0 (54.0%)	1.18	26.1 (40)	28.9 (25)	32.7 (25)	Y: 12.3 (10)
			531.4 (46.0%)
Zn–P	834.1	641.5	529.1 (51.2%)	1.05	38.5 (40)	31.5 (25)	21.3 (25)	Zn: 8.7 (10)
			531.5 (48.8%)
Ce–P	834.3	641.8	529.2 (51.9%)	1.08	28.3 (40)	27.8 (25)	31.4 (25)	Ce: 12.5 (10)
			532.2 (48.1%)

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For O 1s, the lower binding energy at around 528.9–529.1 eV can be ascribed to the lattice oxygen (O²⁻)^15,28 and the BE value at around 530.8–533.0 eV is assigned to the adsorbed oxygen species (O_ad) in the surface which contains hydroxyl (OH⁻), carbonate species (CO₃²⁻) and molecular water. Doped with the fourth components, the binding energy decreases which indicates that there are more electrons around oxygen. It is likely that the fourth components transfer the electronic to the oxygen. Obviously, the intensity of the peaks is different from each other. The presence of surface adsorbed oxygen species suggests the formation of oxygen vacancies in the defected oxides.²⁹ The increasing of the O_ad/O²⁻ ratio upon addition of the fourth elements indicates that the increasing of the amount of defect on perovskite oxide that is favorable for the activation of the catalyst. The binding energy of Mn 2p_3/2 for MnO, Mn₂O₃ and MnO₂ are located at 640.6, 641.9 and 642.2 eV respectively. As reported in other studies,^30,31 the mean oxidation state of Mn ions at the surface layers is extremely difficult to detect by XPS. However, the previous reports suggested that the BE difference between Mn 2p_3/2 and O 1s increase with about 0.6–0.7 eV for the change of the oxidation state between Mn³⁺ and Mn⁴⁺. In this study, as shown in Table 2, the BE difference is in the range of 112.3–113.0 eV which means a change of the Mn⁴⁺/Mn³⁺ ratio.^32,33

Since the binding energy of the Cu 2p_3/2 band in the metal (932.6 eV) and in Cu⁺ (932.4 eV) are almost same, they can be distinguished by different kinetic energy of the Auger Cu LMM line position in Cu⁰ (918.6 eV), Cu⁺ (916.7 eV) or in Cu²⁺ (917.9 eV).^26,34 The Auger electron spectroscopies of Cu LMM of reduced samples are shown in Fig. 3. The profiles are deconvoluted into two peaks. It can be seen that the majority of the copper species exist as Cu⁺ for all samples except for the P sample. The predominance of Cu⁺ in P is in accordance with the report of Jia et al.²⁰ and the results of XRD and N₂O-adsorption mentioned above. The weak Cu⁰ peak could be the explanation for the immeasurable of exposed Cu⁰ in the N₂O-adsorption results for Ce–P (Table 1).

Fig. 3 Cu LMM Auger electron spectroscopy of (a) P; (b) Mg–P; (c) Y–P; (d) Zn–P; (e) Ce–P samples after reduction.

The surface compositions and the nominal concentration of the catalysts are also listed in Table 2. The enrichment of Cu and depletion of La and Mn are observed for P. However, the Mn enrichment is found with the doping of the fourth elements. As Y and Ce entered the perovskite structure, it may lead to further enrichment of Cu on the catalyst surface. For Zn–P, the lack of both Zn and Cu on the surface may indicate that more Zn and Cu enter the bulk of the sample. It is also found that Mg is greatly enriched on the surface of Mg–P.

Fig. 4 shows the binding energy curves of the fourth components. The peak at 49.4 eV is attributed to Mg–O binding,³⁵ 1021.8 eV is assigned to Zn–O binding.³⁰ While the 156.8 eV and 158.6 eV may originate from the Y existed as 3+.³⁶ For Ce–P, six peaks located at 882.3, 888.4, 898.1, 900.7, 907.2 and 916.2 eV can be ascribed to Ce⁴⁺ which is in accordance with the XRD result of the Ce–P.³⁷

Fig. 4 XPS results of Mg 2p, Y 3d, Zn 3d and Ce 3d.

3.3. The reducibility of the prepared catalysts

The TPR profiles of all samples are shown in Fig. 5. It can be seen that the Y–P sample has two reduction peaks (denoted as peak α and peak β) while other samples present only one peak (denoted as peak β) which indicates that there is only one kind of copper spices in the P, Mg–P, Zn–P and Ce–P samples. The peak α at the lower temperature can be attributed to the reduction of highly dispersed surface copper species, whereas the peak at higher temperature is related to the reduction of the bulk-like copper species.³⁸ Upon introduction of the fourth elements, the enrichment of the copper on the surface (Table 2) leads to lowered reduction temperature. Moreover, more O_ad (Table 2) on the surface may also result in lowered reduction temperature which confirms the function of the defection in the perovskites. Y doping can increase the amount of the copper on the surface area which agrees with the N₂O chemisorption.

Fig. 5 TPR profiles of the catalysts.

The H₂ consumption in the TPR experiment is listed in Table 3. The amount of H₂ consumption is in the order of: Zn–P > Y–P > P > Mg–P > Ce–P, which indicates that there are more copper species that can be reduced for Zn–P and Y–P.

Table 3 The H₂ consumption amount in TPR and the H₂ desorption amount in H₂-TPD experiment

Samples	H2 consumption (mmol g^−1)	H2 desorption (μmol g^−1)
		Peak α	Peak β	Total	H2-523^a	H2-523^b
a H2 desorption below 523 K.b H2 desorption below 523 K on per unit are (μmol g^−1 m^−2).
P	9.47	48.2	165.2	213.4	8.7	1.34
Mg–P	9.06	50.6	68.3	118.9	18.9	3.50
Y–P	9.50	60.7	76.8	137.5	40.2	3.56
Zn–P	9.59	35.5	46.8	82.3	24.3	5.93
Ce–P	8.65	69.3	51.6	120.9	25.0	3.47

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3.4. H₂-TPD analysis

The H₂-TPD profiles of the reduced catalysts are shown in Fig. 6. In order to get more insight into the H₂-TPD results, the profiles are deconvoluted into two Gaussian peaks. According to the literature, the lower peak around 400 K can be assigned to the desorption of weakly molecularly adsorbed hydrogen on the Cu⁰ sites. However, such desorption peak are not observed because of less surface content of Cu⁰. The desorption peak (peak α) at lower temperature can be assigned to the desorption of dissociative hydrogen on the surface copper species, while the higher temperature peak (peak β) can be ascribed to desorption of strongly adsorbed dissociative hydrogen on the bulk copper or other metal oxides.^39–41 It can be seen that the desorption peak of H₂ at low temperature shifts toward even lower temperature with the introduction of the fourth components. However, the temperature of the peak β is higher than that for methanol synthesis, which means that these hydrogen might have little effect on the reaction. In addition, the desorption peak of the H₂ presents as a broad peak also suggests the strong spillover in the perovskite catalysts.

Fig. 6 H₂-TPD curves of the catalysts.

The amount of H₂ desorption over the pre-reduced materials is also listed in Table 3. With the introduction of the fourth elements, the total and high temperature desorption amount of hydrogen decreased. However, the desorption amount of H₂ at low temperature increased except Zn–P. It implies that the doping of the fourth element is not favor the desorption of H₂, especially under high temperature. Moreover, the H₂ desorption on the unit surface area below 523 K (test temperature) increased for all the four components samples.

3.5. CO₂-TPD analysis

Two desorption peaks (denoted as peak α and peak β) are observed for all samples (Fig. 7) which could be assigned to weak basic sites (around 400 K) and strong basic sites (around 600 K). The weak basic sites may be ascribed to the linear adsorption of CO₂ while the strong basic sites may result from the bridge adsorption of CO₂.⁴¹ With the introduction of the fourth components, the peak α shifts to higher temperature which indicates the increasing of the strength of the weak basic sites. The strength for the weak basic sites of the catalysts increases in the order of: P < Ce–P < Y–P < Mg–P < Zn–P. However, the peak β shifts to lower temperature which reveals that the addition of the fourth elements decrease the basicity of the strong basic sites of the samples.

Fig. 7 CO₂-TPD curves of the catalysts.

Doping with the fourth elements leads to the change of the amount of the basic sites. The quantitative analysis for the CO₂-TPD based on the relative area of the profiles is listed in Table 4, in which the P sample is assigned as 1.00. For Mg–P, the amount of all basic sites increases due to the alkalinity of Mg. Although the amount of total basic sites and strong basic sites for Y–P increases, the amount of weak basic sites decreases. Moreover, the amount of the weak basic sites increases for Zn–P and Ce–P samples despite the decrease of the amount of the strong basic sites and total basic sites.

Table 4 The basicity calculated from CO₂-TPD data

Samples	Adsorption type and distribution based in CO2-TPD data^a
	Peak α	Peak β	Total
a The amount of basicity of P is assigned as 1.00 to compare with other samples and the values in parentheses are the desorption temperature (K).
P	1.00 (383)	1.00 (614)	1.00
Mg–P	1.30 (396)	1.06 (583)	1.11
Y–P	0.87 (387)	1.67 (595)	1.53
Zn–P	1.56 (401)	0.85 (596)	0.97
Ce–P	1.22 (385)	0.69 (581)	0.78

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3.6. Catalytic performance

The catalytic performances of the catalysts for CO₂ hydrogenation to methanol are summarized in Table 5. The main product is methanol and the by-products are CO and H₂O derived from reverse water gas shift (RWGS) reaction.^34,38,42 It is found that the CO₂ conversion and methanol selectivity are improved for the doped catalysts. With the introduction of Zn, both the CO₂ conversion and the methanol selectivity increased greatly which might due to the fact that CO₂ could be activate on the site of Cu⁺–O–Zn.^43,44 However, the addition of Ce leads to a slight improvement for the catalytic performance. The relationship between the CO₂ conversion and the amount of H₂ desorption on unit surface area below 523 K (Table 3) is shown in Fig. 8. It can be seen that the more H₂ desorbed on the unit area, the more CO₂ conversion. Lower CO₂ conversion may result from lower copper content as well as lower surface area of copper in the system. The results of Fig. 9 indicate that the strength of the weak basic sites play an important role for methanol selectivity. The formate theory may explain the relationship between the catalytic performance and the H₂ desorption as well as the strength of the weakly basic sites in which the formaldehyde is the intermediate species and the basic sites is the key factor for methanol selectivity.⁴³

Table 5 The performance for methanol synthesis from CO₂ hydrogenation over the reduced catalysts^a

Samples	CO2 conversion (%)	Selectivity (C mol%)
		CH3OH	CO	CH4
a Reaction conditions: n(H2)/n(CO2) = 3 : 1, T = 523 K, P = 5.0 MPa, GHSV = 4000 h^−1.
P	1.8	0.7	93.4	5.9
Mg–P	2.8	23.7	68.1	6.5
Y–P	4.6	14.5	82.6	2.9
Zn–P	6.1	51.0	46.4	2.7
Ce–P	2.0	5.0	85.9	9.2

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Fig. 8 The relationship between the CO₂ conversion and the amount of H₂ desorbed on unit surface area below 523 K.

Fig. 9 The relationship between the selectivity for methanol and the strength of the weak basic of the catalysts.

Previous studies have demonstrated that two active centers were involved in the catalytic process of CO₂ hydrogenation for Cu-based catalysts which undergo the formate intermediate process,^26,45 i.e. CO₂ was adsorbed on the surface of the metal oxides such as ZnO and ZrO₂ while the H₂ was adsorbed on the Cu sites. Then the activated species participated in the reaction to produce the aim products.^43,46,47 During the reaction, either methanol or by-product CO was produced from the intermediate, formate species, and the strength of the basic sites decided the products selectivity. With the special perovskite structure, in our study, CO₂ could be adsorbed on the basic sites (perovskite oxide carriers) to form activated CO^*₂ and H₂ may be adsorbed and activated on the Cu sites resulting from the reduction of extra-perovskite and intra-perovskite copper species. The methanol selectivity is only related to the strength of the weak basic sites. The hydrogen supplied from Cu sites may spillover to the basic sites and react with the CO₂* to form the formate intermediate. The formate species formed on the weak basic sites may lead to methanol production, while the intermediate formed on other basic sites may produce CO. The possible reaction mechanism for CO₂ hydrogenation to methanol over the perovskites catalysts can be described in Scheme 1.

Scheme 1 Proposed reaction mechanism of CO₂ hydrogenation to methanol over the perovskite catalysts.

4. Conclusions

Doped La–M–Mn–Cu–O (M = Ce, Mg, Y, Zn) catalysts derived from perovskite-type precursors were prepared by sol–gel method. Introduction of the fourth elements leads to the separation of copper out of the LaMnO₃ perovskite lattice and thus produces more oxygen vacancies. Lower reduction temperature and low temperature adsorption properties are obtained with the increasing of defects for the doped samples. For the reaction of CO₂ hydrogenation to methanol, the CO₂ conversion increases with increasing of the amount of absorbed H₂ on the unit area under 523 K, while the methanol selectivity increases upon increasing of the strength of the weak basic sites. Both the H₂ adsorption amounts on the unit area and weak basic site strength are enhanced considerably as a result of the introduction of Zn such that improved the catalytic performance. Firstly, the Zn doped sample showed the largest H₂ consumption in the TPR result which indicated the existence of the most amount of reducible copper (the active site for the reaction) in the sample. Secondly, the Zn doped sample possessed the maximum amount of H₂ desorption on the unit surface area under 523 K. Thirdly, the samples possessed more weakly basic sites which favored the methanol selectivity. The Ce–P shows an iso-conversion compared with P due to the slight improvement of the H₂ adsorption. However, with the slight enhancement of the weak basic sites, the Ce–P showed almost five times more selectivity towards methanol which implies that it is probably much easier to improve the methanol selectivity for the perovskite catalytic system.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (21343012), the National Key Technology Research and Development Program of the Ministry of Science and Technology (2013BAC11B00) and “Strategic Priority Research Program-Climate Change: Carbon Budget and Related Issues” of the Chinese Academy of Sciences, Grant no. XDA05010109, XDA05010110, XDA05010204.

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