Haijuan Zhanab,
Feng Lia,
Peng Gaoc,
Ning Zhao*a,
Fukui Xiaoa,
Wei Weiac and
Yuhan Sunad
aState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, South Taoyuan Road 27#, Taiyuan 030001, People's Republic of China. E-mail: zhaoning@sxicc.ac.cn; Fax: +86-351-041153; Tel: +86-351-4049612
bUniversity of Chinese Academy of Sciences, Beijing 100049, People's Republic of China
cCenter for Greenhouse Gas and Environmental Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201203, People's Republic of China
dCAS Key Laboratory of Low-carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201203, People's Republic of China
First published on 25th September 2014
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.
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 NOx, CHx, 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 Cuα+ in the structure.19 The study for CO hydrogenation and CO2 hydrogenation over LaMn1−xCuxO3 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.
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 (DCu) and exposed Cu surface area (SCu) 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)
![]() | (1) |
nCu = 2Y | (2) |
SCu = (nCu × N)/(1.4 × 1019 × W)(m2 g−1) | (3) |
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 Eb = 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.
![]() | (4) |
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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.
Samples | Phase symmetry | Size of LaMnO3 crystallites (Å) | Elemental compositiona (ICP-OES) | SBET (m2 g−1) | Dispersionb (%) | SCu (m2 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 | — | — |
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.
Binding energy (eV) | Relative surface concentration of metala (%) | |||||||
---|---|---|---|---|---|---|---|---|
Samples | La 3d5/2 (eV) | Mn 2p3/2 (eV) | O 1s (eV) | Oad/O2 | 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%) |
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 (Oad) 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 Oad/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 2p3/2 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 2p3/2 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 2p3/2 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
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.
Samples | H2 consumption (mmol g−1) | H2 desorption (μmol g−1) | ||||
---|---|---|---|---|---|---|
Peak α | Peak β | Total | H2-523a | H2-523b | ||
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 |
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.
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.
Samples | Adsorption type and distribution based in CO2-TPD dataa | ||
---|---|---|---|
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 |
Samples | CO2 conversion (%) | Selectivity (C mol%) | ||
---|---|---|---|---|
CH3OH | CO | CH4 | ||
a Reaction conditions: n(H2)/n(CO2) = 3![]() ![]() |
||||
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 |
![]() | ||
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.
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Scheme 1 Proposed reaction mechanism of CO2 hydrogenation to methanol over the perovskite catalysts. |
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