Zr catalysts for CO2 hydrogenation: Introduction of Cu2+ at different formation stages of precursors

Catalysis Today 194 (2012) 9–15

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Effect of hydrotalcite-containing precursors on the performance of Cu/Zn/Al/Zr catalysts for CO2 hydrogenation: Introduction of Cu2+ at different formation stages of precursors Peng Gao a,b , Feng Li a , Fukui Xiao a , Ning Zhao a , Wei Wei a,∗ , Liangshu Zhong c , Yuhan Sun a,c,∗ a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China Graduate University of the Chinese Academy of Sciences, Beijing 100049, PR China c Low Carbon Conversion Center, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201203, PR China b

a r t i c l e

i n f o

Article history: Received 29 February 2012 Received in revised form 12 June 2012 Accepted 13 June 2012 Available online 12 July 2012 Keywords: Cu/Zn/Al/Zr catalysts Cu2+ introduction procedure Hydrotalcite-containing precursor CO2 hydrogenation Methanol

a b s t r a c t A series of Cu/Zn/Al/Zr catalyst derived from hydrotalcite-containing precursors were prepared by introducing the Cu2+ at different stages of Zn/Al/Zr hydrotalcite-like compounds formation and reconstruction and the catalytic performance for the synthesis of methanol from CO2 hydrogenation was examined. The results showed that the physicochemical properties of the catalysts, such as the Cu2+ content located in the layer structure of precursor, the exposed Cu surface area, the Cu+ and Cu0 content on the reduced surface and the ratio of Cu+ /Cu0 were greatly influenced by the Cu2+ introduction procedure. The Cu/Zn/Al/Zr catalyst prepared by adding the Cu2+ into the precursor at the reconstructed stage exhibited the best catalytic performance due to the highest exposed Cu surface area and the ratio of Cu+ /Cu0 . Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction As a cheap, nontoxic and abundant C1 feedstock, chemical utilization of CO2 is very attractive and the catalytic hydrogenation of CO2 into valuable methanol has been recognized as one of the most effective and economical ways to fix and utilize a large amount of emitted CO2 [1–3]. Methanol can be used as a fuel additive or clean fuel, and also converted to high-octane gasoline (MTG process), aromatics (MTA process), ethylene and propylene in the methanol to olefins (MTO) process, as well as other useful petrochemicals [1–3]. Compared with state-of-the-art syngas-based methanol synthesis process, the lower by-product content in the production from CO2 hydrogenation may allow a simplified distillation and benefit further chemical conversion [4]. As a result, CO2 as an alternative feedstock for methanol synthesis has received more and more attention for CO2 utilization. The catalysts for CO2 hydrogenation to methanol generally contained Cu and Zn as the main components together with different modifiers (Zr, Ga, Si, Al, B, Cr, Ce, V, Ti, etc.) [5–9]. It is suggested Cu/Zn dispersion and the number of active species plays an important role in the CO2 hydrogenation process [7,8,10]. The pro-

∗ Corresponding authors at: Institute of Coal Chemistry, Chinese Academy of Sciences, South Taoyuan Road 27#, 030001, PR China. Tel.: +86 0351 4049612; fax: +86 0351 4041153. E-mail addresses: [email protected] (W. Wei), [email protected] (Y. Sun).

moter 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 [10–12]. Composite catalysts could be obtained by the controlled thermal decomposition of hydrotalcite-like compounds (HTlcs) with the general formula [M2+ 1−x M3+ x (OH)2 ]x+ (An− )x/n ·mH2 O where M2+ and M3+ are divalent and trivalent cations, respectively [13,14]. Several studies have discussed the advantages of htl phases as precursors for Cu-based catalysts [15–19], such as good dispersion of M2+ and M3+ at an atomic level, homogeneous microstructure, high stability against sintering and high specific surface area [20–22]. However, Cu-containing HTlcs represent a rather peculiar system, probably attributed to the coordination requirements of Cu2+ to form distorted octahedra, introducing Jahn–Teller effect into the layers and, thereby, destabilizing the htl structure [21,23]. According to Behrens et al. [21], some Cu particles are embedded in amorphous after calcination and thus, only a small fraction of these Cu particles was exposed and higher reduction temperature was needed for the copper rich catalysts derived from HTlcs. We have recently demonstrated that the Cu/Zn/Al/Zr catalyst via hydrotalcite-containing precursor, in which most Cu2+ did not enter the layer structure and separated CuO was formed, exhibited the best catalytic performance mainly due to the maximum content of active species compared with two other catalysts derived

0920-5861/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.06.012

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P. Gao et al. / Catalysis Today 194 (2012) 9–15

from phase-pure hydrotalcite-like and conventional rosasite precursors [24]. In addition, the effect of Cu2+ /Zn2+ atomic ratios for the hydrotalcite-containing precursor on the structure and the activity of Cu/Zn/Al/Zr catalysts was also investigated and the optimum Cu2+ /Zn2+ atomic ratios is 2. Furthermore, ageing treatment also played an important role during the preparation process of HTlcs, and their chemical and physical properties could be improved after ageing treatment [25,26]. Furthermore, the morphology, textural properties and nature of interlayer anions could be recovered without any change in the long-range htl structure organization, based on the so-called “memory effect”. The reconstruction of the htl structure by the memory effect took place by the rehydration of the derived mixed metal oxide formed by the calcination of their htl precursor [27–29]. Obviously, adding the Cu2+ into the Zn/Al/Zr HTlcs system at different stages of Zn/Al/Zr HTlcs formation and reconstruction should greatly influence the characteristics of the resulting catalysts and more study is needed to reveal the effect of preparation method. In the present work, a series of Cu/Zn/Al/Zr catalyst derived from hydrotalcite-containing precursors were prepared by introducing the Cu2+ at different stages of Zn/Al/Zr hydrotalcite-like compounds formation and reconstruction, and their physical property and catalytic performance for CO2 hydrogenation to methanol were investigated and correlated to get an insight into the effect of the Cu2+ introduction at different stages of the precursor formation.

2. Experimental 2.1. Preparation of catalysts 2.1.1. Preparation of Zn/Al/Zr HTlcs The Zn/Al/Zr htl material with Zn2+ :Al3+ :Zr4+ = 3:0.7:0.3 was prepared by co-precipitation method reported elsewhere [28]. Typically, two solutions, A and B, were added dropwise to 200 mL of deionized water under vigorous stirring at room temperature (20 ◦ C). Solution A was a metal salts solution of the Zn(NO3 )2 ·6H2 O, Al(NO3 )3 ·9H2 O and ZrO(NO3 )2 ·2H2 O with desired Zn:Al:Zr molar ratio. Solution B contained 2 M NaOH and 0.5 M Na2 CO3 . During the synthesis process, the pH of the suspensions was kept at 10.5 ± 0.2. After precipitation, the resulting slurry was aged at 80 ◦ C for 15 h under stirring and then filtered and washed for several times with deionized water to remove residual sodium. Finally, the filter cakes were dried overnight at 80 ◦ C. The resulting materials were mentioned as ZAZ.

2.1.2. Preparation of Cu/Zn/Al/Zr catalysts Four kinds of Cu/Zn/Al/Zr hydrotalcite-containing materials were prepared by introduction of Cu2+ (Cu2+ :Zn2+ = 2) into Zn/Al/Zr HTlcs at different stages, e.g. (1) by co-precipitation method as described in Section 2.1.1, in which Cu(NO3 )2 ·3H2 O was also added in the metal salt solution A; (2) by two-step precipitation method, in which the Cu(NO3 )2 ·3H2 O solution and the solution B were added dropwise into the resulting slurry obtained in Section 2.1.1 right after the precipitation of Zn/Al/Zr; (3) by post-precipitation method, in which the Cu(NO3 )2 ·3H2 O solution and the solution B were added during the ageing treatment process; (4) by reconstruction method, in which the Cu(NO3 )2 ·3H2 O solution and the solution B were added into reconstructed form of Zn/Al/Zr HTlcs, which were obtained by the rehydration of the derived mixed metal oxide formed by the calcination of their htl precursor [29]. The above four procedures were followed by aging, washing and drying as described in Section 2.1.1. The resulting materials were mentioned as cp-CZAZ, ts-CZAZ, pp-CZAZ and rc-CZAZ, respectively. Then these four precursor materials were calcined at 450 ◦ C for

4 h, and the corresponding mixed metal oxides were denoted as Ccp-CZAZ, Cts-CZAZ, Cpp-CZAZ and Crc-CZAZ, respectively. 2.2. Characterization of catalysts The surface area of samples was determined by N2 adsorption–desorption at liquid nitrogen temperature −195.8 ◦ C, using a Micromeritics Tristar3000 instrument. Sample degassing was carried out at 200 ◦ C 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 K␣ (0.15418 nm) radiation source. The intensity data were collected over a 2 range of 5–75◦ with a 0.05◦ step size and using a counting time of 1 s per point. Thermal decomposition of the catalyst precursors was studied by thermogravimetric (TG) method by using NETZSCH STA449 Thermal Analyzer. Measurements were performed in the temperature range of 30–700 ◦ C with linear temperature program ˇ = 10 ◦ C min−1 in continuous flow of synthetic air (50 mL min−1 ). The exposed copper surface area (SCu ) was determined by dissociative N2 O adsorption and carried out on Micromeritics AutoChem 2920 instrument using the procedure described by Yuan et al. [30]. 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 N2 O (85 mL min−1 ) for 1 h to ensure complete oxidation of Cu. The samples were then flushed with Ar to remove the N2 O and cooled to room temperature. Finally, a pulse of pure H2 was passed over the catalyst at 350 ◦ 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 Eqs. (1) and (2): D=

((2nH2 × MCu )/W ) × 100%

SCu =

X 2nH2 × N 1.4 × 1019 × W

(m2 g−1 )

× 100%

(1)

(2)

where nH2 is the molar number of consumed H2 , D is the dispersion of Cu, MCu is the relative atomic mass (63.546 g mol−1 ), W is the weight of the catalyst, and X is the stoichiometric composition of Cu (wt.%); SCu is the exposed copper surface area per gram catalyst, N is Avogadro’s constant (6.02 × 1023 atoms mol−1 ), and 1.4 × 1019 is the number of copper atoms per square meter [30,31]. The morphology of the samples was observed by a Hitachi S4800 scanning electron microscope (SEM) operated at 20.00 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed over a VG MultiLab 2000 instrument with Mg K␣ radiation (h = 1253.6 eV) under ultrahigh vacuum (10−7 Pa), calibrated internally by the carbon deposit C (1s) (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, and the XPS measurements have been recorded with the exclusion of air contact after reduction. Temperature program reduction (TPR) was carried out in a Utube 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 . The amount of hydrogen consumption was determined by a calibrating run using weighted amount of CuO.

P. Gao et al. / Catalysis Today 194 (2012) 9–15 Table 1 Structure of the as-synthesized Cu/Zn/Al/Zr precursor materials.

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 (0.85 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 350 ◦ C and maintain at 350 ◦ C 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 230–270 ◦ C, 5.0 MPa, n(H2 ):n(CO2 ) = 3:1, GHSV = 7500 mL gcat−1 h−1 . The steady-state activity measurements were taken after at least 24 h on the stream. H2 , CO, CH4 and CO2 were quantitatively analyzed using gas chromatograph equipped with a thermal conductivity detector (TCD, TDX-01 column). The water and methanol in liquids were quantitatively analyzed using a gas chromatograph with a TCD (Propake-Q column). The conversion of CO2 and the carbon-based selectivity values for the hydrogenation products (CH3 OH, CO and hydrocarbons) were calculated by an internal normalization method. The space time yields (STYs) of CH3 OH, which gave the amounts of CH3 OH produced per gram catalyst per hour, were defined as Eq. (3): STYCH3 OH =

WT × X(CH3 OH) t×m

(3)

where WT was the total weight of CH3 OH and H2 O product (g), X(CH3 OH) was the mass fraction of CH3 OH; t was the reaction time (h), and m was the weight of catalyst (g). 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 all the uncalcined samples, typical pattern of the hydrotalcite-like structure is the major phase. Besides, the CuO phase and some extent of zincite-type ZnO phase are also observed. 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 [25,32]. Benito et al. [25] have reported that it

rc-CZAZ

Intensity

pp-CZAZ

ts-CZAZ

•♦

•

♦

10

20

30

11

♦•

40

50

cp-CZAZ ♦ ♦ ♦♦ • ♦• 60

70

2θ Fig. 1. XRD patterns of the precursors of the Cu/Zn/Al/Zr catalysts. () Hydrotalcite; (䊉) CuO; () ZnO.

Sample

Preparation method

˚ a (A)

˚ c (A)

cp-CZAZ ts-CZAZ pp-CZAZ rc-CZAZ ZAZ

Co-precipitation Two-step precipitation Post-precipitation Reconstruction –

3.072 3.071 3.069 3.064 3.067

22.92 22.90 22.88 22.76 22.87

is impossible to hinder the ZnO segregation in Zn/Al HTlcs when the Zn2+ /Al3+ atomic ratio was higher than 2. In addition, the XRD pattern of rc-CZAZ sample shows that the htl structure was fully reconstructed from mixed oxides (Fig. 1). The crystallographic parameters a and c were calculated by employing least-squares refinement assuming a hexagonal crystal system for hydrotalcite-like structure in precursors and the results were listed in Table 1. In addition, the unit cell parameters a and c for Zn/Al/Zr HTlcs (ZAZ) were also shown in Table 1 for comparison. Both a and c parameters decreased in the order of cp-CZAZ > tsCZAZ > pp-CZAZ > ZAZ > rc-CZAZ. The value of a (=2 × d1 1 0 ) is a function of the average radius of the metal cations. According to previous investigation, copper ions may partially go into the layer structure of HTlcs [33,34]. Zhang et al. [14] demonstrated that the lattice a parameter increases with increasing the Cu2+ content in the layered structure of HTlcs, reflecting the fact that the ionic radii (Shannon ionic radii [35]) for Cu2+ , Zn2+ and Al3+ are 0.073, 0.074 and 0.054 nm, respectively. The value of c (=(d0 0 3 + 2d0 0 6 + 3d0 0 9 )) is a function of a number of factors including the average charge of the metal cations, the nature of the interlayer anion, and the water content. The decrease of c parameter can be attributed to enhanced electrostatic interactions between layer and interlayer as the excess charge introduced by the ratio of Al3+ /(Cu2+ + Zn2+ ) is increased. However, both the parameters a and c for rc-CZAZ sample are lower than that for ZAZ material, which can be assigned to the rehydration treatment of Zn/Al/Zr HTlcs. Kooli et al. [29] investigated that the a and c parameters of rehydrated Zn/Al samples were lower than those of the original ones due to the behavior of the Zn2+ . The mixed oxides were not totally rehydrated, and so some could form an oxide phase (ZnO), or an amorphous zinc hydroxide phase undetectable by XRD. Furthermore, both the parameters a and c for rc-CZAZ sample are slightly higher than the value for rehydrated Zn3 Al0.7 Zr0.3 sample in the literature [28], indicating a little Cu2+ exist in the htl structure of Zn3 Al0.7 Zr0.3 sample. Consequently, the more Cu2+ enter the layered structure, the higher the parameters a and c are. It can be concluded that the cp-CZAZ possesses the maximum content of Cu2+ located in the layer structure of HTlcs, and a little Cu2+ exist in the layered structure of rc-CZAZ sample. Fig. 2 shows the XRD patterns of the calcined samples. It can be noticed that the hydrotalcite-like structure completely collapsed after calcination [36], and the intensities of diffraction peaks assigned to ZnO and CuO phases increased. In addition, there are no peaks corresponding to Al2 O3 and ZrO2 , which indicate that Al2 O3 and ZrO2 existed in an amorphous state [9]. As mentioned above, Cu/Zn/Al/Zr catalysts are obtained by calcination and subsequent reduction of the precursors. It was thus of interest to study and compare the thermal behavior of the different precursors. Fig. 3 presents the weight loss rates of precursor materials in air. In general, the thermal decomposition of the hydrotalcites consists of two major steps. The first one, mainly ascribed to the elimination of interlayer and physically adsorbed water molecules, is sharp and occurs at 100–200 ◦ C. The second step, occurring between 200 and 350 ◦ C, is broad 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

12

P. Gao et al. / Catalysis Today 194 (2012) 9–15

Intensity

Table 2 Physicochemical property of the calcined Cu/Zn/Al/Zr samples.

Crc-CZAZ

Sample

SBET (m2 g−1 )

Dispersiona (%)

SCu a (m2 g−1 )

Cpp-CZAZ

Ccp-CZAZ Cts-CZAZ Cpp-CZAZ Crc-CZAZ

31 29 30 27

4.4 4.7 5.2 6.0

16.1 17.2 19.2 21.4

a

Calculated from N2 O dissociative adsorption.

Cts-CZAZ •

♦

♦

10

20

•

•

♦

30

♦•

40

50

♦•

Ccp-CZAZ •♦ • ♦ ♦ • 60

70

2θ Fig. 2. XRD patterns of the calcined Cu/Zn/Al/Zr catalysts. (䊉) CuO and () ZnO.

[37]. A progressive weight loss is observed between 500 and 650 ◦ C, which can be attributed to further decomposition of oxycarbonate Cuv MII w MIII x Oy (CO3 )z formed during the former steps [23,38]. The peaks corresponding to the former two processes of weight loss shift to higher temperature from cp-CZAZ to rc-CZAZ material, which indicates the thermal stability decreases in the order: rc-CZAZ > pp-CZAZ > ts-CZAZ > cp-CZAZ. The introduction of Cu2+ destabilizes the layered structure by Jahn–Teller effect, and the more Cu2+ introduction, the lower the stability of layered structure [21,23]. Therefore, cp-CZAZ and rc-CZAZ samples possess the maximum and minimum Cu2+ content located in the layer structure, respectively, which is in agreement with the XRD analysis. Consequently, the amount of stable Cu-containing oxycarbonate formed during the thermal decomposition of the htl structure decreases in the series: Ccp-CZAZ > Cts-CZAZ > Cpp-CZAZ > Crc-CZAZ. Furthermore, these Cu-containing oxocarbonates species with higher thermal stability were not so reducible, leading to lower catalytic efficiency of those samples [21]. The physicochemical properties of the calcined Cu/Zn/Al/Zr catalysts were summarized in Table 2. Similar values of the BET specific surface areas were obtained for Cts-CZAZ and Cpp-CZAZ

samples. The Ccp-CZAZ and Crc-CZAZ posess the highest and the lowest BET specific surface areas, respectively. Moreover, the exposed Cu surface area and Cu dispersion were measured by N2 O adsorption technique, which increased in the series: CcpCZAZ < Cts-CZAZ < Cpp-CZAZ < Crc-CZAZ (Table 2). The increasing trend of the exposed Cu surface area should be related to the content of Cu2+ located in the layer structure of HTlcs. According to the study by Behrens et al., the Cu particles, which derived from copper rich hydrotalcite-like precursor, are embedded into the amorphous ZnAl2 O4 matrix to a large extent and only a small fraction of these Cu particles is accessible to the gas phase [21]. On the other hand, the stable Cu-containing oxycarbonate formed during the thermal decomposition of the htl structure may be difficult to be reduced. In addition, the value of copper surface area reflects the Cu dispersion. Therefore, the variation trend of the exposed Cu surface area and Cu dispersion is easy to understand by considering the similar trend of the Cu2+ content located in the layered structure. Obviously, it can be expected that the Crc-CZAZ can provide more catalytically active species with high dispersion. SEM images can give detailed information about the structures of the materials and the shape of the individual particles (Fig. 4). A large amount of isolated and agglomerated particles which located on the decomposed lamellar platelets can also be observed from the SEM micrographs of all the calcined samples. However, some hexagonal plate-shaped crystals with crystal size ranging from 200 to 300 nm still existed, on which few or no particles can be found (see Fig. 4a–c). Moreover, fewer amounts of agglomerated particles can be observed for Crc-CZAZ compared with other samples, indicating that the dispersion of well-crystallized particles for Crc-CZAZ is higher.

3.2. XPS investigations

Weight Loss and Weight Loss Rate (a.u.)

275 rc-CZAZ

175 260 167

pp-CZAZ

265 ts-CZAZ

162 245

cp-CZAZ

155 100

200

300

400

500

600

700

o

Temperature ( C) Fig. 3. Thermogravimetry and differential thermogravimetry (TG-DTG) profiles of precursor materials in air.

Fig. 5a illustrates the typical XPS spectrum of Cu 2p3/2 core levels of the reduced samples. All of the spectra consisted of a principal peak around 932.0 eV, which was the characteristic peak of reduced Cu+ /Cu0 species. The Cu2+ has the binding energy (BE) of the Cu 2p3/2 band above 933.5 eV, which was absent on the reduced surface of all the samples. Since the BE of the Cu 2p3/2 band in metal (932.67 eV) and in Cu+ (932.4 eV) are almost the same, they can be distinguished by different kinetic energy (KE) of the Auger Cu LMM line position in metal (918.65 eV), Cu+ (916.8 eV) or in Cu2+ (917.9 eV) [39]. The Cu LMM Auger electron spectroscopies of reduced samples are shown in Fig. 5b. The value around 917.1 eV and 919.2 eV for the species with a binding energy of 932.0 eV value matched with KE of Cu+ and Cu0 , within the error limit, respectively. Surface composition of the reduced catalysts prepared by different methods as determined by XPS results, was summarized in Table 3. The Cu+ , Cu0 content and the ratio of Cu+ /Cu0 increased in the series: Ccp-CZAZ < Cts-CZAZ < Cpp-CZAZ < Crc-CZAZ. The maximum content of both Cu0 (9.2%) and Cu+ (5.9%) and ratio of Cu+ /Cu0 (0.64) is found for Crc-CZAZ. It is a popular viewpoint that both Cu+ and Cu0 species are necessary for catalyzing the CO2 hydrogenation to methanol [40–42]. Therefore, better catalytic performance toward the target reaction is expected for Crc-CZAZ catalyst.

P. Gao et al. / Catalysis Today 194 (2012) 9–15

13

Fig. 4. SEM images of (a) Ccp-CZAZ, (b) Cts-CZAZ, (c) Cpp-CZAZ, and (d) Crc-CZAZ.

Table 3 Surface composition of the reduced catalysts. Catalyst

Cu+ /Cu0

Surface composition 0

Ccp-CZAZ Cts-CZAZ Cpp-CZAZ Crc-CZAZ

+

Cu (at.%)

Cu (at.%)

Zn (at.%)

Al (at.%)

Zr (at.%)

O (at.%)

6.6 8.5 8.7 9.2

3.7 5.0 5.3 5.9

31.7 28.7 26.7 27.4

5.5 7.9 8.4 8.7

2.4 1.8 2.1 1.3

50.2 48.1 48.8 47.6

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. As shown in Fig. 6, the reduction profiles of all the samples prepared by different methods exhibit a broad reduction profiles with shoulders. To gain more insight into the TPR results, the broad band of H2 consumption 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 [43]. The peak positions and their relative areas were summarized in Table 4. Table 4 Temperature of reduction peaks and their contributions to the TPR pattern over Cu/Zn/Al/Zr catalysts. Sample

H2 consumption (mmol g−1 )

Ccp-CZAZ Cts-CZAZ Cpp-CZAZ Crc-CZAZ

8.1 8.5 8.7 9.7

a

TPR peak position [temperature (◦ C)] and contribution (%)a Peak 1

Peak 2

276 (71.0) 267 (80.0) 288 (81.9) 300 (83.9)

310 (29.0) 297 (20.0) 328 (18.1) 339 (16.1)

Values in parentheses are the contribution (%) of each species.

0.56 0.59 0.61 0.64

As shown in Table 4, the contribution of peak 1 to the TPR pattern increased in the order: Ccp-CZAZ < Cts-CZAZ < CppCZAZ < Crc-CZAZ. The result indicates that the amount of easily reducible well-dispersed CuO, from Ccp-CZAZ to Crc-CZAZ, shows an increasing trend. Furthermore, the similar trend of the total amounts of hydrogen consumption can also be found in Table 4, which indicates that cp-CZAZ and rc-CZAZ samples possess the maximum and minimum amount of reducible copper, respectively. The variation trend of the amount of stable Cu2+ oxycarbonate, which is difficult to reduce, as well as the exposed Cu surface area is similar to that of total amounts of hydrogen consumption, according to the TG and N2 O dissociative adsorption analysis, respectively. 3.4. Catalytic performance in the CO2 hydrogenation to methanol The catalytic performance of Cu/Zn/Al/Zr catalysts in the methanol synthesis from CO2 hydrogenation is summarized in Table 5 and Fig. 7. Methanol, carbon monoxide and water are the main products under the reaction conditions, and trace amount of methane and higher hydrocarbons are also detected. As shown in Fig. 7, the CO2 conversion increases in the following order at 230–270 ◦ C: Ccp-CZAZ < Cts-CZAZ < Cpp-CZAZ < Crc-CZAZ. It is suggested that the copper surface area is an important parameter for the catalytic performance of copper-based catalysts [11,44–46]. Guo et al. [43,46] reported that there was a linear relationship

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P. Gao et al. / Catalysis Today 194 (2012) 9–15

932.3

931.9

Peak 1

Cu 2p band

4

Intensity, c.p.s.

Peak 2

Crc-CZAZ

Consumption of H2 (a.u.)

(a)

3

2

Cpp-CZAZ

Cts-CZAZ

1

Ccp-CZAZ 928

932

936

940

944

948

100

Binding energy (eV)

(b)

200

300

400

500

600

o

Temperature ( C)

Cu LMM Auger band

Fig. 6. H2 -TPR profiles of the Cu/Zn/Al/Zr catalysts.

4 -1

CH3OH yield (g g h )

0.32 -1

Intensity, c.p.s.

3

2

912

914

916

918

920

922

0.24 0.20

924

Kinetic energy (eV) Fig. 5. (a) XPS of Cu2p3/2 levels and (b) Cu LMM Auger electron spectroscopy of 1: Ccp-CZAZ, 2: Cts-CZAZ, 3: Cpp-CZAZ and 4:Crc-CZAZ samples after being reduced.

Ccp-CZAZ Cts-CZAZ Cpp-CZAZ Crc-CZAZ

0.16 0.12

CO2 Conversion £¨%£©

1

0.28

25 20 15 10 5 0 230

between CO2 conversion of Cu-based catalysts and their exposed Cu surface areas. In addition, Chinchen et al. [47,48] pointed out that the methanol yield increased with the increase Cu surface area. Similar results were also reported by other researchers [49,50]. Furthermore, it is a popular viewpoint that both Cu+ and Cu0 species are essential to catalyze the CO2 hydrogenation to methanol [40–42]. Chen et al. [51] proposed that the activity of a copperbased catalyst is directly proportional to the total copper exposed (Cu0 and Cu+ ). In our study, the variation trend of catalytic activity is consistent with the trend of the exposed Cu surface area and the Cu+ and Cu0 content demonstrated by N2 O dissociative adsorption and XPS measurements. As illustrated in Table 5, the CH3 OH selectivity increases in the order of Ccp-CZAZ < Cts-CZAZ < Cpp-CZAZ < Crc-CZAZ at 250 ◦ C. It is well known that the synthesis of methanol (4) and the reverse

250

270 o

Temperature£¨ C£© Fig. 7. Comparison of various Cu/Zn/Al/Zr catalysts for CO2 hydrogenation to methanol. Reaction conditions: T = 230–270 ◦ C, P = 5.0 Mpa, GHSV = 7500 mL gcat−1 h−1 , and H2 :CO2 (atomic) = 3:1.

water gas shift (RWGS) (5) are the main reactions during the CO2 hydrogenation process. CO2 + 3H2 → CH3 OH + H2 O

(4)

CO2 + H2 → CO + H2 O

(5) Cu+

Some researchers claimed that the species presence probably determined the high selectivity for CH3 OH and the unusual low selectivity for CO in the operating conditions [52,53]. In fact, it is

Table 5 The activity and selectivity for methanol synthesis from CO2 hydrogenation over Cu/Zn/Al/Zr catalysts. Sample

Ccp-CZAZ Cts-CZAZ Cpp-CZAZ Crc-CZAZ

CO2 conversion (%)

17.1 19.8 20.4 22.2

CH3 OH yield (g gcat−1 h−1 )

Selectivity (C-mol.%) CO

CH3 OH

CH4

49.2 48.3 47.5 45.8

50.0 51.0 51.8 53.6

0.8 0.7 0.7 0.6

Reaction conditions: T = 250 ◦ C, P = 5.0 Mpa, GHSV = 7500 mL gcat−1 h−1 , H2 :CO2 (atomic) = 3:1.

0.22 0.25 0.26 0.30

P. Gao et al. / Catalysis Today 194 (2012) 9–15

well known that RWGS is more favored on metallic copper than on partially oxidized copper. Therefore, the variation trend of CH3 OH selectivity is easy to understand by considering the similar trend of the ratio of Cu+ /Cu0 , which had been demonstrated by XPS measurements. Consequently, similar variation trend of CH3 OH yield can be observed from Fig. 7 at 230–270 ◦ C. The Crc-CZAZ catalyst exhibits the highest activity for methanol synthesis from CO2 hydrogenation, and the CO2 conversion and CH3 OH yield reached 22.2% and 0.30 g gcat−1 h−1 at 250 ◦ C, respectively (Table 5). Therefore, the catalyst prepared by introducing the Cu2+ into the precursor at the reconstructed stage exhibited the best catalytic performance. 4. Conclusions A series of Cu/Zn/Al/Zr catalyst derived from hydrotalcitecontaining precursors were prepared by introducing the Cu2+ at different stages of Zn/Al/Zr HTlcs formation and reconstruction. The results revealed that the physicochemical properties of the precursor and calcined materials were strongly influenced by the procedure of Cu2+ introduction. The content of Cu2+ located in the layered structure of precursor decreased in the order: cp-CZAZ > ts-CZAZ > pp-CZAZ > rc-CZAZ, whereas opposite trends were observed for exposed Cu surface area, the Cu+ and Cu0 content on the reduced surface and the ratio of Cu+ /Cu0 . The sequence of catalytic performance for methanol synthesis from CO2 hydrogenation is in good agreement with the results of physicochemical properties, and the catalyst prepared by introducing the Cu2+ into the precursor at the reconstructed stage exhibited the best catalytic performance. Acknowledgements This work was supported by the Knowledge Innovation Programme of the Chinese Academy of Science (KGCX2-YW-323) and “Strategic Priority Research Program-Climate Change: Carbon Budget and Related Issues” of the Chinese Academy of Sciences (Grant Nos: XDA05010109, XDA05010110 and XDA05010204). L.S. Zhong acknowledges the financial support from Shanghai Municipal Science and Technology Commission, China (Grant No: 11ZR1436200). References [1] G.A. Olah, A. Geoppert, G.K.S. Prakash, Beyond Oil and Gas: The Methanol Economy, 1st ed., Wiley-VCH, Weinheim, 2006, pp. 173–187, 239–245. [2] F. Deng, D.F. Zeng, J. Yang, J.Q. Wang, J. Xu, Y.X. Yang, C.H. Ye, Microporous and Mesoporous Materials 98 (2007) 214–219. [3] J.F. Haw, W.G. Song, D.M. Marcus, J.B. Nicholas, Accounts of Chemical Research 36 (2003) 317–326. [4] F. Pontzen, W. Liebner, V. Gronemann, M. Rothaemel, B. Ahlers, Catalysis Today 171 (2011) 242–250. [5] C. Yang, Z.Y. Ma, N. Zhao, W. Wei, T.D. Hu, Y.H. Sun, Catalysis Today 115 (2006) 222–227. [6] B.J. Liaw, Y.Z. Chen, Applied Catalysis A: General 206 (2001) 245–256. [7] U.R. Pillai, S. Deevi, Applied Catalysis B: Environmental 65 (2006) 110–117. [8] G.X. Qi, X.M. Zheng, J.H. Fei, Z.Y. Hou, Catalysis Letters 72 (2001) 191–196. [9] L.Z. Gao, C.T. Au, Journal of Catalysis 189 (2000) 1–15. [10] X. An, J.L. Li, Y.Z. Zuo, Q. Zhang, D.Z. Wang, J.F. Wang, Catalysis Letters 118 (2007) 264–269. [11] Q. Zhang, Y.Z. Zuo, M.H. Han, J.F. Wang, Y. JinF. Wei, Catalysis Today 150 (2010) 55–60. [12] H.W. Lim, M.J. Park, S.H. Kang, H.J. Chae, J.W. Bae, K.W. Jun, Industrial and Engineering Chemistry Research 48 (2009) 10448–10455.

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