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Effect of promoter SiO2, TiO2 or SiO2-TiO2 on the performance of CuO-ZnO-Al2O3 catalyst for methanol synthesis from CO2 hydrogenation
Applied Catalysis A: General 415–416 (2012) 118–123
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Effect of promoter SiO2 , TiO2 or SiO2 -TiO2 on the performance of CuO-ZnO-Al2 O3 catalyst for methanol synthesis from CO2 hydrogenation Luxiang Zhang ∗ , Yongchun Zhang, Shaoyun Chen Department of Catalysis Chemistry and Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China
a r t i c l e
i n f o
Article history: Received 17 October 2011 Received in revised form 8 December 2011 Accepted 9 December 2011 Available online 17 December 2011 Keywords: Promoter TiO2 SiO2 CO2 Hydrogenation Methanol
a b s t r a c t The influences of SiO2 , TiO2 or SiO2 -TiO2 promoters on the catalytic performance of CuO-ZnO-Al2 O3 catalyst in the methanol synthesis from CO2 hydrogenation were studied. The catalysts were prepared by co-precipitation method, SiO2 and TiO2 were loaded by hydrolyzation of tetraethyl orthosilicate (TEOS) and hydrolyzation of tetra-n-butyl titanate (C16 H36 O4 Ti), respectively. The catalytic performances of the prepared catalysts were investigated under conditions of T = 533 K, P = 2.6 MPa, H2 :CO2 = 3:1 (volume ratio) and SV = 3600 h−1 . The experimental results showed that the promoted catalysts showed a higher performance than CuO-ZnO-Al2 O3 . Especially, the one promoted with SiO2 -TiO2 maximized both activity and methanol selectivity with 40.70% in CO2 conversion and 41.17% in methanol selectivity as compared to the one without promoter (15.81% in CO2 conversion and 23.31% in methanol selectivity). Characterizations of XRD, H2 -TPR, H2 -TPD, NH3 -TPD, CO2 -TPD and SEM revealed that all the promoters improved the CuO dispersion in the catalyst body and improved the adsorption/activation of H2 on the catalyst. Especially SiO2 -TiO2 exhibited higher performance as compared to SiO2 or TiO2 . © 2011 Elsevier B.V. All rights reserved.
1. Introduction As a greenhouse gas, CO2 emission has been considered for causing the environmental problems in the form of greenhouse effect and ozone depletion [1]. However CO2 is a valuable carbon resource for being converted into chemicals such as methanol, methane, syngas (CO + H2 ) and dimethyl ether (DME), etc. [2,3]. So far it has been regarded as an issue for CO2 hydrogenation to methanol, researchers devoted themselves into this area, but CO2 conversion still remained low (≤20%) for the hard activation of CO2 [4–6,2,7]. This situation leads to the need of developing efficient catalysts to improve catalyst activity and methanol selectivity. It was well known [8] that Cu-based catalysts were the typical catalysts for methanol synthesis from CO2 hydrogenation such as CuO-ZnOAl2 O3 , CuO-ZnO-ZrO2 , etc. And supported noble metal catalysts were also considered in this reaction, such as Pd/SiO2 , Li-Pd/SiO2 , etc. The other catalysts such as Fe3 C, SnCl4 , etc. have been tried, too. The detail mechanism remained unclear for the controversy of intermediate specie, active site and reaction route [9,10]. Superfine catalyst [11,12] exhibited the better stability, larger specific surface area (SSA) and higher surface energy, etc. Certain promoter [13] could generate a synergistic effect on the active components, modulate the active sites, prevent the formation of spinel and enhance
∗ Corresponding author. Tel.: +86 411 84986322; fax: +86 411 84986322. E-mail address: [email protected] (L. Zhang). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.12.013
the adsorption of intermediate species, etc. It was also reported [14] that the catalysts supported on the carbon nano-tubes (CNTs) exhibited well catalytic performances. The catalyst prepared with ultrasonic technology exhibited well dispersion of CuO and larger specific surface area [15,16], hence acquired a higher catalytic performance. This present work studied the catalytic performance of ternary CuO-ZnO-Al2 O3 catalyst, promoted with SiO2 , TiO2 or SiO2 -TiO2 , in methanol synthesis from CO2 hydrogenation, aiming to improve the catalytic performance of CuO-ZnO-Al2 O3 catalyst. 2. Experimental 2.1. Catalyst preparation The catalysts were prepared by co-precipitation method with the proportion of CuO/ZnO/Al2 O3 /M = 5/4/1/0.02 (M represents SiO2 , TiO2 or SiO2 -TiO2 , SiO2 /TiO2 = 1/1, mass ratio) in the catalyst body. Firstly, the mass ratio of metal oxides should be converted into the mass ratio of the corresponding metal nitrate, and then a solution containing Cu, Zn and Al nitrates was prepared, after that the oxalate (H2 C2 O4 ·2H2 O) solution, tetraethyl orthosilioate (TEOS), and/or tetra-n-butyl titanate (C16 H36 O4 Ti) were dropped into the above solution in turn under constant stirring at a water bath of 343 K for 4 h, the resultant precipitate was cooled to room temperature and the wet cake obtained by filtration was oven-dried at 383 K for 10 h, and then calcined in air at
L. Zhang et al. / Applied Catalysis A: General 415–416 (2012) 118–123
623 K for 4 h with a heating-up rate of 3.5 K/min, subsequently cooled, crushed and sieved to obtain particles in the range of 20–40 mesh, the obtained catalysts were denoted as 2 wt.% M/CuOZnO-Al2 O3 (M = SiO2 , TiO2 or SiO2 -TiO2 ). Promoter SiO2 and TiO2 were obtained from hydrolyzation of TEOS and hydrolyzation of tetra-n-butyl titanate (C16 H36 O4 Ti), respectively. For comparison, catalyst CuO-ZnO-Al2 O3 was prepared by the same method without addition of tetraethyl orthosilioate and tetra-n-butyl titanate (C16 H36 O4 Ti). 2.2. Catalyst evaluation The prepared catalysts were evaluated in a tubular stainless steel, fixed-bed reactor (10 mm i.d) equipped with a temperatureprogrammed control unit and a K-type thermocouple. Each sample (1.0 g) was loaded between two layers of quartz sands (20–40 mesh) in the reactor and was diluted with the same quartz sands (20–40 mesh) to avoid hot spot due to an exothermal reaction. Before experiment, the catalyst was firstly pre-reduced at 543 K and atmospheric pressure for 2 h by a hydrogen stream (10 vol.% H2 /N2 ) and then cooled to desired temperature. Subsequently the H2 –CO2 reaction mixture (CO2 was dehydrated by molecular ˚ H2 /CO2 = 3/1, molar ratio) controlled by two calibrated sieve 5 A, mass flow controllers (Sevenstar, D08-2B) was introduced at a flow rate of 60 ml/min with the pressure being raised to 2.6 MPa. After leaving the reactor, the effluent gases were heated electrically on-line to avoid the condensation of water, methanol and other hydrocarbons in products, and analyzed by gas chromatograph (FULI, GC-9790, Porapak Q column, TCD). Conversion and selectivity values were calculated by internal standard and mass-balance methods. Similar fixed-bed reactor and the testing methods have been well described in the previous literatures [17–20]. 2.3. Catalyst characterization XRD characterization of powdered samples were performed using a D/max-2400 diffractometer (CuK␣ radiation, 40 kV, 100 mA) at a scan step of 0.02◦ /min from 10◦ to 85◦ . Temperature-programmed reduction (TPR) of powdered samples (≤200 mesh, 30 mg) was performed on a Chemisorptions Analyzer (FINESORB-3010A) with Ar-carried 10 vol.% H2 /Ar (10 ml/min). The temperature was raised from 293 to 773 K for each test at a rate of 10 K/min. The H2 consumption was monitored with a thermal conductivity detector (TCD). CO2 temperature-programmed desorption (CO2 -TPD) characterization was performed on the above apparatus from 293 to 973 K. The catalyst (30 mg) was firstly pre-reduced with 10 vol.% H2 /Ar at 523 K for 2 h, then cooled to room temperature, following that a CO2 (99.99%, purity) stream was introduced for adsorption (40 min). After adsorption, the examined sample was flushed with helium stream (10 ml/min, 20 min) to remove weakly adsorbed CO2 , and then it was heated from room temperature to 973 K at a rate of 10 K/min. The TPD spectra were recorded by the response of thermal conductivity detector. NH3 temperature-programmed desorption (NH3 -TPD) and H2 temperature-programmed desorption (H2 -TPD) were performed as that of CO2 -TPD, but 100 mg sample was used, and Ar was used as a carrier gas in the H2 -TPD. BET surface area of the fresh catalyst was characterized using N2 physical adsorption at its boiling point (77 K), on the same Chemisorptions Analyzer above. The sample (50 mg, ≤200 mesh) loaded in the U-shape quartz tube was heated to 393 K for 20 min to avoid the moisture, and then cooled, following that the 20% N2 /He was introduced for adsorption at 77 K for 20 min, and then desorbed for 20 min.
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Scanning electron microscopic (SEM) studies were performed on NOVA NANOSEM 450 scanning electron microscope, resolution 0.1 nm. Surface area of Cu (SCu ) was determined on a FINESORB-3010A Chemisorptions Analyzer by single-pulse (0.803 ml) N2 O titration (T = 363 K). Before measurements, the samples were pre-reduced in situ at 523 K in a 10 vol.% H2 /Ar for 60 min. After pre-reduction, the samples were “flushed” in the Ar carrier gas for 20 min and cooled to room temperature. SCu was calculated assuming a Cu:N2 O = 2 titration stoichiometry, and a surface atomic density of 1.46 × 1019 Cuat /m2 , respectively [21]. 3. Results and discussion 3.1. Catalytic activity Methanol synthesis from CO2 hydrogenation was tested over the reduced promoted CuO-ZnO-Al2 O3 catalysts (Table 1), for comparison, the activity of CuO-ZnO-Al2 O3 was investigated. The major product was methanol, and the by-products were CO and H2 O [22]. It can be seen that the CO2 conversions and methanol yield over the promoted catalysts are higher than that over CuO-ZnOAl2 O3 , which indicates that SiO2 , TiO2 or SiO2 -TiO2 could enhance the catalytic performances. Especially the SiO2 -TiO2 promoted CuO-ZnO-Al2 O3 catalyst shows the highest CO2 conversion and methanol selectivity among the promoted ones. Further characterizations and analysis are shown in Sections 3.2–3.6 to explore the mechanism of the promotion modification of promoter of the CuO-ZnO-Al2 O3 . It still remains unclear whether Cu0 or Cu+ is the active site on the Cu-Based catalysts, Arena et al. [23] considered Cuı (0 < ı < 10) and two-dimensions Cu0 –Cu+ as active sites, Cong et al. [24] claimed that the high dispersed clusters of metal copper are the active sites in the synthesis of methanol. CO2 + 3H2 CH3 OH + H2 O H 298 K = −49.58 kJ/mol, G29 K = 3.79 kJ/mol
(i)
CO2 + H2 CO + H2 O (reverse water gas shift reaction) H 298 K = 41.19 kJ/mol, G29 K = 28.62 kJ/mol
(ii)
CO + 2H2 CH3 OH H 298 K = −90.77 kJ/mol, G29 K = −24.83 kJ/mol
(iii)
Reactions (i) and (iii) are exothermal while (ii) is endothermic as shown above. Temperature was revealed as a strong influence on all the reactions, the higher temperature would inhibit the formation of methanol while the lower temperature cannot run the catalyst as shown in Table 2. The promoter can improve CuO dispersion in the catalyst body, and enlarged the SBET and SCu of CuO-ZnO-Al2 O3 , as shown in Table 3. Pepe and Polini [25] reported that a linear relationship was found between the catalytic activity and the SCu . 3.2. XRD analyses Fig. 1 shows the XRD patterns of the CuO/ZnO/Al2 O3 promoted with 2 wt.% SiO2 , TiO2 or SiO2 -TiO2 . The diffraction peaks at 35.6◦ , 39.1◦ and 49.0◦ can be ascribed to the presence of CuO phase while 31.8◦ , 34.5◦ , 36.3◦ , 47.5◦ , 56.7◦ , 62.9◦ and 68.1◦ can be ascribed to the presence of ZnO phase. The peaks of Al2 O3 , SiO2 and TiO2 were not observed, it indicates that Al2 O3 , SiO2 or TiO2 were amorphous or well dispersed in the catalyst body. With the addition of SiO2 , TiO2 or SiO2 -TiO2 , the intensity of peaks for CuO and ZnO weakened
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Table 1 Effects of promoters SiO2 , TiO2 or SiO2 -TiO2 on the performance of CuO-ZnO-Al2 O3 catalyst for methanol synthesis from CO2 hydrogenation. Promoter
Conversion of CO2 (%)
Selectivity of CH3 OH (%)
Selectivity of CO (%)
Yield of CH3 OH (%)
Without 2 wt.% SiO2 2 wt.% TiO2 2 wt.% SiO2 -TiO2 SiO2 /TiO2 = 1/1, mass ratio
15.81 20.24 16.10 40.70
23.31 27.15 25.29 41.17
76.69 72.85 74.71 58.83
3.69 5.50 4.07 16.76
Catalyst: x wt.% M/CuO-ZnO-Al2 O3 (x = 0, 2; M = SiO2 , TiO2 , SiO2 -TiO2 ; CuO/ZnO/Al2 O3 = 5/4/1, SiO2 /TiO2 = 1/1, mass ratio). Reaction condition: T = 533 K, P = 2.6 MPa, H2 :CO2 = 3:1 (molar ratio) and SV = 3600 h−1 .
Table 2 Effect of reaction temperature on the performance of catalyst CuO-ZnO-Al2 O3 . Temperature (K)
Conversion of CO2 (%)
Selectivity of CH3 OH (%)
Selectivity of CO (%)
Yield of CH3 OH (%)
493 513 533 553 573
5.17 8.32 15.81 16.69 21.37
34.99 30.86 23.31 12.74 7.68
65.01 69.14 76.69 87.26 92.32
1.81 2.57 3.69 2.13 1.64
Reaction condition: P = 2.6 MPa, SV = 3600 h−1 , H2 :CO2 = 3:1 (molar ratio). Catalyst: CuO/ZnO/Al2 O3 =5/4/1 (mass ratio).
gradually meanwhile the line width broadened slightly, it indicates SiO2 , TiO2 or SiO2 -TiO2 had enhanced the dispersion of CuO and ZnO well in the catalyst body than ever as shown in Fig. 2 (SEM), leading to the larger SBET and SCu as shown in Table 3 and the easier reduction of CuO as shown in Fig. 3 (H2 -TPR). The well-dispersed ZnO can promote the dispersion of CuO in the catalyst body, enhance interaction between CuO and ZnO, and the catalytic activity is related to the interaction between CuO and ZnO [26]. Promoter can modulate the structure and coordination of the catalyst surface, enhance the dispersion of the active sites, and improve the stability of catalyst [27]. Fig. 2 presents the SEM images of the catalysts before and after modification. The results clearly show that the images of the promoted catalysts are quite different from that of CuO/ZnO/Al2 O3 , i.e. small particles show better dispersion than CuO/ZnO/Al2 O3 , in good agreement with the XRD results [28].
3.3. H2 -TPR Fig. 3 shows the H2 -TPR curves of the CuO-ZnO-Al2 O3 catalyst before and after modification. SiO2 display a magnificent effect on the H2 -TPR curves. The peaks shifted to the lower temperature from 554.1 K (before modification) to 549.6 K or 553.8 K (after modification) with the addition of SiO2 or TiO2 , it indicates promoter SiO2 or TiO2 can enhance the dispersion of CuO (Fig. 1 XRD and Fig. 2 SEM), leading to the easier reduction of CuO in the catalyst body. However the hybrid promoter SiO2 -TiO2 enhanced the dispersion of CuO well, while made the reduction temperature higher (556.8 K) as shown in Fig. 4d, it indicates that SiO2 and TiO2 were not simply physically mixed, perhaps an interaction or a synergistic effect occurred between them, which led to the difficult reduction of CuO, and the higher catalytic performances of SiO2 -TiO2 promoted CuOZnO-Al2 O3 [24,29]. 3.4. H2 -TPD Fig. 4 shows the H2 -TPD curves of the pre-reduced catalysts CuO-ZnO-Al2 O3 modified by SiO2 , TiO2 or SiO2 -TiO2 . For comparison, the pre-reduced CuO-ZnO-Al2 O3 was investigated either. Three peaks can be observed on all the H2 -TPD curves due to three adsorptive forms on the surface of the catalysts. Peak ␣ represents the desorption of H2 molecular, while peaks  and ␥ represent two different desorption of dissociated H species [30]. with the addition of the promoters, all the peaks shifted to the higher temperature slightly (from 390.0 K to 398.0 K and 407.3 K for peak ␣, from 651.6 K to 662.4 K for peak , and from 755.9 K to 774.4 K for peak ␥) as shown in Fig. 4b–d, and the corresponding peak area increased, it indicates that all the promoters enhanced the dispersion of CuO (Fig. 1 XRD and Fig. 2 SEM) and enlarged the SBET Table 3 SBET and SCu of catalyst CuO-ZnO-Al2 O3 before and after modification.
Fig. 1. XRD spectra of CuO-ZnO-Al2 O3 catalysts before and after modification by promoters SiO2 , TiO2 or SiO2 -TiO2 . (a) CuO-ZnO-Al2 O3 ; (b) 2 wt.% SiO2 /CuO-ZnOAl2 O3 ; (c) 2 wt.% TiO2 /CuO-ZnO-Al2 O3 ; (d) 2 wt.% SiO2 -TiO2 /CuO-ZnO-Al2 O3 .
Catalysts
BET (m2 /g)
SCu (m2 /gcat )
CuO-ZnO-Al2 O3 2 wt.% SiO2 /CuO-ZnO-Al2 O3 2 wt.% TiO2 /CuO-ZnO-Al2 O3 2 wt.% SiO2 -TiO2 /CuO-ZnO-Al2 O3 , SiO2 :TiO2 = 1:1 (mass ratio)
26.32 33.69 37.77 42.72
5.80 7.47 6.24 8.23
Note: CuO/ZnO/Al2 O3 =5/4/1 (mass ratio). Before SCu measurements, the catalysts should be pre-reduced at 543 K and atmospheric pressure for 60 min by a hydrogen stream (10 vol.% H2 /N2 ).
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Fig. 2. SEM images of CuO-ZnO-Al2 O3 catalysts promoted with SiO2 , TiO2 or SiO2 -TiO2 . (a) CuO-ZnO-Al2 O3 ; (b) 2 wt.% SiO2 /CuO-ZnO-Al2 O3 ; (c) 2 wt.% TiO2 /CuO-ZnO-Al2 O3 ; (d) 2 wt.% SiO2 -TiO2 /CuO-ZnO-Al2 O3 .
Fig. 3. H2 -TPR curves of CuO-ZnO-Al2 O3 catalysts before and after modification by promoters SiO2 , TiO2 or SiO2 -TiO2 . (a) CuO-ZnO-Al2 O3 ; (b) 2 wt.% SiO2 /CuO-ZnOAl2 O3 ; (c) 2 wt.% TiO2 /CuO-ZnO-Al2 O3 ; (d) 2 wt.% SiO2 -TiO2 /CuO-ZnO-Al2 O3 .
Fig. 4. H2 -TPD curves of CuO-ZnO-Al2 O3 catalysts promoted with SiO2 , TiO2 or SiO2 TiO2 . (a) CuO-ZnO-Al2 O3 ; (b) 2 wt.% SiO2 /CuO-ZnO-Al2 O3 ; (c) 2 wt.% TiO2 /CuO-ZnOAl2 O3 ; (d) 2 wt.% SiO2 -TiO2 /CuO-ZnO-Al2 O3 .
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Fig. 5. NH3 -TPD curves of CuO-ZnO-Al2 O3 catalysts promoted with SiO2 , TiO2 or SiO2 -TiO2 . (a) CuO-ZnO-Al2 O3 ; (b) 2 wt.% SiO2 /CuO-ZnO-Al2 O3 ; (c) 2 wt.% TiO2 /CuOZnO-Al2 O3 ; (d) 2 wt.% SiO2 -TiO2 /CuO-ZnO-Al2 O3 .
and SCu (Table 3.), and improved adsorption of H2 molecular and dissociated H species. CO2 conversion was related to the SCu and the adsorption ability of catalyst to H2 [31]. Especially the hybrid promoter SiO2 -TiO2 exhibited magnificent effect on the catalytic performance as compared to SiO2 or TiO2 . The better adsorption strength of H2 molecular and the dissociated H species is beneficial to the hydrogenation of activated CO2 species to form methanol [31]. 3.5. NH3 -TPD Fig. 5 shows the NH3 -TPD curves of CuO-ZnO-Al2 O3 catalysts before and after modification by promoters SiO2 , TiO2 or SiO2 -TiO2 . Two peaks are observed on all the curves, which represent the weak acidic site (denoted as ␣) and the strong acidic site (denoted as ), respectively. With the addition of SiO2 or TiO2 , peak ␣ shifted to the lower temperature (from 438.6 K to 424.6 K), and the area of peak ␣ became larger slightly, it indicated that the slightly lower strength for weak acid sites and the higher concentration of weak acid sites appeared on the surface of the catalyst. On the contrary, the area of peak  became smaller obviously, it indicated that the concentration of strong acid sites decreased remarkably, which is beneficial to the adsorption of CO2 . Promoter SiO2 -TiO2 enlarged area of peak ␣ obviously, while peak ␣ shifted to lower temperature (from 438.6 K to 397.5 K) on curve Fig. 5d, it indicates that promoter SiO2 , TiO2 or SiO2 -TiO2 can modulate the acid sites on the surface of catalyst, especially the promoter SiO2 -TiO2 showed magnificent effect on the strength/concentration of acid sites as compared to SiO2 or TiO2. 3.6. CO2 -TPD Fig. 6 shows the CO2 -TPD curves of CuO-ZnO-Al2 O3 catalysts before and after modification by promoter SiO2 , TiO2 or SiO2 TiO2 . Two desorption peaks are observed (denoted as ␣ and ), these peaks are related to two adsorptive forms, peak ␣ and  represent the linear adsorptive form (O C O M) and the bridge-
C
O M ) (M represents metal bonded adsorptive form (M O atom), respectively [17]. Peak ␣ appears in lower temperature range while peak  locates in the higher temperature range, result
Fig. 6. CO2 -TPD curves of CuO-ZnO-Al2 O3 catalysts promoted with SiO2 , TiO2 or SiO2 -TiO2 . (a) CuO-ZnO-Al2 O3 ; (b) 2 wt.% SiO2 /CuO-ZnO-Al2 O3 ; (c) 2 wt.% TiO2 /CuOZnO-Al2 O3 ; (d) 2 wt.% SiO2 -TiO2 /CuO-ZnO-Al2 O3 .
in the easier desorption for the former and more difficult desorption for the later. For peak ␣, it shifted to the higher temperature (from 341.1 K to 363.7 K) and its corresponding area increased with the addition of SiO2 , TiO2 or SiO2 -TiO2 , which indicates that the promoters improve the linear adsorption of CO2. For peak , the bridge-type adsorption weakened on curve Fig. 6b, and enhanced on curve Fig. 6c. Especially, the adsorption capacity/strength of CO2 on the TiO2 -promoted CuO-ZnO-Al2 O3 catalyst shows better than the others, but the linear adsorption shows various strength, it indicates that the oxygen atoms in molecular CO2 were adsorbed on the various metal atoms, leading to the different adsorption strength and the corresponding peaks such as ␣1 , ␣2 , ␣3 and ␣4 as shown in Fig. 6c. With the addition of SiO2 -TiO2 , the strength of both weak and strong acid sites weakened obviously than did SiO2 or TiO2 . But the concentration of weak acid sites increased remarkably, which is not beneficial to the adsorption of CO2 , and showed the negative effect on the adsorption of CO2 . Jun et al. [28] claimed that the peak at low temperature was rather related with the active site for the CO2 hydrogenation than with high temperature. 4. Conclusions According to the experimental results and the characterizations, some conclusions could be drawn: (1) CuO-ZnO-Al2 O3 catalysts modified with SiO2 , TiO2 or SiO2 -TiO2 exhibited better catalytic performances than the one without promoter. The CO2 conversion and methanol yield increased with the addition of promoters, and the maximum of CO2 conversion and methanol yield were obtained over 2 wt.% SiO2 TiO2 /CuO-ZnO-Al2 O3 ; (2) SiO2 , TiO2 or SiO2 -TiO2 enhanced CuO dispersion in the catalyst body. Both SiO2 and TiO2 led to the easier reduction of CuO, while SiO2 -TiO2 made the reduction of CuO difficult slightly; (3) Promoter SiO2 -TiO2 showed better performance than SiO2 or TiO2 , leading to a weaker acid strength and a higher acid concentration on the surface of the catalyst, which resulted in a weaker adsorption of CO2 while a stronger adsorption of the H2 molecular and the dissociated H species.
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Effect of promoter SiO2 , TiO2 or SiO2 -TiO2 on the performance of CuO-ZnO-Al2 O3 catalyst for methanol synthesis from CO2 hydrogenation Luxiang Zhang ∗ , Yongchun Zhang, Shaoyun Chen Department of Catalysis Chemistry and Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China
a r t i c l e
i n f o
Article history: Received 17 October 2011 Received in revised form 8 December 2011 Accepted 9 December 2011 Available online 17 December 2011 Keywords: Promoter TiO2 SiO2 CO2 Hydrogenation Methanol
a b s t r a c t The influences of SiO2 , TiO2 or SiO2 -TiO2 promoters on the catalytic performance of CuO-ZnO-Al2 O3 catalyst in the methanol synthesis from CO2 hydrogenation were studied. The catalysts were prepared by co-precipitation method, SiO2 and TiO2 were loaded by hydrolyzation of tetraethyl orthosilicate (TEOS) and hydrolyzation of tetra-n-butyl titanate (C16 H36 O4 Ti), respectively. The catalytic performances of the prepared catalysts were investigated under conditions of T = 533 K, P = 2.6 MPa, H2 :CO2 = 3:1 (volume ratio) and SV = 3600 h−1 . The experimental results showed that the promoted catalysts showed a higher performance than CuO-ZnO-Al2 O3 . Especially, the one promoted with SiO2 -TiO2 maximized both activity and methanol selectivity with 40.70% in CO2 conversion and 41.17% in methanol selectivity as compared to the one without promoter (15.81% in CO2 conversion and 23.31% in methanol selectivity). Characterizations of XRD, H2 -TPR, H2 -TPD, NH3 -TPD, CO2 -TPD and SEM revealed that all the promoters improved the CuO dispersion in the catalyst body and improved the adsorption/activation of H2 on the catalyst. Especially SiO2 -TiO2 exhibited higher performance as compared to SiO2 or TiO2 . © 2011 Elsevier B.V. All rights reserved.
1. Introduction As a greenhouse gas, CO2 emission has been considered for causing the environmental problems in the form of greenhouse effect and ozone depletion [1]. However CO2 is a valuable carbon resource for being converted into chemicals such as methanol, methane, syngas (CO + H2 ) and dimethyl ether (DME), etc. [2,3]. So far it has been regarded as an issue for CO2 hydrogenation to methanol, researchers devoted themselves into this area, but CO2 conversion still remained low (≤20%) for the hard activation of CO2 [4–6,2,7]. This situation leads to the need of developing efficient catalysts to improve catalyst activity and methanol selectivity. It was well known [8] that Cu-based catalysts were the typical catalysts for methanol synthesis from CO2 hydrogenation such as CuO-ZnOAl2 O3 , CuO-ZnO-ZrO2 , etc. And supported noble metal catalysts were also considered in this reaction, such as Pd/SiO2 , Li-Pd/SiO2 , etc. The other catalysts such as Fe3 C, SnCl4 , etc. have been tried, too. The detail mechanism remained unclear for the controversy of intermediate specie, active site and reaction route [9,10]. Superfine catalyst [11,12] exhibited the better stability, larger specific surface area (SSA) and higher surface energy, etc. Certain promoter [13] could generate a synergistic effect on the active components, modulate the active sites, prevent the formation of spinel and enhance
∗ Corresponding author. Tel.: +86 411 84986322; fax: +86 411 84986322. E-mail address: [email protected] (L. Zhang). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.12.013
the adsorption of intermediate species, etc. It was also reported [14] that the catalysts supported on the carbon nano-tubes (CNTs) exhibited well catalytic performances. The catalyst prepared with ultrasonic technology exhibited well dispersion of CuO and larger specific surface area [15,16], hence acquired a higher catalytic performance. This present work studied the catalytic performance of ternary CuO-ZnO-Al2 O3 catalyst, promoted with SiO2 , TiO2 or SiO2 -TiO2 , in methanol synthesis from CO2 hydrogenation, aiming to improve the catalytic performance of CuO-ZnO-Al2 O3 catalyst. 2. Experimental 2.1. Catalyst preparation The catalysts were prepared by co-precipitation method with the proportion of CuO/ZnO/Al2 O3 /M = 5/4/1/0.02 (M represents SiO2 , TiO2 or SiO2 -TiO2 , SiO2 /TiO2 = 1/1, mass ratio) in the catalyst body. Firstly, the mass ratio of metal oxides should be converted into the mass ratio of the corresponding metal nitrate, and then a solution containing Cu, Zn and Al nitrates was prepared, after that the oxalate (H2 C2 O4 ·2H2 O) solution, tetraethyl orthosilioate (TEOS), and/or tetra-n-butyl titanate (C16 H36 O4 Ti) were dropped into the above solution in turn under constant stirring at a water bath of 343 K for 4 h, the resultant precipitate was cooled to room temperature and the wet cake obtained by filtration was oven-dried at 383 K for 10 h, and then calcined in air at
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623 K for 4 h with a heating-up rate of 3.5 K/min, subsequently cooled, crushed and sieved to obtain particles in the range of 20–40 mesh, the obtained catalysts were denoted as 2 wt.% M/CuOZnO-Al2 O3 (M = SiO2 , TiO2 or SiO2 -TiO2 ). Promoter SiO2 and TiO2 were obtained from hydrolyzation of TEOS and hydrolyzation of tetra-n-butyl titanate (C16 H36 O4 Ti), respectively. For comparison, catalyst CuO-ZnO-Al2 O3 was prepared by the same method without addition of tetraethyl orthosilioate and tetra-n-butyl titanate (C16 H36 O4 Ti). 2.2. Catalyst evaluation The prepared catalysts were evaluated in a tubular stainless steel, fixed-bed reactor (10 mm i.d) equipped with a temperatureprogrammed control unit and a K-type thermocouple. Each sample (1.0 g) was loaded between two layers of quartz sands (20–40 mesh) in the reactor and was diluted with the same quartz sands (20–40 mesh) to avoid hot spot due to an exothermal reaction. Before experiment, the catalyst was firstly pre-reduced at 543 K and atmospheric pressure for 2 h by a hydrogen stream (10 vol.% H2 /N2 ) and then cooled to desired temperature. Subsequently the H2 –CO2 reaction mixture (CO2 was dehydrated by molecular ˚ H2 /CO2 = 3/1, molar ratio) controlled by two calibrated sieve 5 A, mass flow controllers (Sevenstar, D08-2B) was introduced at a flow rate of 60 ml/min with the pressure being raised to 2.6 MPa. After leaving the reactor, the effluent gases were heated electrically on-line to avoid the condensation of water, methanol and other hydrocarbons in products, and analyzed by gas chromatograph (FULI, GC-9790, Porapak Q column, TCD). Conversion and selectivity values were calculated by internal standard and mass-balance methods. Similar fixed-bed reactor and the testing methods have been well described in the previous literatures [17–20]. 2.3. Catalyst characterization XRD characterization of powdered samples were performed using a D/max-2400 diffractometer (CuK␣ radiation, 40 kV, 100 mA) at a scan step of 0.02◦ /min from 10◦ to 85◦ . Temperature-programmed reduction (TPR) of powdered samples (≤200 mesh, 30 mg) was performed on a Chemisorptions Analyzer (FINESORB-3010A) with Ar-carried 10 vol.% H2 /Ar (10 ml/min). The temperature was raised from 293 to 773 K for each test at a rate of 10 K/min. The H2 consumption was monitored with a thermal conductivity detector (TCD). CO2 temperature-programmed desorption (CO2 -TPD) characterization was performed on the above apparatus from 293 to 973 K. The catalyst (30 mg) was firstly pre-reduced with 10 vol.% H2 /Ar at 523 K for 2 h, then cooled to room temperature, following that a CO2 (99.99%, purity) stream was introduced for adsorption (40 min). After adsorption, the examined sample was flushed with helium stream (10 ml/min, 20 min) to remove weakly adsorbed CO2 , and then it was heated from room temperature to 973 K at a rate of 10 K/min. The TPD spectra were recorded by the response of thermal conductivity detector. NH3 temperature-programmed desorption (NH3 -TPD) and H2 temperature-programmed desorption (H2 -TPD) were performed as that of CO2 -TPD, but 100 mg sample was used, and Ar was used as a carrier gas in the H2 -TPD. BET surface area of the fresh catalyst was characterized using N2 physical adsorption at its boiling point (77 K), on the same Chemisorptions Analyzer above. The sample (50 mg, ≤200 mesh) loaded in the U-shape quartz tube was heated to 393 K for 20 min to avoid the moisture, and then cooled, following that the 20% N2 /He was introduced for adsorption at 77 K for 20 min, and then desorbed for 20 min.
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Scanning electron microscopic (SEM) studies were performed on NOVA NANOSEM 450 scanning electron microscope, resolution 0.1 nm. Surface area of Cu (SCu ) was determined on a FINESORB-3010A Chemisorptions Analyzer by single-pulse (0.803 ml) N2 O titration (T = 363 K). Before measurements, the samples were pre-reduced in situ at 523 K in a 10 vol.% H2 /Ar for 60 min. After pre-reduction, the samples were “flushed” in the Ar carrier gas for 20 min and cooled to room temperature. SCu was calculated assuming a Cu:N2 O = 2 titration stoichiometry, and a surface atomic density of 1.46 × 1019 Cuat /m2 , respectively [21]. 3. Results and discussion 3.1. Catalytic activity Methanol synthesis from CO2 hydrogenation was tested over the reduced promoted CuO-ZnO-Al2 O3 catalysts (Table 1), for comparison, the activity of CuO-ZnO-Al2 O3 was investigated. The major product was methanol, and the by-products were CO and H2 O [22]. It can be seen that the CO2 conversions and methanol yield over the promoted catalysts are higher than that over CuO-ZnOAl2 O3 , which indicates that SiO2 , TiO2 or SiO2 -TiO2 could enhance the catalytic performances. Especially the SiO2 -TiO2 promoted CuO-ZnO-Al2 O3 catalyst shows the highest CO2 conversion and methanol selectivity among the promoted ones. Further characterizations and analysis are shown in Sections 3.2–3.6 to explore the mechanism of the promotion modification of promoter of the CuO-ZnO-Al2 O3 . It still remains unclear whether Cu0 or Cu+ is the active site on the Cu-Based catalysts, Arena et al. [23] considered Cuı (0 < ı < 10) and two-dimensions Cu0 –Cu+ as active sites, Cong et al. [24] claimed that the high dispersed clusters of metal copper are the active sites in the synthesis of methanol. CO2 + 3H2 CH3 OH + H2 O H 298 K = −49.58 kJ/mol, G29 K = 3.79 kJ/mol
(i)
CO2 + H2 CO + H2 O (reverse water gas shift reaction) H 298 K = 41.19 kJ/mol, G29 K = 28.62 kJ/mol
(ii)
CO + 2H2 CH3 OH H 298 K = −90.77 kJ/mol, G29 K = −24.83 kJ/mol
(iii)
Reactions (i) and (iii) are exothermal while (ii) is endothermic as shown above. Temperature was revealed as a strong influence on all the reactions, the higher temperature would inhibit the formation of methanol while the lower temperature cannot run the catalyst as shown in Table 2. The promoter can improve CuO dispersion in the catalyst body, and enlarged the SBET and SCu of CuO-ZnO-Al2 O3 , as shown in Table 3. Pepe and Polini [25] reported that a linear relationship was found between the catalytic activity and the SCu . 3.2. XRD analyses Fig. 1 shows the XRD patterns of the CuO/ZnO/Al2 O3 promoted with 2 wt.% SiO2 , TiO2 or SiO2 -TiO2 . The diffraction peaks at 35.6◦ , 39.1◦ and 49.0◦ can be ascribed to the presence of CuO phase while 31.8◦ , 34.5◦ , 36.3◦ , 47.5◦ , 56.7◦ , 62.9◦ and 68.1◦ can be ascribed to the presence of ZnO phase. The peaks of Al2 O3 , SiO2 and TiO2 were not observed, it indicates that Al2 O3 , SiO2 or TiO2 were amorphous or well dispersed in the catalyst body. With the addition of SiO2 , TiO2 or SiO2 -TiO2 , the intensity of peaks for CuO and ZnO weakened
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Table 1 Effects of promoters SiO2 , TiO2 or SiO2 -TiO2 on the performance of CuO-ZnO-Al2 O3 catalyst for methanol synthesis from CO2 hydrogenation. Promoter
Conversion of CO2 (%)
Selectivity of CH3 OH (%)
Selectivity of CO (%)
Yield of CH3 OH (%)
Without 2 wt.% SiO2 2 wt.% TiO2 2 wt.% SiO2 -TiO2 SiO2 /TiO2 = 1/1, mass ratio
15.81 20.24 16.10 40.70
23.31 27.15 25.29 41.17
76.69 72.85 74.71 58.83
3.69 5.50 4.07 16.76
Catalyst: x wt.% M/CuO-ZnO-Al2 O3 (x = 0, 2; M = SiO2 , TiO2 , SiO2 -TiO2 ; CuO/ZnO/Al2 O3 = 5/4/1, SiO2 /TiO2 = 1/1, mass ratio). Reaction condition: T = 533 K, P = 2.6 MPa, H2 :CO2 = 3:1 (molar ratio) and SV = 3600 h−1 .
Table 2 Effect of reaction temperature on the performance of catalyst CuO-ZnO-Al2 O3 . Temperature (K)
Conversion of CO2 (%)
Selectivity of CH3 OH (%)
Selectivity of CO (%)
Yield of CH3 OH (%)
493 513 533 553 573
5.17 8.32 15.81 16.69 21.37
34.99 30.86 23.31 12.74 7.68
65.01 69.14 76.69 87.26 92.32
1.81 2.57 3.69 2.13 1.64
Reaction condition: P = 2.6 MPa, SV = 3600 h−1 , H2 :CO2 = 3:1 (molar ratio). Catalyst: CuO/ZnO/Al2 O3 =5/4/1 (mass ratio).
gradually meanwhile the line width broadened slightly, it indicates SiO2 , TiO2 or SiO2 -TiO2 had enhanced the dispersion of CuO and ZnO well in the catalyst body than ever as shown in Fig. 2 (SEM), leading to the larger SBET and SCu as shown in Table 3 and the easier reduction of CuO as shown in Fig. 3 (H2 -TPR). The well-dispersed ZnO can promote the dispersion of CuO in the catalyst body, enhance interaction between CuO and ZnO, and the catalytic activity is related to the interaction between CuO and ZnO [26]. Promoter can modulate the structure and coordination of the catalyst surface, enhance the dispersion of the active sites, and improve the stability of catalyst [27]. Fig. 2 presents the SEM images of the catalysts before and after modification. The results clearly show that the images of the promoted catalysts are quite different from that of CuO/ZnO/Al2 O3 , i.e. small particles show better dispersion than CuO/ZnO/Al2 O3 , in good agreement with the XRD results [28].
3.3. H2 -TPR Fig. 3 shows the H2 -TPR curves of the CuO-ZnO-Al2 O3 catalyst before and after modification. SiO2 display a magnificent effect on the H2 -TPR curves. The peaks shifted to the lower temperature from 554.1 K (before modification) to 549.6 K or 553.8 K (after modification) with the addition of SiO2 or TiO2 , it indicates promoter SiO2 or TiO2 can enhance the dispersion of CuO (Fig. 1 XRD and Fig. 2 SEM), leading to the easier reduction of CuO in the catalyst body. However the hybrid promoter SiO2 -TiO2 enhanced the dispersion of CuO well, while made the reduction temperature higher (556.8 K) as shown in Fig. 4d, it indicates that SiO2 and TiO2 were not simply physically mixed, perhaps an interaction or a synergistic effect occurred between them, which led to the difficult reduction of CuO, and the higher catalytic performances of SiO2 -TiO2 promoted CuOZnO-Al2 O3 [24,29]. 3.4. H2 -TPD Fig. 4 shows the H2 -TPD curves of the pre-reduced catalysts CuO-ZnO-Al2 O3 modified by SiO2 , TiO2 or SiO2 -TiO2 . For comparison, the pre-reduced CuO-ZnO-Al2 O3 was investigated either. Three peaks can be observed on all the H2 -TPD curves due to three adsorptive forms on the surface of the catalysts. Peak ␣ represents the desorption of H2 molecular, while peaks  and ␥ represent two different desorption of dissociated H species [30]. with the addition of the promoters, all the peaks shifted to the higher temperature slightly (from 390.0 K to 398.0 K and 407.3 K for peak ␣, from 651.6 K to 662.4 K for peak , and from 755.9 K to 774.4 K for peak ␥) as shown in Fig. 4b–d, and the corresponding peak area increased, it indicates that all the promoters enhanced the dispersion of CuO (Fig. 1 XRD and Fig. 2 SEM) and enlarged the SBET Table 3 SBET and SCu of catalyst CuO-ZnO-Al2 O3 before and after modification.
Fig. 1. XRD spectra of CuO-ZnO-Al2 O3 catalysts before and after modification by promoters SiO2 , TiO2 or SiO2 -TiO2 . (a) CuO-ZnO-Al2 O3 ; (b) 2 wt.% SiO2 /CuO-ZnOAl2 O3 ; (c) 2 wt.% TiO2 /CuO-ZnO-Al2 O3 ; (d) 2 wt.% SiO2 -TiO2 /CuO-ZnO-Al2 O3 .
Catalysts
BET (m2 /g)
SCu (m2 /gcat )
CuO-ZnO-Al2 O3 2 wt.% SiO2 /CuO-ZnO-Al2 O3 2 wt.% TiO2 /CuO-ZnO-Al2 O3 2 wt.% SiO2 -TiO2 /CuO-ZnO-Al2 O3 , SiO2 :TiO2 = 1:1 (mass ratio)
26.32 33.69 37.77 42.72
5.80 7.47 6.24 8.23
Note: CuO/ZnO/Al2 O3 =5/4/1 (mass ratio). Before SCu measurements, the catalysts should be pre-reduced at 543 K and atmospheric pressure for 60 min by a hydrogen stream (10 vol.% H2 /N2 ).
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Fig. 2. SEM images of CuO-ZnO-Al2 O3 catalysts promoted with SiO2 , TiO2 or SiO2 -TiO2 . (a) CuO-ZnO-Al2 O3 ; (b) 2 wt.% SiO2 /CuO-ZnO-Al2 O3 ; (c) 2 wt.% TiO2 /CuO-ZnO-Al2 O3 ; (d) 2 wt.% SiO2 -TiO2 /CuO-ZnO-Al2 O3 .
Fig. 3. H2 -TPR curves of CuO-ZnO-Al2 O3 catalysts before and after modification by promoters SiO2 , TiO2 or SiO2 -TiO2 . (a) CuO-ZnO-Al2 O3 ; (b) 2 wt.% SiO2 /CuO-ZnOAl2 O3 ; (c) 2 wt.% TiO2 /CuO-ZnO-Al2 O3 ; (d) 2 wt.% SiO2 -TiO2 /CuO-ZnO-Al2 O3 .
Fig. 4. H2 -TPD curves of CuO-ZnO-Al2 O3 catalysts promoted with SiO2 , TiO2 or SiO2 TiO2 . (a) CuO-ZnO-Al2 O3 ; (b) 2 wt.% SiO2 /CuO-ZnO-Al2 O3 ; (c) 2 wt.% TiO2 /CuO-ZnOAl2 O3 ; (d) 2 wt.% SiO2 -TiO2 /CuO-ZnO-Al2 O3 .
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Fig. 5. NH3 -TPD curves of CuO-ZnO-Al2 O3 catalysts promoted with SiO2 , TiO2 or SiO2 -TiO2 . (a) CuO-ZnO-Al2 O3 ; (b) 2 wt.% SiO2 /CuO-ZnO-Al2 O3 ; (c) 2 wt.% TiO2 /CuOZnO-Al2 O3 ; (d) 2 wt.% SiO2 -TiO2 /CuO-ZnO-Al2 O3 .
and SCu (Table 3.), and improved adsorption of H2 molecular and dissociated H species. CO2 conversion was related to the SCu and the adsorption ability of catalyst to H2 [31]. Especially the hybrid promoter SiO2 -TiO2 exhibited magnificent effect on the catalytic performance as compared to SiO2 or TiO2 . The better adsorption strength of H2 molecular and the dissociated H species is beneficial to the hydrogenation of activated CO2 species to form methanol [31]. 3.5. NH3 -TPD Fig. 5 shows the NH3 -TPD curves of CuO-ZnO-Al2 O3 catalysts before and after modification by promoters SiO2 , TiO2 or SiO2 -TiO2 . Two peaks are observed on all the curves, which represent the weak acidic site (denoted as ␣) and the strong acidic site (denoted as ), respectively. With the addition of SiO2 or TiO2 , peak ␣ shifted to the lower temperature (from 438.6 K to 424.6 K), and the area of peak ␣ became larger slightly, it indicated that the slightly lower strength for weak acid sites and the higher concentration of weak acid sites appeared on the surface of the catalyst. On the contrary, the area of peak  became smaller obviously, it indicated that the concentration of strong acid sites decreased remarkably, which is beneficial to the adsorption of CO2 . Promoter SiO2 -TiO2 enlarged area of peak ␣ obviously, while peak ␣ shifted to lower temperature (from 438.6 K to 397.5 K) on curve Fig. 5d, it indicates that promoter SiO2 , TiO2 or SiO2 -TiO2 can modulate the acid sites on the surface of catalyst, especially the promoter SiO2 -TiO2 showed magnificent effect on the strength/concentration of acid sites as compared to SiO2 or TiO2. 3.6. CO2 -TPD Fig. 6 shows the CO2 -TPD curves of CuO-ZnO-Al2 O3 catalysts before and after modification by promoter SiO2 , TiO2 or SiO2 TiO2 . Two desorption peaks are observed (denoted as ␣ and ), these peaks are related to two adsorptive forms, peak ␣ and  represent the linear adsorptive form (O C O M) and the bridge-
C
O M ) (M represents metal bonded adsorptive form (M O atom), respectively [17]. Peak ␣ appears in lower temperature range while peak  locates in the higher temperature range, result
Fig. 6. CO2 -TPD curves of CuO-ZnO-Al2 O3 catalysts promoted with SiO2 , TiO2 or SiO2 -TiO2 . (a) CuO-ZnO-Al2 O3 ; (b) 2 wt.% SiO2 /CuO-ZnO-Al2 O3 ; (c) 2 wt.% TiO2 /CuOZnO-Al2 O3 ; (d) 2 wt.% SiO2 -TiO2 /CuO-ZnO-Al2 O3 .
in the easier desorption for the former and more difficult desorption for the later. For peak ␣, it shifted to the higher temperature (from 341.1 K to 363.7 K) and its corresponding area increased with the addition of SiO2 , TiO2 or SiO2 -TiO2 , which indicates that the promoters improve the linear adsorption of CO2. For peak , the bridge-type adsorption weakened on curve Fig. 6b, and enhanced on curve Fig. 6c. Especially, the adsorption capacity/strength of CO2 on the TiO2 -promoted CuO-ZnO-Al2 O3 catalyst shows better than the others, but the linear adsorption shows various strength, it indicates that the oxygen atoms in molecular CO2 were adsorbed on the various metal atoms, leading to the different adsorption strength and the corresponding peaks such as ␣1 , ␣2 , ␣3 and ␣4 as shown in Fig. 6c. With the addition of SiO2 -TiO2 , the strength of both weak and strong acid sites weakened obviously than did SiO2 or TiO2 . But the concentration of weak acid sites increased remarkably, which is not beneficial to the adsorption of CO2 , and showed the negative effect on the adsorption of CO2 . Jun et al. [28] claimed that the peak at low temperature was rather related with the active site for the CO2 hydrogenation than with high temperature. 4. Conclusions According to the experimental results and the characterizations, some conclusions could be drawn: (1) CuO-ZnO-Al2 O3 catalysts modified with SiO2 , TiO2 or SiO2 -TiO2 exhibited better catalytic performances than the one without promoter. The CO2 conversion and methanol yield increased with the addition of promoters, and the maximum of CO2 conversion and methanol yield were obtained over 2 wt.% SiO2 TiO2 /CuO-ZnO-Al2 O3 ; (2) SiO2 , TiO2 or SiO2 -TiO2 enhanced CuO dispersion in the catalyst body. Both SiO2 and TiO2 led to the easier reduction of CuO, while SiO2 -TiO2 made the reduction of CuO difficult slightly; (3) Promoter SiO2 -TiO2 showed better performance than SiO2 or TiO2 , leading to a weaker acid strength and a higher acid concentration on the surface of the catalyst, which resulted in a weaker adsorption of CO2 while a stronger adsorption of the H2 molecular and the dissociated H species.
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