Effects of supports on bimetallic Pd-Cu catalysts for CO2 hydrogenation to methanol

Applied Catalysis A, General 585 (2019) 117210

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Effects of supports on bimetallic Pd-Cu catalysts for CO2 hydrogenation to methanol ⁎

T

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Fawei Lina,b,c,1, Xiao Jiangc,1, Nuttakorn Boreriboonc, Zhihua Wangb, , Chunshan Songc, , Kefa Cenb a

School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, PR China State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, PR China c Clean Fuels & Catalysis Program, EMS Energy Institute, PSU-DUT Joint Center for Energy Research, Departments of Energy and Mineral Engineering and of Chemical Engineering, The Pennsylvania State University, 209 Academic Projects Building, University Park, PA 16802, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Pd-Cu bimetallic catalyst CO2 hydrogenation Methanol Metal-support interaction

Lab-synthesized and commercial materials, TiO2, ZrO2, CeO2, Al2O3 and SiO2, were employed to investigate the effects of support on bimetallic Pd-Cu catalysts for CO2 hydrogenation to methanol. SiO2 was used as a benchmark with negligible MSI (metal-support interaction) and adsorption capacity. TiO2 P25 (TiO2-P1) with coexistence of anatase/rutile phase exhibited the optimal supports among TiO2 serial supports. TiO2-P1, ZrO2, and Al2O3 exhibited significant promotional effects than SiO2 support, and the methanol synthesis activity decreased as: TiO2-P1˜ZrO2 > Al2O3 > CeO2-D˜SiO2. Pd-Cu/TiO2-P1 and Pd-Cu/ZrO2 yielded 1.6-time more CH3OH than Pd-Cu/SiO2. Detailed characterizations demonstrated that methanol formation was mainly correlated to PdCu3 alloy and H2/CO2 adsorption. Pd-Cu/CeO2-D possessed the strongest MSI, but with alloy restructuring to form PdCux, lowered weakly adsorbed ratio of CO2 and surface adsorption ratio of H2/CO2, therefore exhibiting worse catalytic performance. Pd-Cu/TiO2-P1, Pd-Cu/ZrO2, and Pd-Cu/Al2O3 possessed moderate MSI and desirable performance.

1. Introduction Over the past 50 years, the global atmospheric CO2 concentration has increased more than 20% [1], resulting in a global concern of climate change. It was predicted that the atmospheric CO2 concentration will reach ˜570 ppm by the year 2100 [2]. From another perspective, CO2 is a cheap and abundant carbon feedstock. Thus, converting CO2 into value-added chemicals and fuels has become an important research topic, as it, by incorporating renewable energy, offers an alternative to mitigate CO2 emissions and reduce the dependence on the depleting, nonrenewable resources such as petroleum. Unfortunately, CO2 is thermodynamically stable and chemically inert, which makes the activation of CO2 and its conversion rather challenging [3,4]. Most research to date is focusing on the CO2 hydrogenation using hydrogen to synthesize hydrocarbons and oxygenates [5–8]. However, CO2 conversion catalysts normally suffer from low activities [1]. Consequently, CO2 conversion requires harsh reaction conditions and substantial energy input. Methanol (CH3OH) is a primary liquid petrochemical and has

attained wide applications for its convenience in storage and transportation [9]. It can be used as the alternative fuel for internal combustion engines and the hydrogen suppler for fuel cells [10]. Moreover, methanol is commonly used as the platform chemical for the production of valuable chemicals, including formaldehyde, acetic acid, dimethyl ether, methyl tert-butyl ether, olefins, etc. [11,12]. It was anticipated that the demand of methanol will reach 106 MMT by 2023 [13]. Therefore, great efforts have been devoted to exploring efficient and robust catalysts for selective CO2 hydrogenation to methanol in the past decades [9,14]. Previous studies mainly focused on the Cu-Zn based catalysts [15]. However, Cu-Zn based catalysts require higher temperatures to reach meaningful activities, the temperature of which is less favorable for CH3OH formation thermodynamically. Recent studies reported that the supported Pd nanoparticles are beneficial for CH3OH formation from CO2 or CO hydrogenation even at low temperatures [16–21]. The hydrogen spillover effect of Pd can activate reactants efficiently [13,22]. Clearly, the incorporation of Pd might offer avenues to develop catalysts that are active at low temperature.

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Corresponding authors. E-mail addresses: [email protected] (Z. Wang), [email protected] (C. Song). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.apcata.2019.117210 Received 22 June 2019; Received in revised form 9 August 2019; Accepted 16 August 2019 Available online 16 August 2019 0926-860X/ © 2019 Elsevier B.V. All rights reserved.

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rutile, respectively. The CeO2 was prepared by direct calcination of cerium nitrate (Alfa Aesar) at 823 K for 2 h (denoted as CeO2-D). The Pd-Cu bimetallic catalysts supported on these supports were prepared by co-impregnation method using an acetone solution of Pd (CH3COO)2 (Aldrich, > 99.9%) and Cu(NO3)2·2.5H2O (Alfa Aesar, ≥ 98%), and detailed procedure was described elsewhere [20,47]. The Pd and Cu loading were fixed at 5.7 wt% and 10 wt%, respectively. The obtained sample was calcined at 723 K for 5 h with a ramp of 1 K/min in dry air (ca. 100 m L(NTP)/min). The Pd/(Pd + Cu) atomic ratio was fixed at 0.25 at at−1, where an Pd-Cu alloy-induced strong synergetic effect on CH3OH formation was observed in our previous work [20].

Our recent work showed that the bimetallic Pd-Cu catalysts exhibited a strong synergetic effect on CH3OH synthesis from CO2 hydrogenation than the monometallic catalyst [20]. Detailed characterization demonstrate the synergy is attributed to the formation of nanosized alloy particles after reduction [20,23–26]. The addition of Pd in Pd-Cu catalysts can improve the reducibility of CuO [27,28] and enrich the catalyst surface with electrons, which is conducive to activate CO2 molecules by interacting with carbon atom in CO2. Notably, the relatively inert amorphous SiO2 was used as support in our previous work [20], as we aimed at identifying the metal contribution. Characterization results demonstrate that CO2 adsorption capacity of amorphous SiO2 was rather poor, neither was the interaction with metal components. A further improvement of activity performance would be anticipated if modifications are done in these regards. The selection of support materials and corresponding MSI (metalsupport interaction) are of significant importance in terms of tuning the electronic structure on the catalyst surface and the geometry [29,30]. Therefore, this work aims to comparatively exploit the effect of different support materials on the activity and selectivity of CH3OH synthesis from CO2 hydrogenation. ZrO2 is a promising support candidate due to its basicity and abundant oxygen defects [31,32], which can benefit CO2 adsorption capacity [16,33,34]. ZrO2 also possesses excellent hydrothermal stability to adapt to the high temperature and high pressure of CO2 hydrogenation [35]. TiO2 is a typical amphoteric oxide and exhibits similar chemical properties with ZrO2 [36–38]. CeO2 has been widely used as support material for catalysts due to its high oxygen storage capacity, redox property, and interaction with metals [39,40]. Besides, CeO2 is reducible and possesses high intrinsic activity toward CO2 adsorption [41]. As an amphoteric oxide [42] with strong Lewis acidity [43], high surface area, and strong interaction with metal components [44], Al2O3 is also a potential candidate as support and has been widely used to prepare the classic Cu-ZnO catalysts [45,46]. There are various surface Al−OH groups on Al2O3 surface that cause high dispersion of active sites [44]. Above all, supports TiO2, ZrO2, CeO2, and Al2O3 are included in the present comparative study as support materials to prepare Pd-Cu bimetallic catalysts, which were evaluated in CO2 hydrogenation to methanol. The correlation of activity performance and support-induced variations in catalyst structure was examined, with particular focuses on MSI and surface chemical nature. For comparison, the amorphous SiO2 was also included as benchmark.

2.2. Activity tests The CO2 hydrogenation activity was tested in a stainless-steel tube fixed-bed reactor with an internal diameter of 6 mm. Typically, 0.20 g of catalyst was used for each test and diluted by inert amorphous silica particles (0.41˜0.46 g). The reactor temperature was controlled by an electrically heated furnace. The flow rate and pressure were regulated with mass flow controllers and a backpressure regulator. Prior to hydrogenation, the catalyst was reduced by pure H2 (99.995%) at 573 K for 2 h with a flow rate of 40 m L(STP)/min. After reduction, the temperature was reduced to the ambient temperature. The reaction system was then pressurized to 4.1 MPa with the feed gas, CO2/H2/Ar (24/72/ 4, 99.995%), followed by heating up to 523 K at a ramp rate of 1.9 K/ min with GHSV at 3600 mL(STP) g-cat−1 h−1. The heat-tracing pipeline was arranged to avoid methanol condensation. Two online GCs (SRI 8610C) were used to analyze the gaseous products periodically. Ar was added as an internal standard for CO formation rate and CO2 conversion measurement, and CH4 was introduced as an external standard for methanol formation rate measurement. The Ar, CO, and CO2 were analyzed in the online GC/TCD, while the CH3OH and CH4 were analyzed in the online GC/FID. All the results were collected with the average of final 3 data after 14 h stability testing. 2.3. Catalyst characterization The N2 adsorption-desorption isotherms of these catalysts were measured in a Micromeritics TriStarⅡ instrument. Prior to measurement, all the catalysts (ca. 200 mg) were degassed at 523 K for 24 h. Then the pore parameters were analyzed at 77 K. The temperature-programmed reduction (H2-TPR), hydrogen temperature-programmed desorption (H2-TPD), and carbon dioxide temperature-programmed desorption (CO2-TPD) were all conducted using the Micromeritics Autochem 2910 TPD/TPR equipped with a mass spectrometer (Ametek Dycor Dymaxion, DM200 M) to monitor the realtime concentration of hydrogen consumption, hydrogen desorption, and carbon dioxide desorption, respectively. For H2-TPR, ˜100 mg of catalyst was charged in the U-tube. The catalyst was firstly heated to 393 K (10 K/min) under 25 mL/min of pure Ar (> 99.999%) and kept for 1 h to remove adsorbed species on the catalyst surface. Since the Pd-Cu bimetallic catalyst could be reduced at very low temperature, the catalyst was cooled down to 233 K using the isopropanol-liquid nitrogen to prevent the reduction before the initiation of reduction. Subsequently, the flow gas was switched to 5% H2/Ar (20 mL/min). After stabilization, the typical TPR process was started by a ramp of 10 K/min up to 1173 K and kept for 30 min. The hydrogen consumption was recorded from 273 K. The hydrogen uptake was quantified by referring to the results of standard silver oxide (AutoChem Corporation). For H2-TPD, ˜150 mg of catalyst was introduced and reduced at 573 K (5 K/min) for 2 h under 50 mL/min of 5% H2/Ar. The temperature was then cooled down to 233 K using the isopropanol-liquid nitrogen to prevent the desorption of weakly-adsorbed hydrogen species. Subsequently, the flow gas was switched to pure He (> 99.999%,

2. Experimental 2.1. Catalyst preparation TiO2, ZrO2, CeO2, Al2O3, and SiO2 were used as support materials, respectively. All materials, except CeO2, are commercially available, and detailed information is listed in Table 1. These commercial supports were used after pretreatment to remove the impurities. Several kinds of commercial TiO2 were selected to investigate the effect of crystal phase on CO2 hydrogenation. Meanwhile, the TiO2 supports were pretreated at 723 K and 973 K to get different ratio of anatase to Table 1 Commercial support pretreatment method and denotation. Denotation

Source

Pretreatment method

TiO2-P1 TiO2-P2 TiO2-A1 TiO2-A2 TiO2-A TiO2-R ZrO2 Al2O3 SiO2

AEROXIDE@TiO2 P25 Evonik AEROXIDE@TiO2 P25 Evonik Alfa Aesar Alfa Aesar Sigma-Aldrich (anatase) Sigma-Aldrich (rutile) SAINT-GOBAIN Sasol Davisil Grade 62

Calc. Calc. Calc. Calc. Calc. Calc. Calc. Calc. Calc.

at at at at at at at at at

723 K 973 K 723 K 973 K 723 K 723 K 823 K 823 K 823 K

for for for for for for for for for

4h 4h 4h 4h 4h 4h 2h 2h 2h

2

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30 mL/min). When the hydrogen signal in the mass spectrometer was stable (about 30 min), the TPD was initiated by a ramp of 10 K/min up to 1173 K. The hydrogen desorption was recorded from 233 K. For CO2-TPD, ˜150 mg of catalyst was introduced and reduced at the same conditions as the H2-TPD above. Then the flow gas was switched to pure He (> 99.999%, 30 mL/min) to remove hydrogen remained in the reactor. Subsequently, the temperature was cooled down to 523 K. When the H2 signal was stable, the flow gas was switched to 12% CO2/ He (30 mL/min). After 1 h adsorption, the temperature was cooled down to 283 K, and the flow gas was switched to pure He (> 99.999%, 30 mL/min). When the CO2 signal in the mass spectrometer became stable (about 30 min), the TPD was started by a ramp of 10 K/min up to 1173 K. The carbon dioxide desorption was recorded from 283 K. The XRD patterns were collected using a RIGAKU D/MAX 2550/PC X-ray diffractometer with Cu Kα (λ = 0.154059 nm) radiation. The spent catalysts and supports after pretreatment were measured. Due to the dilution by amorphous silica during activity tests, some silica peaks (before 30°) were detected in the XRD patterns of spent catalysts. Data was collected at 40 kV and 40 mA from 10 to 80° (2θ, diffraction angle). The crystal sizes were calculated using Scherrer equation

D=

K×λ β × cosθ

Table 2 CO2 hydrogenation activity and product selectivity over bimetallic Pd-Cu catalysts with different supports. #

1 2 3 4 5 6 7 8 9 10

Support

TiO2-P1 TiO2-P2 TiO2-A1 TiO2-A2 TiO2-A TiO2-R CeO2-D ZrO2 Al2O3 SiO2a

CO2 conv. %

16.4 14.2 9.7 13.9 7.6 7.5 9.9 15.8 12.4 6.6

Product sel./mol%

Formation rate/μmol g-cat−1s−1

CO

CH3OH

CO

CH3OH

74.3 75.2 78.0 80.6 81.3 75.5 71.6 73.2 68.7 66.0

25.7 24.8 22.1 19.4 18.7 24.5 28.4 26.8 31.4 34.0

1.43 1.31 0.93 1.35 0.72 0.64 0.95 1.41 1.03 0.61

0.50 0.43 0.26 0.33 0.17 0.21 0.38 0.52 0.47 0.31

Reaction conditions: 523 K, 4.1 MPa, CO2/H2 = 1/3, GHSV = 3600 ml (STP) gcat−1 h−1. a Data from Ref. [20].

respectively) compared with TiO2-P-supported catalyst with both anatase and rutile. This suggests that the presence of two TiO2 crystal phases is conducive to CO2 hydrogenation activity. The TiO2-P support calcined at higher temperature (TiO2-P2) showed a clear reduction in both CO2 conversion and CH3OH formation rate in comparison to TiO2P1 calcined at relatively lower temperature, whereas a slight reduction of the CH3OH selectivity was evident (i.e., 25.7–24.8 %). By contrast, the TiO2-A2 support, calcined at higher temperature than TiO2-A1, displayed an enhancement in both CO2 conversion and CH3OH formation rate, while a slight decrease for CH3OH selectivity (i.e., 22.1–19.4 %). Among all TiO2-supported catalysts, TiO2-P1-supported one showed the most superior activity performance in terms of CO2 conversion, CH3OH formation rate, and selectivity. Therefore, the TiO2P1 was employed hereafter to compare with other support materials.

(1)

where D is mean size of the ordered (crystalline) domains, K is Scherrer constant, λ is the X-ray wavelength, β is the line broadening at half the maximum intensity (FWHM), after correction, and θ is the Bragg angle (in radian). 3. Results and discussion 3.1. Screening tests of CO2 hydrogenation over TiO2-supported Pd-Cu catalysts 3.1.1. Activity performance on Pd-Cu/TiO2 Different TiO2 supports were initially employed to investigate the CO2 hydrogenation activity. TiO2 P25 (80% anatase and 20% rutile) and another TiO2 with high surface area (˜150 m2/g) were first calcined at 723 and 973 K, respectively, to attain different ratios of crystal phases, namely anatase and rutile. The pure commercial anatase and rutile TiO2 were also employed as benchmark. Fig. 1 illustrates the changes of CO2 conversion and CH3OH formation rate and selectivity over Pd-Cu/TiO2 with different TiO2 supports, and detailed activity performance is listed in Table 2. Evidently, the CO2 hydrogenation was influenced by the TiO2 crystal phases (Fig. 1). Both TiO2-A- and TiO2-R-supported catalysts, with single crystal phase, exhibited lower CO2 conversion (7.6 and 7.5%, respectively) and CH3OH formation rate (0.17 and 0.21 μmol g-cat−1 s−1,

3.1.2. Textural property and crystallite structure The textural properties of all TiO2-supported Pd-Cu catalysts are summarized in Table 3, as well as the information of corresponding pristine support materials in the parentheses. For bare TiO2, the ones calcined at 973 K (i.e., TiO2-P2 and TiO2-A2) showed decreased values in surface area and pore volume, whereas increased values in average pore diameter in comparison to those calcined at 723 K (i.e., TiO2-P1 and TiO2-A2, respectively), suggesting the occurrence of aggregation at higher temperatures [48]. It is reasonable to infer that such temperature-induced variation should reflect on the activity performance in terms of metal dispersion and/or alloy structuring and mass transfer. The loaded catalysts showed decreased values in surface area and pore Table 3 Textural properties of supported Pd–Cu catalysts a. Catalyst

BET surface area/ m2 g-cat−1

Pore volume mL g-cat−1

Pd-Cu/TiO2-P1 Pd-Cu/TiO2-P2 Pd-Cu/TiO2-A1 Pd-Cu/TiO2-A2 Pd-Cu/TiO2-A Pd-Cu/TiO2-R Pd-Cu/CeO2-D Pd-Cu/ZrO2 Pd-Cu/Al2O3 Pd-Cu/SiO2d

38 (43) 13 (13) 70 (101) 12 (11) 12 2 65 (86) 53 (61) 126 (147) 309 (315)

0.23 0.11 0.27 0.10 0.05 0.01 0.18 0.20 0.76 0.80

a

Fig. 1. CO2 conversion and CH3OH formation rate and selectivity over bimetallic Pd-Cu catalysts supported on different TiO2 support materials. Reaction conditions: CO2/H2 = 1/3, 523 K, 4.1 MPa, 3600 mL (STP) g-cat−1 h−1.

b c d

3

(0.15) (0.06) (0.35) (0.11)

(0.24) (0.29) (1.04) (1.10)

b

/

Avg. pore diameter c / nm 29.5 (35.1) 40.3 (51.6) 13.7 (12.5) 23.6 (26.7) 15.0 39.2 9.1 (9.9) 11.5 (12.7) 24.2 (25.3) 9.3 (11.0)

Values in parentheses are for corresponding pristine support materials. BJH desorption cumulative pore volume. BJH desorption average pore diameter. Data from Ref. [20].

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Fig. 2. XRD patterns of spent bimetallic Pd-Cu catalysts supported on (a) TiO2-R, (b) TiO2-P2, (c) TiO2-P1, (d) TiO2-A1, (e) TiO2-A2, and (f) TiO2-A.

volume in comparison to corresponding pristine support materials, which can be attributed to the incorporation of Pd and Cu species into the pores of support materials. However, slight increases of pore volume can be seen for TiO2-P1 and TiO2-P2. As a matter of fact, the XRD patterns (Fig. 2) of these two catalysts confirmed the existence of single metallic Cu0 phase in the bulk, indicating the possibility of inward diffusion of Cu or Pd-Cu alloy into the lattice of support [49,50]. The surface area and pore volume of the catalysts supported on the TiO2 with single crystal phase (TiO2-A and TiO2-R) were both very low, which, other than the single crystal phase in support, likely resulted in the observed lower activities. The XRD patterns of spent catalysts are presented in Fig. 2, wherein the regions for TiO2 phases (2θ, 20–30°) and Pd-Cu alloy phases (2θ, 30–80°) are provided in separate figures for clarity. To explore the phase change and alloy formation after impregnation, the comparison between pristine support and supported catalysts is plotted in Fig. S1. All the diffraction peaks became much weaker after metal impregnation. Both TiO2-R and TiO2-A, the bare support, exhibited the strongest diffraction peaks of rutile and anatase TiO2 phase, respectively [51]. TiO2-A1 and TiO2-A2 displayed a similar diffraction peak in the region of TiO2 phases regardless of pretreatment temperatures, which can correspond to the anatase phase. The anatase peak was intensified when pretreated at 973 K (TiO2-A2) compared with that at 723 K (TiO2-A1) (Fig. 2 left and Fig. S1). This demonstrates that the crystallite size of TiO2 became larger when treated at higher temperatures. The TiO2 P25based support possessed two phases at the same time, which might even interconvert relying on pretreatment temperatures. Interestingly, in the case of TiO2 P25, more rutile phase was formed when pretreated at 973 K (TiO2-P2), which was in striking contrast to the lower-temperature pretreated TiO2-P1. Considering the superior activity performance of TiO2 P25-supported catalysts (Fig. 1), it is suggested the coexistence of anatase and rutile phase is beneficial for CH3OH formation and selectivity, particularly in an anatase-rich environment. In Fig. 2 (right), alloy phase PdCu3 was identified from the XRD pattern of the spent catalysts supported on TiO2-R, TiO2-A1, TiO2-P1, and TiO2-A, indicating the mutual diffusion between Pd and Cu after reduction. The crystallite phases, sizes, and interplanar spacings for spent Pd-Cu catalysts are listed in Table 4. The magnified XRD patterns are shown in Fig. S4(A). Notably, for catalysts supported on TiO2-R and

Table 4 Crystallite phases, sizes and interplanar spacings for spent Pd-Cu catalysts supported on different supports. Catalyst

Metal Phase

2θ(111)/o

Size a/nm

d-spacing b/nm

Pd-Cu/TiO2-P1 Pd-Cu/TiO2-P2 Pd-Cu/TiO2-A1 Pd-Cu/TiO2-A2 Pd-Cu/TiO2-A Pd-Cu/TiO2-R Pd-Cu/CeO2-D Pd-Cu/ZrO2 Pd-Cu/Al2O3

PdCu3 Cu0 PdCu3 Cu0 PdCu3 PdCu3 PdCu3 PdCu3 PdCu3

42.3 43.1 42.3 43.1 42.4 42.3 42.6 42.3 42.3

14 16 14 15 16 24 13 16 11

0.21 – 0.21 – 0.21 0.21 0.21 0.21 0.21

a b

Crystalline sizes determined from Scherrer equation for (111) peak. Interplanar spacings between (111) planes in PdCu3 structure.

TiO2-A, the diffraction peaks of PdCu3(200) (2θ = 49.3°) are much sharper than those of the other catalysts, and an additional peak, corresponding to alloy phase PdCu3(111) (2θ = 42.3°) with face-centered cubic (fcc) structure, emerged in the meantime [52,53]. Such observation implies that the TiO2 support with poor pore volume and surface area is less favorable for particle dispersion, as reflected from both peak intensity and broadening. This consideration is supported by the larger crystallite sizes of Pd-Cu/TiO2-A and Pd-Cu/TiO2-R than those of others (Table 4). For the supports pretreated at 973 K (TiO2-A2 and TiO2-P2), the XRD patterns exhibited the typical diffraction peak of Cu metallic phase at 2θ = 43.1°, corresponding to Cu(111) [54]. However, the patterns barely showed any Pd0 peaks, implying a better dispersion of Pd0 particles in the form of disordered alloy with shortrange ordering/PdCux than that of Cu0. In other words, the introduction of these supports interfere the alloy formation. By contrast, the formation of Pd-Cu alloy could be confirmed for the catalysts supported on TiO2 pretreated at 723 K. These results suggest that the interconversion of anatase and rutile is strongly dependent on the temperature, and the pretreatment of TiO2 support at a higher temperature can inhibit the formation of alloy phase. Our previous work identified the correlation of synergetic effect on CH3OH synthesis with the Pd-Cu alloy formation on silica support [20], which allows us to further infer that the lower catalytic activities of Pd-Cu supported on TiO2-P2 and TiO2-A2, 4

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Fig. 3. CO2 conversion and CH3OH formation rate and selectivity over bimetallic Pd-Cu catalysts with different supports (TiO2, ZrO2, CeO2, Al2O3, SiO2). CO2 hydrogenation: CO2/H2 = 1/3, 523 K, 4.1 MPa, 3600 mL (STP) gcat−1 h−1.

Fig. 4. XRD patterns of spent bimetallic Pd-Cu catalysts supported on: (a) TiO2P1, (b) ZrO2, (c) CeO2-D, and (d) Al2O3.

pretreated at higher temperature, should originate from the prohibition of alloy formation.

database on textual properties, it is not easy to make conclusion between surface area and CO2 hydrogenation activity. Totally, it can be speculated that the effect of textural properties on CO2 conversion and CH3OH formation was negligible compared with metal component. In Fig. 4 and magnified Fig. S4(C), only a broad diffraction peak of PdCu3(111) alloy phase can be observed for these catalysts, in addition to the diffractions of corresponding support materials, implying the dispersion of Pd and Cu in the form of alloy over these support materials. The strong interaction between Pd and Cu, due to electronic perturbation, improves the dispersion of Pd and Cu species on the catalyst surface and suppresses the aggregation of single metal ensemble [59]. Notably, the intensity of diffraction peaks of support materials became much weaker compared with those of corresponding pristine supports (Fig. S1), indicating the reduced crystallinity [60]. The PdCu3 peaks were very weak, especially in the case of Pd-Cu/CeO2-D (Fig. 4c). The crystallite sizes were estimated according to Scherrer Equation (Table 4), which appeared to be higher than the average pore size of corresponding support materials, except for the TiO2-P1- and Al2O3supported catalysts. Moreover, both the surface area and pore volume of TiO2-P1, CeO2-D, and ZrO2 were not comparable to those of Al2O3. It is suggested that the confinement of alloy particle growth is negligible when the surface area and pore volume lie within a small range, even though the pore size of TiO2-P1 was large enough. Yet, the confinement might still function when the surface area and pore volume are considerably increased, as evidenced from the smaller crystallite size of alloy phase in the case of Al2O3 (e.g., 11 nm < 25.3 nm).

3.2. Effect of supports on CO2 hydrogenation to CH3OH 3.2.1. Activity performance over Pd-Cu catalysts supported on TiO2, ZrO2, CeO2, and Al2O3 To investigate the effect of supports on catalytic behavior, TiO2-P1, ZrO2, CeO2-D, and Al2O3 were introduced as support materials. As benchmark, SiO2 was also employed for comparison. As depicted in Fig. 3, SiO2 displayed lowest CO2 conversion (6.6%) and CH3OH formation rate (0.31 μmol g-cat−1s−1). The CO2 conversion decreased in the following order: TiO2-P1 ˜ ZrO2 > Al2O3 > SiO2 ˜ CeO2-D, while the CH3OH selectivity increased in a nearly inverted order: ZrO2 ˜ TiO2P1 < CeO2-D < Al2O3 ˜ SiO2. TiO2-P1 and ZrO2 exhibited the highest CO2 conversion (16.4% and 15.8%, respectively) and CH3OH formation rate (0.50 and 0.52 μmol g-cat−1s−1, respectively), the CH3OH formation rates of which yielded almost 50% more than that of the benchmark SiO2-supported counterpart. Meanwhile, the CO formation rates of these two catalysts were also the highest (1.43 and 1.41 μmol gcat−1s−1, respectively), which led to the lowest CH3OH selectivity among all catalysts. As reported, these introduced support materials possess MSI with either Pd or Cu [55–58], which might be crucial to affect the alloy structuring, tune the chemical surface nature, and alter the adsorption behavior. To verify the presence of the MSI and to clarify its roles in improved activity performance, these catalysts were characterized by various techniques, and results will be discussed in the following sections.

3.2.3. H2-TPR study The effect of different supports on the reduction behavior of these catalysts were investigated by H2-TPR, and the resultant profiles are presented in Fig. 5. The reduction temperature range and the amount of H2 uptake for the main reduction peak, as well as the estimated molar ratio between H2 consumption relative to metal component (H2/M) are listed in Table 5. The TPR profiles of bare supports are also collected for comparison, which displayed negligible reducibility. Generally, the reduction peaks of all catalysts ranged between 280 and 490 K, corresponding to the reduction of Pd and Cu oxides. Pd is susceptible to hydrogen at even ambient temperature [61], while the reduction of crystalline CuO undergoes two steps, including from Cu2+ to Cu+ at ca. 573 K and subsequently to Cu0 at ca. 587.9 K [47,62]. Clearly, the reduction of PdO was retarded due to the combination of Pd and Cu, while the reducibility of CuO was considerably improved. Additionally, the single reduction peak implies the simultaneous reduction instead of separately, also indicative of the strong interaction between Pd and Cu

3.2.2. Textural property and crystallite structure The textural properties of bimetallic Pd-Cu catalysts supported on TiO2-P1, ZrO2, CeO2-D, Al2O3, and SiO2 are listed in Table 3. The SiO2 support exhibited the largest BET surface area and pore volume, while the TiO2-P1 support possessed the largest average pore diameter. Loading Pd and Cu species decreased the BET surface area and pore volume for all these supported catalysts. Fig. S5 depicts the correlation of CH3OH and CO formation rate and CH3OH selectivity as a function of BET surface area, as well as that of CO2 conversion. Usually, higher surface area is beneficial to metal dispersion, and thereof enables more active sites. However, the CO2 conversion seems to deviate from this regularity, which decreases with the increase of surface area; meanwhile, CO formation rate exhibits a similar trend. The CH3OH selectivity increases along with BET surface area, although the CH3OH formation rate remains unchanged in the meantime. Due to limited 5

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[63,64]. CeO2 + 1/2H2 = 1/2 Ce2O3 + 1/2H2O

In the XRD pattern of Pd-Ce/CeO2-D (Fig. 4), the diffractions of PdCu alloy phase were absent, nor were the isolated metallic phase. In our previous work, the in situ XPS investigations demonstrate that the inward diffusion of Pd and Cu into the lattice of CeO2 [64]. This is probably still valid in the present work. Such inward diffusion resulted in promoted oxygen mobility due to the charge neutralization and hydrogen dissociation and spillover mechanism, which caused improved reduction of dispersed CuO clusters as well as CeO2 support at lower temperature [65,66]. These phenomena are indicative of SMSI between CeO2 and metal components [67]. By contrast, the H2 uptake of Pd-Cu/ TiO2-P1 tends to be stoichiometric, reflecting relatively mild interaction. There are several points that can be summarized from above phenomena. Pd-Cu/ZrO2 exhibited similar reduction profile with Pd-Cu/ SiO2, suggesting comparable MSI, and consequently had negligible effect on alloy formation. The tendency from Gaussian-like peak to bellshaped peak also indicated stronger MSI than Pd-Cu/SiO2. However, the bell-shaped reduction profiles of Pd-Cu/TiO2-P1 and Pd-Cu/CeO2-D certified the existence of alloy restructuring, which might result in the formation of stoichiometric PdCux from PdCu3 [47]. Therefore, it can prove the interaction force between support (TiO2 and CeO2) and metal components (Pd-Cu). By contrast, the presence of CeO2 reduction versified from higher H2/M ratio and the absence of Pd-Cu phases in XRD pattern demonstrated strongest interaction for Pd-Cu/CeO2-D. In terms of Pd-Cu/Al2O3, it did not have bell-shaped reduction profile and overstoichiometric H2/M ratio, but the isolated reduction from PdO and CuO in TPR profile also indicated the alloy formation was still affected, suggesting stronger MSI than Pd-Cu/ZrO2 but weaker than Pd-Cu/TiO2P1. Above all, these five catalysts can be divided into three sections based on the strength of MSI. CeO2-D-supported catalyst possesses strongest MSI. TiO2-P1-, ZrO2-, and Al2O3-supported catalysts possess moderate MSI. SiO2-supported catalyst possesses too weak MSI.

Fig. 5. H2-TPR profiles of fresh catalysts and supports. (a) TiO2-P1, (b) ZrO2, (c) CeO2-D, (d) Al2O3, and (e) SiO2. The profiles of corresponding supports are listed as the dash dot line at the bottom of each catalyst. Table 5 Amount of H2 uptake in H2-TPR over fresh bimetallic Pd-Cu catalysts with different supports. Catalyst

Red. Temp. range/ K

H2 uptake/ mmol g-cat−1

H2/M ratio*

Pd-Cu/TiO2-P1 Pd-Cu/ZrO2 Pd-Cu/CeO2-D Pd-Cu/Al2O3 Pd-Cu/SiO2

325-410 330-460 330-435 285-490 320-455

1.84 1.71 2.23 1.64 1.91

1.05 0.97 1.27 0.93 1.09

(2)

* Ratio between the amounts of hydrogen consumed and the reducible species in mole.

due to alloy formation. Previous work also showed a similar single reduction peak, indicating fairly homogeneous alloy formation [47]. Notably, the reduction peaks were different in shape and temperature ranges. Pd-Cu/SiO2 showed a distinctive-single reduction peak at 420 K, which is consistent with previous work [28]. A similar reduction profile was also evident in the case of ZrO2-supported catalyst, which was more like a transition status from Gaussian-like peak to bell-shaped peak. Differently, the reduction temperature range of Pd-Cu/Al2O3 was broader, and the initiation was even lower than other catalysts, as well as emergence of peaks at both lower and higher temperature ranges. This suggests the existence of isolated PdO and CuO. Apparently, the alloy formation was interfered in the presence of Al2O3. The reduction peaks of Pd-Cu/CeO2-D and Pd-Cu/TiO2-P1 shifted towards lower temperatures, indicating that the reducibility of metal oxides was enhanced. Besides, they both showed a bell-shaped peak of these two catalysts, which was totally different from the Gaussian-like peak of PdCu/SiO2, also allowed us to speculate that Pd-Cu alloy had some variation during the alloy structuring, i.e., disordered alloy PdCux with short-range ordering not PdCu3 that favorable for CH3OH formation, suggesting the existence of MSI [47]. Furthermore, the reduction peaks of Pd-Cu/CeO2-D and Pd-Cu/TiO2-P1 displayed clear start and end of reduction, while the tail of peak in the two sides can be observed for rest catalysts. This indicates more ordered fractions in the Pd-Cu alloy of CeO2-D- and TiO2-P1-supported catalysts. As the amount of H2 uptake quantified in Table 5, the H2 uptake of Pd-Cu/Al2O3 was the lowest, indicating worst reducibility. Furthermore, as listed in Table 5, the estimated H2/M ratios of all catalysts were around the stoichiometric ratio 1.0, except that the ratio of Pd-Cu/ CeO2-D was 1.27. As reported, CeO2 can be partially reduced to Ce2O3 in the presence of Pd-Cu via the following reaction equation [47], which should contribute to the excessive hydrogen consumption

3.2.4. Adsorption property Fig. 6 depicts the H2-TPD profiles over bimetallic Pd-Cu catalysts supported on these supports, as well as the TPD profiles of corresponding support materials for comparison. Note that all supports were reduced at 573 K for 2 h with H2 flow for consistency. Pure support materials barely showed desorption peaks (the dash-dot lines in Fig. 6), implying the negligible H2 adsorption capacity. The H2-TPD profiles exhibited significant changes between the catalysts after loading Pd and Cu. Interestingly, all catalysts showed a clear preference towards the adsorption of the species around or below 523 K, which can be considered as weakly-bonded species. Such characteristic adsorption performance agreed with our previous work on the SiO2-supported Pd-Cu catalyst, which was attributed to the PdCu3 alloy formation [20]. Clearly, the resemblance demonstrates that Pd-Cu alloy still exists in those catalysts supported on different materials and functions as the active sites responsible for the adsorption of weaklybonded species during the reaction. Although with similar peak shape, peak intensities differed significantly between catalysts. Such variation might originate from the interaction between Pd-Cu species and support materials. The hydrogen desorption was quantified and the desorption at below 523 K, the reaction temperature, was also analyzed separately, as listed in Table 6. Among these supported catalysts, Pd-Cu/CeO2-D exhibited an extremely high amount of adsorbed hydrogen, which might be related to the reduction of Ce4+ to Ce3+ via the SMSI, accompanied with the generation of abundant oxygen vacancy [64,68]. On the whole, the H2 desorption is correlated with its H2 uptake in H2-TPR results, from PdCu/TiO2-P1, Pd-Cu/ZrO2 to Pd-Cu/CeO2-D. The abnormity of Pd-Cu/ Al2O3 should be ascribed to its excellent acid property. The hydrogen 6

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ZrO2. The improvement of total adsorption capacity can be also attributed to the oxygen vacancy due to enhanced reduction of Ce4+ to Ce3+ in the presence of Pd-Cu via SMSI [64]. Notably, the CO2 desorption occurred at the temperature lower than 523 K dominated for PdCu/TiO2-P1, Pd-Cu/ZrO2, and Pd-Cu/Al2O3, and the proportion varied from 0.73 to 0.79. However, this value dropped drastically to 0.45 for Pd-Cu/CeO2-D, indicating a migration from weakly to strongly adsorbed CO2, which should be unfavorable for hydrogenation reaction. The surface desorption ratios of H2/CO2 were calculated and listed in Table 6. According to the CH3OH formation mechanism, more H2 accumulated on catalyst surface is beneficial to CH3OH formation [9,69]. Interestingly, except for Pd-Cu/ZrO2, the increasing ratios of H2/CO2 always contribute to CO2 conversion, CH3OH formation rate, and CH3OH selectivity (Table 2 and Fig. 3). In the case of Pd-Cu/ZrO2, the second-high CO2 desorption might compensate its lowest H2 adsorption capacity. Additionally, both the CO2-TPD and H2-TPD (shown in Fig. 6) results show that the bimetallic Pd-Cu catalyst supported on CeO2-D possessed strong adsorption capacities for both CO2 and H2, which might lead to the competitive adsorption of CO2 and H2 molecules [70]. This competition is bound to suppress the CO2 hydrogenation activity. In the case of CeO2-D, the CO2 adsorption was improved greatly (i.e., 1.5 times) after metal loading, but the improvement was negligible for TiO2-P1 and ZrO2. These results suggest that CO2 adsorption mainly occurs on the surface of support due to their abundant oxygen defects [71,72]. Moreover, the metal loading-induced MSI might create more sites at the vicinity of alloy particles for CO2 adsorption so that the loss of adsorption capacity due to metal coverage can be retrieved and even improved. In contrast, the total CO2 adsorption capacity of Pd-Cu/ Al2O3 decreased in comparison to that of Al2O3, and the majority loss originated from the type I (Fig. 7D). Nonetheless, the proportion of the species below 523 K on Al2O3-supported catalyst was comparable to that on TiO2-P1-supported catalyst, and even much higher than that on CeO2-D-supported counterpart. This probably resulted from the strong Lewis acid sites on Al2O3 that retained its CO2 adsorption.

Fig. 6. H2-TPD profiles of reduced catalysts and supports: (a) TiO2-P1, (b) ZrO2, (c) CeO2-D, and (d) Al2O3. The profiles of corresponding supports are listed as the dash dot line at the bottom of each catalyst.

3.2.5. Discussion on effect of support on CH3OH synthesis activity As discussed in Section 3.2.1, the Pd-Cu catalysts supported on different support materials with identical loading level and Pd/ (Pd + Cu) atomic ratio exhibited various activity performance, in which the CH3OH synthesis activity decreased in the following order: TiO2-P1 ˜ ZrO2 > Al2O3 > CeO2-D ˜ SiO2. Desirable improvement was achieved by altering catalyst support from initial SiO2 to TiO2 or ZrO2. To explore the origins of such variation, these catalysts were characterized by N2 physisorption, XRD, H2-TPR, and H2-/CO2-TPD. The N2 physisorption results demonstrated the porosity-induced confinement of particle growth, however, the estimated crystallite size from XRD patterns (11–16 nm) barely showed any linkage with the activity performance. Consequently, the diverse porosity of support materials seems unlikely to be an important factor governing the activity performance. H2-TPR profiles pointed to the relationship between Pd and Cu in the form of alloy. Noticeably, both CeO2-D- and TiO2-P1-supported catalysts exhibited a bell-shaped reduction peak, which certified the existence of partially alloy restructuring to form stoichiometric PdCux

desorption occurred at the temperature lower than 523 K exhibited obvious difference for these catalysts, and the proportion varied from 0.34 to 0.61, which seemed irrelevant to its catalytic performance. The CO2 adsorption properties of the same catalysts were investigated by CO2-TPD, and the TPD profiles of reduced catalysts and corresponding supports are depicted in Fig. 7. In sharp contrast to their H2 adsorption performance, all supports materials exhibited strong CO2 desorption peaks, the intensities of which were even comparable to those of the corresponding catalysts. Due to the similar profiles and substantial adsorption capacity between bare support and catalyst, CO2 adsorption took place majorly on the surface of support. A quantitative analysis was carried out, and the desorbed CO2 amount below 523 K and total CO2 desorption were determined, as shown in Table 6. The amount of desorbed CO2 decreases in the order for both bare supports and catalysts: CeO2-D > ZrO2 > Al2O3 > TiO2P1. The CO2 desorption amount of Pd-Cu/CeO2-D is 10 times more than that of Pd-Cu/TiO2-P1, and also 2.7 times more than that of Pd-Cu/

Table 6 Amount of H2 and CO2 desorption over reduced bimetallic Pd-Cu catalyst with different supports.a. Catalyst

Pd-Cu/TiO2-P1 Pd-Cu/ZrO2 Pd-Cu/CeO2-D Pd-Cu/Al2O3 a

Amount of desorbed H2/μmol g-cat−1

Amount of desorbed CO2/μmol g-cat−1

H2/CO2

Total

< 523 K

< 523 K/Total

Total

< 523 K

< 523 K/Total

64.49 54.51 303.09 72.36

22.13 21.05 153.81 44.19

0.34 0.39 0.51 0.61

25.46 (23.33) 107.29 (100.48) 278.13 (183.76) 62.47 (70.16)

19.43 84.24 125.45 45.58

0.76 0.79 0.45 0.73

Values in parentheses are for corresponding pristine support materials. 7

2.53 0.51 1.09 1.59

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Fig. 7. CO2-TPD profiles of reduced catalysts and supports: (a) TiO2-P1, (b) ZrO2, (c) CeO2-D, and (d) Al2O3.

restructuring and “interruption” of alloy formation. In another scenario, relatively moderate interaction can preserve the nano-sized alloy particles and accompanied benefits, and even benefit the catalytic performance. In our previous work, a relatively inert support, SiO2, was used to focus on the alloy effect on the activity performance. A recent follow-up study showed that the SiO2 had negligible capability in terms of H2 or CO2 adsorption, and the characteristic adsorption towards weaklybonded species primarily originated from metallic surface [25]. In sharp contrast, the pristine support materials employed in this work showed stronger ability in CO2 adsorption. Interestingly, the weaklybonded CO2 species appeared to correlate to the activity performance. A considerable reduction of weakly-bonded CO2 was evidenced on the catalyst Pd-Cu/CeO2-D in comparison to other catalysts, which might be related to SMSI in the case of CeO2-D-supported catalysts. Probably, a competition between CO2 and H2 adsorption occurs during the reaction, thereof resulting in a lower activity.

from PdCu3. The barely visible PdCu3 diffraction peak of Pd-Cu/CeO2-D in XRD patterns certificated stronger alloy restructuring effect than PdCu/TiO2-P1. The former one even showed over-stoichiometric H2 uptake, originating from enhanced reduction of CeO2. These results demonstrated that CeO2-D-supported exhibited strongest MSI. ZrO2- and Al2O3-supported catalysts exhibited similar transitional reduction peaks from Gaussian-like peak of Pd-Cu/SiO2 to bell-shaped peak of Pd-Cu/ CeO2-D and Pd-Cu/TiO2-P1, while isolated reduction of PdO and CuO could be observed for Pd-Cu/Al2O3, indicating “interruption” of alloy formation. Totally, these supported catalysts can be divided into three sections with different strength of MSI. Pd-Cu/CeO2-D possesses strongest MSI. Pd-Cu/TiO2-P1, Pd-Cu/ZrO2 and Pd-Cu/Al2O3 possess moderate MSI, while Pd-Cu/SiO2 possesses negligible MSI. Accordingly, the generation of abundant oxygen vacancy from CeO2 reduction induced strongest adsorption capacity of H2 and CO2, which also might cause competitive adsorption. Its adsorbed CO2 was mostly distributed as strongly adsorption, and the total surface adsorption ratio of H2/CO2 was lower than other catalysts. However, alloy restructuring, strongly adsorption, and low H2/CO2 ratio were unfavorable for CH3OH synthesis. Therefore, the lower CH3OH synthesis activity on the CeO2D-supported Pd-Cu catalyst should be related to its strongest MSI. However, the surface H2/CO2 ratio of Pd-Cu/TiO2-P1 was highest, contributing to its desirable catalytic performance. The excellent acidbase property of Al2O3 compensated the possible loss of CO2 adsorption and activation caused by the absence of Pd-Cu alloy. Therefore, though SMSI enabled a characteristic enhancement of adsorption on the surface, it is not beneficial to the catalytic property due to the alloy

4. Conclusions The effect of supports on the CO2 hydrogenation to methanol over bimetallic Pd-Cu catalysts were investigated in the present work. Commercial and self-prepared TiO2, ZrO2, CeO2, Al2O3, and SiO2 were introduced as support materials. The results are summarized below. (1) Among the TiO2 supports, commercial TiO2 P25 supported Pd-Cu exhibited the highest CO2 hydrogenation activity (CH3OH FR.: 8

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0.50 μmol g-cat−1s−1, CH3OH selectivity: 25.7%, and CO2 conv.: 16.4%). Compared with the benchmarked support, SiO2 (CH3OH FR.: 0.31 μmol g-cat−1s−1), a significant improvement was observed for TiO2 P25 (i.e., 0.50 μmol g-cat−1s−1) and ZrO2 (i.e., 0.52 μmol g-cat−1s−1). When put all these supports together, CH3OH synthesis activity of supported Pd-Cu catalysts decreased in the following order: TiO2-P1 ˜ ZrO2 > Al2O3 > CeO2-D ˜ SiO2. (2) XRD results demonstrate that PdCu3(111) alloy phase dominated over the catalyst. Cu0(111) phase was only detected on supports TiO2P2 and TiO2-A2, which were pretreated at high temperature. These resulted in poor catalytic activity. PdCu3(200) alloy phase was observed on supports TiO2-A and TiO2-R, and their crystallite sizes of PdCu3(111) particles were larger than others, which were attributed to the poor metal dispersion due to inferior pore structure. (3) The bell-shaped H2-TPR profiles of CeO2-D- and TiO2-P1-supported Pd-Cu catalysts suggested alloy restructuring to form PdCux from PdCu3. The former one showed extremely weak diffraction peak of PdCu3 and over-stoichiometric H2 uptake, originating metal-facilitated reduction of the Ce4+ to Ce3+. These phenomena suggested the strongest MSI between CeO2-D and each metal component, while the inhibition of catalytic activity for CH3OH formation was occurred. Pd-Cu/TiO2-P1, Pd-Cu/ZrO2, and Pd-Cu/Al2O3 displayed moderate MSI, but the lower activity of Pd-Cu/Al2O3 should arise from alloy interference. (4) The pristine support materials all showed strong CO2 adsorption ability, which was negligible for previous inert SiO2 support. Especially, CeO2-D support displayed extremely strong CO2 adsorption ability due to abundant oxygen vacancy. However, the weakly-bonded CO2 of Pd-Cu/ CeO2-D was greatly reduced compared with other catalysts, which appeared to correlate to the undesirable activity performance. The lower surface adsorption ratio of H2/CO2 was also not beneficial to CH3OH formation. Probably, the coexistence of strongest H2 and CO2 adsorption ability caused a competition during the reaction, resulting in undesirable performance.

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