anatase catalyst by tuning

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Enhanced CO2 methanation activity of Ni/anatase catalyst by tuning strong metal-support interactions Jian Li, Yaping Lin, Xiulian Pan, Dengyun Miao, Ding Ding, Yi Cui, Jinhu Dong, and Xinhe Bao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00401 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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ACS Catalysis

Enhanced CO2 Methanation Activity of Ni/Anatase Catalyst by Tuning Strong MetalSupport Interactions Jian Lia,b#, Yaping Lina,b#, Xiulian Pan*a, Dengyun Miaoa, Ding Dingc, Yi Cuic, Jinhu Donga,b, Xinhe Bao*a a

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, Dalian 116023, Liaoning, China b

University of Chinese Academy of Sciences, Beijing 100049, China

c

Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and

Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China # the authors contribute equally to the present work *E-mail: [email protected] , [email protected] ABSTRACT The strong metal-support interactions (SMSI) has been widely recognized for the platinum-group metals on reducible oxide supports. Herein we report that the catalytic activity of Ni catalyst is significantly suppressed over the conventional anatase (a-TiO2) support due to the SMSI induced formation of titania overlayer around the Ni nanoparticles. Furthermore, CO is the only product in CO2 hydrogenation. In contrast,

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the NH3-treatment and H2-treatment of the a-TiO2 support enhances remarkably the activity of Ni, i.e. CO2 conversion shoots up by one order of magnitude and CO2 is hydrogenated almost exclusively to CH4. X-ray photoelectron spectroscopy (XPS), H2 and CO chemisorption and low temperature electron paramagnetic resonance (EPR) reveal that the enhanced CO2 methanation activity is correlated with the relative concentration of Ti3+ species in the bulk that generated by reduction treatment, which likely have altered the SMSI between Ni and a-TiO2 support. This simple reduction treatment approach may be applicable to modulate the SMSI of other reducible oxide supported metal catalysts. KEYWORDS: CO2, methanation, anatase, nickel-based supported catalyst, strong support-metal interactions 1. INTRODUCTION The strong metal-support interaction (SMSI) is a widely recognized effect in heterogeneous catalysis, which can significantly modulate the catalytic activities. It was first reported by Tauster and co-workers when they observed the suppressed chemisorption of CO and H2 over the TiO2 supported platinum-group metals (PGMs) after high temperature reduction.1 For the reducible oxide supports e.g. TiO2, Nb2O5 and CeO2, the SMSI frequently induces formation of an overlayer of oxides during high temperature reduction, which decorates the metal nanoparticles.2-10 Besides the reduction-induced encapsulation overlayer, Mou and co-workers reported oxidationinduced overlayer for the Au/ZnO catalyst, which was termed as O-SMSI.11 The O-


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ACS Catalysis

SMSI effect was also observed on the HAP supported PGMs such as Pd and Pt catalysts.12 In addition, the oxide overlayer can be induced by certain adsorbates (termed as A-SMSI), which was reported for the TiO2 and Nb2O5 supported Rh catalysts upon CO2-H2 (CO2-riched) treatment at 250 °C.13 In comparison, there are much fewer studies on the SMSI of the non-platinumgroup metals, particularly their catalytic behaviour. Fu et al. proposed metals with a work function higher than 5.3 eV and a surface energy larger than 2 J·m-2 (vs. 0.44 J·m-2 for anatase (101)) could be encapsulated by titania overlayer, and Ni (2.4 J·m-2) also sat on the rim of that region.9, 14-16 Somorjai et al. observed partial encapsulation of metallic Co by TiO2-x, which changed the product selectivity in CO and CO2 hydrogenation.17 Recently, Hernández Mejía et al. studied the SMSI effects on the Fischer-Tropsch synthesis activities of Co supported on TiO2 and Nb2O5 and observed that reduction-oxidation-reduction (ROR) treatments can modify the SMSI.18 Although the formation of TiOx overlayer upon the high-temperature reduction treatment has been reported over single crystal TiOx/Ni (111), Ni/TiO2 (110),5 and Ni/TiO2 (100),7 the SMSI effect on the catalytic activities of Ni/TiO2 is rarely studied and not well understood. Herein we investigate the possibility to tune the SMSI of Ni/TiO2 and its catalytic activities by taking CO2 methanation as a probe reaction since Ni is widely demonstrated to be an active catalyst for CO2 methanation.19-24 CO2 hydrogenation via methanation using renewable hydrogen is an attractive route to utilize the decentralized CO2 as the carbon resources to value-added chemicals


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and fuels.25-35 This may find important applications in the submarines, space crafts and space stations to remove CO2 and to generate O2. However, Ni supported on the conventional anatase (Ni/a-TiO2) exhibits a poor activity for CO2 methanation forming almost exclusively CO.36 We report here that NH3-treatment and H2-treatment of aTiO2 suppress formation of the encapsulation overlayer, consequently enhancing significantly the CO2 methanation activity by one order of magnitude. 2.

EXPERIMENTAL SECTION 2.1. Materials. The anatase TiO2 was purchased from Shanghai Aladdin Bio-Chem

Technology Co., Ltd. Ni(NO3)2·6H2O was purchased from Tianjin Kermel Reagent Company. 2.2. Preparation of Ni/a-TiO2 catalysts. The anatase TiO2 (a-TiO2) was treated at 600 °C for 3 h in NH3, and the resulting sample was denoted as a-TiO2-NH3. Ni was introduced by the wet-impregnation method. Typically, 1 g a-TiO2 or a-TiO2-NH3 was added to 30 mL ethanol solution containing 246 mg Ni (NO3)2·6H2O, which was kept stirring at room temperature. Subsequently, it was dried at 60 °C for 12 h and calcined at 500 °C in static air for 2 h, and were named as NiO/a-TiO2 and NiO/a-TiO2-NH3, respectively. After H2 reduction, the obtained catalysts were denoted as Ni/a-TiO2 and Ni/a-TiO2-NH3. 2.3. Activity test. Typically, 100 mg fresh catalyst (20-40 mesh) was used unless otherwise stated. The catalysts were in situ reduced at 500 °C under a flow of H2 (3000 mL·g-1·h-1) for 2 h at atmospheric pressure prior to reaction. CO2 methanation was


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performed in a fixed-bed reactor under conditions of H2/CO2 = 4, 0.1 MPa, 360 °C and 15000 mL·g-1·h-1. A pre-mixed stream CO2, H2 and Ar (CO2/H2/Ar = 19/76/5, v/v/v) was used as the feed, where Ar acted as the internal standard for online gas chromatograph (GC) analysis (Agilent 7890B). Hayesep Q, and 5A molecular sieves packed columns connected to the thermal conductivity detector (TCD) were used to quantify CO, CO2, CH4 and Ar, and HP-FFAP and HP-AL/S capillary columns were connected to the flame ionization detector (FID) for analysis of CH4 and other possible hydrocarbon products. CO2 conversion was calculated on a carbon atom basis, i.e.

CO2 Conversion(%) =

𝐶𝑂2 𝑖𝑛𝑙𝑒𝑡 - 𝐶𝑂2 𝑜𝑢𝑡𝑙𝑒𝑡 𝐶𝑂2 𝑖𝑛𝑙𝑒𝑡

× 100%,

CH4 selectivity and CO selectivity were calculated as the following: CH4 Selectivity(%) = CO Selectivity(%) =

𝐶𝐻4 𝑜𝑢𝑡𝑙𝑒𝑡 𝐶𝑂2 𝑖𝑛𝑙𝑒𝑡 - 𝐶𝑂2 𝑜𝑢𝑡𝑙𝑒𝑡 𝐶𝑂 𝑜𝑢𝑡𝑙𝑒𝑡 𝐶𝑂2 𝑖𝑛𝑙𝑒𝑡 - 𝐶𝑂2 𝑜𝑢𝑡𝑙𝑒𝑡

× 100%, × 100%,

where CO2 inlet and CO2 outlet represent the molar concentration of CO2 in the feed and effluent, respectively; CH4 outlet and CO outlet represent the molar concentration of CH4 and CO in the effluent, respectively. 2.4. Catalyst characterization. X-ray diffraction (XRD) was carried out on an Empyrean-100 X-ray diffractometer with a Cu Kα (λ = 1.541 Å) mono-chromatic radiation source from 35° to 60° at a scan rate of 1.8°·min-1. Inductively coupled plasma (ICP) was performed to determine the Ni content of supported Ni catalysts using ICP-OES 7300DV apparatus. The specific surface area (SBET) of


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the supports was measured by N2 adsorption-desorption isotherms at -196 °C using Quantachrome instruments. Prior to adsorption, the samples were degassed at 300 °C for 5 h. The Brunauer-Emmett-Teller method was used to calculate the SBET. Low temperature electron paramagnetic resonance (EPR) spectra were collected at 110 K using a Bruker A200 EPR spectrometer operating at the X-band frequency. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was performed on JEM F200 microscope. Hydrogen temperature-programmed reduction (H2-TPR) was carried out in a tubular quartz reactor. 100 mg of samples were placed in the tube and treated with Heat 120 °C for 1 h, then cooled to 50 °C. The sample was heated in 1% H2/Ar at a ramp of 10 °C /min from 50 ° to 800 °C with the effluents monitored by the online mass spectrometer. H2 or CO pulse adsorption experiment was carried out on a Micromeritics Autochem 2920 chemisorption analyzer with a TCD detector. The catalyst samples (50 mg) were reduced with H2 at 500 °C for 2 h, and subsequently flushed with He for 30 min at 550 °C. When the temperature decreased to 20 °C (For CO pulse adsorption experiment, the temperature should decrease to 10 °C), 10 vol% H2/He (5 vol% CO/He) was injected until the system was saturated. The H2 and CO uptake were calculated from the pulse results. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a Thermo ESCALAB 250Xi system with a 162 W monochromatic Al Kα radiation (1486.6 eV). The binding energy was calibrated by contamination carbon C1s peak (284.6 eV) as the


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reference. The quasi-in-situ near ambient pressure X-ray photoelectron spectroscopy (quasi-in-situ NAP-XPS) were conducted using the Specs NAP-XPS system with a PHOIBOS NAP hemispherical energy analyzer. The base pressure of the analysis chamber was ~5  10-10 mbar and the NAP operation were conducted under 1 mbar. An Al Kα photon energy of 1486.6 eV was used and the energy resolution was estimated to be ~0.2 eV. The sample can be heated by a laser-heating equipment from room temperature to about 500 °C in a H2 or O2 atmosphere of 1 mbar, and keep for 1 h. 3. RESULTS AND DISCUSSION Ni was impregnated onto a-TiO2 and the NH3-pretreated a-TiO2 support following the same procedure and under the same conditions. However, Figure 1 shows that these two catalysts exhibit significantly different activities and product selectivities in CO2 methanation. CO and CH4 are the only carbonaceous products detected over these two catalysts. Ni/a-TiO2 gives a low activity. CO2 conversion is only 5%. Furthermore, this catalyst is not active at all for methanation and CO is the only carbon-containing product. In contrast, CO2 conversion shoots up to 58.2% when Ni is supported on the NH3-pretreated a-TiO2 support. More interestingly, CO2 is converted almost exclusively to CH4 over this catalyst (Figure 1b). Furthermore, we compared the product selectivity at the same CO2 conversion (4.9%) by varying the space velocity. As shown in Figure S1, Ni/a-TiO2-NH3 catalyst still exhibits a CH4 selectivity of nearly 90% while CO is still the only product on Ni/a-TiO2 catalyst. Such a significant difference in the activity and selectivity is surprising because the impregnation of nickel


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nitrate and the following calcination have been carried out in air. Furthermore, both catalysts have been prepared following the same procedure and under the same conditions except the support pretreatment step.

Figure 1. Catalytic performance of Ni/a-TiO2 and Ni/a-TiO2-NH3 in CO2 methanation. (a) CO2 conversion; (b) Product selectivity. Reaction condition: 360 °C, 0.1 MPa, CO2/H2 = 1/4 and GHSV = 15000 mL·g-1·h-1. ICP analysis in Table S1 indicates that the Ni loadings over both catalysts are similar (5.20 and 5.17 wt.%). Furthermore, no leaching of Ni species is observed, as the Ni loadings remain practically unchanged after reaction. Figure S2a shows that Ni/a-TiO2-NH3 still exhibits characteristic anatase (a-TiO2) crystal phase as Ni/a-TiO2, which indicates that ammonia treatment does not change the crystal phase of the a-TiO2 support. The specific surface area SBET of a-TiO2 is 96.4 m2·g-1, and upon NH3 treatment SBET is reduced to 42.7 m2·g-1. These results demonstrate that the lower surface area of Ni/a-TiO2-NH3 does not offset the enhanced activity of Ni by NH3 treatment. XPS analysis of a-TiO2-NH3 (Figure S2b) shows the presence of N1s signal at 395.5 eV whereas this is not observed over the original a-TiO2.37-38 However, this N


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signal disappears upon H2 reduction of Ni/a-TiO2-NH3 (Figure S2b) indicating that N species are either reduced to an undetectable concentration or even removed completely during reduction. Therefore, it is reasonable to conclude that the N species may not have played an important role in the reaction. This is in contrast to a previous study on a catalyst with Pd nanoparticles supported on TiO2, where strongly coupled Pd-N species were observed and it was proposed to be responsible for the enhanced nitrobenzene hydrogenation activity and stability of Pd/ N-doped TiO2.39 To investigate the reducibility of Ni species over NiO/a-TiO2 and NiO/a-TiO2-NH3, H2-temperature programmed reduction (H2-TPR) was performed. As shown in Figure 2, no reduction peak is observed below 300 °C. There are three reduction peaks falling in the range of 300-400 °C and 400-500 °C, which are attributed to NiO species interacting strongly with TiO2. 40 Furthermore, the main reduction peak over Ni/a-TiO2NH3 (496 °C) is higher than that over Ni/a-TiO2 (475 °C), suggesting more difficult to reduce for the nickel species supported on a-TiO2-NH3.

Figure 2. H2-TPR profiles of NiO/a-TiO2 and NiO/a-TiO2-NH3 catalysts.


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We turn to the low temperature electron paramagnetic resonance (EPR) to further investigate the properties of a-TiO2 and a-TiO2-NH3. As shown in Figure 3a, no EPR signals are observed for a-TiO2. By contrast, upon NH3 treatment at 600 °C, an EPR signal corresponding to a g-value of 1.987 is observed, which can be assigned to the Ti3+ species.2, 37, 41 Since EPR is a bulk characterization technique and the Ti3+ species on the surface would be readily oxidized to Ti4+ upon exposure to air,41 the EPR signal for g = 1.987 is more likely attributed to bulk Ti3+ species. Furthermore, the intensity of this signal becomes stronger as the NH3-treatment is prolonged. Note that the measurement conditions are kept unchanged and the same amount of catalysts have been used. Therefore, the intensity of this EPR signal g = 1.987 can reflect the relative concentration of bulk Ti3+ species, as it was widely practiced previously,37,

42-43

although it cannot give an absolute concentration quantitatively. For example, Valentin et al. reported that F-dopants induced formation of Ti3+ in the bulk of anatase, and the relative concentration of Ti3+ can be estimated according to the integrated peak areas of EPR spectra.43 Khan et al. compared the intensity of EPR signals to show that doping of Fe3+ can generate defects such as Ti3+ and oxygen vacancies.42 Hoang et al. also compared the relative concentration of Ti3+ of TiO2 from the intensity of the EPR signals between different samples.37 The results in Figure S3 indicate that prolonged NH3-treatment leads to formation of more Ti3+ in a-TiO2-NH3. Interestingly, the corresponding Ni catalysts supported on these a-TiO2-NH3 samples exhibit a stepwise enhanced CO2 methanation activity as the increasing NH3-treatment duration.


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ACS Catalysis

Figure 3. (a) EPR spectra recorded at 110 K for a-TiO2 and a-TiO2-NH3 upon NH3treatment for different duration; (b) Comparison of the CO2 methanation activities of the Ni catalysts supported on a-TiO2 upon different treatments; (c) CO2 conversion as a function of the relative intensity of EPR signal. Reaction condition: 360 °C, 0.1 MPa, CO2/H2 = 1/4 and GHSV = 15000 mL·g-1·h-1. Interestingly, H2 treatment of a-TiO2 supports gives a similar effect. As shown in Figure 3b, a-TiO2 subjected to H2 reduction at 600 ºC for 3 h was used as the support of Ni (denoted as a-TiO2-H2) gives a CO2 conversion 54.0%, which is comparable to that obtained over Ni/a-TiO2-NH3. As shown in Figure S4, EPR analysis of this a-TiO2H2 shows that H2 has a similar effect as NH3-pre-treatment, as it gives an EPR signal at g = 1.955, consistent with a previous study.44 By contrast, those treated by CO2 or inert gas exhibit very poor methanation activity with only about 3.0% CO2 conversion (Figure 3b), and CO selectivities above 90%. Figure 3c shows that the CO2 hydrogenation activity correlates almost linearly with the relative intensity of the EPR signal, regardless of NH3-treatment or H2-treatment of the supports. However, beyond a certain range, CO2 conversion levels off. The above results indicate that the concentration of Ti3+ in the support likely plays a very important role for Ni/a-TiO2


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catalyzing CO2 hydrogenation. Although reduction of the Ni/a-TiO2 at 500 °C for 2 h prior to reaction tests also leads to formation of Ti3+ species on the catalyst surface, as displayed by XPS in Figure S5. The still very low CO2 conversion (Figure 1a) demonstrates that the surface Ti3+ is not the key but the bulk Ti3+ species, as detected by EPR, play a more important role. It could be hypothesized that the presence of a large amount of bulk Ti3+ species may affect the chemical state of Ni species via electronic interactions. However, quasi-insitu XPS analysis shows that there is no significant difference in the chemical state of Ni species between Ni/a-TiO2 and Ni/a-TiO2-NH3 (Figure S6). It indicates that the presence of more bulk Ti3+ species does not modify further the electronic state of Ni species.45 Therefore, there must be other factors which has enhanced remarkably the activity of Ni/a-TiO2-NH3. The presence of SMSI has been widely observed between the Group 8-10 metals and reducible oxide supports, such as titania. It is widely accepted that the surface titania would be partially reduced forming TiOx upon high temperature reduction (usually above 500 °C), which then migrated and consequently forming an overlayer around the metal species.46-49 However, our attempts to directly observe the overlayer by TEM was not successful, in consistent with recent studies,18, 50 likely due to the insufficient contrast between Ni nanoparticles and the oxide support or too thin overlayers. Therefore, quasi-in-situ XPS analysis was conducted to investigate the relative concentration of surface Ni and Ti. We first estimate the mean sizes of metallic


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ACS Catalysis

Ni particles by HAADF-STEM analysis. The results in Figure S7 show a size of 8.7 and 12.9 nm for Ni/a-TiO2 and Ni/a-TiO2-NH3, respectively. In addition, XRD analysis indicates that the mean crystal sizes of Ni are 9.5 and 14.2 nm for Ni/a-TiO2 and Ni/aTiO2-NH3, respectively, in consistence with the TEM results (Table S2). It indicates that Ni species are better dispersed over a-TiO2 and thus the surface concentration of nickel would be expected to be higher than that over Ni/a-TiO2-NH3. However, XPS analysis shows that the atomic ratio of the surface Ni/Ti is comparable over these two samples after H2 reduction (Figure 4), indicating that the Ni species on Ni/a-TiO2 are partially encapsulated by titania. Encapsulation of Ni5,

7, 51

and Co17 particles by

reducible titania support has been reported previously. In addition, it was widely demonstrated that encapsulation of Pt-group metal nanoparticles by reducible oxide was reversible upon reduction-oxidation treatment.1-3, 11-13, 46 This is also observed here. As shown in Figure 3, the atomic ratio of surface Ni/Ti over Ni/a-TiO2 increases from 0.037 to 0.08 upon oxidation in O2 at 500 ºC for 1 h, indicating that some Ni species are unveiled, i.e. the overlayer receding off. In comparison, the surface Ni/Ti ratio only increases to 0.05 over Ni/a-TiO2-NH3, which suggests that the surface Ni species are also partially but less covered by the titania overlayer than Ni/a-TiO2.


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Figure 4. The atomic ratio of surface Ni/Ti over Ni/a-TiO2 and Ni/a-TiO2-NH3 upon reduction and oxidation, which is estimated from quasi-in situ XPS. Reduction condition: 500 °C, 1 h, 1 mbar H2; Oxidation condition: 500 °C, 1 h, 1 mbar O2. Such an overlayer around the metal nanoparticles may provide a protection medium, preventing the aggregation of metal species but also could block the catalytic active sites, and hence retard the activation of reactants.1, 52-54 This is confirmed by H2 chemisorption and CO chemisorption. The H2 uptake and CO uptake of Ni/a-TiO2 are only 0.4 and 2.9 μmol·gcat-1, respectively (Table 1). In comparison, the H2 uptake and CO uptake over Ni/a-TiO2-NH3 increase to 2.4 and 6.1 μmol·gcat-1, respectively, although the Ni particles exhibit a larger size than those over Ni/a-TiO2. Therefore, it is likely that more Ni species are accessible to H2 and CO over Ni/a-TiO2-NH3 than Ni/a-TiO2 considering the similar Ni loadings over both catalysts. This is also validated by further catalytic tests in CO methanation. As shown in Figure 5, Ni/a-TiO2-NH3 is highly active in CO methanation and CO conversion reaches as high as 96.5% whereas it is only 1.8% over Ni/a-TiO2.


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Table 1. The H2 uptake and CO uptake of Ni/a-TiO2 and Ni/a-TiO2-NH3, measured from chemisorption. Sample

H2 uptake (μmol·gcat-1)

CO uptake (μmol·gcat-1)

Ni/a-TiO2

0.4

2.9

Ni/a-TiO2-NH3

2.4

6.1

Figure 5. CO methanation catalytic performances for Ni/a-TiO2 and Ni/a-TiO2-NH3. Reaction condition: 360 °C, 0.1 MPa, CO/H2 = 1/2.5 and GHSV = 15000 mL·g-1·h-1. Although extensive studies have been carried out on the SMSI effects for Pt-group metal catalysts,13, 55-57 it is rarely studied for CO2 hydrogenation. Recently, Christopher and co-workers observed that the selectivity can be controlled via pre-treatment for the supported catalysts of Rh/TiO2 and Rh/Nb2O5.13 In that study, the authors observed that pre-treatment of the supported catalyst with pure H2 at 450 °C for 4 h and then 20CO2:2H2 at 250 °C for 4 h suppressed CH4 formation and enhanced CO formation due to formation of a permeable and stable HCOx-functionalized SMSI (A-SMSI) overlayer. In this study, pre-treatment of a-TiO2 support itself can tune the SMSI


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significantly. Interestingly, further experiments by supporting noble metal Ru on the ammonia-treated a-TiO2 support exhibit a very similar phenomenon in CO2 hydrogenation as Ni/a-TiO2 (Figure S8). CO2 conversion is 17.3% over Ru/a-TiO2 and CO is the only product. In contrast, Ru/a-TiO2-NH3 give CO2 conversion of 65.7% with a 98% CH4 selectivity and only 2% CO selectivity. However, it is not clear if the reduction has the same effects on SMSI between Ru and a-TiO2. In addition, more sophisticated experiments would be required to unveil if there are other structural defect sites in addition to Ti3+, which may play an important role in the altered activity and selectivity upon reduction of a-TiO2 support. 4． CONCLUSION In summary, we show that the conventional a-TiO2 supported Ni catalyst exhibits a poor activity in CO2 methanation due to blocked Ni species by the titania overlayer induced by SMSI. The reaction gives CO as the sole product. By contrast, pre-treatment of a-TiO2 by NH3 and H2 enhances the activity by one order of magnitude in CO2 methanation, leading to almost exclusive formation of CH4. Detailed characterization by EPR, H2-chemisorption and XPS reveals that the pre-reduction of the a-TiO2 support generates a large amount of Ti3+ in the bulk, which is likely the reason for suppressed formation of titania overlayer and also altered SMSI. Thus, more Ni species are accessible to the reactants leading to a high activity and allowing CO2 conversion almost exclusively to methane over Ni/a-TiO2-NH3 catalyst. This simple reduction treatment provides an effective approach to tune the SMSI of metal catalysts over the


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reducible oxide supports and hence their catalytic activities. ASSOCIATED CONTENT Supporting Information This information is available free of charge on the ACS Publications website. Additional experimental data (Figures S1-S8 and Table S1-S2). AUTHOR INFORMATION Corresponding Author Xiulian Pan, Xinhe Bao. E-mail: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We gratefully acknowledge the financial support from the Ministry of Science and Technology of China (No. 2016YFA0202803), the National Science Foundation of China (Grant No. 91645204, 21425312 and 21621063) and the Chinese Academy of Sciences (XDA21020400). We are thankful for the fruitful discussions with Prof. Qiang Fu (DICP). We also thank Guang Zeng (DICP) for his help in EPR measurements. REFERENCES


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TOC:


anatase catalyst by tuning

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