Anatase Catalyst by Tuning

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Research Article Cite This: ACS Catal. 2019, 9, 6342−6348

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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*,†

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†

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China S Supporting Information *

ABSTRACT: Strong metal−support interaction (SMSI) has been widely recognized for platinum-group metals on reducible oxide supports. Herein we report that the catalytic activity of Ni catalyst in CO2 methanation is significantly suppressed over conventional anatase (a-TiO2) support due to the SMSI-induced formation of a titania overlayer around the Ni nanoparticles. Furthermore, CO is the only product . In contrast, the NH3-treatment and H2-treatment of the a-TiO2 support enhance remarkably the activity of Ni, i.e., CO2 conversion increases by 1 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 may be related with the Ti3+ species in the bulk that are 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 metal−support interactions

1. INTRODUCTION Strong metal−support interaction (SMSI) is a widely recognized effect in heterogeneous catalysis, which can significantly modulate catalytic activities. Tauster and coworkers first reported the suppressed chemisorption of CO and H2 over TiO2-supported platinum-group metals (PGMs) after high temperature reduction.1 For reducible oxide supports, e.g., TiO2, Nb2O5 and CeO2, 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 an oxidation-induced overlayer for Au/ZnO catalyst, which was termed as OSMSI.11 The O-SMSI effect was also observed on HAPsupported PGMs such as Pd and Pt catalysts.12 In addition, the oxide overlayer can be induced by certain adsorbates (termed © 2019 American Chemical Society

as A-SMSI), which was reported for TiO2- and Nb2O5supported Rh catalysts upon CO2−H2 (CO2-rich) treatment at 250 °C.13 In comparison, there are much fewer studies on the SMSI of the nonplatinum-group metals, particularly their catalytic behavior. 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 a 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 Mejiá Received: January 26, 2019 Revised: April 16, 2019 Published: May 31, 2019 6342

DOI: 10.1021/acscatal.9b00401 ACS Catal. 2019, 9, 6342−6348

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ACS Catalysis 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 a TiOx overlayer upon 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 less 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 because 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 CO2 as a carbon resource to value-added chemicals and fuels.25−35 This may find important applications in submarines, space crafts, and space stations to remove CO2. 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 a-TiO2 enhances significantly the CO2 methanation activity by 1 order of magnitude by tuning the SMSI.

CO selectivity(%) =

where CO2 inlet and CO2 outlet represent the moles of CO2 in the feed and effluent, respectively, and CH4 outlet and COoutlet represent the moles 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) monochromatic 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 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 SBET. Low temperature electron paramagnetic resonance (EPR) spectra were collected at 110 K using a Bruker A200 EPR spectrometer operated at the X-band frequency. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was performed on a JEM F200 microscope. Hydrogen temperature-programmed reduction (H2-TPR) was carried out in a tubular quartz reactor. A 100 mg of sample was placed in the tube, heated at 120 °C for 1 h, and then cooled to 50 °C. The sample was heated in 1 vol % H2/Ar at a ramp of 10 °C·min−1 from 50° to 800 °C with the effluents monitored by an online mass spectrometer. A 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 uptakes were calculated from the pulse results. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a Thermo ESCALAB 250Xi system with 162 W monochromatic Al Kα radiation (1486.6 eV). The binding energy was calibrated by contamination carbon C 1s peak (284.6 eV) as the reference. Quasi-in situ near ambient pressure X-ray photoelectron spectroscopy (quasi-in situ NAPXPS) was conducted using a 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 was 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 laser-heating equipment from room temperature to about 500 °C in a H2 or O2 atmosphere of 1 mbar and kept for 1 h.

2. EXPERIMENTAL SECTION 2.1. Materials. 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. 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 of a-TiO2 or a-TiO2−NH3 was added to 30 mL of ethanol solution containing 246 mg of 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 was 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 of 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 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 premixed stream CO2, H2, and Ar (19 vol % CO2 + 76 vol % H2 + 5 vol % Ar) 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 sievepacked 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.

3. RESULTS AND DISCUSSION Ni was impregnated onto a-TiO2 and the NH3-pretreated aTiO2 supports 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/aTiO2 gives a low activity. CO2 conversion is only 5%. Furthermore, this catalyst is not active at all for methanation

CO2 inlet − CO2 outlet CO2 conversion(%) = × 100% CO2 inlet

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

COoutlet × 100% CO2 inlet − CO2 outlet

CH4 outlet × 100% CO2 inlet − CO2 outlet 6343

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Figure 1. Catalytic performance of Ni/a-TiO2 and Ni/a-TiO2−NH3 in CO2 methanation. (a) CO2 conversion; (b) product selectivity. Reaction conditions: 360 °C, 0.1 MPa, CO2/H2 = 1/4, and GHSV = 15000 mL·g−1·h−1.

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

and CO is the only carbon-containing product. In contrast, CO2 conversion increases 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 nitrate and the following calcination have been carried out in air for both catalysts. Furthermore, they have been prepared following the same procedures and under the same conditions except the support pretreatment step. 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 N 1s signal at 395.5 eV whereas this is not observed over the original a-TiO2.37,38 However, this N signal disappears upon H2 reduction of Ni/a-TiO2−NH3 (Figure S2b) prior to reaction, 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 a decisive 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/aTiO2 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-TiO2−NH3 (496 °C) is higher than that over Ni/a-

TiO2 (475 °C), suggesting more difficulty to reduce for the nickel species supported on a-TiO2−NH3. We turn to the low temperature electron paramagnetic resonance (EPR) to further investigate the properties of aTiO2 and a-TiO2−NH3. As shown in Figure 3a, no obvious 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 Because 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 has 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. Interestingly, H2 treatment of a-TiO2 supports gives a similar effect as NH3 treatment. As shown in Figure 3b, when a-TiO2 subjected to H2 reduction at 600 °C for 3 h (denoted as aTiO2-H2) is used as the support for Ni catalyst (denoted as Ni/a-TiO2−H2), it 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-TiO2−H2 shows that H2 has an effect similar to that of NH3-pretreatment, as it gives an EPR signal at g = 1.955, consistent with a previous study.44 By contrast, those treated with 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 within a certain range, regardless of NH3-treatment or H2-treatment of the 6344

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Figure 3. (a) EPR spectra recorded at 110 K for a-TiO2 and a-TiO2−NH3 upon NH3-treatment for different durations. (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 conditions: 360 °C, 0.1 MPa, CO2/H2 = 1/4, and GHSV = 15000 mL·g−1·h−1.

supports. However, beyond that 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 catalyzing CO2 hydrogenation. 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. However, CO2 conversion remains low (Figure 1a) demonstrating 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-in situ 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 have remarkably enhanced 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 surface titania is partially reduced, forming TiOx upon high temperature reduction (usually above 500 °C), which then migrates and consequently forms an overlayer around the metal species.46−49 However, our attempts to directly observe the overlayer by TEM was not successful, 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 estimated the mean sizes of metallic 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-TiO 2 and Ni/a-TiO 2−NH3 , respectively, consistent with the TEM results (Table S2). It indicates that Ni species are better dispersed over a-TiO2, and thus the surface concentration of nickel is expected to be higher than that over Ni/a-TiO2−NH3 considering a similar loading. 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/aTiO2 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

Figure 4. Atomic ratio of surface Ni/Ti over Ni/a-TiO2 and Ni/aTiO2−NH3 upon reduction and oxidation, which is estimated from quasi-in situ XPS. Reduction conditions: 500 °C, 1 h, 1 mbar H2; oxidation conditions: 500 °C, 1 h, 1 mbar O2.

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/aTiO2 increases from 0.037 to 0.08 upon oxidation in O2 at 500 °C for 1 h, indicating that some Ni species are unveiled, i.e., the overlayer recedes 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. 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 Table 1. H2 Uptake and CO Uptake of Ni/a-TiO2 and Ni/aTiO2−NH3, Measured from Chemisorption.

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sample

H2 uptake (μmol·gcat−1)

CO uptake (μmol·gcat−1)

Ni/a-TiO2 Ni/a-TiO2−NH3

0.4 2.4

2.9 6.1 DOI: 10.1021/acscatal.9b00401 ACS Catal. 2019, 9, 6342−6348

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ACS Catalysis 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/aTiO2−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.

pretreatment of a-TiO2 by NH3 and H2 enhances the activity by 1 order of magnitude in CO2 methanation, leading to almost exclusive formation of CH4. Detailed characterization by EPR, H2-chemisorption, and XPS reveals that the prereduction of the a-TiO2 support generates a large amount of Ti3+ in the bulk, which is likely the reason for suppressed formation of the titania overlayer and also altered SMSI. Thus, more Ni species are accessible to the reactants, leading to a higher 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 reducible oxide supports and hence their catalytic activities.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b00401. Additional experimental data (Figures S1−S8 and Tables S1 and S2) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Figure 5. CO methanation activities of Ni/a-TiO2 and Ni/a-TiO2− NH3. Reaction conditions: 360 °C, 0.1 MPa, CO/H2 = 1/2.5, and GHSV = 15000 mL·g−1·h−1.

Xiulian Pan: 0000-0002-5906-6675 Yi Cui: 0000-0002-9182-9038 Xinhe Bao: 0000-0001-9404-6429 Author Contributions

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 coworkers observed that the selectivity can be controlled via pretreatment for the supported catalysts of Rh/TiO2 and Rh/ Nb2O5.13 In that study, the authors observed that pretreatment of the supported catalyst with pure H2 at 450 °C for 4 h led to formation of CH4, while treatment with 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, pretreatment of a-TiO2 support itself can tune the SMSI significantly. Interestingly, further experiments by supporting noble metal Ru on the ammonia-treated a-TiO2 support exhibit a phenomenon very similar in CO2 hydrogenation to that for 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 gave CO2 conversion of 65.7% with a 98% CH4 selectivity and only 2% CO selectivity. However, it is not clear if the reduction treatment has the same effects on Ru/a-TiO 2 as that on Ni/a-TiO2. In addition, more sophisticated experiments would be required to reveal if there are other structural defect sites in addition to Ti3+, which may play an important role in the altered activity and selectivity over the reduction-treated a-TiO2 supported catalysts.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Author Contributions #

These authors contributed equally to the present work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 with EPR measurements.

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REFERENCES

(1) Tauster, S.; Fung, S.; Garten, R. L. Strong Metal-Support Interactions. Group 8 Noble Metals Supported on Titanium Dioxide. J. Am. Chem. Soc. 1978, 100, 170−175. (2) Tang, H.; Su, Y.; Zhang, B.; Lee, A. F.; Isaacs, M. A.; Wilson, K.; Li, L.; Ren, Y.; Huang, J.; Haruta, M.; et al. Classical Strong Metal− Support Interactions between Gold Nanoparticles and Titanium Dioxide. Sci. Adv. 2017, 3, e1700231. (3) Tauster, S. Strong Metal-Support Interactions. Acc. Chem. Res. 1987, 20, 389−394. (4) Roberts, S.; Gorte, R. A Study of the Migration and Stability of Titania on a Model Rh Catalyst. J. Catal. 1990, 124, 553−556. (5) Chung, Y.-W.; Xiong, G.; Kao, C.-C. Mechanism of Strong Metal-Support Interaction in Ni/TiO2. J. Catal. 1984, 85, 237−243.

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, 6346

DOI: 10.1021/acscatal.9b00401 ACS Catal. 2019, 9, 6342−6348

Research Article

ACS Catalysis

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DOI: 10.1021/acscatal.9b00401 ACS Catal. 2019, 9, 6342−6348

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DOI: 10.1021/acscatal.9b00401 ACS Catal. 2019, 9, 6342−6348