Molecular-Level Insight into How Hydroxyl Groups Boost Catalytic Activity in CO2 Hydrogenation into Methanol

Article

Molecular-Level Insight into How Hydroxyl Groups Boost Catalytic Activity in CO2 Hydrogenation into Methanol Yuhan Peng, Liangbing Wang, Qiquan Luo, ..., Wensheng Yan, Jinlong Yang, Jie Zeng [email protected] (J.Y.) [email protected] (J.Z.)

HIGHLIGHTS The surfaces of SiC quantum dots are hydroxylated and hydrophilic Hydrophilic SiC quantum dots with superior catalytic activity in CO2 hydrogenation The surface hydroxyl species participates in CO2 hydrogenation

The surfaces of SiC quantum dots are hydroxylated and hydrophilic. The surface hydroxyl species on SiC quantum dots directly participated in CO2 hydrogenation through the addition of H atoms in hydroxyl groups into CO2. The unique reaction path facilitated the activation of CO2 and accordingly boosted the activity for CO2 hydrogenation. Following this understanding, metal hydroxides and layered double hydroxides were found to be highly active catalysts in CO2 hydrogenation.

Peng et al., Chem 4, 1–13 March 8, 2018 ª 2018 Elsevier Inc. https://doi.org/10.1016/j.chempr.2018.01.019

Please cite this article in press as: Peng et al., Molecular-Level Insight into How Hydroxyl Groups Boost Catalytic Activity in CO2 Hydrogenation into Methanol, Chem (2018), https://doi.org/10.1016/j.chempr.2018.01.019

Article

Molecular-Level Insight into How Hydroxyl Groups Boost Catalytic Activity in CO2 Hydrogenation into Methanol Yuhan Peng,1,2 Liangbing Wang,1,2 Qiquan Luo,1,2 Yun Cao,1 Yizhou Dai,1 Zhongling Li,1 Hongliang Li,1 Xusheng Zheng,1 Wensheng Yan,1 Jinlong Yang,1,* and Jie Zeng1,3,*

SUMMARY

The Bigger Picture

Exploring how hydrophilicity regulates catalytic properties at the molecular level remains a grand challenge, although it has great potential to offer guidelines for developing highly efficient catalysts and deepen the mechanistic understanding of heterogeneous catalysis. Here, we provide molecular-level insight into the influence of surface hydroxyl groups on hydrophilic SiC quantum dots (QDs) on CO2 hydrogenation. In CO2 hydrogenation into methanol, SiC QDs exhibited higher catalytic activity and lower activation energy than commercial SiC. Mechanistic studies revealed that the surface hydroxyl species on SiC QDs was directly involved in CO2 hydrogenation through the addition of H atoms in hydroxyl groups into CO2 to form HCOO* as the intermediate. The unique reaction path decreased the energy barrier for the formation of HCOO*, facilitating the activation of CO2. Our understanding of surface hydrophilicity directly instructs the development of efficient catalysts toward CO2 hydrogenation.

Exploring how hydrophilicity regulates catalytic properties at the molecular level remains a grand challenge, although it has great potential to offer guidelines for developing highly efficient catalysts and deepen the mechanistic understanding of heterogeneous catalysis. Here, we provide molecular-level insight into the influence of hydroxyl groups on hydrophilic SiC quantum dots on CO2 hydrogenation into methanol. On the basis of this understanding, metal hydroxides and layered double hydroxides were found to be highly active catalysts in that they exhibited more than oneorder-of-magnitude enhancement in mass activity in relation to their metal oxide counterparts. This work not only provides a guideline for developing efficient catalysts but also advances the mechanistic understanding of heterogeneous catalysis.

INTRODUCTION Because catalysis proceeds on the surface of catalysts, engineering their surface properties serves as viable access to manipulate the catalytic activity, selectivity, and stability. A pivotal intrinsic surface property is hydrophilicity, the affinity of a material for water molecules, which originates from the coverage of surface hydroxyl (–OH) species.1 To this end, efforts have been made to adjust the hydrophilicity (or hydrophobicity) to improve catalytic performance.2–16 The most common understanding of the effect of hydrophilicity on catalytic properties is based on regulating the concentration of reactants accessible to the active sites.17–23 Specifically, the hydrophilic surface is able to enrich hydrophilic reactants such as alcohols around the active sites, whereas the hydrophobic surface facilitates the accumulation of hydrophobic reactants, including esters and aromatic ketones. For instance, the hydrophobic Pd surface derived from C2H2 treatment helps to accumulate more hydrophobic reactants, contributing to enhanced catalytic activity in the hydrogenation of nitrobenzene.17 In the hydrogenation of styrene, the hydrophobic coating of polydimethylsiloxane for Pd/MOF composites facilitates the enrichment of styrene, thereby promoting the interaction between the hydrophobic reactants and Pd sites to improve the catalytic activity.18 Although there are various examples of controlling hydrophilicity over the concentration of reactants from the macroscopic perspective, little attention has been focused on molecular-level insights into how hydrophilicity regulates catalytic properties.24 Exploring the influence of hydrophilicity on catalytic performance at the molecular level has great potential to offer

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guidelines for developing highly efficient catalysts and deepen the mechanistic understanding of heterogeneous catalysis, but this still remains as a challenge. Here, we demonstrate at the molecular level how surface hydroxyl groups on hydrophilic SiC quantum dots (QDs) boost their catalytic activity in CO2 hydrogenation. The surfaces of the SiC QDs were hydroxylated and hydrophilic, whereas the surfaces of commercial SiC were hydrophobic. Because CO2 serves as the most environmentally abundant C1 building block available, the conversion of CO2 into fuels and value-added chemicals represents a pivotal route for alleviating the dearth of fossil fuels and achieving a sustainable carbon cycle. In CO2 hydrogenation into methanol, hydrophilic SiC QDs exhibited mass activity of 169.5 mmol g 1 hr 1 under 32 bar of CO2/H2 mixed gas (CO2:H2 = 1:3) at 150 C, which was more than three orders of magnitude higher than that of hydrophobic commercial SiC. The activation energy for SiC QDs was 48.6 KJ mol 1, about half that (94.7 KJ mol 1) for commercial SiC. According to mechanistic studies, the enhanced activity of SiC QDs was closely correlated with their hydrophilicity. Specifically, the surface hydroxyl species on SiC QDs directly participated in CO2 hydrogenation through the addition of H atoms in hydroxyl groups into CO2 to form HCOO* as the intermediate. In addition, the unique reaction path induces a decrease in the energy barrier for the formation of HCOO*, facilitating the activation of CO2 and accordingly improving the activity for CO2 hydrogenation. More importantly, our understanding of surface hydrophilicity offers a guideline for the development of efficient catalysts toward CO2 hydrogenation. Metal hydroxides and layered double hydroxides (LDHs) were determined to be highly active catalysts, exhibiting more than one-order-of-magnitude enhancement in mass activity in relation to their metal oxide counterparts.

RESULTS Synthesis and Structural Characterization of SiC QDs In a typical synthesis of SiC QDs, commercial SiC powders larger than 100 nm (Figure S1) were etched by nitric acid and hydrofluoric acid. SiC QDs were obtained after centrifugation. As revealed by the X-ray diffraction (XRD) profiles, both commercial SiC and SiC QDs took the cubic (3C) polytype (Figure S2). Figure 1A shows a representative transmission electron microscopy (TEM) image of the SiC QDs obtained. The QDs were highly dispersed and had an average diameter of 3.2 nm. Figure 1B shows a magnified high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of an individual QD. The lattice parameters of 2.5 and 2.2 A˚ were indexed to the {111} and {200} planes, respectively, of 3C SiC. To investigate the different surface properties of commercial SiC and SiC QDs, we conducted Fourier-transform infrared (FTIR) measurements. The FTIR spectrum of commercial SiC revealed a peak at 824 cm 1, which was assigned to the stretching vibration of Si–C (Figure 1C). With regard to SiC QDs, besides the peak at 814 cm 1 for the stretching vibration of Si–C, new peaks at 3,473 and 1,441 cm 1 emerged, corresponding to the stretching vibrations of O–H and Si–O, respectively. The surface of the SiC QDs was hydroxylated, and the hydroxyl species were mainly bonded to surface Si atoms. To investigate the electronic and coordination structures of SiC QDs, we carried out X-ray absorption near-edge spectroscopy (XANES). In the Si L3,2-edge spectrum of SiO2 (Figure 1D), the features at 106.2 and 106.8 eV emerged as the spinorbit doublet associated with the transitions of Si 2p3/2 and 2p1/2 core states, respectively, to the antibonding Si 3s derived states.25–27 The feature at

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1Hefei

National Laboratory for Physical Sciences at the Microscale, Key Laboratory of Strongly-Coupled Quantum Matter Physics of Chinese Academy of Sciences, National Synchrotron Radiation Laboratory, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China

2These 3Lead

authors contributed equally

Contact

*Correspondence: [email protected] (J.Y.), [email protected] (J.Z.) https://doi.org/10.1016/j.chempr.2018.01.019

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Figure 1. Structural Characterizations of SiC QDs (A) TEM image of SiC QDs. The inset in shows the size-distribution diagram of SiC QDs. (B) Magnified HAADF-STEM image of an individual SiC QD. (C) FTIR spectra of commercial SiC and SiC QDs. (D) Si L 3,2 -edge XANES spectra of SiO 2 , commercial SiC, and SiC QDs. (E) C K-edge XANES spectra of commercial SiC and SiC QDs. (F and G) Contact angles of water droplets on commercial SiC (F) and SiC QDs (G) deposited on glass.

108.8 eV was assigned to Si 3d derived states from the hybridization with O 2p orbitals.25–27 Different from that of SiO2, the spectrum of commercial SiC showed a prominent feature at 104.4 eV that arose from the electronic transition of Si 2p to Si 3d in commercial SiC.25–27 As for the spectrum of SiC QDs, the main features were similar to those observed from SiO2, except that the energy of the main features from SiC QDs was downshifted in relation to that from SiO2 (Figure 1D). The similarity in Si L3,2-edge XANES spectra between SiO2 and SiC QDs indicates the existence of Si–O bonds in SiC QDs, consistent with the FTIR result (Figure 1C). In addition, the lowered energy for SiC QDs was derived from the replacement of the bridging oxygen in the Si–O–Si bond of SiO2 with the –OH group in SiC QDs.25–27 As shown in Figure 1E, both commercial SiC and SiC QDs exhibited similar C K-edge spectra. To further characterize the electronic properties of SiC QDs and commercial SiC, we conducted X-ray photoelectron spectroscopy (XPS) measurements. The Si 2p spectrum of commercial SiC exhibited a prominent peak at 100.4 eV, which was assigned to Si–C (Figure S3A). As for SiC QDs, besides the peak for Si–C, a new peak at 102.3 eV appeared, corresponding to Si–O (Figure S3A). In the C 1s spectra of both commercial SiC and SiC QDs, a prominent peak for Si–C was observed (Figure S3B). Moreover, the coverage of hydroxyl species was found to influence the hydrophilicity of commercial SiC and SiC QDs, as revealed in Figures 1F and 1G. Specifically, the contact angle of water droplets on the commercial SiC was 141.1 , implying the hydrophobic nature of the commercial SiC surfaces. In comparison, the contact angle of 42.2 for SiC QDs indicates that the surfaces of SiC QDs were hydrophilic. In addition, the mass fraction of F adsorbed on SiC QDs was determined to be 0.9% (9 mgF /gcatalysts).

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A

B

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Figure 2. Catalytic Performance of Commercial SiC and SiC QDs in CO2 Hydrogenation (A) Time courses of the products for commercial SiC and SiC QDs under 32 bar of CO 2 /H 2 mixed gas (CO 2 :H 2 = 1:3) at 150  C. (B) Comparison of mass activity with commercial SiC and SiC QDs as the catalysts at different temperatures. The reaction tests were pressurized with CO2 /H2 mixed gas (CO 2 :H2 = 1:3, 32 bar). (C) Comparison of TOF with commercial SiC and SiC QDs as the catalysts at different temperatures. (D) The Arrhenius plots of commercial SiC and SiC QDs. The amounts of commercial SiC and SiC QDs were kept at 226.1 and 5.0 mg, respectively, for a catalytic test to ensure that the total surface areas for each catalytic test were the same. (E and F) Comparison of mass activity of fresh and treated SiC QDs in CO 2 hydrogenation in water (E) and DMF (F) under 32 bar of CO 2 /H2 mixed gas (CO2 :H2 = 1:3) at 150  C. (G) Comparison of mass activity of SiC QDs in CO 2 hydrogenation in NaOH aqueous solution with different concentrations under 32 bar of CO 2 /H2 mixed gas (CO2 :H 2 = 1:3) at 150  C. Error bars represent the SD from three independent measurements.

Catalytic Properties of SiC QDs in CO2 Hydrogenation The catalytic properties of the as-obtained SiC QDs were evaluated in comparison with commercial SiC toward CO2 hydrogenation. As indicated by the nitrogen adsorption-desorption isotherms, the Brunauer-Emmett-Teller (BET) surface areas of commercial SiC and SiC QDs were determined to be 14.0 and 633.1 m2 g 1, respectively. To ensure that the total surface areas for each catalytic test were the same, we kept the amounts of commercial SiC and SiC QDs at 226.1 and 5.0 mg, respectively. When the reaction was catalyzed by commercial SiC under 32 bar of CO2/H2 mixed gas (CO2:H2 = 1:3) at 150 C, 0.08 mmol of methanol was produced after 5 hr without the formation of other products (Figure 2A). In comparison, the yield of methanol increased to 4.24 mmol over SiC QDs, and no other products were detected (Figure 2A). To compare the catalytic activity more accurately, we calculated the mass activity and turnover frequency (TOF) of these catalysts on the basis of the reaction profile. The mass activity of SiC QDs reached 169.5 mmol g 1 hr 1, about three orders of magnitude higher than that (0.1 mmol g 1 hr 1) of commercial SiC. In addition, the TOF numbers of commercial SiC and SiC QDs were 0.01 and 0.27 mmol m 2 hr 1 at 150 C, respectively. To further explore the differences in catalytic properties between commercial SiC and SiC QDs, we conducted a series of catalytic tests under 32 bar of CO2/H2 mixed gas (CO2:H2 = 1:3) at different temperatures. As shown in Figures 2B and 2C, SiC QDs were much more active than commercial SiC. Arrhenius plots were obtained on the basis of the linear fitting of lnTOF versus 1,000/T (Figure 2D). The activation

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energy for SiC QDs was 48.6 kJ mol SiC.

1

, about half that (94.7 kJ mol

1

) for commercial

To investigate the origin of the enhanced activity of SiC QDs, we carried out surface treatments on SiC QDs before CO2 hydrogenation. Considering the remaining F adsorbed on SiC QDs, we conducted CO2 hydrogenation over SiC QDs in NaF aqueous solution with different concentrations (0.1, 0.5, or 1.0 mM) to investigate the influence of fluoride species. Despite the varied concentrations of F , all SiC QDs exhibited similar catalytic activity of ca. 150 mmol g 1 hr 1 (Figure S4). Thus, the influence of fluoride species on CO2 hydrogenation over SiC QDs was ignored. In addition, we removed surface hydroxyl groups on SiC QDs by heating at 280 C under a flow of N2 for 3 hr (denoted as treated SiC QDs). As shown in the FTIR spectrum of the treated SiC QDs, the peaks assigned to hydroxyl groups were not observed, indicating the successful removal of hydroxyl groups (Figure S5). When the treated SiC QDs were used in water under 32 bar at 150 C, their mass activity was 160.0 mmol g 1 hr 1, approximating that of SiC QDs without surface treatment (Figure 2E). The FTIR spectrum of treated SiC QDs after CO2 hydrogenation shows the recovery of hydroxyl species (Figure S6). Thus, the small size of SiC QDs endows them with high affinity for hydroxyl groups. To avoid the recovery of hydroxyl groups in water, we replaced water with DMF as the solvent in CO2 hydrogenation. When the fresh and treated SiC QDs were used in DMF under 32 bar at 150 C, their mass activity was 152.2 and 9.5 mmol g 1 hr 1, respectively (Figure 2F). Thus, the involvement of surface hydroxyl groups boosts the catalytic activity of SiC QDs. To further investigate the role of surface hydroxyl groups, we carried out isotope experiments and applied NaOH aqueous solution as the solvent in CO2 hydrogenation. When the reaction was conducted over SiC QDs under 8 bar of CO2 and 24 bar of D2 at 150 C after 5 hr, the ratio of CH3OH:CH2DOH:CHD2OH:CD3OH was determined to be 0.13:0.59:2.60:1.00 on the basis of gas chromatography-mass spectrometry. The H atoms in hydroxyl groups on SiC QDs were proved to participate in CO2 hydrogenation. When the reaction was carried out in NaOH aqueous solution at concentrations of 0.1, 1.0, and 10.0 mM, the mass activities of SiC QDs were 100.5, 53.0, and 32.0 mmol g 1 hr 1, respectively, under 32 bar at 150 C (Figure 2G). The catalytic activity of SiC QDs decreased with the concentration of NaOH. Accordingly, the removal of H from hydroxyl groups was found to decrease the activity. Therefore, the addition of H atoms in hydroxyl groups into CO2 mainly accounts for the importance of surface hydroxyl groups on SiC QDs. To study the stability of SiC QDs, we performed successive rounds of CO2 hydrogenation under 32 bar of CO2/H2 mixed gas (CO2:H2 = 1:3) at 150 C for 5 hr. After ten rounds, 95% of the original reaction activity was preserved, indicating the high catalytic stability of SiC QDs (Figure S7). In addition, the TEM image of the reused catalysts showed that the QD structure was retained without aggregation (Figure S8A). As shown in the FTIR spectrum, hydroxyl species still existed on the surface of SiC QDs (Figure S8B). Collectively, SiC QDs were endowed with high structural and catalytic stability. In addition, we also investigated the stability of commercial SiC after CO2 hydrogenation under 32 bar at 150 C for 5 hr. The TEM image of the commercial SiC showed the retained morphology (Figure S9A). The FTIR spectrum also shows the existence of hydroxyl species (Figure S9B). Moreover, the contact angle of water droplets on the commercial SiC after CO2 hydrogenation was 113.2 , narrower than that (141.1 ) on fresh commercial SiC, but wider than that (42.2 ) on SiC QDs (Figure S9C). Thus, thermal treatment in water activates the commercial SiC surface via the formation of hydroxyl species. In addition, the

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Figure 3. Mechanistic Studies of Remarkable Catalytic Activity for SiC QDs in CO2 Hydrogenation (A) In situ DRIFT spectra of commercial SiC and SiC QDs after treatment with 1 bar of CO 2 at 150  C. (B) In situ XPS spectra of C 1s for commercial SiC and SiC QDs after treatment with 1 bar of CO 2 at 150  C. (C and D) C K-edge XANES spectra for commercial SiC (C) and SiC QDs (D) before and after treatment, respectively, with 1 bar of CO 2 at 150  C.

density of surface hydroxyl species on commercial SiC after reaction is lower than that on SiC QDs, accounting for the lower activity of commercial SiC. Mechanistic Studies of Remarkable Catalytic Activity for SiC QDs in CO2 Hydrogenation The interaction between reactants and SiC QDs was explored via in situ diffuse reflectance infrared Fourier transform (DRIFT) measurements. Figure 3A shows the in situ DRIFT spectra of commercial SiC and SiC QDs after treatment with CO2 at 150 C. With regard to commercial SiC, a peak at 1,362 cm 1 was observed, corresponding to the stretching vibration of CO2d species.28–30 In the spectrum of SiC QDs, the peaks at 2,976, 1,698, and 1,380 cm 1 were assigned to the stretching vibration of C–H and the asymmetrical and symmetrical stretching vibrations of the bidentate O–C–O in HCOO* species, respectively.31 Thus, the –OH group in hydroxylated SiC QDs was directly involved in the hydrogenation of CO2 into HCOO* species. The transformation of CO2 was further explored by in situ XPS and XANES measurements. Before in situ XPS measurements, commercial SiC and SiC QDs were treated with CO2 at 150 C in a reaction cell attached to the XPS end station. As shown in Figure 3B, the C 1s spectra of commercial SiC after CO2 treatment exhibited two typical peaks at 288.4 and 283.0 eV, corresponding to CO2d and Si–C, respectively.32–35 As for SiC QDs, besides the peak at 283.2 eV for Si–C, a peak at 287.5 eV appeared, which was assigned to HCOO* species.32–34 Figures 3C and 3D show the C K-edge XANES profiles of commercial SiC and SiC QDs before and after treatment with CO2

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at 150 C. After CO2 treatment, the profile of commercial SiC revealed intensity enhancement from the feature at 291.1 eV. This result indicates the chemisorption of CO2 on commercial SiC because the energy for excitation of C 1s to 2pu* in the adsorbed CO2 corresponds to 291.1 eV.36 With regard to SiC QDs after exposure to CO2, a strong peak at 287.7 eV was observed, which arose from the excitation of C 1s to p* in HCOO*.37 Thus, SiC QDs induced the generation of HCOO*. To gain molecular-level insight into how hydroxylation of SiC QDs boosts catalytic activity in CO2 hydrogenation, we carried out density functional theory (DFT) calculations. A model of Si-terminated (111) surface of SiC, denoted as SiC(111), was established (Figure S10). When SiC(111) was modified with –OH groups on the surface to mimic the hydrophilic SiC QDs, the optimized configuration was as shown in Figure S11A. Specifically, the oxygen atoms in –OH groups bound to surface Si atoms to form Si–O with adsorption energy of 5.89 eV, in good agreement with the FTIR and XANES results of SiC QDs (Figures 1C and 1D). When H2 molecules are adsorbed on SiC(111), they can spontaneously dissociate into H atoms without energy barriers. Thus, the dissociation of H2 on SiC is not determined by the hydroxyl group. Moreover, because of abundant active sites for hydrogen dissociation on SiC(111), the possible promotion of spillover by hydroxyl groups38–41 is not considered to play a determining role in the enhancement of catalytic activity. The dissociated H atoms are adsorbed on Si atoms rather than C atoms with an adsorption energy of 4.01 eV (Figure S11B). As for CO2 molecules, both C and O atoms are bound to surface Si atoms in a bent geometry with adsorption energy of 3.20 eV (Figure S11C). We screened possible intermediates and reaction channels in CO2 hydrogenation for adsorbed H atoms and H atoms in surface –OH groups on SiC(111). When the H atoms adsorbed on SiC(111) hydrogenate CO2 into HCOO* and COOH*, the energy barriers are 2.10 and 2.21 eV, respectively (Figure S12A). Given that the energy of HCOO* is 1.96 eV lower than that of COOH*, HCOO* is considered to be the stable intermediate. When the H atoms in surface –OH groups on SiC(111) directly add to CO2 to form HCOO* with O atoms left at bridge sites, the energy barrier decreases to 1.36 eV (Figure S12B). The reaction path to generating COOH* via H atoms in surface –OH groups is excluded because COOH* with high energy is unstable and tends to convert back to CO2 spontaneously during the calculation. Therefore, HCOO* serves as the intermediate for both the adsorbed H atoms and the H atoms in surface –OH groups on SiC(111). Moreover, the unique reaction path with H atoms in surface –OH groups reduces the energy barrier to form HCOO*. The distinction in intermediates of CO2d or HCOO* between commercial SiC and SiC QDs reflects the different energy barrier for the first step in CO 2 hydrogenation. After the depletion of H atoms in surface –OH groups, the O atoms left at the bridge sites on SiC(111) are reconstructed at the top sites with an energy barrier of 0.70 eV and bind with the dissociated H atoms to recover –OH groups with an energy barrier of 1.18 eV (Figure S13). Development of Efficient Catalysts for CO2 Hydrogenation according to the Proposed Mechanism On the basis of the above understanding that hydrophilicity boosts activity, hydroxyl-rich nanocrystals are regarded as candidate catalysts toward CO2 hydrogenation. To this end, we synthesized Ni(OH)2,42 CoMn LDHs,43 NiTi LDHs,44 and NiCo LDHs45 (Figures S14–S17). For comparison, these hydroxides and LDHs were calcined in air at 500 C for 5 min to form the metal oxide counterparts,

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Figure 4. Development of Highly Active Catalysts for CO2 Hydrogenation according to the Proposed Mechanism Comparison of Mass Activity of Hydroxides and LDHs against Their Oxide Counterparts The catalytic test was conducted under 32 bar of CO 2 /H 2 mixed gas (CO 2 :H 2 = 1:3) at 150  C. Error bars represent the SD from three independent measurements.

including NiO, MnCo2O4, NiO$TiO2, and NiCo2O4 (Figures S14–S17). The BET areas of Ni(OH)2, NiO, NiCo LDHs, NiCo2O4, NiTi LDHs, NiO$TiO2, CoMn LDHs, and MnCo2O4 were determined to be 38.2, 31.3, 46.8, 29.6, 19.4, 16.5, 83.6, and 62.3 m2 g 1, respectively. The surface area of metal hydroxides was comparable with that of their oxide counterparts. Five milligrams of each catalyst was tested under 32 bar of CO2/H2 mixed gas (CO2:H2 = 1:3) at 150 C. As expected, the metal hydroxides and LDHs exhibited significant enhancement in catalytic activity in relation to their metal oxide counterparts (Figure 4). Specifically, the mass activities of Ni(OH)2, CoMn LDHs, NiTi LDHs, and NiCo LDHs reached 141.4, 176.1, 282.6, and 335.7 mmol g 1 hr 1, respectively, which are 18.4, 16.6, 15.6, and 14.4 times higher than that of NiO, MnCo2O4, NiO$TiO2, and NiCo2O4, respectively (Figures S18 and S19). Besides hydroxyl groups, the enhanced activity also possibly derives from other factors such as hydrogen dissolution, spillover, reducibility, and surface oxygen atoms as a result of the different phases of metal oxides and hydroxides. Therefore, the density of surface hydroxyl groups may be regarded as a potential descriptor of catalytic activity in CO2 hydrogenation.

DISCUSSION We have provided molecular-level insight into the effect of surface hydroxyl groups on hydrophilic SiC QDs on CO2 hydrogenation. The hydrophilic surfaces of SiC QDs were determined to be hydroxylated. The surface hydroxyl species on SiC QDs directly participated in CO2 hydrogenation through the addition of H atoms in –OH groups into CO2 to form HCOO* as the intermediate. The unique reaction path enables a lowered energy barrier for the formation of HCOO*, facilitating the activation of CO2 and thereby enhancing the catalytic activity for CO2 hydrogenation. More importantly, our understanding of surface hydrophilicity instructs the development of efficient catalysts toward CO2 hydrogenation. Metal hydroxides and LDHs were found to be highly active catalysts in that they exhibited more than one-order-of-magnitude enhancement in mass activity in relation to their metal oxide counterparts. This work not only provides a guideline for developing efficient catalysts but also advances the mechanistic understanding of heterogeneous catalysis.

EXPERIMENTAL PROCEDURES Chemicals and Materials Commercial SiC (>99.0%) was obtained from Alfa-Aesar. All other chemicals were of analytical grade and purchased from Shanghai Chemical Reagent. All chemical reagents were used as received without further purification. All aqueous solutions were prepared with deionized (DI) water with a resistivity of 18.2 MU$cm.

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Preparation of Catalysts SiC QDs Three grams of commercial SiC was dissolved in 7.5 mL of nitric acid aqueous solution (65 wt %) and 22.5 mL of hydrofluoric acid aqueous solution (40 wt %). The solution formed was heated at 100 C for 1 hr with stirring at 300 rpm. After the solution was cooled down to room temperature, powders were obtained by centrifugation at 8,000 rpm, washed twice with DI water, and dried at 70 C for 5 hr. Then, the powders were dispersed in 30 mL of DI water, followed by ultrasonic treatment for 1 hr. The SiC QDs were generated by ultrasonic treatment. The solution was centrifuged at 12,000 rpm for 10 min, followed by ultracentrifugation of the supernatant at 100,000 rpm for 30 min. The products were dried at 50 C for 3 hr. Ni(OH)2 and NiO Typically,42 2 mmol of Ni(NO3)2$6H2O, 10 mmol of sodium dodecyl sulfate, and 12 mmol of hexamethylenetetramine were added to a 200-mL autoclave containing 100 mL of deionized water. After the autoclave was sealed, the mixed solution was heated at 120 C for 24 hr. Commercial Ni(OH)2 was collected by centrifugation at 13,000 rpm, washed three times with a mixture of ethanol and water (1:1, v/v), and dried at room temperature overnight. Then, 100 mg of commercial Ni(OH)2 was added into 50 mL of formamide, followed by ultrasonication of the solution for 12 hr. Subsequently, the suspension was centrifuged at 2,000 rpm, and Ni(OH)2 nanosheets were collected by centrifugation of the supernatants at 13,000 rpm for 30 min. In a typical synthesis of NiO, the Ni(OH)2 obtained was calcined in air at 500 C for 5 min. The BET areas of Ni(OH)2 and NiO were 38.2 and 31.3 m2 g 1, respectively. CoMn LDHs and MnCo2O4 Typically,43 Co(NO3)2$6H2O (0.75 mmol, 0.217 g), Mn(NO3)2$4H2O (0.375 mmol, 0.094 g), NaNO3 (1.8 mmol, 0.153 g), and NH4F (5 mmol, 0.185 g) were added to 250 mL of DI water at room temperature under an N2 atmosphere with stirring at 700 rpm for 30 min. Then, 25 mL of H2O2 aqueous solution (30 wt %) was added to the solution. NaOH aqueous solution (0.08 M) was added dropwise to keep the pH of the solution at around 10 under an N2 atmosphere. After the solution was aged for 24 hr, the product was collected by centrifugation at 13,000 rpm, washed three times with a mixture solvent of ethanol and water (1:1, v/v), and dried at room temperature overnight. In a typical synthesis of MnCo2O4, the CoMn LDHs obtained were calcined in air at 500 C for 5 min. The BET areas of CoMn LDHs and MnCo2O4 were 83.6 and 62.3 m2 g 1, respectively. NiTi LDHs and NiO$TiO2 Typically,44 1.1 mL of water, 50 mL of isooctane, 1.80 g of sodium dodecyl sulfate, 1.5 mL of l-butanol, 0.004 mol of Ni(NO3)2$6H2O, and 0.001 mol of TiCl4 were added in sequence into a 100-mL flask, followed by stirring at room temperature for 1 hr. Then, 1.2 g of urea was added to the solution. The solution was heated to 110 C and kept at 110 C for 27 hr. The product was collected by centrifugation at 13,000 rpm, washed three times with a mixture of ethanol and water (1:1, v/v), and dried at 45 C overnight. In a typical synthesis of NiO$TiO2, the NiTi LDHs obtained were calcined in air at 500 C for 5 min. The BET areas of NiTi LDHs and NiO$TiO2 were 19.4 and 16.5 m2 g 1, respectively. NiCo LDHs and NiCo2O4 Typically,45 2.5 mmol of Ni(NO3)2$6H2O, 5.0 mmol of Co(NO3)2$6H2O, 37.5 mL of ethylene glycol, and 15.0 mL of deionized water were added to a 100-mL

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round-bottom flask with magnetic stirring for 10 min. Then, 37.5 mmol of urea was added to the solution. The solution was heated to 90 C and kept at 90 C for 3 hr. The product was collected by centrifugation at 13,000 rpm, washed three times with a mixture of ethanol and water (1:1, v/v), and dried at 60 C overnight. In a typical synthesis of NiCo2O4, the NiCo LDHs obtained were calcined in air at 500 C for 5 min. The BET areas of NiCo LDHs and NiCo2O4 were 46.8 and 29.6 m2 g 1, respectively. Catalytic Tests in CO2 Hydrogenation As indicated by the nitrogen adsorption-desorption isotherms, the BET surface areas of commercial SiC and SiC QDs were determined to be 14.0 and 633.1 m2 g 1, respectively. To ensure the same total surface areas for each catalytic test, the amounts of commercial SiC and SiC QDs were kept at 226.1 and 5.0 mg, respectively. Five milligrams of Ni(OH)2, CoMn LDHs, NiTi LDHs, NiCo LDHs, NiO, MnCo2O4, NiO$TiO2, and NiCo2O4 were used for each catalytic test. The hydrogenation of CO2 was conducted in a 100-mL slurry reactor (Parr Instrument Company). In a typical catalytic test, the reactor was pressurized with CO2 (8 bar) and H2 (24 bar) at room temperature after the addition of 30 mL of H2O and the catalyst into the Teflon inlet. The reaction proceeded under stirring at a rate of 300 rpm at 150 C. After completion of the reaction, the gas phase was determined by gas chromatography with a flame ionization detector. The liquid phase of the reaction mixture was collected by centrifugation at 10,000 rpm for 2 min; 1 mmol of N,N-dimethylformamide was introduced to 1 mL of the reaction mixture as an internal standard. Then, 50 mL of the mixture was dissolved in 0.5 mL of DMSO-d6 for determining the product yield by 1H NMR spectroscopy. Each catalytic test was repeated three times. In Situ XPS Measurement In situ XPS measurements were performed at the photoemission end station at beamline BL10B in the National Synchrotron Radiation Laboratory in Hefei, China. The beamline is connected to a bending magnet and covers photon energies from 100 to 1,000 eV with a resolving power (E/DE) better than 1,000. The end station is composed of four chambers: an analysis chamber, a preparation chamber, a load-lock chamber, and a high-pressure reactor. The analysis chamber, with a base pressure of <2 310 10 torr, is connected to the beamline with a VG Scienta R3000 electron energy analyzer and a twin anode X-ray source. The high-pressure reactor contains a reaction cell where the samples can be treated with different gases up to 20 bar and simultaneously heated up to 650 C. The mass ratio of commercial SiC to SiC QDs was 45.2:1 in order to maintain the same total surface areas for each measurement (the same as that in catalytic tests). After the sample treatment, the reactor can be pumped with the pressure down to <10 8 torr for sample transfer. In the current work, the sample was treated with 1 bar of CO2 at 150 C for 0.5 hr in the high-pressure reactor and then transferred to the analysis chamber for XPS measurement without exposure to air. In Situ DRIFT Tests In situ DRIFT experiments were conducted in an elevated-pressure cell (DiffusIR Accessory PN 041-10XX) with a Fourier transform infrared spectrometer (TENSOR II Sample Compartment RT-DLaTGS) with a wavenumber resolution of 4 cm 1 at 150 C. The mass ratio of commercial SiC to SiC QDs was 45.2:1 in order to maintain the same total surface area for each measurement (the same as that in catalytic tests). After flowing with 1 bar of N2 for 0.5 hr at 150 C, the background

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spectrum of the sample was acquired. For the treatment of the samples with CO2, 1 bar of CO2 was allowed to flow into the cell at a rate of 30 sccm at 150 C for 0.5 hr, followed by 1 bar of N2 at the rate of 30 sccm at 150 C for 1.0 hr. The DRIFT spectrum was obtained for detecting the species on the sample. DFT Calculations All calculations were performed by the plane-wave periodic DFT in the Vienna ab initio simulation package (VASP).46,47 The electron-ion interaction was described with the projector augmented wave (PAW) method.48,49 The electron exchange and correlation energy were treated within the generalized gradient approximation (GGA) in the form of the Perdew-Burke-Ernzerhof (PBE) functional.50 The long-range van der Waals interactions were described by empirical correction in the Grimme scheme (vdW-D2).51 The cut-off energy was set up to 450 eV. The force convergence and total energy convergence were set to be lower than 0.02 eV/A˚ and 10 5 eV, respectively. Electron smearing of s = 0.1 eV was used according to the Gaussian scheme. Brillouin zone sampling was used with a Monkhorst-Pack grid.52 The catalyst was modeled by a six-layer SiC(111) surface. The top four layers were fully relaxed, and the bottom two layers were fixed to their buck positions. The vacuum zone was 15 A˚ in the z direction to separate the slabs. The nudged-elastic-band method was used to locate the transition states of the hydrogenation reactions.53 The computed vibrational frequencies were used to characterize a minimum state without imaginary frequencies or an authentic transition state with only one imaginary frequency. The adsorption energy of adsorbate A on the SiC(111) surface was defined by Eads = (Eslab + EA) Eslab+A, where Eslab+A is the total energy of the slab with adsorbate A, Eslab is the total energy of the slab, and EA is the total energy of the isolated adsorbate A. Thus, the more positive the adsorption energy, the stronger the adsorbate adsorption on the SiC(111) surface. The activation energy was defined as Ea = ETS – EIS, and the reaction energy was calculated by Er = EFS – EIS, where EIS, ETS, and EFS are the total energies of the initial state, transition state, and final state, respectively.

SUPPLEMENTAL INFORMATION Supplemental Information includes 19 figures and can be found with this article online at https://doi.org/10.1016/j.chempr.2018.01.019.

ACKNOWLEDGMENTS This work was supported by the Collaborative Innovation Center of Suzhou Nano Science and Technology of the Chinese Ministry of Science and Technology (2014CB932700), the National Natural Science Foundation of China (21573206), the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (QYZDB-SSW-SLH017), the Anhui Provincial Key Scientific and Technological Project (1704a0902013), the Major Program of Development Foundation of Hefei Center for Physical Science and Technology (2017FXZY002), and Fundamental Research Funds for the Central Universities.

AUTHOR CONTRIBUTIONS Y.P., L.W., and Q.L. contributed equally to this work. Y.P., L.W., and J.Z. designed the studies and wrote the paper. L.W., Y.P., and Y.C. synthesized catalysts. L.W., Y.P., and Y.D. performed catalytic tests. Y.P., Z.L., and H.L. conducted XRD, FTIR, and in situ DRIFT measurements. L.W. and X.Z. conducted in situ XPS measurements. Y.P. and W.Y. conducted XANES measurements. Q.L. and J.Y. conducted DFT calculations. All authors discussed the results and commented on the manuscript.

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DECLARATION OF INTERESTS The authors declare no competing interests. Received: July 14, 2017 Revised: September 8, 2017 Accepted: January 27, 2018 Published: February 22, 2018

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