Selective Hydrogenation of Cinnamaldehyde over Co-Based

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Selective Hydrogenation of Cinnamaldehyde over Co-based Intermetallic Compounds Derived from Layered Double Hydroxides Yusen Yang, Deming Rao, Yudi Chen, Siyuan Dong, BIN WANG, Xin Zhang, and Min Wei ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02755 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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Figure 1. (A) XRD patterns of the as-synthesized LDH precursors: (a) CoMgAl-LDH, (b) CoInMgAl-LDH, (c) CoGaMgAl-LDH. (B) XRD patterns of the resulting Co and Co-based IMCs obtained from LDH precursors: (a) Co, (b) CoIn3, (c) CoGa3. XRD standard cards for Co and Co-based IMCs are shown in the lower part of the panels: Co-PDF#15-0608, CoIn3-PDF#41-0880, and CoGa3-PDF#15-0578. SEM images of the Co and Cobased IMCs: (C1) Co, (C2) CoIn3, (C3) CoGa3, respectively. 34x24mm (300 x 300 DPI)

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Figure 2. STEM images of the as-synthesized Co and Co-based IMCs: (A1) Co, (B1) CoIn3, (C1) CoGa3. The inset shows their size distribution for these Co and Co-based IMCs. TEM images of the as-synthesized Co and Co-based IMCs: (A2) Co, (B2) CoIn3, (C2) CoGa3. The corresponding HRTEM lattice fringe images are shown in (A3), (B3), and (C3), respectively. 37x27mm (300 x 300 DPI)


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Figure 3. (A) Normalized intensity of Co K-edge XANES spectra and (B) corresponding Fourier transform k3weighted EXAFS spectra in R space for pristine Co, CoIn3 and CoGa3, respectively. XPS spectra of (C) Co 2p of CoIn3, (E) Co 2p of CoGa3, (D) In 3d of CoIn3, (F) Ga 2p of CoGa3. 45x39mm (300 x 300 DPI)


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Figure 4. Reaction pathways in the hydrogenation of cinnamaldehyde: (a) cinnamaldehyde (CAL), (b) cinnamyl alcohol (COL), (c) hydrocinnamaldehyde (HCAL), (d) hydrocinnamyl alcohol (HCOL). 46x17mm (300 x 300 DPI)


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Figure 5. (A) Catalytic conversion and (B) corresponding selectivity vs. reaction time for the hydrogenation of CAL over various catalysts: (a) Co, (b) CoIn3, and (c) CoGa3. (C) Comparison of selectivity toward each product for hydrogenation of CAL over: (a) Co, (b) CoIn3, and (c) CoGa3. (D) Catalytic conversion and selectivity vs. recycling times over CoGa3 catalyst. Reaction conditions: CAL/Co = 58 (molar ratio); CAL: 1.0 mL; iso-PrOH: 30 mL; temperature: 100 °C; H2 pressure: 2 MPa. 41x33mm (300 x 300 DPI)


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Figure 6. Fourier-transformed infrared spectra of CAL adsorption on (a) Co, (b) CoIn3 and (c) CoGa3

sample recorded in 1800−1200 cm−1 after flowing CAL for 15 min at 100 °C and subsequently flushing with He for 15 min. 43x36mm (300 x 300 DPI)


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Figure 7. Fourier-transformed infrared spectra of the hydrogenation progress of CAL over (A) Co, (B) CoGa3 sample recorded in 1800−1500 cm−1 and 1200−1000 cm−1 by flowing H2 as a reaction gas. From curve a to curve e in each panel: 0 s, 40 s, 80 s, 160 s and 640 s, respectively. 50x56mm (300 x 300 DPI)


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Figure 8. Energy profiles of acrolein hydrogenation on the surface of (A) Co (111), and (B) CoGa3 (221). Energy profiles of the optimal reaction pathway for acrolein hydrogenation on the surface of (C1) Co (111), and (D1) CoGa3 (221). Energy profiles of over-hydrogenation on the surface of (C2) Co (111), and (D2) CoGa3 (221). Energy of gaseous H2 and acrolein is set as the zero point (the horizontal line in black). 50x54mm (300 x 300 DPI)


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Figure 9. Schematic illustration for reaction mechanism of CAL hydrogenation to COL on the surface of CoGa3. 42x35mm (300 x 300 DPI)


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Table of Contents 14x7mm (300 x 300 DPI)


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Selective Hydrogenation of Cinnamaldehyde over Co-based Intermetallic Compounds Derived from Layered Double Hydroxides Yusen Yang,1 Deming Rao,1,2 Yudi Chen,3 Siyuan Dong,1 Bin Wang,4 Xin Zhang,*1 Min Wei*1

1

State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for

Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China 2

Institute of Science and technology strategy, Jiangxi academy of science, Nanchang 330096, P. R.

China 3

Beijing Center for Physical & Chemical Analysis, Beijing 100089, P. R. China

4

Beijing Research Institute of Chemical Industry, Sinopec Group, Beijing 100013, P. R. China

*

Corresponding authors. Tel: +86-10-64412131; Fax: +86-10-64425385.

E-mail addresses: [email protected] (X. Zhang); [email protected] (M. Wei).

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ABSTRACT: Selective hydrogenation of unsaturated carbonyl compounds plays a key role in the production of fine chemicals and pharmaceutical agents. In this work, two kinds of intermetallic compounds (IMCs: CoIn3 and CoGa3) were prepared via structural topotactic transformation from layered double hydroxides (LDHs) precursors, which exhibited surprisingly high catalytic activity and selectivity toward hydrogenation reaction of α,β-unsaturated aldehydes (C=O vs. C=C). Notably, the CoGa3 catalyst shows a hydrogenation selectivity of 96% from cinnamaldehyde (CAL) to cinnamyl alcohol (COL), significantly higher than CoIn3 (80%) and monometallic Co catalyst (42%). A combination study including XANES, XPS and CO-IR spectra verifies electron transfer from Ga (or In) to Co, leading to the formation of Co−Ga (or Co−In) coordination. FT-IR measurements and DFT calculation studies substantiate that the electropositive element (Ga or In) in IMCs serves as active site and facilitates the adsorption of polarized C=O; whilst C=C adsorption on Co site is extremely depressed, which is responsible for the markedly enhanced selectivity toward hydrogenation of C=O. This work reveals the key role of functional group adsorption in determining the hydrogenation selectivity of α,β-unsaturated aldehydes, which gives an in-depth understanding on structure-property correlation and reaction mechanism. KEYWORDS: Intermetallic compounds, layered double hydroxides, selective hydrogenation, cinnamaldehyde, FT-IR, DFT

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1. INTRODUCTION Selective hydrogenation of α,β-unsaturated aldehydes, such as cinnamaldehyde, is known as a significant chemical transformation in flavorings, perfume and pharmaceutical industries for the production of unsaturated alcohols.1-4 Normally, saturated aldehydes are predominant products since C=C bond undergoes a more thermodynamically and kinetically favorable hydrogenation than C=O;5-10 therefore, the preferential hydrogenation of C=O in α,β-unsaturated aldehydes toward unsaturated alcohols is an extremely difficult chemical process. In order to solve this conundrum, great research efforts have been focused on the design of efficient catalysts to improve the hydrogenation selectivity of C=O.11-18 In recent years, intermetallic compounds (IMCs) with specific geometric and electronic structure have received considerable attention in this area, since a number of studies have demonstrated their promising hydrogenation selectivity toward C=O.19-21 For instance, Ru-Ti and Pt-Sn systems display a high hydrogenation selectivity to crotyl alcohol in the hydrogenation of crotonaldehyde.22-23 Although much progress has been made, the most essential factor influencing hydrogenation selectivity and corresponding reaction mechanism are still rather limited. Based on a large number of experimental measurements and computational studies, it has been recognized in previous research that the electronic structure of IMCs plays a crucial role on the adsorption and activation of functional groups in selective hydrogenation reaction of α,β-unsaturated aldehydes.24-25 The first active metal including noble or transition metals (e.g., Pd, Pt, Ni, or Co) normally enables adsorption and dissociation of both hydrogen and α,β-unsaturated aldehydes;26-29 the second inert metal such as Sn, Ga or In,20, 30-31 serves as a regulator to modify the electronic structure of the first metal via electron transfer between them. The existence of the second metal separates continuous metal sites 3


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of the first metal,32-33 which weakens the hydrogenation of C=C and thus enhances hydrogenation selectivity of C=O. In principle, for selective hydrogenation of α,β-unsaturated aldehydes, the competitive activated adsorption of C=C and C=O imposes a decisive effect on the formation of the final product.34 However, the understanding of C=C and C=O adsorption on the surface of IMCs is extremely deficient, and the identification of intrinsic active site is rather lacking. The fundamental knowledge on activated adsorption of unique functional group is highly important to further push ahead understanding on reaction mechanism of selective hydrogenation, which would give valuable instructions for the development of new catalysts design. In our previous work, we reported a green synthesis route for the preparation of Co-Sn IMCs by virtue of the topotactic transformation process of layered double hydroxides (LDHs) precursors, which showed a high selectivity for the hydrogenation reaction of citral.16 The influence of electron transfer between Co and Sn on adsorption selectivity (C=C vs. C=O) was proposed, but the intrinsic active site for competitive adsorption still remains ambiguous. This inspires us to explore the detailed adsorption of C=C and C=O on the active site of IMCs and to reveal the reaction mechanism of selective hydrogenation. Herein, we further extend this synthesis to the preparation of supported CoIn3 and CoGa3 IMCs based on an LDHs approach: introducing highly-active Co and more electropositive Ga (or In) into LDHs precursors followed by a subsequent co-reduction treatment, and their catalytic performance toward selective hydrogenation of α,β-unsaturated aldehyde (cinnamaldehyde, CAL) were investigated. Noteworthily, flow reaction formulation have attracted much attention on the development of chemoselective hydrogenation, since it improved process profitability and green credentials simultaneously.35-36 However, our main research focuses on material design, structure optimization and structure-property correlation in this work, by using 4


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batch reaction as a facile evaluation method. The conversion of all these three samples (Co, CoIn3 and CoGa3) is close to 100% (100 °C, 2 MPa), but the selectivity toward unsaturated alcohol (cinnamyl alcohol, COL) increases in the following order: Co < CoIn3 < CoGa3. Especially, the CoGa3 IMC catalyst exhibits the optimal catalytic performance with a conversion of 99% and selectivity of 96%, which is superior to previously reported Co-based catalysts and even comparable to the level of noble metals catalysts (e.g., Pt, Au, or Pd). XANES, XPS and computational studies verify the electron transfer from Ga or In to Co, and Ga (in CoGa3) shows a higher electropositivity than In (in CoIn3). FT-IR spectra and DFT calculations give direct evidences that the electropositive element (Ga or In) in IMCs serves as active site and facilitates the adsorption of polarized C=O; whilst the C=C adsorption on Co site is significantly depressed, leading to the markedly enhanced selectivity of IMC catalysts relative to pristine Co catalyst. In addition, the higher electropositivity of Ga in CoGa3 induces a much stronger polarization and adsorption of C=O group, resulting in its larger selectivity to unsaturated alcohol compared with In in CoIn3 IMC. It is expected that the CoGa3 IMC can serves as a highly-efficient catalyst for selective hydrogenation of unsaturated carbonyl compounds.

2. RESULTS AND DISCUSSION 2.1. Structure and morphology characterizations of Co-based IMCs The XRD patterns of CoMgAl-LDH, CoInMgAl-LDH and CoGaMgAl-LDH precursors (Figure 1A) display characteristic reflections at 2θ 11.94°and 23.78°, which are assigned to the (003) and (006) of an LDH phase, respectively. The SEM images (Figure S1A-C) display a sheetlike morphology with a diameter of 2–3 m, indicating a good crystallization. H2-TPR measurements were performed to study the topotactic transformation process of LDHs precursors. 5


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It is observed that CoMgAl-LDH (Figure S2, curve a) shows the lowest reduction temperature at 440 °C, which increases to 650 °C and 770 °C for CoInMgAl-LDH (Figure S2, curve b) and CoGaMgAl-LDH (Figure S2, curve c), respectively. This indicates a strong interaction between Co and Ga (In) during the co-reduction process.30

Figure 1. (A) XRD patterns of the as-synthesized LDH precursors: (a) CoMgAl-LDH, (b) CoInMgAl-LDH, (c) CoGaMgAl-LDH. (B) XRD patterns of the resulting Co and Co-based IMCs obtained from LDH precursors: (a) Co, (b) CoIn3, (c) CoGa3. XRD standard cards for Co and Cobased IMCs are shown in the lower part of the panels: Co-PDF#15-0608, CoIn3-PDF#41-0880, and CoGa3-PDF#15-0578. SEM images of the Co and Co-based IMCs: (C1) Co, (C2) CoIn3, (C3) CoGa3, respectively.

A subsequent reduction treatment in a H2 atmosphere gives rise to topotactic transformation of LDHs precursors to the corresponding Co or Co-based IMCs. The CoMgAl-LDH transfers to a supported metal cobalt phase (Figure 1B, curve a; PDF#15-0608); while CoInMgAl-LDH and CoGaMgAl-LDH transforms to intermetallic compound CoIn3 and CoGa3 (supported in MgO and 6


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Al2O3 matrix), respectively. Their corresponding XRD patterns CoIn3 (Figure 1B, curve b: PDF#410880) and CoGa3 (Figure 1B, curve c; PDF#15-0578) verify the successful synthesis of Co-based IMCs by virtue of the versatility in chemical composition of LDHs precursors. In addition, the LDHs precursors transformed to corresponding Co and CoIn3 (or CoGa3) IMC, without other impurity phase, as shown in Figure 1B. The SEM images (Figure 1C1-C3) of all these reduced samples (Co and Co-based IMCs) display that they inherit the morphological property of original LDHs precursors, with numerous highly-dispersed particles immobilized on the surface of sheet-like substrate revealed by STEM images (Figure 2A1-C1). The mean size of supported Co particles is 18.7 nm; while the supported CoIn3 and CoGa3 particles give an average size of 19.4 and 19.8 nm, respectively. HRTEM images further confirm the single crystalline nature of Co and Co-based IMCs. The lattice spacing of 0.204 nm corresponds to the (111) plane of Co (Figure 2A3), and those of 0.232 nm and 0.211 nm (Figure 2B3 and 2C3) are attributed to the (122) plane of P-4n2 CoIn3 and the (212) plane of P-4n2 CoGa3, respectively. The element contents of these samples were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Table S1). High-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) and electronic energy-loss spectroscopy (EELS) measurements were carried out to further confirm the structure of these samples (Figure S3), which accords well with XRD and HRTEM results. Generally, Co-based IMCs prepared by conventional solid phase reactions give relatively large sizes; however, the Co-based IMCs prepared via LDHs precursors approach display rather small particle size in this work, which would be beneficial to their catalytic performance.

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Figure 2. STEM images of the as-synthesized Co and Co-based IMCs: (A1) Co, (B1) CoIn3, (C1) CoGa3. The inset shows their size distribution for these Co and Co-based IMCs. TEM images of the as-synthesized Co and Co-based IMCs: (A2) Co, (B2) CoIn3, (C2) CoGa3. The corresponding HRTEM lattice fringe images are shown in (A3), (B3), and (C3), respectively. The electronic structure of Co-based IMCs was studied by the X-ray absorption near-edge structure (XANES) spectroscopy, and Figure 3A and 3B show the normalized XANES spectra of Co-based IMCs and the reference Co sample. From Co to CoIn3 and then to CoGa3, the absorption edge of Co K-edge shifts to low photon energy with a decreased intensity of white line (Figure 3A), indicating an increased electronegativity of Co possibly due to electron transfer from In or Ga atoms to Co atoms.37 Figure 3B shows the detailed atomic configuration of Co-based IMCs via Fourier transform of Co K-edge EXAFS oscillations in R space. Figure 3B shows that Co cluster and CoOx might be simultaneously present: the peak at 2.1 Å is attributed to Co cluster and the one at 1.5 Å is due to CoOx in R space. The former peak is rather strong and even conceals the latter one, indicating the predominance of Co cluster. In comparison with the monometallic Co, the intensity of the first 8


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nearest-neighbor distance for Co-based IMCs decreases obviously accompanied with a peak shift to large R value, indicative of a reduced Co–Co coordination owing to the introduction of In or Ga. The Co−Co bond length (Table S2) increases gradually from Co (2.494 Å) to CoIn3 (2.919 Å) and then to CoGa3 (2.972 Å), demonstrating a significantly reduced Co−Co bonding induced by the separation effect of In or Ga. Table S2 lists the curve-fitting data of these samples, showing that the coordination number of Co–Co bond decreases from 12 (Co nanoparticles) to 1 in CoIn3 and CoGa3, and CoIn3 and CoGa3 display the same coordination number of 8 for Co–In or Co–Ga bond. The mean sizes of these samples are very close. This indicates that the coordination number is independent on particle size in this work but is related to intrinsic crystalline property. The bond length and coordination number from EXAFS and theoretical modeling accord well with XRD results (Table S2). The results show that CoIn3 and CoGa3 have rather similar coordination environment, and the introduction of In or Ga notably affects the metallic state of Co atom. X-ray photoelectron spectroscopy (XPS) was used to study the electronic structure information of Co and Co-based IMCs. As shown in Figure S4, the peaks at 778.3 eV and 781.0 eV in pristine Co 2p are attributed to Co0 2p3/2 and Co2+ 2p3/2, respectively.38 For CoIn3 and CoGa3 samples, Co0 2p3/2 peak shifts to a low binding energy of 777.9 eV in CoIn3 and then to 777.3 eV in CoGa3; while Co2+ 2p3/2 moves to a low binding energy of 780.4 eV in CoIn3 and then to 780.1 eV in CoGa3 (Figure 3C and 3E). This indicates an enhanced electronegativity of Co in CoIn3 and CoGa3 relative to pristine Co. According to the In 3d3/2 XPS spectra, the main peak of CoIn3 sample can be fitted into doublet, including In3+ 3d3/2 at 446.4 eV and In0 3d3/2 at 444.6 eV (Figure 3D), both of which are higher than In0 species. Meanwhile, in the case of Ga 2p3/2 spectra, the sample of CoGa3 shows a doublet peak consisting of Ga3+ 2p3/2 at 1119.2 eV and Ga0 2p3/2 at 1118.2 eV (Figure 3F), whose 9


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binding energies are larger than Ga0 species. With the introduction of In (or Ga), the obvious downshift of Co 2p and up-shift of In 3d3/2 (or Ga 2p3/2) substantiate the electron donation from In (or Ga) to Co, in accordance with the formation of Co−In or Co−Ga coordination; and a stronger electron transfer occurs in CoGa3 than in CoIn3. The Bader charge analysis (Table S3) shows that the outermost electron number of Co increases form 9 (pristine Co particles) to 9.43 (CoIn3) and then to 9.57 (CoGa3). Simultaneously, the outermost electron numbers of the three types of In atoms (Figure S5) in CoIn3 are 2.85, 2.85 and 2.87, in contrast to 2.81, 2.81 and 2.81 for the three types of Ga atoms in CoGa3, which indicates a more electropositive property of Ga than In. The Bader charge analysis confirms electron transfer from In (or Ga) to Co, which accords well with XANES and XPS results.

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Figure 3. (A) Normalized intensity of Co K-edge XANES spectra and (B) corresponding Fourier transform k3-weighted EXAFS spectra in R space for pristine Co, CoIn3 and CoGa3, respectively. XPS spectra of (C) Co 2p of CoIn3, (E) Co 2p of CoGa3, (D) In 3d of CoIn3, (F) Ga 2p of CoGa3. In order to give an in-depth insight into surface structures, Fourier-transformed infrared (FTIR) absorption of CO molecule over the pristine Co and Co-based IMCs was carried out (Figure S6). Generally, CO molecule undergoes two different adsorption types on Co0 site: multicoordinated adsorption (below 2000 cm−1) and linear adsorption (above 2000 cm−1);39 but the adsorption of CO on In0 and Ga0 site is not detectable at room temperature. Herein, for the pristine Co (Figure S6, curve a), both the multicoordinated adsorption (1780 cm−1) and the linear adsorption peak (2081 cm−1) are observed. In these cases of CoIn3 and CoGa3 (Figure S6, curve b and c), only linear adsorption of CO is observed, whose band shows a red-shift from 2081 cm−1 (monometallic Co) to 2037 cm−1 (CoIn3) and then to 2009 cm−1 (CoGa3). The results accord well with the electron transfer from Ga (or In) atoms to Co atoms, and Ga donates more electrons than In as confirmed by XPS and XANES. From the viewpoint of geometric structure, the incorporation of In (or Ga) separates continuous Co sites and simultaneously increases the Co−Co bond length of CoIn3 and CoGa3, which therefore inhibits CO multicoordinated adsorption. 2.2. Selective hydrogenation reaction of cinnamaldehyde over Co-based IMCs For this study, cinnamaldehyde hydrogenation, which is a typical selective hydrogenation reaction of α,β-unsaturated aldehydes, was used to evaluate the catalytic performance of the pristine Co and Co-based IMCs. Selective hydrogenation production of cinnamaldehyde (CAL), i.e., cinnamyl alcohol (COL), serves as a promising chemical intermediate widely used in fine chemical synthesis. CAL is a representative molecule consisting of three different types of unsaturated

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functional groups: a C=C bond, a C=O bond and a benzene ring with a large conjugation structure,40 whose structure and the hydrogenation products are displayed in Figure 4. Figure S7 shows the correlation between catalytic performance of CoGa3 sample and reaction conditions (H2 pressure or temperature). Based on the results and the principle of minimum energy consumption, 2 MPa of H2 pressure and 100 °C of reaction temperature were chosen.

Figure 4. Reaction pathways in the hydrogenation of cinnamaldehyde: (a) cinnamaldehyde (CAL), (b) cinnamyl alcohol (COL), (c) hydrocinnamaldehyde (HCAL), (d) hydrocinnamyl alcohol (HCOL). Figure 5A and 5B show the catalytic conversion and COL selectivity over monometallic Co and Co-based IMCs catalysts vs. reaction time. The CAL conversion over these three samples is close to 100% at 8 h, and the reaction rates of Co, CoIn3 and CoGa3 are 59.0, 41.8 and 38.0 mmol g−1 h−1, respectively, which is at a high level compared with previously reported catalysts (Table S4). A linear relationship between −ln(1 − C) and reaction time is obtained over Co, CoIn3, and CoGa3 samples (Figure S8), with rate constants of 0.66, 0.49 and 0.32 h−1, respectively (Table S5), indicating the first-order reaction. However, the selectivity toward COL shows an obvious enhancement in the following sequence: Co (42%) < CoIn3 (80%) < CoGa3 (96%). A conversion of 99% and selectivity of 96% are obtained for the CoGa3 sample, superior to previously reported Co12


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based catalysts and even comparable to the level of noble metal catalysts (see Table S4 for comparison). Moreover, the reaction rate (38.0 mmol g−1 h−1) is also among the highest level. As shown in Figure 5C, the monometallic Co sample prefers to hydrogenate C=C and results in the formation of saturated aldehyde (HCAL, selectivity: 36%) and saturated alcohol (HCOL, selectivity: 22%). In contrast, the formation of HCAL is completely inhibited both in CoIn3 and CoGa3 reaction system, accompanied with a depressed HCOL selectivity of 20% and 4%, respectively. Therefore, the incorporation of In or Ga inhibits the hydrogenation of C=C which is highly active over monometallic Co, and simultaneously enhances the hydrogenation of C=O to produce COL. Additionally, the reusability of CoGa3 sample was tested by 5-time recycling under the same reaction conditions (Figure 5D). The reaction conversion of CAL still maintains 95% and the COL selectivity shows a small decrease from 96% to 92% after 5 recycles, demonstrating a satisfactory stability and recyclability of CoGa3 catalyst. Table S6 shows that isopropanol as solvent has no obvious influence on this reaction. The XRD patterns and TEM images of the used catalyst after 5 cycles do not show obvious change compared with the fresh one, indicating the stability of catalyst (Figure S9). We further evaluated selective hydrogenation of other unsaturated aldehydes over CoGa3 catalyst, in comparison with the monometallic Co sample (Table S7). A similar tendency is observed, demonstrating the universality of CoGa3 catalyst toward this reaction.

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Figure 5. (A) Catalytic conversion and (B) corresponding selectivity vs. reaction time for the hydrogenation of CAL over various catalysts: (a) Co, (b) CoIn3, and (c) CoGa3. (C) Comparison of selectivity toward each product for hydrogenation of CAL over: (a) Co, (b) CoIn3, and (c) CoGa3. (D) Catalytic conversion and selectivity vs. recycling times over CoGa3 catalyst. Reaction conditions: CAL/Co = 58 (molar ratio); CAL: 1.0 mL; iso-PrOH: 30 mL; temperature: 100 °C; H2 pressure: 2 MPa. 2.3. Studies on structure-selectivity correlation over Co-based IMCs It has been recognized that the hydrogenation reactions over transition metal catalysts obey the Horiuti-Polanyi reaction mechanism, which involves hydrogen dissociation followed by consecutive addition of atomic hydrogen to the substrate.41-42 H2-TPD measurements were carried out to study the adsorption properties of hydrogen on the monometallic Co and Co-based IMCs

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(Figure S10). Previous studies have proved that hydrogen dissociation occurs on the surface of cobalt and desorption peaks at different temperatures indicate different modes of hydrogen chemisorption.43 For monometallic Co sample in this work (Figure S10, curve a), two obvious desorption peaks at 78 °C and 162 °C are observed, which are attributed to linear adsorption at Co site below the second layer and bridge adsorption at Co–Co coordination, respectively. In the cases of Co-based IMCs samples (Figure S10, curve b and curve c), two desorption peaks at 78 °C and 127 °C are observed, which are assigned to linear adsorption at Co site below the second layer and linear adsorption at Co site with Co−In or Co−Ga coordination.20 The bridge adsorption mode of H2 disappears on the surface of Co-based IMCs, resulting from the largely decreased Co–Co site confirmed by EXAFS. As shown in Figure S10, CoIn3 and CoGa3 samples show a relatively decreased total integral area of H2-TPD curve, in comparison with pristine Co sample, leading to the reduction of surface concentration of adsorbed hydrogen. Moreover, kinetic experiment was carried out to investigate the effect of H2 pressures on the reaction (Figure S11). It is found that the reaction rate constant is almost constant within H2 pressure of 2−4 MPa (Table S8), indicating the concentration of adsorbed hydrogen is saturated at the reaction pressure (2 MPa). IR characterization, as an important technique, can provide useful information on surface adsorption, intermediates, and reaction pathway.44 Herein, we carried out IR measurements to investigate the adsorption of reactant CAL on the surface of Co, CoIn3 and CoGa3 samples (Figure 6). Pure CAL was flowed into the reaction cell at the reaction temperature (100 °C) for 15 min, followed by purging He flow to remove the gaseous and physically-adsorbed CAL for measurement of the chemical adsorption. For the monometallic Co (Figure 6, curve a), the bands at 1677 cm−1 and 1392 cm−1 are assigned to ν(C=O) and aldehyde C−H rock, respectively.39 For CoIn3 and CoGa3 15


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samples (Figure 6, curve b and c), both of the two bands show a continuous red-shift: ν(C=O) moves to 1661 cm−1 in CoIn3 and then to 1657 cm−1 in CoGa3; while aldehyde C−H rock moves to 1387 cm−1 in CoIn3 and then to 1375 cm−1 in CoGa3. This indicates a stronger adsorption of C=O on the surface of Co-based IMCs than pristine Co. In the cases of CoIn3 and CoGa3 samples, two additional bands at 1626 and 1597 cm−1 are observed, which are assigned to the ν(C=C) adjacent to C=O in CAL. However, these two bands are not found in the sample of monometallic Co. It should be noted that the IR adsorption band of ν(C=C) is rather different from that of ν(C=O): the adsorption of C=O group on catalyst surface (linear adsorption) would result in a red shift of ν(C=O); while C=C group undergoes a bridge adsorption that gives rise to the disappearance of ν(C=C). Therefore, these results above indicate that adsorption and activation of C=C occurs on the surface of monometallic Co, with the absence of observation of ν(C=C) (Figure 6, curve a). However, this is significantly depressed in the cases of CoIn3 and CoGa3 samples, owing to the observation of free state ν(C=C) (Figure 6, curve b and c). In addition, the bands at 1498 and 1452 cm−1 due to ν(C−C) of free benzene ring are observed in all these three samples, indicating benzene ring group does not adsorb on these samples and thus remains an unconstrained state. Based on the discussion above, it is concluded that both C=O and C=C undergo activation adsorption on the surface of monometallic Co sample, which induces a high catalytic activity but a very low selectivity (a high yield of full hydrogenation product HCOL). In contrast, CoIn3 and CoGa3 catalysts show an enhanced activation adsorption of C=O but a largely inhibited C=C adsorption, resulting in a high selectivity toward hydrogenation product of C=O (COL) with absence of hydrogenation product of C=C group (HCAL), as shown in Figure 5C. This obvious variation in activation adsorption of C=O vs. C=C is possibly due to the change in geometrical structure of adsorption sites, which will be further studied in the following 16


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section.

Figure 6. Fourier-transformed infrared spectra of CAL adsorption on (a) Co, (b) CoIn3 and (c) CoGa3 sample recorded in 1800−1200 cm−1 after flowing CAL for 15 min at 100 °C and subsequently flushing with He for 15 min. In order to investigate the process of hydrogenation reaction and formation of intermediates and products, FT-IR spectroscopy was carried out by purging H2 into the reactor at the reaction temperature (100 °C) for detection of hydrogenation process. A comparison study was performed between monometallic Co (without selectivity) and CoGa3 with the optimal selectivity toward hydrogenation product of C=O (COL). According to the buildup and decay of various surface species, detailed information on catalytic reaction mechanism can be obtained. Figure 7 displays FT-IR spectra of hydrogenation process of CAL over Co and CoGa3, respectively, with flushing H2

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at time 0 s, 40 s, 80 s, 160 s and 640 s. For monometallic Co (Figure 7A1 and A2), the characteristic signal of C=O (1677 cm−1) diminishes gradually with continuous inflow of hydrogen, accompanied with a new band at 1060 cm−1 ascribed to aldehyde C−O.45 This indicates the hydrogenation process of C=O group. In addition, another new band at 1085 cm−1 is also found with increasing intensity, which is attributed to ν(C−C) (hydrogenation production of C=C). The results reveal a simultaneous hydrogenation process of both C=C and C=O on the surface of monometallic Co, which leads to an unsatisfactory selectivity. In contrast, in the case of CoGa3 sample, the FT-IR spectra display a different hydrogenation process (Figure 7B1 and B2). The characteristic signal of C=O (1657 cm−1) weakens gradually and completely disappears at 640 s; while the new band at 1060 cm−1 (aldehyde C−O) comes into formation with a rather strong intensity at 640 s. Moreover, the characteristic signals of C=C (1626/1597 cm−1) do not show obvious change from t=0 to t=640 s, excluding the hydrogenation of free state C=C group over CoGa3 sample. With continuous inflow of hydrogen over CoGa3 with adsorbed COL, the characteristic signal of C=C declines gradually and disappears totally at 900 s in FT-IR spectra (Figure S12), without observation of new product signal. This indicates that the COL molecule desorbs from the catalyst surface rather than consecutive hydrogenation. FT-IR spectra demonstrate that only hydrogenation of C=O group occurs on the surface of CoGa3, in agreement with the consequence of the catalytic performance (a high hydrogenation selectivity toward COL).

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Figure 7. Fourier-transformed infrared spectra of the hydrogenation progress of CAL over (A1) and (A2) Co sample, (B1) and (B2) CoGa3 sample recorded in 1800−1500 cm−1 and 1200−1000 cm−1 by flowing H2 as a reaction gas. From curve a to curve e in each panel: 0 s, 40 s, 80 s, 160 s and 640 s, respectively. DFT calculations are further carried out to provide a deep understanding on the geometric and electronic structure of monometallic Co and Co-based IMCs. As shown in Figure S13, CoIn3 and CoGa3 display different density of states with respect to pristine Co: they exhibit a band-gap of −5 and −3 eV, which significantly modify the electronic structure near the Fermi level. This feature is 19


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related with a higher thermodynamic stability of CoIn3 and CoGa3. The different planes of nanocatalysts have great influence on the reaction performance in heterogeneous catalysis. 46-47 In this work, we chose the most stable and preferential facets for calculations, i.e., (111) plane of Co and (221) plane CoGa3 IMC, based on the results of XRD and HRTEM, which can represent the surface property of catalysts. In this work, the calculated models of Co and CoGa3 are constructed on the basis of their XRD patterns. For monometallic Co, each Co atom is coordinated by twelve adjacent Co atoms. With the introduction of Ga, the Co−Co coordination decreases when the Co−Ga coordination increases in CoGa3 (Table S2), which agrees well with the curve fitting EXAFS data. For the purpose of simplifying calculations at a reasonable level, acrolein (with an adjacent C=C group and C=O group) was chosen for the computational study. The initial adsorption state of substrate can be determined by comparing adsorption energy of various adsorption configurations. The adsorption configuration B was chosen as the initial adsorption state, owing to its stronger adsorption energy (−1.298 eV) than that of configuration A (−0.879 eV) (Figure S14). Figure S15 shows the optimal adsorption state of acrolein over Co and CoGa3, with an adsorption energy (Ead) of −1.115 eV and −1.382 eV, respectively. In the Co system, acrolein molecule is parallel to the surface in a planar configuration that allows both C=C and C=O to interact with two Co atoms on the catalyst surface (Figure S15A). In contrast, the adsorption state changes significantly in the case of CoGa3 system, owing to the electron transfer from Ga to Co (confirmed by XANES and XPS). The terminal oxygen atom in C=O group adsorbs on the surface of Ga atom, and C=C group shows a weak interaction with only one Co atom (Figure S15B). The first active hydrogen atom can attach to the primary (Site 1) or secondary (Site 2) carbon atom in C=C bond, or to the carbon (Site 3) or oxygen (Site 4) atom in C=O bond (as shown in Figure S15). Figure 8A and 8B illustrate all of the 20


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possible hydrogenation paths via Path-12, Path-21, Path-34 and Path-43 (see Figure S16 for respective elementary reaction steps), and the activation barriers of each hydrogenation path are evaluated by energy profiles over Co and CoGa3 surfaces. Figure 8A shows the lowest energy barrier of hydrogenation is present along Path-12 on the surface of Co, and the first step is hydrogenation of the carbon atom (Site 1). This confirms that C=C hydrogenation is thermodynamically favorable on the surface of Co.48 On the contrary, in the case of CoGa3 system, Path-43 gives the lowest energy barrier of hydrogenation (Figure 8B), and the first step is hydrogenation of oxygen atom (Site 4) to form alkoxide. Figure 8C1 and D1 show the transition states of the optimal hydrogenation paths (path-12 and path-43) over Co and CoGa3, respectively, from which C=O hydrogenation is more favorable on the surface of CoGa3. In addition, the energy barrier of over-hydrogenation on Co is low (Figure 8C2), resulting in a facile occurrence of over-hydrogenation via Path-124. However, this energy barrier is rather high on the surface of CoGa3 (Figure 8D2) owing to the unstable transition state, and the energy of TS432 is much higher than that of initial state via Path-432. This indicates that over-hydrogenation hardly occurs on the surface of CoGa3, in accordance with the extremely weak adsorption of C=C bond on Co atom (with the observation of free state ν(C=C) in FT-IR spectra). The reaction rates of CAL hydrogenation over Co and CoGa3 samples were obtained via kinetic studies, which were fitted well with the first-order kinetics (Table S9). Apparent activation energy (Ea) on CoGa3 was measured to be 18.6 kJ mol−1 (Figure S17), lower than that on Co (25.1 kJ mol−1), indicating an enhanced catalytic activity of CoGa3 sample. In addition, the energy barrier of TS1 on the surface of CoGa3 is lower than that on Co according to DFT calculations (Figure 8C1 and 8D1), which indicates a more favorable hydrogenation reaction on CoGa3. Therefore, the DFT calculations agree well with the kinetic parameter from experimental studies. 21


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Figure 8. Energy profiles of acrolein hydrogenation on the surface of (A) Co (111), and (B) CoGa3 (221). Energy profiles of the optimal reaction pathway for acrolein hydrogenation on the surface of (C1) Co (111), and (D1) CoGa3 (221). Energy profiles of over-hydrogenation on the surface of (C2) Co (111), and (D2) CoGa3 (221). Energy of gaseous H2 and acrolein is set as the zero point (the horizontal line in black). Based on the results of FT-IR measurements and DFT calculations above, the adsorption state of CAL over CoGa3 as well as the reaction pathway are clearly determined (Figure S18). The initial state (CAL molecule) undergoes activation adsorption of C=O on the surface of CoGa3 (Figure S18, 22


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curve a). Subsequently, one dissociated hydrogen atom attacks the adsorbed O in C=O bond to form the intermediate (alkoxide adspecies). With continuous inflow of hydrogen, the characteristic signal of C=O weakens gradually accompanied with the appearance of a new band (C−O) (Figure S18, curve b), indicating the formation of product (COL) via the intermediate. Finally, the C=O disappears totally and COL molecule desorbs from the catalyst surface (Figure S18, curve c). According to the results above, the reaction mechanism of CAL hydrogenation to COL on the surface of CoGa3 catalyst is proposed in Figure 9. The reaction proceeds in four steps as follows: (1) CAL molecule undergoes activation adsorption on the surface of CoGa3, via terminal O atom in C=O bond attaching to one Ga atom and a weak adsorption (physical adsorption) of C=C bond on one Co atom; (2) hydrogen dissociation occurs on the adjacent Co atoms in CoGa3 to produce active hydrogen atoms; (3) one active hydrogen atom attacks the adsorbed O in C=O bond to form alkoxide adspecie; (4) the alkoxide adspecie is sequentially hydrogenated on the C atom followed by desorption of COL. Therefore, the specific geometrical/electronic structure of CoGa3 results in a unique adsorption state of CAL consisting of both C=O and C=C group, giving rise to an excellent selectivity toward hydrogenation of C=O bond, as revealed by both experimental observations (FTIR spectra and catalytic evaluations) and DFT calculations. The adsorbed configuration of substrate eventually determines the site of hydrogenation and the reaction selectivity. FT-IR spectra demonstrate that activation adsorption and hydrogenation of C=O group (not C=C group) occur on the surface of CoGa3 to produce COL, followed by product desorption from catalyst surface rather than consecutive hydrogenation.

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Figure 9. Schematic illustration for reaction mechanism of CAL hydrogenation to COL on the surface of CoGa3.

3. CONCLUSION Two Co-based IMCs (CoIn3 and CoGa3) were synthesized by a co-reduction approach on the basis of LDHs precursors. The CoGa3 catalyst demonstrates significantly improved catalytic selectivity (96%, 100 °C, 2 MPa) for the hydrogenation of CAL to COL, which is much higher than the monometallic Co catalyst (selectivity: 42%). XANES, XPS and CO-IR confirm electron transfer from Ga (or In) to Co, leading to the formation of Co−Ga (or Co−In) coordination. FT-IR spectra and DFT calculations verify that the electropositive element (Ga or In) in IMCs serves as active site and facilitates the activation adsorption of polarized C=O group. For the monometallic Co sample, both C=O and C=C group undergo activation adsorption on the surface, which causes a low

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selectivity toward COL. In contrast, CoGa3 sample shows an enhanced activation adsorption of C=O and a rather weak physical adsorption of C=C group, resulting in a high selectivity toward hydrogenation product of C=O group (COL). This strategy holds crucial promise for Co-based IMCs as high-performance catalysts toward selective hydrogenation reaction of unsaturated carbonyl compounds.

4. EXPERIMENTAL SECTION 4.1. Materials All the following organic and inorganic reagents were bought from Sigma Aldrich and were used without further purification: Co(NO3)2·6H2O, Mg(NO3)2·6H2O, Al(NO3)3·9H2O, Ga(NO3)3·xH2O, In(NO3)3·xH2O and urea. 4.2. Preparation of Co-based IMCs catalysts Synthesis of CoMgAl-LDH precursors. CoMgAl-LDH was prepared by a urea decomposition way. Urea, Co(NO3)2·6H2O, Mg(NO3)2·6H2O and Al(NO3)3·9H2O were dissolved in deionized water (500 mL) with a concentration of 0.75 M, 0.10 M, 0.05 M and 0.05 M, respectively. The mixed solution was sealed and stirred in a flask at 100 C for 24 h. The resulting precipitate was separated, washed with deionized water and dried in a vacuum oven at 60 C for 12 h. Synthesis of CoMgGaAl-LDH and CoMgInAl-LDH precursors. CoMgGaAl-LDH and CoMgInAl-LDH were synthesized based on a similar method described above, and the molar concentrations of Co(NO3)2·6H2O, Mg(NO3)2·6H2O, Ga(NO3)3·xH2O (or In(NO3)3·xH2O), Al(NO3)3·9H2O and urea were 0.01 M, 0.09 M, 0.03 M, 0.02 M and 0.75 M, respectively. Synthesis of supported Co and Co-based IMCs. 1.0 g of the above LDHs precursors were reduced in a H2/N2 (1/9, v/v) stream at various temperatures (from 600 to 900 C) for 6 h (heating rate: 2 C min−1). This reduction process leads to the structural transformation from LDHs precursors to monometallic Co or Co-based IMCs. Finally, the as-obtained product was cooled to room temperature in a N2 stream. 4.3. Catalytic evaluation Cinnamaldehyde (1 mL), solvent (iso-PrOH, 30 mL), and catalyst (0.05 g, 0.1 mmol of Co) were placed in a stainless-steel hydrogenation reactor. The air in the reactor was thoroughly replaced by 2 MPa H2 five times. As 25


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the temperature of reactor was increased to 100 C, H2 was flowed into the reactor with a pressure of 2 MPa to trigger the hydrogenation reaction. 4.4. Characterizations Hydrogen temperature programmed reduction (H2-TPR) and hydrogen temperature programmed desorption (H2-TPD) were carried out in a quartz tube reactor on a Micromeritics Chemi-Sorb 2720 which is equipped with a thermal conductivity detector (TCD). In the case of H2-TPR program, the sample (100 mg) was sealed, followed by the introduction of a gas mixture of H2 and Ar (1:9, v/v) at 30 mL min−1. The temperature was gradually raised to 900 C at a heating rate of 10 C min−1. In the case of H2-TPD program, the sample (100 mg) was sealed and treated in the reactor in a gas mixture of H2 and Ar (1:9, v/v) at 500 C. Subsequently, Ar was flowed into the reactor at 450 C for 60 min and then cooled down to 25 C for the absorption of H2. After flushing with Ar until a stable baseline was obtained, the temperature was raised to 800 C with a heating rate of 10 C min−1. Powder XRD measurement was conducted on a Rigaku XRD-6000 diffractometer by using Cu Ka radiation (λ = 0.15418 nm) at 40 kV and 40 mA, with a scanning rate of 5 C min−1 and a 2 angle from 5° to 90°. The Zeiss Supra 55 scanning electron microscope (SEM) with an accelerating voltage of 20 kV was used to study the morphology of samples. TEM images were obtained by using a JEOL JEM-2010 high-resolution transmission electron microscopes. Co XAFS measurement was performed at the beam line 1W1B of the Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (CAS). X-ray photoelectron spectra (XPS) were performed on a Thermo VG Escalab 250 X-ray photoelectron spectrometer operated at a pressure of 2 × 10−9 Pa, using monochromatized Al Kα radiation (1486.6 eV) with a beam diameter of 100 μm. FT-IR measurements of CO adsorption as well as the adsorption of cinnamaldehyde were carried out over Bruker infrared spectrometer (Transmission cell) in the reaction cell on a VERTEX 70 spectrometer. The IR reaction cell was equipped with KBr window, installed with inlet and outlet flows, whose temperature was controlled by thermocouples. The sample (50 mg) was pressed into a self-supporting wafer (diameter: 12 mm). Typically, the sample was previously reduced in the reaction cell with H2 at 600−800 °C for 4 h. In the case of CO measurement, the sample was cooled down to room temperature in H2 to obtain a reference baseline signal, followed by the introduction of 1% CO/N2 and collection of infrared signal vs. the reference signal. For the IR measurement of cinnamaldehyde hydrogenation, the sample was cooled down to 100 °C (reaction temperature) in H2, vacuumed, and purged in He to remove the gaseous and weakly adsorbed cinnamaldehyde. The spectra of hydrogenation reaction process were collected at every minute after purging H2 (flow rate: 100 cm3 min−1). 26


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4.5. Computational method First-principles calculations based on spin polarized DFT were performed using Vienna ab-initio simulation package (VASP).49 The projector-augmented wave way and the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional50 were employed for the all calculations. The models of Co, CoGa3, and CoIn3 were constructed in accordance with the XRD standard cards, and their crystal structures are based on the experimental results: Co, Fm3m (225); CoGa3, P-4n2 (118); CoIn3, P-4n2 (118) (see details in Figure S19). A 5 × 5 × 1 k-point sampling in the surface Brillouin zone was performed for the surfaces, and a thickness of 15 Å was set to avoid interactions between the periodic slabs. A cut-off energy of 500 eV and a force threshold on each relaxed atom below 0.05 eV/Å were used. The transition states were located with a CI-NEB way. The adsorption energies (Ead) and energy barriers (Ea) are defined as: Ead = Etotal − (Eg + Eslab)

(1)

Ea =ETS – EIS

(2)

The adsorption energies (Ead) of adsorbed species was calculated from the energy difference between the optimized clean surface (Eslab) and the optimized surface containing adsorbate (Etotal) in gas state (Eg); the energy barrier was obtained from the difference between energy of transition state (ETS) and its corresponding initial state (EIS). The selectivity toward propenol or propanal was denoted by the difference between the hydrogenation barrier and the desorption barrier of propenol or propanal, defined as ΔEa. ΔEa = Ea,hydr − Ea,des = Ea,hydr − |Ead|

(3)

where Ea,hydr and Ea,des are the hydrogenation and desorption barrier of propenol or propanal, respectively. This indicates that the larger the |ΔEa| is, the larger selectivity toward propenol or propanal will be.

AUTHOR INFORMATION Corresponding Authors [email protected] (X. Zhang); [email protected] (M. Wei) ORCID 0000-0003-3559-2096(X. Zhang); 0000-0001-5257-790X (M. Wei) Notes 27


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The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information. Figures S1−S19 and Tables S1−S9. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgment This work was supported by the National Key Research and Development Program (Grant No. 2017YFA0206804), the National Natural Science Foundation of China (NSFC) and the Fundamental Research Funds for the Central Universities (buctylkxj01).

REFERENCES (1) Zhao, M.; Yuan, K.; Wang, Y.; Li, G.; Guo, J.; Gu, L.; Hu, W.; Zhao, H.; Tang, Z. Metal-Organic Frameworks as Selectivity Regulators for Hydrogenation Reactions. Nature 2016, 539, 76−80. (2) Tian, Z.; Xiang, X.; Xie, L.; Li, F. Liquid-Phase Hydrogenation of Cinnamaldehyde: Enhancing Selectivity of Supported Gold Catalysts by Incorporation of Cerium into the Support. Ind. Eng. Chem. Res. 2013, 52, 288−296. (3) Maity, P.; Yamazoe, S.; Tsukuda, T. Dendrimer-Encapsulated Copper Cluster as a Chemoselective and Regenerable Hydrogenation Catalyst. ACS Catal. 2013, 3, 182−185. (4) Kahsar, K. R.; Schwartz, D. K.; Medlin, J. W. Control of Metal Catalyst Selectivity through Specific Noncovalent Molecular Interactions. J. Am. Chem. Soc. 2014, 136, 520−526. (5) Gallezot, P.; Richard, D. Selective Hydrogenation of α,β-Unsaturated Aldehydes. Chem. Rev. 1998, 40, 81−126. (6) Bailón-García, E.; Carrasco-Marín, F.; Pérez-Cadenas, A. F.; Maldonado-Hódar, F. J. Influence of the Pretreatment Conditions on the Development and Performance of Active Sites of Pt/TiO2 Catalysts Used for the Selective Citral Hydrogenation. J. Catal. 2015, 327, 86−95. (7) Wei, W.; Zhao, Y.; Peng, S. C.; Zhang, H. Y.; Bian, Y. P.; Li, H. X.; Li, H. Hollow Ni–Co–B Amorphous Alloy Nanospheres: Facile Fabrication via Vesicle-Assisted Chemical Reduction and Their Enhanced Catalytic Performances. J. Mater. Chem. A 2014, 2, 19253−19259. (8) Wang, H.; Li, X.; Lan, X.; Wang, T. Supported Ultrafine NiCo Bimetallic Alloy Nanoparticles Derived from Bimetal-Organic Frameworks: A Highly Active Catalyst for Furfural Alcohol Hydrogenation. ACS Catal. 2018, 8, 2121−2128. (9) Bai, S.; Bu, L.; Shao, Q.; Zhu, X.; Huang, X. Multicomponent Pt-Based Zigzag Nanowires as Selectivity 28


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Selective Hydrogenation of Cinnamaldehyde over Co-Based

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