Integration of Quantum Confinement and Alloy Effect to Modulate

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Integration of Quantum Confinement and Alloy Effect to Modulate Electronic Properties of RhW Nanocrystals for Improved Catalytic Performance towards CO2 Hydrogenation Wenbo Zhang, Liangbing Wang, Haoyu Liu, Yiping Hao, Hongliang Li, Munir Ullah Khan, and Jie Zeng Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b03967 • Publication Date (Web): 05 Jan 2017 Downloaded from http://pubs.acs.org on January 6, 2017

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Revised ms # nl-2016-03967c

Integration of Quantum Confinement and Alloy Effect to Modulate Electronic Properties of RhW Nanocrystals for Improved Catalytic Performance towards CO2 Hydrogenation

Wenbo Zhang,†,|| Liangbing Wang,†,|| Haoyu Liu,† Yiping Hao,† Hongliang Li,† Munir Ullah Khan,† and Jie Zeng*,†

†

Hefei National Laboratory for Physical Sciences at the Microscale, Key Laboratory of

Strongly-Coupled Quantum Matter Physics of Chinese Academy of Sciences, Hefei Science Center & National Synchrotron Radiation Laboratory, Department of Chemical Physics University of Science and Technology of China, Hefei, Anhui 230026, P. R. China

*To whom correspondence should be addressed. E-mail: [email protected] ||

These authors contributed equally.

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Abstract The d-band center and surface negative charge density generally determine the adsorption and activation of CO2, thus serving as important descriptors of the catalytic activity towards CO2 hydrogenation. Herein, we engineered the d-band center and negative charge density of Rh-based catalysts by tuning their dimensions and introducing non-noble metals to form an alloy. During the hydrogenation of CO2 into methanol, the catalytic activity of Rh75W25 nanosheets was 5.9, 4.0, and 1.7 times as high as that of Rh nanoparticles, Rh nanosheets, and Rh73W27 nanoparticles, respectively. Mechanistic studies reveal that the remarkable activity of Rh75W25 nanosheets is owing to the integration of quantum confinement and alloy effect. Specifically, the quantum confinement in one dimension shifts up the d-band center of Rh75W25 nanosheets, strengthening the adsorption of CO2. Moreover, the alloy effect not only promotes the activation of CO2 to form CO2δ-, but also enhances the adsorption of intermediates to facilitate further hydrogenation of the intermediates into methanol.

Key words: carbon dioxide, rhodium, tungsten, charge transfer, d-band center

Table of Content

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Hydrogenation of CO2 into useful chemicals or fuels is a promising strategy that both helps reduce the amount of CO2 in the atmosphere to mitigate global warming, and alleviates the dependence on fossil fuels to meet the increasing energy demand.1-21 The chemical inertness of CO2 makes this process considerably challenging which requires catalysts to both adsorb and activate CO2 efficiently. Typical heterogeneous catalysts for CO2 hydrogenation include Pt, Au, Rh, Cu, and their alloys. For pure and alloyed transition metals, d-band center is generally regarded as a descriptor of activity for researchers to screen the optimal catalysts towards CO2 hydrogenation.22-26 Specifically, the higher d-band center relative to Fermi level leads to stronger adsorption of CO2 due to the depopulation of anti-bonding states based on theoretical studies. One example to achieve efficient conversion of CO2 is to use the Cu/CeO2, where the interface serves as the active site for CO2 activation.27 Very recently, Pt3Co octapods endowed with sharp-tip effect and alloy effect were found to be remarkably active in transforming CO2 into CO2δ-, as a result of the increased negative charge density on Pt atoms.21 These studies indicate that the catalytic performance for CO2 hydrogenation could be effectively improved by modulating the surface electronic properties of heterogeneous catalysts. Tuning the dimension of nanostructures represents an effective strategy to engineer the surface electronic properties by varying the spatial distribution of electrons. For instance, two-dimensional nanosheets generally exhibit distinct electronic properties relative to their bulk or nanoparticle counterparts.28-30 Typically, when the thickness is controlled down to a few atomic layers, the nanosheets are in the regime of strong quantum confinement, which could alter the d-band center and thus the adsorption of CO2.24 Another strategy to modulate the electronic properties is to form an alloy. In the process of alloying, charge transfer between different metals is induced, resulting in the variation of charge distribution on surface.31-33 The region with high negative charge density is found to benefit the electron transfer to the anti-bonding orbit of the adsorbed CO2, thereby promoting the CO2 activation. Herein, we combined these two strategies to tune the electronic properties of Rh-based nanocrystals in order to enhance the catalytic activity towards CO2 hydrogenation. The obtained Rh75W25 nanosheets exhibited remarkable catalytic activity with the turnover frequency (TOF) number of 592 h-1, which was 5.9, 4.0, and 1.7 times as high as that of Rh nanoparticles, Rh nanosheets, and Rh73W27 nanoparticles, respectively. Mechanistic studies reveal that the remarkable activity of Rh75W25 nanosheets derives from the integration of quantum confinement


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and alloy effect. Specifically, the quantum confinement in one dimension shifts up the d-band center of Rh75W25 nanosheets, strengthening the adsorption of CO2 relative to the nanoparticles. Moreover, the electron transfer from W to Rh in Rh75W25 nanosheets not only benefits the activation of CO2 by forming CO2δ-, but also enhances the adsorption of intermediates to facilitate further hydrogenation of the intermediates into methanol. The enhancement in the adsorption and activation of CO2 for Rh75W25 nanosheets was directly revealed by in-situ diffuse reflectance infrared Fourier transform (DRIFT) spectra. In a typical synthesis, Rh(acac)3 was dissolved in a solution containing diphenyl ether, oleylamine and oleic acid. The mixture was preheated in an oil bath at 220 oC for 5 min, followed by the addition of W(CO)6. Figure 1A shows a representative high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of the obtained ultrathin nanosheets. Most of the nanosheets took a hexagonal morphology with an average edge length of 75 nm. The thickness was directly measured to be ca. 1.8 nm, less than 8 atomic layers thick, from the image of vertical nanosheets which self-assembled into a lamellar structure (Fig. 1B). Figure 1C revealed a high-resolution TEM (HRTEM) image of an individual horizontal nanosheet along the [111] zone axis. The lattice fringe of 2.4 Å was assigned to the 1/3(422) reflection which was generally forbidden for a face-centered cubic (fcc) lattice. The corresponding fast Fourier transform (FFT) pattern (the inset of Fig. 1C) proves a six-fold symmetric structure, implying that flat faces were bounded by (111) planes. Figure 1D shows the side view of nanosheets with the distance between neighboring planes of ~3.5 nm and the thickness of 1.8 nm. To analyze the structures and chemical compositions of the nanostructure, the STEM and STEM-energy dispersive X-ray (EDX) elemental mapping images of nanosheets were shown in Figure 1E, indicating a successful preparation of RhW alloy nanosheets with homogenous distribution of both Rh and W. As revealed by the inductively coupled plasma-atomic emission spectroscopy (ICP-AES), the molar ratio of Rh:W was determined as 75:25. Moreover, the X-ray diffraction (XRD) profile of Rh75W25 nanosheets was similar to that of the standard fcc Rh without obvious peaks assigned to those of body-centered cubic (bcc) W, confirming the fcc structure type for Rh75W25 nanosheets (Fig. 1F). Notably, the (111), (200), and (220) reflections of the Rh75W25 nanosheets shifted to lower angles slightly compared with those of pure Rh, owing to an expansion of the lattice induced by the larger atomic radius of W than that of Rh. Based on Vegard’s law34 which claims the linear relation between the lattice


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parameter and composition, the composition of Rh:W was estimated to be 3:1, consistent with the ICP-AES data. For latter comparison in catalytic and electronic properties, we prepared 6-nm Rh73W27 nanoparticles and Rh ultrathin nanosheets (Figs. S1 and S2). The average lateral size and thickness of Rh nanosheets were 150 and 1.5 nm, respectively (Fig. S1). By varying the amounts of W(CO)6, Rh90W10, Rh82W18, Rh69W31, and Rh64W36 nanosheets were also synthesized (Fig. S3). To track the evolution of Rh75W25 nanosheets, we monitored the reaction by characterizing the morphology of the products by TEM and atomic force microscopy (AFM) at different reaction time. Figure S4A-D shows that the lateral size of the products increased from 5 to 137 nm with the reaction time prolonged from 10 to 60 min. As indicated by AFM, the thickness (~1.8 nm) of the products remained unchanged at different reaction time (Fig. S4E-H). Accordingly, the thickness of nanosheets could not be controlled by tuning the reaction time due to the influence of the capping agent. As such, CO deriving from the decomposition of W(CO)6 not only functioned as a reducing agent, but also served as the capping agent which was adsorbed on the (111) facets and prevented growth along the [111] direction, resulting in the unchanged thickness. This point was supported by the Fourier transform infrared (FTIR) spectra of Rh75W25 nanosheets (Fig. S5A), where the signal of CO could be obviously distinguished at 1900-2100 cm-1, indicating the adsorption of CO molecules on the surface of nanosheets predominantly in a bridge configuration. In addition, we also examined the influence of reaction temperature on the morphology of Rh75W25 nanocrystals. Figure S6A shows that the nanocrystals also took the nanosheet morphology with the thickness of ~1.8 nm at 200 oC. Increasing the temperature to 240 oC led to the formation of nanoparticles because of the accelerated reaction rate (Fig. S6B). The catalytic properties of the as-obtained ultrathin RhW nanosheets in CO2 hydrogenation were evaluated in comparison with those of Rh nanoparticles, Rh nanosheets and Rh73W27 nanoparticles. The mean size of Rh nanoparticles supported on active carbon was estimated to be 4.0 nm (Fig. S7). All of the nanocrystals were loaded on active carbon at a mass loading of 5%, and exposed to ultraviolet (UV)/ozone cleaner at 80 oC for 30 min to clean the surface. FTIR spectra of these nanosheets indicate that the previously adsorbed CO molecules were released after treatment (Fig. S5B). The blank test was performed with only active carbon added, but no product was observed. When the reaction was catalyzed by Rh nanoparticles under 32 bar of


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H2/CO2 mixed gas (H2:CO2 = 3:1) at 150 oC, about 5.0 mmol of methanol and 0.5 mmol of formaldehyde were formed after 5 h (Fig. 2A). Under the same reaction condition, Rh nanosheets exhibited slightly higher activity relative to Rh nanoparticles, with 7.4 mmol of methanol and 0.4 mmol of formaldehyde. In comparison, the reaction over Rh73W27 nanoparticles yielded 10.7 mmol of methanol and 0.4 mmol of formaldehyde. As for Rh75W25 nanosheets, the catalytic activity was significantly improved with 18.0 mmol of methanol generated in the reaction (Fig. 2A). It is worth noting that Rh75W25 nanosheets achieved extremely high selectivity of 97% for methanol due to their uniformly exposed facets. To compare the catalytic activity more accurately, we calculated the TOF numbers based on all metal atoms (denoted as TOFMetals) of these catalysts. As shown in Figure 2B, the TOFMetals numbers of Rh nanoparticles, Rh nanosheets, Rh73W27 nanoparticles, and Rh75W25 nanosheets were 100, 148, 254, and 444 h-1, respectively. Since Rh served as active sites in catalysis, we further calculated the TOF numbers by solely taking Rh atoms into account (denoted as TOFRh). Rh75W25 nanosheets were endowed with the highest TOFRh number of 592 h-1 which was 5.9, 4.0, and 1.7 times as high as that of Rh nanoparticles, Rh nanosheets, and Rh73W27 nanoparticles, respectively (Fig. 2B). Given the higher activity of nanosheets than nanoparticles, we investigated the influence of surface area and dispersion on catalysis. The ratios of surface atoms to total atoms in Rh nanoparticles, Rh nanosheets, Rh73W27 nanoparticles, and Rh75W25 nanosheets range from 20% to 33%, suggesting that the surface area does not determine the catalytic activity. Moreover, both nanoparticles and nanosheets were highly dispersed on active carbon, which indicates the negligible influence of the dispersion. In addition, we also evaluated the catalytic properties of RhW nanosheets with different ratios under 32 bar of H2/CO2 mixed gas (H2:CO2 = 3:1) at 150 oC. Figure S8 revealed a volcano-type relationship between activity and the ratio of Rh:W, with Rh75W25 nanosheets exhibiting the highest activity. The details of catalytic process using Rh75W25 nanosheets were worth further investigation by determining the product yield with time. The methanol yield amounted to almost 20.3 mmol when the reaction time was prolonged to 10 h (Fig. 2C). Furthermore, the selectivity for methanol was maintained at about 97% during the whole process of the reaction. By comparison, when Rh nanoparticles were utilized instead, only 6.7 mmol of methanol was produced together with 0.5 mmol of formaldehyde after 10 h (Fig. S9). Notably, the selectivity of Rh75W25 nanosheets was higher than that of Rh nanoparticles. Moreover, the stability of Rh75W25


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nanosheets was also explored by recycling the catalyst. After six rounds, Rh75W25 nanosheets preserved almost 96% of the original activity and 97% of the selectivity for methanol (Fig. 2D), while retaining their nanosheet morphology according to TEM images (Fig. S10). In addition, more than 95% of the metal was preserved in the reuse tests (Table S1). To rationalize the remarkable catalytic activity of Rh75W25 nanosheets towards CO2 hydrogenation, we conducted density functional theory (DFT) calculations to investigate the electronic properties of Rh and RhW nanocrystals. The models of Rh nanoparticles, Rh nanosheets, Rh73W27 nanoparticles, and Rh75W25 nanosheets were shown in Figure S11. As shown in Figure 3, A and B, both Rh nanoparticles and nanosheets underwent a weak charge redistribution. With the introduction of W atoms, electron transfer from W to Rh atoms was observed for both Rh73W27 nanoparticles and Rh75W25 nanosheets, leading to the formation of Rh atoms with the average charge of -0.2 e and thus promoting the activation of CO2 relative to pure Rh (Fig. 3, C and D). In addition to the Bader charge analysis, we also calculated the d-band centers of different nanocrystals. As shown in Figure 3E, the d-band center (with regard to Fermi level) of Rh nanosheets quantum-confined in one dimension was -1.49 eV, which was 0.09 eV higher than that of Rh nanoparticles with three-dimensional quantum confinement. As for Rh75W25 nanosheets, the d-band center shifted to -1.56 eV, but was still higher relative to that of Rh73W27 nanoparticles (Fig. 3E). It is well-established that the upshift of d-band center leads to the depopulation of the anti-bonding states, resulting in the formation of the stronger bond between the adsorbates and metal surfaces.22-26 Accordingly, one-dimensional quantum confinement shifts up the d-band center of surface atoms, thereby strengthening the adsorption of CO2 on the surface of nanosheets relative to that on nanoparticles. In comparison, alloyed nanowires with two-dimensional quantum confinement were reported to exhibit lower d-band centers which weakened the adsorption of reactants or intermediates.35-37 To experimentally analyze the electronic properties of these catalysts, we conducted X-ray photoelectron spectroscopy (XPS) measurements. As shown in Figure 4A, the binding energies of Rh 3d5/2 in Rh nanoparticles, Rh nanosheets, Rh73W27 nanoparticles, and Rh75W25 nanosheets were 307.2, 307.2, 306.6, and 306.6 eV, respectively. The negative shift of Rh 3d in both Rh73W27 nanoparticles and Rh75W25 nanosheets implies that a charge transfer occurred between Rh and W, resulting in the accumulation of negative charges on Rh atoms in RhW alloys. This result

is

consistent

with

the

Bader

charge

analysis.


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we

conducted

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temperature-programmed desorption (TPD) measurements to confirm the role played by the d-band center in the adsorption of CO2. As shown in Figure 4B, the temperatures for CO2 desorption were 181.4, 209.3, 172.0, and 187.2 oC for Rh nanoparticles, Rh nanosheets, Rh73W27 nanoparticles, and Rh75W25 nanosheets, respectively. This result indicates that the adsorption strength of CO2 increased on going from Rh73W27 nanoparticles to Rh nanoparticles, Rh75W25 nanosheets, and Rh nanosheets, in good agreement with the trend of the d-band center. During CO2 hydrogenation, surface electronic properties such as the d-band center and negative charge density play a pivotal role in determining the adsorption and activation of CO2. Specifically, the higher d-band center leads to stronger adsorption of CO2 via the depopulation of the anti-bonding states.22-26 In addition, the negatively charged surface promotes the activation of CO2 through facilitating the electron transfer from the catalyst surface to the anti-bonding orbit of CO2.21,38-44 Compared with nanoparticles, nanosheets reveal higher activity due to their higher d-band centers and thus stronger adsorption of CO2, induced by quantum confinement in one dimension. With the introduction of W, higher activity is achieved for RhW alloys than that for pure Rh, because the electron transfer from W to Rh atoms makes surface Rh atoms more negatively charged, accordingly promoting the activation of CO2. Therefore, the remarkable catalytic activity of Rh75W25 nanosheets is attributed to the integration of one-dimensional quantum confinement for shifting up d-band center and alloy effect for generating negatively charged Rh atoms. The adsorption and activation of CO2 were directly detected by conducting in-situ DRIFT measurements. Without catalysts, the spectrum of CO2 exhibited two distinct peaks at 667 and 2350 cm-1, corresponding to the flexural vibration and asymmetrical stretching vibration of CO2, respectively (Fig. S12). In the presence of Rh-based catalysts, the flexural vibration peak (667 cm-1) disappeared, along with the occurrence of a new peak at ca. 1400-1600 cm-1, indicating the formation of carboxylate (CO2δ-) species (Fig. 4C).21,45-46 Generation of CO2δ- species is generally considered as the critical step in CO2 hydrogenation.21,38-46 For comparison, we normalized the intensity of the peak assigned to the asymmetrical stretching vibration of CO2. Rh75W25 nanosheets exhibited the maximum peak intensity for CO2δ-, followed by the peaks of Rh73W27 nanoparticles, Rh nanosheets, and Rh nanoparticles (Fig. 4D). Notably, this order is consistent with that of catalytic activities indicated by both theoretical analysis and experiments. To gain further insight into the catalytic mechanism, we carried out in-situ DRIFT


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measurements after exposing Rh nanoparticles, Rh nanosheets, Rh73W27 nanoparticles, and Rh75W25 nanosheets to H2/CO2 mixed gas (H2:CO2 = 3:1, 1 bar) at 150 oC to simulate the reaction conditions. As indicated by the in-situ DRIFT spectra, besides the peaks ranging from 1500 to 1600 cm-1 for CO2δ-, the peaks at 2961, 1684, 1453, and 1047 cm-1 appeared, corresponding to the stretching vibrations of C-H, C=O, and C-O as well as the scissoring vibration of C-H for HCOO* species, respectively (Fig. 5). As such, CO2δ- was hydrogenated to HCOO* as a stable intermediate. Among all the tested catalysts, Rh75W25 nanosheets exhibited the highest ratio of the peak intensity for HCOO* to that for CO2δ-, indicating the strongest capacity to transform CO2δ- into HCOO* and thus the highest activity towards CO2 hydrogenation. In addition, the peaks assigned to the stretching vibrations of C=O and C-O for Rh73W27 nanopaticles and Rh75W25 nanosheets shifted to lower wavenumbers relative to those of Rh nanopaticles and nanosheets (Fig. 5). The red shift indicates that the adsorption of HCOO* on RhW alloys was stronger than that on pure Rh, due to the accumulation of negative charges on Rh atoms in RhW alloys. As such, the negatively charged surface not only promoted the activation of CO2 to form CO2δ-, but also enhanced the adsorption of intermediates to facilitate further hydrogenation of the intermediates into methanol. If HCOO* was to be desorbed from the surface of catalysts because of the weak adsorption, formic acid and formaldehyde would be formed. Otherwise, the adsorbed HCOO* could be further hydrogenated into methanol. As a bonus, the enhanced adsorption of intermediates inhibits the formation of by-products such as formaldehyde, benefiting the selective hydrogenation to methanol. Therefore, the alloy effect in RhW nanocrystals contributes to the improvement in both catalytic activity and selectivity for methanol, relative to pure Rh. In conclusion, we have engineered the d-band center and negative charge density of Rh-based catalysts by tuning their dimensions and introducing non-noble metals to form an alloy. The obtained Rh75W25 nanosheets were found to indeed exhibit much enhanced catalytic activity, which was 5.9, 4.0, and 1.7 times as high as that of Rh nanoparticles, Rh nanosheets and Rh73W27 nanoparticles, respectively. Based on mechanistic studies, the adsorption and activation of CO2 were enhanced for Rh75W25 nanosheets, because of their higher d-band center induced by one-dimensional quantum confinement and negatively charged Rh surface deriving from alloy effect. Moreover, the alloy effect also enhances the adsorption of intermediates to promote further hydrogenation of the intermediates into methanol. This approach not only represents a


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rational design of efficient catalysts for methanol synthesis from CO2 by preparing ultrathin alloy nanosheets, but also demonstrates potential application of manipulating electronic properties in CO2 hydrogenation. REFERENCES (1) Li, P. Z.; Wang, X. J.; Liu, J.; Lim, J. S.; Zou, R.; Zhao, Y. J. Am. Chem. Soc. 2016, 138, 2142. (2) Manzi, A.; Simon, T.; Sonnleitner, C.; Döblinger, M.; Wyrwich, R.; Stern, O.; Stolarczyk, J. K.; Feldmann, J. J. Am. Chem. Soc. 2015, 137, 14007. (3) Kajiwara, T.; Fujii, M.; Tsujimoto, M.; Kobayashi, K.; Higuchi, M.; Tanaka, K.; Kitagawa, S. Angew. Chem. Int. Ed. 2016, 55, 2697. (4) Kornienko, N.; Zhao, Y.; Kiley, C. S.; Zhu, C.; Kim, D.; Lin, S.; Chang, C. J.; Yaghi, O. M.; Yang, P. J. Am. Chem. Soc. 2015, 137, 14129. (5) Meng, X.; Wang, T.; Liu, L.; Ouyang, S.; Li, P.; Hu, H.; Kako, T.; Iwai, H.; Tanaka, A.; Ye, J. Angew. Chem. Int. Ed. 2014, 53, 11478. (6) Studt, F.; Sharafutdinov, I.; Abild-Pedersen, F.; Elkjaer, C. F.; Hummelshøj, J. S.; Dahl, S.; Chorkendorff, I.; Nørskov, J. K. Nat. Chem. 2014, 6, 320. (7) Ozin, G. A. Adv. Mater. 2015, 27, 1957. (8) Gao, S.; Lin, Y.; Jiao, X.; Sun, Y.; Luo, Q.; Zhang, W.; Li, D.; Yang, J.; Xie, Y. Nature 2016, 529, 68. (9) Li, C. S.; Melaet, G.; Ralston, W. T.; An, K.; Brooks, C.; Ye, Y. F.; Liu, Y. S.; Zhu, J.; Guo, J.; Alayoglu, S.; Somorjai, G. A. Nat. Commun. 2015, 6, 6538. (10) Hou, J.; Cheng, H.; Takeda, O.; Zhu, H. Angew. Chem. Int. Ed. 2015, 54, 8480. (11) Behrens, M. Angew. Chem. Int. Ed. 2014, 53, 12022. (12) Liu, X. H.; Ma, J. G.; Niu, Z.; Yang, G. M.; Cheng, P. Angew. Chem. Int. Ed. 2015, 54, 988. (13) Moret, S.; Dyson, P. J.; Laurenczy, G. Nat. Commun. 2014, 5, 4017. (14) Gao, D.; Zhou, H.; Wang, J.; Miao, S.; Yang, F.; Wang, G.; Wang, J.; Bao, X. J. Am. Chem. Soc. 2015, 137, 4288. (15) Xiang, Q.; Cheng, B.; Yu, J. Angew. Chem. Int. Ed. 2015, 54, 11350. (16) He, Z.; Qian, Q.; Ma, J.; Meng, Q.; Zhou, H.; Song, J.; Liu, Z.; Han, B. Angew. Chem. Int. Ed. 2016, 55, 737.


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(17) Matsubu, J. C.; Yang, V. N.; Christopher, P. J. Am. Chem. Soc. 2015, 137, 3076. (18) Zhao, Y.; Chen, G.; Bian, T.; Zhou, C.; Waterhouse, G. I. N.; Wu, L. Z.; Tung, C. H.; Smith, L. J.; O'Hare, D.; Zhang, T. Adv. Mater. 2015, 27, 7824. (19) Beinik, I.; Hellström, M.; Jensen, T. N.; Broqvist, P.; Lauritsen, J. V. Nat. Commun. 2015, 6, 8845. (20) Kim, C.; Jeon, H. S.; Eom, T.; Jee, M. S.; Kim, H.; Friend, C. M.; Min, B. K.; Hwang, Y. J. J. Am. Chem. Soc. 2015, 137, 13844. (21) Khan, M. U.; Wang, L.; Liu, Z.; Gao, Z.; Wang, S.; Li, H.; Zhang, W.; Wang, M.; Wang, Z.; Ma, C.; Zeng, J. Angew. Chem. Int. Ed. 2016, 55, 9548. (22) Hammer, B.; Nørskov, J. K. Adv. Catal. 2000, 45, 71. (23) Nilsson, A.; Pettersson, L. G. M.; Hammer, B.; Bligaard, T.; Christensen, C. H.; Nørskov, J. K. Catal. Lett. 2005, 100, 111. (24) Xiao, J.; Frauenheim, T. J. Phys. Chem. C 2013, 117, 1804. (25) Lim, D. H.; Jo, J. H.; Shin, D. Y.; Wilcox, J.; Ham, H. C.; Nam, S. W. Nanoscale 2014, 6, 5087. (26) Nørskov, J. K.; Abild-Pedersen, F.; Studt, F.; Bligaard, T. Proc. Natl Acad. Sci. USA 2011, 108, 937. (27) Graciani, J.; Mudiyanselage, K.; Xu, F.; Baber, A. E.; Evans, J.; Senanayake, S. D.; Stacchiola, D. J.; Liu, P.; Hrbek, J.; Sanz, J. F.; Rodriguez, J. A. Science 2014, 345, 546. (28) Liu, G. B.; Xiao, D.; Yao, Y.; Xu, X.; Yao, W. Chem. Soc. Rev. 2015, 44, 2643. (29) Sun, Y.; Gao, S.; Xie, Y. Chem. Soc. Rev. 2014, 43, 530. (30) Miró, P.; Audiffred, M.; Heine, T. Chem. Soc. Rev. 2014, 43, 6537. (31) Wang, L.; Zhao, S.; Liu, C.; Li, C.; Li, X.; Li, H.; Wang, Y.; Ma, C.; Li, Z.; Zeng, J. Nano Lett. 2015, 15, 2875. (32) Wang, M.; Wang, L.; Li, H.; Du, W.; Khan, M. U.; Zhao, S.; Ma, C.; Li, Z.; Zeng, J. J. Am. Chem. Soc. 2015, 137, 14027. (33) Zhang, H.; Watanabe, T.; Okumura, M.; Haruta, M.; Toshima, N. Nat. Mater. 2012, 11, 49. (34) Denton, A. R.; Ashcroft, N. W. Phys. Rev. A 1991, 43, 3161. (35) Zhu, H.; Zhang, S.; Guo, S.; Su, D.; Sun, S. J. Am. Chem. Soc. 2013, 135, 7130. (36) Liu, W.; Rodriguez, P.; Borchardt, L.; Foelske, A.; Yuan, J.; Herrmann, A. K.; Geiger, D.; Zheng, Z.; Kaskel, S.; Gaponik, N.; Kötz, R.; Schmidt, T. J.; Eychmüller, A. Angew. Chem.


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Int. Ed. 2013, 52, 9849. (37) Li, H. H.; Zhao, S.; Gong, M.; Cui, C. H.; He, D.; Liang, H. W.; Wu, L.; Yu, S. H. Angew. Chem. Int. Ed. 2013, 52, 7472. (38) Li, Y.; Chan, S. H.; Sun, Q. Nanoscale 2015, 7, 8663. (39) Solymosi, F. J. Mol. Catal. 1991, 65, 337. (40) Dietz, L.; Piccinin, S.; Maestri, M. J. Phys. Chem. C 2015, 119, 4959. (41) Pacansky, J.; Wahlgren, U.; Bagus, P. S. J. Chem. Phys. 1975, 62, 2740. (42) Compton, R. N.; Reinhardt, P. W.; Cooper, C. D. J. Chem. Phys. 1975, 63, 3821. (43) Sommerfeld, T.; Meyer, H. D.; Cederbaum, L. S. Phys. Chem. Chem. Phys. 2004, 6, 42. (44) Freund, H. J.; Roberts, M. W. Surf. Sci. Rep. 1996, 25, 225. (45) Seiferth, O.; Wolter, K.; Dillmann, B.; Klivenyi, G.; Freund, H. J.; Scarano, D.; Zecchina, A. Surf. Sci. 1999, 421, 176. (46) Mudiyanselage, K.; Senanayake, S. D.; Feria, L.; Kundu, S.; Baber, A. E.; Graciani, J.; Vidal, A. B.; Agnoli, S.; Evans, J.; Chang, R.; Axnanda, S.; Liu, Z.; Sanz, J. F.; Liu, P.; Rodriguez, J. A.; Stacchiola, D. J. Angew. Chem. Int. Ed. 2013, 52, 5101.

ASSOCIATED CONTENT Supporting Information. Experimental details, TEM and AFM images, FTIR spectra, products for CO2 hydrogenation over RhW nanosheets with different ratios, comparison of TOFMetals and TOFRh for RhW nanosheets with different ratios, time course of CO2 hydrogenation catalyzed by Rh nanoparticles, carbon balance and residual metal, atomic structures, In-situ DRIFT spectrum of CO2. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions W.Z. and L.W. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS [**]This work was supported by Collaborative Innovation Center of Suzhou Nano Science and Technology, MOST of China (2014CB932700), NSFC under Grant Nos. 21573206 and 51371164, Key Research Program of Frontier Sciences, CAS (QYZDB-SSW-SLH017), Strategic Priority Research Program B of the CAS under Grant No. XDB01020000, Hefei Science Center CAS (2015HSC-UP016), and Fundamental Research Funds for the Central Universities.


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Figure 1. (A, B) HAADF-STEM images of Rh75W25 ultrathin nanosheets. The inset in panel B shows the thickness distribution of Rh75W25 nanosheets. (C, D) HRTEM images of Rh75W25 nanosheets. The inset in panel C shows its corresponding FFT pattern. (E) STEM image and STEM-EDX elemental mapping images of Rh75W25 nanosheets. Green corresponds to W while red corresponds to Rh. (F) XRD pattern of Rh75W25 nanosheets.


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Figure 2. (A) Products of CO2 hydrogenation over Rh nanoparticles, Rh nanosheets, Rh73W27 nanoparticles, and Rh75W25 nanosheets at 150 oC after 5 h. (B) Comparison of TOFMetals and TOFRh for different catalysts. (C) Time course of the CO2 hydrogenation catalyzed by Rh75W25 nanosheets at 150 oC. (D) Relative activity and selectivity of Rh75W25 nanosheets over the course of six rounds of successive reaction. Error bars represent standard deviation from three independent measurements. .


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Figure 3. (A-D) Bader charge analyses of Rh nanoparticles, Rh nanosheets, Rh73W27 nanoparticles, and Rh75W25 nanosheets, respectively. (E) Projected d-density of states of surface atoms in different nanocrystals. The calculated d-band centers are marked with white lines.


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Figure 4. (A) XPS spectra of Rh nanoparticles, Rh nanosheets, Rh73W27 nanoparticles, and Rh75W25 nanosheets. (B) CO2-TPD profiles of different nanocrystals. (C-D) In-situ DRIFT spectra of different nanocrystals. In-situ DRIFT spectra were obtained after the treatment of the samples with CO2 at 150 oC.


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Figure 5. In-situ DRIFT spectra of Rh nanoparticles, Rh nanosheets, Rh73W27 nanoparticles, and Rh75W25 nanosheets after the treatment with H2/CO2 mixed gas (H2:CO2 = 3:1, 1 bar) at 150 oC.


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Integration of Quantum Confinement and Alloy Effect to Modulate

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