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Surface Composition Control of the Binary Au–Ag Catalyst for Enhanced Oxidant-free Dehydrogenation Jianwei Zheng, Jin Qu, Haiqiang Lin, Qian Zhang, Xiang Yuan, Yanhui Yang, and Youzhu Yuan ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01348 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on September 1, 2016

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Surface Composition Control of the Binary Au–Ag Catalyst for Enhanced Oxidant-free Dehydrogenation Jianwei Zhenga,b, Jin Quc, Haiqiang Lina,b, Qian Zhangb, Xiang Yuanb, Yanhui Yangb*, Youzhu Yuana* a

Department of Chemistry, College of Chemistry and Chemical Engineering, State Key

Laboratory of Physical Chemistry for Solid Surfaces, iChEM, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Xiamen University, Xiamen 361005, China b

School of Chemical and Biomedical Engineering, Nanyang Technological University,

Singapore 637459, Singapore c

Centre for Computational Chemistry and Research Institute of Industrial Catalysis, East

China University of Science and Technology, Meilong Road 130, Shanghai 200237, China

1 ACS Paragon Plus Environment

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ABSTRACT: Effectively controlling surface composition of bimetallic catalysts is challenging, yet important as synergistic effect between two metals plays vital roles in heterogeneous catalysis. This study involves methods of loading control, thermal activation, and selective surface etching to modify the bimetallic surface on the Au–Ag/SBA-15 catalyst and gains insight on the nature of the promotional effect of Ag modification over Au catalysts. The binary Au–Ag/SBA-15 catalyst prepared following a stepwise metal-loading procedure is found for the first time to be active in benzyl alcohol dehydrogenation without oxidant and hydrogen acceptor. A direct correlation is established between the surface compositions of the Au–Ag nanoparticles and their intrinsic catalytic activities. Au–Ag element analytical techniques, including X-ray fluorescence, X-ray photoelectron spectroscopy, and high-sensitivity low-energy ion scattering spectroscopy, are applied to acquire compositional information on bulk and surface. As a result, the pronounced improvement of Au catalyst by Ag is from the ensemble with a specific surface composition rather than the bulk composition. The synergy between Au and Ag for benzyl alcohol dehydrogenation is the most pronounced in the case of the Au–Ag surface compositions of 4:1, corresponding to the prepared Au1−Ag0.111 catalyst. The electronic promoting effect is suggested as the natural origin of compositional enhancement by differentiating it from the structural effect. This work demonstrates the importance of surface composition control at an atomic level in developing highly efficient and multi-component catalysts. KEYWORDS: Gold; Silver; Alloy; Dehydrogenation; Benzyl alcohol; Composition


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INTRODUCTION

Au nanoparticles have attracted significant attentions in heterogeneous catalysis due to their superior catalytic performances in various reactions.1–7 A number of explanations to address the catalytic activities of the Au nanoparticles have been proposed such as quantum size effects, charge transfer, support-induced strain, oxygen spillover to or from the support, low coordinated Au atoms, and the synergy between Au and heteroatom.2–7 Synthesizing Au-based nanoparticles with control over composition and structure, and affording potential heterogeneous catalysts are emerging based on the well-developed nanotechnology science and technology. Adding a second metal promoter to the metallic catalysts often leads to diverse properties that emerge with variable combinations, renders to the pronounced improvement of catalytic performances.8–12 Constructing Au-contained alloy nanoparticles improves the catalytic performance (i.e., activity, selectivity, and stability) of Au catalysts.4,7–9 Mou et al. attempted the Au–Ag alloy catalyst in CO oxidation and selective acetylene hydrogenation, showing a better catalytic activity compared to the Au monometallic catalyst.8,13 The Au–Cu alloy catalyst displayed superior activity and selectivity in aerobic alcohol oxidation due to the strong synergistic interaction between Au and Cu.14 Our previous work has revealed that the Au–Ag/SBA-15 catalyst catalyzed the chemoselective hydrogenation of dimethyl oxalate to methyl glycolate or crotonaldehyde to crotyl alcohol at very low reaction temperatures, while the monometallic Au/SBA-15 and Ag/SBA-15 catalysts were almost inactive under the identical conditions. Considerably reduced activation energy barriers indicated that unique active sites were formed to efficiently activate the substrate molecules on the Au–Ag


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alloy nanoparticle surfaces.15,16 One of the most essential steps in synthetic chemistry is the dehydrogenation of alcohols to the corresponding carbonyl compounds.17–23 Oxidative dehydrogenation is the conventional pathway because conversion can be significantly improved by introducing the hydrogen acceptor. Nevertheless, using oxidant (e.g., molecular oxygen, dichromate, or permanganate) as the hydrogen acceptor inevitably causes economic or environmental problems. Therefore, the oxidant-free dehydrogenation route is proposed to avoid the consumption of hydrogen acceptor and the formation of oxidative derivants and provides a promising path for the H2 feedstock supply and chemical storage of hydrogen energy. Ru/γ-Al2O3 was the first reported catalyst for the catalytic dehydrogenation of alcohols to carbonyl compounds without a hydrogen acceptor.24 Kaneda et al. reported that hydrotalcite-supported silver nanoparticles (Ag/HT) were highly effective in acceptor-free dehydrogenation of a wide range of alcohols.20 The Ag/HT catalyst can be reused without any activity and selectivity loss. Evidence clearly indicated that molecular hydrogen could easily be released from the Ag surface because Ag nanoparticles only weakly absorbed hydrogen. The catalytic performances of Au nanoparticles on various supports have been studied for the oxidant-free dehydrogenation of benzyl alcohol to benzaldehyde and hydrogen.25,26 The hydrotalcite support with both strong acidity and basicity afforded the best catalytic activities. The basic sites facilitated the alkoxide intermediate formation. The Brønsted acid sites participated in the transformation of Au hydride formed in the β–H elimination step to molecular hydrogen. A Cu/MgO catalyst with 5 wt. % Cu loading exhibited a remarkable performance with 98% conversion and 97% selectivity for benzyl


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alcohol dehydrogenation to benzaldehyde without hydrogen acceptor.27 The basic sites on MgO can act as nucleophilic active centers and abstract proton from alcohol to form a negatively charged alkoxide intermediate. Most of the previous studies require using acid or basic support, such as MgO, Al2O3, and HT, to complete the benzyl alcohol dehydrogenation. In this study, Au−Ag nanoalloy catalysts supported on inert SBA-15 were attempted to catalyze the dehydrogenation of benzyl alcohol to benzaldehyde and hydrogen in the absence of hydrogen acceptor and base additive. The composition of bimetallic surface was controlled by metal loading, selective leaching, and thermal treatment. High-sensitivity low-energy ion scattering (HS-LEIS) was employed to characterize the surface composition and distribution of Au and Ag on bimetallic nanoparticles. Efforts have also been made to investigate the relationship between the surface compositions of bimetallic nanoparticles and catalytic performance. The synergistic interaction between Au and Ag was also validated in formic acid dehydrogenation.

Experimental Section

2.1. Catalyst Preparation The binary Au−Ag catalysts in the present study were prepared followed the sequential adsorption-reduction method with the aid of NaBH4 solution, which was freshly prepared with ice deionized water.8,13,15 Briefly, Salt solutions of Au3+ (HAuCl4 ⋅ 3H2O, Aldrich) was dissolved in deionized water in the presence of a defined amount of functionalized SBA-15 material followed by the reduction with NaBH4 solution; the suspension was continuously


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stirred for 15 minutes at room temperature, filtered and washed till no Cl– was detected. The obtained solid was then added to silver nitrate (Sigma-Aldrich) solution. Repeat the step of NaBH4 reduction. The received powder was air-calcined at 873 K for 6 h to remove the surfactants followed by reduction under H2 atmosphere at 623 K for 4 h to afford the Aux−Agy/SBA-15 catalysts, where x and y denotes the atomic ratio of Au and Ag, respectively. The loading of Au was fixed to 4 wt. %. Monometallic catalysts Au/SBA-15, Ag/SBA-15, binary catalysts Au−Pd/SBA-15, Au−Pt/SBA-15, and Au−Cu/SBA-15 were prepared in an atomic ratio of 1:0.111 via the similar procedure described above. Au1−Ag0.333/SBA-15 was soaked in diluted nitric acid (37 wt. %, Sigma) for 12 h, subsequently filtered and washed with deionized water until the pH was ca. 7, transferred to the vacuum oven at 323 K and evacuated it overnight. The remaining powder denoted as naked Au. Naked Au was activation under H2 atmosphere at 423 K, 623 K, and 823 K to afford NAu-T (NAu-423, NAu-623, and NAu-823). 2.2. Activity test In a general kinetic activity test of benzyl alcohol dehydrogenation, 5 mL of p-xylene (Sigma), 108 mg of the substrate alcohol (benzyl alcohol, Sigma) and 0.5 mmol of dodecane (Sigma) as the internal standard were put in a three-neck round-bottomed vessel fitted with a reflux condenser. The prereduced catalyst was added to the flask when the reaction temperature reached 393 K. One of the remaining ports served as an inlet for the argon purge, while the third served as a sampling port. The assembly was placed in a paraffin oil bath. Argon was bubbled at a constant flow rate of 10 mL/min and the reaction mixture was vigorously stirred. The reaction was thus carried out under


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semibatch conditions. The outlet gas was connected to a gas chromatograph equided with a six-way valve and a thermal conductivity detector. Liquid samples of the reaction mixture were withdrawed by a microsyringe at regular intervals of time. The products were analyzed using a gas chromatograph (Agilent GC-7890) packed with a HP-5 capillary column (30 m × 0.32 mm × 0.25 µm) and a flame ionization detector with a relative standard deviation (RSD) of less than 2%. The RSD of analysis data for each sampling was less than 3%. The products were also collected and confirmed using a 7890GC−5975MS system. In all tests, no obvious evidence of any over-oxidation product was found; only trace amounts of byproducts were detected with a high benzyl aldehyde selectivity (>99%). For formic acid dehydrogenation, 1.84 g of sodium formate was dissolved in 5 mL of deionized water and the solution was then added into a glass flask loaded with 0.074 g of powder catalyst. Prior to starting the dehydrogenation, 2.16 g of formic acid was introduced into the flask and the mixture was kept vigorously stirring at 363 K. The volume of gas product was obtained using the volume of exhausted water via drainage method. A trace amount of CO was detected in all cases.

RESULTS AND DISCUSSION

For Au-based bimetallic catalysts, Pt, Pd, Cu, and Ag elements were primarily employed to modify the properties of Au. Au–Pt, Au–Pd, Au–Cu, and Au–Ag catalysts were extensively explored for their broad range of catalytic reactions.9,12–14 Figure S1 compares their catalytic activities in benzyl alcohol dehydrogenation. The binary Au–Ag catalyst is highly active, whereas Au–Pd, Au–Pt, and Au–Cu exhibit the inferior activity in spite of


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having similar crystallite sizes (Figure S2). Ag and Au possess remarkably similar lattice constants (JCPDS 4-0784 and JCPDS 4-0783, respectively) and are completely miscible over a wide range of composition,28 therefore, single-phase alloy with any desired composition can be readily synthesized.29 It has been reported that alloying Au with Ag benefited the synergetic interaction between Au and Ag, which played vital roles in CO oxidation8, chemoselective hydrogenation of esters15, oxidative coupling of methanol30. In this work, synergy between Au and Ag was also found important in dehydrogenation of benzyl alcohol. Figure 1 shows benzyl alcohol dehydrogenation over the Au1−Ag0.111 catalyst in the absence of oxidant. Benzyl alcohol can be completely converted using this Ag-modified Au catalyst after 8 h of reaction. Benzyl aldehyde is the dominant dehydrogenation products with selectivity over 99% (Figure 1a). In addition, H2 is detected in the outlet gas and the amount increases as the dehydrogenation proceeds. The monometallic Au catalyst cannot complete the reaction even after 15 h, and pure Ag is inactive (Figure 1b). Control experiment shows that mechanical mixing of pure Au and Ag catalysts does not afford the superior catalytic activity of the Au−Ag bimetallic catalyst. These results demonstrate that the Au and Ag combination can significantly promote the direct benzyl alcohol dehydrogenation, the proximity of Ag to Au is essential to afford the excellent catalytic performance in dehydrogenation. Furthermore, the active site of the Au−Ag bimetallic catalyst should be distinguished from that of the monometallic catalysts. The catalytic activity (shown as the benzyl alcohol yield of bimetallic catalyst normalized to that of the unpromoted monometallic Au catalyst after 3 h reaction) in the


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benzyl alcohol dehydrogenation is compared by changing the Ag content in the mixed Au–Ag alloy catalysts (Figure 1c). The comparison shows the dramatic activity increase of 85% as a result of promoting with 0.06 wt. % Ag (Au1−Ag0.028). The best activity is observed with nearly 0.24 wt. % Ag content (Au1−Ag0.111), which is 142% relative to the Au monometallic catalyst. Further increasing the Ag loading leads to the decreased activity, e.g., the 2.1 wt. % Ag sample (Au1−Ag1). This content dependence on the catalytic activity can be inferred based on the modified structural and electronic analysis of the crystallite surface derived from the variable compositions. The TEM images (Figure S3) reveal the variations in crystal structure of the binary Au–Ag catalysts. Figure 1d, S4 show the statistical results, including crystallite sizes and metal dispersions. The Ag concentration in the binary catalysts is identified as a critical parameter that influences the crystallite size, especially in the range of 0.34–2.1 wt. %. The crystallite size notably decreases, and the metal dispersion gradually increases because of the inverse relationship with the crystallite size. Ag may act as a geometric spacer isolating and stabilizing the Au crystallites. From a comprehensive point of view, adding Ag results in the atomically uniform distribution of Ag in Au catalysts, which stabilizes Au with higher dispersion, affording more active sites to promote the catalytic benzyl alcohol dehydrogenation. The intrinsic activities are calculated by dividing the benzyl aldehyde yield to the amount of active surface atoms (Figure 1c). The intrinsic activities show a typical volcano-type trend when increasing the Ag contents. The sample with the optimum Ag content (i.e., 0.24 wt. %) possesses the highest intrinsic activities (94 h−1). The intrinsic


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activities decrease when the Ag content is higher than 0.24 wt. % and show the value even poorer than that of pure Au in the presence of excess amounts of Ag promoters. The activity of SBA-15-supported bimetallic Au−Ag alloy is superior to that of reported catalyst such as Ag/Al2O3 (~10 h−1), Cu/HT (< 10 h−1), and Pd/HT (36 h−1) for the oxidant-free dehydrogenation of benzyl alcohol.20,23,31 In these studies, acidic or basic supports are necessary to enhance the activity of monometallic catalysts, which are considered to participate the dehydrogenation. In this work, dehydrogenation of benzyl alcohol can be carried out without acidic or basic support using bimetallic alloy, implying the synergistic effect in this Au−Ag alloy catalyst. The Au−Ag catalysts with Ag = 0.06–0.24 wt. % successfully afford relatively high dispersion with excellent intrinsic activities. The geometrical nanostructuring effect may be related to this improvement to some extent. However, their intrinsic activities do not improve for the Ag-rich catalysts (Ag = 0.75–2.1 wt. %) even though the Au contents are similar and the dispersions are reasonably high. Therefore, the structural analysis of the geometrical effect cannot explain this phenomenon well. Ag-doping to the fixed-content Au catalyst increases the total metallic amount and dispersion along with the change on the surface properties, e.g., composition and local connectivity. These modifications may directly change the active sites and electronic properties. Such doping is facilitated for Ag species to atomically distribute in the binary metallic crystallites, replacing Au atom from the surface, where dehydrogenation occurs. Excess depletion of Au from surface is not beneficial for the intrinsic activities, which may be caused by the low activity of Ag atoms and the decrease of surface Au active sites. Considering that the appropriate amount of Ag


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substitution improves the intrinsic activity and metal dispersion, the importance of the surface composition is apparent in the most active sample having 0.24 wt. % of Ag. The surface composition may be dominant in controlling the catalytic activities relative to the nanostructured effect. Therefore, the relationship between the surface composition and the catalytic performance will be explored by varying the surface composition in addition to controlling the bulk loading of Au and Ag. Figure 2 shows the diffuse reflectance UV–vis spectra of Au nanocrystallites with varying amounts of deposited Ag. The samples are visualized by the color change from red brown to orange by Ag doping. The Ag deposition on the Au surface in the UV–vis spectra is characterized by the blue shift of the single plasmon absorption (SPR) bands between 529 and 426 nm. Accordingly, the positions of these SPR bands follow a linear rule as a function of the Ag contents (Figure S5), suggesting the alloying of Ag and Au species. The alloying affords the proximal interaction for the synergistic enhancement, as proven in Figure 1b. The analytical techniques, such as XRF, XPS, and HS-LEIS (Figure 2b) are employed to analyze both surface and bulk compositions. Table 1 shows the assignment of the surface segregation of Ag atoms on the Au–Ag nanoalloy particles. The catalyst surface is Ag-rich relative to Au species, e.g., the catalyst containing a bulk Ag/Au atomic ratio of 1:1 has a surface Ag/Au atomic ratio of 1.4:1 obtained from XPS analysis. The XPS characterization is capable of surface elemental analysis but not able to distinguish the first layer from the subsurface layers. In this study, the outmost surface information about the Au–Ag catalysts is obtained by performing HS-LEIS to track the surface compositions. Interestingly, only the signal of silica can be detected, implying that the nanoparticles are confined in the SBA-15 mesopores.


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The confinement also causes the decrease of BET surface area and mean pore size of SBA-15 (Table S1). The signals of Au and Ag species appear after sputtering the silica shell. The statistical results indicate that Ag is more concentrated on the nanocrystallite surface than on its subsurface. The surface energy of Au (97 meV A–2) is higher than that of Ag (78 meV A–2), which favors the Ag surface enrichment. Meanwhile, the metal–metal bond strengths are in the order of Au–Au > Au–Ag > Ag–Ag, which favors the Au core enrichment.32 Therefore, controlling the thermodynamic stability of the Au–Ag nanoparticles is possible in accomplishing the surface composition control. Suntivich et al.33 confirmed that thermal treatment can control the Au–Pt nanoparticles to attain surface Au enrichment by taking advantage of the difference in the free surface and binding energies with various adsorbates. Previous studies8,34 have shown that the Au–Ag alloys reduced at different temperatures showed different activities in CO oxidation, which might have originated from the surface structure and composition change. It is speculated that the promoting effect in this particular Au−Ag bimetallic catalyst comes from geometrical nanostructuring and electronic interaction. Combined with the fact that the compositional series of the Au–Ag nanoalloy exhibits the activities in a volcanic-type trend, the surface status involving the number of active sites and the coordination environments of the Au atoms on the surface of the nanoalloy particles should shed light on the promoting effect of Au−Ag catalysts in benzyl alcohol dehydrogenation. The surface Ag enrichment can be further achieved by heat-treating calcined Au1−Ag0.111/SBA-15 under H2 atmosphere at different temperatures. Their activities in


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benzyl alcohol dehydrogenation are subsequently compared (Figure 3a). The intrinsic activities show remarkable change and track a volcano-type trend as a function of the activation temperature and the variation of intrinsic activities as a function of the metallic composition. Crystallites with the same bulk composition have similar particle sizes because of the high stability to inhibit the aggregation of the Au–Ag bimetallic precursor after harsh treatment during calcinations at 873 K for 6 h. However, the intrinsic activities of the Au1−Ag0.111 catalysts activated at different temperatures show substantial differences in spite of their similar sizes and bulk compositions. Therefore, ruling out the geometric effect, a correlation can be made between the activated temperature and the surface composition. Surface composition is assumed as the main difference among these catalysts, which causes the changes in activity. A systematic examination of their composition changes on the metallic surface is conducted using the UV–vis absorption spectra of the alloyed particles activated at different temperatures (Figure 3b). In general, both the bulk structure and the surface composition will determine the SPR band position and the plasmon resonance width when the nanoparticles are composed of two different metallic elements. In this study, the analogous width of the plasmon resonance bands is observed due to their similar structure and bulk composition. They also rule out the bulk structure effect on the SPR band position despite the same precursor. Therefore, the blue shifts of the SPR bands due to the surface reconstruction indicate the Ag enrichment with increased activation temperatures. The HS-LEIS results also show that the Ag concentration increases with the activation temperature (Figure 3c). The unusual activity of Au1−Ag0.111 with a fixed bulk composition in benzyl alcohol under different reduction treatments implies that the Ag enrichment


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leading to the consequent surface composition tuning is the origin of the activity changes when the bulk composition is fixed. Notably, Au1−Ag0.111 activated at 623 K affords the highest intrinsic activity among all samples with different bulk compositions activated at 623 K. The Au1−Ag0.111 precursors with similar bulk compositions are activated at other temperatures. In both cases, their intrinsic activities change in similar volcano-type trends, suggesting that Au1−Ag0.111 activated at 623 K should have the optimized surface composition for benzyl alcohol dehydrogenation if the internal domain of the active Au−Ag nanocrystallite only slightly affects the dehydrogenation. Further investigation is conducted to provide experimental evidence on the surface composition-dependent activity by employing both selective chemical etching and thermal activation to modify the surface composition. The Au1−Ag0.333 powder is soaked in diluted nitric acid (37 wt. %, Sigma) to remove Ag species from alloy surfaces (Figure 4). The obtained powder, denoted as naked Au, is thermally treated to emerge Ag back to the NAu-T surfaces (T represents the heat-treating temperature). The XRF analysis proves that 0.17 wt. % of Ag species remains in the naked Au. The XPS and HS-LEIS are combined to distinguish the location of these remaining Ag species and the results are presented in Figure 5a. No Ag signal is observed from the HS-LEIS spectrum, while it appears in the XPS spectrum, evidencing that the Ag species stay in the subsurface of the naked Au catalyst after being soaked in nitric acid. Thermal-treating compromises the blue shift of the SPR bands on another set of NAu-T catalysts (Figure 5b), implying that the Ag atoms embedded in the internal domain/subsurface of particles successfully migrate to the surfaces. The higher the treatment temperature, the more Ag is enriched on surfaces.


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The activity tests of the benzyl alcohol dehydrogenation show that the conversion (Figure S6) continuously increases with the thermal treatment temperature. The removal of surface Ag species leads to the inferior dehydrogenation activity as evidenced in Figure 4. Noting that naked Au possesses a slightly poorer intrinsic activity than pure Au (36 vs. 43) due to the incomplete removal of Ag species from the subsurface layers by surface etching, which may cause the inferior activity. Nonetheless, the catalytic performance of the naked Au catalyst is distinctly enhanced after thermal treatment. There results demonstrate that Ag species on outer surface is more beneficial than the subsurface one. The intrinsic activity as a function of the surface atomic Ag and Au ratio is summarized in Figure 6. The activities change in the volcano-type trend with the ratio increase despite different treatments. However, the intrinsic activity of these catalysts cannot be higher than that of the Au1−Ag0.111 catalyst with the optimum surface composition (Au:Ag = 1:0.25). These experimental evidence have further proven the idea that the surface composition-dependent activity actually functions and demonstrates the ensemble-specific effect. The catalytic active site should be changed to the Au–Ag ensemble form, showing remarkably enhanced intrinsic activity in benzyl alcohol dehydrogenation after introducing the inactive Ag species. Valden et al. have reported that Au crystallite in nanosize showed a quantum effect which can change the band structure of Au catalyst.35 In this study, the metal crystallite size clearly decreases with increasing Ag content (from 4 nm to ~2.2 nm, Figure S4), therefore, the band structure would change to some extent accompanying such a variation in metal particle size. Taking into account the experimental results, the delicate electronic behavior guided by the surface Au–Ag composition and quantum effect is the primary factor determining the


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catalytic activity in this particular reaction (Figure 6). There is an agreement of that the catalytic evolution of the surface active sites is correlated to the surface electronic structure for the bimetallic catalysis. The surface electronic behavior affects the surface adsorption properties. Several bimetallic models for the active sites involving the “synergistic effect” have been suggested for the bimetallic catalysis in redox reactions. Xu et al. observed a strong electronic interaction on the Pt-deposited Au particle catalyst.36 They suggested that the synergy between the Pt and small Au nanoparticles was on their division and collaboration later. In their study, the Pt sites functioned to activate the H2 molecules, whereas the Au sites served to activate the unsaturated substrate molecules by chemisorptions.9 Tsukuda et al.37 revealed a correlation between the enhancement of the catalytic performance by Ag doping and the charge of Au sites in the Au–Ag alloy clusters. They inferred an active site model, wherein the Au sites with anionic character played an essential role in aerobic oxidation reactions. Along with these studies, the adsorption on the Ag-doping Au catalysts is analyzed using optical IR and UV–vis characterizations in this study (Figure S7). Unlike pure Ag, adsorbed bands at ~1660 cm−1 in IR and ~288 nm in the UV–vis spectrum emerge on pure Au and Au1−Ag0.111 catalysts. They correspond to the C=O vibration peak and n–π transition of benzyl aldehyde respectively, indicating that the Au atoms play a vital role in transferring benzyl alcohol into benzyl aldehyde, whereas the Ag atoms lack the capability. By comparing the electronegativity properties of Au and Ag, the Ag atoms should show electronically poor activity, function as electrophilic or Lewis acidic sites, and bring in the intensification of polarizing the C=O bond via the electron lone pair in oxygen. The


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calculation by density functional theory (DFT) (Figure S8, Table 2) is attempted to compare the strength of the interaction between the reaction species and the catalysts. The benzyl alcohol adsorption on the Au–Ag surface is stronger than that on the pure Au surface (−0.23 vs. −0.13 eV), indicating that the Ag-doped Au surface facilitates the reactant adsorption via the interaction of Ag atom on the metallic surface and oxygen atom in the benzyl alcohol. The binary Au–Ag surface shows weaker adsorption energy to the product than the pure Au surface, suggesting that the product can desorb from the Au–Ag surface more easily. Two kinds of formation mechanisms have been proposed for the benzyl alcohol dehydrogenation, namely, metal-alcohoxide38–41 and alkoxide intermediate.27,39,42,43 The former easily generates benzyl aldehyde accompanied with other byproducts such as benzene and toluene. However, the byproduct formation is negligible in this study, therefore, the benzyl alcohol dehydrogenation via an alkoxide intermediate over the Au–Ag alloy is proposed. Figure 7 shows the reaction pathway for direct benzyl alcohol dehydrogenation. The functional group -OH of benzyl alcohol adsorbs on the catalyst surface (i.e., active sites), and the absorbed alcohol is in the form of a negatively charged alkoxide intermediate. The presence of alloy sites is beneficial for substrate adsorption because the electric potential is present between two elements in the alloy sites. The cleavage of the O–H bond generates a surface alkoxy group on the Ag atom site. Subsequently, the adsorbed alkoxy group undergoes the H elimination step, where a benzyl aldehyde molecule is released. Finally, the adsorbed hydrogen desorbs from the metallic ensemble sites and molecular hydrogen is released. The Au–Ag ensembles with specific compositions on the surface affords the


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charge-flow channels for the catalytic benzyl alcohol dehydrogenation. The role of Ag is to provide electronic interchange with Au sites and, in return, create a new reaction pathway for benzyl alcohol dehydrogenation at high rates. The results of this study suggest that the property of the surface active sites can be tuned by the composition-induced promoter, probably for intrinsic reasons of efficient charge transfer during the chemical process. Therefore, synergistic effect of Au−Ag alloy shows great potentials for the dehydrogenation of substrates containing alcohol group. The Ag promotion to the Au catalyst in formic acid dehydrogenation is tested to further validate the speculation (Figure S9). A similar synergy of the Au–Ag alloy is found because the Au–Ag performance is twice higher than that of the pure Au catalyst. The pure Ag catalyst also shows poor activity, which is exactly the same as that in benzyl alcohol dehydrogenation.

CONCLUSIONS

Bimetallic Au–Ag alloy nanoparticles deposited on mesoporous SBA-15 over a wide range of compositions were synthesized. These Au−Ag/SBA-15 catalysts showed high reactivity for direct benzyl alcohol dehydrogenation without using acidic or basic support in the absence of oxidants and base. Three strategies were invoked to control the surface composition of binary Au−Ag nanoparticles: the bulk content, thermal activation, and selective surface etching. The Au–Ag composition on the surface was directly observed to contribute to the anaerobic benzyl alcohol dehydrogenation activity rather than that in the bulk. The intrinsic activities were remarkably enhanced when the Ag atoms partially substituted the Au sites on the metallic surfaces. In addition, subsurface Ag species was


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proven to be detrimental to the reactivity of surface Au. The geometric nanostructuring and electronic alloying effects caused by the surface compositional change were considered for the pronounced change of intrinsic activity, where the latter may play a dominant role. Alloying electronically modified Au and Ag species, which promoted the charge transfer between Au and Ag, facilitated the benzyl alcohol adsorption and hydrogen species formation.

s

ASSOCIATED CONTENT Supporting Information.

Experimental section, activity, TEM, size, dispersion, SPR band statistic, UV−vis and computational models of some catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Authors: *(YH. Y.) Phone: +65-6316 8940. Fax: +65 6794 7553. E-mail: [email protected]; * (YZ. Y.) Phone: +86-592-2181659. Fax: +86 592 2183047. E-mail: [email protected].

ACKNOWLEDGEMENTS

We grateful acknowledge Dr. Yanping Zheng for the HS-LEIS tests and the financial support from the Natural Science Foundation of China (21303141, 21403178, 21473145 and 21503173), the Postgraduate Basic Innovative Research Program of Xiamen University


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(201412G001), and the Program for Innovative Research Team in Chinese Universities (No. IRT_14R31); the Funding from the National Research Foundation, Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise program.

REFERENCES

1. Johnston, P.; Carthey, N.; Hutchings, G. J. J. Am. Chem. Soc. 2015, 137, 14548–14557. 2. Hayashi, T.; Tanaka, K.; Haruta, M. J. Catal. 1998, 178, 566–575. 3. Lopez-Acevedo, O.; Kacprzak, K. A.; Akola, J.; Hakkinen, H. Nat. Chem. 2010, 2, 329–324. 4. Wang, Y.; Zheng, J. M.; Fan, K. N.; Dai, W. L. Green Chem. 2011, 13, 1644–1647. 5. Mavrikakis, M.; Hammer, B.; Nørskov, J. K. Phys. Rev. Let. 1998, 81, 2819–2822. 6. John, J.; Gravel, E.; Hagege, A.; Li, H.; Gacoin, T.; Doris, E. Angew. Chem. Int. Ed. 2011, 50, 7533–7536. 7. Villa, A.; Dimitratos, N.; Chan-Thaw, C. E.; Hammond, C.; Prati, L.; Hutchings, G. J. Acc. Chem. Res. 2015, 48, 1403–1412. 8. Liu, J. H.; Wang, A. Q.; Chi, Y. S.; Lin, H. P.; Mou, C. Y. J. Phys. Chem.B 2005, 109, 40–43. 9. Hong, Y. C.; Sun, K. Q.; Zhang, G. R.; Zhong, R. Y.; Xu, B. Q. Chem. Commun. 2011, 47, 1300–1302. 10. Song, Y. J.; López-De Jesús, Y. M.; Fanson, P. T.; Williams, C. T. J. Phys. Chem. C 2013, 117, 10999–11007. 11. Behrens, M.; Zander, S.; Kurr, P.; Jacobsen, N.; Senker, J.; Koch, G.; Ressler, T.; Fischer,


Page 20 of 35

Page 21 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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R. W.; Schlogl, R. J. Am. Chem. Soc. 2013, 135, 6061–6068. 12. Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Science 2006, 311, 362–365. 13. Liu, X. Y.; Li, Y.; Lee, J. W.; Hong, C.Y.; Mou, C.Y.; Jang, B. W. L. Appl. Catal. A: Gen. 2012, 439-440, 8–14. 14. Li, W. J.; Wang, A. Q.; Liu, X. Y.; Zhang, T. Appl. Catal. A: Gen. 2012, 433-434, 146–151. 15. Zheng, J. W.; Lin, H. Q.; Wang, Y. N.; Zheng, X. L.; Duan, X. P.; Yuan, Y. Z. J. Catal. 2013, 297, 110–118. 16. Lin, H. Q.; Zheng, J. W.; Zheng, X. L.; Gu, Z. Q.; Yuan, Y. Z.; Yang, Y. H., J. Catal. 2015, 330, 135–144. 17. Sugano, Y.; Shiraishi, Y.; Tsukamoto, D.; Ichikawa, S.; Tanaka, S.; Hirai, T. Angew. Chem. Int.Ed. 2013, 52, 5295–5299. 18. Amakawa, K.; Kolen’ko, Y. V.; Villa, A.; Schuster, M. E.; Csepei, L. I.; Weinberg, G.; Wrabetz, S.; Naumann d’Alnoncourt, R.; Girgsdies, F.; Prati, L.; Schlögl, R.; Trunschke, A. ACS Catal. 2013, 3, 1103–1113. 19. Tsunoyama, H.; Sakurai, H.; Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 9374–9375. 20. Mitsudome, T.; Mikami, Y.; Funai, H.; Mizugaki, T.; Jitsukawa, K.; Kaneda, K. Angew. Chem. Int. Ed. 2008, 47, 138–141. 21.Chen, Y. T.; Wang, H. P.; Liu, C. J.; Zeng, Z. Y.; Zhang, H.; Zhou, C. M.; Jia, X. L.; Yang,


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Y. H. J. Catal. 2012, 289, 105–117. 22. Bauer, J. C.; Veith, G. M.; Allard, L. F.; Oyola, Y.; Overbury, S. H.; Dai, S. ACS Catal. 2012, 2, 2537–2546. 23. Shimizu, K.; Sugino, K.; Sawabe, K.; Satsuma, A. Chem. Eur. J. 2009, 15, 2341–2351. 24. Karvembu, R.; Priyarega, S. React. Kinet. Catal. Lett. 2006, 88, 333–338. 25. Fang, W. H.; Zhang, Q. H.; Chen, J.; Deng, W. P.; Wang, Y. Chem. Commun. 2010, 46, 1547–1549. 26. Fang, W. H.; Chen, J.; Zhang, Q. H.; Deng, W. P.; Wang, Y. Chem. Eur. J. 2011, 17, 1247–1256. 27. Marella, R. K.; Prasad Neeli, C. K.; Rao Kamaraju, S. R.; Burri, D. R. Catal. Sci. Technol. 2012, 2, 1833–2838. 28. Okamoto, H.; Massalski, T. B. Phase diagrams of binary gold alloys, ASM International, Metals Park, 1987. 29. Devarajan S.; Vimalan B.; Sampath S. J. Colloid Interf. Sci. 2004, 278, 126−132. 30. Wittstock, A.; Zielasek, V.; Biener, J.; Friend, C. M.; Baumer, M. Science 2010, 327, 319−322. 31. Mitsudome, T.; Mikami, Y.; Ebata, K.; Mizugaki, T.; Jitsukawa, K.; Kaneda, K. Chem. Commun. 2008, 4804–4806. 32.Li, Z. Y.; Wilcoxon, J. P.; Yin, F.; Chen, Y.; Palmer, R. E.; Johnston, R. L. Farad. Disc. 2008, 138, 363–373. 33. Suntivich, J.; Xu, Z.; Carlton, C. E.; Kim, J.; Han, B.; Lee, S. W.; Bonnet, N.; Marzari, N.; Allard, L. F.; Gasteiger, H. A.; Hamad-Schifferli, K.; Shao-Horn, Y. J. Am. Chem. Soc.


Page 22 of 35

Page 23 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

2013, 135, 7985–7991. 34. Wang, A. Q.; Liu, J. H.; Lin, T. S.; Mou, C. Y. J. Catal. 2005, 233, 186–197. 35. Valden, M.; Goodman, D. W. Science 1998, 1647–1650. 36. Zhao, D.; Xu, B.Q. Angew. Chem. Int. Ed. 2006, 118, 5077–5081. 37. Chaki, N. K.; Tsunoyama, H.; Negishi, Y.; Sakurai, H.; Tsukuda, T. J. Phys. Chem. C 2007, 111, 4885–4888. 38. Dimitratos, N.; Lopez-Sanchez, J. A.; Morgan, D.; Carley, A. F.; Tiruvalam, R.; Kiely, C. J.; Bethell, D.; Hutchings, G. J. Phys. Chem. Chem. Phys. 2009, 11, 5142–5153. 39. Gupta, N. K.; Nishimura, S.; Takagaki, A.; Ebitani, K. Green Chem. 2011, 13, 824–827. 40. Takagaki, A.; Takahashi, M.; Nishimura, S.; Ebitani, K. ACS Catal. 2011, 1, 1562–1565. 41. Villa, A.; Wang, D.; Veith, G. M.; Prati, L. J. Catal. 2012, 292, 73–80. 42. Nishimura, S.; Yakita, Y.; Katayama, M.; Higashimine, K.; Ebitani, K. Catal. Sci. Technol. 2013, 3, 351–359. 43. Zhou, C. M.; Chen, Y. T.; Guo, Z.; Wang, X.; Yang, Y. H. Chem. Commun. 2011, 47, 7473–7475.


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Table/Scheme/Figure captions

Table 1.

Physicochemical properties of Au–Ag/SBA-15 catalysts

Table 2.

Absorption energies of benzyl alcohol and benzaldehyde and hydrogen on

Au/SBA-15 and Au–Ag/SBA-15 catalysts by DFT

Figure 1.

(a) Component change curve of benzyl alcohol and benzyl aldehyde as a

function of reaction time ∆t over the Au1–Ag0.111 catalyst. (b) Comparison of the catalytic activities over pure Au, Ag, Au + Ag (mechanism mixing of pure Au and pure Ag), and Au1–Ag0.111 bimetallic catalysts. (c) Size distribution of the Au1−Ag0.111 catalyst (inset, TEM image). (d) Performance and intrinsic activity in benzyl alcohol dehydrogenation as functions of the Ag promoter content. Figure 2.

(a) Diffuse reflectance UV–vis spectra of pure Ag and Au nanocrystallites with

varying amounts of deposited Ag. The number represents the atomic ratio of Au/Ag on Au−Ag/SBA-15. (b) HS-LEIS spectra for SBA-15-supported (1) pure Au, (2) Au1−Ag0.028, (3) Au1−Ag0.111, (4) Au1−Ag0.167, (5) Au1−Ag0.333, (6) Au1−Ag1, and (7) pure Ag at 5 keV 20

Ne+.

Figure 3.

(a) Surface Ag/Au atomic ratio and intrinsic activity for benzyl alcohol

dehydrogenation as functions of activation temperatures to the Au1−Ag0.111 catalyst; (b) diffuse reflectance UV–vis spectra. (c) HS-LEIS spectra of the Au1−Ag0.111 catalyst activated at (1) 423, (2) 523, (3) 623, (4) 723, and (5) 823 K. Figure 4.

Comparison of the catalytic intrinsic activities for benzyl alcohol 24 ACS Paragon Plus Environment

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ACS Catalysis

dehydrogenation. The Au1−Ag0.333 catalyst is soaked in 37 wt. % HNO3 for 12 h to afford the naked Au. The naked Au is heat-treated at a serial temperature under H2 atmosphere. Figure 5.

(a) Joint use of the HS-LEIS and XPS spectra for the naked Au. (b) Diffuse

reflectance UV–vis spectra of the (1) naked Au catalyst, (2) sample (1) activated at 423 K, (3) sample (1) activated at 623 K, and (4) sample (1) activated at 823 K. Figure 6.

Surface construction strategies and snapshot of the benzyl alcohol

dehydrogenation on the nanocrystallite framework. Figure 7.

Plausible schematic steps of the reaction cycle of benzyl alcohol

dehydrogenation on Au−Ag/SBA-15.


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Table 1.

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Physicochemical properties of Au–Ag/SBA-15 catalysts Au loadinga

Ag loadinga

Ag : Au atomic ratio

/ wt. %

/ wt. %

XRF

XPS

HS-LEIS

Au

4.00

−b

0

0

0

Au1−Ag0.028

3.83

0.06

0.03

0.05

0.21

Au1−Ag0.111

3.87

0.24

0.11

0.24

0.25

Au1−Ag0.167

3.72

0.34

0.17

0.28

0.29

Au1−Ag0.333

3.94

0.71

0.33

0.59

0.60

Au1−Ag1

3.88

2.10

0.99

1.40

1.50

Ag

−

2.02

∞c

∞

∞

Catalyst

a

Metal loading determined by XRF. bNot detected. cInfinity.


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Table 2.

Absorption energies of benzyl alcohol and benzaldehyde and hydrogen on

Au/SBA-15 and Au–Ag/SBA-15 catalysts by DFT Catalyst

Bulk ratio of

Surface ratio of

Absorption energy (eV)

Au:Ag

Au:Ag

PhCH2OHa

PhCHO + 2[H]b

Au

∞c

∞

-0.13

-0.18

Au1−Ag0.111

9

3

-0.23

-0.05

a

Benzyl alcohol. bBenzaldehyde and 2 hydrogen atoms. cInfinity.


ACS Catalysis

b 100

120 100

80 H2

80

60

60 40

40

ΟΗ

20

20

0

0 0

2

4

6

8

10

Benzyl alcohol conversion / %

Ο

Amount of H2 / mmol

Component distribution / %

a 100

Au1−Ag0.111 80

Au + Ag

60

Au

40 Ag

20 0 0

12

2

4

6

c 250

10

12

14

16

200

200

150

150

100

100

50

50

d 30

0 0.0 0.3 0.6 2.00 2.05 2.10 Ag content / wt. %

25 Frequency / %

Instrinsic activity / h

−1

250

0

Figure 1.

8 ∆t / h

∆t / h

Benzyl aldehyde weight time yield / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20 15 10

Average size: 4.2 ± 0.4 nm

5 0 2

4 6 8 10 Particle size / nm

12

14

(a) The component change curve of benzyl alcohol and benzyl aldehyde as a

function of reaction time ∆t over Au1−Ag0.111 catalyst. (b) Comparison of catalytic activities over pure Au, Ag, Au+Ag (mechanism mixing of pure Au and pure Ag), and Au1−Ag0.111 bimetallic catalysts.

(c) Performance

and intrinsic

activity

in

benzyl alcohol

dehydrogenation as functions of Ag promoter content. (d) Size distribution of Au1−Ag0.111 catalyst and the insert is its TEM image.


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a

Ag

b

Ag Au−Ag

Au

12

7 1.0 Au

0.5

300

Figure 2.

Signal / a.u.

1.5 F(R∞) / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

400

6 5 4 3 2 1

500 600 700 Wavelength / nm

7 6

8

5 4

4

3 2 1

0 800

2000

2500

3000 Ef / eV

3500

4000

(a) Diffuse reflectance UV−vis spectra of pure Ag and Au nanocrystallites with

varying amounts of deposited Ag. The number represents the atomic ratio of Au/Ag in Au−Ag/SBA-15. (b) HS-LEIS spectra for SBA-15 supported catalysts: (1) pure Au, (2) Au1−Ag0.028, (3) Au1−Ag0.111, (4) Au1−Ag0.167, (5) Au1−Ag0.333, (6) Au1−Ag1, and (7) pure Ag at 5 keV 20Ne+.


0.40

100

0.35

90

0.30

80 70

0.25

60

0.20

50

0.15

Instrinsic activity / h

Surface molar atomic ratio of Ag/Au

a

40 0.10 400

b

600 800 Active temperature / K

c

Au−Ag 1.5

16

5 4 3 1.0

Au Ag 5

12 Signal / a.u.

F(R∞) / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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−1

ACS Catalysis

2 1

4

8

3 4

0.5

2 1

200

300

400

500

600

700

800

0

2000

2500

Wavelength / nm

Figure 3.

3000 Ef / eV

3500

4000

(a) Surface Ag/Au atomic ratio and intrinsic activity for benzyl alcohol

dehydrogenation as functions of activation temperatures to Au1−Ag0.111 catalyst. (b) Diffuse reflectance UV−vis spectra. (c) HS-LEIS spectra of Au1−Ag0.111 catalyst activated at (1) 423 K, (2) 523 K, (3) 623 K, (4) 723 K, and (5) 823 K.


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100 Intrinsic activity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Au Ag

80 60 40 20

Acid soaked Au1Ag0.333

Figure 4.

Naked Au

Thermal pretreatment

NAu-423

NAu-623

NAu-823

Comparison of catalytic intrinsic activity for benzyl alcohol dehydrogenation.

The Au1−Ag0.333 catalyst was soaked in 37 wt. % HNO3 for 12 h to afford naked Au. The naked Au was heated treating at serial temperature under H2 atmosphere.


ACS Catalysis

3600

3200

Ef / eV 2800

2400

1800

1.2

Au−Ag

1600

XPS

2

Ag 0 Au Ag

F(R∞) / a.u.

LEIS Au

1400

b

2000 4

Signal / a.u.

a Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4

0.8

3 2

0.4

-2

1

1200

80 85 90 95 360 370 Binding energy / eV

Figure 5.

380

-4

0.0 400

500 Wavelength / nm

600

(a) Joint use of HS-LEIS and XPS spectra for naked Au. (b) Diffuse reflectance

UV−vis spectra of (1) naked Au catalyst activated at (2) 423 K, (3) 623 K, and (4) 823 K.


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100

Intrisic activity / h−1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

60

40

20 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Surface Ag : Au atomic ratio

Figure 6.

Strategies for surface construction and snapshot of benzyl alcohol

dehydrogenation on the nanocrystallite framework.


ACS Catalysis

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Figure 7.

Plausible schematic steps of a reaction cycle of benzyl alcohol dehydrogenation

on Au−Ag/SBA-15.


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Graphic Abstract Intrisic activity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 80 60 40 20 0.0

0.2

0.4 0.6 0.8 1.0 1.2 1.4 Surface Ag : Au atomic ratio


1.6

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