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Surface Plasmon Aided Ethanol Dehydrogenation Using Ag-Ni Binary Nanoparticles Chanyeon Kim, Bong Lim Suh, Hongseok Yun, Jihan Kim, and Hyunjoo Lee ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00411 • Publication Date (Web): 17 Feb 2017 Downloaded from http://pubs.acs.org on February 20, 2017
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Surface Plasmon Aided Ethanol Dehydrogenation Using Ag-Ni Binary Nanoparticles Chanyeon Kima, Bong Lim Suha, Hongseok Yuna, Jihan Kima and Hyunjoo Leea* a
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea KEYWORDS. plasmonic catalyst, binary nanoparticles, ethanol, dehydrogenation, effective energy barrier
ABSTRACT: Plasmonic metal nanoparticles absorb light energy and release the energy through radiative or non-radiative channels. Surface catalytic reaction would take advantage of the non-radiative energy relaxation of plasmon with enhanced activity. Particularly, binary nanoparticles are interesting because diverse integration would be possible consisting of plasmonic part and catalytic part. Herein, we demonstrated ethanol dehydrogenation under light irradiation using AgNi binary nanoparticles with different shapes, snowman and core-shell, as plasmonic catalysts. The surface plasmon formed in the Ag part enhanced the surface catalytic reaction that occurred at the Ni part, and the shape of the nanoparticles affected the extent of the enhancement. The surface plasmon compensated the thermal energy required to trigger the catalytic reaction. The absorbed light energy was transferred to the catalytic part by the surface plasmon through the non-radiative hot electrons. The effective energy barrier was greatly reduced, from 41.6 kJ/mol for the Ni catalyst to 25.5 kJ/mol for the core-shell nanoparticles and 22.3 kJ/mol for the snowman shaped nanoparticles. These findings can be helpful in designing effective plasmonic catalysts for other thermally driven surface reactions.
INTRODUCTION Localized surface plasmon resonance (LSPR) is an intrinsic phenomenon of metal nanoparticles (NPs), such as Au, Ag and Cu, caused by interaction with incident light. The light energy is absorbed by free electrons on the metal surface, and the absorbed light energy is relaxed through two types of decaying processes of the excited electrons; one is a radiative channel which refers to the emission of light, and the other is a non-radiative channel which induces hot electron and heat generation. Energy relaxation of LSPR via the radiative channel is of particular interest in research fields including lasing, bioimaging, and gas-sensing.1-3 In these applications, the energy relaxation via the non-radiative channel is considered to be an energy loss, while there are applications that take advantage of non-radiative energy relaxation, such as photovoltaic cells and plasmonic catalysts,4-7 to promote electronic bias and chemical reaction under light irradiation. Among those applications utilizing the non-radiative energy relaxation of LSPR, plasmonic catalysts have been recently reported.8-17 In plasmonic catalysts, there are three different pathways of energy transfer depending on the attached mediums, such as adsorbed molecules, semiconductors, and other metal species. The first pathway is energy transfer from the metal to adsorbed molecules on the metal surface; in this case, the metal NPs themselves can catalyze chemical reactions. For example, Au, Ag, or
Cu NPs have been used for alkene epoxidation or H2 dissociation.18-21 The second pathway is energy transfer from the metal to a semiconductor. Using this type of energy transfer, M/TiO2, M/ZnO, M/Fe2O3 and M/CdS (M = Au or Ag) have been extensively studied for photocatalytic organic pollutant degradation, hydrogen production, CO2 reduction, and other plasmon-enhanced chemical reactions.14, 22-25 In these metal-semiconductor hybrid nanostructures, LSPR facilitates the electron transfer from the metal to the semiconductor, or increases the exciton lifetime in the semiconductor by field enhancement, which are the key factors that determine the performance of the photocatalyst.14, 22-25 The third pathway is metal-tometal energy transfer within binary NPs, which consist of two different metals such as Au-Pd, Ag-Pd, and Au-Pt.26-31 In this particular system, the metals in group IB yield surface plasmons, while the attached precious metals, such as Pt and Pd, serve as catalytically active sites. In this category of energy transfer, LSPR can be applied for surface catalytic reaction. For instance, Au-Pd binary NPs showed enhanced catalytic activity upon light irradiation for Pd catalyzed benzyl alcohol oxidation and Suzuki coupling reactions.27, 29 Recently, it has been reported that conventional metal catalysts of Pt, Pd, Ir can have light enhanced catalytic performance through interband transition or photoexcitation of the electrons in adsorbate-metal bonds.32, 33 However, weak interaction of incident light with these conventional metals hinders further improvement.34-36 Therefore, binary NPs consisting of a plasmonic metal and a catalytic metal are particularly interesting
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because they hold great potential for enhancing surface reactions by strengthening interactions with light and reactant molecules. In binary NPs used as plasmonic catalysts, the interfacial contact between the constituent metal species is important for modulating the LSPR effect on catalytic reactions. Recently, it has been reported that there is remarkable field enhancement at the interfacial contact of Au-Ag and Au-Pt binary NPs.37-39 Furthermore, different geometric configurations are known to affect charge transfer within binary NPs, because of the different work functions of each metal constituent.28, 31, 38, 40 The excited free electrons should decay or transfer differently depending on the spatial charge distribution, which varies with the shape of the binary NPs. In this regard, binary NPs with different shapes, such as snowman and core-shell, can be an interesting model system for the fundamental study of the shape-dependent LSPR effect on surface catalytic performance. Recent studies of surface catalytic reactions using binary NPs as plasmonic catalysts have primarily dealt with a handful of liquid phase reactions driven at temperatures below 80˚C, using a batch reactor.26-31 However, it would be more impactful if the reaction could be operated in a continuous flow-type reactor. Investigating the LSPR effect on a gas phase surface reaction in continuous systems can contribute to both scientific understanding and the practical application of plasmonic catalysts. Dehydrogenation of ethanol to acetaldehyde has received considerable attention recently in the bio-refinery field, as a means of upgrading bioethanol, because acetaldehyde is an important raw material for producing valuable chemicals including acetic acid, ethyl acetate, butyl aldehyde, crotonaldehyde, pyridine, etc.41-46 Besides, ethanol dehydrogenation is an endothermic reaction in which overheating of the system is inevitable, because the activation energy is always bigger than the reaction enthalpy. Systems involving endothermic reactions typically require high operating temperatures, which can cause chronic issues, such as operating cost, safety, and thermal degradation of the catalyst. Therefore, supplying the required energy from light energy via LSPR would be a significant approach for alleviating such problems. In this study, we demonstrate ethanol dehydrogenation using a plasmonic catalyst. The model catalysts are Ag-Ni binary NPs with two different shapes: snowman and core-shell. Different geometric configurations of the interfacial contacts between the Ag and Ni in the binary NPs induced different LSPR effects during ethanol dehydrogenation. The temperature at which the heterogeneous reaction initiates and the effective energy barriers were studied for different nanoparticle shapes.
RESULTS AND DISCUSSION Figure 1 shows typical low magnitude images of the Ag-Ni snowman NPs (Ag-Ni SM NPs), Ag@Ni core-shell NPs (Ag@Ni CS NPs), Ag spherical NPs, and Ni spherical NPs. The insets are high resolution images of each shape of NP, which clearly show their different shapes. In the Ag-Ni
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SM NPs (Figure 1a), the typical sizes of the Ni part and the Ag part are 12.2 ± 3.1nm and 11.0 ± 2.3nm, respectively. It should be noted that the shape selectivity of the Ag-Ni SM NPs is 89%, and a little bit of trimer (2%) and monomer (9%) were coexistent. In the Ag@Ni CS NPs (Figure 1b), the Ni shell thickness was 2.1 ± 0.6 nm and the Ag core had an average size of 9.3 ± 1.5 nm. In addition, spherical Ni with an average size of 13.5 ± 3 nm and spherical Ag with an average size of 10.3 ± 2.6 nm were also synthesized (Figure 1c and d). The sizes of the Ag for the Ag-Ni binary NP and the spherical Ag NP were controlled to be similar, to exclude any size effect on the LSPR.
Figure 1. TEM images of typical (a) Ag-Ni snowman (SM) NPs (b) Ag@Ni core-shell (CS) NPs, (c) Ni NPs, and (d) Ag NPs. Insets are HR images for each type of NP.
During the synthesis of the Ag-Ni binary NPs, Ag was reduced first, and then Ni species were reduced and overgrown on the Ag NPs. We previously found that Ni overgrowth with a metastable hexagonal closed packing (hcp) structure was preferred to a ground state lattice structure, face centered cubic (fcc), under synthetic conditions using hexadecylamine intensively.37 Under this specific condition, the overgrown hcp Ni on fcc Ag suffered from structural strain, and thus it was eventually developed as a snowman shape, while Ni shell was formed under the synthetic condition that prefers the formation of fcc Ni. Since the organic surfactants used during the NP syntheses block active sites on the metal NP surface, it is essential to remove the organic surfactants for the NPs to have catalytic activity.47, 48 A surface treatment was carried out to ensure clean NPs surface. First of all, the synthesized NPs were supported over commercial silica nanopowder, and then annealed to remove organic surfactants following the literature protocol.47 In a typical thermal annealing, samples were exposed in the air at 700˚C for 30 sec and quickly cooled down to room temperature. After the first cycle of annealing, there were leftover organic residues in the supported Ag-Ni snowman and Ag@Ni
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core-shell catalysts, which was confirmed in the FT-IR results by the remaining C-H stretching peaks in the range of 2800 to 3000 cm-1, as shown in Figure S1. After repeated annealing processes, the C-H stretching peaks gradually disappeared. Finally, post-reduction was performed to recover the nanoparticle surface which had been possibly oxidized during the annealing processes. The reduction was conducted under 10 % H2/N2 atmosphere at 300 ˚C for 6 hr. Figure S2 shows the temperatureprogrammed reduction (TPR) results of the Ag-Ni binary NPs before and after the reduction treatment. The surface metal oxides were significantly reduced. The Ni sites on the surface were measured by CO chemisorption (Table 1) after the surface treatment processes. Before the surface treatment, there were no accessible sites for CO adsorption due to the surface-capped organic residues. However, after the surface treatment, clean Ni surfaces were available to adsorb CO molecules as evidenced by the CO uptake data in Table 1. The CO uptake levels for a given amount of Ni were controlled by changing the number of the annealing procedure, so that they would be similar to each other. This enabled the comparison of Ni mass activity for the chemical reaction. Table 1. CO chemisorption results for various silica supported nanoparticle catalysts. CO uptakes (µmol/gNi)
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Ag
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0
0
0
0
6.6
21.4
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0
14.8
48.9
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-
46.9
-
-
-
Annealed once then reduced Annealed twice then reduced Annealed 4 times then reduced
Figure 2 shows the TEM images of the silica supported Ag-Ni binary NPs before and after the surface treatment. No severe aggregation of nanoparticles was observed after the surface treatment. The shape of the individual nanoparticles was difficult to identify clearly from the TEM images in Figure 2, due to the rough surface of the silica nano-powder. Nevertheless, through HAADF images and elemental mapping using STEM, the shape and composition of the Ag-Ni binary NPs were observed more clearly before and after the surface treatment, and no noticeable deformation of the Ag-Ni binary NPs was found. The shape preservation of the Ag-Ni binary NPs was also confirmed when as-made nanoparticles deposited on a SiO2coated Au TEM grid were exposed to the same thermal process as the surface treatment (Figure S3).
Figure 2. TEM images for (a) pristine and (b) surface treated Ag-Ni SM/SiO2, (c) pristine and (d) surface treated Ag@Ni CS/SiO2. The insets are HAADF images and elemental mapping for each sample. The white scale bars are 9 nm.
The surfaces of the nanoparticles were further characterized using XPS, as presented in Table S1. Because of the mild reduction condition, the Ni surface still had oxidized species in both the Ag-Ni snowman and core-shell NPs. Nonetheless, the proportions of metallic Ni to oxidized Ni were similar in all of the samples, and the Ag surface was completely reduced in both the Ag-Ni binary NPs. Accordingly, the dominant variables that could modulate the LSPR property of the nanostructures, such as size and composition, could be ruled out, and thus any changes in catalytic activity could be mainly attributed to the shape dependence. The shape dependent optical properties of the silica supported Ag-Ni binary NPs were investigated using UV-DRS as shown in Figure S4. Different geometric configurations of the interfacial contact between the Ni part and Ag part obviously affected the LSPR properties, showing a slightly red-shifted absorption spectra in the core-shell shaped nanoparticles compared to the snowman shape. Catalytic activity tests were carried out using a continuous flow type customized photochemical reactor system (Figure S5) and the details are described in the experimental section. Figure 3 shows the catalytic conversion of ethanol under elevated temperature conditions using various types of supported nanoparticles, with and without light irradiation. Under light irradiation, only the supported Ag-Ni binary NP catalysts which had direct contact between the Ag and Ni exhibited enhancements in ethanol conversion. In particular, the on-set temperature, at which reaction was initiated, was significantly reduced when the Ag-Ni binary NPs were used. More importantly, the on-set temperature was reduced as much as 90˚C, from 200˚C under dark to 110˚C under light irradiation, in the case of the supported Ag-Ni snowman NPs, while the supported Ag@Ni core-shell NPs showed a decrease of 40˚C, from 180˚C to 140˚C. In addition, the selectivity of products during ethanol dehydrogenation was checked (Figure S6). The produced acetaldehyde can be decomposed to CO and CH4. Only a small fraction of the unde-
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ACS Catalysis sirable decomposition was observed. The light irradiation hardly affected the selectivity. 40
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Figure 3. Ethanol dehydrogenation using supported nanoparticle catalysts, (a) ethanol conversion with and without light irradiation. (b) Arrhenius plot of various catalysts and (c) effective energy barrier (Eb) estimated from (b).
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from 34.6 kJ/mol to 25.5 kJ/mol. The reduction in the effective energy barrier indicates that the required thermal energy for ethanol dehydrogenation was compensated by non-radiative energy transfer from the LSPR, which is highly related to the shape of the nanoparticles. Energy relaxation of LSPR can be investigated by measuring emission properties. In the photoluminescence spectra of the Ag-Ni binary NPs (Figure 4a), the maximum of the emission spectrum was more red-shifted and significantly reduced in the case of the snowman in comparison with the core-shell at 375 nm excitation. Differences in energy relaxation were also checked by the timeresolved fluorescence technique (Figure 4b); the decay of fluorescence in the snowman shape was much faster than in the core-shell. The lifetime of fluorescence, which is τ=(kr+knr)-1 where kr and knr are the rate constants of the radiative and non-radiative decay, was estimated as 1.44 ns for Ag-Ni snowman NPs and 2.52 ns for Ag@Ni coreshell NPs. From the different emission behaviors of the Ag-Ni binary NPs, it was deduced that the light energy consumed through the non-radiative channel was bigger in the snowman shape than in the core-shell.
On the other hand, the supported Ni NP catalyst did not show any change, in either the conversion or the onset temperature. There should have been enhancement in the case of supported Ni under light irradiation if oxidized Ni species acted as a semiconductor-type photocatalyst or metallic Ni itself acted as photocatalyst through interband transition. Therefore, these results indicate that the enhancement in the supported Ag-Ni binary NP was induced by LSPR. The supported Ag NP catalyst showed the lowest catalytic activity among the tested samples under the dark condition, and there was no enhancement of catalytic performance (conversion or on-set temperature) under light irradiation. Accordingly, this indicates that the direct energy transfer from the LSPR of Ag to the adsorbed ethanol molecule was not involved in the enhanced ethanol dehydrogenation by the Ag-Ni binary NPs. Besides, the physical mixture of supported Ag and Ni NP catalyst, in which direct contact between Ag and Ni did not occur, also did not show a noticeable change in catalytic activity upon light irradiation. The interesting behavior of the on-set temperature under light irradiation might give us another clue about the energy transfer by LSPR during the catalytic reaction. Effective energy barrier in the presence of a catalyst is one of the most important indices for evaluating the performance of a catalyst. In Figure 3b, Arrhenius plots are presented based on the results in Figure 3a, and effective energy barrier was calculated from the slopes of these Arrhenius plots for various catalysts (Figure 3c). There were significant reductions in the effective energy barrier of the supported Ag-Ni binary NPs under light irradiation; the effective energy barrier was reduced from 40.6 kJ/mol (dark condition) to 22.3 kJ/mol for the supported Ag-Ni snowman NPs catalyst, while the effective energy barrier for the Ag@Ni core-shell NPs catalyst was only reduced
Figure 4. (a) Photoluminescence (PL) spectra and (b) PL decay measured at 528 nm for the snowman and 487 nm for the core-shell by time-resolved fluorescence spectroscopy. The changes in catalytic activity depending on (c) light intensity, and (d) light wavelength. Electric field enhancements estimated by the FDTD method for the (e) snowman NPs and (f) core-shell NPs.
In previous studies on plasmonic catalysts, there have been arguments about the mechanism of non-radiative energy relaxation that induced the enhanced catalytic
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activity. Some studies have claimed that photothermal effect dominantly contributes to the enhanced catalytic performance rather than hot electron transfer.49-53 In our system, light irradiation was not supplying heat during the catalytic reactions. Heat radiation from the Xe lamp itself was cancelled out by adopting a water circulating filter in front of the Xe lamp, which effectively absorbed direct heat radiation from the light source. When a light intensity varied during the surface reaction, a linear dependence was observed between the catalytic reaction rate and the light intensity as shown in Figure 4c. The linear dependence is a signature of hot electron-driven chemical process.54, 55 The surface reaction also occurred under various light wavelengths. When the activity was plotted versus the wavelength, the catalytic activity showed a profile very similar to the absorbance for each shaped NPs, as shown in Figure 4d. These results indicate that hot electrons mediated by LSPR caused the observed light enhancement in the surface reaction. Furthermore, the slopes in Figure 4c show that the snowman NPs has twice larger slope than the core-shell NPs, suggesting that the snowman produces the hot electrons more than the core-shell. This difference is in a good agreement with the electric field enhancement estimated using finite difference time domain (FDTD) in Figure 4e and 4f. It has been known that a larger field enhancement would yield the higher rates of hot electron generation.54, 55 If there was an interfacial transfer of hot electrons from Ag into Ni, Ni would gain extra negative charge by the hot electrons under light irradiation. Microscopic mechanism of ethanol dehydrogenation was estimated using density functional theory (DFT) calculations on neutral and negatively charged Ni surface as shown in Figure S7. The ethanol dehydrogenation has two elementary steps: cleavage of O-H (transition state 1, TS1) and cleavage of αC-H (transition state 2, TS2). The TS2 was the rate-limiting step and it was significantly reduced from 0.93 eV to 0.64 eV when the Ni surface was negatively charged. This is in good agreement with the experimental observation of the significant reduction in effective energy barrier upon light irradiation. Thus, the transfer of hot electrons from Ag to Ni would facilitate the ethanol dehydrogenation under light irradiation. Regarding the LSPR effect on the surface reaction of a catalyst, there was a report that oxidized Cu species on the surface of a Cu nanoparticle had been reduced under light irradiation.20 In the Ag-Ni binary NPs, if the oxidized Ni species were reduced under light irradiation, the catalytic conversion should be changed after turning the light off. In Figure 5a, catalytic conversions were monitored under alternating light irradiation conditions (turning the light off-on-off). When tested near on-set temperatures for both shapes, no enhancement was observed after turning off the light. The catalysts themselves do not seem to be changed under light irradiation.
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Figure 5. (a) Catalytic conversion before and after light irradiation. (b) Temperature programmed oxidation profiles for fresh and used catalysts.
There is the possibility of catalyst deactivation caused by carbonaceous species from side reactions, such as aldol-condensation or the Boudouard reaction.56, 57 Carbonaceous species on the catalyst surface will eventually block active sites. In Figure 5b, temperature programmed oxidation (TPO) was conducted with fresh and used supported nanoparticle catalysts and the results were normalized by the amount of nickel. The ethanol conversions for the used catalysts were 30.6% for the core-shell, 35.8% for the snowman, and 27.5% for the Ni catalysts. It should be noted that the shaded spectra for all the samples in the TPO results are caused by the consumption of oxygen from the oxidation of Ni and Ag. The ethanol conversions were bigger for the Ag-Ni binary NPs than for the Ni, which may imply a higher potential for forming undesirable carbonaceous species. Indeed, the supported Ag@Ni core-shell NPs showed a larger amount of carbonaceous species than the supported Ni NPs after the reaction. However, the supported Ag-Ni snowman NPs showed the highest catalytic conversion but the lowest amount of carbonaceous species among the tested catalysts.
CONCLUSIONS Ethanol dehydrogenation using plasmonic catalysts was demonstrated with different shaped nanoparticles, snowman and core-shell. The plasmonic catalysts (i.e., Ag-Ni SM/SiO2 and Ag@Ni CS/SiO2) exhibited enhanced catalytic conversions of ethanol, and their on-set temperatures were significantly reduced under light irradiation. The reduction of on-set temperature, by up to 90 ˚C for the Ag-Ni snowman NPs, indicated that effective energy barrier to trigger chemical reaction was reduced under light irradiation. Moreover, the reduction in the effective
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energy barrier depended on the nanoparticle shapes, from 40.6 kJ/mol to 22.3 kJ/mol for the snowman shape and from 34.6 kJ/mol to 25.5 kJ/mol for the core-shell shape. From photoluminescence and decay experiments, it was confirmed that the snowman shape had an advantage in terms of the non-radiative relaxation of absorbed light energy. The dependence of the catalytic activity on light intensity and light wavelength confirmed that hot electrons generated via LSPR during light irradiation caused the enhancement in the surface reaction. The observed effects for the plasmonic catalysts in this study can be helpful in designing new catalysts for other catalytic surface reactions using light energy in tandem with thermal energy.
EXPERIMENTAL Synthesis of Ag-Ni snowman NPs. Ag-Ni snowman NPs were synthesized using a modified recipe from our previous study.37 In a typical synthesis, nickel acetylacetonate (40 mg; Sigma Aldrich, 95 %) and silver acetylacetonate (120 mg; Sigma Aldrich, 98 %) were dissolved in mesitylene (5 ml; Sigma Aldrich, 98 %) in a three-neck flask. Into the flask, hexadecylamine (3 g; Sigma Aldrich, 90 %) was added. Then, trioctylphosphine (100 μl; Sigma Aldrich, 90 %) was injected into the reaction mixture under vigorous stirring. The final mixture was heated up to 80 ˚C and kept at that temperature for 30 min. The fully dissolved suspension was heated again to 200 ˚C (ramping rate; 5 ˚C/min). The whole heating process took place under H2 gas flowing (20 ccm) with reflux. After 2 hours at 200 ˚C, the resulting nanoparticle suspension was cooled to room temperature and precipitated by adding acetone. After centrifugation (3500 rpm, 10 min), the supernatant was discarded and the precipitate was redispersed in 10 ml of n-hexane. The solution was further washed with 30 ml of acetone and this washing process was repeated 3 time. Finally, the Ag-Ni NPs were gathered by the density gradient method.58 Shortly, a 3-layer step gradient was made using 0, 50%, and 100% (by volume) nhexane solutions with dichloromethane. Then centrifugation was conducted for 10 minutes at 12,000 rpm. Synthesis of Ag@Ni core-shell NPs. The Ag@Ni coreshell NPs were synthesized following a literature protocol.59 Nickel acetylacetonate (108 mg), silver nitrate (20 mg; Sigma Aldrich, 99+%), and triphenylphosphine (25 mg; Sigma Aldrich, 99 %) were mixed with oleylamine (7 ml; Aldrich, 70 %) under vigorous stirring in a three-neck flask. The mixture was degassed at 80 °C and heated to 190 °C under an argon atmosphere. The resulting nanoparticle suspension was cooled to room temperature after 2 hours. Further washing steps were identical to those for the snowman NPs. Synthesis of Ag and Ni NPs. For the synthesis of spherical Ni NPs, nickel acetylacetonate (500 mg) and trioctylphosphine (2.5ml) were mixed with oleylamine (7 ml) in a three-neck flask. The rest of the steps were same as the procedure for the synthesis of the Ag@Ni core-shell NPs. Spherical Ag NPs were also synthesized by a recipe similar to the core-shell NPs except that 170 mg of Ag
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precursor and 20 ml of oleylamine were used without nickel precursor and triphenylphosphine. The resulting nanoparticles were washed using the identical procedure described above. Surface treatment of the synthesized NPs. Surface treatment was performed on the silica supported nanoparticles. In a typical preparation of the silica supported nanoparticles, each of the synthesized nanoparticles (50 mg Ni basis for snowman, core-shell, and Ni NPs, 50 mg Ag basis for Ag NPs) were dispersed in n-hexane (10 ml; 95 %, Samchun) and mixed with 500 mg of commercially available silica nanopowder (Sigma Aldrich, 12 nm primary nanoparticles, 99.8%) under vigorous stirring. Then, the resulting sludge for each type of nanoparticles was dried in a convection drying oven overnight. Dried solids were grounded and placed in an alumina boat. The first step of the surface treatment was the removal of organic residue on the surface of the nanoparticles using rapid thermal annealing following a literature protocol.47 Specifically, in one cycle of the thermal annealing, a sample in the alumina boat was placed in a pre-heated furnace (700 ˚C, air). After 30 sec, the sample was quickly removed from the furnace and allowed to cool down to room temperature. The number of cycle was different depending on the type of nanoparticles: typically, four times for snowman, two times for core-shell, and one time for spherical Ni and Ag nanoparticles. Annealed samples were reduced at 300 ˚C for 6 hr under 10 % H2/N2 atmosphere. Characterizations. The shape and composition of the Ag-Ni binary NPs were investigated using high-resolution transmission electron microscopy (HR-TEM) and high angle annular dark field scanning TEM (HAADF-STEM) (TECNAI and JEOL, respectively). A Fourier transform infrared (FT-IR) spectrometer (Nicolet iS50, Thermo Fisher Scientific Instrument) was used to observe the removal of the organic residue after the thermal annealing of the samples. Temperature programmed reduction and CO chemisorption experiments were performed using a catalyst analyzer (BELCAT-B, BEL Japan). X-ray photoelectron spectroscopy (XPS) (K-alpha, Thermo VG Scientific) equipped with a monochromatic Al Kα X-ray source (energy resolution: 0.5 eV FWHM) was used to measure the surface properties of the nanoparticles after thermal annealing and post-reduction. In typical analyses using XPS, binding energies were calibrated by the maximum intensity of C 1s signal at 285 eV as a reference. Absorption spectra of the silica supported Ag-Ni binary NPs were obtained using an ultraviolet-diffuse reflectance spectrometer (UV-DRS) (UV3600, Shimadzu). Elemental analysis was conducted using an inductively coupled plasma optical emission spectrometer (ICP-OES 720, Agilent). Photoluminescence and its decay were measured using a fluorescence lifetime spectrometer (FL920, Edinburgh Instruments). Catalytic activity test (Ethanol dehydrogenation). Silica supported nanoparticle catalysts were placed in a cylindrical quartz sample holder (2 cm of diameter). Twenty milligram Ni basis was used for the supported Ag-
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Ni snowman, Ag@Ni core-shell, Ni, and a physical mixture of Ag and Ni NPs. And 25 mg Ag basis was used for the supported Ag. The sample holder was placed in a customized photo-reactor equipped with a sapphire window (2 cm diameter), a proportional-integral-derivative (PID) controlled electrical furnace, a temperature monitor for both inside and outside of the reactor, an insulating jacket, and a 300 W Xe lamp (Figure S5). It should be noted that we applied a water circulating filter in the middle of the light pathway to cancel the effect of IR range light and thermal radiation from the lamp. A feed stream was provided using N2 bubbling (10 ccm) through ethanol (99 %, anhydrous, Aldrich) in the chamber at 25 ˚C. The feed stream was diluted by an additional N2 stream (50 ccm). In this step, a static mixer and check valve were used to stabilize and prevent the backdraft of the feed stream. Catalytic activity was monitored using an online gas chromatograph (YL6100 GC, YL Instrument) equipped with molesieve/PORAPAK N columns (Sigma Aldrich), TCD, and FID with methanizer. Quantitative analysis was performed using N2 internal standard. Light intensity and wavelength-dependent measurements were conducted at 200 ˚C. The light intensity was varied by controlling the electrical power. The wavelength was varied using longpass filters (sharp cut filter, SCHOTT). FDTD calculations. The computational simulation for the silica supported nanoparticles with different shapes was performed using the finite-difference-time-domain (FDTD) method. The software package from FDTD Solutions (Lumerical Solutions, Inc.) with perfectly matched layer (PML) boundary conditions was employed to perform the 3D FDTD simulation. The mesh size was 0.25 nm, and the optical properties of the Ag, Ni, and SiO2 were adopted from Palik data.60 The size of the Ni and Ag domains in the different shapes of nanoparticles were taken from the average values of the experimental size distribution. Because the silica supported nanoparticles were tested in the gas phase, which was mostly N2, the refractive index of the medium was set as 1. Transition State calculation on Nickel surface. The first-principles calculations were performed with the Vienna ab initio simulation program (VASP).61 The projected-augmented wave (PAW)62 method was used to describe the electron–ion interactions, and a plane wave basis set was employed for valence electrons. The generalized gradient approximation method, known as GGAPBE63 was used for the exchange-correlation functional. The DFT + U method was employed to explore electronic structures. Spin alignments of transition-metal systems were applied to the localized d electrons under the influence of strong-correlation Hubbard U potentials64 with the U corrections (the Ni’s U values of 2.0 eV were taken from reference65). A cut-off energy of 400 eV was found to be sufficient to provide convergent results. The k-point mesh for these systems was selected as (3×3×1). The Ni(1 1 1) surface was modeled by a supercell that contains a 2layer slab with a vacuum layer of 15 Å. The coordinates of the initial structures for the nickel surface was taken from the Materials Studio66 and initial geometries were further
relaxed via the DFT simulations. The transition state (TS) of ethanol dehydrogenation was estimated by a climbingimage nudged elastic band method (CI-NEB)67 with seven intermediate images. The NEB with improved tangent estimate was used to determine the minimum energy path and to locate the transition-state structures of elementary reactions involved in ethanol dehydrogenation. The maximum energy geometries obtained with NEB method were then optimized separately using a quasiNewton algorithm.
ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the ACS publications website at DOI: characterizations of Ag-Ni binary NPs (FT-IR, TPR, TEM, UV-DRS, and XPS), a scheme for reactor setup, DFT calculation results and selectivity data.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] Author Contributions C.K. and H.L. developed the project. C.K. carried out experiments and analyses. B.I.S. and J. Kim performed theoretical calculation. H.Y. advised modification of a recipe for Ag-Ni snowman nanoparticles.
ACKNOWLEDGMENT This work was financially supported by the National Research Foundation of Korea (NRF-2015R1A2A2A01004467) funded by the Ministry of Education, Science and Technology and the Saudi Aramco-KAIST CO2 Management Center.
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