Article Cite This: J. Phys. Chem. C 2018, 122, 26613−26622
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Plasmon Catalysis on Bimetallic SurfaceSelective Hydrogenation of Alkynes to Alkanes or Alkenes O. Guselnikova,†,‡ A. Olshtrem,†,‡ Y. Kalachyova,†,‡ I. Panov,§ P. Postnikov,†,‡ V. Svorcik,† and O. Lyutakov*,†,‡ †
Department of Solid State Engineering, University of Chemistry and Technology, 16628 Prague, Czech Republic Research School of Chemistry and Applied Biomedical Sciences, Tomsk Polytechnic University, 634050 Tomsk, Russian Federation § Group of Advanced Materials and Organic Synthesis, Institute of Chemical Process Fundamentals, Czech Academy of Sciences, Rozvojová 1/135, 165 02 Prague, Czech Republic
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‡
S Supporting Information *
ABSTRACT: Utilization of plasmonics as a driving tool for chemical transformation triggering enables to achieve unprecedented results regarding photochemical conversion efficiency and chemical selectivity regulation. In this study, the bimetallic surface plasmon-polariton-supported grating is proposed as an effective background for plasmon-induced hydrogenation of alkynyl groups of absolute chemoselectivity. The periodical bimetallic structure consists of spatially modulated gold layers, covered with the nanometer-thick platinum layer. The alkyne bonds are covalently attached to the surface of the catalytic system through covalent grafting of the bimetallic surface with 4-ethynylbenzenediazonium tosylate, with triple bonds separated from the platinum layer by the benzene rings. The proposed bimetallic structure enables selective hydrogenation of alkyne bonds to alkenyl or alkyl moieties using cyclohexene as a hydrogen source. The selectivity of hydrogenation can be controlled by changing the structure parameters, for example, the thickness of the upper platinum layer.
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plasmon-supported bimetallic heterostructures.27,28 Usually, this approach involves the combination of plasmon-active catalytically active metals.26,29−33 The synergetic effect of bimetallic heterostructures can be attributed to two main factors: enhanced affinity of a specific reactant to the bimetallic surface and/or a significant increase of hot-electron lifetime.30,34−38 In particular, the inherent chemical inertness of plasmonic metals (Au and Ag) is compensated by the inclusion of a second metal which creates new active surface sites.39 Besides this, the addition of Pt or Pd facilitates the separation of plasmon-generated hot electrons and holes across the interface and, in this way, significantly improves plasmontriggering efficiency and hot electron injection.27,30,34−38,40−42 In this study, for the first time, we show plasmon-catalyzed selective hydrogenation of triple carbon−carbon bonds on the surface of an ordered surface plasmon-polariton-supported bimetallic gratings. The present combination of Au/Pt metal heterostructures is expected to significantly increase the hot electron lifetime (through both plasmon−exciton coupling and
INTRODUCTION Implementation of plasmon-based processes opens tremendous possibilities in sensorics, optical modulation, and chemical catalysis.1−6 In the field of plasmon-induced chemical transformations, the plasmon-triggering processes become exponentially involved in the range of advanced applications, with a special focus on the technologically relevant processes, such as water splitting, CO2 reduction, H2 production, or photooxidation of alcohols.7−14 In particular, the plasmonmediated effects can maximize the photochemical conversion efficiency and create new reaction pathways to manipulate product selectivity.15−18 Even the ability to precisely activate a specific reactant or functional group and to regulate the reaction selectivity makes the plasmon catalysis especially attractive.19 Recently demonstrated examples of plasmonbased selectivity include the reduction of the aromatic nitro compound to anilines,20 transformation of nitrobenzene to azobenzene,21 oxidation of benzene,22 epoxidation of propylene,23 and others.24,25 The majority of studies in this field achieves the selectivity by the interplay between plasmon-band strength and wavelength position. Plasmonic nanostructures may also support light sources and reaction environment.26 An alternative way to introduce selectivity in the plasmon triggering of chemical transformation is the application of © 2018 American Chemical Society
Received: July 31, 2018 Revised: September 20, 2018 Published: October 10, 2018 26613
DOI: 10.1021/acs.jpcc.8b07398 J. Phys. Chem. C 2018, 122, 26613−26622
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The Journal of Physical Chemistry C
Figure 1. Schematic presentation of the proposed experimental concept: (i) creation of bimetallic, SPP-supported heterostructure with different Pt thickness (A,B), (ii) grafting of alkyne triple bonds containing chemical moieties, and (iii) utilization of plasmon catalysis for chemically selective hydrogenation of unsaturated carbon bonds (CC).
efficient electron-sink function of Pt), the reaction rate, and to enhance the reaction selectivity.43
After modification, the metal substrates were rinsed under sonication sequentially with water, methanol, and acetone for 10 min and dried in a desiccator under reduced pressure. Plasmon-Driven Hydrogenation of 4-Ethynylphenyl Groups. The plasmon-induced hydrogenation reactions and the corresponding time-dependent surface-enhanced Raman spectroscopy (SERS) measurements were carried out on Au/ Pt grating modified by ADT−CCH using ProRaman-L spectrometer (785 nm laser wavelength). Every time before starting the reaction, spectra of the modified metal surface were recorded. For the hydrogenation reaction, bimetallic gratings were immersed into cyclohexene and illuminated with a laser beam from Raman spectrometer. The online SERS measurements were performed in the reaction media with laser powers of 4.8, 6.0, 7.2, and 9.6 μW/μm2. After hydrogenation of alkyne bonds, the metal gratings were sequentially rinsed with chloroform and acetone and dried. Raman spectra were measured 30 times, each of them with 3 s accumulation time. Measurement Techniques. The X-ray photoelectron spectroscopy (XPS) was performed using an Omicron Nanotechnology ESCAProbeP spectrometer fitted with a monochromated Al Kα X-ray source working at 1486.6 eV. For characterization of the grating’s surfaces, the peak force atomic force microscopy (AFM) technique was applied using the Icon (Brucker) microscope. AFM scratch tests were carried out on a smooth gold film (25 nm) by profiling across a scratch at an angle of 90° relative to the surface. The reported film thicknesses are the mean values from at least 16 measurements performed on three different scratches. Raman scattering was measured on a portable ProRaman-L spectrometer with 785 nm excitation wavelengths. UV−vis absorption spectra were
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EXPERIMENTAL SECTION Materials. Acetic acid (reagent grade, ≥99.0%), diethyl ether, deionized water, methanol [puriss. p.a., absolute, ≥99.8% (GC)], 4-ethynylaniline (97%), p-toluenesulfonic acid monohydrate (ACS reagent, ≥98.5%), chloroplatinic acid hydrate (≥99.9%), and L-ascorbic acid (≥99.0%) were purchased from Sigma-Aldrich and used without further purification. A solution of Su-8 was purchased from Microchem, Germany. Sample Preparation. Grating Preparation. Su-8 films were spin-coated onto freshly cleaned glass substrates and patterned using the linearly polarized excimer laser irradiation, according to the ref 43. Thin Au layers with the thickness of about 25 nm were deposited onto polymer surface using thermal evaporation method (t = 0.15 min, I = 20 mA, speed density = 0.1 nm/min). Created Au gratings were immersed into 10 mM water solution of H2PtCl6, followed by dropwise addition of 100 mM water solution of ascorbic acid with gentle mixing (volume ratio of solutions is 2.5:1, respectively). Au substrates were soaked for 5, 10, 15 (Au/Pt-1), 25, 30, 35, 40 (Au/Pt-2), and 50 min in the reaction mixture and then rinsed with deionized water and dried on the air. Surface Modification. 4-Ethynylbenzenediazonium tosylate (ADT) was synthesized according to the previously published procedure;44 for detailed information, see the Supporting Information. The metals surfaces (Au, Au/Pt-1, and Au/Pt-2) were spontaneously modified by soaking in 2 mM freshly prepared aqueous solution of ADT−CCH for 20 min.44 26614
DOI: 10.1021/acs.jpcc.8b07398 J. Phys. Chem. C 2018, 122, 26613−26622
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first 5 min), the Pt layer grows linearly with a speed of ca 0.4 nm/min. Taking into account the previous optimization results, two Pt thicknesses corresponding to “low” and “high” Pt amounts were chosen for further experiments: 4.7 nm (hereinafter referred as Au/Pt-1) and 15.1 nm (hereinafter referred as Au/Pt-2). The results of XPS data are presented in Table S1, summarizing the surface element concentrations. On the pristine Au surface, only Au (Au 4f, 84.6 eV), C (C 1s, 284.6 eV), and O (O 1s, 532.2 eV) (from the underlying polymer film) peaks were observed. Chemical reduction of H2PtCl6 for 15 min (Au/Pt-1) led to the appearance of apparent Pt XPS signal (Pt 4f, 71.2 eV). Prolongation of the reaction time to 40 min (Au/Pt-2) results in an increase of surface Pt concentration with a simultaneous decrease of Au concentration. Subsequent grafting of 4-ethynylphenyl functional groups (results are presented in Table S1) led to a reduction of the Pt surface concentration and an increase of carbon concentration, both confirming the success of the grafting procedure. Raman spectroscopy (see Figure S2) confirms the successful covalent grafting by the appearance of absorption bands associated with 4-ethynylbenzene functional groups, namely, 2010 cm−1 (CCH stretch), 1590 cm−1 (Ar ring stretch), 1180 cm−1 (C−H in-plane deformations), 1130 cm−1 (−C−CC stretch), 720 cm −1 (C−H out of plane deformations), and 505−560 cm−1 (Ar ring vibrations). During the reduction process, new peaks at 1580 and 490 cm−1 were not observed, the appearance of which could indicate strong interaction of the unsaturated bond with the Pt surface and Pt−carbon stretch vibration, according to refs 52 and 53. In this way, a possible formation of an active complex of the CC group with Pt was excluded. The crystalline structure of SPP-active gratings was studied by XRD before and after Pt deposition. In the spectra shown in Figure S3, distinct lines of the cubic Pt crystalline phase in both cases of Au/Pt-1 and Au/Pt-2 are visible. As can be expected, the most pronounced Pt peaks were observed in the case of Au/Pt-2. The positions of Au-related peaks, also observed on the pristine Au grating, remain unchanged after Pt deposition so that the structure of the underlying Au crystalline phase remains unchanged. Deposition of an additional thin layer of Pt will significantly affect the SPP wavelength position and strength. Results of UV−vis spectroscopy, measured on the pristine Au grating, Au/Pt-1, and Au/Pt-2 samples, are presented in Figure 2B. Pristine Au grating shows a pronounced SPP-related absorption band, located near 760 nm. The addition of a thin Pt layer led to the significant broadening of the absorption band and its red shift (which coincides with the literature data29,54). Further increase of the Pt thickness does not affect the width of the plasmon peak, but the wavelength shift becomes more pronounced (which was also observed earlier29,54). The bold arrow in Figure 2B shows the laser wavelength, which was used for the plasmon catalysis on bimetallic heterostructure. Despite the observed shift of plasmon resonance and because of the broadening of plasmon absorption, the further-used 785 nm wavelength was found to be suitable for an effective plasmon catalysis activation. Changes in the surface morphology after the Pt deposition and ADT (ADT−CCH) grafting were checked using the AFM technique (Figure 3). The pristine Au grating surface represents well-ordered periodical surface with a sinusoidal shape. The addition of a Pt layer does not affect the grating parameters, in terms of periodicity and amplitude, but
measured by using a Lambda 25 UV−vis−NIR spectrometer (PerkinElmer, USA) with a scanning rate of 480 nm min−1.
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RESULTS AND DISCUSSION We focused on the preparation of a plasmon-supported structure, potentially capable to effectively prolong the lifetime of the plasmon-induced hot electrons and to make them available for the subsequent chemical reactions with a possible contribution to plasmon-catalysis selectivity control. Plasmontriggering was performed through the surface plasmonpolariton excitation on the ordered bimetallic grating, where the lifetime of hot electrons can be enhanced through the plasmon−exciton coupling and additional decay pathway provided by the Pt layer.26,45−47 The proposed experimental concept is schematically depicted in Figure 1. As a starting point, the Au grating, which is able to support the SPP excitation effectively, was used. The Au surface was further coated by thin Pt layer via reduction of chloroplatinic acid by ascorbic acid in water solutions.48 Then, the −C6H4−CCH organic moieties were covalently grafted to the bimetallic surface via the spontaneous reaction of arenediazonium tosylates with metal surfaces.49 Created samples were immersed in cyclohexene and illuminated at 785 nm wavelength, corresponding to the SPP absorption band. In our experimental concept, the SPP excitation activates the grafted organic moieties, whereas cyclohexene will serve as a hydrogen source, according to the literature data.50,51 The generation of the hydrogen species from the cyclohexene was tested by mass spectrometry (see the Supporting InformationFigure S1 and related discussion) and appearance of benzene peak in the reaction mixture, after the plasmon triggering convincingly proves the production of hydrogen. It should also be noted that grafted 4-ethynyl groups on the benzene ring are not in direct contact with a Pt layer. Therefore, the reaction path of hydrogenation is expected to be different from the traditional noble metal catalytic systems used for unsaturated carbon bond reduction. The successful deposition of the Pt additional layer was controlled using the XPS and AFM scratch test techniques. Increase of bimetallic structure thickness because of Pt deposition is plotted in Figure 2A as a function of H2PtCl6 reduction time. As is evident, after the initiation period (ca.
Figure 2. (A) Dependence of Pt thickness on the Au surface of the H2PtCl6 exposure time and (B) UV−vis spectra of pristine Au grating, and Au grating with Pt adlayers (Au/Pt-1 and Au/Pt-2 samples). 26615
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significantly changes the surface nanoroughness. This effect is more pronounced in the case of larger amounts of Pt. Further grafting of ADT−CCH led to the appearance of spotlike surface features, indicating the growth of polyphenylene organic layer. In the next step, the samples grafted by −C6H4−CCH moieties were immersed in the cyclohexene and triggered by 785 nm laser wavelength with a simultaneous plasmon-induced hydrogenation of ethynyl groups and online collection of SERS spectra. Time-resolved SERS spectra, corresponding to plasmon-induced chemical transformations, are presented in Figure 4. On the initial stage of the reaction, the attention deserves the Raman band assigned to CC bond, located at 2010 cm−1, which was used for the determination of triple bond conversion. In the SERS spectra of Au/Pt-1 and Au/Pt-2 gratings modified by ADT−CCH, the 2010 cm−1 vibration band is well visible, but its amplitude decreases during the illumination, indicating the hydrogenation of the triple bond. The hydrogenation dynamic and character of formed moieties were identified by additional features on the SERS spectra. In the case of Au/Pt-1 (Figure 4A), a pronounced peak, located at 1650 cm−1 (CC stretching vibrations), appears with the amplitude gradually increasing with plasmon triggering time and demonstrating the formation of CC double bonds. Simultaneously, the several closely spaced peaks in the 3000− 3250 cm−1 region, corresponding to ethenyl C−H group unsymmetrical and symmetrical vibrations, increase too. Therefore, the observed spectral changes corresponding to particular chemical groups show that the proceeding chemical transformations can be attributed to the hydrogenation of triple bonds to appropriate double bonds. The reaction conversion is considered as a complete one because the ethynyl-related peak entirely disappeared. Prolongation of illumination time or increasing of laser power density does not
Figure 3. AFM images of gratings morphology: (A) pristine Au grating; (B,B′) Au/Pt-1 and Au/Pt-2 gratings created by 15 and 40 min hydrogenation of H2PtCl6; (C,C′) Au/Pt-1 and Au/Pt-2 gratings after diazonium salt grafting; and (D,D′) Au/Pt-1 and Au/Pt-2 gratings after plasmon-induced hydrogenation of alkynes triple bonds.
Figure 4. Pristine and time-dependent (time in s) SERS spectra of plasmon-assisted hydrogenation of phenylacetylene groups on: (A) Au/Pt-1 and (B) Au/Pt-2 gratings under continuous exposure to laser illumination (wavelength785 nm, intensity7.2 μW/μm2). 26616
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Figure 5. Time-depending intensity of disappearing (at 2010 and 720 cm−1, C−H stretching of −CCH) and arising Raman bands [3230 and 1650 cm−1, alkene groups in the case of Au/Pt-1 (A) or 2910 and 1480 cm−1, alkyl groups in the case of Au/Pt-2 (B)] measured at laser powers (4.8 and 9.6 μW/μm2).
lead to any further spectral changes, indicating that the formed double bonds do not tend to be involved in a further hydrogenation reaction. Significantly different results (compared to Au/Pt-1) were observed in the case of plasmon triggering of the samples with “thicker” Pt layer (Au/Pt-2). SERS peak (2010 cm−1) corresponding to the triple bond of ethynyl moieties decreases, but the CC bond-related signal does not appear. Instead, several Raman bands, typical for the saturated hydrocarbon (1480 cm−1 C−H bending, 720 cm−1 C−H deformations, 2780−2950 cm−1 C−H unsymmetrical and symmetrical stretch) appeared with amplitudes increasing with prolongation of plasmon triggering time (Figure 4B). Observed spectral changes convincingly prove that in the case of Au/Pt-2 samples, the full hydrogenation of triple bonds occurred, leading to alkane formation. It should also be noted that in the online SERS measurements of CC hydrogenation on Au/Pt2, no peaks assigned to alkene were found. The reaction conversion rate can also be estimated as a complete one because the CC related vibration band entirely disappeared, and further illumination or increasing of laser intensity in the cyclohexene excess do not lead to any further spectral changes.
Additional XPS measurements were also performed after the reaction completion (Table S1) to eliminate potential mistakes because of degradation or detachment of surface groups. Because no significant differences in the surface element concentrations (C, Au, and O) were observed, the undergoing chemical transformation has to be attributed to the hydrogenation of CC bond and possible effects of side chemical transformations be excluded. We also performed the additional UV−vis measurements of the Au/Pt-1 and Au/Pt-2 gratings, grafted by −C6H4−CCH moieties before and after plasmontriggered selective hydrogenation of alkynes triple bonds. Results are presented in Figure S4 and indicate that the grafting of −C6H4−CCH slightly shifts the wavelength position of SPP-related absorption peak. Such a shift can be attributed to the changes of a dielectric medium, surrounding the plasmon-supported structure. The selective hydrogenation of ethynyl groups and formation of CC or C−C bonds also lead to the slight shift of SPP absorption band, probably because of the change of dipole moments of molecules, closely attached to the plasmonic surface. Effect of the Pt layer was studied in a control experiment with the plasmon triggering of −C6H4−CCH moieties 26617
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Figure 6. (A,B) First-order reaction kinetic curves of plasmon-induced hydrogenation of phenylacetylene groups on Au/Pt-1 and Au/Pt-2 gratings at different laser powers 4.8, 7.2, and 9.6 μW/μm2; (C,D) calculated first-order reaction constants K as a function of applied laser power for Au/Pt1 and Au/Pt-2 gratings correspondingly.
function of the laser power density, and it decreases with the increasing laser power in both cases. The kinetic of acetylene and phenylacetylene hydrogenation has been extensively discussed before in refs 55 and 56. There are two possible ways for the hydrogenation of phenylacetylene to the corresponding styrene and ethylbenzene: direct hydrogenation of the CC to C−C or two-step pathway including the formation of a single- or multiple-adsorbed hydrogen species. Taking into consideration the absence of Raman peak characteristic for the CC bond on the present time-resolved SERS spectra of Au/Pt-2 during plasmoninduced hydrogenation, we assume that, in our case, the hydrogenation proceeds in a one-step way. It should additionally be noted that the hydrogenation of ethynyl groups to ethenyl, observed on the Au/Pt-1 substrates, proceeds much faster than the hydrogenation to the ethyl groups in the case of Au/Pt-2 substrate. The assumption of the first-order reaction kinetics was used for the calculation (1) of hydrogenation rate constants, through the linear fit of ln(Ct/C0) = ln(It/I0) = Kt + b dependence, where It and I0 are Raman band intensities at time t and the reaction start (t = 0), respectively; Ct and C0 are the corresponding concentrations of the hydrogenated chemical group; and K is the reaction constant.57 The calculation was performed using intensities of 1650 cm−1 peak for Au/Pt-1 and 1480 cm−1 peak for Au/Pt-2. Obtained data are presented in Figure 6, which also shows the dependence of the reaction constants K on the applied laser intensities. The absolute values of reaction constants (Figure 6C,D) are significantly affected by applied laser power densities for both bimetallic substrates. An increase in the laser power leads to a significant increase in the reaction rate. Additionally, this phenomenon was founded to be more pronounced in the case of Au/Pt-1
grafted to the Au grating surface. The experimental arrangement was the same as in the previous case of bimetallic structure but without the Pt layer. Time-resolved SERS spectra, measured online after illumination with 785 nm wavelength at the highest laser power density of 9.6 μW/μm2, are presented in Figure S5. As is evident, no spectral changes were observed, indicating the absence of any process associated with the hydrogenation of ethynyl groups. Thus, we can conclude that the role of Pt in hydrogenation is crucial, although the triple bond is separated from the bimetallic surface by a benzene ring. Probably, Pt layer initiates the formation of active hydrogen from cyclohexene, which is further involved in the reaction with plasmon-activated ethynyl groups. To avoid potential errors in assessing the high value of chemical selectivity, we performed a series of reproducibility measurements. The results presented in Figure S6 show that the chemical selectivity is absolutely reproducible, whereas the intensity of formation and the growth of characteristic C−C or CC Raman bands coincide with an accuracy of up to 95% between similar samples. Figure 5A,B displays the intensities of Raman bands as a function of laser triggering time between 4.8 and 9.6 μW/μm2 power densities. The Raman bands related to a decrease in the amount of ethynyl groups and formation of ethenyl (for Au/ Pt-1) or ethyl groups (for Au/Pt-2) are plotted, corresponding to the following wave numbers: 2010 cm−1 (CC str), 720 cm−1 (C−H def), 1650 cm−1 (CC str), and 3230 cm−1 (C− H asym and sym str) cm−1 (Figure 5A) and 2010 cm −1 (CC str), 720 cm−1 (C−H def), 1480 cm−1 (CC str), and 2910 cm−1 (C−H asym and sym str) (Figure 5B). It is evident that the reaction starts immediately after the SPP excitation. The time required for the full conversion of ethynyl groups is a 26618
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affect the reaction selectivity, and (v) the reaction was much faster in the case of an Au/Pt-1 substrate, with a thinner platinum layer, on which the unsaturated double carbon bonds were formed. Recently, the focus of catalytically active plasmon-supported metal nanostructure synthesis is increasingly shifted toward bimetallic systems.33,58,59 Introduction of the bimetallic compound allows to significantly enhance the catalytic performance of pure active catalytic metals.40 In particular, taken separately, the metals show little catalytic activity (or its absence), whereas their combination leads to an almost 100% conversion under irradiation with the corresponding light wavelength.40 This is not surprising because for the major of plasmonic photocatalysts, fast charge recombination competes with charge separation, resulting in low photocatalytic efficiency. However, for bimetallic structures, the theoretical simulation shows that surface plasmon resonance enhanced the hot electron transfer from plasmonic to catalytic layer, creating the opportunity to start the chemical transformation.36 Moreover, the mean transfer of hot electrons from the plasmonic core to the catalytical shell was also observed experimentally.28,36 Additionally, utilization of the bimetallic plasmon-supported structure allows to introduce the selectivity in the chemical reaction. The plasmonic nanostructures absorb the light energy, and the excited hot electrons can activate the substrate or transformation, whereas the addition of a catalytical metal is responsible for the reaction selectivity.31 In our case, absence of the chemical transformation on pristine Au surface demonstrates that the implementation of Pt surface is necessary for the desired transformation. The fact that chemical transformation proceeds when the excitation wavelength is coincident with the SPP absorption band is strongly supportive of the conclusion that plasmon excitation is responsible for the hydrogenation of 4-ethynyl groups and the formation of active hydrogen species. The addition of thin Pt layer on the Au grating surface can modify the dominant channel of plasmon decay, although, for example, the introduction of a faster plasmon decay channel through the additional metal layer and thus a large fraction of the electromagnetic energy concentrated in the nanostructure is dissipated through the thin Pt shell via the formation of energetic charge carriers.27 Another issue that we demonstrated is that the dissipated energy through the Pt shell is able to perform chemical work (drive the chemical transformation). The low catalytic activity in the case of thinner Pt thickness can be attributed to the limited number of catalytic active sites for hydrogen production because of the formation of thin nonstoichiometric intermetallic phase, which prevents the appearance of catalytically active sites, necessary for the formation of active hydrogen species. However, the increase of Pt thickness creates catalytic sites on the samples surface, which are pumped by hot electrons under plasmon excitation and are responsible for the appearance of active hydrogen.27 The synergetic effect between the plasmonic enhancement of Au and the cocatalysis of Pt for photocatalytic hydrogen production was reported in several works.29 In turn, the gigantic electromagnetic energy concentration near the plasmonic surface also activates the grafted 4-ethynyl groups, which reacts with active hydrogen. The observed chemical selectivity is a function of the grafted organic moiety activation as well as the initial energy, which receive the active hydrogen species. In conclusion, our work presents the synergistic effect between the plasmonic enhance-
bimetallic structure, and even in this case, the reaction was completed much faster. The impact of the Pt thickness on the reaction rate and selectivity was further evaluated, and the results are presented in Figures S7−S9 (only “selected graphs” are given) as a series of SERS measurements. In turn, the estimated values of the reaction constant are given in Figure 7. As is evident, the
Figure 7. Calculated first-order reaction constants K as a function of Pt layer thickness at laser power 7.2 μW/μm2.
plasmon triggering starts from the thickness of Pt above 2 nm and leads to the formation of CC bonds. Further increasing of Pt thickness promotes the reaction rate up to 9.1 nm thickness of the Pt layer, with the formation of the same hydrogenation products. However, further increase of Pt thickness results in the shift of reaction selectivity, and only formation of C−C bonds was detected when the Pt thickness exceeds 10 nm. Despite our attempts to detect intermediate samples on which hydrogenation led to the simultaneous formation of single and double bonds, a similar effect was not observed. A further increase in Pt thickness, above 12 nm, led to a gradual decrease in the reaction rate. At the Pt thickness of more than 17 nm, any informative Raman peaks from grafted organics were not detected because of the suppression of SPP and loss of SERS activity. The observed sharp dependence of chemical selectivity on the Pt thickness can be explained by a complex mechanism, which includes the interaction of two metals, at which the hot electrons, excited in the Au grating, reach the catalytically active sites of Pt; simultaneously, the plasmon energy is “smeared” and averaged along the grating because of SPP propagation, grafted organic moieties undergo the strongly enhanced intensity of electromagnetic field near the metal surface and interact with the active hydrogen, formed on the reaction sites of Pt. Similar dependences of chemical selectivity and reaction rate were observed in a number of works31,40 and in the future may give the opportunity for an additional mechanism for regulating selectivity in the chemical transformation. Therefore, we can summarize our results as follows: (i) plasmon activation on bimetallic structure in cyclohexene led to the hydrogenation of surface phenylacetylene moieties, (ii) the reaction products (saturated or unsaturated carbon bonds) were defined by the initial structure of the bimetallic plasmon substrates, (iii) absolute chemoselectivity of the reaction was observed, (iv) the illumination time or laser intensity do not 26619
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ment of Au and the cocatalysis of Pt which gives some guidance for bimetallic catalyst design and utilization.
CONCLUSIONS In this article, the bimetallic periodical structure, able to support the plasmon-polariton excitation and propagation, was proposed for the selective hydrogenation of alkyne triple bonds. The proposed bimetallic heterostructure consists of plasmon-active Au grating, further covered with the Pt layer. To demonstrate the catalytic activity, 4-ethynylphenyl groups were covalently grafted to the Pt surface via diazonium chemistry and illuminated with 785 nm wavelength, corresponding to the maximum of surface plasmon polariton resonance in the cyclohexene solution. It was observed that the surface plasmon polariton effectively activates the process of 4ethynylphenyl hydrogenation. The structure of hydrogenation products was strongly affected by the parameters of the plasmon-supported structure. The “thinner” Pt layer leads to the formation of CC bonds (without the further hydrogenation under the subsequent illumination or increasing of laser intensity), whereas the entire hydrogenation (formation of C−C bonds) of ethynyl groups is observed in the case of “thicker” Pt layer (without the detection of double bonds during the plasmon catalysis procedure). ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b07398. General experimental remarks; synthesis and characterization of ADT (ADT−CCH); results of mass spectrometry analysis of postreaction mixture; XPS results of gold/platinum gratings before and after modification and hydrogenation; Raman spectra of unmodified and modified Au/Pt gratings; X-ray diffraction spectra; UV−vis spectra of gratings before and after modification and hydrogenation; time-dependent SERS spectra of modified grating after illumination; reproducibility of SERS study; time-dependent SERS spectra of plasmon-assisted hydrogenation of phenylacetylene groups gold/platinum grating with different Pt thicknesses; and first-order reaction kinetic curves of plasmon-induced hydrogenation of phenylacetylene groups on gold gratings with different Pt thickness (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
O. Lyutakov: 0000-0001-8781-9796 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the GACR under the project P108/12/G108 and by Tomsk Polytechnic University (Project VIU-RSCABS-196/2018). 26620
DOI: 10.1021/acs.jpcc.8b07398 J. Phys. Chem. C 2018, 122, 26613−26622
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