Plasmon Catalysis on Bimetallic SurfaceâSelective Hydrogenation of

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Plasmon Catalysis on Bimetallic Surface - Selective Hydrogenation of Alkynes to Alkanes or Alkenes Olga Guselnikova, Anastasiya Olshtrem, Yevgeniya Kalachyova, Illia Panov, Pavel S. Postnikov, Vaclav Svorcik, and Oleksiy Lyutakov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07398 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 15, 2018

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The Journal of Physical Chemistry

Plasmon Catalysis on Bimetallic Surface - Selective Hydrogenation Of Alkynes To Alkanes Or Alkenes O. Guselnikovaa,b, A. Olshtrema,b, Y. Kalachyovaa,b, I. Panovc, P. Postnikova,b, V. Svorcika, O. Lyutakova,b a

Department of Solid State Engineering, University of Chemistry and Technology, 16628 Prague,

Czech Republic b

Research School of Chemistry and Applied Biomedical Sciences, Tomsk Polytechnic University,

Russian Federation c

Group of Advanced Materials and Organic Synthesis, Institute of Chemical Process

Fundamentals, Czech Academy of Sciences, Rozvojová 1/135, 165 02 Prague, Czech Republic ____________________ * Corresponding author: [email protected]

Abstract Utilization of plasmonics as a driving tool for chemical transformations triggering enable to achieve the 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 the covalent grafting of the bimetallic surface with 4ethynylbenzenediazonium 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 of structure parameters, e.g., the thickness of the upper platinum layer.

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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 photo-oxidation of alcohols [7-14]. In particular, the plasmon-mediated 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 plasmon-based 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 achieve 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 the selectivity in the plasmon-triggering of chemical transformation is the application of 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 specific reactant to the bimetallic surface and/or 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 plasmon-triggering 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 metals heterostructures is expected to significantly increase the hot electron lifetime (through both plasmon-exciton coupling and efficient electronsink function of Pt), the reaction rate and to enhance the reaction selectivity [43].


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Experimental 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 %), 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. Samples 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 [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 was synthesized according to the previously published procedure [44], for detailed information see SI. 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-C≡CH for 20 min [44]. After the 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 SERS measurements were carried out on Au/Pt grating modified by ADT-C≡CH 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, the bimetallic gratings were immersed into cyclohexene and illuminated with a laser beam from Raman spectrometer. The on-line SERS measurements were performed in the reaction media with the 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, acetone and dried. Raman spectra were measured 30 times, each of them with 3 s accumulation time Measurement techniques 3 ACS Paragon Plus Environment


The X-ray photoelectron spectroscopy (XPS) was performed using an Omicron Nanotechnology ESCAProbeP spectrometer fitted with monochromated Al K Alpha X-ray source working at 1486.6 eV. For characterization of the grating’s surfaces, the peak force AFM technique was applied using the Icon (Brucker) microscope. AFM scratch tests were carried out on smooth gold film (25 nm) by profiling across a scratch at an angle of 900 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 portable ProRaman-L spectrometer Raman spectrometer with 785 nm excitation wavelengths. UV/Vis absorption spectra were measured by using a Lambda 25 UV/Vis/NIR Spectrometer (PerkinElmer, USA) at a scanning rate of 480 nm min-1.

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. Plasmon-triggering was performed through the surface plasmon-polariton 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 Pt layer [26, 45-47]. The proposed experimental concept is schematically depicted in the Fig. 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-C≡CH 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 the cyclohexene and illuminated at 785 nm wavelength, corresponding to SPP absorption band. In our experimental concept, the SPP excitation activates the grafted organic moieties, while 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 the mass spectrometry (see Supplementary Information - Fig. S1 and related discussion) and appearance of benzene peak in the reaction mixture, after the plasmon triggering convincingly proofs the production of hydrogen. It should also be noted that grafted 4-ethynyl groups on the benzene ring are not in the 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 bonds reduction. 4 ACS Paragon Plus Environment

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The successful deposition of Pt additional layer was controlled using the XPS and AFM scratch test techniques. Increase of bimetallic structure thickness, due to Pt deposition is plotted in the Fig. 2A as a function of H2PtCl6 reduction time. As is evident, after the initiation period (ca 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 the Tab. S1, summarizing the surface elements concentrations. On the pristine Au surface only Au (Au 4f, 84.6 eV), C (C1s, 284.6 eV) and O (O1s, 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 reaction time to 40 min (Au/Pt-2) results in an increase of surface Pt concentration with simultaneous decrease of Au concentration. Subsequent grafting of 4-ethynylphenyl functional groups (results are presented in the Tab. S1) led to a reduction of Pt surface concentration and an increase of carbon concentration, both confirming the success of the grafting procedure. Raman spectroscopy (see Fig. S2 confirms successful covalent grafting by appearance of absorption bands associated with 4-ethynylbenzene functional groups, namely: 2010 cm-1 (C≡CH stretch), 1590 cm-1 (Ar ring stretch), 1180 cm-1 (C-H in-plane deformations), 1130 cm-1 (-C-C≡C stretch), 720 cm-1 (CH 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 [52,53]. In this way, a possible formation of an active complex of C≡C group with Pt was excluded. The crystalline structure of SPP-active gratings was studied by XRD before and after the Pt deposition. In the spectra shown in the Fig. 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 the Pt deposition so that the structure of 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 the Fig. 2B. Pristine Au grating shows a pronounced SPP related absorption band, located near 760 nm. The addition of thin Pt layer led to the significant broadening of the absorption band and its redshift (which coincides with the literature data [29, 54]. Further increase of Pt thickness does not affect the width of the plasmon peak, but the wavelength shift becomes more pronounced (which was also observed earlier [29,54]). 5 ACS Paragon Plus Environment


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The bold arrow in the Fig. 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 effective plasmon catalysis activation. The

changes

in

the

surface

morphology

after

the

Pt

deposition

and

4-

ethynylbenzenediazonium tosylate (ADT-C≡CH) grafting were checked using the AFM technique (Fig. 3). The pristine Au grating surface represents well-ordered periodical surface with a sinusoidal shape. The addition of Pt layer does not affect grating parameters, in terms of periodicity and amplitude, but significantly change the surface nanoroughness. This effect is more pronounced in the case of the larger amount of Pt. Further grafting of ADT-C≡CH led to the appearance of spottylike surface features, indicating the growth of polyphenylene organic layer. In the next step, the samples grafted by –C6H4-C≡CH moieties were immersed in the cyclohexene and triggered by 785 nm laser wavelength with 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 the Fig. 4. On the initial stage of the reaction, the attention deserves the Raman band assigned to C≡C 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-C≡CH, 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 (Fig. 4A), a pronounced peak, located at the 1650 cm-1 (C=C stretching vibrations) appears with the amplitude gradually increasing with plasmon triggering time and demonstrating the formation of C=C 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. So that, 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 complete one since the ethynyl-related peak entirely disappeared. Prolongation of illumination time or increasing of laser power density does not 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 C=C bond related signal does not appear. 6 ACS Paragon Plus Environment

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Instead, several Raman bands, typical for 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 (Fig. 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 C≡C hydrogenation on Au/Pt-2 no peaks assigned to alkene were found. The reaction conversion rate can also be estimated as complete one since the C≡C 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 (Tab. S1), in order to eliminate potential mistakes due to degradation or detachment of surface groups. Since no significant differences in the surface elements concentrations (C, Au, O) were observed, the undergoing chemical transformation has to be attributed to the hydrogenation of C≡C 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-C≡CH moieties before and after plasmon triggered selective hydrogenation of alkynes triple bonds. Results are presented in the Fig. S4 and indicate that the grafting of –C6H4-C≡CH slightly shift the the wavelength position of SPP related absorption peak. Such a shift can be attributed to the changes of dielectricum medium, surrounding the plasmon-supported structure. The selective hydrogenation of ethynyl groups and formation of С=С or C-C bonds also leads 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 Pt layer was studied in a control experiment with the plasmon triggering of –C6H4C≡CH moieties 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 the Fig. S5. As is evident, no spectral changes were observed, indicating the absence of any process associated with 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. In order to avoid potential errors in assessing the high value of chemical selectivity, we performed a series of reproducibility measurements. The results are presented in Fig. S6 show that the chemical



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selectivity is absolutely reproducible, while the intensity of formation and growth of characteristic C-C or C=C Raman bands coincide with an accuracy of up to 95% between the similar samples. Figs 5A and 5B display 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 following wave numbers: 2010 cm-1 (C≡C str), 720 cm-1 (C-H def), 1650 cm-1 (C=C str) and 3230 cm-1 (C-H asym and sym str) cm-1 (Fig. 5A) and 2010 cm -1 (C≡C str) , 720 cm-1 (C-H def), 1480 cm-1 (C=C str) and 2910 cm-1 (C-H asym and sym str) (Fig. 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 function of the laser power density, and it decreases with increasing laser power in both cases. The kinetic of acetylene and phenylacetylene hydrogenation has been extensively discussed before in [55,56]. There are two possible ways for the hydrogenation of phenylacetylene to the corresponding styrene and ethylbenzene: direct hydrogenation of the C≡C 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 C=C bond on the present timeresolved SERS spectra of Au/Pt-2 during plasmon-induced 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 calculation (1) of hydrogenation rate constants, through the linear fit of the

ln (Ct/C0)=ln (It/I0)= Kt+b

dependence,where It and I0 are Raman band intensities at the 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 the intensities of 1650 cm-1 peak for Au/Pt-1 and 1480 cm-1 peak for Au/Pt-2. Obtained data are presented in the Fig. 6, which also shows the dependence of the reaction constants K on the applied laser intensities. The absolute values of reaction constants (Fig. 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 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 results are presented in the Fig. S7, S8 and S9 (only “selected graphs” are given) as a series of SERS measurements. In turn, the estimated values of reaction constant are given in the Fig. 7. As is 8 ACS Paragon Plus Environment

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evident, the plasmon triggering starts from the thickness of Pt above 2 nm and lead to the formation of C=C bonds. Further increasing of Pt thickness promote the reaction rate up to 9.1 nm thickness of 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, due to the suppression of SPP and loss of SERS activity. The observed sharp dependence of chemical selectivity on the Pt thickness can be explained by the 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 plasmon energy is “smeared” and averaged along the grating due to 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 works [31,40] and in the future may give the opportunity for an additional mechanism for regulating selectivity in the chemical transformation. So, we can summarize our results as follow: (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 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 plasmonsupported metal nanostructure synthesis is increasingly shifting toward bimetallic systems [33,58,59]. Introduction of the bimetallic compound allows to significantly enhance the catalytic performance of pure active catalytical metals [40]. In particular, taken separately, the metals show little catalytical activity (or its absence), while their combination leads to an almost 100% conversion under the irradiation with corresponded light wavelength [40]. This is not surprising since 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 electrons 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 9 ACS Paragon Plus Environment


was also observed experimentally [28, 36]. Additionally, utilization of the bimetallic plasmonsupported 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, while the addition of catalytical metal is responsible 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 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 formation of active hydrogen species. The addition of thin Pt layer on the Au grating surface can modify the dominant channel of plasmon decay, though, for example, 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 through the Pt shell energy is able to perform chemical work (drive the chemical transformation). The low catalytical activity in the case of thinner Pt thickness can be attributed to the limited number of catalytical active sites for hydrogen production, due to the formation of thin non stoichiometric intermetallic phase, which prevents the appearance of catalytical active sites, necessary for the formation of active hydrogen species. However, increase of Pt thickness creates the catalytical sites on the samples surface, which are pumped by hot electrons under the plasmon excitation and are responsible for the appearance of active hydrogen [27]. The synergetic effect between the plasmonic enhancement of Au and the co-catalysis of Pt for photocatalytic hydrogen production was reported in several works [29]. In turn, the gigantic electromagnetic energy concentration near the plasmonic surface also activate the grafted 4-ethynyl groups, which reacts with active hydrogen. The observed chemical selectivity is a function of grafted organic moieties activation as well as initial energy, which receive the active hydrogen species. In conclusion, our work presents the synergistic effect between the plasmonic enhancement of Au and the co-catalysis of Pt which gives some guidance for bimetallic catalyst design and utilization.

Conclusion In this paper, the bimetallic periodical structure, able to support the plasmon-polariton excitation and propagation was proposed for selective hydrogenation of alkynes triple bonds. The proposed bimetallic heterostructure consists of plasmon-active Au grating, further covered with Pt layer. To demonstrate the catalytic activity, 4-ethynylphenyl groups were covalently grafted to Pt 10 ACS Paragon Plus Environment

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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 surface plasmon polariton effectively activate the process of 4-ethynylphenyl hydrogenation. The structure of hydrogenation products was strongly affected by the parameters of the plasmonsupported structure. The “thinner” Pt layer leads to the formation of С=С bonds (without the further hydrogenation under the subsequent illumination or increasing of laser intensity), while 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).

Acknowledgment This work was supported by the GACR under the project P108/12/G108 and by Tomsk Polytechnic University (Project VIU-RSCABS-196/2018).

Supporting Information. General experimental remarks; Synthesis and characterization

of 4-ethynylbenzenediazonium

tosylate (ADT-C≡CH); 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 SERS study; time-dependent SERS spectra of plasmon-assisted hydrogenation of phenylacetylene groups gold/platinum grating with different Pt thickness; first order reaction kinetic curves of plasmon-induced hydrogenation of phenylacetylene groups on gold gratings with different Pt thickness.

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plasmon-induced

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reactions

using

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Figure caption Fig. 1 Schematic presentation of proposed experimental concept: (i) creation of bimetallic, SPP supported heterostructure with different Pt thickness (A and B), (ii) grafting of alkynes triple bonds containing chemical moieties, (iii) utilization of plasmon catalysis for chemically selective hydrogenation of unsaturated carbon bonds (C≡C). Fig. 2 (A) – dependence of Pt thickness on the Au surface on the H2PtCl6 exposure time , (B) – UV-Vis spectra of pristine Au grating, and Au grating with Pt adlayers (Au/Pt-1 and Au/Pt-2 samples). Fig. 3. AFM images of gratings morphology: (A) – pristine Au grating, (B and B’)– Au/Pt-1 and Au/Pt-2 gratings created by 15 and 40 min hydrogenation of H2PtCl6, (C and C’)– Au/Pt-1 and Au/Pt-2 gratings after diazonium salt grafting, (D and D’) – Au/Pt-1 and Au/Pt-2 gratings after plasmoninduced hydrogenation of alkynes triple bonds. 16 ACS Paragon Plus Environment

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Fig. 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 (wavelength - 785 nm, intensity - 7.2 µW/µm2). Fig. 5 Time-depending intensity of disappearing (at 2010 and 720 cm-1, C-H stretching of -C≡CH) 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) Fig. 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/Pt-1 and Au/Pt-2 gratings correspondingly. Fig. 7 Calculated first order reaction constants K as a function of Pt layer thickness at laser power 7.2 µW/µm2



Fig. 1.


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Fig. 2.



Fig. 3.


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Fig. 4



Fig. 5


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Fig. 6



Fig. 7


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Plasmon Catalysis on Bimetallic SurfaceâSelective Hydrogenation of

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