Highly Oriented Pyrolytic Graphite Catalysts: from

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Bimetallic Pd-Au/HOPG Catalysts: from Composition to Pairwise Parahydrogen Addition Selectivity Andrey V. Bukhtiyarov, Dudari B. Burueva, Igor P. Prosvirin, Alexander Yu. Klyushin, Maxim A. Panafidin, Kirill V. Kovtunov, Valerii I. Bukhtiyarov, and Igor V. Koptyug J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06281 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 27, 2018

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The Journal of Physical Chemistry

Bimetallic Pd-Au/HOPG Catalysts: from Composition to Pairwise Parahydrogen Addition Selectivity Andrey V. Bukhtiyarov,*,† Dudari B. Burueva,‡,§ Igor P. Prosvirin,†,§ Alexander Yu. Klyushin,∥,⊥ Maxim A. Panafidin,† Kirill V. Kovtunov,*,‡,§ Valerii I. Bukhtiyarov,† Igor V. Koptyug,‡,§ †

Boreskov Institute of Catalysis SB RAS, 5 Acad. Lavrentiev Ave., 630090 Novosibirsk, Russia

‡

International Tomography Center SB RAS, 3A Institutskaya St., 630090 Novosibirsk, Russia

§

Novosibirsk State University, 2 Pirogov St., 630090 Novosibirsk, Russia

∥Fritz

Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany

⊥Helmholtz-Zentrum

Berlin für Materialien und Energie GmbH, Albert-Einstein-Str. 15, 12489

Berlin, Germany KEYWORDS: bimetallic catalysts, parahydrogen, XPS, STM, Pd-Au

ABSTRACT

The model Pd and Au mono- and bimetallic (Pd-Au) catalysts were prepared using vapor deposition of metals (Au and/or Pd) under ultra-high vacuum conditions on the defective highly

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oriented pyrolytic graphite (HOPG) surface. The model catalysts were investigated using the Xray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) at each stage of preparation procedure. For the preparation of bimetallic catalysts, different procedures were used to get different structures of PdAu particles – Aushell-Pdcore or alloyed. All prepared catalysts showed rather narrow particles size distribution with an average particles size in the range of 4-7 nm. Parahydrogen-enhanced nuclear magnetic resonance spectroscopy was used as a tool for the investigation of Pd-Au/HOPG, Pd/HOPG and Au/HOPG model catalysts in propyne hydrogenation. In contrast to Au sample, Pd, PdAualloy, and Aushell-Pdcore samples were shown to have catalytic activity in propyne conversion, and pairwise hydrogen addition routes were observed. Moreover, bimetallic samples demonstrated the 2- to 5-fold higher activity in pairwise hydrogen addition in comparison to the monometallic Pd sample. It was shown that the structures of bimetallic Pd-Au particles supported on HOPG strongly affected their activities and/or selectivities in propyne hydrogenation reaction: the catalyst with Aushell-Pdcore structure demonstrated higher pairwise selectivity than that with PdAualloy structure. Thus, the reported approach can be used as an effective tool for the synergistic effects investigations in hydrogenation reactions over model bimetallic Pd-Au catalysts, where the active component is supported on a planar support.

INTRODUCTION Bimetallic catalysts often demonstrate improved catalytic properties (selectivity, activity, and stability) in comparison with monometallic ones.1–4 In particular, the Pd-Au, probably the most studied system, has shown exceptional catalytic performance for the direct formation of hydrogen peroxide from the H2/O2 mixture,5 acetoxylation of ethylene to vinyl acetate,6


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hydrogenation of hydrocarbons,7 etc. However, the nature of synergistic effects in different catalytic reactions is still under discussion.8,9 It is well known that addition of a second metal leads to the formation of the active sites with a specific geometry (ensemble effect) and/or modification of the electronic properties of active metal (ligand effect). Therefore the formation of the specific surface composition of bimetallic Pd-Au particles plays the crucial role in synergistic effect.6,7,10–12 Obviously, the detailed study of bimetallic Pd-Au catalysts with controlled particle size distribution, Au/Pd ratio and particles structure (“core-shell”/alloyed) could help to understand the nature of active sites and to optimize the catalyst composition for the best activity, selectivity, and stability. Using model catalysts with metal particles deposited on a planar support could help to get more reliable data concerning the surface structure and chemical composition of active metals depending on different treatments13–16 since in a case of the “real” catalysts, where nanoparticles of active metals are deposited on high surface area support, the application of surface sensitive techniques is limited due to low surface concentration of the active component. From the other side, the total amount of active sites in model systems is much lower compared to “real” catalysts. Thus, catalytic testing of model systems under the conditions which are normally used for “real” catalysts leads to negligible conversion/products yields. In such case, some specific tools or approaches for catalytic activity investigations of model systems could be used, such as microreactors.17,18 In this work, the catalytic performance of planar model systems was examined with the use of parahydrogen-induced polarization (PHIP) technique.19 The principle of PHIP technique is to create a non-equilibrium population of nuclear spin levels for molecules through the catalytic hydrogenation with parahydrogen – spin isomer of hydrogen with total nuclear spin I=0.20 PHIP effect arises in product molecules only in case of


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pairwise addition of parahydrogen molecule to an unsaturated precursor, that is, when two H atoms of a single parahydrogen molecule end up in the same product molecule. The product and/or reaction intermediate then exhibit strongly enhanced (by up to 4-5 orders of magnitude, in theory) NMR signals in comparison with a spectrum at thermal equilibrium.19 The PHIP effects can be observed for both homogeneous21 and heterogeneous22 catalysts. The latter approach named HET-PHIP (parahydrogen-induced polarization in heterogeneous reactions) attracts considerable interest due to the possibility to produce hyperpolarized gases and liquids which are not contaminated with the catalyst and can be used in various magnetic resonance imaging applications.23–25 Throughout the last decade, the scope of catalysts suitable for HET-PHIP was significantly expanded: from bulk metal catalysts26 to single-site catalysts27. Moreover, the series of systematic investigations revealed the influence the nature of the active metal,26,28 particle size distribution,29 the interaction between metal and support30,31 on the efficiency of pairwise hydrogen addition. However, the nature of many heterogeneous catalysts is such that the contribution of the pairwise hydrogen addition is rather low (1-3%), which is explained by the migration of H atoms on the catalyst surface, leading to significant losses of the correlation between H atoms of parahydrogen molecule.26 One of the promising ways to increase the pairwise addition efficiency of HET-PHIP catalysts is to implement the single-site nature of a homogeneous one in HET-catalysts and utilize the heterogeneous catalysts with the well-defined single-site structure of active centers.27 At first, the immobilization approach was utilized, and immobilized metal complexes were successfully used as catalysts for PHIP.32,33 However, they suffer from catalyst deactivation and leaching of the metal. So, single-atom alloy (SAA) catalysts attracted considerable attention.34 Recently, it was shown that Pt-Sn35 bimetallic catalyst provided a ~3000-fold increase in the pairwise selectivity compared to the monometallic Pt, and


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produced HP propane with 11% pairwise selectivity, but the overall conversion estimated as 0.5% is not sufficient for MRI applications. However, in recent work, the bimetallic Pd-In36 catalytic system was utilized and the significant the contribution of pairwise hydrogen addition of 9% was achieved for propene along with high conversion of 20% during selective heterogeneous hydrogenation of propyne. Nevertheless, the more detailed insight into the bimetallic catalyst structure and its effect on the pairwise route would be useful for designing an efficient catalytic system with the large contribution of the pairwise addition route. Most recently, Wang et al. demonstrated that both overall conversion and pairwise hydrogen addition activity were correlated with composition and morphology of Pd-Au bimetallic supported particles.37 It was found that the random PdAu alloy structure provided larger PHIP effects compared to Pdcore-Aushell and monometallic Pd. Since PHIP approach provides very high sensitivity in NMR, it seems promising to solve the inverse issue of revealing the changes in catalytic composition/structure with the use of PHIP technique. The recent results provide prerequisites for this – the PHIP technique was successfully utilized to investigate strong metalsupport interaction (SMSI) effects along with transmission electron microscopy and X-ray photoelectron spectroscopy, and it was shown that PHIP effects are much more sensitive to the onset of SMSI effects than the catalytic activity or chemisorption measurements.30 Here, we report for the first time the results of the investigation using PHIP approach of hydrogenation reaction over the model Pd and Au mono- and bimetallic (Pd-Au) catalysts supported on highly oriented pyrolytic graphite (HOPG) via physical vapor deposition. The primary goal of the work was to check the potential of application of PHIP technique as a tool for investigation of synergetic effects in model systems with metal particles deposited on HOPG. The enhanced pairwise hydrogenation activity of the bimetallic Pd-Au catalyst was observed,


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and the correlation between activity in pairwise hydrogen addition and surface structure (“coreshell”/alloyed) of model Pd-Au bimetallic catalysts was established. MATERIALS AND METHODS XPS measurements and catalysts preparation The preparation and characterization of model mono- and bimetallic catalyst samples (Au/HOPG, Pd/HOPG, and Pd-Au/HOPG) were performed inside a photoelectron spectrometer (SPECS, Germany) with three main chambers: load lock, preparation, and analyzer chambers. For the catalysts preparation the Pd and Au foils with a high purity (0.9999) were used. The small pieces of foils were mounted inside tantalum crucible and evaporated using Electron Beam Evaporator Omicron EFM3. Monometallic Au/HOPG and Pd/HOPG model catalysts were prepared using the earlier reported three-step procedure for monometallic gold and silver particles which includes surface defects formation on the HOPG surface via smooth Ar+ sputtering (t = 3-4 sec, P(Ar) = 3×10-6 mbar and accelerating voltage of 0.5 kV) followed by Au or Pd deposition (carried out in a preparation chamber of SPECS spectrometer using Omicron EFM3 electron beam evaporator) and defects annealing (mainly interlayer defects) at 300 °C under ultra-high vacuum conditions.44,45 The bimetallic Pd-Au/HOPG model catalyst with alloyed Pd-Au particles was prepared by physical vapor deposition (PVD) of Pd on Au/HOPG matrix with the subsequent annealing at UHV to 400 °C to form Pd-Au alloyed particles.46 The bimetallic Au-Pd/HOPG model catalyst with Aushell-Pdcore particles was prepared by PVD of Au on Pd/HOPG matrix. Analyzer chamber is equipped with the hemispherical analyzer PHOIBOS150-MCD-9, the ellipsoidal monochromator FOCUS 500, and the X-ray source XR 50M with double Al/Ag anode. In the present work, AlKα (hν = 1486.74 eV, 200 W) was used as primary


6

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irradiation. The binding energy (BE) scale was pre-calibrated using the positions of Au4f7/2 (84.0 eV) and Cu2p3/2 (932.7 eV) photoelectron lines from metallic gold and copper foils. The pressure in the analysis chamber did not exceed 5×10-9 mbar. Spectral analysis and data processing were performed with XPS Peak 4.1 software.38 The position of C1s line at BE = 284.5 eV from the support (HOPG) was used as an internal standard for calibration of other measured peaks.39 For quantitative analysis, integral intensities of measured peaks in XPS spectra (Au4f, C1s, and Pd3d) were corrected by their respective atomic sensitivity factors.40 For the peak fitting of Pd3d, the Au4d5/2 peak was subtracted from spectra because of their overlapping. The shape and integral intensity of Au4d5/2 were calculated from a less intense Au4d3/2 peak. The depth profiling of Au and Pd in the bimetallic model Pd-Au/HOPG catalysts after catalytic testing was performed at the ISISS beamline at synchrotron radiation source BESSY II/HZB (Berlin, Germany).18 With the aim to estimate Pd/Au, Pd/C, and Au/C atomic ratios on the surface, all XPS lines from gold, palladium, and carbon were measured at different excitation energies to provide the same kinetic energy of 300, 450 and 600 eV. To determine the positions of the peaks in the Au4f and Pd3d spectra, XPS peaks were calibrated against the C1s spectra taken at the same primary excitation energies as Au4f and Pd3d. For the quantitative analysis, the integral intensities of Au4f, Pd3d, and C1s lines were corrected using ionization cross-section data taken from the work of Yeh and Lindau41 and also were normalized with respect to current and photon flux. For the estimation of the analysis depth at different kinetic energies of 300, 450 and 600 eV, the average value of inelastic mean free path (IMFP) of electrons in metallic gold and palladium was assumed as 5.9, 7.6 and 9.2 Å, respectively.42 Scanning tunneling microscopy


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STM measurements of Au/HOPG, Pd/HOPG, and Pd-Au/HOPG samples (fresh and after catalytic testing) were carried out using a UHV 7000 VT microscope (RHK Technology, USA) operating in constant current mode. For all images, the cut Pt-Ir tips were used. The standard samples of Si (111) single crystal with 7×7 reconstruction and clean HOPG were utilized for scanner calibration. Catalytic tests Commercially available hydrogen and propyne were used as received. For PHIP experiments, hydrogen gas was enriched with parahydrogen up to 90.5% using Bruker parahydrogen generator BPHG-90. The mixture of propyne/p-H2 with 1 : 4 ratio was used. All hydrogenation experiments were performed at atmospheric pressure. The 1H NMR spectra were acquired on a 300 MHz Bruker AV 300 NMR spectrometer using a π/4 rf pulse (PASADENA protocol).43 In PASADENA experiments the catalysts were placed at the bottom of 10 mm NMR tube situated inside the NMR spectrometer. The sample was heated under He flow from room temperature to 130 °C and held for 10 min. The gas flow rate was measured using an Aalborg rotameter and was varied from 1.3 to 5.1 mL/s. A scheme of the experimental setup is presented in Figure 1.


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Figure 1. A scheme of the experimental setup RESULTS AND DISCUSSION Recently it was shown that mainly the particles with “core-shell” structures are formed on the HOPG surface before final annealing at UHV to 400 °C.46 All samples were investigated using XPS and STM at all stages of preparation procedure. The regularities of preparation and characterization of bimetallic Pd-Au nanoparticles supported on HOPG by STM and XPS were discussed in a recently published paper.46 The STM images of all model catalysts are presented in Figure 2. All samples are characterized by a rather narrow particle size distribution with the mean particle size in the range of 4-7 nm. The mean particle size () was determined according to the equation:

∑ , ∑

where Ni is the number of particles with diameter di.


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Figure 2. STM images (100×100 nm), the particle size distributions and mean particles size of prepared model catalysts: (a) – Au/HOPG; (b) – Pd/HOPG; (c) – PdAualloy/HOPG and (d) – Aushell-Pdcore/HOPG. Tunneling parameters: (a) 0.30 nA, -1500 mV; (b) 0.46 nA, -1400 mV; (c) 0.50 nA, -1500 mV; (d) 0.75 nA, -1500 mV. Table 1 shows Au/C, Pd/C, and Pd/Au (for bimetallic sample) atomic ratios for the model Au/HOPG, Pd/HOPG and Pd-Au/HOPG samples calculated from XPS spectra taken at each step of preparation procedure. Four monometallic (Pd/HOPG and Au/HOPG) samples with the different amount of deposited metals were prepared at the first step. Two monometallic Au/HOPG (Au/C = 0.006) and Pd/HOPG (Pd/C = 0.018) samples were then used as matrices for the second metal deposition. As noted earlier in recently published work, this step leads to the formation of the bimetallic particles with a “core-shell” structure on the HOPG surface.46,47 The amount of second metal deposited on the surface was varied by the duration of evaporation to get similar Pd/Au ratios for both bimetallic samples, which was around 1.5-1.6 (Table 1). It should be mentioned that bimetallic samples have different Pd/C atomic ratio, i.e., different amounts of deposited palladium, which is equal to 0.008 and 0.018 for PdAualloy/HOPG and AushellPdcore/HOPG model catalysts, respectively. Then the PdAualloy/HOPG sample was annealed up to 400 °C at UHV to form the bimetallic particles with alloyed structure. The decrease of Pd/Au atomic ratio to 1.2 after annealing indicates the metals redistribution (Au segregation on the surface of bimetallic nanoparticles), i.e., the PdAu alloy formation.46–49 Table 1. Atomic ratios of elements calculated from XPS data measured at SPECS (using AlKα source).

Sample

Monometallic Me/HOPG

Me2 deposition (“core-shell” structure)

Annealing at 400°C (alloy formation)

After catalytic testing


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Me/C

Me2/C

Pd/Au

Pd/Au

Pd/Au

Pd/C

Au/HOPG

Au/C = 0.012

–

–

–

–

-

Pd/HOPG PdAualloy/ HOPG Aushell-Pdcore/ HOPG

Pd/C = 0.022

–

–

–

–

Au/C = 0.006

Pd/C = 0.008

1.5

1.2

1.5

0.006

Pd/C = 0.018

Au/C = 0.010

1.6

–

1.0

0.007

The XPS spectra of Au4f and Pd3d measured for prepared samples are presented in Figure 3. Pd3d and Au4f core-level spectra demonstrated that Pd and Au exist on the HOPG surface in two states with different binding energies (BE) values. In the recently published papers45,46 the Pd3d5/2 peak at 337.2 eV and Au4f7/2 peak at 85.0 eV were assigned to Pd, Au and/or Pd-Au bimetallic nanoclusters attached to carbon defected sites, which were formed during smooth Ar+ sputtering of the HOPG surface. In case of model monometallic Au/HOPG and Pd/HOPG catalysts, BE of main species of Pd3d5/2 and Au4f7/2 were 335.6 and 84.2 eV respectively, which correspond to Pd0

7,46,50

and Au0

46,51

species. The same values were obtained in the case

of Aushell-Pdcore/HOPG catalysts indicating the absence of alloyed particles on the HOPG surface. For bimetallic PdAualloy/HOPG sample, XPS spectra show the shift of Pd3d5/2 and Au4f7/2 peaks to lower binding energies (Figure 3) confirming the Pd-Au alloy formation.46,48,49 Thus, two monometallic Au/HOPG and Pd/HOPG, and two bimetallic PdAu/HOPG samples with different particle structures (“Aushell-Pdcore” and alloyed) were prepared with rather narrow particle size distributions and similar average particles size.


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335.6

Pd3d

84.2

Au4f 337.2

85.0

Aushell- Pdcore/HOPG 83.8

335.2

PdAualloy/HOPG

Pd/HOPG

346 344 342 340 338 336 334 332

Binding energy (eV)

Au/HOPG

92

90

88

86

84

82

Binding energy (eV)

Figure 3. XPS spectra of Au4f and Pd3d for all prepared mono- and bimetallic model catalysts measured using AlKα source (hν = 1486.74 eV). To probe the catalytic activity of model planar catalysts, the gas-phase hydrogenation of propyne was carried out. The catalyst slab was placed at the bottom of NMR tube maintained at 130 °C (Figure 1). The reaction mixture (propyne : p-H2 enriched gas = 1:4) was continuously supplied to the catalyst and NMR spectra of the product mixture were acquired using a π/4 rf pulse (PASADENA43 protocol). First, reference experiments with pristine HOPG were performed and no catalytic activity in propyne hydrogenation was found. Notably, for all samples no propene signals were observed in the thermal NMR spectra even after interruption of the gas flow. However, the moderate hyperpolarized signals of CH and CH2 groups of propene (signals 4, 3 and 5 in Figure 4) were observed in the 1H NMR spectra acquired during the gas flowing over the Pd, PdAualloy, and Aushell-Pdcore samples. It means that for all Pd-based samples


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propene was formed, but the thermal signal of produced propene did not exceed the NMR noise level. Also, it was shown that Au/HOPG sample was completely inactive and no hyperpolarized products were detected under the same experimental conditions (Figure 4b). For Pd-containing samples, the upper estimate of propyne conversion was calculated from the signal-to-noise ratio as 1-3%. Such low conversions could be explained by the very low metal loading of the model samples used. Importantly, the activity of model Aushell-Pdcore catalyst means that the Pd active sites should be present at the bimetallic particles surface, despite the Aushell-Pdcore structure of formed particles, since the monometallic gold catalyst is fully inactive under the same experimental conditions. Also, the different intensities of signals 3 and 5 (Figure 4) indicate that the hydrogenation over Pd, PdAualloy, and Aushell-Pdcore samples proceeds preferentially through syn-addition of hydrogen that can be an evidence of single-site nature of active sites.


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Figure 4. (a) Scheme of propyne hydrogenation. (b)-(e) 1H NMR spectra acquired in propyne hydrogenation with parahydrogen over Au/HOPG (b), Pd/HOPG (c), PdAualloy/HOPG (d), and Aushell-Pdcore/HOPG (e) samples at 130 °C while the gas was flowing with 2.5 mL/s flow rate. All spectra were acquired with 64 acquisitions and are presented on the same vertical scale. It is important to note that both bimetallic PdAuаlloy and Aushell-Pdcore samples demonstrate the factor of 2-5 stronger polarized signals of propene in comparison to monometallic Pd sample (Figure 4c-e). It was mentioned above, that Wang et al. demonstrated that both overall conversion and pairwise hydrogen addition activity were correlated with composition and morphology of Pd-Au bimetallic supported particles for the heterogeneous hydrogenation of propene to propane.37 Opposite to obtained results presented in Figure 4, the Wang et al. found that the random PdAu alloy structure provided larger PHIP effects compared to Pdcore-Aushell and monometallic Pd, that without doubt resulted from the both completely different catalytic systems (titania supported nanoparticles vs 2D planar catalytic systems) and substrates natures (propene vs propyne) used. Our observations could be explained by the higher activity of bimetallic catalysts in pairwise hydrogen addition under the assumption that the overall conversion to propene for both bimetallic catalysts is the same. This assumption can be verified by the fact that the Pd/C ratios for both samples after testing are almost the same (0.006-0.007). The higher selectivity of bimetallic samples in pairwise hydrogen addition compared to monometallic ones can be explained by the lower mobility of H atoms which are formed during H2 adsorption on metal center on the catalyst surface leading to preservation of spin correlation between the hydrogen atoms of parahydrogen. The restricted mobility of H atoms is caused by Pd isolation by inactive in hydrogenation Au atoms on the surface, i.e. by the disruption of continuous Pd ensembles by gold. The higher activity in pairwise hydrogenation of Aushell-


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Pdcore/HOPG relative to PdAualloy/HOPG sample (Figure 4d-e) also confirms Pd sites isolation by Au atoms since the Pd/Au ratios obtained by XPS after catalytic testing demonstrate the stronger dilution of Pd sites by Au atoms for the most active Aushell-Pdcore/HOPG sample (Table 1). Importantly, STM data obtained after catalytic testing show that for all model catalysts both the particle size distributions and the mean particle size remained the same. As an example, STM images showed the invariance of mean particle size of 4.7 nm before and after catalytic reaction for PdAualloy/HOPG catalyst (Figure S1). In addition, the stability test for PdAualloy/HOPG sample was performed: the catalyst was exposed to the reaction mixture under the same experimental conditions (130 °C, gas flow rate 2.5 mL/s) during 50 min (Figure S2). It was shown that after 50 min NMR spectrum of product mixture did not change and this stable performance of PdAualloy catalyst can be an indirect confirmation of the invariance of catalyst particle size. This allows us to suggest that model bimetallic Pd-Au/HOPG catalysts are stable against sintering under propyne hydrogenation conditions and the Pd/Au atomic ratio changes (before/after catalytic tests, Table 1) can be assigned to the transformation of the surface composition/structure of bimetallic particles under the influence of reaction mixture. In case of the PdAualloy/HOPG, the Pd/Au atomic ratio returns to the value detected before the stage of alloy formation (~ 1.5) indicating Pd segregation on the surface of bimetallic particles (Table 1). Analysis of C1s spectra shows the appearance of the shoulder at the higher BE together with broadening of the peak indicating the additional carbon species formation during the catalytic testing (Figure S3). In contrast to the sample with alloyed bimetallic particles, for model AushellPdcore/HOPG catalysts the Pd/Au atomic ratio decreased from 1.6 to 1.0 after catalytic testing (Table 1). It could be explained by the deposition (accumulation) of carbon species on the sample surface under reaction conditions, in which case the structure of the Aushell-Pdcore


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particles was not changed after catalytic testing and the intensity of Pd3d (Pd atoms located in the “core”) was attenuated stronger by the adsorbed carbon species than the intensity of Au4f (Au atoms located in the “shell”). Moreover, attenuation for Pd could be even higher due to the fact that CHx-species formation occurrs at the Pd sites predominantly.52,53 The Pd/Au atomic ratios calculated from XPS data measured after catalytic testing of bimetallic model catalysts are 1.5 and 1.0 for PdAualloy/HOPG and Aushell-Pdcore/HOPG model catalysts, respectively (Table 1), whereas the Pd/C atomic ratios are similar – 0.006 and 0.007, respectively. Basically the “working” bimetallic catalysts (after all redistributions of metals under reaction conditions) have similar amounts of Pd atoms on the surface, but in case of Aushell-Pdcore/HOPG catalysts the palladium on the surface is more diluted by gold atoms in comparison with PdAualloy/HOPG. This allow us to suggest that difference in activity of these two catalysts is determined by the presence and/or higher concentration of specific Pd active sites on the surface of bimetallic particles with the Aushell-Pdcore structure. To study the structure of bimetallic PdAu particles after the catalytic reaction, bimetallic model catalysts were investigated at ISISS station at the Berlin synchrotron radiation source BESSY II18 – the use of synchrotron radiation made possible the non-destructive depth profiling by variation of excitation energy. Thus, the Au4f, Pd3d and C1s core-level spectra were measured at different excitation energies to provide the same kinetic energy of 300, 450 and 600 eV for all analyzed photoelectron spectra. The dependences of Pd/Au and Pd/C atomic ratios on the depth of analysis calculated from XPS spectra measured for PdAualloy/HOPG and Aushell-Pdcore/HOPG model catalysts after catalytic testing are presented in Figure 5. With increasing analysis depth, the Pd/C atomic ratio increases while the Pd/Au atomic ratio decreases for both model bimetallic samples. This means that the particles surface is enriched with Pd atoms for both samples after


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the catalytic reaction. At the lowest depth of analysis (kinetic energy 300eV ~ 6Å) the Pd/C atomic ratio for less active PdAualloy/HOPG is about 2 times higher than for the most active in PHIP Aushell-Pdcore model catalyst. At the same time the Pd/Au atomic ratio is about 6 times higher, which confirms the suggestion concerning the higher dilution of Pd by Au atoms on the surface of bimetallic nanoparticles in case of Aushell-Pdcore/HOPG model catalysts. Depth of analysis (Å) ~ 5.9

~ 7.6

Depth of analysis (Å) ~ 9.2

~ 7.6

~ 5.9

~ 9.2

0.012 12 0.010

PdAualloy

10

Aushell-Pdcore 0.008

8

Pd/C

Pd/Au

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


6

0.006

4

0.004

2 0.002 0 300

400

500

600

Kinetic energy (eV)

300

400

500

600

Kinetic energy (eV)

Figure 5. Pd/Au and Pd/C atomic ratios calculated from XPS spectra measured for PdAualloy/HOPG and Aushell-Pdcore/HOPG model catalysts after catalytic reaction depending on the depth of analysis. The data presented clearly demonstrate that the initial structure of bimetallic PdAu particles (alloy or core-shell) strongly affects their activity in propyne hydrogenation reaction. It means that the appearance of some specific active sites of Pd is necessary to reach the best catalytic properties. Utilization of parahydrogen-induced polarization technique gives an opportunity to investigate the synergistic effects in hydrogenation reactions over model bimetallic catalysts depending on Pd/Au ratio and particles structure.


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CONCLUSIONS A set of Pd and Au mono- and bimetallic (Pd-Au) model catalysts supported on highly oriented pyrolytic graphite (HOPG) were prepared. All samples were tested in gas-phase hydrogenation of propyne. Pd, PdAualloy, and Aushell-Pdcore samples showed catalytic activity in propyne conversion, and PHIP effects were observed in 1H NMR spectra for propene, in contrast to Au/HOPG sample which produced no product or NMR signal enhancement. Bimetallic PdAualloy and Aushell-Pdcore samples demonstrate the 2- to 5-fold higher activity in pairwise hydrogen addition in comparison with monometallic Pd sample. Thus, the synergistic effect has been demonstrated in comparison with monometallic Au and Pd samples. Moreover, it was shown that the structure of bimetallic PdAu particles supported on HOPG also strongly affects their activity in pairwise hydrogen addition. Utilization of parahydrogen-induced polarization technique helped to investigate the synergistic effects in hydrogenation reactions at model bimetallic catalysts depending on Pd/Au ratios and particles structure.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional NMR, STM and XPS data (PDF). AUTHOR INFORMATION Corresponding Author


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*Kirill V. Kovtunov, PhD, International Tomography Center SB RAS, 3A Institutskaya St., 630090 Novosibirsk, Russia, e-mail address: [email protected] *Andrey V. Bukhtiyarov, PhD, Boreskov Institute of Catalysis SB RAS, 5 Acad. Lavrentiev Ave., 630090 Novosibirsk, Russia, e-mail address: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Russian Science Foundation ACKNOWLEDGMENT BIC team would like to thank Russian Science Foundation (grant # 14-23-00146) for the financial support of the preparation and characterization of model mono- and bimetallic PdAu/HOPG catalysts. D.B.B and K.V.K. acknowledge the grant from the Russian Science Foundation (17-73-20030) for the support of heterogeneous hydrogenation experiments. We also thank Helmholtz-Zentrum Berlin (HZB) for the allocation of synchrotron radiation beamtime at the ISISS beamline. I.V.K thanks SB RAS integrated research program (# 0333-2018-0006 / II.1.13) for parahydrogen activation studies.

ABBREVIATIONS


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Graphic for Table of Content (TOC)


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Highly Oriented Pyrolytic Graphite Catalysts: from

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