Identification of Active Sites in Oxidation Reaction from Real-Time

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Identification of Active Sites in Oxidation Reaction from RealTime Probing of Adsorbate Motion over Pd Nanoparticles Ahmed Ghalgaoui, Ridha Horchani, Jijin Wang, Aimeric Ouvrard, Serge Carrez, and Bernard Bourguignon J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02215 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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The Journal of Physical Chemistry Letters

Identification of Active Sites in Oxidation Reaction from Real-Time Probing of Adsorbate Motion over Pd Nanoparticles Ahmed Ghalgaoui †‡*, Ridha Horchani#, Jijin Wang†, Aimeric Ouvrard†, Serge Carrez†, Bernard Bourguignon† †Institut des Sciences Moléculaires d’Orsay (ISMO), CNRS, Univ. Paris-Sud, Université ParisSaclay, F-91405 Orsay, France. ‡ Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Strasse 2 a, 12489 Berlin, Germany. #

College of Arts and Applied Science, Dhofar University Salalah, Oman.

Corresponding Author *[email protected]

ACS Paragon Plus Environment

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Obtaining insight into the type of surface sites involved in a reaction is essential to understand catalytic mechanisms at atomic level and a key for understanding selectivity in surface-catalyzed reactions. Here we use ultrafast broad-band vibrational spectroscopy to follow in real-time diffusion of CO molecules over a palladium nanoparticle surface toward an active site. Site-tosite hopping is triggered by laser excitation of electrons and followed in real-time from subpicosecond changes in the vibrational spectra. CO photoexcitation occurs in 400 fs and hopping from NP facets to edges follows within ̴ 1ps. Kinetic modelling allows to quantify the contribution of different facet sites to the catalytic reaction. These results provide useful insights for understanding the mechanism of chemical reactions catalyzed by metal NPs.

TOC GRAPHICS

KEYWORDS: catalysis; metal nanoparticles; active site determination; photocatalysis; CO oxidation; sites contribution


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Adsorbate motion on a surface is critically important for many surface chemical reactions, because it is the way for adsorbates to meet a reaction partner or to reach an active site before reaction takes place. Surface motion occurs if molecules can overcome an energetic barrier and is typically a thermally promoted process with rates increasing with increasing temperature. Ultrafast laser excitation that excites electrons in the metal1 or localized surface plasmons in metallic nanoparticles (NPs)2-4 to high electronic temperatures has been shown to enable a catalytic reaction. Direct observation of surface reactions has been resolved in real-time using the surface sensitive nonlinear optical sum frequency generation spectroscopy (SFG). This technique has been successfully utilized to monitor transient excitation of external adsorbate vibrational modes that couple to diffusion5 and to desorption reaction6. Supported catalysts are considered the most promising systems for creating advanced catalysts with enhanced activity and selectivity.7,8 These crystalline particles which typically are dispersed on a high-surface-area oxide support expose a variety of nonequivalent adsorption sites which do not have the same activity or selectivity, giving rise to the concept of structure sensitivity.9-11 Identification of active sites is a challenging task that requires the combination of several approaches.11-14 The nature of the active site where bond breaking occurs is different for different chemical bonds.15 Molecules may dissociate at low-coordinated sites including steps, kinks, open “rough” faces, or they may dissociate after surface or subsurface diffusion from a site neighboring on an active site.16 A direct observation of the catalytic performance of specific sites has long been lacking. A key challenge is to observe molecular hopping to the specific sites where reaction takes place with time and spatial resolutions. More details of the reaction can be captured, eventually allowing to design a catalyst improving activity and selectivity.


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Here we report the direct real-time observation of CO diffusion on NP surface which provides sub-nanometer spatial resolution and sub-picosecond temporal resolution. Using time resolved vibrational spectroscopy we provide a real time “snapshot” of the process by monitoring selectively the time dependence of CO population on different adsorption sites. Time domain calculation allows to quantify and differentiate CO hopping from (100) and (111) facets toward NP edges.

Figure 1. (a) Schematic of the different adsorption sites on Pd nanoparticles. (b) SFG spectra of CO adsorbed on 3.5 nm sized Pd deposited on 2 ML MgO/Ag(100) under 10-4 mbar of CO, inset: CO at edges between (100) and (111) facets (2030 cm-1) and on-top sites (2090 cm-1). (c) Illustration of the shortest distance between two adjacent CO: Case (I): on top site on the (111) facet and bridge site at the edge between (100) and (111) facets (4.44 Å); Case (II): bridge site


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on the (100) or (111) facets and bridge site at the edge between (100) and (111) facets (2.81 Å). (d) Pump probe spectra of CO adsorbed on 1 ML equivalent thickness of Pd as a function of pump-probe delay from 0 to 700 fs. Inset: zoom of CO at edges and on-top sites. Figure. 1(a) shows the model surface employed in this study which consists of palladium (Pd) NPs supported on a well-ordered thin magnesium oxide (MgO) film grown on a Ag(100) single crystal.17,18 NPs exhibit a large (100) top facet parallel to the substrate and smaller lateral (111) facets.19,20 These crystalline particles also exhibit several types of defects, such as steps, edges, kinks and corners, which are expected to have different bond enthalpies and desorption energies. These defects are found to be more reactive than highly coordinated metal atoms.21 Vibrational SFG spectroscopy allows to discriminate 3 types of sites depending on CO pressure and coverage (Figure S1). The main peak at 1985 cm-1 corresponds to CO on bridge sites of the main (100) facet. The band of bridge sites on lateral (111) facets are expected to overlap the (100) band, but their intensity should be smaller because NPs are rather flat, so lateral facets are small. In addition, because of the anisotropic character of SFG, CO adsorbed on the side facets should produce a smaller signal (75%) since the molecule is tilted by 60° with respect to the surface normal. The frequency at 2030 cm-1 corresponds to CO at edges and other defects, while the band at 2090 cm-1 corresponds to CO molecules linearly adsorbed in a-top geometry on (111) facets.17,18 The latter appear only at CO pressure > 10-4 mbar (Figure 1(b)) and indicate a compressed coverage of 0.63 ML. Their intensity is small as expected for a lateral facet. Observation of CO motion between different NP sites provides sub-nanometer spatial resolution. Figure 1(c) shows the different possible distances between two adjacent CO molecules: in case I the distance between a-top site on a (111) facet, and bridge site located at the edge between (100) and (111) facets, is

√

~ 4.44 Å with the lattice constant of the substrate a=


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3.89 Å. In case II the distance decreases to 2.81 Å between bridge site of the main facet and edge site of the nanoparticle (Figure S2). Note that the relative SFG intensities of the CO bands do not reflect the relative concentrations of the corresponding species because of intensity transfer between neighboring bands due to dipole coupling, and differences in the dynamic dipole moment of different modes as a function of the local adsorption geometry and the electronic coupling to the metal.22

Figure 2. Peak intensity of CO as a function of pump-probe delay: (a) CO on bridge sites located between (100) and (111) facets; (b) CO on a-top sites of (111) facet; (c) CO on the bridge sites located at the (100) facet for two different sizes of Pd NPs. (d) Two-temperature model: time dependent electron and phonon temperatures after excitation at time = 0 by a pump pulse of 19 J m-2. Temperatures of the frustrated rotation (electronic coupling time, 0.1 ps) and the frustrated translation (electronic coupling time, 4 ps) are also shown as computed using the so-called friction model. Further details of this model can be found in the Supporting Information. We studied the dynamics of CO diffusion in ultra-short time scale on 3.5 nm sized Pd NPs, for which photodesorption was estimated to be weak.23 Figure 1(d) shows SFG spectra for different delays between pump and IR probe lasers. As recently reported, CO photodesorption


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occurs mainly from the bridge sites.23 While the bridge band decreases strongly, the edge band increases slightly while atop band decreases slightly (Figure. 1(d); inset). The initial coverage (negative delay) results from the balance of photodesorption and readsorption. In our experimental conditions (laser repetition rate and fluence, CO pressure, surface temperature of 300 K) it is not maximal even at edges, which makes it possible for CO to diffuse from facet sites to edge sites in response to laser excitation. Most changes occur in a narrow temporal window (less than 400 ± 100 fs) (Figure S3). The edge continues to increase more slowly until ~ 1ps (Figure S4), similar to the case of CO on Pt(553) on which CO could jump between sites within a picosecond time scale.5 Thermal equilibrium between electrons and phonons is reached in a time of ~ 1ps (Figure. 2(d)) as shown by the two temperature model (see Supporting Information), suggesting that the sub-picosecond response of CO is purely electron-mediated. Return to normal is first governed by diffusion and is faster for edges (30% in the first 5 ps) than for facets (only a few %). The latter have been depopulated by photodesorption and cannot be refilled by diffusion alone. Thermal diffusion is expected on a fast time scale (< 100 ps) (~1016 and ~1011 times faster than at 40 K and 100 K respectively.24-26 In order to quantify the ultrafast transient response of CO to laser irradiation we follow the time evolution of CO population at different adsorption sites (Figure 2). The fact that edge sites (Figure 2(a)) have gained intensity while linearly bonded (Figure 2(b)) and bridge sites (Figure. 2(c)) have lost intensity indicates that CO motion from the facet(s) to edges occurs in addition to photodesorption. Both CO photodesorption and motion have been shown to be largely driven by frustrated rotation.5,6 The continuing decrease of bridge CO population until >~1 ps matches the time scale of the changes in electron and frustrated rotation temperatures (Figure 2(d)). It can be interpreted as the coexistence of both vibrationally hot CO and CO in a weakly adsorbed


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precursor state prior to desorption or readsorption.27,28 Vibrationally hot CO disappear with hot electrons. After this time, no more excitation takes place while CO molecules in the precursor state may refill facet bridge sites.

Figure 3. (a) Time evolution of the calculated hopping probability as a function of Pd NP size. (b) Time evolution of the calculated desorption rate for low CO coverage (0.2 ML) as a function of Pd NPs size. (c) Same as (b) for high CO coverage (0.6 ML). (d) Calculated CO hopping probability for 3.5 nm sized Pd NPs at the bridge site of the (111) and (100) facets and at the ontop site, labeled B111, B100 and Top respectively. Total bridge is the sum of B111 and B100. The CO coverage is 0.5 ML and 0.63 ML for the (100) and (111) facets respectively, the on-top CO coverage is 0.16 ML. Calculations were performed for hopping and desorption mediated by excitation of the frustrated rotation mode, using the time dependent adsorbate temperature of Figure 2(d).


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The rate of an activated process is calculated by an Arrhenius type expression assuming first

order kinetics, = − = /( ) , where θ(t) is the time dependent surface coverage, k0 is a pre-exponential factor, Ea is the activation barrier to desorption or diffusion. We introduce here a new formula to calculate the rate of the reaction in the case of a nanoparticle (see Supporting Information): &-. / %&' ) 01 ' )% '( )% ( ' *+, ( ( ) ' %

" (#)($%&'

(, ) = ()exp (−

)*+, (

),

where D is the NP diameter, 23 the activation energy in case of a single crystal, 4 is a shape factor, D0 denotes the critical size (D0 < 2 nm) at which all atoms of the crystal are located on its surface and Sb = ∆67 /7 is the bulk solid-vapor transition entropy, where ∆67 and 7 are the bulk enthalpy of vaporization and the boiling temperature, respectively. R(D, Tads) is equal to the first order kinetic rate in the case of a single crystal when → ∞. The diffusion barrier on transition metal surfaces is generally estimated to be 12 % of the magnitude of the adsorbate binding energy.29 It has recently been shown that this is still valid in the case of transition metal NPs.30 CO binding energy was experimentally found to be size dependent; the initial heat of adsorption decreases from 126 kJ mol-1 for 8 nm size down to 106 kJ mol-1 for ~1.8 nm sized NPs.31,32 The diffusion barrier is then estimated as E~150 meV for the largest NPs, and 130 meV in the case of ~1.8 nm NPs. We have calculated the time-dependence of both desorption and hopping probabilities controlled by the frustrated rotation mode. We used a coupling time of frustrated rotation to hot electrons of τe=0.1 ps (Figure 2(d)), in agreement with the coupling time previously reported for this mode,5,6 and a prefactor k0 of 1012 s-1,


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assuming that it is the same as for a single crystal (see Supporting Information). Results of the calculations are shown in Figure 3. The hopping probability on picosecond time scale shows only a slight decrease from single crystal to Pd NP size of 6 nm, and a more significant decrease between 4 and 2 nm (Figure 3(a)). The desorption rate increases with increasing NP size and also with CO coverage (Figure 3(b) and 3(c)) in agreement with experiment.23 The increased desorption rate with coverage is linked to CO desorption activation and binding energies with increasing CO coverage (Figure S5). The increased rate with NPs size was attributed to the increase of electrons available for photoexcitation for the large particles33 as well as the effect of electron confinement which promotes the relaxation to phonons rather than the excitation of adsorbed molecules for the small particles.23


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Figure 4. (a) Pump-probe spectra of CO oxidation reaction triggered by the pump pulse on 3.5 nm sized Pd for different pump pulse energies at a delay of 1 ps. NPs are preexposed to O2 and CO pressures of 1 and 1.5 mbar ensure full readsorption of reactants between two pulses. (b) Facet sites contribution to the catalytic CO oxidation deduced from SFG intensity dependence as a function of the pump laser energy in (a). (c) Pictorial view of real time “snapshots” at different times for CO photoexcitation and diffusion. CO on the bridge site of (100) and (111) facets has more probability to interact with O to from CO2 than CO adsorbed on top geometry.


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A difficulty to evaluate experimentally the relative importance of bridge and on-top sites for diffusion in more details, including NP size effect comes from the fact that relative SFG intensities at facets and edges do not scale with the number of adsorption sites, due to presumably different hyperpolarizabilities at facets and edges.17 However, real-time calculation of the hopping probability from the two facet sides with occupancies of 0.5 ML and 0.63 ML for the (100) and (111) facets has been done by excitation of the frustrated rotational mode using CO adsorption energies of 1.47 eV and 1.15 eV for the bridge and on-top sites respectively.34 Figure. 3(d) shows that bridge CO contributes much more to the filling of edge sites than on-top CO. Recently, it has been concluded that oxygen preferentially accumulates at the edges of the NPs.16 Therefore CO motion to NP edges appears to be a key step of catalytic oxidation, leading us to examine the contributions of bridge and a-top sites to CO oxidation in pump-probe experiments. Figure 4(a) shows SFG spectra recorded under O2 and CO pressures of 1 and 1.5 mbar respectively on 3.5 nm sized Pd NPs at a delay of 1 ps, for which all species on the surface are activated35 and oxidation reaction can proceed. Oxygen was preadsorbed at a dose of 10-4 mbar for 20 minutes to avoid surface poisoning by CO. At these pressures and in the presence of oxygen, full compression of the CO layer results in a strong atop band. By contrast, the band of CO at edges has vanished, presumably because edges are occupied by oxygen atoms. The bridge band is broader than the atop band, which can be ascribed to the overlapping contributions of (111) and (100) facets. At room temperature oxidation proceeds extremely slowly on Pd, so only the pump beam triggers oxidation. Figure 4(a) shows that by increasing the pump energy from 0 µJ to 45 µJ the intensity of bridge CO decays down to about 17 % of its original intensity at the highest pump energy, much faster than on top CO (50 %) (Figure 4(a) and 4(b)). As expected in


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conditions of oxidation where those CO which have moved to edges react with O atoms, the band of CO at edges does not appear. In conclusion the strong difference between bridge and atop sites observed for CO hopping to edges is also present in oxidation conditions, evidencing the role of CO motion on the catalytic reaction. The larger bridge contribution to the catalytic CO oxidation reaction reflects the dependence of the hopping rate from different facet sites to the edges. The distance between initial and final sites is certainly a relevant parameter. Our findings are summarized in a simple picture depicted in Figure 4(c). In the absence of oxygen, the short laser pulse excitation of NP electrons leads to CO photoexcitation and photodesorption at a time scale of 400 ± 100 fs. At ̴ 1ps time scale a portion of CO molecules has moved away from the bridge and the on-top sites of the NPs facet towards less coordinated bridge sites located between (100) and (111) facets. At this time the species at edges are already activated. If oxygen is present, CO start to collide with O leading to the formation of OC-O in the transition state attempting to form CO2 with larger contribution from the bridge sites. Recently femtosecond xray laser pulses were used to probe CO oxidation on Ru in real time, showing a new transient electronic state on a time scale slightly larger than CO motion and ascribed to the OC-O transition state.35 Time scales of electronic excitation, CO diffusion and reaction were similar to the ones obtained for Pd NPs in the present work. Using broad-band SFG vibrational spectroscopy we have examined the excitation dynamics and oxidation of CO catalyzed by Pd NPs on subpicosecond time scale. By monitoring the timedependence of CO populations on relevant sites, we can quantify and differentiate the contribution of molecules from these sites to surface reactions. Both electronic excitation and surface diffusion are shown to be essential. The ability to probe specific sites with high spatial and temporal resolutions provides unique tools for a deeper understanding of surface chemical


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reactions in heterogeneous catalysis. Real-time laser vibrational and electronic spectroscopies have the potential to tackle the transition state in chemical reactions at surfaces. ASSOCIATED CONTENT Supporting Information. Details of experiments and theoretical models, SFG spectra of CO adsorbed on Pd NPs as a function of CO pressure for bridge sites (a) and edges (b) (Figure S1).distance between adjacent CO (Figure S2), Ultrafast dynamics of CO on bridge site of 3.5 nm sized Pd (Figure S3), edge site (Figure S4), CO desorption activation energy for a truncated octahedron shaped Pd nanoparticle as a function of Pd NP size and CO coverage (Figure S5). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Ahmed Ghalgaoui: 0000-0002-6978-9690 Ridha Horchani: 0000-0001-7603-2904 Jijin Wang: 0000-0002-2467-4785 Aimeric Ouvrard: 0000-0003-3652-1222 Bernard Bourguignon: 0000-0002-4171-6828 Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT We gratefully acknowledge The French National Agency for Research (Agence Nationale de Recherche) for financial support through the PNANO program (Projet CANA ANR-06-NANO0031). REFERENCES (1) Frischkorn, C.; Wolf, M. Femtochemistry at Metal Surfaces: Nonadiabatic Reaction Dynamics. Chem. Rev. 2006, 106, 4207-4233. (2) Brongersma, M. L.; Halas, N. J.; Nordlander, P. Plasmon-Induced Hot Carrier Science and Technology. Nat. Nanotechnol. 2015, 10, 25-34. (3) Zhang, Y.; He, S.; Guo, W.; Hu, Y.; Huang, J.; Mulcahy, J. R.; Wei, W. D. SurfacePlasmon-Driven Hot Electron Photochemistry. Chem. Rev. 2018, 118, 2927-2954. (4) Christopher, P.; Xin, H.; Linic, S.; Visible-Light-Enhanced Catalytic Oxidation Reactions on Plasmonic Silver Nanostructures. Nat. Chem. 2011, 3, 467-472. (5) Backus, E. H. G.; Eichler, A.; Kleyn, A. W.; Bonn, M. Real-Time Observation of Molecular Motion on a Surface. Science 2005, 310, 1790-1793. (6) Fournier, F.; Zheng, W.; Carrez, S.; Dubost, H.; Bourguignon, B. Vibrational Dynamics of Adsorbed Molecules Under Conditions of Photodesorption: Pump-Probe SFG Spectra of CO/Pt(111). J. Phys. Chem. 2004, 121, 4839-4847. (7) Xia, Y.; Yang, H.; Campbell, C. T. Nanoparticles for Catalysis. Acc. Chem. Res. 2013, 46, 1671-1672. (8) Zaera, F. Nanostructured Materials for Applications in Heterogeneous Catalysis. Chem. Rev. Soc. 2013, 42, 2746. (9) Van Santen, R. A. Complementary Structure Sensitive and Insensitive Catalytic Relationships. Acc. Chem. Res. 2009, 42, 57-66. (10) Lee, I.; Delbecq, F.; Morales, R.; Albitler, M. A.; Zaera, F. Tuning Selectivity in Catalysis by Controlling Particle Shape. Nat. Mater. 2009, 8, 132-138. (11) Kale, M. J.; Christopher, P. Utilizing Quantitative in Situ FTIR Spectroscopy to Identify Well-Coordinated Pt Atoms as the Active Site for CO Oxidation on Al2O3 Supported Pt Catalysts. ACS Catal. 2016, 6, 5599-5609. (12) Ding, K.; Guelec, A.; Johnson, A. M.; Schweitzer, N. M.; Stucky, G. D.; Stair, P. C. Identification of Active Sites in CO Oxidation and Water-Gas Shift over Supported Pt Catalysts. Science 2015, 350, 189-192. (13) Behrens, M.; Studt, F.; Kasatkin, I.; Kühl, S.; Hävecker, M.; Abild-Pedersen, F.; Zander, S.; Girgsdies, F.; Kurr, P.; et al. The Active Site of Methanol Synthesis over Cu/ZnO/Al2O3 Industrial Catalysts. Science 2012, 336, 893-897. (14) Calle-Vallejo, F.; Tymoczko, J.; Colic, V.; Vu, Q. H.; Pohl, M. D.; Morgenstern, K.; Loffreda, D.; Sautet, P.; Schuhman, W.; et al. Finding Optimal Surface Sites on Heterogeneous Catalysts by Counting Nearest Neighbors. Science 2015, 350, 185-189. (15) Somorjai, G. A.; McCrea, K. R.; Zhu, J. Active Sites in Heterogeneous Catalysis: Development of Molecular Concepts and Future Challenges. Top. Catal. 2002, 18, 157166.


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(34) Yudanov, I. V.; Sahnoun, R.; Neyman, K. M.; Rösch, N.; Hoffmann, J.; Schauerman, S.; Johánek, V.; Unterhalt, H.; Rupprechter, G.; Libuda, J.; et al. CO Adsorption on Pd Nanoparticles: Density Functional and Vibrational Spectroscopy Studies. J. Phys. Chem. B 2003, 107, 255-264. (35) Öström, H.; Öberg, H.; Xin, H.; LaRue, J.; Beye, M.; Dell’Angela, M.; Gladh, J.; Ng, M. L.; Selberg, J. A.; et al. Probing the Transition State Region in Catalytic CO Oxidation on Ru. Science 2015, 347, 971-981.


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Identification of Active Sites in Oxidation Reaction from Real-Time

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