CO Activation Determines Ultrafast Dynamics of CO Oxidation

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CO Activation Determines Ultrafast Dynamics of CO Oxidation Reaction on Pd Nanoparticles Sourav Banerjee, Gowrav Munithimhaiah Narasimhaiah, Anish Mukhopadhyay, and Atanu Bhattacharya J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07719 • Publication Date (Web): 26 Oct 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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The Journal of Physical Chemistry

CO Activation Determines Ultrafast Dynamics of CO Oxidation Reaction on Pd Nanoparticles Sourav Banerjee, Gowrav Munithimhaiah Narasimhaiah, Anish Mukhopadhyay and Atanu Bhattacharya* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, India 560012. *Correspondence should be sent to [email protected] or [email protected] *Corresponding author’s phone number: 91-80-22933349.

Abstract: Femtosecond two-pulse correlation spectroscopy, temperature program desorption spectroscopy, and density functional theory calculations are used to elucidate the mechanisms and the ultrafast dynamics of carbon monoxide (CO) oxidation reaction on palladium nanoparticle (Pd NP) surfaces. Two different time-scales for the CO oxidation reaction on Pd NPs are observed: a fast channel (time constant ~ 7 picosecond) is found for the oxidation reaction when strongly bound CO is involved and a slow channel (time constant ~ 15 picosecond) is found for the oxidation reaction when weakly bound CO is involved. Temperature programmed desorption spectroscopy confirms that the fast and the slow channels are associated with CO oxidations mostly at (111) facets and at edges of the Pd NPs, respectively. This is the first report on heterogeneous gas phase ultrafast dynamics study of CO oxidation reaction on transition metal (which is active catalyst in catalytic converter) NP surfaces.

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I. Introduction: The catalytic oxidation of carbon monoxide (CO) on transition metal surfaces is technologically very important, being a key reaction, for example, in automotive exhaust catalysts (catalytic converter).1,2 Despite a two fold increase in the number of automobiles, mobile CO emissions in the United States of America have continuously decreased since the three-way catalytic converter was introduced in 1975.3 Complete understanding of the surface reaction associated with the CO oxidation, which ideally can help one improve the efficiency of this highly demanding catalytic process, requires insight into the elementary reaction steps on the time scale (femtosecond to picosecond) of making and breaking of bonds. As transition metals (such as Pt, Rh, Pd, etc.) play crucial role in the CO treatment application of automotive exhaust, several ultrafast studies have been undertaken, so far, on transition metal single crystal surfaces,413 to obtain real time dynamical information of the catalytic CO oxidation reaction. However, automobile catalytic converter does not make use of bulk (or single crystal extended) transition metal catalyst; instead, transition metal nanoparticles dispersed on metal oxide support are used. Metal nanoparticles (NPs) show remarkable changes of electronic properties as compared to the bulk metals or extended single crystal metal surfaces.14 In particular, size and electronic confinement in the NPs can influence chemistry proceeding on them,15 which may not be realized on extended single crystal metal surfaces. Quite surprisingly, thus far, “no” ultrafast dynamics study of the CO oxidation reaction has been undertaken on metal oxide-supported transition metal NP surfaces. Misewich, Heinz, Ho and others,6,7,16-21 over twenty years ago, demonstrated, for the first time, that the femtosecond two pulse correlation (2PC) spectroscopy can efficiently be utilized to study surface chemical dynamics at flat single crystal surfaces on the ultrafast time scale. Several observations are evident in the single crystal-based experimental results. Femtosecond (fs) optical pulse within a wavelength range of ~700-900 nm is not directly absorbed by the adsorbates; instead, the energy is deposited onto the metal surface, rapidly creating transient hot electron distribution that can effectively couple energy into the (adsorbate) reaction coordinate, either directly or through phonon, on an ultrafast time scale. Subsequent bond cleavage or formation is believed to occur by pathways similar to those in thermally excited surface reactions. A clear link between thermal surface chemical reactions and hot electrons (which can be created by femtosecond pulse excitation) has also been established by both experiment and theory.22-24 Furthermore, two distinct mechanisms of energy transfer/flow have been identified in fs pulse-induced surface chemical reactions: (1) Desorption/reaction induced by multiple electronic transition (DIMET), and (2) Frictional energy transfer from electronic and phonon baths of the substrate to the adsorbate reaction coordinate.25 The first attempt to study fs pulse-induced chemistry at metal NP surfaces was made by Hotzel and co-workers26 to explore the desorption dynamics of H2O from quartz-supported Ag NPs. The results were interpreted as due to coupling of adsorbates to the NP lattice and were closely linked to the lattice cooling times of a few 100 picosecond (ps). Later, on a different support, namely Al2O3/NiAl(110), desorption of NO from (NO)2-covered Ag NPs was studied by Menzel and his coworkers.27 The timescale of NO desorption was found to be less than 100 fs. In contrast to Hotzel’s study, NO desorption was interpreted as due to direct influence of strongly non-thermal hot-electrons. These limited results clearly demonstrate that our current

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understanding of ultrafast elementary chemical steps and their controlling factors in gas phase heterogeneous catalytic reactions on supported NP surfaces is insufficient. Here we report on an in-depth investigation of the ultrafast dynamics of CO oxidation reaction on supported palladium (Pd) NPs. This is the first report on ultrafast dynamics study of CO oxidation reaction on transition metal NP surfaces. Two different time-scales for the CO oxidation reaction are observed: oxidation of strongly bound CO exhibits a fast channel (time constant ~7 picosecond) and weakly bound CO features a slow oxidation reaction channel (time constant ~15 picosecond). Based on the present study we elucidate two different mechanisms of CO oxidation reactions on Pd NP surfaces: the fast channel of the CO oxidation is associated mostly with the CO oxidation reaction on the (111) facets of the Pd NPs and the slow channel of the CO oxidation reaction is associated mostly with the CO oxidation reaction at the edges of the NPs.

II. Methods A. Synthesis, Characterization, and Thermal Chemistry of Pd NPs: Palladium NPs were synthesized by complexation of palladium(II) acetate, Pd(OAc)2, with the core of reverse micelles of polystyrene-block-poly(2-vinylpyridine), PS-b-P2VP, (PS and P2VPaverage molecular weights, Mn, equal to 34,000 and 18,000 g mol–1, respectively). The reserve micelle was prepared by dissolving the PS-b-P2VP in anhydrous toluene (0.5 % wt) inside nitrogen glove box and then by stirring at 80°C for 12 h followed by stirring at room temperature for 2 days. 0.5 Pd(OAc)2 moles per mole P2VP monomer unit was added at room temperature. Single layer of the salt-loaded reverse micelles were deposited ex-situ on the ultrahigh vacuum (UHV)-prepared fully-oxidized TiO2(110) surface by spin coating at 4400 rpm. Microwave oxygen plasma treatment was used to remove the encapsulating polymer. Subsequent UHV-annealing helped reduce the Pd particles to neutral form. We have investigated the effectiveness of plasma treatment followed by UHV-annealing for the final preparation of the clean Pd-TiO2(110) sample. More details of the synthesis, deposition and characterization procedures and results are given in Supporting Information S1. Figure 1a and 1b show Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) images, respectively, recorded after the UHV annealing of Pd-TiO2(110) sample on which ultrafast surface chemical dynamics study of CO oxidation was carried out. The diameter and height distributions are found to be centered at ~ 11-nm and ~ 4-nm, respectively, as revealed by SEM and AFM, respectively. Figure 1 indicates that NPs are somewhat flatter than a hemisphere, with a most probable diameter-to-height aspect ratio of ~ 2.75. These particles remain pseudohexagonally arrayed even after extensive high temperature surface treatment (upto 900 K) and femtosecond pulse-induced desorption experiments, indicating that this particle distribution is robust. For the temperature programmed desorption (TPD) measurements, natural carbon monoxide (CO: mass 28 amu) and isotopic oxygen (18O2) gases were delivered to the surface with an effusive beam formed in a stainless steel tube downstream from a 5 µm aperture. In the work presented here, we recorded mass signal associated with C16O18O (represented simply as CO2) using a quadrupole mass spectrometer (QMS) to monitor thermally activated and 3



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femtosecond pulse-induced CO oxidation reactions. Immediately prior to adsorption, the PdTiO2(110) sample was flash annealed to 900 K. Desorption rates of gases of interest were measured at a constant temperature ramp rate of 2.0 K s–1 rate.

Figure 1: (a) AFM and (b) SEM images of clean Pd-TiO2(110) surface on which the ultrafast dynamics of the CO oxidation reaction has been studied. Gaussian fits to the height and diameter distributions obtained from AFM and SEM studies, respectively, are also given.

B. Ultrafast Dynamics Measurement: Femtosecond pulses (800 nm, 35 fs) were generated by a regenerative amplifier system (Spectra Physics). For the two pulse correlation experiments (see Supporting Information S2), fundamental pulses were split in a 50% beam splitter and then two cross-polarized beams were recombined after delaying one beam by a programmable delay stage (Newport). Finally, desorption yield was measured as a function of the delay between the two laser pulses with a quadrupole mass spectrometer.

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Photodesorption measurements were initiated by opening a computer-controlled shutter. The shutter remained open for 100-200 laser shots before closing; a shot is defined as a pair of laser pulses in two-pulse correlation measurement or a single pulse if one of the two beams is blocked. The repetition rate of the laser was chosen to be 5 Hz so that the period between shots is 200 ms. No change in bulk substrate temperature was observed during laser irradiation (at 5 Hz). Ion pulses from the mass spectrometer were collected by a multi channel scalar (MCS) card.

Figure 2: (a) Temporal histogram of the QMS signal of CO2 after the first laser shot. (b) Depletion curve (with an exponential fit) of CO2 product as a function of laser shot

A typical temporal histogram (pseudo time-of-flight spectrum) of desorbing molecules after a laser shot is shown in Figure 2a, taking illustrative example of CO2 yield for O2-exposure temperature 300 K. The maximum desorption yield occurs with the first shot, falling exponentially with additional shots. Plotting the desorption yield vs. the laser shot number results in a depletion curve like the one shown in Figure 2b, again taking an illustrative example of the CO2 yield for O2-exposure temperature 300 K. All femtosecond pulse-induced desorption experiments were performed at substrate temperature 98 K only. C. Density Functional Theory (DFT) Calculations: All density functional theory (DFT) calculations were performed within the generalized gradient approximation28 (with PBE functional) to study the mechanisms of the CO oxidation reaction on the Pd(111) and (1x2)missing row Pd(110) surfaces using the CASTEP program.29,30 Ionic cores were described by ultrasoft pseudopotentials31 and the Kohn-Sham one electron states were expanded in a plane wave basis set up to 300 eV. We used the k-point sampling by using 2 x 2 x 1 Monkhorst-Pack mesh.32 We also tested higher energy cut-off and higher order Monkhorst-Pack mesh to verify the accuracy of the present method.33 The supercell approach was employed to model periodic geometries. The surfaces were modeled by three-layer slabs of Pd metal atoms for both surfaces. Particularly, a 3 x 3 supercell with three layers containing 27 metal atoms was used for the Pd(111) system and a 2x2 supercell

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with three layers containing 18 metal atoms was used for the (1x2)-missing row Pd(110) system. The slabs were separated by 15 Å of vacuum. In all the calculations, only the top surface layer of metal atoms was allowed to relax. As shown in one of our recent works,33 the above setup provides sufficient accuracy for the study of catalytic reactions on transition metal surfaces. Transition states were searched using quadratic synchronous transit (QST) method.34

III. Results and Discussion: A. Thermally Promoted CO Oxidation. Before monitoring the femtosecond laser pulseinduced CO oxidation reaction on Pd NP surfaces, thermally activated CO oxidation reaction was studied by the temperature programmed desorption (TPD) spectroscopy. TPD profiles were obtained after saturation dosage of O2 at two different temperatures (either 300 K or 98 K), followed by saturation dosage of CO at 98 K. Figure 3a illustrates the CO2 TPD profiles for two different O2 exposure temperatures (namely 98 K and 300 K). In these plots, the relatively steep peak at ~170 K remains approximately constant in both cases. The other two relatively broad peaks (one at ~227 K and another at ~368 K) are seen only when O2 is dosed at 300 K.

Figure 3: (a) Two measurements of CO2 TPD from Pd-TiO2(110): red and blue profiles obtained at O2 exposure temperature 98 and 300 K, respectively. (b) The red and blue TPD profiles feature unreacted CO, recoded simultaneously with respective (presented by the same color) CO2 TPD profiles; the black TPD profile shows CO desorption from Pd-TiO2 sample, not exposed to O2. For all TPD measurements, the temperature ramp rate was 2.0 K s–1.

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Analogous desorption profiles of unreacted CO (Figure 3b) were also collected simultaneously with the respective CO2 product in order to investigate the correlation between the CO2 product and the nature of the bonding of the reactive CO component. These CO TPD profiles are compared with that obtained from a CO-only adlayer surface (i.e., when the molecular O2 was not exposed prior to CO exposure). The CO desorption plot, obtained from CO-only adlayer surface, exhibits a broad desorption feature ranging from 100 to 560 K, with two distinct peaks at 134 and 487 K. By comparing with the TPD profile recorded for a saturated CO-only adlayer on fully oxidized TiO2(110) surface (not shown here), the 134 K peak is identified as CO desorption from the TiO2(110) support surface. On the other hand, the higher temperature desorption features (in the 193-560 K range) closely resemble those observed in CO-TPD from Pd NPs grown by physical vapor deposition on Al2O3/NiAl(110).35 These high temperature broad desorption features of CO result from widely-ranging binding energies associated with the diversity of CO adsorption sites36-38 (e.g., atop, bridge, and hollow sites: these sites are depicted schematically in Figure 4a, taking an example of (111) facet of Pd, for reader’s perusal). Here we note that among different surface sites available for CO adsorption on the Pd surface, hollow sites are the most stably bound sites for CO and atop sites are the most loosely bound sites for CO.39 DFT calculation shows that the chemisorption energy for the CO residing at bridging site is approximately 0.18 eV less than that for CO residing at hollow site.39 Furthermore, atop site exhibits chemisorption energy of CO approximately 0.47 eV less than that for CO at bridging site.39 As a result, loosely bound CO (residing at atop and bridge sites) desorbs from the Pd NP surface at relatively lower temperature range (in ~ 200-235 K range) and strongly bound CO (residing at hollow sites) desorbs from the Pd NP surface at relatively higher temperature range (featuring high temperature peak in CO TPD profile at 498 K).40-43 Comparison of the unreacted CO TPD profiles with that obtained from CO-only adlayer surface (Figure 3b) directly suggests that it is mostly loosely bound CO molecules which are reactive when O2 is dosed at 98 K, rendering CO2 desorption profile (Figure 3a) with a single steep peak at 170 K. On the other hand, both loosely and strongly bound CO are reactive when O2 is dosed at 300 K, rendering the CO2 desorption profile with three peaks, at 170 K, 227 K and 368 K, respectively. Therefore, the activation energy associated with the CO oxidation reaction on Pd NP surfaces strongly depends on the site where reactive CO resides. This experimental observation is consistent with the DFT results which indicate that the activation energy for the CO oxidation reaction is ~0.6 eV less, when CO resides at the top site (weakly bound) as compared to the hollow site (tightly bound).39 It is clearly evident in Figure 3a and 3b that two different O2 exposure temperatures directly control efficacy of the thermally promoted CO oxidation reaction on Pd NP surfaces. Different O2-exposure temperature results in different extent of dissociation of molecular O2 on the Pd surface and thereby, controls the structure of the atomic O-adlayer on the NP surfaces. Since exact nature of the atomic O-adlayer plays an important role in the catalytic CO-oxidation reaction,44 it is an important task to probe the nature of the atomic O-adlayer structure on the Pd NP surfaces, in details, to understand the O2-exposure temperature dependent CO oxidation reactions discussed above.

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Figure 4: (a) schematically atop, bridge and hollow sites are shown. (b) A p(2x2)-O adlayer structure is depicted schematically on Pd(111) surface. Here O atoms occupy the hollow sites. (c) A typical example of Pd NP is depicted to show (111) facets and edges. (d) Edge sites of Pd NPs are comparable to (1x2)-missing row Pd(110) surfaces. A typical example of dissociation of O2 at an edge of (1x2)-missing row Pd(110) surface is shown. (e) A zigzag-O adlayer structure is schematically shown on (1x2)-missing row Pd(110) surface.

With regard to the dissociation of molecular O2, previous single crystal studies evidence that the (111) surface of Pd is unique among other low index faces.45,46 For Pd(100)47,48 and Pd(110)49,50, both O2 TPD and electron energy loss spectroscopy (EELS) indicate that the O2 dissociates immediately upon adsorption even at temperatures below 100 K.48,51 Therefore, O2TPD profiles following adsorption of sub-monolayer quantities of O2 at 100 K show no lowtemperature (in 100-200 K range) desorption feature: molecular O2 is found to desorb above ~ 650 K (formed by recombination of two adsorbed oxygen atoms).52 In contrast, adsorption temperature 220 K is found to be the lowest limit to complete the dissociation of molecular O2 on the single crystal Pd(111) surface.45 Therefore, O2-TPD profiles, obtained following

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adsorption of sub-monolayer quantities of O2 at 100 K on the Pd(111) surface, exhibit lowtemperature (in 100-200 K range) desorption features (comprising molecular peroxo and superoxo states of O2).45,46

Figure 5: O2 TPD profiles obtained from Pd-TiO2(110) as a function of O2 dosage. The temperature ramp rate was 2 K s-1. Normalized integrated O2 desorption yield (in the 100-200 K range) as a function of O2 exposure at 98 K is shown in top-right figure.

Our exposure dependent O2 TPD profiles, obtained following O2 exposure at 98 K (see Figure 5), show exclusively low-temperature O2 desorption features (in 100-200 K range, essentially identical to that observed for Pd(111)) that develop very rapidly with exposure in the low coverage limit. This confirms existence of significant (111) texturing on the Pd NP surfaces because only on (111) facets molecular O2 are stable at 98 K. Undoubtedly, further investigation (e.g., scanning tunneling microscopy and infrared spectroscopy) will be required to determine the degree of (111) texturing53,54 (currently these facilities are not available in our laboratory). However, having no evidence of recombinative (O + O = O2) O2 desorption from Pd NP surfaces in the high temperature range (up to ~ 900 K), particularly at low exposure limit of O2, confirms that (111) facets cover dominating fraction of the Pd NP topography. Molecular O2 survives (without dissociation) only on the (111) facets of Pd NPs at 98 K. At O2-exposure temperature 300 K, however, molecular O2 dissociates immediately, rendering atomic O-adlayer on (111) facets. Recent scanning tunneling microcopy (STM) study shows that saturation-exposure of molecular O2 to physical vapor deposited-Pd NPs with dominant (111) facets at 300 K leads to formation of a p(2x2)-O overlayer structures on the (111) facets, just as in the case of extended single crystal Pd(111) surfaces54 (for reader’s perusal, a p(2x2)-O

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structure on (111) facet is schematically depicted in Figure 4b). We also anticipate formation of similar atomic O-adlayer on dominant (111) facets of Pd NPs at O2 exposure temperature 300 K. Although we have already concluded that Pd NPs, synthesized for the present study, exhibit dominant (111) facets, here, we must note that reactivity of Pd NPs even with significant (111) textures may deviate from that of the extended single crystal Pd(111) surface because NPs have high density of low coordination sites, such as edges (see Figure 4c for schematic representation of a model Pd particle with dominant (111) facets and with associated edges), which are, in principle, absent in the case of extended single crystal surfaces. Therefore, to understand the reactivity of the Pd NPs towards the thermally activated CO oxidation reaction fully, we also have to evaluate the role of edges in the CO oxidation reaction. Interesting effect of edge sites of Pd NPs with dominant (111) facets has recently been determined by Hansen et al.54 Their STM study shows that the adsorption of saturation exposure of oxygen (O2) at 300 K on Pd NPs with dominant (111) facets results in formation of p(2x2)-O overlayer structures on (111) facets. However, at very low exposure (at the same surface temperature), O2 dissociates only at edges of the Pd NPs, leading to occupation of the fcc sites just next to the particle edges. We have also drawn similar conclusion from our TPD measurements presented below.

Figure 6: (a) Several measurements of CO2 TPD from Pd-TiO2(110) surface for different exposure of O2 at 300 K. (b) Respective unreacted CO TPD profiles, recorded simultaneously with respective CO2 TPD profiles. In both figures, increasing numerical numbers denotes increasing exposure of O2. Furthermore, in both figures, the TPD profile no. 1 is obtained when no O2 was dosed prior to exposure of CO.

Figure 6a illustrates the TPD profiles of the CO2 product for varied O2 exposures. In each case, the sample was first exposed to different dosage of O2 at 300 K, then the sample was cooled down to 98 K and subsequently, was exposed to saturation dosage of CO. Figure 6b 10


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illustrates the residual CO TPD spectra recorded simultaneously with that of correlated CO2 product. At the large O2 exposure (profile no. 8 in Figure 6a), three CO2 desorption features are evident, as shown earlier (Figure 3a). At smaller exposure of O2, the high temperature peak at 368 K disappears: CO2 desorption profile occurs only in low temperature peaks at 170 K and 227 K. The residual CO desorption in the range of 200-600 K (see Figure 6b), on the other hand, exhibits features which are characteristics of CO desorption from purely Pd NP surfaces. Figure 6b also plots CO desorption for CO-only adlayer surface (profile no. 1 in Figure 6b) in order to probe the reactive CO components in the CO oxidation reaction. Comparison of residual CO TPD profiles and CO TPD profile obtained from CO-only adlayer surface suggests that at large exposure of O2 both strongly (residing at hollow surface sites) and loosely (residing at atop and bridge surface sites) bonded CO molecules are reactive and take part in the catalytic CO oxidation reaction. However, upon small exposure of O2, only loosely bonded (atop and bridge) CO molecules are reactive. Based on the above TPD results and previous STM study47,48 we model our CO oxidation results obtained on Pd NP surfaces in the following way. O2 dissociates completely on Pd NPs at surface temperature 300 K. However, when the Pd NP surfaces are exposed to very small dosage of O2 at 300 K, O2 molecules dissociate only at the edge sites (as also concluded using STM study54) and oxygen atoms preferentially occupy the fcc sites just next to the edges of Pd NPs (this is schematically shown in Figure 4d). Subsequent saturation exposure of CO at 98 K leads to the formation of CO overlayer structures both on the dominant (111) facets and at the edges. Here we note that dominant (111) facets possess atop, bridging and three fold hollow sites for CO adsorption. On the other hand, edges possess only atop and bridging sites which are weakly bonding sites for CO adsorption (see Figure 4d). Therefore, at very low exposure of O2, only loosely bound CO molecules, residing at atop and bridge sites of edges, get opportunity to react with the edge O atoms to form CO2. This edge-specific CO oxidation mechanism is responsible for the low temperature peaks at 170 K observed in CO2 TPD profiles. The edge-specific CO oxidation channel discussed above remains active at large exposure limit of O2 at 300 K. In addition to that, a p(2x2)-O overlayer structure is also formed on the dominant (111) facets, as shown in recent STM study.54 Subsequently, saturation exposure of CO at 98 K creates mixed (CO+O) adlayer structure on the dominant (111) facets as well as at edge sites. On the dominant (111) facets, CO molecules, residing at three fold hollow sites react with O atoms which are also present at three fold hollow sites of the same facet and as a result, a new high temperature desorption peak at 385 K shows up in CO2 TPD profile. The same can occur on extended Pd(111) single crystal surface and this is why the 387 K peak is also present in CO2 TPD profiles for both Pd NPs with dominant (111) facets and extended single crystal Pd(111) surfaces (not shown here). Having (qualitatively) understood the mechanism of CO oxidation reaction at O2 exposure temperature 300 K, it is now an obvious question: what is the mechanism of CO oxidation at O2 exposure temperature 98 K? Before we address this question, we can try to predict possible mechanism based on the topographic model of the Pd NPs given above. The dominant (111) facets of Pd NPs should not dissociate O2 at 98 K because molecular O2 is stable on the (111) facets at this temperature. However, O2 molecules can dissociate at edge sites of Pd NPs even at 98 K. The edge sites of Pd NP with dominant (111) facets behave like a (1x2)-

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missing row reconstructed Pd(110) surface, as shown in Figure 4e. This argument was given previously by Hansen et al.54 to explain the role of edge sites in preferential dissociation of O2 at the edge sites over the (111) terrace sites. We have closely followed their argument, first, to predict the CO oxidation reaction mechanism at O2-exposure temperature 98 K and then, to corroborate the predicted mechanism using the TPD measurement. From a simplistic point of view, a (1x2)-missing row reconstructed Pd(110) surface is similar to the nano-crystal edges if only the outermost few rows of Pd atoms are considered (compare Figure 4c, d and e, to notice the similarity between edge sites of Pd NPs and a (1x2)missing row Pd(110) single crystal surface). In both systems, a close packed row of Pd atoms is situated between two (111) facets. Therefore, adsorption behavior of O2 and CO on the nanocrystal edges can be easily modeled using adsorption of these adsorbates on a (1x2)-missing row reconstructed Pd(110) surface. The Pd(110) surfaces are well-known to dissociate molecular O2 completely at low temperature (even at 100 K).49,50 In addition, Recent STM and DFT studies on the (1x2)-missing row reconstructed Pt(110)55 and Pd(110)56-59 surfaces evidence that (upon O2 dissociative adsorption) O atoms occupy the threefold hollow sites (called (111)-micro facets) along the ridge upon dissociation of molecular O2 (as shown in Figure 4e). Among different atomic O-edge configurations, recent DFT study confirms that the most favorable structure exhibits adsorbed O atoms occupying every second fcc three-fold hollow site at both sides of the edge, forming a zigzag structure as depicted in Figure 4e.54 Based on these results, we also predict that similar zigzag structure of O atoms at the edge sites is formed upon dissociative adsorption of O2 at 98 K on Pd NP surfaces. Subsequent saturation exposure of CO at 98 K populates both dominant (111) facet sites and the edge sites. However, edge-O atoms stay in close proximity with CO residing at the edge sites only. The edge sites are quite different from (111) facets: only atop and bridge sites are available for CO adorption at the edges, whereas, three-fold hollow, atop and bridge sites are available on (111) facets for CO adsorption. As a result, one can easily speculate that it would the weakly bound CO (atop and bridging) which would react to form CO2 following saturation dosage of CO on Pd NP surfaces which were preexposed to saturation dosage of O2 at 98 K. Indeed, above speculation is evident in CO2 TPD profiles obtained for 98 K O2-exposure temperature (see Figure 7a). The TPD profiles for the CO2 product and the residual (unreacted) CO, recorded following saturation exposure of O2 at 98 K followed by two different dosage of CO at 98 K, are illustrated in Figure 7. At large exposure (higher than 0.7 θsat,CO) of CO, the CO2 TPD (red profile in Figure 7a) shows a low temperature steep peak at 170 K. On the contrary, a high temperature peak at 400 K shows up (blue profile in Figure 7a) at very low exposure (smaller than 0.3 θsat,CO) of CO. Figures 7b and 7c, on the other hand, confirm that it is only loosely bound CO (e.g., atop and bridging CO) molecules which are reactive at larger exposure of CO and more stably bound CO (undoubtedly the ones which reside at hollow sites) molecules are reactive for very low exposure of CO. This observation is remarkable because when Pd surface is saturated with CO, both loosely bound (atop and bridging) and strongly bound (hollow) sites are occupied by CO; however, only CO molecules, occupying loosely bound surface sites (atop and bridging), get opportunity to react with O-atoms. The CO molecules, residing at hollow sites, do not get opportunity to react with O-atoms. This is because, at 98 K, O overlayer structure is formed only at the edges of the NPs. Dominant (111) facets do not help formation of p(2x2)-O overlayer at 98 K because molecular O2 is stable on (111) facets at this

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low temperature. Subsequent saturation dosage of CO fills up both edge- as well as (111) facetsites of Pd NPs. However, CO molecules, which reside at the edges of Pd NPs, react with neighboring O-atoms at the edges, resulting in CO2 TPD profile with only peak at 170 K. Undoubtedly, these reactive CO molecules reside at atop and bridging sites present at the edges of the Pd NPs.

Figure 7: (a) Two CO2 TPD profiles obtained following two different exposures of CO: red and blue profiles are recorded at 0.7 θsat,CO and 0.3 θsat,CO exposures of CO, respectively (where θsat,CO is defined as saturation exposure of CO). (b) and (c) represent unreacted CO TPD profiles: red and blue profiles are recorded simultaneously with respective CO2 profiles at 0.7 θsat,CO and 0.3 θsat,CO exposures of CO, respectively; black profile is obtained when sample was not exposed to O2.

At low dosage of CO, on the other hand, only (stably bound) hollow sites present on (111) facets are occupied by CO but atop/bridging sites (including those at edges) are not occupied by CO.40-43 Therefore, at low dosage of CO, only strongly bound CO, which resides

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mostly on (111) facets, first, migrates to the edge sites and then, reacts with edge-O to produce CO2, resulting in CO2 desorption profile with a peak at high temperature (~400 K).

Figure 8: (a) CO2 TPD profiles obtained following saturation exposure of O2 at different O2exposure temperatures. (b) Integrated CO2 yield is plotted as a function of O2-exposure temperature.

Having understood the mechanisms of the CO oxidation reactions for two O2-exposure temperatures on Pd NP surfaces, in the present context, it is also intriguing to examine the effects of O2 exposure temperatures other than 98 and 300 K on the activity of Pd NPs towards the oxidation of CO. Figure 8a illustrates the TPD profiles of the CO2 product for different O2 exposure temperatures. In each case, the sample was first saturated with O2 (θsat,O2) at the specified surface temperature, then the sample was cooled to 98 K and subsequently, was saturated with CO (θsat,CO) at 98 K. Figure 8a clearly evidences that desorption profiles for CO2 obtained for O2 adsorption temperatures 98 and 200 K are quite similar: they both contain one steep peak at ~170 K. When O2 is exposed at 300 K, the steep peak at 170 K remains at the same position (with similar intensity). In addition, two new and relatively broad desorption features at 227 and 368 K, respectively, show up in the TPD profile for CO2. At O2 exposure temperatures 400K and above (e.g., 500 K, not shown in Figure 8a), on the other hand, CO2 desorption profiles comprise features similar to that observed for O2-exposure temperature 300 K. Figure 8b plots integrated yield of CO2 as a function of O2-exposure temperature, which suggests that efficient thermal combustion of CO on Pd NP surfaces requires higher O2-exposure temperature. Above 300 K, CO2 production is efficient.

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Table 1: Summary of mechanisms of the CO oxidation reactions on Pd NP surfaces, as revealed by present TPD measurements, previous STM study and present DFT calculations. The CO oxidation reaction on Pd NPs involves different reactive CO components.

Therefore, in brief, based on current TPD measurements and previous STM study, we conclude several mechanisms for thermally promoted CO oxidation reaction on Pd NP surfaces, which are summarized in Table 1. One important conclusion, which is evident in Table 1, is that CO, residing at edge sites, reacts with edge-O at low temperature (170 K) and CO, residing at hollow sites present at (111) facets, reacts with O atom (present either at edge site or at (111) facets) at high temperature (above 350 K). This result directly points to existence of different activation energy barriers associated with edge-specific CO oxidation and (111) facet-specific 15



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CO oxidation reactions. To support this conclusion further via theoretical calculations (based on DFT), we have compared and contrasted adsorbate adlayer structures and CO oxidation activation energies associated with Pd(111) and (1x2)-missing row Pd(110) surfaces. Here we remind us that slab model Pd(111) surface represents (111) facets of Pd NPs and slab model (1x2)-missing row Pd(110) surface represents edges of Pd NPs, as discussed earlier in this article. We have considered these two model systems because it is impossible to carry out DFT calculations on a “real” nanocrystal (with ~11 nm diameter and ~4 nm height), as this would require a very large unit cell. Previous experimental44,54 and theoretical54 works evidence that the p(2x2)-O structure, in which O atoms occupy three fold hollow sites, is the most stable oxygen atom adlayer structure on the Pd(111) surface and the zigzag-O structure, in which oxygen atoms occupy every second fcc threefold hollow site at both sides of the edge, is the most stable oxygenadlayer configuration on the (1x2)-missing row Pd(110) surface. Therefore, we have first optimized these two O-adlayers on model Pd(111) and (1x2)-missing row Pd(110) surfaces, respectively, and then have re-optimized the (CO+O) mixed adlayer structure by considering only one CO molecule per surface unit cell. Respective most stable optimized (CO+O) adlayer structures are depicted in Figure 9. Here, we note that among different (CO+O) configurations, the most stable (associated with lowest energy) (O+CO)-structure on the Pd(111) surface is the one with CO and O both residing at three fold hollow sites and that on the (1x2)-missing row Pd(110) surface is the one with O atoms residing at (111) microfacets of the edges and with CO residing at bridging sites of the edges. The activation barriers and the exothermicities associated with the CO oxidation reaction on Pd(111) and (1x2)-missing row Pd(110) surfaces are given in Figure 9a and b, respectively. These results, in close agreement with experimental observations, corroborate the fact that the activation energy associated with the edge-specific CO oxidation reaction is much lower (DFT predicts ~0.6 eV lower energy) than that of (111) facet-specific CO oxidation reaction.

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Figure 9: DFT results: Most stable (CO+O) adlayer structure, activation barrier associated with the CO oxidation reaction, and transition state as well as CO2 product structures on (a) Pd(111) surface and (b) (1x2)-missing row Pd(110) surface. Here slab models of Pd(111) surface and (1x2)-missing row Pd(110) surface represent the (111) facets and the edges of Pd NPs, respectively.

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B. Ultrafast Dynamics of CO Oxidation. Ultrafast laser surface spectroscopy provides an opportunity to take a real-time snapshot of the chemical reaction and to observe the related dynamics for well-defined initial adsorbate structures. For last two decades, the femtosecond two pulse correlation (2PC) spectroscopy has been efficiently utilized to study the surface chemical dynamics at flat single crystal surfaces on the ultrafast time scale. If one carries over the concepts developed over the two decades from 2PC study of femtosecond pulse-induced processes at single crystal surfaces to that at nano-scale surfaces, one can speculatively depict possible post-irradiation energy transfer mechanisms at the supported metal NP surface, as shown in Figure 10.15 Each channel is self-explanatory and will be again discussed later in this article.

Figure 10: Schematic overview of the possible energy transfer and dissipation processes involved in a photoinduced molecular dynamics on a supported nanoparticle. Corresponding possible time scales are also given.15

Generally, under femtosecond-pulse excitation, the first-shot yield ( YFS ) depends on the incident fluence ( F ) in a highly nonlinear fashion (Figure 11a). Empirically, the fluence dependence may be described by a power law, YFS ∝ F n . Typically, n > 2 for desorption induced by a femtosecond pulse is observed,20,60,61 consistent with a mechanism where high transient adsorbate temperatures, which may greatly exceed the thermal-desorption temperature, are needed to drive the desorption process. The CO oxidation reaction on Pd-TiO2 surface, induced by femtosecond laser pulses, results in efficient desorption of the product CO2. For (CO+O)/PdTiO2(110) surface, which is prepared by saturation exposure of O2 at 300 K followed by saturation exposure of CO at 98 K, n estimated to be 3. We note that the fluence dependent

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femtosecond pulse-induced desorption measurement is performed at substrate temperature 98 K after preparing the (CO+O)/Pd-TiO2(110) surface.

Figure 11: (a) Fluence dependence of CO2 yield from Pd NPs obtained at the saturation coverage regime of adsorbates. (b) Two-pulse correlation (2PC) traces of ultrafast CO oxidation reaction dynamics from Pd NPs at saturation coverage regime of adsorbates.

Two pulse correlation (2PC) measurement of the desorption-yield directly probes the dynamics of energy flow in the adsorbate-substrate complex in real time. In this measurement, the 800 nm excitation pulse is split into two parts separated by a delay, and the photodesorption yield is measured as a function of the delay as described in the Methods section. Because the absorption of the light by CO or O atoms is negligible at this wavelength, the energy absorbed in the substrate must be transferred to the reaction coordinate through coupling of the substrate electrons and/or phonons to the adsorbate-substrate vibrational modes.60,62,24 The strengths of the couplings among the various degrees of freedom of the system determine the time response of the system to femtosecond pulse excitation. By varying the delay between the two excitation pulses, the relaxation time scale(s) of the excitation driving the reaction can be measured. In general, when a sufficiently short relaxation time, less than 1 ps, is observed, strong coupling of the adsorbate to the electronic degrees of freedom of the substrate is indicated. More quantitative information can be inferred by numerical simulation of the 2PC measurements based on two temperature model (2TM) for the surface chemical reaction dynamics. Figure 11b plots the data for 2PC measurements of the desorbing CO2 product for two different O2 exposure temperatures (98 K and 300K). In these measurements, Pd NP surfaces are first saturated with O2 at the specified temperature, then the NP surfaces are saturated with CO at 98 K and finally femtosecond pulse pair (pump+probe)-induced CO2 desorption yield is measured at substrate temperature 98 K using QMS integrated with MCS-card. We find that both

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2PC data are well-described by single exponential decay: time constants (τ), extracted from exponential fits to the data, are given in Figure 11b. The time constants (τ) obtained from the 2PC measurements for O2-exposure temperatures 200 K and 400 K, respectively, are shown in Table 2. The data clearly shows that fs pulse-induced CO oxidation reaction dynamics observed for O2-exposure temperature 200 K remains similar to that observed for O2-exposure temperature 98 K. Similarly, ultrafast dynamics of CO oxidation reaction observed for O2-exposure temperature 400 K is quite similar to that observed for O2-exposure temperature 300 K. Furthermore, the 2PC decay time depends strongly on the O2-exposure temperature. We observe a factor of ~2 increase in time constant (τ) when the O2-exposure temperature is decreased from 300 K to 98 K.

Table 2: Two Pulse Correlation Dynamics of CO Oxidation Reaction on Pd NP Surfaces. O2-exposure Temperature 98 K

τ (ps) 15.0±1.4

First-shot Yield of CO2 (Counts) 15±1

200 K

15.3±1.5

16±1

300 K

7.3±1.1

27±2

400 K

7.5±1.3

29±2

As CO oxidation reaction in time domain has already been studied on the single crystal Pd(111) surface,12 it is quite instructive that we begin the analysis of our present ultrafast spectroscopy results by comparing the femtosecond pulse-induced CO oxidation reaction on Pd NP surfaces with that observed for the single crystal Pd(111) surface. Currently, this comparison can be performed only for high coverage limit of CO and for O2-exposure temperature 300 K, because present femtosecond pulse-induced CO oxidation reaction on Pd NP surfaces could only be performed at saturation exposure of CO (very low counts at the CO2 mass channel did not allow us to record the CO2 desorption yield at lower exposure of CO) and previous single crystal Pd(111) study was performed only for O2-exposure temperature 300 K. Here, we also note that the CO oxidation reaction in time domain has also been explored on the single crystal Ru(001) surface.11 A detailed comparison of femtosecond pulse-induced CO oxidation reaction on Pd(111) with that on Ru(001) surface can be found in literature12 and that is why we have not repeated the same here.

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We find one similarity and several differences between 2PC data recorded on the single crystal Pd(111) and Pd NP surfaces for high CO coverage limit and 300 K O2-exposure temperature. The 2PC data obtained from single crystal Pd(111) surface is fitted by biexponential decay, with fast and slow time constants found to be ~6 and ~50 ps, respectively. On the other hand, single exponential fitting is used for the 2PC data recoded from Pd NPs (with dominant (111) facets): the time constant is found to be ~7 ps for O2-exposure temperature 300 K. This time constant is quite similar to the fast time constant observed for single crystal Pd(111) surface. Not observing the slow decay in 2PC data recorded on Pd NP surfaces suggests that only single CO oxidation reaction mechanism is active on Pd NP surfaces. On the other hand, two different CO oxidation reaction mechanisms are involved on the single crystal Pd(111) surface. More rigorous comparison of femtosecond pulse-induced CO oxidation reactions between single crystal Pd(111) and Pd NP surfaces can be only performed by monitoring the coverage dependence and O2-exposure temperature dependence of the oxidation dynamics on both surfaces. For this, however, we have to increase the size of the Pd NPs and the particle density to improve the counts at the CO2 mass channel. In fact, we have already started working on the synthesis of Pd NPs which will give us larger particle size and higher particle density. After comparing Pd(111)- and Pd NP-results, we shall now focus on finding correlation between the TPD and the 2PC measurements. At the saturation exposure limit of CO, TPD measurements have already established that the nature of reactive CO involved in the CO oxidation reaction changes as we change the O2-exposure temperature (see Table 1). At large CO exposure limit, two different mechanisms of CO oxidation reaction have been established via TPD measurements and DFT calculations: (a) only loosely bound CO, residing at atop and bridge sites of NP edges, react with fcc edge-O atoms at both O2-exposure temperatures 98 and 200 K (resulting in edge-specific CO oxidation reaction); and (b) at O2-exposure temperatures 300 and 400 K, on the other hand, both loosely and strongly bound CO molecules (residing at NP edges and on dominant (111) facets) react with edge- and (111)-O atoms. The 2PC measurements for O2-exposure temperatures 98, 200, 300 and 400 K, therefore, estimate the time scale of the CO oxidation reaction in which different CO reactants are involved. The CO oxidation reaction is relatively a fast (time constant ~7 ps) process when CO molecules, residing at three-fold hollow sites (of (111) facets) of Pd NPs, react with O atoms. It is interesting to note that similar time constant is also found for CO oxidation reaction at single crystal Pd(111) surface as well, pointing to similarity between the reactivity of (111) facets of Pd NPs and single crystal Pd(111) surfaces.12 On the other hand, the CO oxidation reaction is relatively a slow (time constant ~ 15 ps) process when CO molecules, residing at atop/bridge sites (of edges) of Pd NPs, react with O atoms. Therefore, we have been able to monitor edge- and (111) facetspecific ultrafast CO oxidation reaction dynamics on Pd NP surfaces by the present 2PC measurements. After having estimated the two different time scales for the CO oxidation reaction associated with two different reactive CO components on Pd NP surfaces, we shall now focus on mechanistic details of femtosecond pulse-induced CO oxidation reaction on Pd NP surfaces. Thus far, resonant hot electron transfer to the low-lying metal-O antibonding states is believed to be the rate determining step for the ultrafast CO oxidation reaction on Ru and Pd single crystal surfaces.4-13 On the contrary, our present work unambiguously shows that activation of CO is the rate limiting step for the CO oxidation reaction on Pd NP surfaces. If electron-mediated

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activation of the Pd-O bond were the rate limiting step in the CO oxidation reaction on Pd NP surfaces, the oxidation reaction should have taken place as soon as the O atoms is vibrationally activated by the hot electrons. As a result, reaction dynamics should not have depended on the activation of CO. Quite interestingly, however, the 2PC traces, depicted in Figure 11b and Table 2, show that the dynamics of the reaction between activated O with CO strongly depends on bonding strength (more specifically NP-CO coupling, which will be discussed shortly) of CO with Pd NP surfaces.

Figure 12: Results of density functional theory (DFT) calculations: (a) Antibonding states of O appears just above Fermi level (within 1 eV from the Fermi level); (b) Antibonding states of CO appears well above the Fermi level (above 2 eV from the Fermi level). In both figures, FermiDirac distribution functions (right coordinate axis) at 300 and 6000 K demonstrate that with increasing electron bath temperature, antibonding O states are more easily populated than the CO antibonding states. Therefore, O atoms can be easily activated through direct coupling with hot electron bath. The same is relatively difficult for the activation of CO molecules.

To obtain further insight into the microscopic mechanism of the direct role of hot electron in activation of O and CO adsorbates, we have carried out density functional theory calculations for (CO+O) adlayer surfaces on slab model Pd(111) and (1x2)-missing row (110) surfaces, to model (111) facets and edges of NPs, respectively. Optimized adlayer structures are already presented earlier in this article. Here we analyze the local density of states (DOS) produced upon adsorption of O and CO adsorbates. An unoccupied antibonding state (with respect to the NP-O bond) right above the Fermi level ε F (within 1 eV from the Fermi level) is induced following adsorption of O atoms on both (111) and (1x2)-missing row (110) surfaces (see Figure 12). Hot metal electrons (created by fs pulse excitation) can easily populate this antibonding state, as 22


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shown in Figure 12, by taking illustrative example of Fermi-Dirac distributions ( f ( ε ) ) at two different electronic bath temperatures (namely, 300 K and 6000 K). More details and relevance of these two electronic temperatures will be discussed shortly. In comparison with the O-DOS, the CO-DOS plots for both Pd(111) and (1x2)-missing row Pd(110) surfaces, on the other hand, show that the unoccupied antibonding state (with respect to NP-CO bond) lies well above the Fermi level (above 2 eV from the Fermi level). These results suggest that weakening (activation) of NP-O bond directly by hot electrons is much easier than that of NP-CO bond due to low-lying (very close to the Fermi level) antibonding state of O-adlayers. Therefore, based on previous fs chemical dynamics studies of CO oxidation reactions on single crystal surfaces4-13 and present DFT results, we believe that the time scale for the Pd-O bond activation is on the order of a few hundred fs because it is purely hot electron-mediated. Given this information, we anticipate that 2 fold increase in reaction time for the mixed (CO+O) adlayer prepared at O2 exposure temperature 98 K (and 200 K) with respect to that at O2exposure temperature 300 K (and 400 K) is the result of the decrease in coupling strength of NP…..CO. The qualitative relationship between the substrate-adsorbate coupling strength and the 2PC decay time constant can be understood intuitively; when the coupling of the adsorbates to the substrate becomes weaker, the response of the adsorbate to the laser induced heating becomes also slower. Slower adsorbate heating results in a slower 2PC decay time constant. This observation is consistent with recent molecular dynamics study of femtosecond laser pulseinduced photodesorption of O2 from Ag(110).63 We shall further investigate these energy transfer rates using two-temperature model (2TM). Femtosecond laser-induced chemical dynamics at extended single crystal surfaces is often analyzed using empirical friction model which includes coupled heat baths for the electron, phonon, and adsorbate degrees of freedom, each characterized by its own temperature (as schematically shown in Figure 10, taking an illustrative example of possible channels relevant to metal NP).25 Under this model, the surface chemical dynamics is described by frictional coupling resulting in a delayed energy transfer between the electron, phonon, and adsorbate subsystems. Finally, the surface chemical reaction rate is calculated by an Arrhenius expression. Employment of similar model for NP surfaces may not be straight forward. The reason can be both technical as well as scientific. One of the important technical difficulties in employing 2TM for NP surfaces is that experimentally it is difficult to measure the fluence absorbed by supported metal NPs. Absorbed fluence decides peak electron and phonon temperatures, which ultimately control surface chemical dynamics of adsorbates on metal NP surfaces. Furthermore, many novel effects (such as, confinement of electronic excitation, Schottky barrier, etc., as elaborately discussed in ref. 11) may also affect the chemical dynamics on NP surfaces. Systematically these effects have not been incorporated, yet, in 2TM-based simulation of chemical dynamics on metal NP surfaces.

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Figure 13: Simulation results obtained with the thermodynamical and optical parameters given in Table 3 (for O2-exposure temperature 300 K): (a) Maximum electron phonon and adsorbate temperatures at the NP surface as a function of the delay between the two pulses; (b) Simulated time-dependent surface electronic (bold red line), surface phonon (dot blue line) and surface adsorbate (green dash line) temperatures, when delay between two pulses is 0 ps (top figure) and 1 ps (bottom figure). Here, adsorbate temperatures are calculated (plotted in both (a) and (b) figures) considering phonon-only model (see text for details).

In spite of technical and scientific difficulties in employing 2TM model for the prediction of ultrafast chemical dynamics at metal NPs, we have used this model semi-quantitatively and have further analyzed the 2PC measurement data presented above. Initially, absorption of photons ( hν ) in the substrate creates a non-equilibrium electron–hole pair distribution which thermalizes by rapid electron–electron scattering. The electronic system can subsequently be described by an electron temperature Tel. The hot electron gas relaxes by electron–phonon coupling to the lattice phonons (Tph) and by heat diffusion into the bulk. The time evolution of the electron and lattice temperature (Tel and Tph) can be written as,64 Cel

∂ Tel ∂t

∂ ∂ κ Tel − g (Tel − Tph ) + S ………. (1) ∂z ∂z ∂ C ph Tph = g (Tel − Tph ) ………. (2) ∂t

=

Here, Cel and C ph are the electron and lattice heat capacities. κ and g refer to the thermal conductivity and the electron-phonon coupling constant, respectively. Furthermore, lateral diffusion is neglected here since the beam diameter is much larger than the electron diffusion length. This reduces the dimensionality to the distance z along the surface normal. The laser source term is given by,

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S ( z, t ) =

1   t − t1  2  t − t2   exp  − z   F1 sech 2   + F2 sech    ………. (3) λ  Z   2 BλZ  B   B 

Here, t1 and t 2 are arrival times for two pulses, F1 and F2 are absorbed fluences, λZ is optical penetration depth and finally, B is temporal width of the pulses.

Table 3: Values of parameters used in the two-temperature model simulations. Parameter Electronic specific heat offset Electronic specific heat slope Thermal conductivity Electron-phonon coupling constant Initial temperature Incident Fluence Wavelength of the Incident light Pulse FWHM Initial Coverage Vibrational frequency

Symbol γ0 γ1 κ g T F λ B ϴ νabs

Value 7.8849 x 104 249.14 72 8.95 x 1017 98 5 800 35 0.75 1.0 x1013

Dimensions J K-1 m-3 J K-2 m-3 W K-1 m-1 W K-1 m-3 K mJ cm-2 nm fs ML s-1

Because of its low heat capacity, the electronic system is heated by the first pulse to several thousands of kelvin before equilibrating with the lattice vibrational modes by electron– phonon coupling. The second pulse drives the system even further from equilibrium, resulting in a spike of the maximum electronic temperature Tel,max. This spike in Tel,max causes the dip in maximum phonon temperature Tph,max. This is shown in Figure 13a, given our experimental conditions for two pulse excitations. The thermodynamical and optical properties of palladium are given in Table 3. Because of its low heat capacity, the electronic system is heated to ~5500 K before equilibrating with lattice vibrational modes by electron phonon coupling within ~1.7 ps, as shown in Figure 13b. Energy transfer from the electron and phonon system to the adsorbate (in its simplest form) can be described by frictional coupling of the electron and phonon heat baths of substrate to that of the adsorbate (defined as Tads ): dU ads = ηel (U el − U ads ) + η ph (U ph − U ads ) ………. (4) dt where, the energy U ads of the oscillator in a heat hath of the temperature Tads can be expressed as, 25



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U ads =

hυ

e

ads hυads / k BTads

−1

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………. (5)

Here, υads refers to the frequency of the vibration along the reaction coordinate, which in case of CO oxidation reaction is assumed to be 1×1013 s −1 (approximately corresponding to ~570 cm-1 for the unstable normal mode associated with the CO oxidation transition state, calculated at 1  DFT level of theory). The friction coefficients ( η ) are inverse energy relaxation times η =  τ  of the adsorbate-substrate vibrational mode which has been used to express adsorbate vibrational excitation and relaxation.

After determining the adsorbate temperature, the desorption rate is calculated by an Arrhenius type expression (the Polanyi-Wigner (PW) rate equation) assuming first order kinetics, R (t ) = −

dθ −E = θν PW e a dt

( k BTads ( t ) )

……… (5)

Here, θ is the coverage, Ea is the activation energy for CO oxidation reaction, and ν PW is the prefactor. Finally, the yield measured in the experiment is the time-integrated reaction rate, R(t ) .

Figure 14: Experimental (circle with error bar) and simulated (best fit: bold line) first shot yield from 2PC measurement at substrate temperature 98 K, taking (a) pure electronic model and (b) pure phonon model. Respective O2 exposure temperatures are also indicated. Theoretical fit in (b) is shown from 500 femtosecond to 100 picosecond.

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To begin the discussion of the simulation and its fitting to the experimental results, first, we consider a model where the adsorbate is coupled to the photoexcitation only through electron interactions (considering η ph = 0 in equation 4). Such a model might be expected to perform well on the basis of the previous results for CO oxidation reaction on single crystal Ru(0001) and Pd(111) surfaces.11,12 To test this pure electronic model, ηel , Ea , and ν PW , were varied to best simulate the 2PC results (see Figure 14a for fitting); however, a good fit could not be obtained. The simulation results, which run closest to the experimental results shown in Figure 14a, exemplify the failing of this pure electronic model. The parameters for this simulation are τ el = ηel −1 = 5 ps , Ea = 1.0 eV and ν PW = 1× 1010 s −1 for O2-exposure temperature 300 K, and

τ el = η el −1 = 15 ps , Ea = 0.5 eV and ν PW = 1× 1010 s −1 for O2-exposure temperature 98 K. In a second model, simulations were performed with a pure adsorbate coupling to phonon (i.e., ηel = 0 in equation 4). In this phonon-only model, Ea and η ph are treated as unconstrained adjustable parameter with a reasonable ν PW = 1× 1013 s −1 prefactor.65,66 A very good fit to the experimental results (see Figure 14b) was obtained with τ ph = η ph −1 = 5 ps and Ea = 1.5 eV for

O2-exposure temperature 300 K, and τ ph = η ph −1 = 15 ps and Ea = 0.9 eV for O2-exposure temperature 98 K. The estimated Ea is in very good agreement with the DFT-computed activation energy of CO oxidation reaction (see Figure 9) both for 300 K and 98 K O2-exposure temperatures. Furthermore, a phonon-adsorbate coupling time of (τ ph = η ph −1 ) 15 ps is estimated for CO oxidation reaction at O2-exposure temperature 98 K and the same is estimated to be 5 ps at O2-exposure temperature 300 K. These timescales are in good agreement with the timeconstants recovered from exponential fitting to experimental results (see Figure 11). Here we note that one prediction of the pure phonon-only model is not observed in the experimental data: the dip near zero time delay between two pulses (compare experimental results and simulation results in Figure 14b near zero delay), which is the consequence of the dip in Tph,max (see Figure 13a), amplified by the exponential factor in equation 5. At present we are unable to explain this mismatch without ambiguity. The mismatch could be due to several reasons: (1) systematic errors in the experiments due to interference effects around zero delay may hinder a final conclusion, (2) absence of a dip in the two pulse correlation measurements for nanoparticles surfaces may question validity of the two temperature model, (3) near zero delay, electronic coupling can compensate the dip due to coupling with phonon. We aim to disentangle these issues in near future. Therefore, based on present simulation, we rule out the possibility of pure electronmediated rate determining step in the CO oxidation reaction on the Pd NP surfaces (at saturation CO coverage limit). Here we note that a combined electron-phonon model fails to provide good fit to the experimental results (see Supporting Information S3). The rate determining step of the CO oxidation reaction on Pd NP surfaces must be a phonon-mediated process. However, Oactivation (which must occur before CO is oxidized) is pure electron-mediated process due to presence of very low-lying NP-O antibonding states (present very close to the Fermi level). In the end, we comment on nonlinear fluence dependence observed for CO2 yield (as shown in Figure 11(a)). Figure 13a exhibits change of maximum adsorbate temperature as a 27



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function of delay time, which is the consequence of the coupling of the adsorbates to the phonon of substrate. This change is further amplified by the exponential factor in equation 5 to render yield of the reaction. The exponential dependence of yield on the adsorbate temperature is the primary reason of nonlinear fluence dependence of the CO2 yield observed in 2PC measurement.

IV. Conclusions: In the present work, using temperature programmed desorption (TPD) spectroscopy and density functional theory calculations, we have identified weakly and strongly bound CO adsorbates which react with O atoms on the Pd NP surfaces. Weakly bound CO molecules mostly reside at edges of the Pd NPs and strongly bound CO molecules resides on (111) facets of the Pd NPs. The CO oxidation reaction dynamics are measured using femtosecond two-pulse correlation spectroscopy. The reaction time scale for the oxidation of weakly bound CO is found to be ~15 ps, and ~7 ps is found for oxidation of strongly bound CO. These experimental results are analyzed with the help of the Density Functional Theory calculations and two temperaturebased simulations. Pure phonon-mediated energy transfer from the metal nanoparticles to the adsorbates is identified to play the key role in the rate-determining step of femtosecond pulseinduced CO oxidation reaction at the saturation CO coverage limit. However, activation of O atoms is identified as relatively fast (presumably occurs in several hundred femtosecond) electron-mediated process. This is the first report on ultrafast dynamics of CO oxidation reaction on supported catalytically active metal nanoparticle surfaces.

Supporting Information: S1: Details of synthesis and characterization of the nanoparticles given; S2: Details of the pumpprobe optical set-up given; S3: A combined electron-phonon model for the 2PC data presented. This information is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments: This research was supported jointly by the DST Nano Mission (SR/NM/NS-1117/2013(G)), UGC CAS, DAE BRNS (2013/20/37P/3/BRNS/2750), and ISRO STC (STC/P-333). Authors thank Professor A. G. Samuelson (IPC, IISc) for donating cluster facility which was used to perform DFT-based computational work presented in this article.

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Table and Figure Captions: Table 1: Summary of mechanisms of the CO oxidation reactions on Pd NP surfaces, as revealed by present TPD measurements, previous STM study and present DFT calculations. The CO oxidation reaction on Pd NPs involves different reactive CO components.

Table 2: Two Pulse Correlation Dynamics of CO Oxidation Reaction on Pd NP Surfaces. Table 3: Values of parameters used in the two-temperature model simulations.

Figure 1: (a) AFM and (b) SEM images of clean Pd-TiO2(110) surface on which the ultrafast dynamics of the CO oxidation reaction has been studied. Gaussian fits to the height and diameter distributions obtained from AFM and SEM studies, respectively, are also given. Figure 2: (a) Temporal histogram of the QMS signal of CO2 after the first laser shot. (b) Depletion curve (with an exponential fit) of CO2 product as a function of laser shot.

Figure 3: (a) Two measurements of CO2 TPD from Pd-TiO2(110): red and blue profiles obtained at O2 exposure temperature 98 and 300 K, respectively. (b) The red and blue TPD profiles feature unreacted CO, recoded simultaneously with respective (presented by the same color) CO2 TPD profiles; the black TPD profile shows CO desorption from Pd-TiO2 sample, not exposed to O2. For all TPD measurements, the temperature ramp rate was 2.0 K s–1. Figure 4: (a) schematically atop, bridge and hollow sites are shown. (b) A p(2x2)-O adlayer structure is depicted schematically on Pd(111) surface. Here O atoms occupy the hollow sites. (c) A typical example of Pd NP is depicted to show (111) facets and edges. (d) Edge sites of Pd NPs are comparable to (1x2)-missing row Pd(110) surfaces. A typical example of dissociation of O2 at an edge of (1x2)-missing row Pd(110) surface is shown. (e) A zigzag-O adlayer structure is schematically shown on (1x2)-missing row Pd(110) surface.

Figure 5: O2 TPD profiles obtained from Pd-TiO2(110) as a function of O2 dosage. The temperature ramp rate was 2 K s-1. Normalized integrated O2 desorption yield (in the 100-200 K range) as a function of O2 exposure at 98 K is shown in top-right figure.

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Figure 6: (a) Several measurements of CO2 TPD from Pd-TiO2(110) surface for different exposure of O2 at 300 K. (b) Respective unreacted CO TPD profiles, recorded simultaneously with respective CO2 TPD profiles. In both figures, increasing numerical numbers denotes increasing exposure of O2. Furthermore, in both figures, the TPD profile no. 1 is obtained when no O2 was dosed prior to exposure of CO. Figure 7: (a) Two CO2 TPD profiles obtained following two different exposures of CO: red and blue profiles are recorded at 0.7 θsat,CO and 0.3 θsat,CO exposures of CO, respectively (where θsat,CO is defined as saturation exposure of CO). (b) and (c) represent unreacted CO TPD profiles: red and blue profiles are recorded simultaneously with respective CO2 profiles at 0.7 θsat,CO and 0.3 θsat,CO exposures of CO, respectively; black profile is obtained when sample was not exposed to O2. Figure 8: (a) CO2 TPD profiles obtained following saturation exposure of O2 at different O2exposure temperatures. (b) Integrated CO2 yield is plotted as a function of O2-exposure temperature.

Figure 9: DFT results: Most stable (CO+O) adlayer structure, activation barrier associated with the CO oxidation reaction, and transition state as well as CO2 product structures on (a) Pd(111) surface and (b) (1x2)-missing row Pd(110) surface. Here slab models of Pd(111) surface and (1x2)-missing row Pd(110) surface represent the (111) facets and the edges of Pd NPs, respectively.

Figure 10: Schematic overview of the possible energy transfer and dissipation processes involved in a photoinduced molecular dynamics on a supported nanoparticle. Corresponding possible time scales are also given.15

Figure 11: (a) Fluence dependence of CO2 yield from Pd NPs obtained at the saturation coverage regime of adsorbates. (b) Two-pulse correlation (2PC) traces of ultrafast CO oxidation reaction dynamics from Pd NPs at saturation coverage regime of adsorbates. Figure 12: Results of density functional theory (DFT) calculations: (a) Antibonding states of O appears just above Fermi level (within 1 eV from the Fermi level); (b) Antibonding states of CO appears well above the Fermi level (above 2 eV from the Fermi level). In both figures, FermiDirac distribution functions (right coordinate axis) at 300 and 6000 K demonstrate that with increasing electron bath temperature, antibonding O states are more easily populated than the CO antibonding states. Therefore, O atoms can be easily activated through direct coupling with hot electron bath. The same is relatively difficult for the activation of CO molecules.

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Figure 13: Simulation results obtained with the thermodynamical and optical parameters given in Table 3 (for O2-exposure temperature 300 K): (a) Maximum electron phonon and adsorbate temperatures at the NP surface as a function of the delay between the two pulses; (b) Simulated time-dependent surface electronic (bold red line), surface phonon (dot blue line) and surface adsorbate (green dash line) temperatures, when delay between two pulses is 0 ps (top figure) and 1 ps (bottom figure). Here, adsorbate temperatures are calculated (plotted in both (a) and (b) figures) considering phonon-only model (see text for details). Figure 14: Experimental (circle with error bar) and simulated (best fit: bold line) first shot yield from 2PC measurement at substrate temperature 98 K, taking (a) pure electronic model and (b) pure phonon model. Respective O2 exposure temperatures are also indicated. Theoretical fit in (b) is shown from 500 femtosecond to 100 picosecond.

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CO Activation Determines Ultrafast Dynamics of CO Oxidation

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