CO Chemisorption on Ultrathin MgO-Supported Palladium

Article pubs.acs.org/JPCC

CO Chemisorption on Ultrathin MgO-Supported Palladium Nanoparticles Aimeric Ouvrard,*,† Ahmed Ghalgaoui,†,⊥ Carine Michel,‡ Clemens Barth,§ Jijin Wang,† Serge Carrez,† Wanquan Zheng,† Claude R. Henry,§ and Bernard Bourguignon† †

Institut des Sciences Moléculaires d’Orsay (ISMO), CNRS, Université Paris-Sud, Université Paris-Saclay, F-91405 Orsay, France Univ Lyon, Ens de Lyon, CNRS UMR 5182, Université Claude Bernard Lyon, Laboratoire de Chimie, F69342, Lyon, France § CNRS, Aix-Marseille Université, CINaM UMR 7325, 13288 Cedex 09 Marseille (France) ‡

S Supporting Information *

ABSTRACT: The adsorption of CO on Pd nanoparticles (NP) supported on MgO ultrathin films is studied by sum frequency generation (SFG) vibrational spectroscopy at room temperature as a function of CO coverage for NP lateral sizes ranging from 3 to 6 nm. While the spectroscopic signature of CO on (100) top facets dominates the spectra, other new bands are attributed to (111) facets, edges, and defects. CO remains strongly bonded for all sizes, but spectroscopic and kinetic parameters evolve as NP size decreases. It is only for the smallest NPs ∼3 nm in diameter that the deviation from Pd(100) single crystal is important. Modeling of adsorption kinetics, dipolar coupling and DFT calculations allow rationalizing the observations: the size dependent Pd−Pd bond extension by the substrate is the main parameter that controls CO frequency in the low coverage limit. SFG sensitivity is found to increase up to 10−4 ML as NP size decreases. This makes SFG particularly well suited to probe molecular adsorption in the submonolayer range on small NPs of 3 nm in diameter and below. These results motivate one to probe CO under catalytic conditions and to follow electron transfer in pump−probe experiments.

■

INTRODUCTION The adsorption and desorption of reactants at gold1−5 and palladium nanoparticles6−14 have been studied to a large extent on single oxide crystal surfaces and ultrathin films to understand fundamental mechanisms involved in catalytic reactions. A very important finding of all such work is that a decrease of the NP’s size onto a few nanometers modifies their site distribution, the number of low coordinated metal atoms and subsurface oxidation, which in turn has a massive impact onto the catalytic properties of the NPs like the adsorption characteristics.1,5,15 One reactant that has been considered to a large extend in heterogeneous model catalysis is CO because of its importance in society. Microscopy,5,16,17 Density functional theory (DFT) calculations,8,18,19 microcalorimetry16 and molecular beam experiments20,21 were extensively used in the past to characterize oxide surfaces and supported NPs as well as the adsorption and desorption of CO on those surface systems. Among all these techniques, adsorption and desorption experiments with optical vibrational spectroscopy1,4,7,13,22 provides valuable information about (i) the molecule−metal interaction (chemical bonding) and (ii) the molecule−molecule (dipolar) interaction via analyzing the CO internal stretch frequency. The molecule−molecule (dipolar) interaction can be determined by modeling, which allows disentangling the two effects. The change of chemical bonding strength with the type of © XXXX American Chemical Society

metallic substrate, the adsorption sites and the CO coverage, are generally the strongest contribution to the CO frequency shift.18,23,24 CO binding on transition metals involves both electron donation from the 5σ CO highest occupied molecular orbital to Pd sp-bands and back-donation from the Pd d-band to CO antibonding 2π* lowest unoccupied molecular orbital.25,26 The d-band central position, Pauli repulsion, and interaction of sp- and d-bands with 5σ and 2π* CO orbitals have to be taken into account for a complete description.27,28 The other contribution to CO frequency shift is dipolar interaction between adsorbates through vibrational coupling, which have been intensively studied in the past on single crystals.29−31 On transition metals of the Pt group, both dipolar and chemisorption effects contribute to blue shift the frequency,32,33 while for coinage metals (Au, Ag, Cu), they contribute in opposite directions.3,4,34 The NP size effect on the CO frequency was already observed on Pd,7,9 Au,3,4 and Pt35 NPs. However, the role of dipolar and chemical contributions has not been disentangled so far although the NP size effect on chemisorption needs to be added to the chemical and dipolar effects of CO adsorption. Reduction of the CO adsorption energy at small Pd NPs Received: October 20, 2016 Revised: February 15, 2017 Published: February 15, 2017 A

DOI: 10.1021/acs.jpcc.6b10595 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

the 1800−2100 cm−1 spectral region. A supercontinuum seeded BBO optical parametric amplifier (Spectra Physics, USA) pumped by a 800 nm Ti:Sa laser/amplifier system (Coherent, USA), with duration 120 fs, repetition rate 1 kHz, and energy 800 μJ/pulse, is used for SFG experiments. Experimental details are given elsewhere.47 SFG is induced by two p−polarized collinear laser beams, i.e., a near-infrared pulse, called “visible” (ωVIS = 12376 cm−1, 7 cm−1 fwhm, 3 ps duration), and an infrared pulse (ωIR = 1950 cm−1, ∼150 cm−1 fwhm, 140 fs duration) covering the spectral region of CO internal stretch frequencies, that are spatially and temporally overlapped on the surface sample. SFG experiments are done at room temperature upon continuous CO exposure. The procedure to monitor CO adsorption by SFG is as follows: the sample is heated up to 540 K in 4 min under residual CO pressure in the chamber (pCO < 10−10 mbar) in order to desorb all CO.47,48 We let the sample cool down for 32 min to almost room temperature. Afterward, the CO pressure is increased to 2.8 × 10−9 mbar (5N purity, Air Liquide, France) and SFG spectra are recorded during CO adsorption. At this pressure CO adsorption reaches saturation at 300 K at a coverage close to 0.5 ML on Pd single crystal after 20−30 min. CO pressure is then raised step by step up to 10−3 mbar in order to reach higher coverages. Figure S1 in the Supporting Information (section SI-1: Protocol for SFG Experiments) summarizes the evolution of temperature and pressure during SFG experiments. It also shows the calculated evolution of the sticking coefficient as a function of time, i.e., temperature. Most of the SFG spectra in literature are modeled with eq 1 where homogeneously broadened bands are assumed:

(4 nm) behaves very similarly to bulk Pd(100) whereas deviations become extremely significant when the NP diameter decreases below 3−4 nm.

■

EXPERIMENTAL METHODS NP growth and SFG experiments are performed at room temperature in an ultrahigh vacuum chamber (2 × 10−10 mbar base pressure) at the ISMO institute. To obtain the geometric information on the supported Pd NPs, noncontact AFM (ncAFM) experiments are conducted under UHV conditions (4 × 10−10 mbar base pressure) at room temperature with a room temperature AFM/STM (Scienta-Omicron, Taunusstein, Germany) and a digital EasyPLL demodulator (NanoSurf, Liestal, Switzerland) at the CINaM institute. Mechanically polished Ag(100) single crystals (Mateck GmbH, Germany) are cleaned by repeated cycles of 30 min Ar+ sputtering (700 eV, 20 μA) and annealing at 755 K for 30 min. Sulfur and carbon on cleaned Ag surface are below the spectroscopy limit of Auger detection. Crystal quality is checked using LEED and nc-AFM. Atomically flat silver terraces of 20 to 100 nm can be observed.43,44 Two self-made effusion cells are used for Mg (549 K, 0.25 ML/min) and Pd (1418 K, 0.5 ML/min) evaporation. The growth of MgO thin films in the lowtemperature growth mode44 is done by evaporating Mg inside the UHV chamber backfilled with 5 × 10−8 mbar O2 (>99.8% purity, Linde MINICAN, Munich, Germany) onto the Ag(100) substrate kept at 573 K. More details of MgO film growth and quality optimization are given elsewhere.43,44 Pd is deposited onto the MgO film at 473 K with a nominal thickness ranging from 0.5 to 16 ML. For 16 ML thick films, NPs coalesce according to Auger analysis (not shown here). Sample temperature during growth is measured by C-type thermocouple and with an infrared pyrometer (Modline 5, precision ±0.5 K, Ircon inc, USA) during SFG experiments. SFG45,46 is used to probe CO adsorption as a function of CO coverage on Pd NPs. Internal CO stretch vibration is probed in

iϕ

ISFG(ω) ∝ IR(ω) χNR e +

∑ q

Aq ω − ωq + i Γq

2

(1)

The parameter χNR is the 2nd order macroscopic nonlinear susceptibility arising from the substrate surface and called “nonresonant signal”, owing to its very large bandwidth with respect to vibrational bands, with a phase difference ϕ relative to the second term of the equation, which is the resonant 2nd order nonlinear macroscopic susceptibility of adsorbed molecules. It corresponds to the sum over different homogeneously broadened vibrational bands, assumed to have a Lorentzian line-shape. Aq, ωq and Γq are the amplitude, central frequency and half-width at half-maximum of vibrational mode q. Aq is proportional to CO coverage (θ) and to the molecular hyperpolarizability. IR(ω) is the IR laser spectrum, which is simultaneously recorded on a ZnSe reference sample. All spectra are normalized by reference spectra, allowing a direct comparison of band intensities over a 250 cm−1 spectral range. Nonlinear curve fitting of experimental spectra is applied to extract fitting parameters with their uncertainty.

■

COMPUTATIONAL METHODS Dipolar Coupling Calculations. Dipolar coupling between adsorbates is calculated from a model of NP size and spatial distributions and CO adlayer structure to evidence the contribution of dipolar coupling to NP size effect on CO frequency shift. A classical electrostatic model has been developed for molecules on single crystal surfaces.29,49 We have adapted it to the particular case of NPs. The main difference with respect to single crystal is the loss of the B


Article


Figure 1. (a) nc-AFM topography image (100 × 100 nm2) of 2.7 ML Pd on a 3 ML MgO film. The density of NP dNP is ∼1.6 ± 0.3 × 1012 cm−2 covering 35% of the MgO film. (b) Height and diameter statistics extracted from part a after removing tip convolution. (c) AFM constant height mode image (30 × 30 nm2). The NPs’ top facet are rectangular, as expected from their known truncated octahedron shape. The measured aspect ratio (width/height) is around 4:1 ± 0.5. (d) Schematic representation of a Pd NP on the ultrathin MgO film.

significant change of αe has been reported between gas and adsorbed phases on various metal surfaces.18,31,32,47 Therefore, the electronic polarizability αe is set to its gas phase value of 3 Å3. The SFG spectrum is calculated using eq 1, where the resonant part is replaced by the sum of all CO susceptibilities ∑i , j χi(2) (ω). Smaller bands corresponding to other facets, ,j edges and defects are included as additional Lorentzian bands in eq 1. Additional details of the model are given elsewhere.47 A scaling parameter is introduced to take into account the possible change of chemical contribution with respect to Pd(100): the chemical contribution is interpreted classically as a consequence of a competition between CO molecules for back-donated electrons from Pd. The limited size of NPs might increase this competition. Calculated spectra are compared to the experimental ones, allowing to extract the vibrational polarizability αν and the scaling parameter. This procedure has been successfully applied for CO/Pd(100).47 Singleton frequencies used in the calculation are taken from Table S1 given in the Supporting Information (section SI-3: Additional SFG Results). DFT Calculations. Density functional theory (DFT) calculations are performed using the Vienna ab initio simulation Package (VASP).51 The exchange-correlation energy and potential is calculated within the generalized gradient approximation (PBE functional).52 Electron−ion interactions are described by the projector augmented wave method (PAW) introduced by Blöchl53 and adapted by Kresse and Joubert.54 A tight convergence of the plane-wave expansion is obtained with an energy cutoff of 400 eV. A Gaussian smearing of 0.2 eV is used to help electronic convergence (threshold of 10−6 eV). Structural relaxations are performed until the atomic forces are less than 0.01 eV/Å. Frequencies are computed within the harmonic approximation. The allocation of the Bader charges is

translation symmetry in the case of the NPs: each molecule has its own dipolar environment, and therefore its own dipolar frequency shift. As a consequence dipolar interactions must be calculated for each adsorbate and the spectrum must be summed over all molecules. However, because dipolar interactions are at short-range, summation does not need to be extended over a very large number of NPs. In practice, calculations are limited over 5 × 5 equal square domains as shown in Figure S2 in the Supporting Information (SI-2: Dipolar coupling calculation methods). Each domain contains a single NP. According to a NP density of dNP = 1.6 ± 0.5 × 1012 cm−2, the size of the square is 1/dNP = 7.91 nm. NP size and position in the domain are randomly generated, in order to reproduce the experimental distributions found on AFM images (Figure 1). Additional details about dipolar modeling are provided in the Supporting Information (section SI-2: Dipolar Coupling Calculation Methods). SFG spectra are calculated according to the formalism of Cho et al.50 to obtain the perturbed second-order susceptibility χi,j(2) (ω) for each CO molecule at position {i, j} on a (100) Pd facet χi(2) (ω) = ,j

χR(2) (ω) [1 + αv(ω)Ui , j][1 + αeUi , j]2

(2)

χ(2) R

where (ω) is the nonlinear susceptibility in the absence of dipolar coupling. If there would be no chemical contribution to the frequency shift, χ(2) R (ω) would be the susceptibility at zero coverage corresponding to the singleton frequency. In the presence of a chemical contribution, it must be corrected by the chemical frequency shift.47 Ui , j = ∑m , n ≠ i , j um , n is the summed dipolar perturbation due to all other molecules. αν(ω) is the molecular polarizability, which is composed of electronic and vibrational polarizabilities, αe (Å3) and αυ (Å3), respectively. No C


Article


Table 1. Estimated Diameter (D), Height (h), Total (NT) and Facet (NF) Atom Number and Facet to Edge (NE) Atom Ratio for Different Equivalent Thicknesses of Pd Assuming a Constant D/h Aspect Ratio Pd (ML) 0.5 1 4 16 a

± ± ± ±

0.01 0.025 0.1 0.4a

D (nm) 2.9 ± 0.3 3.6 ± 0.4 5.8 ± 0.6 −

h (nm)

h (ML)

NT

NF

NF/NE

± ± ± ±

3−4 4−6 6−9 16

410 ± 210 820 ± 380 3300 ± 1380 −

110 ± 30 180 ± 40 440 ± 90 −

1.2 ± 0.2 1.5 ± 0.2 2.3 ± 0.3 −

0.7 0.9 1.5 3.1

0.2 0.2 0.3 0.1a

Coalesced NPs.

Figure 2. SFG spectra of CO adsorbed on Pd NPs as a function of CO dose or pressure for Pd deposits of: (a) 16 ML, (b) 4 ML, (c) 1 ML, and (d) 0.5 ML. All spectra have been divided by their reference spectrum and shifted vertically for clarity. Some spectra have been highlighted: close to zerocoverage, before and after band splitting at 1930 cm−1, at saturation under at 2.8 × 10−9 mbar, and at equilibrium at 10−7 and 10−3 mbar.

interactions between the periodic images of the deposited Pd62 and Pd128 NPs, respectively. The cell parameter used is the one of silver (aAg = 4.16 Å). Only the bottom layer is kept fixed during the geometry calculations. Gas phase calculations of the isolated NPs are done keeping the same cells. The Brillouinzone integration are performed at the Γ point.

computed using the routine developed in the group of Henkelmann.55−57 The surface of Pd(100) is modeled using a p(2√2 × 2√2) supercell slab with a vacuum interlayer of at least 16 Å, and an optimized lattice parameter of aPd = 3.96 Å. The Brillouin-zone integration are performed using a Monkhorst−Pack mesh of 3 × 3 × 1 K points.58 During structural relaxation, the bottom layers are kept fixed at the bulk position to simulate the underlying bulk while the two upper layers are relaxed. A CO molecule is adsorbed on the upper surface, at the most stable bridge position in agreement with previous experimental48,59 and theoretical results.60 This corresponds to a coverage of θ = 0.125 ML. Those slabs are also deposited on a two-layer MgO thin film with the Pd atoms in O-top positions.61,62 As this thin film is supported over a Ag(100) single crystal, we use the silver lattice parameter (aAg = 4.16 Å) rather than the MgO one. We also included the silver support. To proper describe the silver Fermi level, six layers of Ag(100) are required. The four bottom layers are kept frozen at the silver bulk position while all the MgO(100) and Pd(100) layers are free to relax. For the Pd NPs supported on MgO/Ag(100) ultrathin films, supercells p(7 × 6) and p(9 × 8) are used to limit the side

■

EXPERIMENTAL RESULTS AFM Characterization of Pd Nanoparticles. A nc-AFM topography image obtained on a 2.7 ML thick Pd film is shown in Figure 1a. The substrate temperature is kept at 473 K during the deposition.63 Randomly distributed NPs with a high density of 1.6 ± 0.3 × 1012 NPs/cm2 can be found. This density of NPs is ten times higher than the one on the bulk MgO(100) surface,64 which is due to a higher defect density that can be regularly observed on the ultrathin MgO films. A reasonably narrow size distribution of ±20% is observed (Figure 1b). We use the constant height mode of the nc-AFM to image the shape of the NP’s top facets with a tip−surface convolution almost at zero (Figure 1c).65 The NP shape is known to be a truncated octahedron and we find an aspect ratio (width/ height) of 4:1 ± 0.5. Because of the specific epitaxy on the D


Article


Figure 3. (a, b) Deconvoluted frequencies, (c, d) intensities and (e, f) bandwidths of the CO bridge band as a function of CO dose (L) and increasing CO pressure up to 10−3 mbar for different NP sizes (4, 1, and 0.5 ML of Pd). SFG results on Pd(100) from our previous work are reported in black.47 Lines are guides to the eye. Accurate deconvolution of the two bands corresponding to the simultaneous filling of (100) and (111) facets could not be done below 1.5 Langmuir.

MgO(100) substrate, the observed flat terrace orientation is mostly (100) as it can be indeed seen in Figure 1d. The NPs are flatter than on the bulk MgO(100) surface, on which the aspect ratio was measured by nc-AFM (2:1),65 TEM (1.4:1),66 X-ray scattering,67 and grazing incidence small angle X-ray scattering (1.6:1).68 In comparison to all experimental observations, theoretical simulations predict a 2:1 aspect ratio on bulk MgO(100).69 As the tip−surface convolution can be almost neglected in the constant height mode, this larger aspect ratio must be due to the ultrathin MgO film where the Mg−O distance is compressed by the underlying Ag crystal resulting into a charge transfer to/from silver into/from the NPs.70,71 In Table 1 the geometrical parameters of Pd NPs for various equivalent thicknesses of Pd deposits are summarized. Theses values are extrapolated from the nc-AFM images in Figure 1, assuming that the size aspect ratio is not size dependent. CO Spectroscopy by SFG. SFG spectra of CO adsortion are presented in Figure 2 as a function of CO dose measured in Langmuir (1 L = 10−6 Torr × s) (i) during adsorption at 2.8 × 10−9 mbar and (ii) in equilibrium with gas phase CO at pressures of up to 10−3 mbar, both for 16, 4, 1, and 0.5 ML Pd

deposits. Several CO bands can be identified: the main band shifts with increasing coverage from 1877 to 1990 cm−1, depending on NP size. A second band overlaps the former. Its intensity seems to rise together with the main band, so that it is impossible to separate them until a CO dose of ≈1−1.5 L, where the second band saturates with a frequency that remains below 1930 cm−1. From the band intensity ratio of ≈1:0.2 observed around 4 L, the site occupancy ratio is approximately 1:0.45. Weaker bands appear in the 2040−2100 cm range at large exposure. In the case of CO/Pd(100) single crystal, CO adsorption results in the progressive filling of the c(2√2 × √2) R45° surface structure, which is saturated at 0.5 ML.72,73 Then compression of the CO layer upon additional CO adsorption occurs through the appearance of denser CO rows that insert between c(2√2 × √2) R45° domains. Orbital hybridization shifts the frequency from 2143 cm−1 in the gas phase down to 1890 cm−1 (zero-coverage limit) for bridge-bonded sites. Upon an increase of CO coverage, the frequency shifts up to 1990 cm−1 for θ = 0.8 ML.23 According to reported frequencies of adsorbed CO on Pd(100),32,74 Pd(111),23,24,75 and E


Article


intensity, only one band was necessary to fit spectra. Because SFG depends non linearly on coverage, the band intensity below 1 L is much larger than the sum of the two bands above 1 L. This suggests that CO moves from one site to the other around 1 L. The observed intensities are consistent with the interpretation that two distinct bands grow and shift simultaneously. They cannot be separated until 1 L of CO. At 4 Langmuir they correspond to relative CO coverages of 1 and 0.45, resulting in SFG relative intensities of 1 and 0.452 = 0.2. If they would overlap, their intensity would be 1.452 = 2.1, which is slightly larger than observed at 1 Langmuir. The singleton frequency is extrapolated at zero coverage from the fitted values. It shifts from 1888 down to 1872 cm−1 as the Pd deposit varies from 16 to 0.5 ML of Pd. CO frequencies close to zero coverage, at half-coverage and at the maximum coverage reached under 10−3 mbar of CO as a function of Pd deposit are gathered in Table S1 in the Supporting Information (section SI-3: Additional SFG results). The frequency red-shift for smaller NP size can be explained by several factors: reduction of dipolar interactions between CO molecules, modification of chemisorption (as indicated by the red-shift of the singleton frequency), lower CO coverage at a given exposure,10 reduced lateral interactions between CO molecules on smaller facets (as reported for Pd/MgO9). We shall discuss the relative importance of these factors in the next sections. Parts e and f of Figure 3 reveal that the CO half-bandwidth (Γ) varies much as a function of NP size and CO coverage. This may result from inhomogeneous broadening due to broader NP size and adsorption site distributions. However, homogeneous broadening might also contribute as a result of a change of CO chemisorption with size, which may modify vibrational relaxation. Homogeneous broadening is directly related to the T2 decoherence time which governs the decay of SFG intensity. T2 can be measured by recording the SFG intensity as a function of the delay between the visible pulse and the femtosecond IR pulse, providing the visible pulse is not temporally shaped and has a duration of 120 fs. We found experimentally T2 = 480 fs (620 fs) for CO on 1 ML (4 ML) Pd deposit, to be compared to T2 = 690 fs for CO on Pd(100).47 In Table 2, we compile the measured bandwidth

supported Pd NPs,7,16,75 the strongest band can be attributed to CO adsorbed on bridge sites on the (100) facets: it behaves like the bulk (100) surfaces, with a frequency that shifts to the red as NP size decreases. Its large intensity is in agreement with the NP shape described above where the (100) facet on top of the NP is the larger one. The other bands are assigned to CO at atop sites of (111) facets, edges, and defects.7,24,32 They are absent on single crystal (100) surfaces but are already present on the 16 ML Pd coalesced layer, which is expected to exhibit defects at the boundaries between coalesced NPs.47 The band that overlaps the (100) bridge band in the low coverage range, might correspond to CO on the (111) facets. It is absent from the 16 ML deposits (Figure 2a) due to a coalescence of NPs, but it is present for all other Pd deposits. Its frequency remains smaller than 1930 cm−1 in agreement with the maximal bridge site frequency on (111) single crystal. Features observed on the Pd(100) single crystal can be found also on the larger NPs:47 (i) at zero CO coverage, a nonresonant (NR) signal is observed, which disappears as coverage increases. This nonresonant signal increases up to 50 times for the smaller NPs, for which it becomes larger than the vibrational bands. Size dependent NR intensity is discussed in the DFT Calculations section. (ii) Bridge CO band raises in intensity and frequency up to half-coverage corresponding to saturation of the uncompressed c(2√2 × √2) R45° phase. (iii) When CO adlayer compression occurs above 0.5 ML,32,47 a sudden frequency shift (⩾7 cm−1) and broadening are observed, and the band of compressed CO rises. The band of uncompressed CO disappears progressively in favor of the new band of compressed CO. Half-coverage corresponds to a maximum intensity on (100) single crystal and for a 4 ML deposit (Figure 2b). For smaller NPs (Figure 2c,d), the intensity maximum at 0.5 ML disappears, perhaps because the compression structures reported for single crystal become larger than the (100) facet size, indicating that compression occurs differently on the smaller NPs.72,73 The spectrum corresponding to 0.5 ML is highlighted in Figure 2 with a thick green line. Note that for all NP sizes, the maximum pressure used in our work (10−3 mbar) does not allow to reach the coverage limit. Fits can satisfactorily be made using eq 1, down to the lowest coverage in order to extrapolate the singleton frequency at zero CO coverage. Two fitted spectra at low and half-coverage on 0.5 and 1 ML of Pd are displayed in Figure S3, parts a and b, respectively, in the Supporting Information (section SI-3: Additional SFG Results). Fitting parameters for the bridge band are plotted in Figure 3 as a function of CO dose and pressure for different NP sizes. No significant effect of CO coverage is found on the relative phase of NR contribution with respect to vibrational bands. Results obtained on Pd(100) single crystal are reported in black.47 The Figure is divided in two columns: on the left (a, c, e) as a function of CO dose during adsorption under 2.8 × 10−9 mbar, and on the right (b, d, f) as a function of CO pressure up to 10−3 mbar. The overlap of the (100) and (111) bridge bands could not be interpreted quantitatively. Acurate deconvolution would imply to unravel the adsorption kinetics and the frequency dependence of CO on the local coverage on each facet. We found it impossible to do because the frequencies are too close to each other, especially at low CO dose where a single band is sufficient to fit the spectra. We show in Figure 3a a phenomenological fit with only one band below ≈1 L and two bands above 1 L for 4 and 1 ML. For 0.5 ML of Pd NPs, due to band broadening and lower SFG

Table 2. δSFG, Full Width at Half Maximum (FWHM = 2 Γ) of the Bridge Band of (100) Facets under 10−8 mbar of CO Compared to the Calculated Value (δFIT) Derived from the Measured Decoherence Lifetime T2 (FWHM = 1/(πcT2)) Pd (ML) 0.5 1 4 (100)

δSFG (cm−1)

T2 (fs)

δFIT (cm−1)

± ± ± ±

− 480 ± 50 620 ± 30 690 ± 2047

− 22 ± 2 17 ± 1 15.4 ± 0.7

60 36 28 17

8 5 4 2

under 10−8 mbar of CO, the measured decoherence time T2, and the homogeneous broadening that corresponds to T2 assuming a Lorentzian line shape. It can be seen that spectrally resolved and time-resolved data are very similar for Pd(100), but suggests an important inhomogeneous broadening for small NPs (1 ML of Pd). Inhomogeneous broadening can originate from NP size distribution and from adsorption site distribution over the NP facet. NP size distribution contribution to the inhomogeneous broadening is discussed in the Supporting Information (SI-5: Inhomogeneous broadening by NP size distribution) using modeling dipolar coupling between F


Article


Figure 4. Fitted CO adsorption kinetics below half coverage as a function of CO dose (cm−2) on (a) Pd(100)47 and (b−d) Pd deposits of 4, 1, and 0.5 ML. Black points correspond to CO coverage (cm−2) on (100) facets deduced from the CO frequency using a linear relationship. Red curves are K ) is displayed in the best fits using the solution of eq S3. CO half-coverage is represented by a green line, while the initial adsorption rate (s = s300 0 blue.

CO Adsorption Kinetics. The CO frequency shift as a function of CO dose is modeled by fitting adsorption kinetics on the Pd NP (100) facets in the low coverage range (0−0.5 ML) when the surface is exposed to 2.8 × 10−9 mbar of CO. With this, we can extract valuable information about the sticking coefficient, adsorption kinetics, CO coverage and the capture zone surrounding the NPs. We assume that the CO frequency varies linearly with coverage between 0 and 0.5 ML, as observed experimentally for Pd(100) single crystal. For 1 and 4 ML of Pd, for which the two bands of (111) and (100) facets overlap, the CO frequency is approximated by the average of the two frequencies used in the spectral deconvolution (weighted by band intensity). As mentioned below, this approximation seems satisfactory. A half-coverage has two spectral signatures: one is the maximum of SFG intensity at 0.5 ML. It is present on single crystal and large NPs. The second one is visible at all NP sizes (broadening and appearance of the compressed site immediately above 0.5 ML). Figure 4 shows the CO coverage as a function of CO dose for Pd(100),47 and for 4, 1, and 0.5 ML of Pd, obtained using the identification of 0.5 ML and the assumption of linear relationship between frequency and coverage. From this we can extract the adsorption kinetics as a function of NP size by fitting data using a Langmuir, first-order, nondissociative adsorption kinetic model. Details of the modeling are given in the Supporting Information (section SI-4: Adsorption Kinetics Modeling). The fits are the red curves in Figure 4a-d and blue lines represent the initial adsorption rate (s = s0300 K). Derivatives of the fits of Figure 4 are equal to the sticking coefficient as a function of CO dose (Figure 5).

adsorbates. Vibrational relaxation is known to be controlled on metals by the coupling of vibrational excitations with electron− hole pairs. The increase of bandwidth (decrease of T2) shows that as NP size decreases this coupling increases significantly. This is presumably related to the calculated increase of d-band density of states around Fermi level as NP size decreases (see the DFT Results section). To summarize this section, adsorption of CO on (100) facets of NPs is clearly visible in SFG spectra. The spectra reveal that the CO frequency shifts to the red at all CO coverages as NP size decreases a signature that chemisorption gradually changes with size. Adsorption on the (100) facets is very similar to Pd(100) single crystal. Unfortunately, a fully quantitative analysis is complicated by the overlap with another band, which is assigned to the lateral (111) facets. A collection of defect and edge sites also appears on a coalesced Pd layer as well as on Pd NPs. An important observation is that SFG is able to detect very small quantities of CO. Taking into account the NP coverage on the MgO film (7−8%), we observe that the CO detection limit is 2 × 10−4 ML (global coverage). For CO on a Pd(100) single crystal, the smallest detectable coverage is 100 times larger (1.5 × 10−2 ML). Local field enhancement by NPs presumably plays an important role in this increased sensitivity like in surface enhanced Raman spectroscopy. It makes SFG spectroscopy an extremely sensitive vibrational probe for very small NPs. Detection can presumably be further improved by tuning the visible frequency to a resonant feature of the NPs (e.g., plasmon resonance). This would be particularly interesting for the study of heterogeneous catalysis on even smaller NPs. G


Article


The model also allows evaluating the contribution of NP size distribution to CO band inhomogeneous broadening. In Figure 6, we show the relationship between the frequency of the bridge sites and the CO coverage on the (100) facet. To fit adsorption kinetics in agreement to results on Pd(100), we use a linear relationship between CO frequency vs CO coverage. Below 0.5 ML of CO, the linearity found for Pd(100) remains satisfying in the case of the NPs. Extrapolation of the linear relationship above 0.5 ML remains correct for 4 and 1 ML of Pd. However, it is not valid for 0.5 ML of Pd, for which extrapolation leads to an unrealistic maximal CO coverage larger than 1. The behavior of the frequencies of compressed and uncompressed CO is examined below in the light of the dipolar model. We first show results of calculations done assuming the vibrational polarizability (αν(θ) = 0.10 to 0.22 Å3 from 0 to 0.5 ML of CO) and the chemical contribution of Pd(100) (which increases with CO coverage from 0 to 56 cm−1).47 Figure 7 presents: (a−d) the 2D spatial distribution of the dipolar shift across the (100) facet, (e) its averaged value as a function of CO coverage in comparison to the chemical contribution of Pd(100), (f and g) the spectral distribution of the CO frequency for 0.5 and 4 ML of Pd, and (h) the CO total frequency shift as a function of CO coverage. Parts a−d of Figures 7 show that all molecules do not have the same frequency shift, due to the absence of neighbors outside the NPs. On 0.5 ML of Pd, almost all CO on the facet are affected, while on 4 ML of Pd ∼50% of CO at the center remains unaffected. For Figure 7e in the 0.5−0.667 ML CO coverage range in which both uncompressed and compressed CO sites coexist, dipolar calculations clearly show the different dipolar environment of both sites evidenced by a shift of a few wavenumbers already found for Pd(100) single crystal in our previous work.47 This shift increases by 4 to 5 cm−1 with respect to Pd(100) for the smaller NPs investigated. Reduction of dipolar effects at the NP border on large NPs has a stronger impact on uncompressed than on compressed CO. Frequency distributions (f, g) are asymmetric, again due to NP border effects. For all NP sizes, the frequency shift dependence on coverage remains linear (Figure 7h), but the slope is reduced by up to 6−7% for 0.5 ML of Pd compared to larger nanoparticles, due to the reduction of dipolar coupling. The results in Figure 7h can be compared to experimental values previously shown in Figure 6a−d. For coalesced NPs, SFG frequencies and dipolar calculations are in perfect agreement and similar to Pd(100). A remarkable result is the fact that the linearity is still valid above 0.5 ML. At 4 ML of Pd, the dipolar calculation, using the same fitting parameters as the single crystal, underestimates very slightly the linear frequency vs. coverage relationship. For 1 ML of Pd, dipolar calculations underestimate SFG frequencies by 7−8%, specially above 0.5 ML of CO. On 0.5 ML of Pd, extrapolation of the linear frequency vs coverage relationship cannot be done over the whole coverage range. Over a second set of calculations, either αν or the chemical contribution are adjusted in order to fit experimental spectra. All SFG spectra as a function of CO coverage and NP size can be satisfactorily fitted. If the chemical contribution is fixed to its value for Pd(100) (blue curve in Figure 8), αν is found to increase moderately by 21% as NP size decreases down to a deposit of 1 ML of Pd, and then by another 26% for 0.5 ML of Pd. If instead αν is fixed (red curve in Figure 8b), the scaling factor of the chemical contribution has to be increased by 9%

Figure 5. Sticking coefficient (derivative of the fits from Figure 4) as a function of CO dose.

The initial increase of s at low CO dose is due to the slight decrease of temperature during our experiment (Figure S1 in the Supporting Information (section SI-1: Protocol for SFG Experiments). The sticking coefficient then decreases normally after a maximum value at around 4 to 6 × 1014 cm−2 CO dose (0.6−0.9 L) (instead of zero under perfectly isothermal conditions). The variation of s0 and k are summarized in Table 3. The initial sticking coefficient is 0.65 for Pd(100) and Table 3. Initial Sticking Coefficient s0, Precursor State Factor k, under 2.8 × 10−9 mbara s0

Pd (ML) 0.5 1 4 16 (c) Pd(100) a

0.78 0.73 0.67 0.55 0.65

± ± ± ± ±

k 0.05 0.05 0.08 0.1 0.05

0.06 0.012 0.015 0.025 0.025

± ± ± ± ±

0.01 0.005 0.005 0.005 0.005

Key: (c) coalesced NPs.

coalesced NPs and increases as NP size decreases. The precursor factor k is equal to ∼0.025 on Pd(100), indicating that impinging molecules have time to diffuse significantly before they adsorb. Diffusion is maximal for 1 ML Pd. Surprisingly, the precursor factor increases significantly on the smaller NPs. The CO capture zone is known to spill over the NPs onto the MgO layer. In the kinetic analysis presented above, the presence of free MgO between NPs is not taken into account explicitly: as a result, s0 includes the spill over effect, namely the adsorption of CO molecules that first impinge on MgO and then diffuse to the NPs. The increase of s0 at small size is resulting from a capture zone contribution. A rough estimation leads to a less than 1 nm large capture zone around the NPs. Schalow et al. have found 1.7 ± 0.7 nm on Fe3O410 and Matolin et al. 0.5−3.3 nm on Al2O3 (see refs 73−75 in ref 10). A much larger value was reported for Pd NPs on bulk MgO surface.76 Additional investigations at different surface temperatures and NP sizes would be required to confirm such a small capture zone size compared to other equivalent systems.

■

THEORETICAL RESULTS Dipolar Coupling Calculations. In this section, we estimate the dipolar contribution to the observed CO frequency shift as a function of CO coverage and NP size. H


Article


Figure 6. Relationship between CO frequency and CO coverage for (100) facets: experimental data points (normal, uncompressed CO in black, compressed CO in red), linear fit in the range 0−0.5 ML and its extrapolation above 0.5 ML (green line), dipolar calculations using the vibrational polarizability and the chemical contribution of Pd(100) for uncompressed (blue line) and compressed sites (cyan line).

Figure 7. Results of dipolar calculations as a function of NP size using the same vibrational polarizability and chemical contribution as Pd(100). (a− d) 2D distribution of dipolar shift on the NP top facet. (e) averaged value of dipolar contribution for both uncompressed and compressed CO and chemical contribution for Pd(100) from.32 (f, g) CO frequency distribution and (h) total CO frequency shift (dipolar + chemical contribution).

for 1 ML of Pd and another 18% for 0.5 ML of Pd. The conclusion is that chemisorption changes more according to the

criteria of frequency variation with CO coverage between 0.5 and 1 ML of Pd than between 1 ML of Pd and Pd(111), either I


Article


Figure 8. CO vibrational polarizability αν (Å3) and chemical contribution (cm−1) as a function of the Pd deposit (ML) for 0.75 ML of CO.

Figure 9. CO adsorbed on a Pd(100)/MgO(100) periodic slab (left) and on Pd NPs deposited on two layers of MgO(100) (Pd62 (central) and Pd128 (right)). Color code: C in blue, Pd in yellow, O in red, and Mg in white.

through the chemical contribution or through the CO polarizability. The latter is itself related to chemisorption, since it depends on parameters such as molecular orbital occupation and bond lengths. Our dipolar coupling model allows us to evaluate the contribution of NP size distribution to inhomogeneous CO band broadening. It shows that the main contribution is the chemical effects rather than to dipolar coupling as demonstrated in Figure S4 in the Supporting Information (section SI5: Inhomogeneous Broadening by NP Size Distribution). DFT Calculations. On the coalesced Pd NPs (16 layers of Pd), the CO vibration wavenumber at low CO coverage is equivalent to the one obtained on a Pd(100) single crystal (see Figures 3 and 6 and Table S1). Decreasing the size of the Pd NPs leads to a red-shift of CO wavenumber at low coverage from 1888 down to 1872 cm−1 (Table S1) for NPs of 16 to 3− 4 layers height for 16 to 0.5 ML of Pd (See Table 1). In this part, we aim at understanding the origin of this decrease with the help of DFT computations. Experimentally, the Pd NPs are supported on an ultrathin layer of MgO(100) deposited on Ag(100). On the one hand, since the 16 ML of Pd and the single crystal behave the same way, the coalesced nanoparticles can be modeled with a thick slab of Pd(100). Using a p(2√2 × 2√2) supercell, one CO corresponds then to a coverage of 0.125 ML. Once adsorbed on a bridge site (the most stable situation), its vibration wavenumber is 1870 cm−1. On the other hand, the 0.5 ML Pd NPs can be represented by a Pd NP on a thin-film of MgO(100), using the cell parameter of the underlying Ag (aAg = 4.16 Å). As shown on Figure 9, we have chosen a Pd128 NP that has a thickness of three-layers and a pyramidal shape. It exhibits (100) top and bottom facets and (111) side facets. The largest (100) facet is interacting with the oxide through Pd−O bonds. When the CO is adsorbed on a bridge site at the center of the top (100) facet, its exhibits a 1850 cm−1 wavenumber. The comparison of those two extreme cases reproduce nicely the 18 cm−1 shift observed experimentally (Table S1). While the CO vibration is sensitive to the Pd model, other parameters are much less sensitive such as the adsorption energy (∼−1.95 eV), the C−O distance (1.18) and the negative charge on the CO (−0.17 e). To better assess the origin of this shift, we have built a series of simpler models of the small NPs. First, since the Pd128 particle is smaller than the smallest experimental NPs, we need to probe the effect of the finite-size. It is difficult to model a higher size particle, thus, we have

chosen to compare the Pd128 NP with a smaller NP (Pd62) and with a three-layer Pd(100) slab, all deposited on MgO(100) (see Figure 9 and Table 4). The deposited NPs exhibit Table 4. Frequency of Vibration (cm−1) of CO Adsorbed on Palladium Clusters and Slabs under Three Different Conditionsa Pd62 Pd128 3 layers slab (4.16 Å) 6 layers slab (3.96 Å)

entry i

entry ii

entry iii

1845 1850 1852 −

1845 1850 1850 −

1862 1865 1857 1870

a

Key (i) deposited on 2 layers of MgO(100); (ii) isolated but frozen in the geometry adopted on MgO(100); (iii) isolated and relaxed in gas phase.

differences in structure compared with the simpler slab model (Pd−Pd distances) but the CO stretch is pretty similar for the three models: 1845, 1850, and 1852 cm−1. In particular, the larger particle yields to a wavenumber that is closer to the one obtained on the three-layer slab deposited on MgO. Thus, 0.5 ML Pd NPs can be represented by a three-layer Pd slab deposited on MgO. Second, we aim at assessing the effect of the Ag underlying support. Its inclusion does not affect much the resulting CO wavenumber according to the comparison of (i) Pd three-layers slab deposited on MgO/Ag (1850 cm−1) and (ii) Pd three-layer slab deposited on MgO (1852 cm−1). As expected, electronic transfer through the Ag-MgO interface can be excluded. Third, let us consider the influence of the MgO thin-film. When this film is removed and the Pd positions are kept constant, the CO wavenumber is almost not affected (see entries i and ii of Table 4), while when the Pd clusters are relaxed in their optimized positions (entry iii of Table 4), the CO wavenumber is considerably blue-shifted, by around +15 cm−1. The epitaxial growth on the MgO(100) ultrathin film leads to strong geometrical deformations due to the large mismatch between the Pd lattice (aPd = 3.96 Å) and the MgO(100) lattice we used (aAg = 4.16 Å): the interacting layer of Pd62 and Pd128 is expanded in the parallel plane by 3−4% relative to the optimized isolated shape, while the upper layer is almost not modified. This Pd−Pd bond expansion in the parallel plane is in agreement with experimental66,67,77 and J


Article

The Journal of Physical Chemistry C theoretical works78−80 for Pd NPs supported on MgO(100) bulk. To better correlate the strain with the CO wavenumber, strained Pd(100) slab models are also considered. In Figure 10,

In addition, the change of DOS presumably modifies the surface susceptibility of Pd. Nonresonant (NR) SFG contribution corresponds to electronic transitions of Pd from states located around Fermi level from −1.55 eV (800 nm) in the d-band, up to empty states above Fermi level (sp-bands). Therefore, we expect qualitatively that the calculated upshift of d-band for smaller NPs results in an increased NR intensity, which is observed. Other causes of changes of the NR are electrostatic effects (change of the surface work function), change of surface polarizability, permanent dipole of adsorbates, and surface relaxation induced by adsorption.81 In conclusion, the Pd128 deposited on MgO(100) is the most reasonable model for the smallest NPs. On the other hand, the large coalesced NPs which are not affected by the support can be modeled using the six-layer slab model with the equilibrium lattice parameter of Pd (3.96 Å). Between those two systems, we predict a shift of 20 cm−1, in good agreement with the experimental shift of 18 cm−1. We attribute this shift to the stronger mechanical strain exerted by the MgO support on the smaller NPs. The in-plane strain contribution is larger than the interplane one, but the inclusion of both is required through the inclusion of MgO to fully account for the observed shift. The larger the NP, the less sensitive it is to the strain induced by the support, and the closer it is to the Pd single crystal situation, explaining the experimental blue-shift we observe for the CO vibration upon the increase of the NP size.

Figure 10. Computed CO wavenumber (cm−1) as a function of the lattice parameter (Å). In red, the Pd(100) six-layer slab; in black, the Pd(100) three-layer slab.

we report the CO vibration wavenumber as a function of the lattice parameter for two different slabs, a thin slab of three layers and a thick slab of six layers. It is clear that the thickness does not influence much the wavenumber considering the two curves. However, the CO vibration wavenumber noticeably decreases with the increase of the lattice parameter. For instance, a 5% expansion of the lattice parameter from the one of Pd (aPd = 3.96 Å) to the one of silver (aAg = 4.16 Å) leads to a red shift (11 cm−1 for the three-layer slab and 13 cm−1 for the six-layer slab). This variation cannot be related to variation in adsorption energies, which do not depend much on the thickness or the strain, all values clustering around −1.95 ± 0.04 eV. Nor can it be related to the CO bond distance (1.18 Å) or the CO Bader charges (−0.17 e to −0.18 e). However, this shift can be related to the modification of the Pd d-band since the central energy is pushed up by 0.4 eV upon Pd−Pd extension. Yet, a strained 3 layers Pd slab is not enough to model the small NP deposited on MgO. From the six layers to the strained three layers, we observe a 13 cm−1 shift, to compare with the 20 cm−1 shift we obtained with Pd128 on MgO. The addition of the MgO support allows us including the interplane strain on the Pd slab and reach the experimental shift of 18 cm−1. This interplane strain is less important than the inplane one. It is also more difficult to rationalize since the structural modifications we observe are opposite in the threelayer slab (extension of the interlayer distance) and in the Pd128 NP (compression of this interlayer distance), but they appear as essential when comparing entry (iii) and (ii) of Table 4. In other words, the good agreement between the three-layer slab and the Pd128 deposited on MgO does not rely on similarity in the local distortion of Pd and appears as rather fortuitous. That is why we propose to rather keep the Pd128 on MgO model to represent the small NPs. In a nutshell, the MgO support constrains the Pd structure. This strain pushes up the d-band, inducing a stronger overlap of Pd orbitals with the CO 2π* orbitals that explains the observed red-shift of CO singleton frequency. Such modification of CO chemisorption for small NPs was suggested by Neyman and co-workers8 using DFT calculations.

■

CONCLUSION SFG experiments are done to probe the CO adsorption on Pd NPs of various sizes (from 3 to 6 nm) over a wide CO pressure range (from 10−9 to 10−3 mbar). Vibrational spectroscopy and adsorption kinetics at such NPs are compared to those ones of the Pd single crystal: at Pd NPs, new spectroscopic bands assigned to (111) facets and NP edges appear in addition to the regular bridge sites of the top (100) facet, which dominates the spectra in the pressure range studied here (

CO Chemisorption on Ultrathin MgO-Supported Palladium

Recommend Documents