Article pubs.acs.org/JPCC
CO Adsorption on Pd(100) Revisited by Sum Frequency Generation: Evidence for Two Adsorption Sites in the Compression Stage Aimeric Ouvrard,* Jijin Wang, Ahmed Ghalgaoui, Sven Nave, Serge Carrez, Wanquan Zheng, Henri Dubost, and Bernard Bourguignon Institut des Sciences Moléculaires d’Orsay, CNRS-UMR 8214, Université Paris-Sud, F-91405 Orsay, France ABSTRACT: Sum frequency generation (SFG) and low-energy electron diffraction (LEED) have been used to revisit CO adsorption on Pd(100) from very low coverages up to saturation at 300 K. Below 0.5 ML, variations of SFG frequency and intensity with coverage are consistent with IRAS results from the literature. Novel observations are done above 0.5 ML, where the CO adlayer compression takes place. The existing compression model postulates the coexistence of compressed and uncompressed CO. We observe two bands in the spectral region of bridge sites and assign them to compressed and uncompressed CO. Both types of CO behave very differently: the molecular hyperpolarizability at compressed sites is smaller by a factor of 2 than at uncompressed sites. The frequency of uncompressed CO red-shifts during compression as the partial coverage decreases, while that of compressed CO continues to blue-shift as coverage increases. In the time domain, the coexistence of compressed and uncompressed sites results in oscillations in the decay of SFG intensity. A strong decrease from 690 to 222 fs of the phase relaxation time of uncompressed CO is observed during compression, indicating a stronger coupling to the substrate. These results are complemented by calculations of dipole−dipole interactions and DFT VASP calculations. While continuing blue-shift of compressed sites reflects a combination of increasing dipolar coupling and chemisorption change with coverage like below 0.5 ML, the very large red-shift amplitude of uncompressed CO indicates a large chemical contribution opposite to compressed CO. DFT VASP calculations allow us to follow the surface structure evolution from 0.5 to 0.67 ML and CO frequency changes with coverage. Pd atoms below compressed CO rows are pushed up, and compressed CO is tilted by 8− 9° with respect to the surface normal. A frequency split between compressed and uncompressed CO is found in agreement with experimental data. These results suggest that while compressed CO is less strongly bonded as compression proceeds the remaining uncompressed CO is more strongly bonded.
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INTRODUCTION For decades, surface science has focused attention on the study of CO adsorption on transition metals1−7 including singlecrystal palladium surfaces8−14 or supported palladium nanoparticles.15−23 Understanding of the mechanisms of heterogeneous catalysis has been a continuing motivation of research, with an increasing emphasis on complex nanostructured surfaces. In this work CO adsorption on Pd(100) is revisited with sum frequency generation (SFG) spectroscopy and forms the basis for understanding CO adsorption on Pd nanoparticles on MgO thin films, to be presented in a forthcoming paper. Using combined SFG spectroscopy and low-energy electron diffraction (LEED), supported with dipolar and DFT VASP calculations, allowed us to observe distinctly two types of CO present in the compression stage, which turn out to be bonded very differently. Internal stretch frequency has often been used with great success to determine adsorption site coordination24,25 by analogy with IR spectra of metal carbonyl compounds and CO coverage12,25 since Pd−C vibration far in the infrared is harder to access with conventional vibrational spectroscopies.26,27 Internal CO frequency measurements offer valuable information about molecule−metal (chemical bonding) and © 2014 American Chemical Society
molecule−molecule (dipolar) interactions. For CO/Pd(100), orbital hybridization shifts the frequency from 2143 cm−1 in the gas phase down to 1890 cm−1 (zero-coverage limit) for bridgebonded sites.24 Upon an increase in CO coverage, the frequency shifts up to 1990 cm−1 for a coverage close to θ = 0.8 ML. G. Blyholder has first given a comprehensive description of CO binding on transition metals that involves both electron donation from the 5σ CO molecular orbital to Pd sp-bands and back-donation from the Pd d-band to the CO antibonding 2π* orbital.3,8 More recently, this model has been refined by taking into account the d-band central position, Pauli repulsion, and interaction of s−p and d-bands with 5σ and 2π* CO orbitals, allowing us to predict more accurately adsorption energy, adsorption site, and dissociative adsorption as we move from one metal to another in the periodic table.28−30 Dipolar interactions have been intensively studied in the past on single crystals.1,12,25,31−34 They are not strong enough to explain the observed large blue-shift of CO frequency with coverage,1,12 indicating that a change of chemical bonding with CO coverage Received: January 26, 2014 Revised: July 21, 2014 Published: July 30, 2014 19688
dx.doi.org/10.1021/jp500912p | J. Phys. Chem. C 2014, 118, 19688−19700
The Journal of Physical Chemistry C
Article
internal stretch frequencies that are spatially and temporally overlapped on the surface sample. Thanks to a second-order nonlinear optical process, a signal is generated at the sum frequency ωSFG = ωVIS + ωIR.39−41 The up-converted vibrational spectrum of the region covered by the IR pulse is recorded by a CCD camera after passing the SFG beam through edge filters, to reject the visible beam, and through a polychromator (
