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Reactivity of Surface Species in Heterogeneous Catalysts Probed by In Situ X-ray Absorption Techniques

Heterogeneous chemical reactions at vapor/solid interfaces play an important role in many processes in the environment and technology. Ambient pressure X-ray photoelectron spectroscopy (APXPS) is a valuable tool to investigate the elemental composition and chemical specificity of surfaces and adsorbates on the molecular scale at pressures of up to 130 mbar. In this review we summarize the historical development of APXPS since its introduction over forty years ago, discuss different approaches to minimize scattering of electrons by gas molecules, and give a comprehensive overview about the experimental systems (vapor/solid interfaces) that have been studied so far. We also present several examples for the application of APXPS to environmental science, heterogeneous catalysis, and electrochemistry.

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Investigation of solid/vapor interfaces using ambient pressure X-ray photoelectron spectroscopy

Vibrational sum-frequency generation (VSFG) spectroscopy is a powerful tool to study interfaces. Recently, multiplex heterodyne-detected VSFG (HD-VSFG) has been developed, which enables the direct measurement of complex second-order nonlinear susceptibility [χ(2)]. HD-VSFG has remarkable advantages over conventional VSFG. For example, the imaginary part of χ(2) [Imχ(2)] obtained with this interferometric technique is the direct counterpart to the infrared [Imχ(1)] and Raman [Imχ(3)] spectra in the bulk, and it is free from the spectral deformation inevitable in conventional VSFG [|χ(2)|2] spectra. The Imχ(2) signal is obtained with a sign that contains unambiguous information about the up/down orientation of interfacial molecules. Furthermore, HD-VSFG can be straightforwardly extended to time-resolved measurements when combined with photoexcitation. In this review, we describe the present status of experiments and applications of multiplex HD-VSFG spectroscopy, in particular with regard to the orientation...

Structure and Dynamics of Interfacial Water Studied by Heterodyne-Detected Vibrational Sum-Frequency Generation

Recent progress on sum-frequency spectroscopy

In the last ten years, two-dimensional infrared spectroscopy has become an important technique for studying molecular structures and dynamics. We report the implementation of heterodyne detected two-dimensional sum-frequency generation (HD 2D SFG) spectroscopy, which is the analog of 2D infrared (2D IR) spectroscopy, but is selective to noncentrosymmetric systems such as interfaces. We implement the technique using mid-IR pulse shaping, which enables rapid scanning, phase cycling, and automatic phasing. Absorptive spectra are obtained, that have the highest frequency resolution possible, from which we extract the rephasing and nonrephasing signals that are sometimes preferred. Using this technique, we measure the vibrational mode of CO adsorbed on a polycrystalline Pt surface. The 2D spectrum reveals a significant inhomogenous contribution to the spectral line shape, which is quantified by simulations. This observation indicates that the surface conformation and environment of CO molecules is more complicated than the simple “atop” configuration assumed in previous work. Our method can be straightforwardly incorporated into many existing SFG spectrometers. The technique enables one to quantify inhomogeneity, vibrational couplings, spectral diffusion, chemical exchange, and many other properties analogous to 2D IR spectroscopy, but specifically for interfaces.

https://www.pnas.org/content/pnas/108/52/20902.full.pdf

Adding a dimension to the infrared spectra of interfaces using heterodyne detected 2D sum-frequency generation (HD 2D SFG) spectroscopy.

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Nonadiabatic coupling between C-O stretching and Pt substrate electrons enhanced by frustrated mode excitations

Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has been applied to the investigation of NO adsorption on Pt(111) under various pressures of NO (up to 1 Torr) at room temperature. Under 10−7 Torr, molecular NO occupies the most stable fcc-hollow sites and partially occupies the energetically unfavorable atop sites. NO reversibly desorbs from the atop sites after evacuation. At NO pressures higher than 10−6 Torr, however, irreversible adsorption of atomic oxygen takes place via NO dissociation, leading to the formation of NO+O domains. This result is consistent with near-edge X-ray absorption fine structure (NEXAFS) and scanning tunneling microscopy (STM) results.

Irreversible Change in the NO Adsorption State on Pt(111) under High Pressure Studied by AP-XPS, NEXAFS, and STM†

We studied the mechanism of the N + NO reaction on Rh(111) surfaces by means of near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Atomic nitrogen layers on Rh(111) were titrated with NO at various temperatures. Below 350 K, the chemisorbed NO monomer does not react with N, while the NO dimer formed in the second layer acts as an extrinsic precursor to the reaction: N(a) + (NO)2(a) → N2O(g) + NO(a). Because of a dominant role of the precursor-mediated mechanism, the reaction proceeds slower with an increase in temperature. Above 350 K, the reaction switches to a different path: N(a) + NO(a) → N2(g) + O(a).

Mechanism of N + NO Reaction on Rh(111) Surfaces: A precursor-Mediated Reaction

Electron and coherent phonon dynamics at potassium-covered Cu(111) surfaces have been studied by using femtosecond time-resolved second-harmonic generation (TRSHG). At the coverages from 0.22 to 0.35 monolayer (ML), TRSHG traces show the oscillatory component with a frequency of $3.05\ifmmode\pm\else\textpm\fi{}0.05\text{ }\text{THz}$. The amplitude of this component decreases as coverage increases higher than $\ensuremath{\sim}0.35$ ML, whereas another oscillating component with a frequency of $1.26\ifmmode\pm\else\textpm\fi{}0.03\text{ }\text{THz}$ grows. Both components are ascribed to K-Cu stretching motion. The spectral changes with coverage suggest that the overlayer structure varies with lateral compression. The fast transient peak in TRSHG traces at around zero delay changes its sign from negative to positive when coverage exceeds $\ensuremath{\sim}0.22$ ML. Since the quantum-well state (QWS) is partly filled at around this coverage, electrons in the QWS are principally responsible for the transition of the electronic response. Furthermore, the excitation photon energy dependence of TRSHG traces indicates that the excitation of substrate $d$-band electrons, giving rise to rapid charge fluctuations in the QWS, generates the coherent K-Cu stretching vibration. Consequently, the QWS plays a major role in the electronic and nuclear dynamics induced by pump pulses at $h\ensuremath{\nu}=2.2\text{ }\text{eV}$.

Ultrafast electron and lattice dynamics at potassium-covered Cu(111) surfaces

We have investigated the mechanism of enhanced absorption intensities of vibrational bands of adsorbates on copper meshes with subwavelength holes by measuring and simulating temporal profiles of infrared pulses transmitted through the meshes. As reported previously [Williams et al., J. Phys. Chem. B, 2003, 107, 11871], the absorption intensities of CH stretching bands of alkanethiolate adsorbed on the mesh increase substantially with decreasing hole size. The enhancements of absorption intensities are associated with temporal delays of infrared pulses transmitted through the mesh. Finite difference time domain calculations reproduce the observed pulse delays as a function of hole size. These facts indicate that the delays of transmitted pulses are not caused by coupling of infrared radiation to surface plasmon polaritons propagating on the front and rear surfaces of the mesh, but they are caused by the reduction in group velocity owing to coupling to waveguide modes of mesh holes. Consequently, the strong enhancements of the absorption intensities are attributed to adsorbates inside the holes rather than to those on the mesh surfaces that have been proposed previously.

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Ikuyo Nakai

Papers

Nonadiabatic coupling between C-O stretching and Pt substrate electrons enhanced by frustrated mode excitations

Irreversible Change in the NO Adsorption State on Pt(111) under High Pressure Studied by AP-XPS, NEXAFS, and STM†

Mechanism of N + NO Reaction on Rh(111) Surfaces: A precursor-Mediated Reaction

Ultrafast electron and lattice dynamics at potassium-covered Cu(111) surfaces

Mechanism of enhancement in absorbance of vibrational bands of adsorbates at a metal mesh with subwavelength hole arrays