Heike Arnolds

Real-Time Infrared Detection of Cyanide Flip on Silver-Alumina NOx Removal Catalyst

A Jump on Catalyst Kinetics A common method for interrogating fast reaction kinetics is to create an abrupt initiation point, and, for reactions catalyzed by metals on oxide supports, one option is to induce a rapid temperature jump, provided that the part of the catalyst undergoing heating can be probed locally. Thibault-Starzyk et al. (p. 1048) used femtosecond laser pulses to heat catalysts for nitrogen oxide removal and followed the reaction with Fourier-transform infrared spectroscopy. These catalysts are designed to remove NO in fuel-efficient lean-burn engines through reaction with CO, rather than with hydrocarbons. A cyanide reaction intermediate was identified that moves from the silver nanoparticle to the alumina surface. Rapid initiation of a catalytic reaction through laser heating enables the spectroscopic detection of a key intermediate. Spectroscopic studies of the mechanistic steps that occur on supported precious metal catalysts used in industrial and automotive applications are hampered by a dearth of suitable experimental methods. We used femtosecond laser excitation followed by nanosecond time-resolved in situ Fourier-transform infrared spectroscopy to initiate a catalytic reaction on alumina-supported silver catalysts, which are of interest in minimizing nitrogen oxide emissions from fuel-efficient lean-burn engines. We found that the key intermediate step in the reaction between carbon monoxide and nitric oxide is the flip of a cyanide group from a silver nanoparticle to the alumina support (with a lifetime of 2 microseconds), which indicates the central role played by the interface between the metal particle and the oxide support.

Broadband femtosecond sum-frequency spectroscopy of CO on Ru{101̄0} in the frequency and time domains

CO on Ru[1010] was investigated by broadband femtosecond sum-frequency spectroscopy at 200 K. Approximately half of the frequency shift of 71 cm(-1) over the coverage range from 0.15 to 1.22 monolayers is shown to originate from dipole-dipole coupling, with the remainder due to a chemical shift. Despite low adlayer-surface registration at the highest coverages, the linewidth of the C-O stretch is comparatively low, and is described by homogeneous broadening according to sum-frequency free-induction decay measurements in the time domain. This can be explained by the dominance of the CO dipole coupling strength over the static disorder present in a coincidence structure. As the coverage decreases below 0.3 monolayer, the linewidth increases considerably, indicative of inhomogeneous broadening. Supported by a concomitant frequency change we suggest that at low coverages CO molecules form chains of irregular length in the [0001] direction, as has been shown for other surfaces with similar symmetry.

The determination of an inhomogeneous linewidth for a strongly coupled adsorbate system.

We present a set of experiments that provide a complete mapping of coherent and incoherent vibrational relaxation times for a molecule on a metal surface, CO/Ir{111}. Included is the first detection of a midinfrared photon echo from a metallic surface, some 15 years after the analogous measurement on a semiconductor surface, which sets a precedent for the ability to manipulate and rephase polarization on a subpicosecond time scale on surfaces. For the C-O stretch in a strongly dipole-coupled CO layer we obtain a total linewidth of 5.6 cm-1, composed of a homogeneous width of 2.7 cm-1 and an inhomogeneous contribution of 3.0 cm-1. Pure dephasing is negligible at liquid nitrogen temperatures, making CO/Ir{111} an attractive model system for quantum computing.

In situ characterization of ultrafast laser pulses for sum frequency surface studies

Femtosecond laser pulses used for studies of metal surfaces in ultrahigh vacuum can be characterized by sum frequency generation. Deposition of an ultrathin alkali metal film enhances the surface nonlinear response by two orders of magnitude and allows the measurement of all necessary laser pulse parameters in 30 min at a repetition rate of 10 Hz.

Femtosecond near-infrared laser desorption of multilayer benzene on Pt{111}: spatial origin of hyperthermal desorption

The intense (1012 W cm−2, 800 nm) near-infrared laser desorption of benzene adsorbed on Pt{111} is reported. Velocity distributions are measured up to an adsorbate coverage of 20 monolayers. The velocity distributions reveal up to three distinct features: a hyperthermal feature with translational energy of 0.2 eV, a thermal feature and a subthermal feature. The translational energy of the hyperthermal feature is relatively independent of coverage and maintains constant desorption intensity from 4 to 20 ML. Experiments employing layered films of C6H6 and C6D6 reveal that the hyperthermal feature originates in the layers closest to the multilayer–vacuum interface.

Hot hole-induced dissociation of NO dimers on a copper surface.

We use reflection-absorption infrared spectroscopy (RAIRS) to study the photochemistry of NO on Cu(110) in the UV-visible range. We observe that the only photoactive species of NO on Cu(110) is the NO dimer, which is asymmetrically bound to the surface. RAIRS shows that photoinduced dissociation proceeds via breaking of the weak N-N bond of the dimer, photodesorbing one NO(g) to the gas phase and leaving one NO(ads) adsorbed on the surface in a metastable atop position. We model the measured wavelength-dependent cross sections assuming both electron- and hole-induced processes and find that the photochemistry can be described by either electron attachment to a level 0.3 eV above the Fermi energy E(F) or hole attachment to a level 2.2 eV below E(F). While there is no experimental or theoretical evidence for an electron attachment level so close to E(F), an occupied NO-related molecular orbital is known to exist at E(F) - 2.52 eV on the Cu(111) surface [I. Kinoshita, A. Misu, and T. Munakata, J. Chem. Phys. 102, 2970 (1995)]. We, therefore, propose that photoinduced dissociation of NO dimers on Cu(110) in the visible wavelength region proceeds by the creation of hot holes at the top of the copper d-band.

Stabilization of Insulin by Adsorption on a Hydrophobic Silane Self-Assembled Monolayer.

The interaction between many proteins and hydrophobic functionalized surfaces is known to induce β-sheet and amyloid fibril formation. In particular, insulin has served as a model peptide to understand such fibrillation, but the early stages of insulin misfolding and the influence of the surface have not been followed in detail under the acidic conditions relevant to the synthesis and purification of insulin. Here we compare the adsorption of human insulin on a hydrophobic (-CH3-terminated) silane self-assembled monolayer to a hydrophilic (-NH3(+)-terminated) layer. We monitor the secondary structure of insulin with Fourier transform infrared attenuated total reflection and side-chain orientation with sum frequency spectroscopy. Adsorbed insulin retains a close-to-native secondary structure on both hydrophobic and hydrophilic surfaces for extended periods at room temperature and converts to a β-sheet-rich structure only at elevated temperature. We propose that the known acid stabilization of human insulin and the protection of the aggregation-prone hydrophobic domains on the insulin monomer by adsorption on the hydrophobic surface work together to inhibit fibril formation at room temperature.

The structure of insulin at the air/water interface: monomers or dimers?

The hydrophobic character of the air/water interface affects the oligomeric composition of insulin. By using interface-specific vibrational sum frequency spectroscopy and calculations of insulin monomer and dimer second-order nonlinear susceptibilities χ((2)), we show that insulin monomers segregate to the air/water interface.

FOURIER TRANSFORM NMR AT SINGLE CRYSTAL SURFACES: 6LI ON RU(001)

Abstract NMR was performed on lithium adsorbates (10 −5 of a monolayer) on a ruthenium single crystal surface. For the first time, free induction decay and spin echo measurements were performed, yielding the transverse relaxation times T 2 ∗ and T 2 . The experiments utilize highly polarized adsorbates and a new beam-foil detection scheme, with single photon counting and phase coherent summation. A short T 2 time of 17 ms is found and contrasted with the much longer T 1 time of 760 ms under conditions where the adsorbate is highly mobile. The reduced dimensionality of the system is the probable cause for this effect.

Theory of SERS enhancement: general discussion

Rohit Chikkaraddy opened the discussion of the Introductory Lecture: Regarding quantifying the chemical enhancement, you showed a systematic change in the SERS enhancement for halide substituted molecules due to charge transfer from the metal. Is the extra enhancement due to an inherent increase in the Raman cross-section of the molecule? How do you go about referencing, as the charge transfer changes the vibrational frequency?