Kinetics of HCN Decomposition on the Pt(111) Surface by Time-Dependent Infrared Spectroscopy

Time-dependent reflection absorption infrared spectroscopy has been used to investigate the kinetics of HCN decomposition on the Pt(111) surface over the temperature range of 120 to 135 K. At these low temperatures, HCN bonds at an atop site with the HCN axis perpendicular to the surface, which gives rise to an intense C–H stretch at ∼3300 cm^–1. Further support for this HCN adsorption geometry is obtained through HCN/CO coadsorption experiments in which both molecules are seen to compete for the atop sites. The disappearance of the C–H stretch peak of HCN at low temperatures is indicative of dissociation to produce adsorbed H and CN. When the decrease in HCN coverage is followed for a sufficiently long time, the data deviate from the expected first-order rate law, and the temperature dependence of the rate constant deviates from the Arrhenius form. Over a more restricted coverage range, simpler behavior is observed, and an activation energy for HCN dissociation of 0.33 eV is obtained

Molecular Oxygen Network as a Template for Adsorption of Ammonia on Pt(111)

Low-temperature scanning tunneling microscopy (STM) was used to observe a mixed NH₃–O₂ overlayer on Pt(111). At adsorption temperatures below 50 K, the chemisorbed O₂ molecules form an ordered network at high coverages. The STM images reveal that this network features a distributed set of holes corresponding to on-top sites of the Pt lattice that are surrounded by two or three O₂ molecules. Different hole–hole distances are observed with 0.73 nm most common. These holes in the O₂ network act as preferential adsorption sites for the ammonia molecules leading to the formation of an NH₃–O₂ complex that serves as a precursor to ammonia oxydehydrogenation

Molecular Oxygen Network as a Template for Adsorption of Ammonia on Pt(111)

Low-temperature scanning tunneling microscopy (STM) was used to observe a mixed NH₃–O₂ overlayer on Pt(111). At adsorption temperatures below 50 K, the chemisorbed O₂ molecules form an ordered network at high coverages. The STM images reveal that this network features a distributed set of holes corresponding to on-top sites of the Pt lattice that are surrounded by two or three O₂ molecules. Different hole–hole distances are observed with 0.73 nm most common. These holes in the O₂ network act as preferential adsorption sites for the ammonia molecules leading to the formation of an NH₃–O₂ complex that serves as a precursor to ammonia oxydehydrogenation

Adsorption and Hydrogenation of Acrolein on Ru(001)

Temperature-programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) were used to study the adsorption and hydrogenation of acrolein on Ru(001). At low coverages, acrolein adsorbs on the surface at 90 K mostly via the CO bond and completely decomposes to CO around 460 K. As the coverage increases, adsorption via the CC bond predominates and most of the acrolein either desorbs molecularly or decomposes to CO and H₂. However, a small amount of the acrolein also self-hydrogenates to yield all the possible hydrogenation products, propanal, 2-propenol, and 1-propanol, with TPRS peak temperatures of 180, 210, and 280 K respectively, with propanal having the highest yield. Co-adsorption with hydrogen enhances the adsorption via the CC bond and the yield of all the hydrogenation products. The formation of propanal and 1-propanol was also confirmed by RAIRS to occur at approximately the same temperatures as observed with TPRS, with the intensity of the RAIRS peaks indicating that the extent of hydrogenation is significantly higher than the yields obtained from TPRS

Single-Molecule Dynamics in the Presence of Strong Intermolecular Interactions

In contrast to conventional spectroscopic studies of adsorbates at high coverage that provide only spatially averaged information, we have characterized the laterally confined shuttling dynamics of a single molecule under the influence of intermolecular interactions by vibrational spectroscopy using a scanning tunneling microscope. The bridge sites on Pt(111) are only occupied by a CO molecule that is surrounded by four other CO molecules at on-top sites. The bridge-site CO undergoes laterally confined shuttling toward an adjacent on-top site to transiently occupy a metastable site, which is slightly displaced from the center of an on-top site through repulsive interaction with adjacent on-top CO molecules. Analysis of action spectra for the shuttling events reveals the C–O stretch frequency of the metastable CO. We also constructed a modified potential energy surface incorporating the intermolecular interaction, which reveals the underlying mechanism and provides a new way to experimentally determine detailed information on the energetics of the metastable state

Direct Pathway to Molecular Photodissociation on Metal Surfaces Using Visible Light

We demonstrate molecular photodissociation on single-crystalline metal substrates, driven by visible-light irradiation. The visible-light-induced photodissociation on metal substrates has long been thought to never occur, either because visible-light energy is much smaller than the optical energy gap between the frontier electronic states of the molecule or because the molecular excited states have short lifetimes due to the strong hybridization between the adsorbate molecular orbitals (MOs) and metal substrate. The S–S bond in dimethyl disulfide adsorbed on both Cu(111) and Ag(111) surfaces was dissociated through direct electronic excitation from the HOMO-derived MO (the nonbonding lone-pair type orbitals on the S atoms (n_S)) to the LUMO-derived MO (the antibonding orbital localized on the S–S bond (σ*_SS)) by irradiation with visible light. A combination of scanning tunneling microscopy and density functional theory calculations revealed that visible-light-induced photodissociation becomes possible due to the interfacial electronic structures constructed by the hybridization between molecular orbitals and the metal substrate states. The molecule–metal hybridization decreases the gap between the HOMO- and LUMO-derived MOs into the visible-light energy region and forms LUMO-derived MOs that have less overlap with the metal substrate, which results in longer excited-state lifetimes

Spectroscopic Identification of Surface Intermediates in the Decomposition of Methylamine on Ru(001)

The thermal decomposition of methylamine on Ru(001) was studied with reflection absorption infrared spectroscopy (RAIRS) and temperature programmed reaction spectroscopy (TPRS). After the multilayer methylamine desorbs at 150 K, the RAIR spectra of the remaining monolayer methylamine undergo small changes due to structural rearrangements but do not indicate any chemical changes until 250 K. The results are in agreement with recent theoretical investigations indicating that CH₃ dehydrogenation to produce H_<i>x</i>CNH₂ species occurs before N–H and C–N bond scission. Experimental spectra of ¹³C- and ¹⁵N-substituted methylamine combined with DFT calculations of H_<i>x</i>CNH_<i>y</i> fragments attached to a Ru₁₉ cluster to simulate the RAIR spectra, including isotopic shifts, clarified the spectral assignments

Spectroscopic Identification of Surface Intermediates in the Dehydrogenation of Ethylamine on Pt(111)

Reflection absorption infrared spectroscopy, temperature-programmed desorption, and density functional theory (DFT) have been used to study the surface chemistry and thermal decomposition of ethylamine (CH₃CH₂NH₂) on Pt(111). Ethylamine adsorbs molecularly at 85 K, is stable up to 300 K, and is partially dehydrogenated at 330 K to form aminovinylidene (CCHNH₂), a stable surface intermediate that partially desorbs as acetonitrile (CH₃CN) at 340–360 K. DFT simulations using various surface models confirm the structure of aminovinylidene. Upon annealing to 420 K, undesorbed aminovinylidene undergoes further dehydrogenation that results in the scission of the remaining C–H bond and the formation of a second surface intermediate called aminoethynyl with the structure CCNH₂, bonded to the surface through both C atoms. The assignment of this intermediate species is supported by comparison between experimental and simulated spectra of the isotopically labeled species. Further annealing to temperatures above 500 K shows that the C–N bond remains intact as the desorption of HCN is observed