Edward B. Kollin

Carbon monoxide oxidation on the Pt(111) surface: Temperature programmed reaction of coadsorbed atomic oxygen and carbon monoxide

Carbon dioxide formation from coadsorbed atomic oxygen and molecular carbon monoxide has been characterized using temperature programmed reaction spectroscopy over a wide range of initial oxygen and carbon monoxide coverages. The experiments were performed in an apparatus containing Auger electron spectroscopy, low energy electron diffraction, and a multiplexed mass spectrometer for the temperature programmed reaction experiments. A single reaction limited CO2 peak is observed in the 320–340 K temperature range over a wide range of initial atomic oxygen and molecular CO coverages, suggesting that a single reaction mechanism dominates. The activation energy for CO2 formation ranges from 166 kJ/mol (40 kcal/mol) for small surface concentrations of reactive adsorbed atomic oxygen and CO (0.4×1014/cm2) to 68 kJ/mol (17 kcal/mol) for larger surface concentrations of reactive adsorbed atomic oxygen and CO (2.5×1014/cm2). Low energy electron diffraction results indicate that adsorbed atomic oxygen forms islands ...

The hydrogen-oxygen reaction over the Pt(111) surface: Transient titration of adsorbed oxygen with hydrogen

Abstract The kinetics of the hydrogen-oxygen reaction have been characterized on a Pt(111) surface over the 300–450 K temperature range by titration of adsorbed atomic oxygen with hydrogen in the 1.3 × 10 −6 to 1.3 × 10 −7 Pa (10 −8 to 10 −9 Torr) pressure range. These experiments were performed in an apparatus equipped with Auger electron spectroscopy, low energy electron diffraction, and a multiplexed mass spectrometer used for titration and thermal desorption measurements. The hydrogen-oxygen reaction has been studied by monitoring the water formation rate as a function of time for various temperatures, hydrogen pressures, and initial adsorbed oxygen concentrations. Adsorbed atomic oxygen forms islands of ordered atomic oxygen with a (2 × 2) structure under the conditions used during these experiments. Hydrogen reacts rapidly with adsorbed atomic oxygen to form water above 300 K. Typically reaction probabilities per incident hydrogen molecule were as high as 0.5 over the temperature range studied. Isotope exchange experiments indicate statistical amounts of HDO are formed during titration of adsorbed oxygen with H 2 -D 2 mixtures. This isotope result coupled with low temperature spectroscopic studies (1) suggests that water formation proceeds via sequential addition of hydrogen first to adsorbed atomic oxygen, then to adsorbed hydroxyl to form the product water. Neither the concerted atomic hydrogen addition mechanism nor the H 2 (a) + O(a) → OH(a) + H(a) mechanism can be rigorously excluded, however, several observations suggest they are not major pathways. The titration data indicate that the reaction is basically first order in incident hydrogen over the full range of adsorbed oxygen concentrations. The water formation rate is not a unique function of oxygen coverage, but also depends on the initial surface oxygen concentration (the largest oxygen coverage attained before the reaction begins). This result demonstrates that all of the adsorbed atomic oxygen is not available for reaction. A simple reaction model is proposed based on the assumption that the island structure of the adsorbed atomic oxygen limits the availability of oxygen for reaction; this simple model rationalizes the qualitative features of the titration data obtained. The model suggests that the key parameters affecting the behavior of the water formation reaction are the size and shape of the oxygen islands and the availability of adsorbed atomic hydrogen in the reaction zone.

Ammonia adsorption on the Pt(111) AND Pt(S)-6(111) × (111) surfaces

Abstract Ammonia adsorption on the Pt(111) and stepped Pt(S)-6(111) × (111) surfaces has been studied using thermal desorption spectroscopy, isotope exchange thermal desorption, and steady-state decomposition measurements. Isotope exchange experiments indicate that molecular ammonia adsorbs without dissociation on both the Pt(111) and Pt(S)-6(111) × (111) surfaces below 400 K. That is, ammonia adsorption is not significantly affected by the presence of (111) type step sites below 400 K. Three distinct desorption peaks are observed following adsorption at 90 K. Multilayers of solid ammonia desorb in a narrow peak at 100 K. The weakly chemisorbed high coverage-low temperature form of molecular ammonia desorbs at 150 K with first order kinetics and a heat of desorption of 36 ± 3 kj mol (8.6 ± 0.6 kcal mol ) . The chemisorbed low coverage-high temperature form of molecular ammonia desorbs in a broad peak in the 170 to 450 K temperature range. Steady-state ammonia decomposition occurs above 400 K on the Pt(111) surface indicating dissociative adsorption does occur at elevated temperatures.

Ethylidyne formation rates on the Pt(111) surface

Abstract The isothermal rates of ethylidyne formation from adsorbed ethylene have been characterized on a Pt(111) surface from 230 to 280 K using transient near edge X-ray absorption fine structure (T-NEXAFS). These studies establish the feasibility of T-NEXAFS as a method for measuring surface reaction rates in the 10 −2 to 10 −5 monolayer/s range. Since direct measurements of surface concentrations are used to establish the rates of the surface reaction no desorbing products are required for characterization of the isothermal reaction rates. The ethylidyne formation rates are first order in ethylene coverage. A substantial isotope effect is observed, perhydroethylene reacts about twice as fast as perdeuteroethylene. The activation energies are 14.4±0.7 kcal/mol (perhydro) and 16.7±1.0 kcal/mol (perdeutero) for the alehydrogenation of adsorbed ethylene.

Adsorption and thermal decomposition of CH3SH on the Pt(111) surface

Abstract Adsorption, desorption and thermal decomposition of methanethiol (CH 3 SH) on a clean and on a (2 × 2)-S covered Pt(111) surface have been studied using temperature programmed desorption, high resolution electron energy loss spectroscopy, and X-ray photoelectron spectroscopy as a function of temperature and coverage on the Pt(111) surface. Vibrational spectroscopy has been used to characterize the structure and bonding of surface intermediates formed during dehydrogenation of adsorbed methanethiol. The identity of surface intermediate species which form during thermal dehydrogenation is determined by both temperature and the availability of free Pt sites. Low coverages strongly favor low temperature dehydrogenation and decomposition reactions. Initial S-H bond activation does not occur at 110 K on crowded surfaces while complete decomposition of methanethiol is observed for 5% of a monolayer on a clean surface. For a saturated monolayer of methanethiol, XPS results indicate that about 60% of the carbon remains on the surface after heating to 750 K indicating that 40% of the carbon desorbs as CH 4 , and C 2 H 4 . The fractional yield of volatile organic products increases with increasing initial coverage of methanethiol up to monolayer coverage. The structure and geometry of the adsorbed intermediates formed by CH 3 SH decomposition also depends on coverage and temperature. Thermal dehydrogenation generates first CH 3 S, then CH 2 S as previously reported. In addition a new intermediate σ-bonded SCH is identified in this work. Similar species form on the (2 × 2)-S pre-sulfided surface; however, in the pre-sulfided case the S-C bonds tend to be oriented more closely along the surface normal than they are on the clean surface. Methanethiol molecules remain intact on the pre-sulfided surface up to 180 K.

The reduction of nitric oxide with hydrogen on the Pt(111) surface

Abstract The reduction of nitric oxide with hydrogen was studied using steady-state reactivity measurements in the range 10 −9 to 10 −8 Torr on the Pt(111) surface over the temperature range 100 to 800 K. The reaction products observed were nitrogen, ammonia, and water. The platinum surface was characterized using low-energy electron diffraction, Auger electron spectroscopy, and thermal desorption. Adsorption and desorption of reactants and products were studied previously on the Pt(111) surface using thermal desorption spectroscopy and electron energy loss spectroscopy. Reduction of nitric oxide occurs rapidly above 400 K on this platinum surface. In excess nitric oxide the dominant product is molecular nitrogen. Below 550 K, in excess nitric oxide, the nitrogen product inhibits nitric oxide reduction. Above 550 K the dissociation of nitric oxide appears to limit both ammonia and nitrogen formation. In excess hydrogen, ammonia is the dominant product from 300 to 500 K; above about 500 K, nitrogen formation dominates the reaction. The formation of an ammonia-nitric oxide surface complex appears to inhibit ammonia formation in the region 300–400 K. Competition is observed between the two reaction branches in the 450- to 550-K regime suggesting a common precursor for both reaction pathways.