John L. Gland

Oxygen interactions with the Pt(111) surface

Abstract Oxygen interaction with the Pt(111) surface has been studied by low energy electron loss (EELS), ultraviolet photoemission (UPS) and thermal desorption (TDS) spectroscopies over the temperature range 100 to 1400 K. Three states of oxygen have been characterized; molecular adsorption predominates below 120 K, adsorbed atomic oxygen predominates in the 150 to 500 K temperature range, while subsurface “oxide” formation may occur in the 1000 to 1200 K temperature range. (Oxide decomposition begins near 1250 K.) The adsorbed molecular state is bound by about 37 kJ/mol to the surface. This weakly adsorbed species has a primary vibrational frequency characteristic of a peroxo species (O 2 −2 ) indicating an oxygen-oxygen single bond in the adsorbed state. This bonding configuration is also supported by the observed positive change in work function of +0.8 eV and the UPS spectra which indicate that the orbitals derived from the π g ∗ orbitals of molecular oxygen are filled in the adsorbed dioxygen complex. Adsorbed atomic oxygen forms a single adsorbed species on the Pt(111) surface as indicated by the single vibrational frequency observed at 490 cm −1 along with the ordered (2 × 2) LEED pattern. The desorption of atomic oxygen is more complex since the heat of desorption [ d ln(desorption rate) d ( 1 RT ) ] decreases from about 500 to 300 kJ/mol with increasing coverages in the low coverage range. In this same coverage region the UPS spectra suggest that non-local electronic effects may be important in determining oxygenplatinum chemistry. Subsurface “oxide” has a single vibrational frequency near 760 cm −1 indicating the formation of a single type of oxide. Thermal decomposition spectra of the oxide also suggest the formation of a simple chemical system since the oxide decomposition curves are smooth above their 1250 K origin.

Molecular and atomic adsorption of oxygen on the Pt(111) and Pt(S)-12(111) × (111) surfaces

Abstract Oxygen adsorption and desorption were studied on the Pt(111) and Pt(S)-12(111) × (111) surfaces over the 100 to 1300 K temperature range using thermal desorption. Auger electron spectroscopy, X-ray photoemission spectroscopy and low energy electron diffraction. Isotope exchange experiments indicate that the low temperature desorption peak (150 K) results from molecular adsorption while the high temperature peak (400–600 K) results from atomic adsorption. Chemical titration experiments indicate that little or no dissociation of adsorbed molecular oxygen occurs at 100 K, that is, oxygen dissociation on platinum is activated. Adsorption into the molecular state is not activated as indicated by large initial sticking coefficients observed at 100 K. Molecular oxygen also adsorbs on these surfaces with preadsorbed (2 × 2) atomic oxygen overlayers. Adsorption of oxygen above 170 K proceeds rapidly to a coverage of about 3.8 × 1014 oxygen atoms cm 2 (the (2 × 2)O structure). Further oxygen can be adsorbed up to a coverage of about 8 × 10 14 atoms cm 2 using extended temperature cycling around 150 K or external atomization. The kinetics of desorption for both the low temperature and high temperature peaks are complex, making extraction of detailed information difficult. Adsorption of molecular oxygen on top of a previously formed (2 × 2)O structure results in simple desorption kinetics due to the drastically reduced probability of atomization during desorption.

The interaction of water with the Pt(111) surface

Abstract The interaction of water with a platinum (111) surface has been examined by thermal desorption (TDS), ultraviolet photoemission (UPS), and X-ray photoemission (XPS) spectroscopies. UPS and XPS results indicate that water adsorbs molecularly at 100K. TDS studies show that water desorbs with approximately zero order kinetics and that a monolayer of water has a maximum desorption rate at 180 K. Heavier coverages exhibit additional desorption from multilayer water (ice) which peaks at temperatures near 165 K. The coadsorption of atomic oxygen with water leads to an increase in the desorption temperature of water of about 30 K. XPS studies of this surface indicate that adsorbed OH species are involved in the reaction mechanism which increases the water desorption temperature.

Binding states and decomposition of NO on single crystal planes of Pt

Nitric oxide desorption and reaction kinetics are compared on the (111), (110),and (100) planes of platinum using temperature programmed desorption mass spectrometry. NO exhibits large crystallographic anisotropies with the (100) plane having stronger bonding and much higher decomposition activity than the (110) or (111) planes. The desorption activation energies for the major tightly bound states are 36, 33.5, and 25 kcal mole−1 on the (100), (110), and (111) planes respectively. Pre-exponential factors for these states on the (110) and (111) planes are 1 × 1016±0.5s−1. The major tightly bound state on the (100) plane dissociates to yield 50% N2 and O2, but all other states all planes desorb without significant decomposition. The fraction decomposed is less than 2% on the Pt(111) surface.

Nitric oxide adsorption on the Pt(111) surface

The adsorption of nitric oxide has been studied on the Pt(111) surface using electron energy loss spectroscopy (EELS), thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) over the temperature range 100 K to 1100 K. The chemisorption of nitric oxide is predominantly molecular on the Pt(111) surface, however a small amount of dissociation occurs with heating. The low temperature vibrational spectrum for a saturation coverage of adsorbed NO is dominated by two nitrogen-oxygen stretching bands at 1490 and 1710 cm−1. These bands can be rationalized using either a site model or a dimerization model. The low frequency mode could be assigned either to adsorption in a multiply bonded bridge site or to an adsorbed nitric oxide monomer. The high frequency mode could be assigned either to singly bound nitric oxide adsorbed in a terminal configuration or to nitric oxide dimers. The authors feel that data available favors the adsorption site model. The thermal desorption spectrum resulting from NO adsorption is complex. At least three distinct desorption processes are observed as well as some dissociation to form molecular nitrogen and oxygen. The desorption peaks below 350 K apparently correspond to desorption from nitric oxide adsorbed in a terminal configuration. As desorption proceeds with increasing temperature the adsorbed nitric oxide changes from an adsorbed configuration containing primarily terminally bonded nitric oxide at saturation coverage to bridge bonded nitric oxide at lower coverages. Some of the nitric oxide which desorbs in the high temperature shoulders above, 350 K is adsorbed molecularly in a distinct configuration as evidenced by the observation of new EELS bands at 1600 and 1820 cm−1. The thermal desorption results suggest some of this adsorbed nitric oxide is also dissociatively adsorbed as evidenced by the observation that some of this nitric oxide exchanges oxygen with preadsorbed oxygen eighteen. The nitric oxide which adsorbs above 350 K is responsible for most of the nitrogen formation which occurs with heating.

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 adsorption of oxygen on a stepped platinum single crystal surface

Abstract The adsorption of oxygen on the Pt(S)-[12(111) × (111) surface has been studied by Auger electron spectroscopy, low energy electron diffraction and thermal desorption spectroscopy. Two types of adsorbed oxygen have been identified by thermal desorption spectroscopy and low energy electron diffraction: (a) atoms adsorbed on step sites; (b) atoms adsorbed on terrace sites. The kinetics of adsorption into these two states can be modeled by considering sequential filling of the two adsorbed atomic states from a mobile adsorbed molecular precursor state. Adsorption on the step sites occurs more rapidly than adsorption onto the terraces. The sticking coefficient for oxygen adsorption is initially 0.4 on the step sites and drops when the step sites are saturated. The heat of desorption from the step site (45 ± 4 kcal/mole) is about 15% larger than the heat of desorption from the terraces.

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.

Carbon monoxide adsorption on the kinked Pt(321) surface

Abstract Carbon monoxide adsorption and desorption were studied on the kinked Pt(321) surface using high resolution electron energy loss spectroscopy, thermal desorption spectroscopy, Auger electron spectroscopy, and low energy electron diffraction. Two chemisorbed states are observed for CO adsorption on the Pt(321) surface. Carbon monoxide adsorbed along the rough steps (which have a high density of kinks) has an adsorption energy of 151 kJ/mol (36 kcal/mol). Carbon monoxide adsorbed on the terraces has an adsorption energy of 96 kJ/mol (23 kcal/mol). Both terminal and bridge bonded CO adsorb on the atomically rough steps although terminal binding is the dominant adsorption mode. Adsorption along the rough steps and terraces does not occur sequentially at 100K indicating that the CO mobility is restricted on the rough Pt(321) surface. However, sequential desorption does occur since CO desorbs from the terraces before it desorbs from steps. Molecular adsorption of CO on the Pt(321) kinked surface clearly dominates. In fact, weakening of the carbon-oxygen bond does not occur contrary to several recent suggestions that surface defects on platinum weaken the carbon-oxygen bond.

Adsorption and decomposition of nitric oxide and ammonia on a stepped platinum single crystal surface

Abstract The adsorption of nitric oxide and ammonia on the Pt(S)-[12(111) × (111)] surface has been studied using low energy electron diffraction, Auger electron spectroscopy, and thermal desorption spectroscopy; at saturation the adsorbed NO forms a 2 × 2 overlayer structure. Auger results and approximate desorption results confirm that the saturated surface corresponds to a coverage of about 3.8 × 10 14 adsorbed nitrogen containing species per square centimeter ( θ max ) or about 1 4 of the platinum surface atom concentration. Partial dissociation of the desorbing NO occurs as evidenced by thermal desorption of N2 and O 2 from the surface. The fraction of adsorbed nitrogen desorbed as N2 decreases from 100% below 0.1 θ max to about 30% at saturation. A surface saturated with ammonia corresponds to about 9 × 10 13 nitrogen containing species per cm 2 . Significant dissociation of the desorbing NH 3 occurs at all coverages studied as evidenced by thermal desorption of N 2 and H 2 .