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 × 10^16±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.     

Interactions between NO and CO on Pt(111)

Temperature programmed desorption (TPD) of coadsorbed NO and CO on Pt(111) shows that no reaction occurs (less than 2%) up to the desorption temperature of NO. At 100 K, adsorption is competitive, but neither gas displaces the other from the surface. Coadsorbed CO causes the NO desorption temperature to be lowered by as much as 100 K, but NO does not affect the CO desorption temperature. TPD spectra for NO depend on which gas is adsorbed first, indicating that equilibrium between species is not established on the surface during desorption. Electron energy loss spectra show that the vibrational spectrum of each gas is only weakly affected by the other. When NO is adsorbed first, CO does not affect the ratio of bridged and terminal NO but lowers the frequencies of the bridged NO by approximately 50 cm^?1 and lowers the intensities of vibrational peaks of both species by a factor of about four. When CO is adsorbed first, the ratio of terminal to bridged NO increases for given coverage of NO, and the frequency of the bridged NO remains at the pure NO value. These results are explained in terms of CO island formation, repulsive interactions between NO and CO, and low adsorbate mobilities.     

Reflection-absorption infrared and thermal desorption studies of carbon monoxide and hydrogen adsorbed on rhodium

Reflection-absorption infrared spectroscopic and thermal desorption techniques have been used to study the interaction of mixtures of carbon monoxide and hydrogen with evaporated rhodium films. For equimolar mixtures near 10^?9 Torr, hydrogen adsorbed much more rapidly, but long exposure times or increases in CO pressures to 10^?6 Torr led to its partial, but never complete, displacement by adsorbed carbon monoxide. Hydrogen desorption spectra taken during the displacement process showed two peaks which was consistent with a cooperative interaction between adsorbed CO and H species. In contrast to previous transmission studies of CO adsorption on small rhodium particles, the present reflection—absorption infrared study of the film system showed a single absorption band at 2075 ±10 cm^?1. While explanations for the discrepancy in terms of particle size effects are possible it is considered more likely that all CO molecules are linearly bound to individual Rh atoms in the present situation. In our work, increases in CO pressure (especially above 10^?6 Torr) were accompanied by an upward frequency shift (from 2065 cm^?1 to 2085 cm^?1) and a narrowing in half width (from 25 to 17 cm^?1). Several possible explanations for the latter unusual effect are discussed.     

Visible light induced photodesorption of NO from the α-Cr2O3(0001) surface

Nitric oxide chemistry and photochemistry on the Cr-terminated surface of α-Cr₂O₃(0001) were examined using temperature programmed desorption (TPD), sticking coefficient measurements and photodesorption. NO exposed to α-Cr₂O₃(0001) at 100 K binds at surface Cr cation sites forming a strongly bound surface species that thermally desorbs at 320–340 K, depending on coverage. No thermal decomposition was detected in TPD in agreement with previous results in the literature. Sticking probability measurements at 100 K indicated near unity sticking for NO up to coverages of ~ 1.3 ML, with additional adsorption with higher exposures at decreased sticking probability. These results suggest that some Cr cation sites on the α-Cr₂O₃(0001) surface were capable of binding more than one NO molecule, although it is unclear whether this was as separate NO molecules or as dimers. Photodesorption of adsorbed NO was examined for surface coverages below the 1 ML point. Both visible and UV light were shown to photodesorb NO without detectable NO photodecomposition. Visible light photodesorption of NO occurred with a greater cross section than estimated using UV light. The visible light photodesorption event was not associated with bandgap excitation in α-Cr₂O₃(0001), but instead was linked to excitation of a surface Cr^3 +–NO^? charge transfer complex. These results illustrate that localized photoabsorption events at surface sites with unique optical properties (relative to the bulk) can result in unexpected surface photochemistry.     

Reactivity of monolayer V2O5 films on TiO2(1 1 0) produced via the oxidation of vapor-deposited vanadium

The growth, and reactivity of monolayer V₂O₅ films supported on TiO₂(1 1 0) produced via the oxidation of vapor-deposited vanadium were studied using X-ray photoelectron spectroscopy and temperature programmed desorption (TPD). Oxidation of vapor-deposited vanadium in 10⁻⁷ Torr of O₂ at 600 K produced vanadia films that contained primarily V³⁺, while oxidation in 10⁻³ Torr at 400 K produced films that contained primarily V⁵⁺. The reactivity of the supported vanadia layers for the oxidation of methanol to formaldehyde was studied using TPD. The activity for this reaction was found to be a function of the oxidation state of the vanadium cations in the film.     

A TPD and IR study of co-adsorption of NO and oxygen on Ru(001)

The adsorption of NO on Ru(001), and its co-adsorption with oxygen, has been investigated using temperature programmed desorption and infra-red reflection-absorption spectroscopy (IRAS). Only one band due to the NO stretch of adsorbed NO could be observed and was assigned to the linearly bound v₂ species. Pre-adsorption of oxygen caused surprisingly little effect on the total desorbed flux of NO and its dissociation products in the TPD spectrum. The main effect of oxygen was to shift intensity from the v₁ peak to a new desorption peak v₁(O), which is accompanied by a splitting in the IRAS spectrum of the v₂ species. The absence of the band observed in the “bridging region” in EELS experiments and the small influence of the pre-adsorbed oxygen on the v₂ species is discussed in the light of current models for this adsorption system.     

The NO + CO reaction on Pt(100)

The interaction of NO with CO and with H₂ on Pt(100) was studied by temperature programmed desorption (TPD), isothermal desorption mass spectrometry, and low energy electron diffraction (LEED), TPD of NO and CO coadsorbed at 120 K yields almost complete reaction with both N₂ and CO₂ products desorbing as sharp, simultaneous peaks at ≈ 410 K. with full widths at half maximum as narrow as 3 K. Isothermal desorption mass spectrometry yields N₂ and CO₂ rates that exhibit a maximum with time. Both experiments indicate that the reaction mechanism is autocatalytic. Annealing NO-CO adlayers formed at 120 K to temperatures above 300 K causes the subsequent N₂ and CO₂ TPD peaks to broaden.'TPD of NO coadsorbed with H₂ yields sharp N₂ and H₂O product peaks that closely resemble the N₂ and CO₂ peaks observed in the NO + CO reaction. LEED experiments during TPD and isothermal desorption showed that the (1 × 1) → hex substrate phase transformation sometimes accompanies desorption of N₂ and CO₂. The TPD and isothermal desorption results can be fit by two simple models: chemical autocatalysis, in which an intermediate chemical species participates in a “chain propagation” reaction, and structural autocatalysis, which involves the formation of a reactive intermediate structure involving Pt atom displacements.     

The reduction of NO with D2 over the (110) surface of iridium

The heterogeneously catalyzed reaction between NO and D₂ to produce N₂, ND₃ and D₂O over Ir(110) was investigated under ultra-high vacuum conditions for partial pressures of the reactants between 5 × 10^?8 and 1 × 10^?6Torr, total pressures between 10^?7 and 10^?6 Torr, and surface temperatures between 300 and 1000 K. Mass spectrometry, LEED, UPS, XPS and AES measurements were used to study this reacting system. In addition, the competitive coadsorption of NO and deuterium was investigated via thermal desorption mass spectrometry and contact potential difference measurements to gain further insight into the observed steady state rates of reaction. Depending on the ratio of partial pressures ($\text{R }\text{P}{}_{\mathrm{<mtext>D</mtext>}}{}_2\text{P}{}_{\mathrm{<mtext>NO</mtext>}}$), the rate of reduction of NO to N₂ shows a pronounced enhancement when the surface is heated above a critical temperature. As the surface is cooled, the rate maintains a high value independent of temperature until a lower critical temperature is reached, where the rate drops precipitously. This hysteresis is due to a change in the structure and composition of the surface. For sufficiently large values of R and for an “activated” surface, N₂ and ND₃ are produced competitively between 470 and 630 K. Empirical models of the different regimes of the steady state reaction are presented with interpretations of these models.     

Vibrations measured at surfaces by HREELS : An updated review.

The adsorption of NO on Ru(001), and its co-adsorption with oxygen, has been studied by LEED, TPD, EELS and IRAS (with particular emphasis on the vibrational spectroscopies) over a wide range of temperatures. It has been well established (ref.1,2) that the adsorption of NO on Ru(001) at room temperature is initially dissociative with molecular adsorption taking place only after a dissociative layer is formed. It therefore seemed appropriate to study the effect of oxygen co-adsorption on the adsorption of NO under well defined conditions. The dissociation layer is characterized by a (2×2) LEED structure and is found to influence the subsequent molecular adsorption of NO in exactly the same way as a saturated, pre-chemisorbed layer of oxygen. One effect of oxygen co-adsorption is to suppress the v₁ intensity in the desorption spectra (ref.1) (the v₂ peak remains essentially unchanged), while simultaneously producing a new TPD peak of NO (v₁(O)) with lower binding energy. In the corresponding EELS spectra these two species (v₁ and v₁ (O)) are clearly distinguished.Our inability to observe anything other than the absorption band at ～1800cm^?1 in the IRAS experiments led us to repeat the EELS experiments (ref.2) together with TPD to elucidate more clearly the nature of the v₁ molecular species (ref.1). The results indicate that the adsorption of NO at low temperature (～ 95K) initially produces a species of NO with an N-O stretch frequency of 1400cm^?1. This is the only species observed up to an exposure of 0.5 Langmuirs, and on warming to room temperature it dissociates $\text{completely}$ to produce the (2×2) pattern described above. Pre-adsorption of oxygen prevents the formation of this species of NO. Only subsequent to the saturation of this species during adsorption at low temperature do two bands at ～1490cm^?1 and ～1810cm^?1, associated with the v₁ and v₂ molecular species (ref.1), appear. We suggest that the low temperature, low coverage species is lying down, and the molecular species v₁, and v₂ are both adsorbed on “on top” sites but corresponding to the bent and linear forms of the NO molecule, respectively.     

The adsorption of oxygen on gold

The oxidation of gold has been studied under UHV conditions by AES, XPS, and TDS. The previously reported adsorbed oxygen state, which formed by heating the sample above 600 K in 10^?5 Torr of oxygen and which remained after subsequent heating to 1100 K in vacuo, has been shown to result from the reaction of oxygen with silicon diffusing from the bulk. No oxygen adsorption was detected on a clean sample for oxygen pressures up to 10^?4 Torr and sample temperatures between 300–600 K. Chemisorption of oxygen atoms could be induced by placing a hot platinum filament close to the sample during exposure to oxygen. The activation energy for desorption of this oxygen state was estimated from the thermal desorption spectra to be about 163 kJ mol^?1. The chemisorbed oxygen atoms and the oxygen associated with silicon were distinguished by different O(1s) binding energies (529.2 and 532.3 eV respectively) and by different O(KVV) Auger fine structure.     

Electron beam interactions with CO on W {100} studied by Auger electron spectroscopy

The interaction of 2500 eV electrons with carbon monoxide chemisorbed on tungsten {100} was investigated by rapid-scan Auger electron spectroscopy. When no α state was present the O and C signals from the β state of CO were invariant during electron bombardment, giving an upper limit estimate for the electron stimulated desorption cross section, Q_β of 2 × 10^?21 cm². With the crystal at room temperature and saturated with CO, however, electron-beam induced accumulation of carbon was observed and characterised, the rate of the process being independent of CO pressure at pressures above 2 × 10^?8 Torr. At 450 K the rate was found to be pressure dependent up to at least 6 × 10^?7 Torr. A model is proposed for the accumulation process, which is based on electron beam dissociation of α₂-CO to form adsorbed carbon and gaseous O and the creation of new sites for further α₂-CO adsorption; it is in quantitative agreement with the results and yields a cross section for ESD of α₂-CO (Q_α2 = 1.55×10^?18cm²) in close agreement with direct measurements.     

Surface characterization of supported Pt,Rh, and alloy particles by CO chemisorption

Temperature programmed desorption (TPD) of CO is used to determine surface areas, binding states, and changes upon oxidation for 10–1000 Å particles of Pt, Rh, and Pt-Rh alloy on amorphous SiO₂. A low area sample configuration is used to obtain rapid and uniform heating and cooling in an ultra-high vacuum system. It is shown that both metals exhibit a higher CO binding state for small particles, but, as particle size increases, this state disappears and is replaced by a more weakly bound state. These states are suggested to be associated with (111) and higher surface free energy planes on these surfaces, heating Rh above 700 K in O₂ at 10^?6 Torr produces an oxide on which the CO saturation coverage is at least a factor of 10 lower than on the reduced surface. For Pt, oxidation produces only a small decrease in CO coverage, although the binding energy of CO increases on the oxygen treated surface. The difference in desorption temperatures for CO on Pt and Rh is consistent with previous experiments which show that an oxidation-reduction cycle produces a surface layer which is enriched in Rh and that the oxidized alloy contains no Pt atoms.     

Adsorption of nitrogen on platinum

The adsorption and desorption of nitrogen on a platinum filament have been studied by thermal desorption techniques. Nitrogen adsorption becomes significant only after any carbon contamination is removed from the surface by heating the platinum filament in oxygen, and after the CO content in the background gas is reduced substantially. At room temperature nitrogen populates an atomic tightly bound β-state, E^≠ = 19 kcal mole^?1. The saturation coverage of the (3-state is 4.5 × 10¹⁴ atoms cm^?2. Formation of the (β-state is a zero order process in the pressure range studied. At 90 K two additional α₁- and α₂-desorption peaks are observed. The activation energy for desorption for the α₂-state is 7.4 kcal mole^?1 at low coverage decreasing to 3 kcal mole^?1 at saturation of this state, 6 × 10 molecules cm^?2. The maximum total coverage in the α-states was 1.2 × 10¹⁵ molecules cm^?2. A replacement process between the β- and α-states has been observed where each atom in the (β-state excludes two molecules from the α-state.     

An investigation of the effect of pre-dosed O atoms on the adsorption of NO on Pt{2 1 1}

Reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) have been used to investigate the effect of pre-dosed O atoms on the adsorption of NO on Pt{2 1 1} at room temperature. RAIRS experiments show that no new species are formed when NO is adsorbed onto a Pt{2 1 1} surface that has been pre-dosed with oxygen and no species are lost from the spectra, compared to spectra recorded for NO adsorption on the clean Pt{2 1 1} surface. However pre-dosed oxygen atoms do influence the frequency and intensity of several of the observed infrared bands. In stark contrast, pre-dosed O has a large effect on the TPD spectra. In particular N₂ and N₂O desorption, seen following NO adsorption on the clean Pt{2 1 1} surface, is completely inhibited. This effect has been assigned to the blocking of NO dissociation by the pre-adsorbed O atoms. A new NO desorption peak, not seen for NO adsorption on the clean Pt{2 1 1} surface, is also observed in TPD spectra recorded following NO adsorption on an oxygen pre-dosed Pt{2 1 1} surface.     

Mechanism of catalytic reduction of NO by H2 or CO on a Pd foil; Role of chemisorbed nitrogen on Pd

The adsorption of NO and its reaction with H₂ over polycrystalline Pd were investigated using flash desorption technique and ultraviolet photoelectron spectroscopy under 10^?5 Pa pressure range of reactants and surface temperatures between 300 and 900 K. NO was adsorbed dissociatively onto the Pd surface above 500 K, and the heat of dissociative adsorption was ca. 126 kJ/mol. Atomic nitrogen was observed to accumulate on the Pd surface during the NO-H₂ reaction, whose desorption rate exhibited second order kinetics and is expressed as follows: V_d = 10^{?9.8 ± 0.3}exp(?67(kJ/mol)/RT) (cm²/atom·s). Hydrogenation of the adsorbed nitrogen proceeded rapidly at 485 K. It was confirmed from these results that formation of N₂ and NH₃ in the NO-H₂ reaction proceeds through this atomically adsorbed nitrogen. Pd-N bond energy and enthalpies of some intermediate states of the NO-H₂ reaction were estimated.     

The adsorption and reaction of H2O on clean and oxygen covered Ag(110)

The adsorption and reaction of water on clean and oxygen covered Ag(110) surfaces has been studied with high resolution electron energy loss (EELS), temperature programmed desorption (TPD), and X-ray photoelectron (XPS) spectroscopy. Non-dissociative adsorption of water was observed on both surfaces at 100 K. The vibrational spectra of these adsorbates at 100 K compared favorably to infrared absorption spectra of ice Ih. Both surfaces exhibited a desorption state at 170 K representative of multilayer H₂O desorption. Desorption states due to hydrogen-bonded and non-hydrogen-bonded water molecules at 200 and 240 K, respectively, were observed from the surface predosed with oxygen. EEL spectra of the 240 K state showed features at 550 and 840 cm^?1 which were assigned to restricted rotations of the adsorbed molecule. The reaction of adsorbed H₂O with pre-adsorbed oxygen to produce adsorbed hydroxyl groups was observed by EELS in the temperature range 205 to 255 K. The adsorbed hydroxyl groups recombined at 320 K to yield both a TPD water peak at 320 K and adsorbed atomic oxygen. XPS results indicated that water reacted completely with adsorbed oxygen to form OH with no residual atomic oxygen. Solvation between hydrogen-bonded H₂O molecules and hydroxyl groups is proposed to account for the results of this work and earlier work showing complete isotopic exchange between H₂¹⁶O_(a) and ¹⁸O_(a).     

The effect of ferroelectric polarization on the interaction of water and methanol with the surface of LiNbO3(0 0 0 1)

Water and methanol temperature programmed desorption (TPD) measurements were performed on the positive (c⁺) and negative (c⁻) surfaces of poled ferroelectric lithium niobate (LiNbO₃) single crystals. The results indicate that the molecule-surface interactions are both coverage and polarization-dependent. From a comparison of the TPD spectra for the positive and negative surfaces, it is shown that the desorption temperatures of water and methanol are consistently lower on the negative surface by 15 K and 20 K, respectively. The TPD spectra were simulated using the Polanyi-Wigner equation with a coverage-dependent energy term. These calculations show that the polarization dependence of the desorption temperature is due to a difference in the zero-coverage desorption energies on the two surfaces equal to a few kJ per mole. The mechanism for the polarization effect is explored with in situ pyroelectric voltage measurements, which indicate that a surface voltage of ±2 mV develops in the LiNbO₃(0 0 0 1) samples during TPD measurements. The magnitude of the pyroelectric-induced surface charge is heating rate dependent.     

The chemisorption and decomposition of NO on the (110) surface of iridium

The chemisorption of NO on Ir(110) has been studied with thermal desorption mass spectrometry (including isotopic exchange experiments), X-ray and UV-photoelectron spectroscopies, Auger electron spectroscopy,LEED and CPD measurements. Chemisorption of NO proceeds by precursor kinetics with the initial probability of adsorption equal to unity independent of surface temperature. Saturation coverage of molecular NO corresponds to 9.6 × 10¹⁴ cm^?2 below 300 K. Approximately 35% of the saturated layer desorbs as NO in two well separated features of equal integrated intensity in the thermal desorption spectra. The balance of the NO desorbs as N₂ and O₂ with desorption of N₂ beginning after the low-temperature peak of NO has desorbed almost completely. Molecular NO desorbs with activation energies of 23.4–28.9 and 32.5–40.1 kcal mole^?1, assuming the preexponential factor for both processes is between 10¹³–10¹⁶ s^?1. At low coverages of NO, N₂ desorbs with an activation energy of 36–45 kcal mole^?1, assuming the preexponential factor is between 10^?2 and 10 cm²s^?1. Levels at 13.5, 10.4 and 8.5 eV below the Fermi level are observed with HeI UPS, associated with the 4σ, 5σ and 1π orbitals of NO, respectively. Core levels of NO appear at 531.5 eV [O(1s)] and 400.2 eV [N(1s)], and do not shift in the presence of oxygen. Oxygen overlayers tend to stabilize chemisorbed NO as reflected in thermal desorption spectra and a downshift in the 1π level to 9.5 eV.     

Adsorption and epitaxial growth of benzene on ZnO(1010) studied by thermal desorption spectroscopy,LEED and UPS

The adsorption and condensation of benzene on ZnO(101&#x0304;0) was investigated by thermal desorption spectroscopy and LEED. The first monolayer shows an ordered c(2 × 2) super-structure. First order desorption is observed. The desorption energy and frequency factor decrease from 73 to 56 kJ mole^?1 and from ~10¹⁵ to ~10¹² s^?1, respectively, with coverage increasing to 0.85. The second layer is more weakly bound. Two-dimensional (2D) island formation is deduced from peak shape analysis. Near completion, the second layer converts to a more tightly bound configuration as deduced from a sudden shift of the desorption peak and the formation of an additional c(4 × 3) LEED pattern. This pattern which can be identified as a property of bulk benzene is preserved upon epitaxial growth of the 3D benzene crystal. Angular resolved UPS measurements indicate the benzene molecules of the first layer to be arranged in an oblique position of low symmetry.     

Ellipsometric study of oxygen adsorption and the carbon monoxide-oxygen interaction on ordered and damaged Ag(111)

The adsorption of oxygen on Ag(111) has been studied by ellipsometry in conjunction with AES and LEED. The oxygen pressure varied between 10^?5 and 10^?3 Torr and the crystal temperature between room temperature and 250° C. Changes in the Auger spectrum and the LEED pattern upon oxygen adsorption are very small. Oxygen coverages were derived from the changes in the ellipsometric parameter Δ. At room temperature a maximum coverage is reached within a few minutes. Its value increases with the damage produced by the preceding argon ion bombardment. The sticking coefficient derived from the initial rate of Δ-change amounts to 3 × 10^?5 for well-annealed surfaces and 2.5 ? 5 × 10^?4 for damaged surfaces. After evacuation no desorption takes place. Other types of adsorption, associated with much larger changes in Δ, were observed upon bombardment with oxygen ions and with oxygen activated by a hot filament. The reaction of CO with adsorbed oxygen was studied ellipsometrically at room temperature in the CO pressure range 10^?7–10^?6 Torr. The initial reaction rate is proportional to the CO pressure. The reaction probability (number of oxygen atoms removed per incident CO molecule) is 0.36.