Oxygen adsorption on the kinked Pt(321) surface

Oxygen adsorption and desorption were characterized on the kinked Pt(321) surface using high resolution electron energy loss spectroscopy and thermal desorption spectroscopy. Molecular oxygen adsorbs mainly as a peroxo-like species at 100K with a heat of desorption of about 22 k^J/mol. Some of the molecular oxygen also adsorbs dissociatively at 100K. Atomic oxygen is adsorbed in three states. One state is due to adsorption on the terraces and another state is due to adsorption along the rough step sites. The heat of desorption of both of these states approximately equal and decreases from 290 k^J/mol to 195kJ/mol with increasing coverage. Atomic oxygen is also observed to adsorb in another state which is interpreted as adsorption at an on-top site.     

Molecular and atomic oxygen adsorption on the kinked Pt(321) surface

Oxygen adsorption and desorption were characterized on the kinked Pt(321) surface using high resolution electron energy loss spectroscopy, thermal desorption spectroscopy and Auger electron spectroscopy. Some dissociation of molecular oxygen occurs even at 100 K on the (321) surface indicating that the activation barrier for dissociation is smaller on the Pt(321) surface than on the Pt(111) surface. Molecular oxygen can be adsorbed at 100 K but only in the presence of some adsorbed atomic oxygen. The dominance of the v(OO) molecular oxygen stretching mode in the 810 to 880 cm^?1 range indicates that the molecular oxygen adsorbs as a peroxo-like species with the OO axis parallel or nearly parallel to the surface, as observed previously on the Pt(111) surface [Gland et al., Surface Sci. 95 (1980) 587]. The existence of at least two types of peroxo-like molecular oxygen is suggested by both the unusual breadth of the v(OO) stretching mode and breadth of the molecular oxygen desorption peak. Atomic oxygen is adsorbed more strongly on the rough step sites than on the smooth (111) terraces, as indicated by the increased thermal stability of atomic oxygen adsorbed along the rough step sites. The two forms of adsorbed atomic oxygen can be easily distinguished by vibrational spectroscopy since oxygen adsorbed along the rough step sites causes a v(PtO) stretching mode at 560 cm^?1, while the v(PtO) stretching mode for atomic oxygen adsorbed on the (111) terraces appears at 490 cm^?1, a value typical of the (111) surface. Two desorption peaks are observed during atomic oxygen recombination and desorption from the Pt(321) surface. These desorption peaks do not correlate with the presence of the two types of adsorbed atomic oxygen. Rather, the first order low temperature peak is a result of the fact that about three times more atomic oxygen can be adsorbed on the Pt(321) surface than on the Pt(111) surface (where only a second order peak is observed). The heat of desorption for atomic oxygen decreases from about 290kJ/mol (70 kcal/mol) to about 196 kJ/mol (47 kcal/mol) with increasing coverage. Preliminary results concerning adsorption of molecular oxygen from the gas phase in an excited state are also briefly discussed.     

binding states of CO and H2 on clean and oxidized (111)Pt

The binding states and sticking coefficients of CO and H₂ on clean and oxide covered (111)Pt are examined using flash desorption mass spectrometry and Auger electron spectroscopy (AES). On the clean surface at 78 K there is one major binding state of CO with a desorption activation energy which decreases with coverage plus a second smaller state, while H₂ exhibits three binding states with peak temperatures of 140, 230 and 310 K and saturation density ratios of 0.5 : 1 : 1. Desorption kinetics of CO are consistent with a first order state with a normal pre-exponential factor of 10^{13 ± 1} sec^?1, while all three peaks of H₂ are broader than expected. Interpretations in terms of anomalous pre-exponential factors, coverage dependent desorption activation energies, and desorption orders are considered. On the oxidized surface saturation densities of both gases are nearly identical to those on the clean surface, but desorption temperatures are increased significantly and the initial sticking coefficient on the oxide decreases slightly for CO and increases slightly for H₂.     

The chemisorption of nitrogen on the (001) surface of ruthenium

High resolution electron energy loss spectroscopy (EELS), thermal desorption mass spectrometry (TDMS) and low energy electron diffraction (LEED) have been used to investigate the molecular chemisorption of N₂ on Ru(001) at 75 K and 95 K. Adsorption at 95 K produces a single chemisorbed state, and, at saturation, a (√3x√3) R30° LEED pattern is observed. Adsorption at 75 K produces an additional chemisorbed state of lower binding energy, and the probability of adsorption increases by a factor of two from its zero coverage value when the second chemisorbed state begins to populate. EEL spectra recorded for all coverages at 75 K show only two dipolar modes — ν(RuN₂) at 280–300 cm^?1 and ν(NN) at 2200–2250 cm^?1 — indicating adsorption at on-top sites with the axis of the molecular standing perpendicular to the surface. The intensities of these loss features increase and ν(NN) decreases with increasing surface coverage of both chemisorbed states.     

Experimental study of the vibrations of acetylene chemisorbed on Ni(111)

High resolution electron energy-loss measurements of normal and deuterated acetylene chemisorbed on Ni(111) have been obtained. Observed vibrational modes are identified using the frequency shifts for the deuterated species and comparisons to the free molecule and a di-cobalt compound of acetylene. These vibrational frequencies indicate that chemisorbed acetylene is strongly rehybridized having a state of hybridization between ~sp^2.5 and sp³. Consideration of the types of modes observed, their assignments and the surface selection rule suggests a molecular orientation with the C-C bond axis slightly skewed relative to the surface and with the plane of the distorted molecule normal to the surface. A bonding geometry is proposed which has the carbon atoms residing above two adjacent 3 fold hollow sites of the Ni surface. This molecular geometry differs from that deduced previously by electron energy-loss spectroscopy for molecularly adsorbed acetylene on Pt(111).     

Adsorption of CO on W(001) by high resolution electron spectroscopy

The vibrational modes induced by CO on W(001) at temperatures ? 350 K are detected by means of electron energy loss spectroscopy with resolution in the 6–7 meV range. Two β adsorption regimes are identified depending on coverage. Heating at various increasing temperatures reveals coverage dependant irreversible surface structure modifications. The β spectra after adsorption or desorption are discussed in terms of the usual questions of multiple β states, dissociation, and reconstruction. The α₁ and α₂ states are detected both by their WC and CO frequencies. A small signal is assigned to a new a-state, named α₃, which may explain some thermal desorption results.     

Adsorption and reaction of nitric oxide and oxygen on Rh(111)

Adsorption of NO and O₂ on Rh(111) has been studied by TPD and XPS. Both gases adsorb molecularly at 120 K. At low coverages (θ_NO < 0.3) NO dissociates completely upon heating to form N₂ and O₂ which have peak desorption temperatures at 710 and 1310 K., respectively. At higher NO coverages NO desorbs at 455 K and a new N₂ state obeying first order kinetics appears at 470 K. At saturation, 55% of the adsorbed NO decomposes. Preadsorbed oxygen inhibits NO decomposition and produces new N₂ and NO desorption states, both at 400 K. The saturation coverage of NO on Rh(111) is approximately 0.67 of the surface atom density. Oxygen on Rh(111) has two strongly bound states with peak temperatures of 840 and 1125 K with a saturation coverage ratio of 1:2. Desorption parameters for the 1125 peak vary strongly with coverage and, assuming second-order kinetics, yield an activation energy of $\text{85 ± 5}\text{kcal}\text{mol}$ and a pre-exponential factor of 2.0 cm² s^?1 in the limit of zero coverage. A molecular state desorbing at 150 K and the 840 K state fill concurrently. The saturation coverage of atomic oxygen on Rh(111) is approximately 0.83 times the surface atom density. The behavior of NO on Rh and Pt low index planes is compared.     

Vibrational characterization of carbon monoxide adsorption on sulfur modified Ni(100) surfaces

Carbon monoxide adsorption has been studied on a series of presulfided Ni(100) surfaces using vibrational spectroscopy. The sulfided Ni(100) surfaces were characterized using Auger electron spectroscopy and low energy electron diffraction, binding states were isolated by heating CO-dosed surfaces to prescribed temperatures, corresponding to the desorption temperatures of the CO. Adsorption of CO on Ni(100) with a p(2 × 2) array of sulfur lead to CO stretching frequencies of 1740 and 1930 cm^?1 corresponding to desorption temperatures of 370 and 290 K, respectively. Adsorption of CO into the c(2 × 2)S structure resulted in a CO stretching frequency of 2115 cm^?1 and a desorption peak near 140 K. The binding sites on the p(2 × 2)S structure were interpreted as metal four-fold hollows and bridging sites. The high frequency state was interpreted as weak bonding into the four-fold hollow with back donation into the π¹ orbital on CO restricted by stearic hindrance due to adsorbed sulfur. Both the thermal desorption and vibrational results indicated that local CO-sulfur interactions are dominant on the presulfided Ni(100) surface in the coverage range studied.     

High coverage states of oxygen adsorbed on Pt(100) and Pt(111) surfaces

Oxygen adsorption on the Pt(100) and Pt(111) surfaces was investigated using X-ray photo-emission and thermal desorption spectroscopies. Low pressure (ca. 10^?5 Pa) oxygen dosing at near ambient crystal temperature resulted in the formation of dissociated adsorbed species at saturation coverages of nominally 0.2–0.25 monolayer on both surfaces. The combination of higher pressure (ca. 10^?3 Pa) and higher surface temperature (570 K) dosing produced a three to five times higher saturation coverage than the low pressure dosing. The effect of dosing condition on the saturation coverage appears to reconcile apparent discrepancies for the Pt(100) surface in the literature. Characterization by XPS of the higher coverage state for oxygen showed that it is in the same chemical state as the oxygen adsorbed at very low coverage. Angle-resolved XPS has shown that in all cases the oxygen appears to reside on the surface with no significant penetration of oxygen into the bulk, as would be characteristic of oxidation. However, some penetration on the surface by oxygen, such as by a place-exchange type restructuring of the first two atomic layers, cannot be entirely ruled out.     

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.     

Vibrational EELS,XPS and UPS study of CO chemisorbed on Pt10Ni90(111): Evidence for the existence of different sites

CO adsorption on the (111) face of a Pt₁₀Ni₉₀ alloy single crystal has been investigated at room temperature by vibrational electron energy loss spectroscopy (EELS) and photoelectron spectroscopy (XPS and UPS). Two well separated CO stretching modes develop at 2070 and 1820 ± 10 cm^?1, with their intensities reaching 64 and 36% respectively of the total intensity at saturation coverage. They are attributed to CO adspecies in terminal and bridge bonded configuration respectively. The UPS spectra of 4σ, 5σ and 1π molecular orbitais of adsorbed CO show complex features which may be resolved into two components having the main characteristics of CO adsorbed on pure Pt(111) and Ni(111) respectively. Such behaviour is also observed by XPS on C 1s on O 1s peaks. Their respective contributions, in both XPS and UPS spectra are about 64 and 36% of the whole spectrum. Finally compared to Ni(111) — on which CO adsorbs mainly in bridge configuration — the alloying with 10% Pt has generated the appearance of a large number of new sites for CO chemisorption associated with the presence of Pt atoms at the surface. The large amount of terminal CO adspecies is interpreted in terms of considerable surface enrichment of the alloy in platinum.     

The adsorption of oxygen on a stepped platinum single crystal surface

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.     

Adsorption and desorption of ammonia,hydrogen, and nitrogen on ruthenium (0001)

The adsorption of ammonia, hydrogen, and nitrogen on a Ru(0001) surface have been investigated by Auger electron spectroscopy, low-energy electron diffraction, and thermal flash desorption. The adsorption of ammonia on Ru(0001) can be divided into a low temperature mode (100 K) and a higher temperature mode (300–500 K). For a crystal temperature of 100 K the ammonia adsorbs into two weakly bound molecular γ states with s = 0.2. The ammonia desorbs as NH₃ molecules with desorption energies of 0.32 and 0.46 eV. At 300–500 K adsorption occurs via an activated process with a low sticking probability (s ? 2 × 10^?4).This adsorption is accompanied by dissociation and formation of an apparent (2 × 2) LEED pattern. Hydrogen adsorbs readily (s = 0.4) on Ru(0001) at 100 K and desorbs with 2nd order kinetics in the temperature range 350–450 K. Nitrogen does not appreciably adsorb on Ru(0001) even at 100 K; maximum nitrogen coverage obtained was estimated to be <2% of a monolayer. Changes in the ammonia flash desorption spectra after hydrogen preadsorption at 100 K will be discussed.     

C2H4在清洁和有Cs覆盖的Ru(0001)表面吸附的TDS研究

用热脱附谱(TDS)方法研究了乙烯(C₂H₄)在Ru(0001)表面上的吸附.在低温下(200K以下)乙烯可以在清洁及有Cs的Ru(0001)表面上以分子状态稳定吸附,在衬底温度升高至200K以上时,乙烯发生了脱氢分解反应,乙烯分解后的主要产物为乙炔(C₂H₂).在清洁的Ru(0001)表面,乙烯有两种吸附状态,脱附温度分别为275K和360K.而乙炔的脱附温度为350K.在Ru(0001)表面有Cs的存在时,乙烯分解 关键词： 乙烯 钌(0001)表面 铯钌(0001)表面乙烯 钌(0001)表面 铯钌(0001)表面     

Formation and hydrogenation of p(2 × 2)-N on Pt(1 1 1)

The formation of a well-ordered p(2 × 2) overlayer of atomic nitrogen on the Pt(1 1 1) surface and its reaction with hydrogen were characterized with reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The p(2 × 2)-N overlayer is formed by exposure of ammonia to a surface at 85 K that is covered with 0.44 monolayer (ML) of molecular oxygen and then heating to 400 K. The reaction between ammonia and oxygen produces water, which desorbs below 400 K. The only desorption product observed above 400 K is molecular nitrogen, which has a peak desorption temperature of 453 K. The absence of oxygen after the 400 K anneal is confirmed with AES. Although atomic nitrogen can also be produced on the surface through the reaction of ammonia with an atomic, rather than molecular, oxygen overlayer at a saturation coverage of 0.25 ML, the yield of surface nitrogen is significantly less, as indicated by the N₂ TPD peak area. Atomic nitrogen readily reacts with hydrogen to produce the NH species, which is characterized with RAIRS by an intense and narrow (FWHM ∼ 4 cm⁻¹) peak at 3322 cm⁻¹. The areas of the H₂ TPD peak associated with NH dissociation and the XPS N 1s peak associated with the NH species indicate that not all of the surface N atoms can be converted to NH by the methods used here.     

Temperature programmed desorption investigations of hydrogen and ammonia reactions on GaN

We report data for chemisorption and reaction of deuterium and isotopically labeled ammonia on single-crystalline GaN films grown on sapphire substrates. Temperature programmed desorption (TPD) and Auger electron spectroscopy (AES) studies, following exposure of the clean GaN film at room temperature to the probe reactant species, were conducted under UHV conditions. Deuterium desorption took place over a wide temperature range, 525–;800 K, with molecular deuterium as the only product. At low exposures, two distinct deuterium desorption peaks at ∼ 660 and 770 K were observed. The deuterium desorption peak at 660 K shifted to lower temperatures with increasing D adatom coverages. TPD experiments after ammonia adsorption on GaN revealed small amounts of hydrogen desorbed at ∼ 600 K and over a range 660–;770 K, suggesting partial decomposition of ammonia. Molecular ammonia desorption was observed at ∼ 560 and 600 K, with the low temperature desorption state growing with increasing ammonia exposures. Further studies on deuterium-precovered GaN films indicated that ammonia production resulted from recombination of NH_x species and hydrogen adatoms on the surface.     

The reflection-absorption infrared spectrum of the dioxygen species adsorbed on platinum recorded by FT-IR spectroscopy

The reflection-absorption infrared spectrum of oxygen adsorbed on a recrystallized Pt foil at 80K shows a single band at 875 cm^?1 at saturation coverage. The spectrum produced by an equilibrated mixture of oxygen isotopes confirms that the adsorbate is molecular. The apparent effective charge, e*, calculated from the RAIR spectrum is in good agreement with that calculated from the electron energy loss (EEL) spectrum reported previously. The coverage dependent frequency shift can be accounted for by dipole-dipole coupling. The large inherent bandwidth is probably due to enhanced vibrational relaxation via electron-hole pair formation.     

Adsorption of hydrogen on a Pt(111) surface

The H₂/Pt(111) system has been studied with LEED, ELS, thermal desorption spectroscopy and contact potential measurements. At 150 K H₂ was found to adsorb with an initial sticking coefficient of about 0.1, yielding an atomic H:Pt ratio of about 0.8:1 at saturation. H₂/D₂ exchange experiments gave evidence that adsorption is completely dissociative. No exrea LEED spots due to adsorbed hydrogen were observed, but the adsorbate was found to strongly damp the secondary Bragg maxima in the I/V spectrum of the specular beam. The primary Bragg maxima were slightly increased in intensity and shifted to somewhat lower energy. A new characteristic electron energy loss at ?15.4 eV was recorded upon hydrogen adsorption. The thermal desorption spectra were characterized by a high temperature (β₂-) state desorbing with second order kinetics below 400 K and a low temperature (β₂-) state that fills up, in the main, after the first peak saturates. The β₂-state is associated with an activation energy for desorption E1 of 9.5 kcal/mole. The decrease E1 with increasing coverage and the formation of the β₁-state are interpreted in terms of a lateral interaction model. The anomalous structure in the thermal desorption spectra is attributed to domains of non-equilibrium configuration. The work function change Δ? was found to have a small positive maximum (～ 2 mV) at very low hydrogen doses (attributed to structural imperfections) and then to decrease continuously to a value of ?230 mV at saturation. The variation of Δ? with coverage is stronger than linear. The isosteric heats of adsorption as derived from adsorption isotherms recorded via Δ? compared well with the results of the analysis of the thermal desorption spectra.     

Adsorption and desorption of no from Rh{111} and Rh{331} surfaces

The adsorption and desorption chemistry of NO on the clean Rh{111} and Rh{331} single crystal surfaces was followed with SIMS, XPS, and LEED. Results suggest dissociative NO adsorption occurs at step and/or defect sites. At saturation coverage there was ～ 10 times more dissociated species on the Rh{331} surface at 300 K than on the Rh{111} surface. On both surfaces two molecular states of NO_ads have been identified as β₁, and β₂ which possess different chemical reactivity. Under the condition of saturation coverage the β₁ and β₂ states are populated on the Rh{111} surface in a different proportion than on the Rh{331} surface. Further, their population on both surfaces is coverage and temperature dependent. When the sample is heated to desorb the saturation overlayer formed on the Rh{111} and Rh{331} crystal surfaces, approximately 50% of the overlayer is found to desorb below ? 400 K primarily from the β₂ state, molecularly as NO(g). Between 300 and 400 K the β₁ state dissociates as binding sites necessary to coordinate N_ads and O_ads are freed by desorption of NO(g).     

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.