Potassium and potassium oxide monolayers on the platinum (111) and stepped (755) crystal surfaces: A LEED,AES, and TDS study

The adsorption of potassium and the coadsorption of potassium and oxygen on the Pt(111) and stepped Pt(755) crystal surfaces were studied by AES, LEED, and TDS. Pure potassium adlayers were found by LEED to be hexagonally ordered on Pt(111) at coverages of θ = _K0.9–;1. The monolayer coverage was 5.4 × 10¹⁴K atoms/cm² (0.36 times the atomic density of the Pt(111) surface). Orientational reordering of the adlayers, similar to the behavior of noble gas phase transitions on metals, was observed. The heat of desorption of K decreased, due to depolarization effects, from 60 kcal/mole at θ_K <0.1, to 25 kcal/mole at θ_K = 1 on both Pt(111) and Pt(755). Exposure to oxygen thermally stabilizes a potassium monolayer, increasing the heat of desorption from 25 to 50 kcal/mole. Both potassium and oxygen were found to desorb simultaneously indicating strong interactions in the adsorbed overlayer. LEED results on Pt(111) further indicate that a planar K₂O layer may be formed by annealing coadsorbed potassium and oxygen to 750 K.     

Adsorption of H2O on Al(111)

The adsorption of H₂O on Al(111) has been studied by ESDIAD (electron stimulated desorption ion angular distributions), LEED (low energy electron diffraction), AES (Auger electron spectroscopy) and thermal desorption in the temperature range 80–700 K. At 80 K, H₂O is adsorbed predominantly in molecular form, and the ESDIAD patterns indicate that bonding occurs through the O atom, with the molecular axis tilted away from the surface normal. Some of the H₂O adsorbed at 80 K on clean Al(111) can be desorbed in molecular form, but a considerable fraction dissociates upon heating into OH_ads and hydrogen, which leaves the surface as H₂. Following adsorption of H₂O onto oxygen-precovered Al(111), additional OH_ads is formed upon heating (perhaps via a hydrogen abstraction reaction), and H₂ desorbs at temperatures considerably higher than that seen for H₂O on clean Al(111). The general behavior of H₂O adsorption on clean and oxygen-precovered Al(111) (θ_O ? monolayer) is rather similar at low temperature, but much higher reactivity for dissociative adsorption of H₂O to form OH _adsis noted on the oxygen-dosed surface around room temperature.     

A photoelectron spectroscopic investigation of the interaction between H2O and oxygen on Ag(110)

The adsorption and reaction of H₂O with adsorbed oxygen atoms on Ag(110) was examined by UPS. In agreement with previous EELS results, H₂O formed multilayers of ice upon adsorption at 140 K. The ice layers could be easily distinguished from monolayer coverages of chemisorbed H₂O (present above 160 K) by UPS. The ice layers produced (1) strong attenuation of the emission from the Ag d-bands, (2) a nearly 2 eV shift of H₂O valence levels to higher binding energy and (3) strong attenuation of emission from the H₂O 3a₁ orbital. H₂O was observed to react stoichiometrically with O(a) above 250 K to produce a pure layer of adsorbed hydroxyl species. The UPS spectra for these species exhibited features at ?5.8 and ?8.7 eV, as well as strong features above the d-bands. These spectra were compared with those for OH(a) on other surfaces, and the difficulties of identifying OH by UPS due to contamination by excess H₂O are discussed.     

Adsorption behavior of H2O on clean and oxygen precovered Ni(s)(111)

Photoelectron spectroscopy (UPS), thermal desorption spectroscopy (TDS), isotope exchange experiments, work function change (δφ) and LEED were used to study the adsorption and dissociation behavior of H₂O on a clean and oxygen precovered stepped Ni(s)[12(111) × (111)] surface. On the clean Ni(111) terraces fractional monolayers of H₂O are adsorbed weakly in a single adsorption state with a desorption peak temperature of 180 K, just above that of the ice multilayer desorption peak (T_m = 155 K). In the angular resolved UPS spectra three H₂O induced emission maxima at 6.2, 8.5 and 12.3 eV below E_F were found for θ ≈ 0.5. Angular and polarization dependent UPS measurements show that the C_2v symmetry of the H₂O gas-phase molecule is not conserved for H₂O(ad) on Ni(s)(111). Although the Δφ suggest a bonding of H₂O to Ni via the negative end of the H₂O dipole, the O atom, no hints for a preferred orientation of the H₂O molecular axes were found in the UPS, neither for the existence of water dimers nor for a long range ordered H₂O bilayer. These results give evidence that the molecular H₂O axis is more or less inclined with respect to the surface normal with an azimuthally random distribution. H₂O adsorption at step sites of the Ni(s)(111) surface leads in TDS to a desorption maximum at T_m = 225 K; the binding energy of H₂O to Ni is enhanced by about 30% compared to H₂O adsorbed on the terraces. Oxygen precoverage causes a significant increase of the H₂O desorption energy from the Ni(111) terraces by about 50%, suggesting a strong interaction between H₂O and O(ad). Work function measurements for H₂O+O demonstrate an increase of the effective H₂O dipole moment which suggests a reorientation of the H₂O dipole in the presence of O(ad), from inclined to a more perpendicular position. Although TDS and Δφ suggest a significant lateral interaction between H₂O+O(ad), no changes in the molecular binding energies in UPS and no “isotope exchange” between ¹⁸O(ad) and H₂¹⁶O(ad) could be observed. Also, dissociation of H₂O could neither be detected on the oxygen precovered Ni(s)(111) nor on the clean terraces.     

H2O interaction with clean and oxygen precovered Ni(110)

The interaction of water vapour with clean as well as with oxygen precovered Ni(110) surfaces was studied at 150 and 273 K, using UPS, ΔΦ, TDS, and ELS. The He(I) (He(II)) excited UPS indicate a molecular adsorption of H₂O on Ni(110) at 150 K, showing three water-induced peaks at 6.5, 9.5 and 12.2 eV below E_F (6.8, 9.4 and 12.7 eV below E_F). The dramatic decrease of the Ni d-band intensity at higher exposures, as well as the course of the work function change, demonstrates the formation of H₂O multilayers (ice). The observed energy shift of all water-induced UPS peaks relative to the Fermi level (ΔE_max = 1.5 eVat 200 L) with increasing coverage is related to extra-atomic relaxation effects. The activation energies of desorption were estimated as 14.9 and 17.3 kcal/mole. From the ELS measurements we conclude a great sensitivity of H₂O for electron beam induced dissociation. At 273 K water adsorbs on Ni(110) only in the presence of oxygen, with two peaks at 5.7 and 9.3 eV below E_F (He(II)), being interpreted as due to hydroxyl species (OH)^δ? on the surface. A kinetic model for the H₂O adsorption on oxygen precovered Ni(110) surfaces is proposed, and verified by a simple Monte Carlo calculation leading to the same dependence of the maximum amount of adsorbed H₂O on the oxygen precoverage as revealed by work function measurements. On heating, some of the (OH)^δ? recombines and desorbs as H₂O at ? 320 K, leaving behind an oxygen covered Ni surface.     

Adsorption of H2O on clean and on boron-contaminated Rh surfaces

The adsorption and dissociation of H₂O on Rh(111) and Rh foil surfaces have been studied in UHV using Auger electron, electron energy loss (in the electronic range) and thermal desorption spectroscopy. H₂O adsorbs weakly on clean Rh samples at 110 K. The adsorption is accompanied by the appearance of a broad loss feature at 14–14.5 eV. At higher exposures new losses appeared at 8.6 and 10.5 eV. The desorption of H₂O took place in two stages, with T_p = 183 K (β, chemisorption) and 158 K (α, multilayer formation). There was no indication of dissociation of H₂O on a clean Rh(111) surface. Similar results were obtained for a clean Rh foil. However, when small amounts of boron segregated on the surface of Rh, they exerted a dramatic influence on the adsorptive properties of this surface and caused the dissociation of H₂O. This was exhibited by the formation of H₂, by the buildup of surface oxygen, by the appearance of an intense new loss at 9.4 eV, identified as B-O surface species, and by the development of “boron-oxide”-like Auger fine structure.     

Interaction of H2O with a clean and oxygen precovered Ni(110) surface studied by XPS

The adsorption and reaction of H₂O on clean and oxygen precovered Ni(110) surfaces was studied by XPS from 100 to 520 K. At low temperature (T<150 K), a multilayer adsorption of H₂O on the clean surface with nearly constant sticking coefficient was observed. The O 1s binding energy shifted with coverage from 533.5 to 534.4 eV. H₂O adsorption on an oxygen precovered Ni(110) surface in the temperature range from 150 to 300 K leads to an O 1s double peak with maxima at 531.0 and 532.6 eV for T=150 K (530.8 and 532.8 eV at 300 K), proposed to be due to hydrogen bonded O_ads… HOH species on the surface. For T>350 K, only one sharp peak at 530.0 eV binding energy was detected, due to a dissociation of H₂O into O_ads and H₂. The s-shaped O 1s intensity-exposure curves are discussed on the basis of an autocatalytic process with a temperature dependent precursor state.     

Photoemission studies of clean and oxygen-covered Pt 6(111) × (100)

Employing the enhanced sensitivity obtained by using synchrotron radiation near the Cooper minimum for the 5d valence electrons, we have located the oxygen 2p and 2s levels for oxygen chemisorbed on a Pt 6(111) × (100) crystal. We find the oxygen 2p level located ?6 eV with a FWHM of 3 eV and the 2s at ?21.6 eV. A factor of four difference in saturation coverage is measured between temperatures of 300 and 120 K, but the position and width of the 2p level is independent of temperature. We observe also the 1b₁ orbital of weakly adsorbed H₂O molecules, which has pure O 2p parentage; from the intensity of this orbital, we are able to suggest why it is difficult to observe the oxygen 2p signal at low photon energies. In addition, we note a strong preferential attenuation in the Pt states near E_f for the adsorbed H₂O in spite of the weak nature of the bond.     

Methanol decomposition on platinum (111)

The interaction of methanol with clean and oxygen-covered Pt(111) surfaces has been examined with high resolution electron loss spectroscopy (EELS) and thermal desorption spectroscopy (TDS). On the clean Pt(111) surface, methanol dehydrogenated above 140 K to form adsorbed carbon monoxide and hydrogen. On a Pt(111)-p(2 × 2)O surface, methanol formed a methoxy species (CH₃O) and adsorbed water. The methoxy species was unstable above 170 K and decomposed to form adsorbed CO and hydrogen. Above room temperature, hydrogen and carbon monoxide desorbed near 360 and 470 K, respectively. The instability of methanol and methoxy groups on the Pt surface is in agreement with the dehydrogenation reaction observed on W, Ru, Pd and Ni surfaces at low pressures. This is in contrast with the higher stability of methoxy groups on silver and copper surfaces, where decomposition to formaldehyde and hydrogen occurs. The hypothesis is proposed that metals with low heats of adsorption of CO and H₂ (Ag, Cu) may selectively form formaldehyde via the methoxy intermediate, whereas other metals with high CO and H₂ chemisorption heats rapidly dehydrogenate methoxy species below room temperature.     

CO and O coadsorption on Pt3Sn studied by DFT: Changes in the adsorptive properties of the surface with alloying and coverage

Adsorption of CO and coadsorption of O and CO on Pt₃Sn(1 1 1) was studied using periodic DFT calculations. Calculations were performed on Pt(1 1 1) by using the same set of parameters and their results were used as reference basis. The calculations showed that the most stable configuration with the minimum energy for coadsorption of CO and O is CO adsorbed atop Pt and O adsorbed on fcc Pt2Sn hollow site and that the decrease in the adsorption strength of the system at a total surface coverage of 0.5 ML is by 0.063 eV as a result of coadsorption, with respect to the adsorption of one species individually. Results show that the interaction between the adsorbed CO and O is short range on PtSn alloy, contrary to that on pure Pt, and this is mainly related to stronger Sn–O bonds compared to Pt–O bonds which eventually reduce the surface strain at the coadsorption structure. There is a pronounced effect of total surface concentration on the adsorption energy of coadsorbed species; the adsorption strength is not directly proportional to the surface coverage but is also related to the distribution of the coadsorbed species on the surface.     

Nitric oxide reduction by CO ON Rh(111): Temperature programmed reaction

The reaction of NO with CO on Rh(111) has been studied with temperature programmed reaction (TPR). Comparisons are made with the reaction of O₂ with CO and the reaction of NO with H₂. The rate-determining step for both CO oxidation reactions is CO(a) + O(a) → CO₂(g). Repulsive interactions between adsorbed CO and adsorbed nitrogen atoms lead to desorption of CO in a peak at 415 K which is in the temperature range where the reaction between CO(a) and O(a) produces CO₂(g). Thus the extent of reaction of CO(a) with NO(a) is less than that between CO(a) and O(a) due to the lower coverage of CO caused by adsorbed N atoms and NO. A similar repulsive interaction between NO(a) and H(a) suppresses the NO + H₂ reaction. CO + NO reaction behavior on Rh(111) is compared to that observed on Pt(111).     

Hydrogen-induced low temperature CO displacement from the Pt(111) surface

A new low temperature displacement mechanism for CO on the Pt(111) surface has been observed in the presence of high pressures of hydrogen (0.001 to 0.1 Torr H₂). Temperature-programmed fluorescence yield near-edge spectroscopy (TP FYNES) was used to continuously monitor the CO coverage as a function of temperature both with and without hydrogen. For hydrogen pressures above 0.01 Torr, removal of CO begins at 130 K (E_d = 10.6 kcal/mol) instead of near the desorption temperature of 400 K (E_d = 26 kcal/mol). The large decrease in CO desorption energy appears to be caused by substantial repulsive interactions in the compressed monolayer induced by coadsorbed hydrogen. The new low temperature CO desorption channel appears to be caused by displacement of the compressed CO adlayer by coadsorbed hydrogen. In addition, the desorption activation energy for the main desorption channel of CO near 400 K is lowered by ~ 1 kcal/mol for hydrogen pressures in the 0.001 to 0.1 Torr range. These new results clearly emphasize the importance of in-situ methods capable of performing kinetic experiments at high pressures on well characterized adsorbed monolayers on single crystal surfaces. High coverages of coadsorbed hydrogen resulting from substantial overpressures may substantially modify desorption activation energies and thus coverages and kinetic pathways available even for strongly chemisorbed species. These phenomena may play an important role in surface reactions which occur at high pressure.     

The adsorption of CO,O2, and H2 on Pt: II. Ultraviolet photoelectron spectroscopy studies

Ultraviolet photoelectron spectroscopy (UPS) has been used to study the chemisorption of CO, O₂, and H₂ on platinum. Three single crystal surfaces ((111), 6(111) × (100), and 6(111) × (111)) and two polycrystalline surfaces were studied. These studies yielded three important results. First, the most dominant change in the Pt valence band upon gas adsorption was a decrease in the height of the peak immediately below the Fermi level. This decrease was nearly identical for all three gases studied. Second, CO adsorption resulted in the formation of a resonance state ～8 eV below the Fermi level which was attributed to CO molecular orbitals. In contrast, no dominant resonance states were observed for adsorbed O or H. The lack of an O resonance state on platinum is in contrast to the results observed for O adsorbed on Fe and Ni and suggests important differences between the OPt chemisorption bond and the OFe and ONi chemisorption bonds. Finally, adsorption of CO at steps or defects led to a decrease in work function while its adsorption on terraces led to an increase in work function. For H, adsorption at steps led to an increase in work function while adsorption on terraces led to a decrease in work function. The adsorption of O led to an increase in work function on all of the surfaces studied.     

The adsorption of H2O on clean and oxygen-dosed silver single crystal surfaces

The adsorption of H₂O on the surface of a single-crystal sphere of silver with exposed (111), (100) and (112) facets has been examined using ESDIAD (electron stimulated desorption ion angular distribution), LEED (low energy electron diffraction) and TDS (thermal desorption spectroscopy). The purpose of the study was (a) to examine the influence of substrate geometry for adsorption of H₂O on a metal surface for which the adsorbate-substrate interaction is weak, and (b) to study the influence of a surface impurity, oxygen, on the surface chemistry and local bonding structure of H₂O on Ag. We have found no evidence for either long-range or short-range local bonding order for adsorbed H₂O at 80 K on any of the surfaces studied. This appears to be a consequence, in part, of the lattice mismatch between the Ag crystal structure and the two-dimensional H₂O ice crystal structure. Adsorbed H₂O reacts with preadsorbed oxygen to form OH species which are bonded with the molecular axis perpendicular to Ag(111) and (100) but “inclined” on (112) surfaces, as identified using ESDIAD. The “inclined” OH species are associated with atomic steps on the (112) surface.     

The coadsorption of CO and H2 on Fe(100)

The coadsorption of CO and hydrogen on an Fe(100) surface was studied by temperature programmed desorption and X-ray photoelectron spectroscopy. It was found that CO adsorption blocked the subsequent dissociative adsorption of H₂, although it did not seem to affect the hydrogen binding energy. Preadsorption of hydrogen was observed to reduce the binding energy of CO subsequently adsorbed and to inhibit the dissociation of CO. A new surface species was identified in a coadsorbed layer of CO and hydrogen. This species was evidenced by the formation of a desorption peak for H₂ at 475 K when CO was adsorbed subsequent to H₂ adsorption.     

Adsorption sites in coadsorption systems determined by photoemission spectroscopy: K and CO coadsorbed on Rh(111)

The adsorption sites of coadsorbed K and CO on the Rh(111) surface have been determined using high-resolution core-level spectroscopy, low-energy electron diffraction and site-resolved photoelectron diffraction. For both a (2×2)-2CO–1K and a -6CO–1K structure, we find that the CO molecules occupy threefold hollow sites and the K atoms on-top sites, contrary to the adsorption sites of K (threefold hollow site) and CO (on-top site below 0.5 monolayers) if adsorbed alone on Rh(111). Deposition of K onto a CO precovered surface is found to induce large shifts towards lower binding energy of the C and O 1s core levels (0.7 eV for C 1s and 1.5 eV for O 1s). The major part of these shifts is shown to arise from the K-induced site change of the CO molecules. This finding may be of importance in the interpretation of XPS data of related co-adsorption systems. Finally, it is suggested that the C and O 1s binding energies provide useful fingerprints of the CO adsorption site also for co-adsorption systems.     

First principle investigation of H2Se,H2Te and PH3 sensing based on graphene oxide

Detecting toxic gases is of great importance to protect our health and preserve the quality of life. In this work, graphene (G) and graphene oxide with three different modifications (G–O, G–OH, and G–O–OH) have been used to detect hydrogen selenide (H₂Se), hydrogen telluride (H₂Te), and phosphine (PH₃) molecules based on Atomistic ToolKit Virtual NanoLab (ATK-VNL) package. The adsorption energy ($E_{\mathrm{ads}}$), adsorption distance (D), charge transfer (ΔQ), density of states (DOS), and band structure have been investigated to confirm the adsorption of H₂Se, H₂Te, and PH₃ on the surface of G, G–O, G–OH, and G–O–OH systems. The results of G revealed highest $E_{\mathrm{ads}}$ for the case of H₂Te with −0.143 eV. After the functionalization of G surface, the adsorption parameters reflected an improvement due to the presence of the functional groups. Particularly, the highest adsorption energy was found between G–O system and H₂Se gas with $E_{\mathrm{ads}}$ of −0.319 eV. The smallest adsorption distance was found between G–OH system and H₂Se gas. The highest charge transfer was found for the case of H₂Se gas adsorbed on G–O–OH system. By thorough comparison of the adsorption energy, adsorption distance, and charge transfer between G, G–O, G–OH, and G–O–OH systems and the three gases, G–O–OH system can be considered as a potential sensor for H₂Se gas.     

DFT study of self-coupling reaction of CF2(ads) coadsorbed on Cu(111) surface for forming CF2=CF2(g)

Total energy calculations based on the density functional theory (DFT) with ultrasoft pseudopotential, generalized gradient spin-polarized approximation and the partial structural constraint path minimization (PSCPM) method were carried out to establish the energetically more favorable reaction pathways for the self-coupling reaction of coadsorbed CF_2(ads) leading to the formation of CF₂=CF_2(ads) on the Cu(111) surface. In addition, the calculated electronic properties, namely partial density of states (PDOS), suggest that the initial breaking of the Cu(111)–CF_2(ads) bond associating with the electron delocalization on the Cu(111) surface and the electron transfer from Cu(111) to both units of CF_2(ads) are factors controlling the energy barrier for self-coupling reaction. Finally, the calculated energy barrier (0.310 eV) for the self-coupling reaction of CF_2(ads) coadsorbed on the Cu(111) surface in comparison with that (0.204 eV) for the single α-fluoride elimination of adsorbed CF_3(ads) on the Cu(111) surface qualitatively manifests that the formation of CF₂ = CF_2(g) at 250 K is limited by the self-coupling reaction of coadsorbed CF_2(ads) instead of the single α-fluoride elimination of adsorbed CF_3(ads).     

Structural effects on water formation from coadsorbed H + O on Rh(100)

The effect of adsorbate coverage, adsorption sequence and temperature on the structure, composition and reactivity of coadsorbed layers, produced by dissociative adsorption of O₂ and H₂ at 200 K on a Rh(100) surface, has been studied by combined TPD, XPS and LEED measurements. The emphasis is on the impact of the structure and composition of the mixed O + H layers on the synthesis of hydroxyl and water as a result of the O + H surface reaction. The difference in the O 1s binding energies of adsorbed O (529.9 eV) and OH species (530.8 eV) was used as a fingerprint to monitor the formation of the OH species. The H₂O TPD spectra show substantial variations of the desorption temperature range and the amount of water evolved with coadsorbate coverage and structure: from 270 to 350 K and from 0 to 0.08 ML, respectively. It has been found that dense O + H adlayers, where the O coverage is in the range 0.25-0.4 ML, favor the formation of stable OH species. The maximum amount of stable hydroxyl OH species ( 0.16 ML) can be produced by heating of these dense adlayers to 260 K. This results in reordering of the adspecies to form a new O + OH − (2 × 6) structure, where hydroxyls react readily to evolve 0.08 ML of water in a sharp desorption peak at 280 K. The effect of the adlayer density and restructuring on the production of OH and H₂O is discussed.     

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.