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1.
The reaction of CO 2 and H 2O to form bicarbonate (HCO −3) was examined on the nearly perfect and vacuum annealed surfaces of TiO 2(110) with temperature programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS) and high resolution electron energy loss spectrometry (HREELS). The vacuum annealed TiO 2(110) surface possesses oxygen vacancy sites that are manifested in electronic EELS by a loss feature at 0.75 V. These oxygen vacancy sites bind CO 2 only slightly more strongly (TPD peak at 166 K) than do the five-coordinated Ti 4+ sites (TPD peak at 137 K) typical of the nearly perfect TiO 2(110) surface. Vibrational HREELS indicates that CO 2 is linearly bound at the latter sites with a νa(OCO) frequency similar to the gas phase value. In contrast, oxygen vacancies dissociate H 2O to bridging OH groups which recombine to liberate H 2O in TPD at 490 K. No evidence for a reaction between CO 2 and H 2O is detected on the nearly perfect surface. In sequentially dosed experiments on the vacuum annealed surface at 110 K, CO 2 adsorption is blocked by the presence of preadsorbed H 2O, adsorbed CO 2 is displaced by postdosed H 2O, and there is little or no evidence for bicarbonate formation in either case. However, when CO 2 and H 2O are simultaneously dosed, a new CO 2 TPD state is observed at 213 K, and the 166 K state associated with CO 2 at the vacancies is absent. SSIMS was used to tentatively assign the 213 K CO 2 TPD state to a bicarbonate species. The 213 K CO 2 TPD state is not formed if the vacancy sites are filled with OH groups prior to simultaneous CO 2+H 2O exposure. Sticking coefficient measurements suggest that CO 2 adsorption at 110 K is precursor-mediated, as is known to be the case for H 2O adsorption on TiO 2(110). A model explaining the circumstances under which the proposed bicarbonate species is formed involves the surface catalyzed conversion of a precursor-bound H 2O–CO 2 van der Waals complex to carbonic acid, which then reacts at unoccupied oxygen vacancies to generate bicarbonate, but falls apart to CO 2 and H 2O in the absence of these sites. This model is consistent with the conditions under which bicarbonate is formed on powdered TiO 2, and is similar to the mechanism by which water catalyzes carbonic acid formation in aqueous solution. 相似文献
2.
The interaction between submonolayer titania coverages and Pt foil has been studied by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). The submonolayer titania can be fully oxidized to TiO 2 at 923 K under 10 −8 Torr O 2, and partially oxidized to TiO x at lower oxidation temperatures. The oxidized surface can be reduced by annealing to 1000 K or higher, or by heating in H 2 at 823 K, or by interacting with surface carbon formed from acetone decomposition. Under certain conditions (e.g., hydrogen reduction at 923 K), the surface titania can be fully reduced to metallic Ti which diffuses into bulk Pt readily. The reduced metallic Ti can resurface when the surface is oxidized at 923 K. Both XPS and HREELS data indicate the existence of subsurface oxygen, which plays an important role for the diffusion of Ti into and out of the Pt foil. Although no special interfacial active sites were revealed by HREELS studies of adsorbed acetone and CO, some TPD and XPS data suggest the presence of sites active for acetone decomposition. 相似文献
3.
The adsorption and dissociation of CH 2I 2 were studied at 110 K with the aim of generating CH 2 species on the Ru(001) surface. The methods used included X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), temperature programmed desorption (TPD), Auger electron spectroscopy (AES) and work function measurements. Adsorption of CH 2I 2 is characterized by a work function decrease (0.96 eV at monolayer), indicating that adsorbed CH 2I 2 has a positive outward dipole moment. Three adsorption states were distinguished: a multilayer ( Tp=200 K), a weakly bonded state ( Tp=220 K) and an irreversibly adsorbed state. A new feature is the formation of CH 3I, which desorbs with Tp=160 K. The adsorption of CH 2I 2 at 110 K is dissociative at submonolayer, but molecular at higher coverages. Dissociation of the monolayer to CH 2 and I proceeded at 198–230 K, as indicated by a shift in the I(3d 5/2) binding energy from 620.6 eV to 619.9 eV. A fraction of adsorbed CH 2 is self-hydrogenated into CH 4 ( Tp=220 K), and another one is coupled to di-σ-bonded ethylene, which — instead of desorption — is converted to ethylidyne at 220–300 K. Illumination of the adsorbed CH 2I 2 initiated the dissociation of CH 2I 2 monolayer even at 110 K, and affected the reaction pathways of CH 2. 相似文献
4.
The interaction of oxygen and different coverages of potassium on Ru(001) has been investigated by thermal desorption spectroscopy (TDS), metastable quenching spectroscopy (MQS), electron stimulated desorption spectroscopy (ESD), and work-function change measurements. The results show that this is a complex surface system with several different oxides forming, depending on the surface stoichiometry and temperature. While we cannot uniquely identify all the surface species, our interpretation of the present data combined with previous information is as follows. For potassium coverages up to about three monolayers (θ K ≈ 1), exposure to oxygen initially gives oxygen atoms on the surface. Further exposure produces some surface monoxide ions O 2−, which are converted with additional exposure to Superoxide ions O −2 and possibly peroxide ions O 2−2. Thermal annealing causes strong changes in the surface oxide composition, and with potassium multilayers (θ K ≈ 10) all the oxides diffuse beneath the K surface layer with annealing to only 300 K. K 2O and K 2O 2 are found to desorb together in the 600–700 K region. 相似文献
5.
The oxidation of CoGa(1 0 0) at 700 K was studied by means of high resolution electron energy loss spectroscopy (EELS), scanning tunneling microscopy, low energy electron diffraction and Auger electron spectroscopy (AES). At 700 K, thin well-ordered β-Ga 2O 3 films grow on CoGa(1 0 0). The EEL spectrum of the Ga-oxide films exhibit Fuchs–Kliewer phonons at 305, 455, 645, and 785 cm −1. For low oxygen exposure (<0.2 L), the growth of oxide-islands starts at step edges and on defects. The oxide films have the shape of long, rectangular islands and are oriented in the [1 0 0] and [0 1 0] directions of the substrate. For higher oxygen exposure, islands of β-Ga 2O 3 are found also on the terraces. After an exposure of 200 L O 2 at 700 K, the CoGa(1 0 0) surface is homogeneously covered with a thin film of β-Ga 2O 3. 相似文献
6.
The interaction of HNCO with oxygen dosed Rh(111) surface has been investigated by Auger electron, electron energy loss and thermal desorption spectroscopy. The presence of adsorbed oxygen exerted no apparent influence on the weakly adsorbed HNCO ( Tp = 130 K). It promoted, however, the dissociative adsorption of HNCO by forming a strong O—H bond which prevented the associative desorption of HNCO. As a result no H 2 and NH 3 formation occurred, in contrast with the clean surface, and the surface concentration of irreversibly bonded NCO was also increased. New products of the surface reaction were H 2O and CO 2, in addition to CO and N 2 observed on a clean surface. From the behavior of the losses characteristic for the adsorbed NCO it appeared that the preadsorbed oxygen exerted a significant stabilizing effect on the NCO bonded to the Rh. 相似文献
7.
Adsorption and decomposition of triethylindium (TEI: (C 2H 5) 3In) on a GaP(0 0 1)-(2×1) surface have been studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). It is found from the TPD result that ethyl radical and ethylene are evolved at about 300–400 and 450–550 K, respectively, as decomposition products of TEI on the surface. This result is quite different from that on the GaP(0 0 1)-(2×4) surface. The activation energy of desorption of ethyl radical is estimated to be about 93 kJ/mol. It is suggested that TEI is adsorbed molecularly on the surface at 100 K and that some of TEI molecules are dissociated into C 2H 5 to form P–C 2H 5 bonds at 300 K. The vibration modes related to ethyl group are decreased in intensity at about 300–400 and 450–550 K, which is consistent with the TPD result. The TEI molecules (including mono- and di-ethylindium) are not evolved from the surface. Based on the TPD and HREELS results, the decomposition mechanism of TEI on the GaP(0 0 1)-(2×1) surface is discussed and compared with that on the (2×4) surface. 相似文献
8.
VO 2表面氧缺陷的存在对VO 2材料具有显著的电子掺杂效应, 极大地影响材料的本征电子结构和相变性质. 通过2, 3, 5, 6-四氟-7, 7', 8, 8'-四氰二甲基对苯醌(F 4TCNQ)分子表面吸附反应, 可以有效消除表面氧缺陷及其电子掺杂效应. 利用同步辐射光电子能谱和X射线吸收谱原位研究了修复过程中电子结构的变化以及界面的化学反应, 发现这种方式使得VO 2薄膜样品氩刻后得到的V 3+失去电子成功地被氧化成原先的V 4+, 同时F 4TCNQ分子吸附引起电子由衬底向分子层转移, 界面形成带负电荷的分子离子物种. 受电化学性质的制约, F 4TCNQ分子吸附反应修复氧缺陷较氧气氛退火更安全有效, 不会引起表面过度氧化形成V 2O 5. 相似文献
10.
Adsorption and thermally-induced dissociation of disilane (Si 2H 6) on clean Ge(001)2 × 1 surfaces have been investigated using a combination of Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), reflection high-energy electron diffraction (RHEED), and scanning tunneling microscopy (STM). With initial Si 2H 6 exposure at room temperature, the Si surface coverage increased monotonically, the EELS surface dangling bond peak intensities continuously decreased, and the intensity of half-order RHEED diffraction rods decreased. The low-coverage Si 2H 6 sticking probability at 300 K on Ge(001) was found to be 0.5 while the saturation coverage was 0.5 ML. A new EELS feature, GSH, involving Si-H and Ge-H bond states was observed at Si 2H 6 exposures φ 3.4 × 10 13 cm −2. In contrast to Si 2H 6 -saturated Si(001), the saturated Ge(001) surface significant fraction of dimerized bonds. Adsorbed overlayers were highly disordered with the primary species on saturated surfaces being SiH 2, GeH, and undissociated SiH 3· Si 2H 6-saturated Ge(001)2 × 1 substrates were annealed for l min at temperatures Ta between 425 and 825 K. Admolecules were mobile at Ta = 545 K giving rise to significant ordering in one-dimensional chains. By Ta = 605 K, essentially all of the admolecules were captured into coarsened islands. Dangling-bond EELS peaks reappeared by 625 K and the intensities of the half-order RHEED diffraction rods increased. Ge segregation to the surface, which began at Ta 625 K, occurred rapidly at Ta 675 K. All H was desorbed by 725 K. 相似文献
11.
The reactions of Si(100) and Si(111) surfaces at 700 °C (973 K) with ethylene (C 2H 4) at a pressure of 1.3×10 −4 Pa for various periods of time were studied by using Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS). For a C 2H 4 exposure level, the amount of C on the (111) surface was larger than that on the (100) surface. The formation of β-SiC grain was deduced by comparing the C KLL spectra from the sample subjected to various C 2H 4 exposure levels, and from β-SiC crystal. 相似文献
12.
High-resolution vibrational electron energy loss spectroscopy, low-energy electron diffraction and Auger electron spectroscopy have been used to study the interactions of nitrogen with the Pd(110) surface. At 120 K, N 2 is chemisorbed molecularly on the Pd(110) surface, and the (2 × 1)-N 2 structure is formed. Most probably, the N 2 molecules are chemisorbed in the on-top sites of the bulk-like Pd(110) surface in the upright-linear structure. The Pd---N 2 bond energy is estimated to be ˜ 6 kcal/mol. The Pd---N 2 and N---N stretching vibrations of N 2 admolecules on Pd(110) are observed at 30 and 278 meV, respectively. The primary-energy dependence and angle dependence of their excitation cross sections agree reasonably well with the prediction of the dipole theory. The electron beam-induced effects are briefly discussed. 相似文献
13.
The effects of potassium on the adsorption and dissociation of CH 3Cl on a Pd(100) surface has been investigated by ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), electron energy loss spectroscopy (in the electronic range EELS), temperature-programmed desorption (TPD) and work function change. In contrast to the clean surface, the adsorption of CH 3Cl caused a significant work function increase, 0.9-1.4 eV, of potassium-dosed Pd. Preadsorbed K enhanced the binding energy of CH 3C1 to the surface and induced the dissociation of adsorbed molecules. The extent of the dissociation increased almost linearly with the potassium content. The appearance of a new emission in the UPS spectrum at 9.2 eV, attributed to adsorbed CH 3 species, and the low-temperature formation of ethane suggest that a fraction of adsorbed CH 3Cl dissociates even at 115–125 K on potassium-dosed Pd(100). At the same time, a significant part of adsorbed CH 3 radical is stabilized, the reaction of which occurs only at 250–300 K. By means of TPD measurements, H 2, CH 4, C 2H 6, C 2H 4, KCl and K were detected in the desorbing gases. The results are interpreted by assuming a through-metal electronic interaction at low potassium coverage and by a direct interaction of the Cl in the adsorbed CH 3Cl with potassium at high potassium coverage. The latter proposal is supported by the electron excited Auger fine structure of the Cl signal and by the formation of KCl in the desorbing gases. 相似文献
14.
The corrosion resistance of uranium is greatly enhanced by alloying with niobium. In this study the initial stages of corrosion of a specific uranium-base alloy (U–14.1 at.% Nb) by O 2 or D 2O have been examined using the surface specific techniques of X-ray photoelectron spectroscopy (XPS), thermal programmed desorption (TPD), static secondary-ion mass spectroscopy (SSIMS), and sputtered neutrals mass spectroscopy (SNMS). XPS studies of the U–14.1 at.% Nb surface following oxidation using O 2 at 300 K indicate production of a thin oxide overlayer of stoichiometric UO 2.0 intermixed with Nb 2O 5. The same stoichiometry is exhibited for uranium when the oxide is prepared at 500 K with O 2; although, niobium is much less oxidized exhibiting a mixture of NbO and Nb. Contrary to previous XPS literature, SNMS depth profiling studies reveal that oxidation by O 2 is much greater (as judged by oxide layer thickness) than that exhibited by D 2O. An oxide layer thickness of less than 20 Å was created using D 2O as an oxidant at 300 K with exposures >3500 L (oxide layers created from O 2 are significantly greater at much smaller exposures). Formation of a critical density of Nb 2O 5 is suggested to be responsible for the enhanced corrosion resistance by preventing diffusion of O − (O 2−) or OD −/OH − into the oxide/metal interface region. The domains of stability of hydroxyl formation have also been followed using TPD, SSIMS and XPS. Maximal surface hydroxyl concentrations ( Θrel=0.30) are obtained at a surface temperature of 175 K for these experimental conditions. 相似文献
15.
The interaction of CCl 4 molecules with Fe(0 0 1) surfaces was investigated by spin-polarized ion scattering spectroscopy (SP-ISS). It was observed that CCl 4 molecules adsorb dissociatively on the surface at ambient temperature (290 K), and consequently, iron and chlorine were major surface constituents. It was found that the chlorine adatoms are located atop of iron atoms of the second surface layer (hollow sites of the surface). It is indicated that the spin state of iron atoms at the surface is not affected by exposure to a CCl 4 atmosphere, while almost no spin is induced in the chlorine adatoms. Similar behavior is observed in the spin states of iron and chlorine on an oxygen preadsorbed-Fe(0 0 1) surface. The difference in the spin states of iron and chlorine clarifies the local property of the incidence ion neutralization and element selectivity of SP-ISS in this CCl 4/Fe system. 相似文献
16.
We study the adsorption and reaction of CO 2 as a function of temperature between 100 and 700 K in the presence of Na on a Pd(111) surface using high resolution electron energy loss spectroscopy. While CO 2 does not react with a clean Pd(111) surface, we find various reaction channels on the Na precovered Pd(111) surface depending on the Na coverage. At intermediate coverage a bent CO 28− species with characteristic vibrational bands can be unambiguously identified. This species is stable up to 200 K, and dissociates into CO and oxygen similar to its behaviour on other surfaces, and as reported in a previous photoemission study [Wambach et al., Surface Sci. 209 (1989) 159]. In case the surface has been oxygen contaminated before Na and CO 2 exposure surface carbonates can be observed. 相似文献
17.
The oxidation of hydrazine on the clean Pt(111) surface has been investigated by temperature-programmed reaction spectroscopy (TPRS) in the temperature range 130–800 K. Direct reaction of molecular oxygen is observed on the Pt(111) surface for the first time, as indicated by the desorption of nitrogen beginning at 130 K with a maximum rate at 145 K, below the molecular oxygen dissociation temperature. Direct reaction of hydrazine with adsorbed molecular oxygen results in the formation of water and nitrogen. With excess hydrazine, all surface oxygen is reacted, forming water. When only adsorbed atomic oxygen is present, the low-temperature nitrogen yield decreases by a factor of 3 and the peak nitrogen desorption temperature increases to 170 K. No high-temperature (450–650 K) nitrogen desorption characteristic of nitrogen atom recombination is seen, indicating that during oxidation the nitrogen-nitrogen bond in hydrazine remains intact, as observed previously for hydrazine decomposition on the Pt(111) surface and hydrazine oxidation on rhodium. Two water desorption peaks are observed, characteristic of desorption-limited (175 K) and reaction-limited (200 K) water evolution from the Pt(111) surface. For low coverages of hydrazine, only the reaction-limited water desorption is observed, previously attributed to water formed from adsorbed hydroxyl groups. When excess hydrazine is adsorbed, the usual hydrazine decomposition products, H 2, N 2 and NH 3, are also observed. No nitrogen oxide species (NO, NO 2 and N 2O) were observed in these experiments, even when excess oxygen was available on the surface. 相似文献
18.
The kinetics of atomic carbon and oxygen buildup on a Ni(100) surface exposed to carbon monoxide at high temperatures have been investigated by Auger electron spectroscopy. The experimental data, taken at different sample temperatures (453 , T 573 K) and at different CO partial pressures (3 ×10 −7 , P co , 3 ×10 −1 mbar) allowed the identification of the CO dissociation mechanism. By fitting the experimental data with a set of rate equations describing CO dissociation, CO reduction of surface oxygen, and C and O recombination, we have been able to determine the pre-exponential factors and the activation energies of these processes. 相似文献
19.
The low work-function ZrO/W(100) surface was examined with the aim of understanding the reducing mechanism of the work function. Low-energy electron diffraction (LEED) was employed to analyze the surface atomic arrangement, and X-ray photoelectron spectroscopy (XPS) was used to identify the surface chemical condition. The ZrO/W(100) surface was made as follows: (i) around three monolayers of Zr were deposited on a clean W(100) surface, (ii) the sample was heat treated in an oxygen ambience of 1.3x10 −5 Pa for several tens of minutes at 1500 K, and (iii) the sample was flash heated at 2000 K in ultrahigh vacuum (UHV). During heat treatment in O 2, the deposited Zr was oxidized to ZrO 2, and the LEED pattern formed was p(2×1). The work function increased to 5.3 eV. Subsequent flash heating in UHV changed the p(2×1) LEED pattern into a c(4×2) pattern, and transformed ZrO 2 into the so-called Zr–O complex, the oxidized level of which is between ZrO 2 and metallic Zr. A drastic decrease in the work function to 2.7 eV ensued. The angular dependence of XPS showed that the Zr–O complex segregated within a few monolayers at the surface. 相似文献
20.
The influence of pre-dosed oxygen on NO–C 2H 4 interactions on the surface of stepped Pt(3 3 2) has been investigated using Fourier transform infrared reflection–absorption spectroscopy (FTIR-RAS) and thermal desorption spectroscopy (TDS). The presence of oxygen significantly suppresses the adsorption of NO on the steps of Pt(3 3 2), leading to a very specific adsorption state for NO molecules when oxygen–NO co-adlayers are annealed to 350 K (assigned as atop NO on step edges). An oxygen-exchange reaction also takes place between these two kinds of adsorbed molecules, but there appears to be no other chemical reaction, which can result in the formation of higher-valence NOx. C2H4 molecules which are post-dosed at 250 K to adlayers consisting of 18O and NO do not have strong interactions with either the NO or the 18O atoms. In particular, interactions which may result in the formation of new surface species that are intermediates for N2 production appear to be absent. However, C2H4 is oxidized to C18O2 by 18O atoms at higher annealing temperature. This reaction scavenges surface 18O atoms quickly, and the adsorption of NO molecules on step sites is therefore quickly restored. As a consequence, NO dissociation on steps proceeds very effectively, giving rise to N2 desorption which closely resembles that following only NO exposure on a clean Pt(3 3 2), both in peak intensity and desorption temperature. It is concluded that the presence of 18O2 in the selective catalytic reduction (SCR) of NO with C2H4 on the surface of Pt(3 3 2) does not play a role of activating reactants. 相似文献
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