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1.
The chemistry of methyl species resulting from the decomposition of dimethylmercury (DMM) and dimethylzinc (DMZ) on Pt(111) in the range 300–400 K has been investigated by temperature prograrnmed desorption (TPD) and Auger electron spectroscopy (AES). In each case at 300 K, dissociative adsorption of the precursor results in the formation of an adlayer of methylmetal (CH3M) moieties. These species are thermally stable to around 350 K before decomposing to yield mainly gaseous products, methane and hydrogen, and surface bound metal atoms. For DMM, subsequent heating to 400 K or direct dissociative adsorption at 400 K results in the formation of ethylidyne species. Ethylidyne formation is not observed in the thermal chemistry of DMZ at temperatures below 400 K and only transiently in the chemistry at 400 K. Complementary TPD and AES data indicate that, for DMM, desorption of the mercury atoms produced by CH3Hg decomposition is the limiting factor in allowing the prevailing C1 species to couple to form ethylidyne. In contrast, AES evidence indicates that zinc atoms remain on the surface to temperatures in excess of 750 K and hence prevent C---C coupling by blocking surface sites. 相似文献
2.
Christopher R. Arumainayagam C. Emil Tripa Jiazhan Xu John T. Yates Jr. 《Surface science》1996,360(1-3):121-127
Fourier transform infrared reflection absorption spectroscopy (FT-IRAS) has been used to probe the non-dissociative adsorption of N2 on an atomically clean Pt(111) single crystal. In contradiction to a previous IRAS study of nitrogen adsorption on a Pt(111) foil at 120 K, no nitrogen infrared (IR) band was observed on a fully annealed Pt(111) surface at 90 K. Following Ar+ ion bombardment, adsorption of nitrogen at 90 K produces an intense IR band at 2222 cm−1 attributed to the N---N stretching mode of molecular nitrogen adsorbed on defect sites produced by ion bombardment. Annealing the Ar+ ion sputtered surface to a temperature above 750 K completely suppresses the adsorption of nitrogen at 90 K. Based on these and other results, we postulate that nitrogen adsorbs at 90 K mainly on monovacancies on platinum. We suggest that this specific adsorption occurs by sigma donation from nitrogen to the base of monovacancy sites which possess a low d-electron density compared to surface Pt atoms. 相似文献
3.
The adsorption and decomposition of NO on Pd(110) 总被引:1,自引:0,他引:1
The sticking probability of nitric oxide (NO) on Pd(110) and the relative selectivity of the surface to nitrogen (N2) and nitrous oxide (N2O) production has been measured as a function of coverage and as a function of surface and gas temperatures using a molecular beam. It is found that, at low temperatures (<440 K), molecular adsorption occurs with an initial sticking probability of 0.40 ± 0.02, rising quickly to a maximum of about 0.48 ± 0.02 as coverage increases before falling towards saturation. Following adsorption at 170 K four distinct adsorption sites can be identified by subsequent TPD. Hence, if beaming occurs at a temperature above the TPD peak due to a given site, then that site cannot be populated and the saturation coverage is found to be reduced. At higher temperatures (440–650 K) the sticking probability is seen to decrease continuously as a function of coverage. At a given NO uptake, the sticking probability falls with temperature indicating that the rate of NO desorption is significant in this temperature range. In addition, dissociation occurs leading to the desorption of nitrogen and nitrous oxide leaving only oxygen adatoms on the surface. The oxygen adatoms poison further reaction but can be cleaned off, even at the lowest temperature at which dissociation occurs, by hydrogen or carbon monoxide. At the low temperature end of this range more nitrous oxide is produced than nitrogen but this ratio falls with temperature until, above 600 K, there is 100% selectivity to the production of nitrogen which we propose is due to the low lifetime of molecular NO on the surface. However, at such high temperatures, reaction only occurs on a few sites probably located at the few step edges present on the crystal. 相似文献
4.
The adsorption and reaction of methylacetylene (H3CC≡CH) on Pt(111) and the p(2×2) and
surface alloys were investigated with temperature programmed desorption, Auger electron spectroscopy and low energy electron diffraction. Hydrogenation of methylacetylene to form propylene is the most favored reaction pathway on all three surfaces accounting for ca 20% of the adsorbed monolayer. Addition of Sn to the Pt(111) surface to form these two ordered surface alloys suppresses the decomposition of methylacetylene to surface carbon. The alloy surfaces also greatly increase the amount of reversibly adsorbed methylacetylene, from none on Pt(111) to 60% of the adsorbed layer on the
surface alloy. Methylacetylene reaction also leads to a small amount of desorption of benzene, along with butane, butene, isobutylene and ethylene. There is some difference in the yield of these other reaction products depending the Sn concentration, with the (2×2)-Sn/Pt(111) surface alloy having the highest selectivity for these. Despite previous experiments showing cyclotrimerization of acetylene to form benzene on the Pt–Sn surface alloys, the analogous reaction of methylacetylene on the alloy surfaces was not observed, that is, cyclotrimerization of methylacetylene to form trimethylbenzene. It is proposed that this and the high yield of propylene is due to facile dehydrogenation of methylacetylene because of the relatively weak H–CH2CCH bond compared to acetylene. The desorption of several C4 hydrocarbon products at low (<170 K) temperature indicates that some minor pathway involving C–C bond breaking is possible on these surfaces. 相似文献
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5.
M. Bowker R. P. Holroyd R. G. Sharpe J. S. Corneille S. M. Francis D. W. Goodman 《Surface science》1997,370(2-3):113-124
The adsorption and decomposition of ethanol on Pd(110) has been studied by use of a molecular beam reactor and temperature programmed desorption. It is found that the major pathway for ethanol decomposition occurs via a surface ethoxy to a methyl group, carbon monoxide and hydrogen adatoms. The methyl groups can either produce methane (which they do with a high selectivity for adsorption below 250 K) or can further decompose (which they do with a high selectivity for adsorption above 350 K) resulting in surface carbon. If adsorption occurs above 250 K a high temperature (450 K) hydrogen peak is observed in TPD, resulting from the decomposition of stable hydrocarbon fragments. A competing pathway also exists which involves C---O bond scission of the ethoxy, probably caused by a critical ensemble of palladium atoms at steps, defects or due to a local surface reconstruction. The presence of oxygen does not significantly alter the decomposition pathway above 250 K except that water and, above 380 K, carbon dioxide are produced by reaction of the oxygen adatoms with hydrogen adatoms and adsorbed carbon monoxide respectively. Below 250 K, some ethanol can form acetate which decomposes around 400 K to produce carbon dioxide and hydrogen. 相似文献
6.
Infrared reflection absorption spectroscopy together with mass spectrometry has been used to investigate the interaction of NO and CO on Pt{1 0 0}, initially prepared in the reconstructed ‘hex’ phase, under ambient pressures of these gases, in the temperature range 300-500 K. The results allow the local and total coverages of adsorbed CO and NO to be related to the rate of reaction to produce gas phase CO2, and provide insight into the species present on the surface during the so-called low temperature oscillatory reaction regime of this process. At temperatures below that at which NO dissociation occurs (approximately 390-400 K) adsorption is controlled by the non-reactive displacement of NO by CO and results in a CO-poisoned surface. Above 400 K when significant CO2 production occurs, the NO coverage increases to produce a surface with NO and CO fully intermixed; the increase in NO coverage is attributed to the higher rate of NO arrival from the gas phase (with a partial pressure ratio of PNO:PCO>1) at free surface sites created by NO dissociation and subsequent reaction with CO. The competition between these two processes of non-reactive NO displacement by CO and reactive displacement of CO by NO is proposed to determine the parameter space of the low temperature oscillatory regime. Rapid equilibration between bridged and atop CO species leads to them appearing to exhibit identical reaction behaviour. Particularly at the lowest reaction temperatures (around 400 K), islands of pure CO may coexist on the surface but not participate in the reaction. Under conditions corresponding to the high temperature oscillatory regime, small quantities of absorbed CO, but no NO, are seen on the surface. 相似文献
7.
Thomas Yorisaki Aashani Tillekaratne Chuhei Oshima Shigeki Otani 《Surface science》2009,603(19):3011-1207
The adsorption of carbon monoxide on the LaB6(1 0 0) and LaB6(1 1 1) surfaces was studied experimentally with the techniques of reflection absorption infrared spectroscopy and X-ray photoelectron spectroscopy. The interaction of CO with the two surfaces was also studied with density functional theory. Both surfaces adsorb CO molecularly at low temperatures but in markedly different forms. On the LaB6(1 1 1) surface CO initially adsorbs at 90 K in a form that yields a CO stretching mode at 1502-1512 cm−1. With gentle annealing to 120 K, the CO switches to a bonding environment characterized by multiple CO stretch values from 1980 to 2080 cm−1, assigned to one, two, or three CO molecules terminally bonded to the B atoms of a triangular B3 unit at the (1 1 1) surface. In contrast, on the LaB6(1 0 0) surface only a single CO stretch is observed at 2094 cm−1, which is assigned to an atop CO molecule bonded to a La atom. The maximum intensity of the CO stretch vibration on the (1 0 0) surface is higher than on the (1 1 1) surface by a factor of 5. This difference is related to the different orientations of the CO molecules on the two surfaces and to reduced screening of the CO dynamic dipole moment on the (1 0 0) surface, where the bonding occurs further from the surface plane. On LaB6(1 0 0), XPS measurements indicate that CO dissociates on the surface at temperatures above 400 K. 相似文献
8.
We report a study of silane adsorption on the Si(111)7 × 7 surface. We have been interested in the first stages of chemisorption at room temperature. Reactive sites of the unit cell have been clearly identified on Scanning Tunneling Microscopy (STM) images: the reaction involves the rest atom and the adjacent adatom of the DAS structure with preferential adsorption on the center adatom. We propose an original chemisorption mechanism which leads to the formation of two SiH2 species by chemisorption and involves the breaking of Si---Si backbonds of the adatom. 相似文献
9.
The adsorption and reaction of methanol and ethanol on a preoxidised Cu(100) surface was studied with Reflection-Absorption Infra-Red Spectroscopy (RAIRS). Both alcohols reacted with the modified surface well below room temperature, undergoing OH bond scission to form alkoxide species. The alkoxides were stable up to 340–360 K at which stage they desorbed as aldehydes. Vibrational assignments of the observed modes were made with reference to the RAIRS spectra of the alcohols and their deuterium substituted analogues on Cu(100) together with literature values for the gaseous, liquid and solid alcohols. Application of the metal surface selection rule to the alkoxide spectra indicate that in both cases the C---O bond lies perpendicular to the surface. The methoxy species is therefore assigned to an upright C3v configuration. Oxygen precoverage is found to govern the amount of alkoxide formation which passes through a maximum at a precoverage of about 150 L. 相似文献
10.
Glycine on Pt(111): a TDS and XPS study 总被引:1,自引:0,他引:1
The adsorption and desorption of in situ deposited glycine on Pt(111) were investigated with thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). Glycine adsorbs intact on Pt(111) at all coverages at temperatures below 250 K. The collected results suggest that the glycine molecules adsorb predominantly in the zwitterionic state both in the first monolayer and in multilayers. Upon heating, intact molecules start to desorb from multilayers around 325 K. The second (and possibly third) layer(s) are somewhat more strongly bound than the subsequent layers. The multilayer desorption follows zero order kinetics with an activation energy of 0.87 eV molecule−1. From the first saturated monolayer approximately half of the molecules desorbs intact with a desorption peak at 360 K, while the other half dissociates before desorption. Below 0.25 monolayer all molecules dissociate upon heating. The dissociation reactions lead to H2, CO2, and H2O desorption around 375 K and CO desorption around 450 K. This is well below the reported gas phase decomposition temperature of glycine, but well above the thermal desorption temperatures of the individual H2, CO2, and H2O species on Pt(111), i.e. the dissociation is catalyzed by the surface and H2, CO2, and H2O immediately desorb upon dissociation. For temperatures above 500 K the remaining residues of the dissociated molecules undergo a series of reactions leading to desorption of, for example, H2CN, N2 and C2N2, leaving only carbon left on the surface at 900 K. Comparison with previously reported studies of this system show substantial agreement but also distinct differences. 相似文献
11.
Optical sum-frequency generation (SFG) is used to characterize CO stretching vibrations on Pt(111) and Pt(110) surfaces. Different adsorption sites (terminal, bridge and step sites) are identified in the SFG spectra of CO on Pt(111), in good quantitative agreement with previous infrared reflection-absorption experiments on this system. For CO on Pt(110) we only observe CO molecules on terminal sites. The measured CO stretching vibration frequencies on Pt(110), both for low and high coverages, are at variance with the results of previous infrared studies. Our SFG results for CO on Pt(110) are confirmed by independent EELS measurements which, in addition, also reveal the frustrated rotational mode and the metal-CO vibration. The measured frequency of 2065 cm−1 for low CO coverage on Pt(110)-(1 × 2) is consistent with a previously proposed empirical relation between the frequency of an isolated adsorbed CO molecule and the coordination number of the binding Pt surface atom. 相似文献
12.
The chemical behaviour of 3-hexyne on oxygen modified Ru(0 0 1) surfaces has been analysed under ultrahigh-vacuum, using reflection-absorption infrared spectroscopy (RAIRS). The effects of oxygen coverage, 3-hexyne exposure and adsorption temperature were studied. Two modified Ru(0 0 1) surfaces were prepared: Ru(0 0 1)-(2 × 2)-O and Ru(0 0 1)-(2 × 1)-O that correspond to oxygen coverages (θO) of 0.25 and 0.5 ML, respectively. The striking result is the direct bonding to an O atom when the modified surfaces are exposed to a very low dose (0.2 L) of 3-hexyne at low temperature (100 K). For θO = 0.25 ML, an unsaturated oxametallacycle [Ru-O-C(C2H5)C(C2H5)-Ru] is proposed, identified by RAIRS for the first time, through the νCC and νCO modes. Further decomposition at 110 K yields smaller oxygenated intermediates, such as acetyl [μ3-η2(C,O)-CH3CO], co-adsorbed with a small amount of carbon monoxide and non-dissociated species. The temperature at which a fraction of molecules undergoes complete C-C and C-H bond breaking is thus much lower than on clean Ru(0 0 1). The ultimate decomposition product observed by RAIRS at 220 K is methylidyne [CH]. Another key observation was that the adsorption temperature is not determinant of the reaction route, contrarily to what occurs on clean Ru(0 0 1): even when 3- hexyne strikes the surface at a rather high temperature (220 K), the multiple bond does not break completely. For θO = 0.5 ML, a saturated oxametallacycle [Ru-O-CH(C2H5)-CH(C2H5)-Ru] is also proposed at 100 K, identified by the νasO-C-C (at 1043 cm−1) and νsO-C-C (at 897 cm−1) modes, showing that some decomposition with C-H bond breaking occurs. For this oxygen coverage, the reaction temperatures are lower, and the intermediate surface species are less stable. 相似文献
13.
John W. Peck Daniel I. Mahon David E. Beck Barbara Bansenaur Bruce E. Koel 《Surface science》1998,410(2-3):214-227
The adsorption and reaction of methyl nitrite (CH3ONO, CD3ONO) on Pt(111) was studied using HREELS, UPS, TPD, AES, and LEED. Adsorption of methyl nitrite on Pt(111) at 105 K forms a chemisorbed monolayer with a coverage of 0.25 ML, a physisorbed second layer with the same coverage that desorbs at 134 K, and a condensed multilayer that desorbs at 117 K. The Pt(111) surface is very reactive towards chemisorbed methyl nitrite; adsorption in the monolayer is completely irreversible. CH3ONO dissociates to form NO and an intermediate which subsequently decomposes to yield CO and H2 at low coverages and methanol for CH3ONO coverages above one-half monolayer. We propose that a methoxy intermediate is formed. At least some C–O bond breaking occurs during decomposition to leave carbon on the surface after TPD. UPS and HREELS show that some methyl nitrite decomposition occurs below 110 K and all of the methyl nitrite in the monolayer is decomposed by 165 K. Intermediates from methyl nitrite decomposition are also relatively unstable on the Pt(111) surface since coadsorbed NO, CO and H are formed below 225 K. 相似文献
14.
The adsorption and thermal decomposition of N-methylaniline (NMA) on the Pt(1 1 1) surface has been studied with reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). NMA adsorbs molecularly at 85 K through the nitrogen lone pair and is stable up to 300 K. At temperatures of 300–350 K it converts to two or more surface intermediates including the N-methyleneaniline (NMEA) species. This NMEA intermediate dissociates upon annealing to 450 K, and further annealing leads to the desorption of HCN and H2, leaving only C on the surface at 800 K. 相似文献
15.
The surface chemistry of acrolein and of crotonaldehyde on Pt(111) single-crystal surfaces was investigated under vacuum by temperature-programmed desorption (TPD) and reflection-absorption infrared (RAIRS) spectroscopies. The main thermal decomposition path seen for both compounds was the expected decarbonylation of the unsaturated aldehyde to carbon monoxide and the corresponding olefin (ethene and propene, respectively), but small amounts of propene and ketene were detected in the case of acrolein as well. The RAIRS data indicate that while acrolein initially adsorbs with its plane parallel to the surface and interacts mainly via the carbonyl group, crotonaldehyde adopts a more complex geometry where the main interaction to the metal is via a rehybridization of the C=C double bond. It is suggested here that the changes in adsorption geometry induced by substitutions in the C=C double bond may be responsible for the observed changes in the subsequent reactivity of the adsorbed unsaturated aldehydes. 相似文献
16.
A quantitative analysis of infrared absorption spectra to determine coadsorbate induced relative changes of the vibrational polarizability αv of an adsorbate mode and of the dielectric screening ? due to this extra species is presented. Four (ternary) coadsorption systems consisting of the Ru(0 0 0 1)-(2 × 2)-(X + CO + O) layer with additional coadsorbates X = H, NO, CO, or O (all of them occupying the remaining empty fcc site) have been studied with FT-IRAS, TDS, LEED and work function change measurements. On-top CO is thereby used as a probe molecule to monitor coadsorbate effects on the dielectric properties of the layer. The vibrational polarizability αv associated with the internal C-O stretch mode (νC-O) of on-top CO is lowered by all coadsorbates. The dielectric screening ? within the adsorbate layer is reduced in the presence of the atomic coadsorbates O and H whereas an increase of ? is found for the molecular coadsorbates, threefold coordinated CO and NO. The derived changes of αv and dielectric screening ?, as well as the involved line shifts of νC-O and νRu-CO can be understood in terms of the standard Blyholder backbonding model, i.e. CO 5σ charge donation to the metal combined with a backdonation to electronic states with 2π∗ character. 相似文献
17.
Denise C. Ford 《Surface science》2005,587(3):159-174
The adsorption of several atomic (H, O, N, S, and C) and molecular (N2, HCN, CO, NO, and NH3) species and molecular fragments (CN, CNH2, NH2, NH, CH3, CH2, CH, HNO, NOH, and OH) on the (1 1 1) facet of platinum, an important industrial and fuel cell catalyst, was studied using self-consistent periodic density functional theory (DFT-GGA) calculations at a coverage of 1/4 ML. The best binding site, energy, and position, as well as an estimated diffusion barrier, of each species were determined. The binding strength for all the species can be ordered as follows: N2 < NH3 < HCN < NO < CO < CH3 < OH < NH2 < H < CN < NH < O < HNO < CH2 < NOH < CNH2 < N < S < CH < C. Although the atomic species generally preferred fcc sites, there was no clear trend in site preference by the molecular species or molecular fragments. The vibrational frequencies of all the stable adsorbates in their best and second best adsorption sites were calculated and found to be in good agreement with experimental values reported in the literature. Finally, the decomposition thermochemistry of NOH, HNO, NO, NH3, N2, CO, and CH3 was analyzed. 相似文献
18.
Josep M. Ricart Marie P. Habas Anna Clotet Daniel Curulla Francesc Illas 《Surface science》2000,460(1-3):170-181
The adsorption of CO2 on the clean and potassium-precovered Pt(111) surface has been studied by means of the cluster model approach within the hybrid B3LYP density functional theory-based method. On the clean surface, CO2 is undistorted and weakly bound. The equilibrium position of this physisorbed species appears at a rather large distance from the surface. However, when coadsorbed K atoms are included in the model, a chemisorbed, bent CO2 species on top of a surface Pt atom is found. The presence of the coadsorbed K is found to be necessary to promote CO2 chemisorption and activation, the key step in activating the CO2 molecule being a direct interaction with the coadsorbate. In addition, the calculated vibrational frequencies for this chemisorbed species are in agreement with available experimental data. 相似文献
19.
J. Lauterbach R. W. Boyle M. Schick W. J. Mitchell B. Meng W. H. Weinberg 《Surface science》1996,350(1-3):32-44
The adsorption of CO on Ir(111) has been investigated with Fourier transform infrared reflection-absorption spectroscopy, temperature programmed desorption, and low-energy electron diffraction. At sample temperatures between 90 and 350 K, only a single absorption band, above 2000 cm−1, has been observed at all CO coverages. For fractional coverages above approximately 0.2, the bandwidth becomes as narrow as 5.5 cm−1. The linewidth is attributed mainly to inhomogeneous broadening at low CO coverages and to the creation of electron-hole pairs at higher CO coverages. The coverage-dependent frequency shift of the IR band can be described quantitatively using an improved dipolar coupling model. The contribution of the dipole shift and the chemical shift to the total frequency shift were separated using isotopic mixtures of CO. The chemical shift is positive with a constant value of approximately 12 cm−1 for all coverages, whereas the dipole shift increases with coverage up to a value of 36 cm−1 at a coverage of 0.5 ML. 相似文献
20.
J. Lohmüller H. H. Bertschat H. Granzer H. Haas G. Schatz ISOLDE Collaboration PPE Division CERN CH- Geneva- Switzerland W. -D. Zeitz 《Surface science》1996,360(1-3):213-220
Chemisorption and desorption of isolated bromine adatoms on the Si(100)-(2 × 1) surface were investigated with nuclear methods. Br adsorption sites at low coverages of 10−3 monolayers (ML) were characterised by measuring the nuclear quadrupole interaction with perturbed γγ-angular correlation (PAC) of 77Br→77Se probe atoms. Below room temperature, two distinct adsorption sites for Br are revealed by PAC. One of them disappears after isochronal annealing above 300 K. The more stable probe-atom state is associated with single Br atoms saturating a dangling bond of the surface, while the less stable state is attributed to adsorption of Br at a bridge site. The potential barrier between the two adsorption sites is estimated to be 0.9(1) eV. At temperatures above 550 K, the fraction of atoms on distinct sites decreases, presumably due to surface diffusion. By measuring the γ-activity of the sample, complete desorption of the 77Br atoms was observed above 620 K. 相似文献