Temperature Programmed Desorption State Research Articles

Strong Temperature Dependence in the Reactivity of H2 on RuO2(110).

Understanding the reactivity of H2 is of critical importance in controlling and optimizing many heterogeneous catalytic processes, particularly in cases where its adsorption on the catalyst surface is rate-limiting. In this work, we examine the temperature-dependent adsorption of H2/D2 on the clean RuO2(110) surface using the King and Wells molecular beam approach, temperature-programmed desorption (TPD), and scanning tunneling microscopy (STM). We show that the adsorption probability of H2/D2 on this surface is highly temperature-dependent, decreasing from ∼0.4 below 25 K to <0.01 at 300 K. Both STM and TPD reveal that adsorption (molecular or dissociative) is severely limited once the temperature exceeds the trailing edge temperature of the H2 TPD state (∼150 K). The presence of coadsorbed water or oxygen does not appear to alter this situation. Previous literature reports of extensive RuO2(110) surface hydroxylation from H2/D2 exposures at 300 K may instead be the result of background contamination brought about by chamber backfilling.

Methylene bromide chemistry and photochemistry on rutile TiO 2(110)

The chemistry and photochemistry of methylene bromide (CD 2Br 2) on the rutile TiO 2(110) surface was probed using temperature programmed desorption (TPD). CD 2Br 2 desorbed in three desorption states at 145, 160 and 250 K tentatively assigned to desorption from the multilayer, from an η 1-CD 2Br 2 species and a bridging η 2-CD 2Br 2 species, respectively. The latter two TPD states presumably involve binding of CD 2Br 2 molecules to the surface through Br coordination at five-coordinate Ti 4+ surface sites. The 160 and 250 K TPD states saturated at coverages of 1.0 and 0.33 ML, respectively, where 1 ML is equivalent to the surface Ti 4+ site density (5.2 × 10 14 cm − 2 ). No thermal decomposition of CD 2Br 2 was observed on either the clean surface or with preadsorbed O 2. UV irradiation of CD 2Br 2 on TiO 2(110) resulted in predominately photodesorption, with trace amounts of photodecomposition evidenced in TPD. The rate of CD 2Br 2 photodesorption from TiO 2(110) occurred with a low cross section (~ 2 × 10 − 21 cm 2) similar to that expected from direct optical excitation of CD 2Br 2. This observation suggests that charge carriers generated in TiO 2(110) were no more effective in activating adsorbed CD 2Br 2 molecules than would be expected through direct molecular excitation. These findings suggest that photocatalytic destruction of halocarbons such as CD 2Br 2 on TiO 2 may preferentially occur though indirect processes (such as OH radical attack) as opposed to direct electron transfer processes involving charge carriers generated in TiO 2 by bandgap excitation.

Photochemistry of methyl bromide on the α-Cr 2O 3(0001) surface

The photochemical properties of the Cr-terminated α-Cr 2O 3(0001) surface were explored using methyl bromide (CH 3Br) as a probe molecule. CH 3Br adsorbed and desorbed molecularly from the Cr-terminated α-Cr 2O 3(0001) surface without detectable thermal decomposition. Temperature programmed desorption (TPD) revealed a CH 3Br desorption state at 240 K for coverages up to 0.5 ML, followed by more weakly bound molecules desorbing at 175 K for coverages up to 1 ML. Multilayer exposures led to desorption at ~ 130 K. The CH 3Br sticking coefficient was unity at 105 K for coverages up to monolayer saturation, but decreased as the multilayer formed. In contrast, pre-oxidation of the surface (using an oxygen plasma source) led to capping of surface Cr 3+ sites and near complete removal of CH 3Br TPD states above 150 K. The photochemistry of chemisorbed CH 3Br was explored on the Cr-terminated surface using post-irradiation TPD and photon stimulated desorption (PSD). Irradiation of adsorbed CH 3Br with broad band light from a Hg arc lamp resulted in both photodesorption and photodecomposition of the parent molecule at a combined cross section of ~ 10 − 22 cm 2. Photodissociation of the CH 3–Br bond was evidenced by both CH 3 detected in PSD and Br atoms left on the surface. Use of a 385 nm cut-off filter effectively shut down the photodissociation pathway but not the parent molecule photodesorption process. From these observations it is inferred that d-to-d transitions in α-Cr 2O 3, occurring at photon energies < 3 eV, do not significantly promote photodecomposition of adsorbed CH 3Br. It is unclear to what extent band-to-band versus direct CH 3Br photolysis play in CH 3–Br bond dissociation initiated by more energetic photons.

Photochemistry of 1,1,1-Trifluoroacetone on Rutile TiO2(110)

The ultraviolet (UV) photon-induced photodecomposition of 1,1,1-trifluoroacetone (TFA) adsorbed on the rutile TiO2(110) surface has been investigated with photon stimulated desorption (PSD), temperature programmed desorption (TPD), and density functional theory (DFT). TFA adsorbed molecularly on the reduced surface (8% oxygen vacancies) in states desorbing below 300 K with trace thermal decomposition observed in TPD. Adsorption of TFA on a preoxidized TiO2(110) surface (accomplished by pre-exposure with 20 L O2) led to formation of a new TFA desorption state at 350 K, assigned to decomposition of a TFA-diolate species ((CF3)(CH3)COO). No TFA photochemistry was detected on the reduced surface. UV irradiation of TFA on the oxidized surface depleted TFA in the 350 K state with TFA molecules in other TPD states unaffected. PSD measurements revealed that both carbonyl substituents (CH3 and CF3), as well as CO, were liberated during UV exposure at 95 K. Postirradiation TPD showed evidence for both acetate (evolving as ketene at 650 K) and trifluoroacetate (evolving as CO2 at 600 K) as surface-bound photodecomposition products. The CO PSD product was not due to adsorbed CO, to mass spectrometer cracking of a CO-containing PSD product, or from background effects, but originated from complete fragmentation of an unidentified adsorbed TFA species. Thermodynamic analysis using DFT indicated that the photodecomposition of the TFA-diolate was likely not driven by thermodynamics alone as both pathways (CH3 + trifluoroacetate and CF3 + acetate) were detected when thermodynamics shows a clear preference for only one (CF3 + acetate). These observations are in contrast to the photochemical behavior of acetone, butanone, and acetaldehyde on TiO2(110), where only one of the two carbonyl substituent groups was observed, with a stoichiometric amount of carboxylate containing the other substituent left on the surface. We conclude that fluorination significantly alters the electronic structure of adsorbed carbonyls on TiO2(110) in such a way as to promote multiple channels of photofragmentation. Factors that dictate the partitioning between the three TFA channels are not related to photon energy (above that of the TiO2 band gap) but likely to the electronic structure of the charge transfer excited state.

Acetone Chemistry on Oxidized and Reduced TiO2(110)

The chemistry of acetone on the oxidized and reduced surfaces of TiO2(110) was examined using temperature-programmed desorption (TPD) and high-resolution electron energy-loss spectroscopy (HREELS). The reduced surface was prepared with about 7% oxygen vacancy sites by annealing in ultrahigh vacuum (UHV) at 850 K, and the oxidized surface was prepared by exposure of the reduced surface to molecular oxygen at 95 K followed by heating the surface to a variety of temperatures between 200 and 500 K. Acetone adsorbs molecularly on the reduced surface with no evidence for either decomposition or preferential binding at vacancy sites. On the basis of HREELS, the majority of acetone molecules adsorbed in an η1 configuration at Ti4+ sites through interaction of lone-pair electrons on the carbonyl oxygen atom. Repulsive acetone−acetone interactions shift the desorption peak from 345 K at low coverage to 175 K as the first layer saturates with a coverage of ∼1 ML. In contrast, about 7% of the acetone adlayer decomposes when the surface is pretreated with molecular oxygen. Acetate is among the detected decomposition products but only comprises about one-third of the amount of decomposed acetone, and its yield depends on the temperature at which the O2-exposed surface was preheated to prior to acetone adsorption. Aside from the small level of irreversible decomposition, about 0.25 ML of acetone is stabilized to 375 K by coadsorbed oxygen. These acetone species exhibit an HREELS spectrum unlike that of η1-acetone or of any other species proposed to exist from the interaction of acetone with TiO2 powders. On the basis of the presence of extensive 16O/18O exchange between acetone and coadsorbed oxygen in the 375 K acetone TPD state, it is proposed that an acetone−oxygen complex forms on the TiO2(110) surface through nucleophilic attack of oxygen on the carbonyl carbon atom of acetone. The reaction between acetone and oxygen is initiated at temperatures as low as 135 K based on HREELS. The major decomposition pathway of the acetone−oxygen complex liberates acetone in the 375 K TPD peak. This species may be a key intermediate in acetone thermal and photolytic chemistry on TiO2 surfaces.

Photodesorption and Trapping of Molecular Oxygen at the TiO2(110)−Water Ice Interface

By means of temperature programmed desorption (TPD), static secondary ion mass spectroscopy (SSIMS), and electron energy loss spectroscopy (EELS), we have investigated further the states of oxygen adsorbed on rutile TiO2. Previous work has shown that annealing the (110) surface in a vacuum produces isolated bridging oxygen vacancies, and that these vacancies are intimately connected with molecular and dissociative oxygen adsorption channels. We find that at 115 K illumination of the oxygen-exposed surface with 4.1 eV photons results in photodesorption of the oxygen associated with a TPD state centered on 410 K, in contrast to the remaining oxygen destined for a dissociative channel. An unusual effect of water overlayers on the photochemical properties of the O/TiO2(110) system is explored. For thick overlayers, it is possible to generate via UV irradiation a previously unobserved oxygen TPD state. Evidence is presented for this new O2 TPD state originating from the photolysis of an O2−water adduct.

Three-dimensional view of the thermal desorption reaction: copper on rhenium(0001)

The application of the Polanyi–Wigner equation to describe the rate R des of a thermal desorption process from a homogeneous surface is common practice in surface science temperature-programmed desorption (TPD) studies. Current evaluation methods (known as line-shape analyses) deduce from series of thermal desorption traces [coverage ( Θ)-dependent] energetic and kinetic parameters such as desorption energy, desorption order or frequency factor. The TPD spectra themselves, i.e., the function R des( T), as well as the output of such an analysis (e.g., the remaining coverage Θ res as a function of T for a given initial coverage), represent parametrized two-dimensional (2D) graphs. It will be shown in this work by means of selected examples [taken from noble metal desorption from a Re(0001) surface] that the 2D graphs are nothing but projections of R des on to either a rate–temperature or a rate–coverage plane. By using the three-dimensional (3D) representation of R des in T, Θ space, valuable information on phase transitions occurring during the desorption process, on multiple TPD states and interlayer interactions can easily be visualized and exploited.

Interaction of Molecular Oxygen with the Vacuum-Annealed TiO2(110) Surface: Molecular and Dissociative Channels

We have examined the interaction of molecular oxygen with the TiO2(110) surface using temperature-programmed desorption (TPD), isotopic labeling studies, sticking probability measurements, and electron energy loss spectroscopy (ELS). Molecular oxygen does not adsorb on the TiO2(110) surface in the temperature range between 100 and 300 K unless surface oxygen vacancy sites are present. These vacancy defects are generated by annealing the crystal at 850 K, and can be quantified reliably using water TPD. Adsorption of O2 at 120 K on a TiO2(110) surface with 8% oxygen vacancies (about 4 × 1013 sites/cm2) occurs with an initial sticking probability of 0.5−0.6 that diminishes as the surface is saturated. The saturation coverage at 120 K, as estimated by TPD uptake measurements, is approximately three times the surface vacancy population. Coverage-dependent TPD shows little or no O2 desorption below a coverage of 4 × 1013 molecules/cm2 (the vacancy population), presumably due to dissociative filling of the vacancy sites in a 1:1 ratio. Above a coverage of 4 × 1013 molecules/cm2, a first-order O2 TPD peak appears at 410 K. Oxygen molecules in this peak do not scramble oxygen atoms with either the surface or with other coadsorbed oxygen molecules. Sequential exposures of 16O2 and 18O2 at 120 K indicate that each adsorbed O2 molecule, irrespective of its adsorption sequence, has equivalent probabilities with respect to its neighbors to follow the two channels (molecular and dissociative), suggesting that O2 adsorption is not only precursor-mediated, as the sticking probability measurements indicate, but that all O2 molecules reside in this precursor state at 120 K. This precursor state may be associated with a weak 145 K O2 TPD state observed at high O2 exposures. ELS measurements suggest charge transfer from the surface to the O2 molecule based on disappearance of the vacancy loss feature at 0.8 eV, and the appearance of a 2.8 eV loss that can be assigned to an adsorbed O2- species based on comparisons with Ti−O2 inorganic complexes in the literature. Utilizing results from recent spin-polarized DFT calculations in the literature, we propose a model where three O2 molecules are bound in the vicinity of each vacancy site at 120 K. For adsorption temperatures above 150 K, the dissociation channel completely dominates and the surface adsorbs oxygen in a 1:1 ratio with each vacancy site. ELS measurements indicate that the vacancies are filled, and the remaining oxygen adatom, which is apparent in TPD, is transparent in ELS. On the basis of the variety of oxygen adsorption states observed in this study, further work is needed in order to determine which oxygen-related species play important roles in chemical and photochemical oxidation processes on TiO2 surfaces.

High-resolution electron energy loss spectroscopy evidence for electron beam-induced decomposition of trimethylsilane adsorbed on Si(100)

The effects of incident electrons on trimethylsilane ((CH 3) 3SiH) adsorbed onto Si(100) at 110 K are examined using high-resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). TPD from trimethylsilane-covered Si(100) show hydrogen desorption from silicon monohydride states at 780 K ( β 1); at high trimethylsilane exposures, two low-temperature (130 K and 155 K) molecular TPD states appear. The absence of silicon dihydride species on the trimethylsilane covered Si(100) surface was demonstrated by comparing the data obtained from this surface with that obtained from disilane-covered Si(100). HREELS and TPD data indicate that carbon deposition on Si(100) is enhanced by irradiating the trimethylsilane covered surface only when the molecular states are present.

Evidence for oxygen adatoms on TiO 2(110) resulting from O 2 dissociation at vacancy sites

Annealing TiO 2(110) in vacuum at high temperature (above about 800 K) generates oxygen vacancy sites that are associated with reduced surface cations. Numerous studies have shown that these sites can be oxidized by exposure to molecular oxygen, but the mechanism and temperature dependence of this oxidation process are not well understood. We present results that suggest low temperature (<600 K) O 2 exposure oxidizes oxygen vacancies but also leaves oxygen-containing species on the surface that we propose are oxygen adatoms. The presence of these oxygen adatoms is evident in the temperature-programmed desorption (TPD) spectrum of water. Oxidizing the vacuum annealed surface at 700 K produces a fully oxidized TiO 2(110) surface that gives a single monolayer TPD state for water at 270 K. Exposing the vacuum annealed surface to O 2 at temperatures between 90 and 600 K followed by water adsorption at 90 K results in a new water TPD state 25 K higher in temperature. Similar results were obtained using ammonia instead of water. Isotopic labeling experiments, in which the vacuum annealed surface was dosed with 18O 2 at 135 K followed by H 2 16O at 135 K, indicate that the new water TPD state results from recombinative desorption, whereas no such effect is observed for the surface exposed to 18O 2 at 700 K. The effect on water is also absent in TPD if the low temperature O 2 treated surface is heated to 600 K prior to water adsorption at 90 K, suggesting that the oxygen adatoms desorb from the surface or diffuse into the bulk. We propose that at low temperatures, O 2 dissociates at oxygen vacancies filling each defect site with one O atom and depositing a second O adatom at a five-coordinate Ti 4+ site or that O 2 interacts with surface hydroxyl groups resulting in O 2 dissociation and the presence of the O adatom. The new dissociative water chemistry results from the interaction of water molecules with these oxygen adatoms. After high temperature (>600 K) O 2 exposure, no dissociative water chemistry is observed, suggesting that these oxygen adatoms are not present on the surface. The presence of surface O adatoms may explain inconsistencies in the literature regarding the reactivity of water, and potentially other species, on TiO 2(110). These results also detail the importance of sample preparation techniques on the chemistry which can occur at a solid surface.

Electron-induced decomposition of methanol on the vacuum-annealed surface of TiO 2(110)

The 100 eV electron-induced decomposition (EID) of methanol adsorbed on the vacuum-annealed surface of TiO 2(110) at 135 K was examined with temperature-programmed desorption (TPD) and electron-stimulated desorption (ESD). By annealing at 850 K, a TiO 2(110) surface was reproducibly prepared with an oxygen vacancy coverage of about 0.08 ML (where 1 ML=5.2×10 14 sites cm −2). In the absence of electron irradiation, CH 3OH adsorbed on the vacuum-annealed surface in three main TPD states: a molecular state at 295 K and two dissociative states at 350 and 480 K. The 480 K state was assigned to methoxyls at oxygen vacancy sites, and the 350 K state was due to methoxyls at non-vacancy sites. The surface coverages in these states for the saturated monolayer were 0.40 ML (295 K), 0.15 ML (350 K) and 0.08 ML (480 K). Although CH 3OH dissociated on the surface, no irreversible decomposition was observed, and CH 3OH was the only desorption product in TPD. By heating a multilayer CH 3OH exposure to 197, 310 and 410 K, followed by recooling to 135 K, methanol adlayers could be prepared containing only the saturated monolayer, only both types of methoxyl and only the methoxyls at vacancies, respectively. Given these preparation conditions, the 100 eV EID of each methanol-related species was examined. Using CD 3OD, the major positive ESD ions detected from multilayer methanol were D +, O +/CD 2 + and OD +/CD 3 + (the latter were mostly O + and OD + based on results with CH 3OH) with weaker signals from C +, CD +, CO +, DCO + and CD 2OD +. However, the monolayer gave D + and O +, with weak signals from OD + and CD +. The EID cross-section for molecularly adsorbed CH 3OH (1.7×10 −16 cm 2) was only a factor of three less than the literature values for the total dissociative cross-section in the gas phase suggesting that the TiO 2(110) surace had little or no influence on the dissociative ionization process. No carbon-containing surface products were detected in post-irradiation TPD associated with EID of molecular CH 3OH, including no additional methoxyl formation. The initial EID cross-sections for the two types of methoxyls were approximately equivalent regardless of the surface condition, but were a factor of 5 greater in the presence of CH 3OH (3.0–3.4×10 −15 cm 2) than in its absence (5.8–6.2×10 −16 cm 2). EID of both vacancy and non-vacancy methoxyl resulted in H 2CO products bound at the same sites, but vacancy-bound H 2CO was resistant to further EID, whereas non-vacancy H 2CO was decomposed with further electron exposure. Total D + ESD cross-sections were several orders of magnitude lower than those measured by post-irradiation TPD, suggesting that the major EID channels involved ejection of neutral species. These results demonstrated the ability of low-energy electrons to active organics adsorbed on oxide surfaces with high cross-sections, and suggest that the EID cross-sections and products for surface organics depend on the coverage, adsorption state and adsorption site as in the case of methanol in TiO 2(110). Based on these conclusions, low-energy electrons produced from adsorption of ionizing radiation may play a significant role in the radiocatalytic destruction of organics over oxide catalysts.

Evidence for bicarbonate formation on vacuum annealed TiO 2(110) resulting from a precursor-mediated interaction between CO 2 and H 2O

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 ν a(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.

Effects of low-energy electron irradiation on submonolayer ammonia adsorbed on Pt(111)

The effects of electron impact on ammonia-covered Pt(111) have been studied using temperature-programmed desorption (TPD) and electron-stimulated desorption (ESD). For coverages below one monolayer, ammonia adsorbs on the surface in two distinct TPD states: the α-state is broad and desorbs over the temperature range 150–350 K, and the β-state appears as a sharper peak at 150 K. The β-state was seen to be damaged by electron-beam impact much more readily than the α-state, resulting in the formation of atomically adsorbed N on the surface. The mass 28 recombinative nitrogen desorption TPD peak appearing at 550 K exhibited second-order desorption kinetics, further confirming the presence of atomically adsorbed nitrogen. The ESD kinetic energy distributions (KEDs) were obtained for m/e=1 amu, which exhibited broad peaks generally. The H+ KEDs were analyzed using empirical curve fits, with the resulting conclusion that the H+ KEDs contain contributions from at least three different hydrogen-containing surface species. We believe that these three H+ KED peaks are due to ESD from adsorbed NH3, NH2 and H. The ESD cross-section for NH3 removal was measured in three different ways, all of which were found to be in general agreement, and which gave an averaged cross-section value of Qtot=4×10−17 cm2. © 1998 John Wiley & Sons, Ltd.

Effects of low‐energy electron irradiation on submonolayer ammonia adsorbed on Pt(111)

The effects of electron impact on ammonia-covered Pt(111) have been studied using temperature-programmed desorption (TPD) and electron-stimulated desorption (ESD). For coverages below one monolayer, ammonia adsorbs on the surface in two distinct TPD states: the α-state is broad and desorbs over the temperature range 150–350 K, and the β-state appears as a sharper peak at 150 K. The β-state was seen to be damaged by electron-beam impact much more readily than the α-state, resulting in the formation of atomically adsorbed N on the surface. The mass 28 recombinative nitrogen desorption TPD peak appearing at 550 K exhibited second-order desorption kinetics, further confirming the presence of atomically adsorbed nitrogen. The ESD kinetic energy distributions (KEDs) were obtained for m/e=1 amu, which exhibited broad peaks generally. The H+ KEDs were analyzed using empirical curve fits, with the resulting conclusion that the H+ KEDs contain contributions from at least three different hydrogen-containing surface species. We believe that these three H+ KED peaks are due to ESD from adsorbed NH3, NH2 and H. The ESD cross-section for NH3 removal was measured in three different ways, all of which were found to be in general agreement, and which gave an averaged cross-section value of Qtot=4×10−17 cm2. © 1998 John Wiley & Sons, Ltd.

Adsorption and decomposition of formic acid on the NiO(111)-p(2 × 2) surface: TPD and steady state kinetics studies

The adsorption and decomposition of formic acid on NiO(111)-p(2 × 2) films grown on Ni(111) single crystal surface were studied by temperature-programmed desorption (TPD) spectroscopy. Exposure of formic acid at 163 K resulted in both molecular adsorption and dissociation to formate. The adsorbed formate underwent further dissociation to H 2, CO 2 and CO. H 2 and CO 2 desorbed at the same temperatures of 340, 390 and 520 K, while CO desorbed at 415 and 520 K. The desorption features varied with the formic acid exposure. Two reaction channels were identified for the decomposition of formate under equilibrium with gas-phase formic acid with a pressure of 2.5 × 10 −4Pa, one preferentially producing H 2 and CO 2 with an activation energy of 22 ± 2 kJ mol −1 and the other preferentially producing CO and H 2O with an activation energy of 16 ± 2 kJ mol −1. The order of both reaction paths was 0.5 with respect to the pressure of formic acid.

Structural Sensitivity in the Dissociation of Water on TiO2 Single-Crystal Surfaces

Temperature-programmed desorption (TPD) and oxygen isotopic labeling studies were used to probe the dissociation of water on the (100) and (110) surfaces of TiO2 (rutile). Water TPD spectra from these two surfaces were distinctive. Three monolayer desorption states were observed for the (100) surface (at 205, 250, and 305 K), while only a single desorption state was observed for the (110) surface (at 270 K). TPD experiments on the surfaces enriched with 18O revealed that water desorbing in the 305 K state from TiO2(100) was isotopically scrambled with the lattice oxygen atoms, strongly suggesting that this TPD state resulted from recombination of surface hydroxyl groups. Isotopic scrambling was not observed for any other desorption state on either surface (in the absence of defects). Since very little water desorption occurred from the (110) surface in the temperature range in which exchange was observed on the (100) surface and since previous HREELS work (Henderson, M. A. Surf. Sci. 1996, 355, 151) indic...

Comparison of H2 Desorption Kinetics from Si(111)7×7 and Si(100)2×l

ABSTRACTThe desorption kinetics of hydrogen from the β1 H2 -TPD state on Si(111)7×7 and Si(100)2×l were studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. Isothermal LITD studies of H2 desorption from Si(111)7×7 revealed second-order kinetics with a desorption activation energy of Ed = 62 ±4 kcal/mol and a preexponential factor of Vd = 92 ±10 cm2 /s. In contrast, H2 desorption from Si(100)2×l revealed first-order kinetics with an activation energy of Ed = 58 ±2 kcal/mol and a preexponential factor of Vd = 5.5 ±0.5 × 1015 s−1. The desorption kinetics yield similar upper limits for the Si-H bond energies but different desorption mechanisms on Si(lll)7×7 and Si(100)2×l.

Adsorption of H 2, N 2, NH 3 and PF 3 on ultra-thin Fe films on W(100)

The morphologies of ultra-thin Fe films (0.1 to 10 monolayer) on W(100) and the chemisorption properties of H 2, N 2, NH 3 and PF 3on these films have been investigated by Auger electron spectroscopy (AES), temperature programmed desorption (TPD) and low energy electron diffraction (LEED). For ⩽ 1 ML coverage dosed at T < 200 K, Fe forms an epitaxial overlayer on W(100) and annealing to 1100 K does not change the AES, LEED and chemisorption properties. Above 1 ML, Fe grows layer-by-layer at 200 K. However, annealing to 1100 K results in the formation of three-dimensional Fe islands over the first monolayer and alters the AES, LEED, and chemisorption properties. Generally, the overlayers with ⩽ 2 ML Fe coverage do not behave like a superposition of pure Fe and W but have unique properties. On monolayer Fe films there is an increase in both the adsorption bond strength and the sticking probability of N 2 compared to bulk Fe. For H 2, there is a high temperature TPD state attributed to absorption of H at the metal-metal interface between Fe and W.

Potassium and its coadsorption with carbon monoxide on Pt(111): A SSIMS/TPD study

Temperature programmed desorption (TPD) and static secondary ion mass spectrometry (SSIMS) have been used to study the chemisorption properties of carbon monoxide on potassium-predosed Pt(111). Nonexponential secondary ion yields of potassium containing ions indicate a local interaction between potassium and nearest-neighbor CO molecules. In TPD, two different CO desorption states are observed in the presence of potassium. The high temperature CO TPD state is accompanied by simultaneous potassium desorption. The magnitude of the coincident potassium and CO TPD peak is related to the amount of CO in the high temperature state. Long-range (beyond nearest-neighbor) as well as short-range (nearest-neighbor) potassium effects on adsorbed CO are supported.

The influence of adsorbate–adsorbate interactions on surface structure: The coadsorption of CO and H2 on Rh(100)

The coadsorption of CO and H2 on Rh(100) at 100 K has been studied using temperature programmed desorption (TPD), low energy electron diffraction (LEED), electron energy loss spectroscopy (EELS), and temperature programmed EELS (TP-EELS). The preferred binding sites, long ranged order, and degree of segregation are dependent on the order of adsorption. When H2 is exposed to a CO preexposed surface, segregation of the surface species (atomic H and CO) is observed. The postdosed H2 causes isolated CO molecules to change from the top site to the bridge site, and compresses the c(2×2) CO islands that develop during the CO preexposure. When CO is exposed to a H2 saturated surface (one hydrogen per surface Rh atom) an intimately mixed c(2×2) CO and H structure is formed with all the CO molecules occupying the top site. Strong repulsive CO–H interactions in this mixed adlayer result in two new low temperature H2 TPD states. During the desorption of the lowest temperature H2 TPD peak, the c(2×2) LEED pattern streaks and the CO molecules shift from the top site to the bridge site. It is proposed that the preferred binding site for hydrogen in the c(2×2) bridge bonding CO structure is different from the fourfold hollow site preferred in the c(2×2) top bound CO structure. After the second H2 TPD peak, the remaining adsorbed H and CO segregate, and the CO regions are compressed. The compression is relaxed as the hydrogen desorbs. The development of the surface structures and their influence on the H2 TPD can be qualitatively understood in terms of precursor adsorption of both CO on H covered surfaces and H2 on CO covered surfaces, strong CO–H repulsions, and local poisoning of H2 dissociation by CO.