The adsorption and desorption of ethanol ices from a model grain surface

Reflection absorption infrared spectroscopy (RAIRS) and temperature programed desorption (TPD) have been used to probe the adsorption and desorption of ethanol on highly ordered pyrolytic graphite (HOPG) at 98 K. RAIR spectra for ethanol show that it forms physisorbed multilayers on the surface at 98 K. Annealing multilayer ethanol ices (exposures >50 L) beyond 120 K gives rise to a change in morphology before crystallization within the ice occurs. TPD shows that ethanol adsorbs and desorbs molecularly on the HOPG surface and shows four different species in desorption. At low coverage, desorption of monolayer ethanol is observed and is described by first-order kinetics. With increasing coverage, a second TPD peak is observed at a lower temperature, which is assigned to an ethanol bilayer. When the coverage is further increased, a second multilayer, less strongly bound to the underlying ethanol ice film, is observed. This peak dominates the TPD spectra with increasing coverage and is characterized by fractional-order kinetics and a desorption energy of 56.3+/-1.7 kJ mol(-1). At exposures exceeding 50 L, formation of crystalline ethanol is also observed as a high temperature shoulder on the TPD spectrum at 160 K.     

Ni/Pt(111) bimetallic surfaces: unique chemistry at monolayer ni coverage.

We have utilized the dehydrogenation and hydrogenation of cyclohexene as probe reactions to compare the chemical reactivity of Ni overlayers that are grown epitaxially on a Pt(111) surface. The reaction pathways of cyclohexene were investigated using temperature-programmed desorption, high-resolution electron energy loss (HREELS), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Our results provide conclusive spectroscopic evidence that the adsorption and subsequent reactions of cyclohexene are unique on the monolayer Ni surface as compared to those on the clean Pt(111) surface or the thick Ni(111) film. HREELS and NEXAFS studies show that cyclohexene is weakly pi-bonded on monolayer Ni/Pt(111) but di-sigma-bonded to Pt(111) and Ni(111). In addition, a new hydrogenation pathway is detected on the monolayer Ni surface at temperatures as low as 245 K. By exposing the monolayer Ni/Pt(111) surface to D2 prior to the adsorption of cyclohexene, the total yield of the normal and deuterated cyclohexanes increases by approximately 5-fold. Furthermore, the reaction pathway for the complete decomposition of cyclohexene to atomic carbon and hydrogen, which has a selectivity of 69% on the thick Ni(111) film, is nearly negligible (<2%) on the monolayer Ni surface. Overall, the unique chemistry of the monolayer Ni/Pt(111) surface can be explained by the weaker interaction between adsorbates and the monolayer Ni film. These results also point out the possibility of manipulating the chemical properties of metals by controlling the overlayer thickness.     

Experimental and theoretical study of reactivity trends for methanol on Co/Pt(111) and Ni/Pt(111) bimetallic surfaces

Methanol was used as a probe molecule to examine the reforming activity of oxygenates on NiPt(111) and CoPt(111) bimetallic surfaces, utilizing density functional theory (DFT) modeling, temperature-programmed desorption, and high-resolution electron energy loss spectroscopy (HREELS). DFT results revealed a correlation between the methanol and methoxy binding energies and the surface d-band center of various NiPt(111) and CoPt(111) bimetallic surfaces. Consistent with DFT predictions, increased production of H2 and CO from methanol was observed on a Ni surface monolayer on Pt(111), designated as Ni-Pt-Pt(111), as compared to the subsurface monolayer Pt-Ni-Pt(111) surface. HREELS was used to verify the presence and subsequent decomposition of methoxy intermediates on NiPt(111) and CoPt(111) bimetallic surfaces. On Ni-Pt-Pt(111) the methoxy species decomposed to a formaldehyde intermediate below 300 K; this species reacted at approximately 300 K to form CO and H2. On Co-Pt-Pt(111), methoxy was stable up to approximately 350 K and decomposed to form CO and H2. Overall, trends in methanol reactivity on NiPt(111) bimetallic surfaces were similar to those previously determined for ethanol and ethylene glycol.     

Water adsorption, desorption, and clustering on FeO(111)

The adsorption of water on FeO(111) is investigated using temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). Well-ordered 2 ML thick FeO(111) films are grown epitaxially on a Pt(111) substrate. Water adsorbs molecularly on FeO(111) and desorbs with a well resolved monolayer peak. IRAS measurements as a function of coverage are performed for water deposited at 30 and 135 K. For all coverages (0.2 ML and greater), the adsorbed water exhibits significant hydrogen bonding. Differences in IRAS spectra for water adsorbed at 30 and 135 K are subtle but suggest that water adsorbed at 135 K is well ordered. Monolayer nitrogen TPD spectra from water covered FeO(111) surfaces are used to investigate the clustering of the water as a function of deposition or annealing temperature. Temperature dependent water overlayer structures result from differences in water diffusion rates on bare FeO(111) and on water adsorbed on FeO(111). Features in the nitrogen TPD spectra allow the monolayer wetting and 2-dimensional (2D) ordering of water on FeO(111) to be followed. Voids in a partially disordered first water layer exist for water deposited below 120 K and ordered 2D islands are found when depositing water above 120 K.     

Adsorption and thermal decomposition of 2-octylthieno[3,4-b]thiophene on Au(111)

The adsorption and thermal stability of 2-octylthieno[3,4-b]thiophene (OTTP) on the Au(111) surfaces have been studied using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). UHV-STM studies revealed that the vapor-deposited OTTP on Au(111) generated disordered adlayers with monolayer thickness even at saturation coverage. XPS and TPD studies indicated that OTTP molecules on Au(111) are stable up to 450K and further heating of the sample resulted in thermal decomposition to produce H(2) and H(2)S via C-S bond scission in the thieno-thiophene rings. Dehydrogenation continues to occur above 600K and the molecules were ultimately transformed to carbon clusters at 900K. Highly resolved air-STM images showed that OTTP adlayers on Au(111) prepared from solution are composed of a well-ordered and low-coverage phase where the molecules lie flat on the surface, which can be assigned as a (9×2√33)R5° structure. Finally, based on analysis of STM, TPD, and XPS results, we propose a thermal decomposition mechanism of OTTP on Au(111) as a function of annealing temperature.     

Site competition during coadsorption of acetone with methanol and water on TiO2(110)

The competitive interaction between acetone and two solvent molecules (methanol and water) for surface sites on rutile TiO(2)(110) was studied using temperature-programmed desorption (TPD). On a vacuum-annealed TiO(2)(110) surface, which possessed ~5% oxygen vacancy sites, excess methanol displaced preadsorbed acetone molecules to weakly bound and physisorbed desorption states below 200 K. In contrast, acetone molecules were stabilized on an oxidized surface against displacement by methanol through formation of acetone diolate species. The behavior of acetone with methanol differs from the interactions between acetone and water which are less competitive. Examination of acetone + methanol and acetone + water multilayer combinations shows that acetone is more compatible in water-ice films than in methanol-ice films, presumably because water has greater potential as a hydrogen-bond donor than does methanol. Acetone molecules displaced from the TiO(2)(110) surface by water are more likely to be retained in the near-surface region, in turn having a greater opportunity to revisit the surface, than when methanol is used as a coadsorbate.     

Crystalline ice growth on Pt(111) and Pd(111): nonwetting growth on a hydrophobic water monolayer

The growth of crystalline ice films on Pt(111) and Pd(111) is investigated using temperature programed desorption of the water films and of rare gases adsorbed on the water films. The water monolayer wets both Pt(111) and Pd(111) at all temperatures investigated [e.g., 20-155 K for Pt(111)]. However, crystalline ice films grown at higher temperatures (e.g., T>135 K) do not wet the monolayer. Similar results are obtained for crystalline ice films of D2O and H2O. Amorphous water films, which initially wet the surface, crystallize and dewet, exposing the water monolayer when they are annealed at higher temperatures. Thinner films crystallize and dewet at lower temperatures than thicker films. For samples sputtered with energetic Xe atoms to prepare ice crystallites surrounded by bare Pt(111), subsequent annealing of the films causes water molecules to diffuse off the ice crystallites to reform the water monolayer. A simple model suggests that, for crystalline films grown at high temperatures, the ice crystallites are initially widely separated with typical distances between crystallites of approximately 14 nm or more. The experimental results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on both Pt(111) and Pd(111).     

Two liquid phases of water in the deeply supercooled region and their roles in crystallization and formation of LiCl solution

The properties of supercooled liquid water and the mechanism of crystallization in it were investigated using time-of-flight secondary ion mass spectrometry and reflection absorption infrared spectroscopy. The self-diffusion of the water molecules commences at 136 K, and then the liquid-liquid phase transition occurs at 160-165 K. The latter is evidenced not only by the occurrence of fluidity but also by the formation of a LiCl solution. The infrared absorption band also changes drastically above 160 K due to crystallization of water (on the Au film) and the formation of LiCl solution (on the LiCl film). The immediate crystallization and dissolution of LiCl are thought to be characteristic of normal water that is created in a deeply supercooled region, indicating that viscous liquid water (T > 136 K) is transformed into supercooled liquid water at around 160 K. The crystallization kinetics is different between these two phases because the former (latter) involves nuclear growth (spontaneous nucleation). Without nuclei, crystallization is quenched below 160 K in the present experiment. It is suggested that the viscous liquid phase coexists at the surface or grain boundaries of metastable ice Ic.     

1-Phenyl-1-propyne on Cu(111): TOFMS TPD, XPS, UPS, and 2PPE Studies

The bonding properties of 1-phenyl-1-propyne (PP, C6H5CCCH3) on Cu(111) at 100 K have been studied using temperature-programmed desorption (TPD), and X-ray, ultraviolet, and two-photon photoemission spectroscopies (XPS, UPS, and 2PPE). In TPD, there is no evidence for dissociation. Multilayer desorption occurs at 187 K, and monolayer desorption occurs at 320 (83.5 kJ/mol) and 390 K (102.4 kJ/mol), with the latter dominating. Based on the calibrated C(1s) XPS, the saturation monolayer coverage is one PP per four surface Cu atoms. The broad and asymmetric C(1s) intensity profile of the monolayer can be resolved into three symmetric components, with peaks at 283.6, 284.5, and 285.2 eV and intensities of 2:6:1, respectively. These are attributed, respectively, to acetylenic carbons bound to Cu, phenyl, and methyl carbons. The monolayer valence band ultraviolet photoemission spectrum profile contains four resonances attributable to PP perturbed by interactions with the Cu(111) substrate. With the exception of the highest occupied molecular orbital (HOMO) that is shifted by 0.4 eV, these are uniformly shifted by 1 eV further from the Fermi level for the multilayer. Calculated electron density plots of the occupied orbitals coupled with UPS profiles suggest a spectator role for the phenyl group and bonding to Cu via the acetylenic carbons. The adsorption of 1.0 monolayer (ML) of PP on Cu(111) lowers the work function by 0.85 eV. Using 2PPE, two unoccupied orbitals were identified at 1.0 (U1*-LUMO) and 0.6 eV (U2*-image state) below the vacuum level. A chemisorption model consistent with these spectroscopic results and the major chemisorption peak in TPD involve di-sigma-bonding of the acetylenic carbons to a pair of second-nearest neighbor surface Cu atoms (cross-bridge).     

Roles of deeply supercooled ethanol in crystallization and solvation of LiI

The mechanisms of glass-liquid transition and crystallization of amorphous solid ethanol were investigated through detailed analyses of the interaction with LiI using time-of-flight secondary ion mass spectrometry and reflection absorption infrared spectroscopy. The LiI species adsorbed on the surface are incorporated into the bulk of ethanol at temperatures higher than 100 K, concomitantly with the reorganization of the ethanol molecules at the surface. This behavior is explicable by self-diffusion of the ethanol molecules as a consequence of the glass-liquid transition. The resulting liquid is a distinct phase, as revealed from the similarity of the IR absorption band to that of amorphous solid ethanol rather than liquid ethanol. The liquid-liquid phase transition occurs at 130 K, and a supercooled liquid ethanol is formed, as evidenced by formation of a metastable LiI solution when ethanol is deposited on the LiI film. The supercooled liquid ethanol is unstable, so that it crystallizes immediately at 130 K on the Ni(111) substrate. The film morphology changes continuously, even after crystallization, and the film abruptly becomes smoother before film evaporation. This behavior implies that crystallization is not completed and that a liquidlike phase coexists.     

Adsorption of oxygenates on alkanethiol-functionalized Pd(111) surfaces: mechanistic insights into the role of self-assembled monolayers on catalysis

Recent work shows that coating a supported palladium catalyst with a self-assembled monolayer (SAM) of alkanethiols can dramatically improve selectivity in the hydrogenation of 1-epoxy-3-butene (EpB) to 1-epoxybutane. Here, we present the results of surface-level investigations of the adsorption of EpB and related molecules on SAM-coated Pd(111), with an aim of identifying mechanistic explanations for the observed catalytic behavior. Alkanethiol SAM-covered Pd(111) surfaces were prepared by conventional techniques and transferred to ultrahigh vacuum, where they were characterized using Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) of EpB and other probe molecules. Whereas previous studies have shown that EpB undergoes rapid decomposition via epoxide ring opening on uncoated Pd(111), TPD studies show that EpB does not undergo substantial ring opening on SAM-covered surfaces but rather desorbs intact at temperatures less than 300 K. Systematic comparisons of EpB desorption spectra to spectra for other C(4) oxygenates suggest that the SAM creates a kinetic barrier to epoxide ring-opening reactions that does not exist on the uncoated surface. The EpB desorption spectra as a function of exposure show behavior similar to the desorption of olefins from Pd(111), indicating that the binding of the olefin functionality, in contrast to that of the epoxide ring, is not significantly perturbed. EpB desorption spectra from surfaces with less well-ordered SAMs show the presence of weakly bound states not observed on well-ordered SAM surfaces. The lower activity observed on catalysts covered with less well-ordered SAMs is hypothesized to occur due to partial confinement of adsorbates into these weakly bound, less active states.     

Reflection absorption infrared spectroscopy and temperature programmed desorption investigations of the interaction of methanol with a graphite surface

Reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) have been used to investigate the adsorption of methanol (CH(3)OH) on the highly oriented pyrolytic graphite (HOPG) surface. RAIRS shows that CH(3)OH is physisorbed at all exposures and that crystalline CH(3)OH can be formed, provided that the surface temperature and coverage are high enough. It is not possible to distinguish CH(3)OH that is closely associated with the HOPG surface from CH(3)OH adsorbed in multilayers using RAIRS. In contrast, TPD data show three peaks for the desorption of CH(3)OH. Initial adsorption leads to the observation of a peak assigned to the desorption of a monolayer. Subsequent adsorption leads to the formation of multilayers on the surface and two TPD peaks are observed which can be assigned to the desorption of multilayer CH(3)OH. The first of these shows a fractional order desorption, assigned to the presence of hydrogen bonding in the overlayer. The higher temperature multilayer desorption peak is only observed following very high exposures of CH(3)OH to the surface and can be assigned to the desorption of crystalline CH(3)OH.     

Nondissociative chemisorption of short chain alkanethiols on Au(111)

The adsorption of methanethiol and n-propanethiol on the Au(111) surface has been studied by temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and low-temperature scanning tunneling microscopy (LT-STM). Methanethiol desorbs molecularly from the chemisorbed monolayer at temperatures below 220 K in three overlapping desorption processes. No evidence for S-H or C-S bond cleavage has been found on the basis of three types of observations: (1) A mixture of chemisorbed CH3SD and CD3SH does not yield CD3SD, (2) no sulfur remains after desorption, and (3) no residual surface species remain when the adsorbed layer is heated to 300 K as measured by STM. On the other hand, when defects are introduced on the surface by ion bombardment, the desorption temperature of CH3SH is extended to 300 K and a small amount of dimethyl disulfide is observed to desorb at 410 K, indicating that S-H bond scission occurs on defect sites on Au(111) followed by dimerization of CH3S(a) species. Propanethiol also adsorbs nondissociatively on the Au(111) surface and desorbs from the surface below 250 K.     

TPD and FT-IRAS investigation of ethylene oxide (EtO) adsorption on a Au(211) stepped surface

Adsorption of ethylene oxide, CH(2)CH(2)O (EtO), on a Au(211) stepped surface was studied by temperature programmed desorption (TPD) and Fourier transform infrared reflection-absorption spectroscopy (FT-IRAS). Ethylene oxide was completely reversibly adsorbed, and desorbed molecularly during TPD following adsorption on Au(211) at 85 K. EtO TPD peaks appeared at 115 K from the multilayer film and 140 and 170 K from the monolayer. Desorption at 140 K was attributed to EtO desorption from terrace sites, and that at 170 K to EtO desorption from step sites. Desorption activation energies and corresponding adsorption energies were estimated to be 8.4 and 10.3 kcal mol(-1), respectively. The EtO ring (C(2)O) deformation band appeared in IRAS at 865 cm(-1) for EtO in multilayer films and when adsorbed in the monolayer at terrace sites. The stronger chemisorption bonding of EtO at Au step sites slightly weakens the bonding within the molecule and causes a small red-shift of this band to 850 cm(-1) for adsorption at step sites. EtO presumably binds via the oxygen atom to the surface, and observation of the EtO-ring absorption band in IRAS establishes that the molecular ring plane of EtO adsorbed at step and terrace sites is nearly upright with respect to the crystal surface plane.     

Reforming of oxygenates for H2 production: correlating reactivity of ethylene glycol and ethanol on Pt(111) and Ni/Pt(111) with surface d-band center

The dehydrogenation and decarbonylation of ethylene glycol and ethanol were studied using temperature programmed desorption (TPD) on Pt(111) and Ni/Pt(111) bimetallic surfaces, as probe reactions for the reforming of oxygenates for the production of H2 for fuel cells. Ethylene glycol reacted via dehydrogenation to form CO and H2, corresponding to the desired reforming reaction, and via total decomposition to produce C(ad), O(ad), and H2. Ethanol reacted by three reaction pathways, dehydrogenation, decarbonylation, and total decomposition, producing CO, H2, CH4, C(ad), and O(ad). Surfaces prepared by deposition of a monolayer of Ni on Pt(111) at 300 K, designated Ni-Pt-Pt(111), displayed increased reforming activity compared to Pt(111), subsurface monolayer Pt-Ni-Pt(111), and thick Ni/Pt(111). Reforming activity was correlated with the d-band center of the surfaces and displayed a linear trend for both ethylene glycol and ethanol, with activity increasing as the surface d-band center moved closer to the Fermi level. This trend was opposite to that previously observed for hydrogenation reactions, where increased activity occurred on subsurface monolayers as the d-band center shifted away from the Fermi level. Extrapolation of the correlation between activity and the surface d-band center of bimetallic systems may provide useful predictions for the selection and rational design of bimetallic catalysts for the reforming of oxygenates.     

Adsorption and reaction of methanol on stoichiometric and defective SrTiO3(100) surfaces

The adsorption and reaction of methanol (CH(3)OH) on stoichiometric (TiO(2)-terminated) and reduced SrTiO(3)(100) surfaces have been investigated using temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and first-principles density-functional calculations. Methanol adsorbs mostly nondissociatively on the stoichiometric SrTiO(3)(100) surface that contains predominately Ti(4+) cations. Desorption of a monolayer methanol from the stoichiometric surface is observed at approximately 250 K, whereas desorption of a multilayer methanol is found to occur at approximately 140 K. Theoretical calculations predict weak adsorption of methanol on TiO(2)-terminated SrTiO(3)(100) surfaces, in agreement with the experimental results. However, the reduced SrTiO(3)(100) surface containing Ti(3+) cations exhibits higher reactivity toward adsorbed methanol, and H(2), CH(4), and CO are the major decomposition products. The surface defects on the reduced SrTiO(3)(100) surface are partially reoxidized upon saturation exposure of CH(3)OH onto this surface at 300 K.     

Glass-liquid transition of carbon dioxide and its effect on water segregation

The interactions between CO(2) and D(2)O molecules have been investigated by using time-of-flight secondary ion mass spectrometry in the temperature rage 13-120 K. The monolayer of CO(2) tends to wet or intermix with the D(2)O film below 40 K and dewets the surface above 60 K. The water nanoclusters deposited on the CO(2) multilayers also start to segregate at 50-60 K and are finally incorporated in the bulk at 85-90 K, where the morphology of the film changes abruptly together with the desorption rate of the CO(2) molecules. The break at 85 K should be caused by the occurrence of the fluidized film whereas the glass-transition temperature of CO(2), as determined from the onset of translational molecular diffusion, is assigned to 50 K. This behavior may be related to the ultraviscous nature of the supercooled liquid, arising from the decoupling between the translational molecular diffusion and viscosity. The He(+) irradiation of the mixed CO(2)-D(2)O ice and the D(2)(+) irradiation of the CO(2) ice at 13 K do not yield any surface residues assignable to H(2)CO(3) and its precursors above 100 K. This result may be related to the segregation between the CO(2) and D(2)O molecules.     

Methanol oxidation on a Pt(111)-OH/O surface

The methanol oxidation on a hydroxylated Pt (Pt(111)-OH) surface has been investigated by means of infrared reflection absorption spectroscopy (IRAS) in ultra-high vacuum (UHV) and in acidic solution. The Pt(111)-OH surface in UHV was prepared by introducing water molecules on a Pt(111)-(2 x 2)-O surface and annealed at temperature higher than 160 K. Methanol was then, introduced to the Pt(111)-OH surface to show the dependence of the reaction intermediate on the annealing temperature. At an annealing temperature below 160 K, IR bands assignable to methanol overlayer were observed and no detectable intermediates, such as CO, formaldehyde and formate, were formed, suggesting that methanol molecules remain stable on Pt(111) surface without dissociation at this temperature region. At an annealing temperature above 160 K, on the other hand, CO and formate were observed. In addition, the oxidation of CO on Pt(111)-OH showed no sign of formate formation, indicating that formate is not derived from CO, but from a direct oxidation of methanol. Methanol oxidation was carried out in 0.1 mol dm(-3) HClO(4) solution on Pt(111) with a flow cell configuration and showed the formation of formate. These results indicate that the formate is the dominant non-CO intermediate both in UHV and in acidic solution, and the preadsorbed oxygen-containing species, in particular OH adsorbates, on Pt(111) surface plays a very important role in the formate formation process in methanol oxidation reaction.     

The application of diffuse reflectance infrared spectroscopy and temperature-programmed desorption to investigate the interaction of methanol on eta-alumina

The adsorption of methanol and its subsequent transformation to form dimethyl ether (DME) on a commercial grade eta-alumina catalyst has been investigated using a combination of mass selective temperature-programmed desorption (TPD) and diffuse reflectance infrared spectroscopy (DRIFTS). The infrared spectrum of a saturated overlayer of methanol on eta-alumina shows the surface to be comprised of associatively adsorbed methanol and chemisorbed methoxy species. TPD shows methanol and DME to desorb with respective maxima at 380 and 480 K, with desorption detectable for both molecules up to ca. 700 K. At 673 K, infrared spectroscopy reveals the formation of a formate species; the spectral line width of the antisymmetric C-O stretch indicates the adoption of a high symmetry adsorbed state. Conventional TPD using a tubular reactor, combined with mass spectrometric analysis of the gas stream exiting the IR cell, indicate hydrogen and methane evolution to be associated with formation of the surface formate group and CO evolution with its decomposition. A reaction scheme is proposed for the generation and decomposition of this important reaction intermediate. The overall processes involved in (i) the adsorption/desorption of methanol, (ii) the transformation of methanol to DME, and (iii) the formation and decomposition of formate species are discussed within the context of a recently developed four-site model for the Lewis acidity of eta-alumina.     

Adsorption, desorption, and diffusion of nitrogen in a model nanoporous material. I. Surface limited desorption kinetics in amorphous solid water

The adsorption and desorption kinetics of N2 on porous amorphous solid water (ASW) films were studied using molecular beam techniques, temperature programed desorption (TPD), and reflection-absorption infrared spectroscopy. The ASW films were grown on Pt(111) at 23 K by ballistic deposition from a collimated H2O beam at various incident angles to control the film porosity. The experimental results show that the N2 condensation coefficient is essentially unity until near saturation, independent of the ASW film thickness indicating that N2 transport within the porous films is rapid. The TPD results show that the desorption of a fixed dose of N2 shifts to higher temperature with ASW film thickness. Kinetic analysis of the TPD spectra shows that a film thickness rescaling of the coverage-dependent activation energy curve results in a single master curve. Simulation of the TPD spectra using this master curve results in a quantitative fit to the experiments over a wide range of ASW thicknesses (up to 1000 layers, approximately 0.5 microm). The success of the rescaling model indicates that N2 transport within the porous film is rapid enough to maintain a uniform distribution throughout the film on a time scale faster than desorption.