Water Temperature Programmed Desorption Research Articles

Readsorption and diffusion-limited TPD of water from zeolite Linde 4A

Coupled effects due to diffusion and readsorption of desorbed species on temperature-programmed desorption (TPD) data have been analyzed and compared with experimental curves during TPD of water from zeolite Linde 4A. The effects of different heating rates and carrier-gas flow are considered explicitly. A simple model is presented which allows straightforward extraction of kinetic parameters from the experimental data, even in the presence of effects due to readsorption and diffusion. In this system, it is found that an extensive readsorption of gas occurs within the zeolite pellet and the local adsorbate coverage is governed by equilibrium adsorption. We found that neglecting the effects of diffusion can cause activation energy falsifications of the order of 10%. When readsorption and diffusion are not taken into account, calculated pre-exponential factors are very small and convey no direct physical meaning.

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.

Structure and geometry of water adsorbed on the MgO(100) surface

Adsorption of D 2O on the MgO(100)/Mo(100) surface has been studied using temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS) and low energy electron diffraction (LEED). The epitaxially grown MgO(100) films show virtually identical water TPD spectra as the single crystal MgO(100) surface. The first, chemisorbed layer is adsorbed with the plane of the molecule nominally parallel to the surface. A (3 × 2) LEED pattern with missing spots at (0, 1 2 ) was also observed. Based on the LEED and RAIRS results, a real space structure for the D 2O adsorption on the MgO(100) surface is proposed. This structure implies an absolute coverage of 0.67 ML (D 2O molecules per Mg cation) for the first, chemisorbed layer.

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...

Dehydroxylation of the surface of magnesium oxide by temperature programmed desorption

The surface of magnesia acquires adsorptive-catalytic reactivity only after high temperature (HT) dehydration. Temperature-programmed desorption (TPD) of water reveals at least four energetically different forms of adsorbed water. The most significant HT desorption (above 1000 K) exposes low-coordinated (LC) MgO sites which are active sites for hydrogen chemisorption. Water and hydrogen TPD measurements allow the evaluation of the LC site concentration at about 7.5 × 10 17 and 7.1 × 10 17 site m −2, respectively, which corresponds to about 7% of the total ion surface population. The concentration of the LC sites confirms the much higher heterogeneity of the real MgO surface than that based on simple surface area modelling (about 2.1 × 10 17 site m −2). A dehydration mechanism for the energetically heterogeneous surface of magnesia is postulated.

The chemisorption of H 2O and O 2 on Cu films on ZnO(0001)-O

The interactions of water and oxygen with well-defined Cu films on the oxygen-terminated ZnO(0001)-O surface has been studied, mainly using temperature-programmed desorption (TPD) and low-energy He + ion scattering (LEIS). The Cu films, which were from submonolayer to multilayer in coverage, had been structurally characterized in previous studies using XPS, LEIS, ARXPS, LEED, work function measurements and CO adsorption. Submonolayer films, deposited at low temperatures in the form of 2D islands, adsorb water and oxygen much like the Cu(110) surface. Upon annealing to high temperatures, these films show a shift in the Cu-derived water TPD peak from 168 down to 158 K, consistent with a conversion of the Cu film from one offering Cu(110)-like sites to one offering Cu(111)-like sites. Even heavy doses of Cu show a water TPD peak due to free ZnO(0001)-O sites, consistent with a film structure where thick Cu islands only cover most, but not all of the surface, with clean ZnO patches between. The fraction of the ZnO surface which is exposed (as measured by this water TPD peak's area) increases with the pre-annealing temperature of the Cu film above 550 K, due to 3D clustering of the Cu. Between 130 K and 370 K, the influence of submonolayer Cu islands on the water desorption temperature from these free ZnO sites decreases, suggesting that small Cu 2D islands coalesce into larger 2D islands. Oxygen-dosed, submonolayer Cu films showed no clear evidence for a hydroxyl state in water TPD.

The effects of water coadsorption on the adsorption of oxygen over metal oxides: I. Temperature-programmed desorption study of Co 3O 4

Adsorption of oxygen and water and the effects of coadsorption of water on oxygen adsorption over Co 3O 4 were studied by means of temperature-programmed desorption (TPD). There were at least three kinds of adsorbed oxygen on the Co 3O 4 samples calcined at 823–973 K, which desorbed at 300–400, 450–600, and >600 K (denoted α, β, and γ oxygen, respectively). γ Oxygen inhibited the adsorption of α and β oxygen in the same degree. N 2O reacted with evacuated surface to evolve almost equimolar N 2 at room temperature. TPD chromatograms of oxygen from the sample with oxygen adsorbate derived from N 2O decomposition well agreed with those from the sample with a corresponding amount of oxygen adsorbate formed from oxygen preadsorption in regard to peak shape and peak maximum temperature. The TPD experiments using 18O 2 revealed that oxygen exchange between oxygen adsorbates and oxide ions of the Co 3O 4 sample took place above 323 K, and the equilibration of isotopes among oxygen adsorbates was observed even below 323 K. Dissociative adsorption of oxygen appears to be compatible with the experimental results. TPD of water from Co 3O 4 preadsorbed with varied amount of water disclosed the presence of four kinds of adsorbed water which desorbed at 300–450, 450–550, 550–700, and >700 K. Preadsorption of water at 473 K brought about the slight decrease of γ oxygen and the rearrangement of α and β oxygen to ε oxygen (350–450 K) without change of the total amount of oxygen desorbed.