We have examined the interaction of molecular oxygen with the TiO 2 (110) surface using temperatureprogrammed desorption (TPD), isotopic labeling studies, sticking probability measurements, and electron energy loss spectroscopy (ELS). Molecular oxygen does not adsorb on the TiO 2 (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 O 2 at 120 K on a TiO 2 (110) surface with 8% oxygen vacancies (about 4 × 10 13 sites/cm 2 ) 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 O 2 desorption below a coverage of 4 × 10 13 molecules/cm 2 (the vacancy population), presumably due to dissociative filling of the vacancy sites in a 1:1 ratio. Above a coverage of 4 × 10 13 molecules/cm 2 , a first-order O 2 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 16 O 2 and 18 O 2 at 120 K indicate that each adsorbed O 2 molecule, irrespective of its adsorption sequence, has equivalent probabilities with respect to its neighbors to follow the two channels (molecular and dissociative), suggesting that O 2 adsorption is not only precursor-mediated, as the sticking probability measurements indicate, but that all O 2 molecules reside in this precursor state at 120 K. This precursor state may be associated with a weak 145 K O 2 TPD state observed at high O 2 exposures. ELS measurements suggest charge transfer from the surface to the O 2 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 O 2species based on comparisons with Ti-O 2 inorganic complexes in the literature. Utilizing results from recent spin-polarized DFT calculations in the literature, we propose a model where three O 2 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 TiO 2 surfaces.

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In this study we show that molecular oxygen reacts with bridging OH (OHbr) groups formed as a result of water dissociation at oxygen vacancy defects on the surface of rutile TiO2(110). The electronic structure of an oxygen vacancy defect on TiO2(110) is essentially the same as that of electron trap states detected on photoexcited or sensitized TiO2 photocatalysts, being Ti3+ in nature. Electron energy loss spectroscopy (EELS) measurements, in agreement with valence band photoemission results in the literature, indicate that water dissociation at oxygen vacancy sites has little or no impact on the electronic structure of these sites. Temperature programmed desorption (TPD) measurements show that O2 adsorbed at 120 K reacts with near unity reaction probability with OHbr groups on TiO2(110) to form an unidentified intermediate that decomposes to generate terminal OH groups at nondefect sites. Commensurate with this process, the electronic defect associated with the original oxygen vacancy defect (Ti3+) is oxidized. Vibrational EELS results indicate that the reaction between O2 and OHbr occurs at about 230 K, whereas electronic EELS results suggest that charge is transferred away from the vacancies at 90 K. Detailed TPD experiments in which the precoverage of water was varied indicate that chemisorption of O2 at cation sites on the TiO2(110) surface is not required in order for the reaction between O2 and OHbr to occur, which implies a direct interaction between weakly bound (physisorbed) O2 and the OHbr groups. In agreement with this conclusion, we find that second-layer water, which selectively hydrogen-bonds to bridging O2- sites and bridging OH groups, blocks the reaction of O2 with OHbr groups and prevents oxidation of the vacancy-related Ti3+ electronic state. These results suggest that the electron scavenging role of O2 in photocatalysis may involve a direct reaction between O2 and trapped electrons located at bridging OH groups. Our studies suggest that the negative influence of high water concentrations in gas-phase heterogeneous photocatalysis studies results from hydrogen-bonded water blocking access of O2 to trapped electrons located at surface OH groups.

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The intrinsic mechanism of the selective catalytic reduction (SCR) reaction over a Cu-exchanged SAPO-34 catalyst at low temperature was studied by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), coupled with mass spectrometry to measure inlet and outlet gas concentrations. The evolution of the surface intermediates, as well as the reactivity of NH3 with surface NO x species and NO x with surface NH3 species, was evaluated. In terms of NO x adsorption, surface nitrates and nitrites are the main NO x adsorption species at low temperature. When NO was exposed to the sample with NH3 preadsorbed, surface NH3 was not reactive because of the low surface coverage of nitrates and nitrites. However, the reactivity is significantly enhanced by the inclusion of O2 in the feed, which promotes an increase in the concentration of surface nitrates and nitrites. DRIFTS results also reveal that the low temperature SCR reaction involves the formation of an NH4NO3 intermediate and its subsequent reduction by NO. The NH4NO3 was formed on Lewis acid sites on the Cu-SAPO-34 sample. The Brønsted acid sites act as an NH3 reservoir that supplies additional NH3 via migration to the Lewis acid sites for the SCR reaction. The migration of NH3 between different acid sites was confirmed in an NH3-temperature-programmed desorption (TPD) study. The presence of NO in the feed reduces surface NH4NO3 to produce N2 at temperatures as low as 100 °C. Since NH4NO3 is typically considered an inhibitor, the onset temperature of the reaction between NO and NH4NO3 is much lower than that reported for other SCR zeolite catalysts; therefore, it is likely the key factor that results in the low temperature SCR activity of Cu-SAPO-34.

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The effect(s) of SO 2 on the two types of active sites on Cu-SSZ-13 NH 3 −SCR catalysts, Z2Cu and ZCuOH, were investigated. Two Cu-SSZ-13 catalysts with Si:Al ratios of 6 and 30 were synthesized, and they provide very different distributions of these two active sites. Inductively coupled plasma optical emission spectroscopy (ICP-OES), H 2 temperature-programmed reduction (H 2 -TPR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were utilized to characterize catalyst samples and quantify the amounts of total Cu, Z2Cu and ZCuOH. In situ DRIFTS results show that Z2Cu and ZCuOH responses to low-temperature (<200 °C) SO 2 poisoning were site-dependent. Results of SO 2 and SO 2 + NH 3 temperatureprogrammed desorption (TPD) and DRIFTS experiments, supplemented with DFT calculations, revealed that the different observed responses correspond to different sulfur intermediates that form. On Z2Cu sites, SO 2 only adsorbs when it is cofed with NH 3 via formation of ammonium sulfate, with its fingerprint TPD feature at 380 °C. However, low-temperature interaction between SO 2 and ZCuOH leads to copper bisulfite species formation, which can be further oxidized to form copper bisulfate with increasing temperature. In terms of low-temperature SCR functionality, the activity of both Cu-SSZ-13 samples were found to be significantly inhibited by SO 2 . However, in terms of regeneration (i.e., desulfation) behavior, Cu-SSZ-13 with a Si:Al = 30 (higher ZCuOH compared to Z2Cu) seemed to require higher desulfation temperatures (>550 °C). Therefore, compared with Z2Cu, ZCuOH sites are more susceptible to severe low-temperature SO 2 poisoning because of the formation of more stable bisulfite and ultimately bisulfate species.

Metal ions exchanged on zeolites represent a unique bridge between heterogeneous solid materials and homogeneous inorganic chemistry. The complexing of exchanged metal ions with H2O or NO, is of particular relevance for a number of reactions, including the ubiquitous presence of both gases in pollution remediation technologies. Here, we interrogate the molecular structure of Pd cations in SSZ-13 zeolites and their interaction with H2O and NO using experimental and computational analyses. Density functional theory (DFT) and spectroscopic characterization establish that Pd cations preferentially populate two Al (2Al) sites in the six-membered ring as PdII. In situ spectroscopic and kinetic analyses follow the Pd coordination environment and reactivity as a function of environmental conditions, and molecular structures are rationalized through ab initio molecular dynamics and first-principles thermodynamic modeling. Experiment and computational modeling together reveal that, at temperatures <573 K, Pd ions are solvated and mobilized by H2O molecules, promoting catalytic CO oxidation, and form molecular complexes akin to their Pd homogeneous analogues. Exposure to NO promotes transformation from 2Al → 1Al charge-compensated H2O-solvated Pd-nitrosyl complexes, which desorb NO at higher temperatures and inhibit CO adsorption and oxidation. A comparison with Pd-BEA and Pd-ZSM-5 zeolites demonstrates a heterogeneous distribution of Pd-NO complexes under dry conditions that coalesce into homogeneous H2O-solvated Pd-nitrosyl complexes upon exposure to H2O.