Unexpected Nondissociative Binding of N₂O on Oxygen Vacancies on a Rutile TiO₂(110)-1×1

Abstract

The interaction of N2O with oxygen vacancies (VO’s) on a partially reduced rutile TiO2(110)-1×1 surface was investigated using temperature-programmed desorption (TPD). Contrary to a common belief that VO on a rutile TiO2(110) is a dissociation site for N2O, our results indicate that N2O does not dissociate to form N2(g) and O(a). In TPD, N2O desorption shows two peaks with maxima at 135 and 175 K that are assigned to N2O desorption from Ti4+ and VO sites, respectively, with absolute coverages determined to be 5.4 × 1014 N2O/cm2 and 2.3 × 1013 N2O/cm2, respectively, on the TiO2(110)-1×1 surface used (VO concentration of 5%, 2.6 × 1013/cm2). When VO’s are passivated by dissociative adsorption of H2O, the N2O desorption peak at 175 K disappears, evidencing that the peak is related to VO-bonded N2O. The absence of N2O dissociation on VO’s is supported by a number of observations. First, the integrated amount of N2O desorbed from the substrate during TPD vs the amount of N2O dosed at 70 K shows a straight line with no offset, indicating no loss of N2O due to the N2 formation. Second, N2O scattering experiments at 300–350 K indicate no change in the VO concentration as determined from the H2O TPD spectra. Third, N2O uptake experiments at 70–90 K show that the N2 desorption feature is observed from TiO2(110) surfaces without VO’s, suggesting a possible contribution from background N2 adsorption. On the basis of the above observations, we conclude that N2O does not dissociate on VO sites on TiO2(110), in contrast with the currently accepted view.

Photocatalytic Conversion of Nitric Oxide on Titanium Dioxide: Cryotrapping of Reaction Products for Online Monitoring by Mass Spectrometry

“…However, the loss of an oxygen atom requires a sufficiently long N 2 O residence time at an oxygen vacancy; otherwise, it can desorb as N 2 O (gas) (step 12). Kim et al reported that N 2 O does not dissociate on TiO 2 surface to form N 2 and O. Moreover, it has been reported that N 2 O molecules are only weakly bound to the TiO 2 surface with an adsorption energy of 7.73 kcal/mol; thus, it is likely that in this photocatalyzed reaction most of the N 2 O (ads) were desorbed as N 2 O (gas) to yield the observed MS signal at m / z 44.…”

“…However, the loss of an oxygen atom requires a sufficiently long N 2 O residence time at an oxygen vacancy; otherwise, it can desorb as N 2 O (gas) (step 12). Kim et al 14 13 thus, it is likely that in this photocatalyzed reaction most of the N 2 O (ads) were desorbed as N 2 O (gas) to yield the observed MS signal at m/z 44.…”

“…Various catalytic processes have been developed to convert NO x gases (i.e., NO and NO 2 ) into nitrogen (N 2 ) and oxygen (O 2 ) or into nitrates (NO 3 – ) for NO x abatement. − However, a majority of the catalysts used in converting NO x gases into harmless gaseous species (i.e., N 2 and O 2 ) contain expensive precious metals (e.g., Rh, Pt, or Pd) and perform most efficiently at high temperatures (>200 °C). , An ideal catalyst for NO x conversion is expected to convert or store NO x gases at lower temperatures (e.g., ambient conditions during the cold start of automobile engines) . Titanium dioxide (TiO 2 ) is among the major photocatalysts used for catalytic conversion of NO x gases into nitrous oxide (N 2 O) and N 2 . , Previous studies have shown promising results where TiO 2 catalysts were utilized for NO x conversion in the presence of solar light. − …”

“…MS allows for direct mass-to-charge ratio ( m / z ) analysis and hence, it can be used for molecular weight determination of NO x species and other potential NO photocatalytic products. − ,,,, In addition, GC/MS systems provide a higher degree of specificity and selectivity for NO x separation prior to final analysis. The use of conventional open tubular GC columns for analysis of NO x gas mixtures is challenging due to poor separation of NO from N 2 and O 2 .…”

Temperature-programmed desorption study of NO reactions on rutile TiO2(110)-1 × 1

“…With an increasing number (> 10) of sputter-anneal cycles, the TiO 2 (110) crystal turned dark blue and the V O concentration increased to ~5% (~2.6 × 10 13 cm -2 ) (this sample is referred to as r-TiO 2 ). We have also prepared hydroxylated [22][23][24] and oxidized [25,26] [27]. The absolute NO dose(in NO/cm 2 ) was estimated from the plot of integrated area of NO TPD spectra obtained from multilayers of NO dosed on oxidized TiO 2 against the calibrated dose of NO by assuming that the sticking coefficient is unity at the dose temperature.…”