Ammonia Formation from NO Reaction with Surface Hydroxyls on Rutile TiO₂(110)-1 × 1

Kim, Boseong,Kay, Bruce D.,Dohná,lek, Zdenek,Kim, Yu Kwon American Chemical Society 2015 The Journal of Physical Chemistry Part C Vol.119 No.2

<P>The reaction of NO with the hydroxylated rutile TiO2(110)-1 x 1 surface (h-TiO2) was investigated as a function of NO coverage using temperature-programmed desorption. Our results show that NO reaction with h-TiO2 leads to formation of NH3, which is observed to desorb at similar to 400 K. Interestingly, the amount of NH3 produced depends nonlinearly on the dose of NO. The yield increases up to a saturation value of similar to 1.3 x 10 (13) NH3/cm(2) at a NO dose of 5 x 10(-13) NO/cm(2), but subsequently decreases at higher NO doses. Preadsorbed H2O is found to have a negligible effect on the NH3 desorption yield. Additionally, no NH3 is formed in the absence of surface hydroxyls (HObs) upon coadsorption of NO and (HO)-O-2 on a stoichiometric TiO2(110) (s-TiO2(110)). On the basis of these observations, we conclude that nitrogen from NO has a strong preference to react with HObs on the bridge-bonded oxygen rows (but not with H2O) to form NH3. The absolute NH3 yield is limited by competing reactions of HOb species with titanium-bound oxygen adatoms to form H2O. Our results provide new mechanistic insight about the interactions of NO with hydroxyl groups on TiO2(110).</P>

Low-Temperature Desorption of N₂O from NO on Rutile TiO₂(110)-1 × 1

Kim, Boseong,Li, Zhenjun,Kay, Bruce D.,Dohná,lek, Zdenek,Kim, Yu Kwon American Chemical Society 2014 The Journal of Physical Chemistry Part C Vol.118 No.18

<P>We find that NO dosed on rutile TiO<SUB>2</SUB>(110)-1 × 1 at substrate temperatures as low as 50 K readily reacts to produce N<SUB>2</SUB>O, which desorbs promptly from the surface leaving an oxygen adatom behind. The desorption rate of N<SUB>2</SUB>O reaches a maximum value after 1–2 s at an NO flux of 1.2 × 10<SUP>14</SUP> NO/cm<SUP>2</SUP>·sec and then decreases rapidly as the initially clean, reduced TiO<SUB>2</SUB>(110) surface with ∼5% oxygen vacancies (V<SUB>O</SUB>’s) becomes covered with oxygen adatoms and unreacted NO. The maximum desorption rate is also found to increase as the substrate temperature is raised up to about 100 K. Interestingly, the N<SUB>2</SUB>O desorption during the low-temperature (LT) NO dose is strongly suppressed when molecular oxygen is predosed, whereas it persists on the surface with V<SUB>O</SUB>’s passivated by surface hydroxyls. Our results show that the surface charge, not the V<SUB>O</SUB> sites, plays a dominant role in the LT N<SUB>2</SUB>O desorption induced by a facile NO reduction at such low temperatures.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2014/jpccck.2014.118.issue-18/jp501179y/production/images/medium/jp-2014-01179y_0006.gif'></P>

The effect of oxygen vacancies on the binding interactions of NH₃ with rutile TiO₂(110)-1 × 1

Kim, Boseong,Li, Zhenjun,Kay, Bruce D.,Dohná,lek, Zdenek,Kim, Yu Kwon The Royal Society of Chemistry 2012 Physical chemistry chemical physics Vol.14 No.43

<P>A series of NH<SUB>3</SUB> temperature-programmed desorption (TPD) spectra were taken after dosing NH<SUB>3</SUB> at 70 K on rutile TiO<SUB>2</SUB>(110)-1 × 1 surfaces with oxygen vacancy (V<SUB>O</SUB>) concentrations of ∼0% (p-TiO<SUB>2</SUB>) and 5% (r-TiO<SUB>2</SUB>), respectively, to study the effect of V<SUB>O</SUB>s on the desorption energy of NH<SUB>3</SUB> as a function of coverage, <I>&thetas;</I>. Our results show that in the zero coverage limit, the desorption energy of NH<SUB>3</SUB> on r-TiO<SUB>2</SUB> is 115 kJ mol<SUP>−1</SUP>, which is 10 kJ mol<SUP>−1</SUP> less than that on p-TiO<SUB>2</SUB>. The desorption energy from the Ti<SUP>4+</SUP> sites decreases with increasing <I>&thetas;</I> due to repulsive NH<SUB>3</SUB>–NH<SUB>3</SUB> interactions and approaches ∼55 kJ mol<SUP>−1</SUP> upon the saturation of Ti<SUP>4+</SUP> sites (<I>&thetas;</I> = 1 monolayer, ML) on both p- and r-TiO<SUB>2</SUB>. The absolute monolayer saturation coverage is determined to be about 10% smaller on r-TiO<SUB>2</SUB> than that on p-TiO<SUB>2</SUB>. Additionally, the trailing edges of the NH<SUB>3</SUB> TPD spectra on the hydroxylated TiO<SUB>2</SUB>(110) (h-TiO<SUB>2</SUB>) appear to be the same as that on r-TiO<SUB>2</SUB> while those on oxidized TiO<SUB>2</SUB>(110) (o-TiO<SUB>2</SUB>) shift to higher temperatures. We present a detailed analysis of the results and reconcile the observed differences based on the repulsive adsorbate–adsorbate dipole interactions between neighboring NH<SUB>3</SUB> molecules and the surface charge associated with the presence of V<SUB>O</SUB>s.</P> <P>Graphic Abstract</P><P>A series of NH<SUB>3</SUB> temperature-programmed desorption (TPD) spectra were taken after dosing NH<SUB>3</SUB> at 70 K on rutile TiO<SUB>2</SUB>(110)-1 × 1 surfaces with oxygen vacancy (V<SUB>O</SUB>) concentrations of ∼0% (p-TiO<SUB>2</SUB>) and 5% (r-TiO<SUB>2</SUB>), respectively, to study the effect of V<SUB>O</SUB>'s on the desorption energy of NH<SUB>3</SUB> as a function of coverage, <I>&thetas;</I>. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cp42754k'> </P>

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

Kim, Boseong,Li, Zhenjun,Kay, Bruce D.,Dohná,lek, Zdenek,Kim, Yu Kwon American Chemical Society 2012 The Journal of Physical Chemistry Part C Vol. No.

<P>The interaction of N<SUB>2</SUB>O with oxygen vacancies (V<SUB>O</SUB>’s) on a partially reduced rutile TiO<SUB>2</SUB>(110)-1×1 surface was investigated using temperature-programmed desorption (TPD). Contrary to a common belief that V<SUB>O</SUB> on a rutile TiO<SUB>2</SUB>(110) is a dissociation site for N<SUB>2</SUB>O, our results indicate that N<SUB>2</SUB>O does not dissociate to form N<SUB>2</SUB>(g) and O(a). In TPD, N<SUB>2</SUB>O desorption shows two peaks with maxima at 135 and 175 K that are assigned to N<SUB>2</SUB>O desorption from Ti<SUP>4+</SUP> and V<SUB>O</SUB> sites, respectively, with absolute coverages determined to be 5.4 × 10<SUP>14</SUP> N<SUB>2</SUB>O/cm<SUP>2</SUP> and 2.3 × 10<SUP>13</SUP> N<SUB>2</SUB>O/cm<SUP>2</SUP>, respectively, on the TiO<SUB>2</SUB>(110)-1×1 surface used (V<SUB>O</SUB> concentration of 5%, 2.6 × 10<SUP>13</SUP>/cm<SUP>2</SUP>). When V<SUB>O</SUB>’s are passivated by dissociative adsorption of H<SUB>2</SUB>O, the N<SUB>2</SUB>O desorption peak at 175 K disappears, evidencing that the peak is related to V<SUB>O</SUB>-bonded N<SUB>2</SUB>O. The absence of N<SUB>2</SUB>O dissociation on V<SUB>O</SUB>’s is supported by a number of observations. First, the integrated amount of N<SUB>2</SUB>O desorbed from the substrate during TPD vs the amount of N<SUB>2</SUB>O dosed at 70 K shows a straight line with no offset, indicating no loss of N<SUB>2</SUB>O due to the N<SUB>2</SUB> formation. Second, N<SUB>2</SUB>O scattering experiments at 300–350 K indicate no change in the V<SUB>O</SUB> concentration as determined from the H<SUB>2</SUB>O TPD spectra. Third, N<SUB>2</SUB>O uptake experiments at 70–90 K show that the N<SUB>2</SUB> desorption feature is observed from TiO<SUB>2</SUB>(110) surfaces without V<SUB>O</SUB>’s, suggesting a possible contribution from background N<SUB>2</SUB> adsorption. On the basis of the above observations, we conclude that N<SUB>2</SUB>O does not dissociate on V<SUB>O</SUB> sites on TiO<SUB>2</SUB>(110), in contrast with the currently accepted view.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2012/jpccck.2012.116.issue-1/jp210636j/production/images/medium/jp-2011-10636j_0007.gif'></P>

The Adsorption and Desorption of NH₃ on Rutile TiO₂(110)-1×1 Surfaces

김보성,Zhenjun Li,Bruce D. Kay,Zdenek Dohnalek,김유권 한국진공학회 2012 한국진공학회 학술발표회초록집 Vol.2012 No.2

Origin of temperature-dependent reduction channels of NO into N₂O on TiO₂(110) surfaces

Yu Kwon KIM,Zdenek DOHNALEK,Janos SZANYI,Bruce D. KAY 한국진공학회 2016 한국진공학회 학술발표회초록집 Vol.2016 No.8

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

Kim, Boseong,Dohná,lek, Zdenek,Szanyi, Já,nos,Kay, Bruce D.,Kim, Yu Kwon Elsevier 2016 Surface science Vol.652 No.-

<P><B>Abstract</B></P> <P>Systematic temperature-programmed desorption (TPD) studies of NO adsorption and reactions on rutile TiO<SUB>2</SUB>(110)-1×1 surface reveal several distinct reaction channels in a temperature range of 50–500K. NO readily reacts on TiO<SUB>2</SUB>(110) to form N<SUB>2</SUB>O, which desorbs between 50 and 200K (LT N<SUB>2</SUB>O channels), which leaves the TiO<SUB>2</SUB> surface populated with adsorbed oxygen atoms (O<SUB>a</SUB>) as a by-product of N<SUB>2</SUB>O formation. In addition, we observe simultaneous desorption peaks of NO and N<SUB>2</SUB>O at 270K (HT1 N<SUB>2</SUB>O) and 400K (HT2 N<SUB>2</SUB>O), respectively, both of which are attributed to reaction-limited processes. No N-derived reaction product desorbs from TiO<SUB>2</SUB>(110) surface above 500K or higher, while the surface may be populated with O<SUB>a</SUB>'s and oxidized products such as NO<SUB>2</SUB> and NO<SUB>3</SUB>. The adsorbate-free TiO<SUB>2</SUB> surface with oxygen vacancies can be regenerated by prolonged annealing at 850K or higher. Detailed analysis of the three N<SUB>2</SUB>O desorption yields reveals that the surface species for the HT channels are likely to be various forms of NO dimers.</P> <P><B>Highlights</B></P> <P> <UL> <LI> N<SUB>2</SUB>O desorption from NO/TiO<SUB>2</SUB> is enhanced in the presence of oxygen vacancies (V<SUB>O</SUB>'s). </LI> <LI> Overall N<SUB>2</SUB>O yield saturates above a threshold NO dose. </LI> <LI> N<SUB>2</SUB>O yields on <I>h</I>-(or <I>r</I>-) TiO<SUB>2</SUB> are about the same with changes in desorption channels. </LI> <LI> Stabilization of NO in the presence of hydroxyls enhances LT and HT2 N<SUB>2</SUB>O channels. </LI> <LI> Oxidation of NO into NO<SUB>2</SUB> and NO<SUB>3</SUB> decreases the N<SUB>2</SUB>O desorption yield. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>