Article pubs.acs.org/JPCC
Ammonia Formation from NO Reaction with Surface Hydroxyls on Rutile TiO2(110)‑1 × 1 Boseong Kim,† Bruce D. Kay,‡ Zdenek Dohnálek,‡ and Yu Kwon Kim*,† †
Department of Energy Systems Research and Department of Chemistry, Ajou University, Suwon 443-749, South Korea Chemical and Materials Sciences Division, Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, P.O. Box 999, Mail Stop K8-88, Richland, Washington 99352, United States
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‡
ABSTRACT: The reaction of NO with the hydroxylated rutile TiO2(110)-1 × 1 surface (h-TiO2) was investigated as a function of NO coverage using temperatureprogrammed desorption. Our results show that NO reaction with h-TiO2 leads to formation of NH3, which is observed to desorb at ∼400 K. Interestingly, the amount of NH3 produced depends nonlinearly on the dose of NO. The yield increases up to a saturation value of ∼1.3 × 1013 NH3/cm2 at a NO dose of 5 × 1013 NO/cm2, 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 (HOb’s) upon coadsorption of NO and H2O 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 HOb’s 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) .
1. INTRODUCTION
Understanding the interactions of NO with surface hydroxyl groups is also of fundamental importance, since they are abundant in many TiO2-based catalysts prepared by hydrolysis. Also, in the catalytic reduction of NO with NH3, N−H bonds in NH3 are being broken, leading to surface hydroxyls for a subsequent decomposition of NO into N2 and H2O.6,27 Thus, understanding the reactions of hydroxyls with NO is needed to provide insight into the mechanism of catalytic reduction of NO with NH3. In addition, hydroxyl species on TiO2 have charges associated them and can be actively involved in catalytic reactions.28−30 Recently, it has also been reported that surface hydroxyls can trap NO on TiO2 at room temperature,19 suggesting that they interact strongly with NO and possibly facilitate its dissociation. On TiO2(110), surface hydroxyls (HOb’s) can easily form by dissociative adsorption of H2O on oxygen vacancies (VO’s) on bridge-bonded oxygen (Ob) rows. Many theoretical and experimental studies have shown that there is charge associated with the HOb’s similar to the VO’s.28,31 In this report, we show that NO interacts with HOb’s but not with H2O on hydroxylated TiO2(110) to produce NH3, under temperature-programmed desorption (TPD) reaction conditions. We show that the amount of NH3 depends nonlinearly on the coverage of NO. It increases up to a saturation value of
Nitrogen oxides (mainly NO and NO2), or NOx, are common air pollutants formed when fuels are burned at high temperatures, as in a combustion process. The primary sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels. Developing efficient catalytic processes for NOx conversion to environmentally benign N2 and water represent one of the critical challenges in catalysis. It has been shown that NOx can be removed by a catalytic reduction over mixed oxide catalysts,1,2 such as V2O5/TiO2,3 WO3/TiO2,4 and V2O5-WO3/TiO2.5 Thus, as a support to the de-NOx catalysts,6 NO and NO2 reduction has been extensively studied on both anatase TiO2 powders 7−13 and prototypical single crystalline rutile TiO2(110)14−22 surfaces. For NO, which is our interest in this study, many important aspects of the reaction on TiO2 have been revealed over the past decades. On both oxidized14 and reduced16 TiO2, NO molecules have been found to react with each other to form N2O while leaving an oxygen atom on the surface. Additionally, UV irradiation has been observed to initiate photoinduced decomposition of NO to N2O, which desorbs from the surface at 110 K.7 However, it is also recognized that the reactivity of NO with TiO2 is strongly influenced by surface defects such as oxygen vacancies and Ti3+ interstitials.15,16 The charges associated with such defects can have a profound effect on the adsorption and subsequent reactions of electronegative adsorbates like NO and O2.23−26 © 2014 American Chemical Society
Received: November 1, 2014 Revised: December 15, 2014 Published: December 17, 2014 1130
DOI: 10.1021/jp5109619 J. Phys. Chem. C 2015, 119, 1130−1135
Article
The Journal of Physical Chemistry C ∼1.3 × 1013 NH3/cm2 at the NO dose of 5 × 1013 NO/cm2, but subsequently decreases at higher NO doses.
2. EXPERIMENTAL DETAILS All experiments were performed in an ultra-high-vacuum (UHV) chamber equipped with an effusive molecular beam for gas dosing. The base pressure was maintained below 1 × 10−10 Torr throughout the experiment. A single crystal rutile TiO2(110) substrate (10 × 10 × 1 mm, Princeton Scientific) was attached to a sample holder as described previously.32 In this configuration, the sample temperature was controlled between 50 and 1000 K by a resistive heating in combination with cooling by a UHV He cryostat. Well-ordered rutile TiO2(110)-1 × 1 was prepared by repeated cycles of Ne+-sputtering at 300 K and annealing to 850 K, as confirmed by a characteristic 1 × 1 low-energy electron diffraction (LEED) pattern. The VO concentration on the as-prepared TiO2 surface (r-TiO2) was determined to be 2.6 × 1013/cm2 (∼5% of surface Ti4+ sites) from the integrated area of the characteristic recombinative H2O TPD peak at 500 K.33 This does not include H2O dissociated over other type of defects such as steps.34 The r-TiO2 was further exposed to H2O (and O2) to obtain hydroxylated (h-TiO2), stoichiometric (sTiO2), and oxidized (o-TiO2) TiO2(110).24,25,35,36 The h-TiO2 with HOb’s was prepared by dosing 1 ML of H2O on r-TiO2 at
