Ammonia Formation from NO Reaction with Surface Hydroxyls on Rutile TiO 2 (110)‑1 × 1 Boseong Kim, † Bruce D. Kay, ‡ Zdenek Dohna ́ lek, ‡ 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 ABSTRACT: The reaction of NO with the hydroxylated rutile TiO 2 (110)-1 × 1 surface (h-TiO 2 ) was investigated as a function of NO coverage using temperature- programmed desorption. Our results show that NO reaction with h-TiO 2 leads to formation of NH 3 , which is observed to desorb at ∼400 K. Interestingly, the amount of NH 3 produced depends nonlinearly on the dose of NO. The yield increases up to a saturation value of ∼1.3 × 10 13 NH 3 /cm 2 at a NO dose of 5 × 10 13 NO/cm 2 , but subsequently decreases at higher NO doses. Preadsorbed H 2 O is found to have a negligible effect on the NH 3 desorption yield. Additionally, no NH 3 is formed in the absence of surface hydroxyls (HO b ’s) upon coadsorption of NO and H 2 O on a stoichiometric TiO 2 (110) (s-TiO 2 (110)). On the basis of these observations, we conclude that nitrogen from NO has a strong preference to react with HO b ’s on the bridge-bonded oxygen rows (but not with H 2 O) to form NH 3 . The absolute NH 3 yield is limited by competing reactions of HO b species with titanium-bound oxygen adatoms to form H 2 O. Our results provide new mechanistic insight about the interactions of NO with hydroxyl groups on TiO 2 (110) . 1. INTRODUCTION Nitrogen oxides (mainly NO and NO 2 ), or NO x , are common air pollutants formed when fuels are burned at high temperatures, as in a combustion process. The primary sources of NO x are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels. Developing efficient catalytic processes for NO x conversion to environ- mentally benign N 2 and water represent one of the critical challenges in catalysis. It has been shown that NO x can be removed by a catalytic reduction over mixed oxide catalysts, 1,2 such as V 2 O 5 /TiO 2 , 3 WO 3 /TiO 2 , 4 and V 2 O 5 -WO 3 /TiO 2 . 5 Thus, as a support to the de-NO x catalysts, 6 NO and NO 2 reduction has been extensively studied on both anatase TiO 2 powders 7-13 and prototypical single crystalline rutile TiO 2 (110) 14-22 surfaces. For NO, which is our interest in this study, many important aspects of the reaction on TiO 2 have been revealed over the past decades. On both oxidized 14 and reduced 16 TiO 2 , NO molecules have been found to react with each other to form N 2 O while leaving an oxygen atom on the surface. Additionally, UV irradiation has been observed to initiate photoinduced decomposition of NO to N 2 O, which desorbs from the surface at 110 K. 7 However, it is also recognized that the reactivity of NO with TiO 2 is strongly influenced by surface defects such as oxygen vacancies and Ti 3+ 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 O 2 . 23-26 Understanding the interactions of NO with surface hydroxyl groups is also of fundamental importance, since they are abundant in many TiO 2 -based catalysts prepared by hydrolysis. Also, in the catalytic reduction of NO with NH 3 ,N-H bonds in NH 3 are being broken, leading to surface hydroxyls for a subsequent decomposition of NO into N 2 and H 2 O. 6,27 Thus, understanding the reactions of hydroxyls with NO is needed to provide insight into the mechanism of catalytic reduction of NO with NH 3 . In addition, hydroxyl species on TiO 2 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 TiO 2 at room temperature, 19 suggesting that they interact strongly with NO and possibly facilitate its dissociation. On TiO 2 (110), surface hydroxyls (HO b ’s) can easily form by dissociative adsorption of H 2 O on oxygen vacancies (V O ’s) on bridge-bonded oxygen (O b ) rows. Many theoretical and experimental studies have shown that there is charge associated with the HO b ’s similar to the V O ’s. 28,31 In this report, we show that NO interacts with HO b ’s but not with H 2 O on hydroxylated TiO 2 (110) to produce NH 3 , under temperature-programmed desorption (TPD) reaction condi- tions. We show that the amount of NH 3 depends nonlinearly on the coverage of NO. It increases up to a saturation value of Received: November 1, 2014 Revised: December 15, 2014 Published: December 17, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 1130 DOI: 10.1021/jp5109619 J. Phys. Chem. C 2015, 119, 1130-1135