Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc Characterization of peroxo species on TiO x /Rh(111) single crystal K. Mudiyanselage, H. Idriss ⁎ Catalysis Department, SABIC-Corporate Research and Development (CRD) at KAUST, 23955 Thuwal, Saudi Arabia ARTICLE INFO Keywords: Peroxo Hydroperoxo Hydrogen peroxide TiO 2 Photocatalytic water splitting Infrared reflection absorption spectroscopy ABSTRACT Identification and characterization of peroxo species (eOeOe and eOeOeH), which has been proposed to accumulate on surfaces during photocatalytic water splitting and hence inhibit the reaction rate, is important to understand this catalytic process. In this study, peroxo species were prepared and characterized on model TiO x / Rh(111) systems. The TiO x /Rh(111) systems were prepared by chemical vapor deposition of TiO x on Rh(111) using titanium tetraisopropoxide (TTIP – (Ti[OCH(CH 3 ) 2 ] 4 )) as a precursor for TiO 2 and characterized with Auger electron spectroscopy. Peroxo species were obtained by exposing the TiO x /Rh(111) systems to H 2 O 2 at 300 K and characterized by infrared reflection absorption spectroscopy (IRRAS) and temperature programmed desorption (TPD). Peroxo species formed on both TiO x clusters and films on Rh(111) decompose completely by 700 K. In order to study their possible reactions, alkenes (ethylene, propylene and cyclooctene, separately) were co-dosed and the surface was subsequently heated at different temperatures. No evidences for the formation of reaction intermediates were observed by IRRAS. TPD experiments of the same systems further confirmed the absence of epoxidation products between the peroxo species and adsorbed olefins. 1. Introduction During photo-irradiation of aqueous suspensions of metal-loaded TiO 2 powders, evolution of a small amount of H 2 is observed due to water-splitting but O 2 is not detected [1]. The main explanation pro- vided in previous studies for the absence of O 2 evolution is that oxi- dation of water forms stable (inactive) hydrogen peroxide/peroxo “like” species instead of making O 2 under photocatalytic water-splitting conditions [2]. Many studies have reported that peroxo species are built up on catalyst surfaces, probably, inhibiting the water-splitting process [1–4]. Hence, identification and characterization of surface peroxo species and investigating possible methods for their removal or de- composition are important to understand the overall water-splitting process and to drive the reaction for continuous production of H 2 . Mainly two types of peroxo species, peroxo (eOeO) and hydro- peroxo (eOeOeH) with different adsorption geometries, formed during photocatalytic water-splitting reaction and direct dosing of hy- drogen peroxide (H 2 O 2 ) were reported on TiO 2 -based photocatalytic systems. The possible adsorption geometries of peroxo and hydroperoxo species on TiO 2 -based systems are shown in Scheme 1 [3]. One of the possible ways to remove adsorbed H 2 O 2 /peroxo species is through alkene epoxidation since H 2 O 2 is used as an oxidant in the industrial propylene epoxidation reaction [5–9]. Hydroperoxo (eOeOeH) has been identified as an active species in the epoxidation reaction of alkenes whereas peroxo (eOeOe) was found to be inactive [6,8,10–12]. Adsorbed peroxo species on photocatalysts were identified using vibrational spectroscopic techniques. Mainly attenuated total reflection (ATR) infrared spectroscopy for liquid–solid systems and transmission infrared spectroscopy for gas–solid systems have been used for the identification of peroxo species [13–17]. In addition, Raman spectro- scopy has also been applied on TiO 2 systems [10]. The assignment of OeO stretching vibrational frequencies observed for peroxo and hy- droperoxo species is ambiguous, and it depends on the nature of the surface, changes with adsorption geometries, and reaction conditions. In general, the OeO stretch peaks for peroxo species are broad and observed in a wide frequency range. The OeO stretches observed at 940–820 cm -1 and 800–740 cm -1 on H 2 O 2 -treated TiO 2 were attrib- uted to stretching vibrations of the OeO bonds of Ti–η 2 -peroxo and Ti–μ-peroxo/on-top hydroperoxo, respectively [18]. Munuera et al. assigned the peaks in the range 800–932 cm -1 to the OeO stretching mode of η 2 -peroxo species [1]. Nakamura et al. reported the peak at 943 cm -1 for surface peroxo, and 838 cm -1 for hydroperoxo species [14]. In a later study, Nakamura et al. assigned 838 and 812 cm -1 bands to OeO stretching modes of surface hydroperoxo, TiOOH, and peroxo, TiOOTi, respectively, under different experimental conditions [15,16]. A peak observed at 835 cm -1 on anatase film in contact with FeCl 3 aqueous solution was assigned to OeO stretching mode of TiOOH https://doi.org/10.1016/j.susc.2018.10.014 Received 29 July 2018; Received in revised form 7 October 2018; Accepted 10 October 2018 ⁎ Corresponding author. E-mail address: idrissh@sabic.com (H. Idriss). Surface Science 680 (2019) 61–67 Available online 11 October 2018 0039-6028/ © 2018 Elsevier B.V. All rights reserved. T