Applied Catalysis B: Environmental 207 (2017) 166–173 Contents lists available at ScienceDirect Applied Catalysis B: Environmental j ourna l h omepa ge: www.elsevier.com/locate/apcatb CO oxidation over Pd supported catalysts —In situ study of the electric and catalytic properties V. Bratan, C. Munteanu, C. Hornoiu ∗ , A. Vasile, F. Papa, R. State, S. Preda, D. Culita, N.I. Ionescu Institute of Physical Chemistry “Ilie Murgulescu” of the Romanian Academy – Splaiul Independentei 202, 060021 Bucharest, Romania a r t i c l e i n f o Article history: Received 22 November 2016 Received in revised form 16 January 2017 Accepted 5 February 2017 Available online 6 February 2017 Keywords: CO oxidation Pd/SnO2/TiO2 and Pd/TiO2 catalysts Electrical conductivity a b s t r a c t In this paper, the CO oxidation on Pd/TiO 2 and Pd/SnO 2 /TiO 2 catalysts was studied using a non- conventional tool: AC electrical measurements in operando conditions. The structural and textural properties of the obtained powders have been characterized using BET, XRD, SEM, and CO chemisorp- tion. Their redox properties were tested using the TPR technique. The surface dynamics was studied by electrical conductivity measurements under similar conditions with those encountered in the practical use in catalysis and correlated with their performances in CO oxidation. © 2017 Elsevier B.V. All rights reserved. 1. Introduction During the past several years a major attention was focused on the detection and removal of carbon monoxide, resulted from auto- motive emissions and industrial processes. The purpose of such research is to improve the quality of the environment. The cat- alytic oxidation is the main solution for the CO abatement. Many transition metal catalysts, such as Pd, Pt, Rh, and Au have been demonstrated to be very efficient in CO oxidation at low temper- atures [1–5]. Noble metal catalysts usually require temperatures above 100 ◦ C for efficient operation. Pd-supported catalysts are used in commercial three-way cat- alysts, for CO and hydrocarbons oxidation, especially for that of methane [6–8]. Most Pd-containing catalysts used in CO oxidation have been supported on metal oxides, such as CeO 2 , Co 3 O 4 , Mn 2 O 3 , SnO 2 , and TiO 2 (Pd is one of the most active metals for interacting with the surface of oxides). The influence of the metal oxide sup- port on the enhancement of the catalytic activity is well known and extensively studied [1,3,9,10]. Based on the previous studies, key factors that determine the behavior of the catalyst were found to be the oxygen storage capac- ity and the release properties of the support [11]. The competitive adsorption of CO and oxygen on Pd surface suppresses CO oxida- tion at lower temperatures [7]. The choice of a proper support that ∗ Corresponding author. E-mail address: chornoiu@icf.ro (C. Hornoiu). activates oxygen improves the catalytic cycles. This is the case of Pd/CeO 2 , Pd/TiO 2 and Pd/SnO 2 [12–14]. On the other hand, the contribution of the two controlling fac- tors for the low-temperature oxidation of CO on Pd-supported catalysts are not sufficiently clarified, namely the valence state and the size of the metallic particles. Some studies considered that oxi- dized Pd plays the main role in the oxidation process, acting as an oxygen reservoir [8,15–17]. Other studies considered that the active phase in CO oxidation is the metallic Pd [18–21]. Anyway, a lower dispersion of Pd seems to be preferable for a higher activ- ity, because highly dispersed Pd particles are easily oxidized but difficult to reduce them after [8,11]. The surface oxidation or reduction of semiconducting oxides- based catalysts could be investigated by following the evolution of the electrical properties of the solid in relation with a specific atmo- sphere. The surface conductivity may be increased or decreased by a chemisorbed substance. For example, in the case of n-type semicon- ductors, the surface conductivity is increased by electron transfer from a chemisorbed species to the solid (leading to a positively charged adsorbate) and decreased by electron transfer from solid surface to the adsorbate (leading to a negatively charged adsor- bate). Thus, exposure to oxygen of an n-type semiconductor leads to the decrease of the conductivity with respect to that in an inert atmosphere, as a result of oxygen ions adsorption (the electron trapped by the adsorbed oxygen are provided by the surface of the oxide). On the contrary, the interaction with a reducing gas (such CO, H 2 ) results either in the formation of anion vacancies (acting as electron donors) or in the consumption of the oxygen ad-ions. Both http://dx.doi.org/10.1016/j.apcatb.2017.02.017 0926-3373/© 2017 Elsevier B.V. All rights reserved.