Methane oxidation on Pd/YSZ by electrochemical promotion C. Jiménez-Borja a, ⁎, S. Brosda b, 1 , M. Makri b, 1 , F. Sapountzi b, 1 , F. Dorado a, 2 , J.L. Valverde a, 2 , C.G. Vayenas b, 1 a Department of Chemical Engineering, (UCLM), Avda. Camilo José Cela 12, 13071 Ciudad Real, Spain b Department of Chemical Engineering, University of Patras, Caratheodory St1, GR-26504 Patras, Greece abstract article info Article history: Received 7 September 2011 Received in revised form 22 December 2011 Accepted 5 March 2012 Available online 28 March 2012 Keywords: Methane combustion Pd catalyst-electrode Electrochemical promotion AC impedance spectroscopy Electrochemical investigations have been carried out on Pd/YSZ catalyst-electrodes for deep oxidation of methane in excess of oxygen. A wet impregnation technique has been used to achieve highly active Pd catalyst films. It was found that co-feeding of ethylene in oxidizing gas mixtures enhances the rate of methane conversion and allows for electrochemical promotion of methane oxidation at temperatures as low as 320 °C. The electro- chemical promotion behavior has been studied with long term potentiostatic transients together with AC imped- ance measurements during polarization. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Natural gas is an attractive energy source with industrial interest as it is the cleanest burning fossil fuel, due to its low carbon content and low levels of impurities: sulfur and nitrogen compounds. The main component (85–95%) of natural gas is methane which has the highest hydrogen to carbon ratio of all hydrocarbons; and thus the combustion of methane yields the lowest amount of CO 2 per unit of produced energy [1]. Methane combustion can be achieved through conventional flame combustion or via catalytic reaction. However, catalytic combustion has been shown to be highly efficient and effec- tive in reduction of pollutants such as carbon monoxide and nitrogen oxides [2]. Therefore, and taking into account the soaring gasoline prices and the increasingly strict emission limits, catalytic combus- tion of methane has been extensively studied for natural gas fuelled vehicles (NGVs) and as an alternative to conventional thermal com- bustion in gas power plants [3]. There is a general consensus that pal- ladium based catalysts have shown a very high activity for the catalytic combustion of methane and consequently it has been widely reported as the catalyst of choice [4,5]. Studies of methane combustion are of special importance partly because of the relative difficulty of activating this particular hydrocar- bon [6]. In this sense, the phenomenon of Electrochemical Promotion of Catalysis (EPOC) has been applied for the enhancement in catalytic activity for a wide variety of catalytic reactions [7]. This phenomenon, also known as the NEMCA effect (Non-Faradaic Electrochemical Modification of Catalytic Activity), was introduced first by the group of Vayenas in the early 80's [8]. EPOC allows for improving the cata- lytic performance of a metal catalyst in a very pronounced and con- trolled manner by the application of low current or potentials between a catalyst (which serves also as the working electrode) sup- ported on a solid electrolyte and a counter electrode also supported on the same electrolyte. This effect is based on the change of the work function due to electrochemical pumping of promoting ionic species (O δ - ) from the solid electrolyte to the metal gas interface [9] as confirmed by several surface spectroscopic and electrochemical techniques. These backspillover ionic species form an overall neutral double layer at the metal/gas interface and affect catalytic rates by modifying the binding energy of chemisorbed reactants and interme- diates primarily via lateral electrostatic interactions [7]. Two main parameters are commonly used to quantify the magnitude of the NEMCA phenomenon [7], the rate enhancement ratio (ρ) and the Faradaic efficiency (Λ), both defined as: ρ ¼ r=r 0 ð1Þ Λ ¼ r-r 0  . I=nF ð Þ¼ Δr= I=nF ð Þ ð2Þ where r 0 is the catalytic reaction rate under open circuit conditions, r is the reaction rate under polarization, n is the number of charge car- ried by the ions (n= 2 with an O 2 - conductor), Δr is the catalytic reac- tion rate change induced by a current I, and F the Faraday's constant. The electrochemically promoted methane oxidation has been pre- viously studied for Rh [10,11], Pt [12] and Pd catalyst-electrodes [13–16]. In both previous studies on Pd [13,14] the reaction was Solid State Ionics 225 (2012) 376–381 ⁎ Corresponding author. Tel.: + 34 926 295300; fax: + 34 926 295256. E-mail addresses: Carmen.JBorja@uclm.es (C. Jiménez-Borja), cgvayenas@upatras.gr (C.G. Vayenas). 1 Tel.: +30 2610 997269; fax: +30 2610 997576. 2 Tel.: +34 926 295300; fax: +34 926 295256. 0167-2738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2012.03.004 Contents lists available at SciVerse ScienceDirect Solid State Ionics journal homepage: www.elsevier.com/locate/ssi