Temperature programmed desorption of oxygen from Pd films interfaced with Y2O3-doped ZrO2

Abstract

The origin of the effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) or Electrochemical Promotion was investigated via temperature-programmed-desorption (TPD) of oxygen, from polycrystalline Pd films deposited on 8 mol%Y 2 O 3 -stabilized-ZrO 2 (YSZ), an O 2-conductor, under high-vacuum conditions and temperatures between 50 and 250°C. Oxygen was adsorbed both via the gas phase and electrochemically, as O 2-, via electrical current application between the Pd catalyst film and a Au counter electrode. Gaseous oxygen adsorption gives two adsorbed atomic oxygen species desorbing at about 300°C (state b 1 ) and 340-500°C (state b 2 ). The creation of the low temperature peak is favored at high exposure times (exposure [1 kL) and low adsorption temperatures (T ads \ 200°C). The decrease of the open circuit potential (or catalyst work function) during the adsorption at high exposure times, indicates the formation of subsurface oxygen species which desorbs at higher temperatures (above 450°C). The desorption peak of this subsurface oxygen is not clear due to the wide peaks of the TPD spectra. The TPD spectra after electrochemical O 2-pumping to the Pd catalyst film show two peaks (at 350 and 430°C) corresponding to spillover O ads and O dÀ ads according to the reaction:The formation of the spillover O dÀ ads oxygen species is an intermediate stage before the formation of the atomic adsorbed oxygen, O ads . Mixed gaseous and electrochemical adsorption was carried out in order to simulate the Electrochemical Promotion conditions. The initial surface coverage with oxygen from the gas phase plays a very important role on the high or low effect of polarization. In general mixed adsorption leads to much higher oxygen coverages compare with that observed either under gaseous or electrochemical adsorption. The binding strength of the atomic adsorbed oxygen (state b 2 ) was investigated as a function of applied potential. It was found that the binding energy decreases linearly with increasing catalyst potential and work function. Similar behavior has been observed for oxygen adsorption on Pt, Ag and Au deposited on YSZ in previous studies.Keywords Temperature programmed desorption Á Oxygen adsorption on Pd Á Electrochemical promotion Á Electrochemical adsorption Á Palladium supported catalyst Á Spillover-backspillover of oxygen Á NEMCA effect 1 Introduction

Recent developments and trends in the electrochemical promotion of catalysis (EPOC)

“…Besides the results which have been already reported in the literature till 2001 and reviewed by Professor Vayenas et al in the ''Electrochemical Promotion of Catalysis'' book [28] several new studies have been reported till the end of 2008 using various techniques and methods such as TPD [29,30,32,47,72], TPR [29], CV [21], FTIR [21], SEM-EDX [21], AC impedance spectroscopy [39], STM [73,74], work function measurements and theoretical considerations [18,75,76], transients experiments and kinetic studies [34,35], quantum mechanical calculations and Monte Carlo simulations [77] and general thermodynamic considerations [18,78,79]. The use of oxygen temperature programmed desorption both under UHV and atmospheric pressure conditions has proven [29,30] the existence of two different and distinct oxygen adsorption states on the catalyst surface when oxygen ion conductors used like YSZ.…”

“…2 which shows the general behaviour of oxygen adsorption on polycrystalline Pt deposited on O 2-solid electrolytes, such as YSZ. Temperature Programmed Desorption (TPD) of oxygen under Ultra High Vacuum (UHV) conditions has been used extensively to study the difference of non-stoichiometric lattice oxygen from oxygen originating from the gas phase [28][29][30][31][32]. Figure 2a presents the O 2 TPD spectra on Pt/YSZ when oxygen is supplied from the gas phase.…”