TEMPERATURE-PROGRAMMED DESORPTION OF OXYGEN FROM PT FILMS INTERFACED WITH Y2O3-DOPED ZRO2

The origin of the effect of nonfaradaic electrochemical modification o f catalytic activity (NEMCA) or electrochemical promotion was investig ated via temperature-programmed desorption (TPD) of oxygen from polycr ystalline Pt films deposited on 8 mol% Y2O3-stabilized ZrO2 (YSZ), an O2- conductor, under high-vacuum conditions and temperatures of 600 to 900 K. Oxygen was adsorbed both via the gas phase and electrochemical ly, as O2-, via electrical current application between the Pt catalyst film and a Au counter electrode. Gaseous oxygen adsorption gives a si ngle adsorption state (T-p approximate to 720-730 K) but electrochemic al or mixed gaseous-electrochemical adsorption was found to cause sign ificant oxygen backspillover from the YSZ solid electrolyte onto the P t surface and the creation of two distinct oxygen adsorption states, i .e., a strongly bonded ionic state (T-p approximate to 750-780 K) and a weakly bonded state (T-p approximate to 675-685 K). The creation of these two states is also manifest by potentiometric work function meas urements and high temperature cyclic voltammetry. These results provid e a straightforward explanation of the effect of electrochemical promo tion on Pt deposited on O2- conducting solid electrolytes. The observe d pronounced catalytic rate enhancement in electrochemical promotion s tudies is due to the high reactivity of the weakly bonded oxygen state , while strongly bonded ionic oxygen acts asa sacrificial promoter. Th e binding strength and average dipole moment of the weakly bonded oxyg en state was investigated as a function of applied potential. It was f ound that the binding energy of adsorbed oxygen decreases linearly wit h increasing catalyst potential and work function in agreement with re cent ab initio quantum mechanical calculations. (C) 1998 Academic Pres s.