Ilan Riess

Abstract:

A novel method is discussed for determining series of elementary steps in the reduction process of oxygen on an oxide.[1,2] The method is based on exposure of the oxide, first to ¹⁶O₂, and then to ¹⁸O₂ while monitoring the rate at which ¹⁶O¹⁸O molecules are generated and evaporate into the gas stream, under short time conditions. The parameters to be changed are oxygen partial pressure, P(O₂) (being the same for both isotopes) and acceptor doping level [A] of the oxide. ¹⁸O₂ can be applied in the form of a pulse or a step function. The rate of ¹⁶O¹⁸O generation is shown to depend on P(O₂)^m1 [A] ^m2. Another parameter that can be determined is J₀, the rate of the forward reaction in the slow step of the series which depends on P(O₂)^m3 [A] ^m4. The indices {m₁,m₂, m₃, m₄} are, in most cases, typical for a particular series of elementary steps. The series to be identified consist of fast steps ending with a relative slow one. This method is then different from the one based on the time dependence of the concentrations of ¹⁶O₂, ¹⁶O¹⁸O and ¹⁸O₂ in the gas phase.[3,4] The method is quite sensitive and even changing the source for electrons from the valence band to the conduction band changes the value of the exponents {m1,…,m4}.
The analysis assumes that the dependence of the concentrations of point defects (oxygen vacancies and electrons) in the outer most layer of the oxide on P(O₂) and [A], is known. The method was applied so far under the conditions that the P(O₂) and [A] dependence is the same as in the deep, neutral bulk. This is shown to be indeed the case under many prevailing conditions.[5] Other P(O₂) and [A] dependence of the concentrations of point defects in the outer most layer of the oxide bulk are also presented.[5] Thus it is possible to determine series of elementary steps on all type of oxides which are undoped or acceptor doped.
The method is not limited to oxygen isotope exchange and can readily be extended to other isotopes e.g. ³⁵Cl₂ and ³⁷Cl₂ exchange. Exchange of H₂ and D₂ requires special attention due to the mass effect on the chemistry of hydrogen and we show how to cope with it.[2]

1. I. Riess, Solid State Ionics, 280 (2015) 51.
2. I. Riess, Solid State Ionics, 302 (2017) 7.
3. K. Klier et al. J. Catal. 2 (1963) 479.
4. G.K. Boreskov, Adv. Catal. 15, (1964) 285.
5. I. Riess, Solid State Ionics, 329 (2019) 95.