561. The surface and materials science of tin oxide

Fig. 1 Scheme of electron transfer across the adsorbate/oxide interface in the course of the heterogeneous catalytic oxidation of hydrocarbon molecules by gas phase oxygen.

Fig. 2 (a) Lennard-Jones model for physisorption and chemisorption of a molecule. An energy barrier EA has to be overcome for a physisorbed molecule to reach the chemisorption well. Typical adsorption isobars are shown in (b). The solid lines are equilibrium physisorption and a chemisorption isobar, the dashed line represents irreversible chemisorption. A maximum coverage of chemisorbed molecules is obtainable at a temperature Tmax. Below Tmax the chemisorption is irreversible because the rate of desorption becomes negligible.

Fig. 3 Sn-O phase diagram.

Table 1 Summary of physical properties of the transparent conducting oxides, In2O3, ZnO and SnO2.

Matthias Batzill, et al, Progress in Surface Science 79, 47 (2005)
https://doi.org/10.1016/j.progsurf.2005.09.002

(1) Critical for triggering a gas response are not the lattice oxygen concentration but chemisorbed (or ionosorbed) oxygen and other molecules with a net electric charge.
(2) The catalytic cycle on oxides may include the reaction of lattice oxygen from the catalyst surface with the reactant and replacement of the lattice oxygen by gas phase oxygen at the end of the catalytic cycle. This is the so-called Mars-van Krevelen mechanism.
(3) In the case of SnO2 these are commonly Sb as a cation dopant and F as an anion dopant.
(4) Mo5+ sites play an important role in methanol and ethanol oxidation to formaldehyde and acetaldehyde or acetic acid.
(5) When a molecule adsorbs at the surface electrons can be transferred to this molecule if the lowest lying unoccupied molecular orbitals of the adsorbate complex lie below the Fermi level (acceptor levels) of the solid and vice versa electrons are donated to the solid if the highest occupied orbitals lie above the Fermi-level of the solid (donor levels).
(6) In the Lennard-Jones model the rate of chemisorption is determined by an activation barrier between a physisorbed state and the chemisorbed state and an activation barrier of desorption.
(7) These are (i) catalytic (chemical) sensitization and (ii) electronic sensitization. Chemical sensitization occurs if the supported clusters catalyze reactions and reaction products subsequently spill-over from the clusters onto the semiconducting oxide support. Electronic sensitization occurs due to the alignment of the Fermi energy of the support with the additive.
(8) Reactions of organic molecules at oxide surfaces are described in the framework of surface acid-base reactions where the oxygen anions can act as Brønsted or Lewis base sites and the metal cations are described as Lewis acid sites.
(9) O2(gas)↔O2(ad)↔O2-(ad)↔O-(ad)↔O2-(ad)↔O2-(lattice).
(10) The conversion rate of methanol to formaldehyde was found to be strongly dependent on the reduction state of the sample.
(11) Oxygen vacancies and other defect sites at the surface may play an important role for water dissociation.