Defect chemistry in ruthenium-supported tin dioxide: a spectromagnetic approach

The reactivity towards CO and air of SnO2 defects, singly and doubly ionized oxygen vacancies (V(o)/· V(o)/··), bivalent tin centers, was studied in ruthenium-supported tin oxide (Ru/SnO2), and compared with that in pure SnO2. Electron paramagnetic resonance and X-ray photoelectron spectroscopy studies demonstrated that CO treatment produces V(o)/· and V(o)/·· defects in SnO2, some V(o)/· transferring their electrons to Sn4+ centers; instead in Ru/SnO2, some V(o)/· defects transferred their electrons to Ru(n+) centers (n=0,1,2,3) and no one bivalent tin center was observed. When contacted with air, pure SnO2 transferred a part of the electrons of V(o)/· and of bivalent tin centers to O2, instead Ru/SnO2 emptied all V(o)/· defects transferring the electrons from V(o)/· to ruthenium and to O2. The via ruthenium transfer increases the number of electrons exchanged between SnO2 and the surrounding atmosphere and gave a rationale for the higher sensitivity towards CO displayed by transition metal-doped SnO2 with respect to pure SnO2.