CHARACTERIZATION OF CU(II) SITES IN CU SNO2 CATALYSTS BY ELECTRON-SPIN ECHO ENVELOPE MODULATION SPECTROSCOPY/

The Cu(LI) sites in different preparations of tin oxide catalysts with low Cu(II) contents were characterized by EPR spectroscopy and electr on spin echo envelope modulation (ESEEM) spectroscopy. The catalysts w ere prepared by two methods: (a) coprecipitation of a mixed oxide gel from aqueous solutions containing both tin(IV) and copper(II) ions and (b) by the sorption of Cu2+ cations onto tin(IV) oxide gel from aqueo us solution. The samples were studied both before and after calcinatio n. The EPR spectrum showed that from each type of preparation two majo r types of Cu(II) species, termed A and B, were generated, Prior to an y thermal treatment the major species in both preparations was A, wher eas after calcination at 573-1073 K the major species was B. Whilst th e EPR spectrum of species A showed that it is static (on the EPR time scale) both at 100 K and at ambient temperatures, species B showed dyn amic effects above 100 K which we attribute to a dynamic Jahn-Teller e ffect, The immediate environment of the Cu(II) was investigated in det ail by following modulation from low-abundance Sn-117.119 nuclei and f rom H-1 nuclei in water and/or hydroxyl groups. In the latter we focus ed on the H-1 combination harmonics generated in the two- and four-pul se ESEEM experiments, From these experiments we concluded that in spec ies A the Cu(II) is hydrated and situated on the external surface, coo rdinated either directly to a surface oxygen or via a hydrogen bond. I n species B the Cu(II) is well incorporated into the SnO2 lattice, it has very few protons in its vicinity, and some of the copper ions have an OH in their first coordination shell. This assignment was further substantiated by the inaccessibility of the Cu(II) species in B to ads orbed ammonia. The major difference between the two preparations is th e significant amount of species B in the coprecipitated material prior to calcination.