Characterization of Cu(II) sites in Cu/SnO2 catalysts by electron spin echo envelope modulation spectroscopy

Khalid Matar, Dongyuan Zhao, Daniella Goldfarb, Wan Azelee, Wayne Daniel, Philip G. Harrison

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Abstract

The Cu(II) sites in different preparations of tin oxide catalysts with low Cu(II) contents were characterized by EPR spectroscopy and electron spin echo envelope modulation (ESEEM) spectroscopy. The catalysts were 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 aqueous solution. The samples were studied both before and after calcination. The EPR spectrum showed that from each type of preparation two major types of Cu(II) species, termed A and B, were generated. Prior to any thermal treatment the major species in both preparations was A, whereas after calcination at 573-1073 K the major species was B. Whilst the 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 dynamic effects above 100 K which we attribute to a dynamic Jahn-Teller effect. The immediate environment of the Cu(II) was investigated in detail by following modulation from low-abundance 117.119Sn nuclei and from 1H nuclei in water and/or hydroxyl groups. In the latter we focused on the 1H combination harmonics generated in the two- and four-pulse ESEEM experiments. From these experiments we concluded that in species A the Cu(II) is hydrated and situated on the external surface, coordinated either directly to a surface oxygen or via a hydrogen bond. In 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 adsorbed ammonia. The major difference between the two preparations is the significant amount of species B in the coprecipitated material prior to calcination.

Original languageEnglish
Pages (from-to)9966-9973
Number of pages8
JournalJournal of Physical Chemistry
Volume99
Issue number24
DOIs
Publication statusPublished - 1 Jan 1995
Externally publishedYes

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