NITRIC-OXIDE REDUCTION BY CO ON CU TIO2 CATALYSTS

Characterization by high-resolution transmission electron microscopy of differently pretreated Cu/TiO2 samples and IR and mass spectroscopic studies of NO-CO interaction on them from room temperature to 473 K are presented. All samples are characterized by Cu particles well dispersed on TiO2 surface. The adsorption of CO produces an absorption in the 2112-2130 cm-1 frequency range; the intensity and position of the band strongly depends on the sample pretreatment, and it is strongly affected by NO interaction. NO dosing on a sample reduced at low temperature (LTR) precovered with CO leads to the formation of some CO2 but the reaction is not complete after heating at 473 K; on a sample reduced at high temperature (HTR) large amounts of N2O and CO2 are detected already after short contact times at room temperature. By heating at 473 K an almost complete conversion to CO2 and N2 is observed. On both LTR and HTR samples neither NO nor isocyanate-adsorbed species are detected. Reaction products are observed if the sample HTR is contacted, after outgassing of a previous interaction mixture without desorption of the oxygen atoms produced in the NO dissociation, with a 1:1 NO-CO mixture or with CO and then with NO. If NO is preadsorbed prior to CO dosing in a second run of the reaction, almost no N2O and CO2 are observed. The experimental data indicate that HTR Cu/TiO2 is significantly more active than LTR Cu/TiO2, the activity being related to an easy dissociation of the NO molecules, determined by an electron transfer from reduced titania to antibonding orbitals of the NO. Free reduced copper sites are needed for CO activation, the reaction being to some extent inhibited by NO oxidation of the copper. (C) 1994 Academic Press. Inc.