PREPARATION, CHARACTERIZATION, AND ACTIVITY OF CU/TIO2 CATALYSTS .1. INFLUENCE OF THE PREPARATION METHOD ON THE DISPERSION OF COPPER IN CU/TIO2

摘要

Different techniques have been used to characterize Cu/TiO2 (Degussa P-25) catalysts with copper loadings of 2, 4, and 8 wt% Cu, prepared by wet impregnation (labeled I) or by chemisorption-hydrolysis (labeled C). X-ray diffraction patterns of the 8% I sample show, before calcination, the diffraction lines of the monoclinic modification of Cu-2(OH)(3)NO3, while after calcination quite large CuO crystallites are observed. In the 8% C sample Cu-2(OH)(3)NO3 is not found before calcination and smaller CuO crystallites are observed thereafter. High-resolution transmission electron microscopy indicates a different behavior of the two calcined samples to electron beam exposure: on I samples the electron beam produces an amorphous overlayer, covering in a continuous way the TiO2 crystallites, whereas on C samples small particles nucleate at the surface of the titania particles. Temperature programmed reduction (TPR) of the C samples shows three narrow peaks in the range 435-470 K, very well resolved in the sample with 4% of copper, while TPR of I samples shows a broad unresolved peak at 440-530 K. The adsorption of CO on the two kinds of reduced samples produces infrared absorption bands in the 2070-2140 cm(-1) wavenumber range with intensity and position strongly dependent on the preparation method and on the sample pretreatments. On all the I samples, there is a band at 2113-2130 cm(-1) which is reduced in intensity with increasing the reduction temperature. On the C samples, three well-resolved bands at 2126, 2103, and 2071 cm(-1) are detected, the last two of which gradually reduce in intensity with decreasing CO pressure. The 2103 cm(-1) band is assigned to CO adsorption on Cu-0 step and edge sites, the 2071 cm(-1) band is assigned to CO adsorbed on Cu(lll) microfacets and on sites at the edges of small copper particles, and the 2126 cm(-1) band, observed also on I samples, is assigned to CO adsorption on isolated Cu atoms and/or two-dimensional small Cu clusters on the surface of titania. The 2103 and 2071 cm(-1) bands are always absent on I samples, indicating that the wet impregnation method leads to samples in which three-dimensional copper particles are almost completely lacking. (C) 1997 Academic Press [References: 34]