Surface acidic properties of supported binary oxides containing CuO coupled with Ga2O3 and SnO2 studied by complementary techniques

Silica–alumina (SA) was used as support for dispersing CuO, Ga2O3, SnO2 oxide phases and the corresponding binary couplings (CuGa/SA and CuSn/SA samples). The acidity of uncovered SA and of SA-supported oxides was studied from a qualitative (nature of acid sites) and quantitative (number, acid strength, and strength distribution of acid sites) points of view by an integrated approach. The adsorption and desorption of basic probes (ammonia and pyridine) onto the acidic surfaces were monitored by the coupled volumetric–calorimetric technique, and by XPS and FT-IR spectroscopy. The results show that the Brönsted acidity of the SA support was converted into predominantly Lewis acidity upon metal oxide deposition. The high dispersion of the copper oxide phase was responsible for this phenomenon. Differences of acid strength distribution were noticed among the samples. Moderate acidity was associated with the Cu sites (100 kJ mol−1 < qdiff < 150 kJ mol−1), whereas the most acidic fraction of the sites (qdiff > 150 kJ mol−1) increased with the presence of Ga and Sn. The experimental results are discussed in the light of the intrinsic acidity of the Cu, Ga, and Sn metal ions derived from electronic parameters.

The integrated use of several analytical techniques (XPS, FT-IR, and adsorption calorimetry) employed for the study of the acidic properties of dispersed metal oxide surfaces, CuO, Ga2O3, SnO2 and their relevant binary combinations over an acidic silico-alumina, is presented. Interesting relationships between the intrinsic electronegativity of the metal ions constituting the oxides and the mean acid strength of the surfaces have been observed. Figure optionsDownload as PowerPoint slide