Development of the acidity of zirconia-supported niobia catalysts

A series of NbO_x/ZrO₂ catalysts containing ≤ 2.67 wt % Nb (≈ 80% nominal surface coverage) was prepared by incipient wetness impregnation from niobium oxalate and oxalic acid solution. Isopropanol decomposition was used as test reaction to assay the acidity of the solids and to seek a correlation between acidity and catalytic performance. Lewis acidity was found for all systems, while significant Brönsted acidity was only developed for the Nb/Al₂O₃ catalysts. In the latter case, Brönsted acid sites were detected after a critical Nb loading. The decomposition of 2-propanol on NbO_x/ZrO₂ catalysts was directly related to the presence and abundance of Brönsted acid sites. By comparing the reported structure of Nb species with the observed isopropanol decomposition activity and the abundance of Brönsted acid sites, monomeric species did not exhibit Brönsted acidity and were essentially inactive. The catalytic activity was associated with polymeric Nb species.