The roles of phosphate and tungstate species in surface acidities of TiO2-ZrO2 binary oxides – A comparison study

Abstract

Porous tungstated and phosphated TiO2-ZrO2 (TZ) binary oxides with high and strong acidity were successfully prepared by means of sol-gel or impregnation approaches. In addition, the influences of the two types of modifiers on the microstructures and acidity were systematically examined, compared, and clarified. The TZ oxide derived from a surfactant-templating method exhibited a high surface area of 195 m2/g with a pore size of 6.3 nm. Moreover, it had a high acidity of 859 µmol/g with a density of 4.4 µmol/nm2 because of defective surface. Phosphation significantly increased the acidity to 1547 µmol/g and showed the highest acid density of 6.7 µmol/nm2 at a surface P density of 22.7P/nm2. On the other hand, tungstated compounds just showed the highest acidity of 972 µmol/g and the highest acid density of 4.8 µmol/nm2 at 4.7 W/nm2. Compared to tungstate species, phosphate anions are more capable of promoting the acidity because they are able to distort the host network and inhibit elemental rearrangement. While Lewis acidity prevailed in the tungstated compounds, Brønsted acidity was dominant in the phosphated oxides. The WO and POH groups were responsible for strong acidity in the modified compounds. Phosphated compounds formed strong Brønsted acid sites on the POH groups with a particular strength, and tungstation produced Lewis acid sites with a continuous strength on the metal ions adjacent to the tungstate moieties. Cyclic NH3 adsorption-desorption processes revealed that the active sites for NH3 adsorption were stable in both the tungstate and phosphate modified compounds, revealing that these solid acids are promising as the adsorbents for removal of base gases.