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

Porous tungstated and phosphated TiO ₂ -ZrO ₂ (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 m ² /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/nm ² because of defective surface. Phosphation significantly increased the acidity to 1547 µmol/g and showed the highest acid density of 6.7 µmol/nm ² at a surface P density of 22.7P/nm ² . On the other hand, tungstated compounds just showed the highest acidity of 972 µmol/g and the highest acid density of 4.8 µmol/nm ² at 4.7 W/nm ² . 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 W[dbnd]O and P–OH groups were responsible for strong acidity in the modified compounds. Phosphated compounds formed strong Brønsted acid sites on the P–OH 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 NH ₃ adsorption-desorption processes revealed that the active sites for NH ₃ 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.