Tin oxide surfaces. Part 12.—A comparison of the nature of tin(IV) oxide, tin(IV) oxide–silica and tin(IV) oxide–palladium oxide: surface hydroxyl groups and ammonia adsorption

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The chemical nature of the surfaces of tin(IV) oxide, tin(IV) oxide–silica and tin(IV) oxide–palladium oxide have been compared by surface dehydroxylation, deuterium exchange and ammonia adsorption. Physisorbed and hydrogen-bonded molecular water is lost from all three oxides at temperatures up to ca. 350 K. Further significant loss of water does not occur until ca. 480 K, when condensation of adjacent surface hydroxyl groups occurs. At higher temperatures, a sharp band at 3715 cm–1 is observed for tin(IV) oxide–silica, which is assigned to the v(OH) stretching vibration of isolated surface Si—OH groups. This band is present even at 723 K, when essentially all other surface hydroxyl groups had desorbed. The thermal dehydroxylation behaviour of tin(IV) oxide and tin(IV) oxide–palladium oxide is similar, although serious transmission problems occur above 523 K for tin(IV) oxide. All three oxides undergo surface deuteration with deuterium oxide at 523 K, although exchange with tin(IV) oxide–silica is not complete until 633 K. Ammonia adsorption demonstrates that tin(IV) oxide and tin(IV) oxide–palladium oxide are predominantly Lewis acidic, although weak Brönsted acidity can be observed in the presence of water vapour. Strong Brönsted-acid sites can be produced by protonation with hydrogen chloride. Surface amide groups are formed on tin(IV) oxide–palladium oxide after evacuation at 500 K, probably on palladium sites. Tin(IV) oxide–silica exhibits both Lewis and Brönsted acidity, the amount of Brönsted acidity with respect to Lewis acidity decreasing with increasing temperature.