Investigation of hydrothermal routes to mixed-metal cerium titanium oxides and metal oxidation state assignment using XANES.

Abstract

The reaction between TiF(3) or TiO(2) and Ce(3+) in sodium hydroxide solutions yields highly crystalline NaCeTi(2)O(6) at room temperature and under mild hydrothermal conditions (T < or = 240 degrees C). There is no evidence for the formation of ternary Ce-Ti-O materials by this method, and the use of bases other than NaOH always produces poorly crystalline materials. The material NaCeTi(2)O(6) has a distorted perovskite structure with sodium and cerium ions randomly occupying the A sites: Pnma, a = 5.4517(8) A, b = 7.7292(6) A, c =5.4573(3) A. XANES spectroscopy at the Ti K edge and Ce L(III) edge, with reference to crystalline model compounds, reveals that cerium is found solely as Ce(III) and titanium as Ti(IV) in NaCeTi(2)O(6). Isomorphous substitution of Ce(3+) by Nd(3+) or Ti(4+) by V(4+) is found to be very facile under hydrothermal conditions (at a temperature of 240 degrees C), by addition of appropriate amounts of metal salts to the hydrothermal reaction mixtures. The series NaCe(1-x)Nd(x)Ti2O6 (0 < or = x < or = 1) and NaCeTi(2-x)V(x)O6 (0 <or = x < or = 1.8) can be formed directly in one step, and all materials were characterized by powder X-ray diffraction. The oxidation states of the substituent elements were determined using XANES spectroscopy, which further demonstrates that isomorphous substitution has taken place, with all vanadium present as V(IV) when the B site is substituted. Scanning electron microscopy analysis of the solids shows them to be made up of irregularly shaped particles made up of clusters of submicron-sized crystallites but that on firing in excess of 1000 degrees C the particle size is increased to approximately 10 microm and the particles have distinct, sharp edges.