Studies of the surface of titanium dioxide. Part 5.—Thermal desorption of hydrogen

Abstract

The thermal desorption of hydrogen adsorbed on titanium dioxide has been studied at temperatures between 298 and 1073 K by two techniques: (i) temperature-programmed desorption (t.p.d.), which involves measuring the pressure of H₂ evolved when the sample is heated in vacuo, and (ii) measuring the hydrogen content, which involves adding the amounts of hydrogen desorbed at increasingly elevated temperatures in a stream of helium. Chemisorption of hydrogen on titanium dioxide took place only when hydrogen was introduced above 623 K. The onset of hydrogen desorption occurred at 523 K on anatase and 573 K on rutile. Anatase gave two H₂ desorption peaks[(I) at 623–673 K and (II) at 773–823 K], while rutile gave a single broad peak [(III) at 923–973 K]. Anatase released oxygen when heated above 923 K in vacuo, probably due to the decomposition of surface lattice oxygens, even after the sample was calcined at 1073 K in a stream of oxygen. The hydrogen content on anatase was much larger than that on rutile; most of the hydrogen on rutile was removed as water molecules above 773 K. From the t.p.d. experiment involving chemisorbed oxygen on the hydrogen-reduced titanium dioxide, it was inferred that hydrogen (I) is bonded to the surface and hydrogen (II) and (III) to the bulk, which has open channels. E.s.r. measurements of hydrogen-reduced titanium dioxide showed no change in the broad signal before and after the desorption run. On the basis of these results the interaction of hydrogen with titanium dioxide is discussed in connection with the formation of surface hydroxy groups.