HYDROGEN RELEASE FROM Ni/FAUJASITE CATALYSTS

Mono- and multimetallic Ni/NaY samples have been studied by static and dynamic techniques. The reducibility of Ni is dramatically enhanced by Pt clusters in supercages and by Mn ions blocking small zeolite cages. No H-2 is trapped by NaY, but for Ni/NaY, NiPt/NaY, or NiPtMn/NaY more H-2 is released in temperature programmed "desorption" (TPD) than can be adsorbed at 25 degrees C. Two phenomena are responsible for the apparent discrepancy between ad- and desorption: (1) Ni inside small cages can ad- and desorb hydrogen only when cage windows are widened by thermal vibration to permit passage of H-2. (2) Above 500 degrees C the original reduction process Ni2+ + H-2 = Ni-0 + 2H(+) is reversed, i.e. Ni atoms are oxidized by hydroxyl groups. This is confirmed by the H-2 consumption in subsequent reduction. The high-temperature TPD feature can be used to estimate the amount of reduced Ni that is located in small cages.