Journal of Catalysis 191, 364–372 (2000) doi:10.1006/jcat.2000.2811, available online at http://www.idealibrary.com on Enhanced Reduction of Fe 2 O 3 Caused by Migration of TM Ions out of Zeolite Channels Olga E. Lebedeva 1 and Wolfgang M. H. Sachtler 2 V. N. Ipatieff L aboratory, Center for Catalysis and Surface Science, Department of Chemistry, Northwestern University, Evanston, Illinois 60201 Received August 10, 1999; revised October 25, 1999; accepted January 10, 2000 Theoretical models do not predict significant hydrogen spillover from metal particles to the surface of a nonconducting oxide in the absence of a high concentration of OH groups. This is confirmed by measuring the reduction enhancement of hematite powder that is intimately mixed with H-zeolite-supported Pt, Pd, Rh, or Co. For Pt, Pd, or Rh in various zeolites, marked shifts of the H 2 TPR peak to lowertemperature are found compared to the same peak forpure Fe 2 O 3 . Such reduction enhancement is, however, only observed with precalcined mixtures, neverwith mixtures that were only reduced after grinding. This eliminates H spillover along zeolite walls as an effective mechanism. Instead, transition metals (TMs) in zeolite cavities form oxide particles which react with the protons to form TM ions during calcination. These ions migrate out of the zeolite to the iron oxide, where they can be reduced to TM metals, now positioned directly on the Fe 2 O 3 . This thermodynamically permit- ted type of H spillover to the oxide thus requires direct contact between the metal and the reducible oxide. The mechanism has general validity;however, marked differences between metals and different zeolites illustrate which factors are crucial for TM ion mo- bility through zeolite channels and over hematite particles. While migration of TM oxide clusters through zeolite channels is faster than migration of TM ions, ligated ions move faster than naked ions, and theirmigration can be accelerated by exposure to water vapor. c  2000 Academic Press Key Words: permitted and forbidden hydrogen spillover;ion mi- gration out of zeolites; enhanced reduction of iron oxide; diffusion across solid-phase boundaries. 1. INTRODUCTION Manycatalyticand solid-state chemicalprocessesinvolve migration of atoms along the surface or in the bulk. A case in point is the reduction of hematite, Fe 2 O 3 , to magnetite, Fe 3 O 4 . For powders which were ground to introduce lat- tice defects, this process involves removal of oxygen from the surface and migration of Fe 2+ ions to octahedral and tetrahedral positions in the interior (1, 2). If hydrogen is 1 On leave from Kazakh State National University, Almaty, Republic of Kazakhstan. 2 To whom correspondence should be addressed. Fax: (847) 467-1018. E-mail: wmhs@nwu.edu. used as the reductant, H 2 molecules must first dissociate at the surface; this step is greatly enhanced by the presence of Fe 0 or other transition metals (TMs). It is generally as- sumed that H atoms migrate from the TM particles to the reducible oxide; the term “hydrogen spillover” was intro- duced byBoudart in 1969(3).Numerousobservationshave been attributed to thisphenomenon (4,5).A significant en- hancement ofthe reduction rate isachieved bydepositingPt particleson the surface ofFe 2 O 3 ;temperature-programmed reduction (TPR) data show that this lowers the tempera- ture at which Fe 2 O 3 isreduced bymore than 100 ◦ C (6). This spillover process can be described as the transformation of an H atom,chemisorbed on the Ptsurface,into a proton and an electron, with the proton bonded to an O 2− ion of the oxide and the electron reducing an Fe 3+ ion to an Fe 2+ ion: H ads → H + + e − . As a reducible oxide is also a semiconductor, further mi- gration of the H atom over the surface of the oxide is then interpreted as migration of a proton–electron pair. As this model requires a finite rate of electron migration, it does not predict any mobility of spilt-over H atoms on the sur- face of an electrically isolating oxide, such as SiO 2 ,Al 2 O 3 , or a zeolite, except at very high temperatures. Claims have, however, been made in the literature that hydrogen spillover including migration over large distances takes place at low temperatures over the surfaces of SiO 2 , Al 2 O 3 , or glass; see for instance Refs. (7, 8). No identifi- cation of the state of the migrating species has, however, been presented. As electron migration appears unlikely, one could imagine that an H atom, adsorbed on the sur- face of a TM, might react with the proton end of an ad- jacent hydroxyl group of the oxide, forming an (H 2 ) + ion attached to the same O 2− ion.For the reaction ofa free pro- ton with an H atom spectroscopic data predict an exother- micity of 2.6507 eV = 255.7 kJ mol −1 (9). When considering that the energy of an H atom on a TM surface is roughly 255 kJ/mol lower than that of a free H atom, such a process appears thermodynamically plausible. For the H migration over the oxidesurface,itrequires,however,ahigh densityof 0021-9517/00 $35.00 Copyright c  2000 by Academic Press All rights of reproduction in any form reserved. 364