Enhanced Reduction of Fe2O3 Caused by Migration of TM Ions out of Zeolite Channels

Abstract

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 H2 TPR peak to lower temperature are found compared to the same peak for pure Fe2O3. Such reduction enhancement is, however, only observed with precalcined mixtures, never with 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 Fe2O3. This thermodynamically permitted 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 mobility 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 their migration can be accelerated by exposure to water vapor.