Agglomeration and Phase Transition of a Nanophase Iron Oxide Catalyst

Abstract

The agglomeration and phase transition behavior of a nanophase iron oxyhydroxide catalyst with a mean particle diameter of ∼30 Å was examined using thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and Mössbauer spectroscopy. Unlike other FeOOH phases, which decompose to Fe 2O 3 at T < 300°C, there is no obvious phase transition between room temperature and 900°C for the 30-Å catalyst. After annealing in air for 24 h from 250 to 350°C, only part of the 30-Å catalyst sample is converted to larger α-Fe 2O 3 particles ( d > 100 Å). Both the relative fraction and average particle size of the α-Fe 2O 3 phase increase with annealing temperature. The phase transition rate is dependent on the moisture content of the catalyst surface. After identical annealing at 250°C for 24 h in air, the as-received sample consists of 65% 30-Å phase and 34% α-Fe 2O 3 phase, whereas the sample exposed to air for 5 days before annealing consists of 26% 30-Å phase and 73% α-Fe 2O 3 phase. The results indicate that due to moist-air exposure, the catalyst particles become linked by water molecules adsorbed at the surface coordination unsaturated (CUS) sites. At elevated temperatures, these water molecules are evolved from the particle linkages, facilitating both agglomeration of the 30-Å particles and the phase transition to α-Fe 3O 3.