Reduction Behavior of Metals Supported on Hydrous Titanium Oxide Ion-Exchange Materials

Abstract

Hydrous titanium oxide (HTO) ion-exchange materials provide a unique and versatile platform for obtaining high loadings of supported transition metal cations with essentially atomic dispersion. Strong interactions between the atomically dispersed metal cations and the support alter the reduction behavior of metal cations as compared to other support materials and preparation methods. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) measurements of the reduction of Rh, Ni, and Fe supported on HTO materials provide a representative cross section of possible behaviors. The results suggest that metals with high melting points and low surface mobility, such as Rh, Pt, and Pd, should retain high dispersions even after reduction at temperatures up to 300°C. Lower melting point metals such as Ni, Co, and Cu, that exhibit moderate heats of oxide formation, will likely undergo extensive sintering at temperatures above 250°C, largely negating the high dispersions characteristic of the freshly ion-exchanged state. Finally, lower melting point metals with very high heats of oxide formation, such as Fe and Cr, will be difficult to reduce due to unfavorable thermodynamics in the presence of the support. The results indicate that for certain types of metals, HTO support materials allow the preparation of catalysts with both high loadings and high metal dispersions. This capability provides a new pathway toward more efficient usage of expensive noble metals that is unavailable using conventional catalyst supports and preparation methods.