Occurrence and Mechanism of Formation of CeOCl in Pd/CeO2 Catalysts

The formation of the CeOCl phase in a Pd(Cl)/CeO2 catalyst during reduction in hydrogen was studied by HRTEM, XRD, and low-temperature N2 adsorption (BET). The results indicate that CeOCl crystallites nucleate at (110) faces of CeO2 crystals at T≤300°C and then grow by coalescence of existing nuclei and attachment of Ce–O–Cl species continuously formed at the ceria surface. Lateral growth of CeOCl results in rapid decline of the BET surface of the catalyst between 300 and 500°C due to the “enwrapping” of the grains of the catalyst. The mechanism of an ordered CeOCl–CeO2 transformation in an oxidizing atmosphere is proposed.