Effect of gold addition by the recharge method on silver supported catalysts in the catalytic wet air oxidation (CWAO) of phenol

Abstract

Catalysts Ag/ZrO₂–CeO₂ and Au/ZrO₂–CeO₂ were synthesized by a deposition–precipitation method and Ag–Au/ZrO₂–CeO₂ was prepared using a recharge method for the second metal (Au). The materials were characterized by physisorption of N₂, XRD, ICP, UV-vis RDS, H2-TPR, XPS and TEM. The results obtained show that the specific areas for monometallic materials were 29–37 m² g⁻¹ and 27–74 m² g⁻¹ for bimetallics. The tetragonal crystal phase of ZrO₂ stabilizes when CeO₂ quantity increases. Using XPS an increment in Ce³⁺ species abundance was determined for bimetallic catalysts in contrast to the monometallic ones; according to the Ag 3d region, this metal oxidation was observed when augmenting the content of CeO₂ in the materials, and with Au the opposite effect was produced. It was determined by TEM, that the average size of the metallic particles was smaller at bimetallic catalysts due the preparation method. Catalytic activity was evaluated by CWAO of phenol, the Ag–Au/ZrO₂–CeO₂ catalyst with 20% wt of cerium reached a degradation of 100% within an hour, being the most active catalyst. Maleic, formic and oxalic acid were identified as reaction intermediates; and at the end of the reaction acetic acid was identified as the main by-product, because it is the most refractory and the conditions for oxidation must be more severe.