First Knowledge on the Formation of Novel Core−Shell Structures in PdCu Catalysts and Their Influence on the Prevention of Catalyst Deactivation

Abstract

TiO2 supported PdCu catalysts (PdCu/TiO2) showed a good long-time stability (160 h) with high activity (X = 60%) and selectivity (ca. 95%) in the gas phase acetoxylation of toluene to benzyl acetate. Characterization of these catalysts was mainly carried out using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). These methods provided new insights into the dynamic changes occurring during time-on-stream particularly in the catalyst structure and the nature of carbon deposition. Migration of Cu from deeper layers to the near-surface region during the reaction could be observed. In addition, the Pd particles also grew significantly during the reaction. Interestingly, formation of novel core−shell structures with Pd core and CuO shell was observed during time-on-stream. Such core−shell arrangement observed for the first time in PdCu solids was found to influence the nature of carbon deposits during the catalytic reaction showing a layer-like carbon structure. This observation made on PdCu/TiO2 is in contrast to the deposits found on PdSb/TiO2, where carbide-like carbon is generated and caused easy deactivation of the samples. The formation of this layer-like kind of carbon in the present PdCu solid seems to weaken the interaction between catalytically active surface species (Pd and/or Cu) and carbon atoms from deposited coke, which in turn leads to a stable catalytic performance.