Infrared and temperature-programmed desorption study of acetophenone on Pd/SiO₂

The adsorption and reaction of acetophenone and isotope-labeled acetophenone were investigated on reduced Pd/SiO₂, Pd black, and oxygen-dosed Pd/SiO₂ via thermal desorption mass spectrometry and FT-IR. The support can strongly influence the decomposition reaction of acetophenone on Pd/SiO₂. Acetophenone is initially coordinated to Pd/SiO₂ in the η¹(O) configuration at room temperature. The η¹-acetophenone either desorbs without reaction or converts to η²(O,C)-acetophenone during annealing of the adlayer. The η²-acetophenone starts to decompose and/or desorb at temperatures higher than 50°C. Acetophenone decomposes to benzene, toluene, CO, H₂, and hydrocarbon fragments on Pd catalysts during the course of TPD experiments. Benzene desorbs in α, β, and γ states, respectively, centered at 135, 180, and 250°C. The α state is from the benzene decomposition product of acetophenone adsorbed on the interface between Pd and SiO₂. The β state is from the benzene adsorbed on the Pd metal surface. The γ state comes partly from the benzene adsorbed on the Pd surface. Some part of the γ state comes from the decomposition reaction of the acetophenone that migrates from the support to the Pd particle. Toluene desorbs in α and γ states centered at 135 and 250°C. The α state toluene comes from the reaction of the β state benzene and acetyl species at the interface between Pd and SiO₂. The γ state toluene comes from the reaction of the β state benzene and acetyl species produced from the decomposition reaction of the acetophenone that migrates from the support to the Pd particle.