Controlled site coverage of strong metal-support interaction (SMSI) on Pd NP catalysts

Strong metal-support interaction catalysts have been shown to improve desired product selectivity at the cost of fractional rates due to active site coverage. The goal of this study was to determine if the active site coverage of metallic nanoparticles could be controlled to lower levels than have been previously reported in SMSI catalysts with the aim of improving the rate while maintaining high selectivity. 2Pd-XTi/SiO2 (2 wt% Pd, X wt% Ti) strong metal-support interaction (SMSI) catalysts with Ti loadings between 0-1.0 wt% were synthesized to control Pd nanoparticle coverage. Calcination at 450 degrees C and reduction at 550 degrees C were sufficient for forming similar to 2 nm sized Pd particles in all catalysts. Increasing the Ti loading from 0.1 to 1.0 wt% increased the surface coverage from 40 to 85% at a fixed reduction temperature of 550 degrees C. The IR spectra of the SMSI catalysts were similar with a high fraction of linear bonded CO which was much higher than that of Pd nanoparticles of similar size. The SMSI overlayer could be removed by oxidation at 350 degrees C and re-reduction at 200 degrees C. EXAFS of the oxidized catalysts indicates that nearly full oxidation of the metallic nanoparticle was required to remove the SMSI overlayer. Oxidation temperatures from 30 to 300 degrees C partially oxidized the Pd nanoparticles and subsequent re-reduction at 200 degrees C partially decreases the SMSI coverage. The fractional surface coverage was determined by measuring the rate of propylene hydrogenation with and without the SMSI overlayer. Increasing the reduction temperature from 200 to 550 degrees C increased the SMSI coverage from 0 to 85% depending on the Ti loading and temperature. After reduction at 550 degrees C and oxidation at 350 degrees C, the range of coverages varied between similar to 10% with 0.1 wt% Ti after re-reduction at 300 degrees C and similar to 85% with 1 wt% Ti after reduction at 550 degrees C.