The effect of the bimetallic Pd-Pb structures on direct oxidative esterification of methacrolein with methanol

Zn Modification of Pd/TiO2/Ti Catalyst for CO Oxidation

The main goal of this study was to modify the activity of Pd/TiO₂/Ti catalyst in the reaction of CO oxidation by the addition of Zn. Plasma electrolytic oxidation (PEO) of Ti wire was conducted to produce a uniform porous layer of TiO₂. A mixture of Pd and Zn was then introduced by means of adsorption. After reduction treatment, the activity of the samples was examined by oxidation of 5% CO in a temperature range from 80-350 °C. Model catalysts with sufficient amounts of the metals for physico-chemical investigation were prepared to further investigate the reaction between Pd and Zn during CO oxidation. The structures and compositions of the samples were investigated using scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), inductively coupled plasma mass spectrometry (ICP-MS), Fourier transform infrared (FTIR), and X-ray diffraction (XRD). Modification of Pd/TiO₂/Ti catalyst by Zn with a Pd:Zn atomic ratio of 2:1 decreased the temperature of complete CO oxidation from 220 °C for Pd/TiO₂/Ti to 180 °C for Pd-Zn/TiO₂/Ti. The temperature of 50% CO conversion on Pd-Zn(2:1)/TiO₂/Ti was around 55 °C lower than in the reaction on monometallic Pd catalyst. The addition of Zn to the Pd catalyst lowered the binding energy of CO on the surface and improved the dissociative adsorption of oxygen, facilitating the oxidation of CO. FTIR showed that the bridging form of adsorbed CO is preferred on bimetallic systems. Analysis of the surface compositions of the samples (SEM-EDS, TOF-SIMS) showed higher amounts of oxygen on the bimetallic systems.

Controlled Metal-Support Interactions in Au/CeO2-Mg(OH)2 Catalysts Activating the Direct Oxidative Esterification of Methacrolein with Methanol to Methyl Methacrylate

The strong metal-support interaction (SMSI) between the three components in Au/CeO₂-Mg(OH)₂ can be controlled by the relative composition of CeO₂ and Mg(OH)₂ and by the calcination temperature for the direct oxidative esterification of methacrolein (MACR) with methanol to methyl methacrylate (MMA). The composition ratio of CeO₂ and Mg(OH)₂ in the catalyst affects the catalytic performance dramatically. An Au/CeO₂ catalyst without Mg(OH)₂ esterified MACR to a hemiacetal species without MMA production, which confirmed that Mg(OH)₂ is a prerequisite for successful oxidative esterification. When Au/Mg(OH)₂ was used without CeO₂, the direct oxidative esterification of MACR was successful and produced MMA, the desired product. However, the MMA selectivity was much lower (72.5%) than that with Au/CeO₂-Mg(OH)₂ catalysts, which have an MMA selectivity of 93.9-99.8%, depending on the relative composition of CeO₂ and Mg(OH)₂. In addition, depending on the calcination temperature, the crystallinity of the CeO₂-Mg(OH)₂ and the surface acidity/basicity can be remarkably changed. Consequently, the Au-nanoparticle-supported catalysts exhibited different MACR conversions and MMA selectivities. The catalytic behavior can be explained by the different metal-support interactions between the three components depending on the composition ratio of CeO₂ and Mg(OH)₂ and the calcination temperature. These differences were evidenced by X-ray diffraction, X-ray photoelectron spectroscopy, and CO₂ temperature-programmed desorption. The present study provides new insights into the design of SMSI-induced supported metal catalysts for the development of multifunctional heterogeneous catalysts.