Effect of Reduction Atmosphere on Structure and Catalytic Performance of PtIn/Mg(Al)O/ZnO for Propane Dehydrogenation

The effect of reduction atmospheres, H2/N2, C3H8/H2/N2, C3H8 and CO, on the structure and propane direct dehydrogenation performance of PtIn/Mg(Al)O/ZnO catalyst derived from ZnO-supported PtIn-hydrotalcite was studied. The physicochemical properties of the as-prepared and used catalytic system were characterized by various characterization methods. The results show that the dehydrogenation performance, especially the stability of the PtIn/Mg(Al)O/ZnO catalyst, was significantly improved along with the change in reduction atmosphere. The highest catalytic activity (51% of propane conversion and 97% propylene selectivity), resistance toward coke deposition, and stability for more than 30 h were achieved with the H2/N2-reduced catalyst. The optimal dehydrogenation performance and coke resistance are mainly related to the high Pt dispersion and In0/In3+ molar ratio, strong Pt–In interaction and small metal particle size, depending on the nature of the reduction atmospheres. The reconstruction of meixnerite favors the stability and coke resistance to some extent.

Characterization of Nanometer-Sized Metal Clusters

Characterization of nanometer-sized particles dispersed onto a porous support is critical for understanding performance of heterogenous catalysts. The electron microscope offers a unique capability for measuring the cluster size and chemistry of these nanometer-sized metal clusters. Cluster size and size distribution can be readily determined by using high angle annular darkfield or z-contrast microscopy. However, as catalysts become more complex with additional components that modify the metal activity, determination of the chemistry of individual clusters could be interesting. Conducting such studies is not straightforward and some of the problems associated with making such measurements will be discussed using model catalysts and a VG HB 601UX dedicated STEM.Problems associated with this type of measurement can include the instability of the small metal particle under the intense, focused electron beam as well as spurious signals from the area surrounding the region of interest.

Image Simulation of Small Pt Particles and its Application to Lattice Spacing Measurements in Catalysts

The technique of high resolution imaging is important for characterizing the structure of small metal particle catalysts and nanophase materials. For bimetallic systems, it is possible to use local lattice parameter measurements to identify alloy compositions in ensembles of nanometer sized metal particles [1]. However, determining alloy composition is challenging because changes in lattice parameters of only a few percent must be reliably detected. We have performed measurements of the apparent d(111) fringe spacing on both simulated and experimental HREM images from Pt particles in the size range 15 - 35 Å. A series of initial image simulations of Pt cubeoctahedrons with 17 Å (201 atoms) and 34 Å (1289 atoms) in diameter have been studied in order to understand the effect of different parameters on the accuracy of lattice spacing. The clusters were built using the Rhodius program developed by Botana et al [2]. Starting with a bulk crystal we create cubeoctahedra by applying successive cuts along either the (111) or (100) direction. A supercell size of 50Å was selected and the particles were oriented at or close to the [110]. Images were calculated by the multi-slice techniques using both the CERIUS and EMS applications. The supercell was divided into 20 slices and the following parameters were used in the calculation Eo = 400 kV, Cs = 1mm, Δf= -320 Å, focal spread = 80 Å, convergence = 0.5 mrad and atomic vibration = 0.35Å.