Investigation Of Palladium Ceria Catalysts For Selective Hydrogenation Of Acetylene In Ethylene

Author : Anish Dasgupta
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (113 download)

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Download or read book Synthesis, Characterization and Catalytic Testing of Palladium and Nickel Based Intermetallic Compounds for Acetylene Semi-hydrogenation written by Anish Dasgupta and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Intermetallic compounds often offer opportunities for synthesis of site isolated catalysts having precise active site coordination and morphology. They are also resistant to segregation effects (compared to random alloys) because of the high heat of formation of their unique electronically stabilized crystal structures. Due to predictable, periodic and easily reproducible nature of the isolated active sites, suitably chosen intermetallic compounds can act as excellent model systems for studying catalytic structure function correlation in different small molecule chemistries. In the current dissertation I utilized H2-D2 exchange in addition to ethylene and acetylene hydrogenation as the probe reactions of choice. My ultimate target is to understand and quantify the effect of active site (transition metal) isolation during the semi-hydrogenation of acetylene in an ethylene rich stream, which is a key step in industrial polyethylene synthesisa multi-hundred-billion-dollar industry. I studied two different classes of catalysts for this chemistry: Pd-(M)-Zn (M=Cu, Ag, Au) -brass intermetallics and Ni-Ga intermetallics. I used micron scale bulk materials as model catalysts because they provide greater compositional control and allows for studying the intrinsic catalytic property of the intermetallic, free from support and ligand effects.A significant part of the text is dedicated to the synthesis, characterization and catalytic study of the bulk Pd-(M)-Zn ternary -brass phase materials because of its unique adaptability in terms of developing a family of closely related model catalysts having Pd-M-Pd active sites (M=Pd, Zn, Au, Cu and Ag) which can change the semi-hydrogenation activity and selectivity by two orders of magnitude even when only one atom is changed per 52 atom unit cell. As an extension of this project I also report several Pd-Ni-Zn and Pd-Pt-Zn ternary -brass phases and determine their atomic site occupancy factors.During the course of my research I realized there were no reported methods for synthesizing Pd-Zn and Ni-Zn -brass intermetallics as catalytically relevant supported nano-particles (NPs). I developed and optimized a hybrid generalized synthesis approach for SiO2 supported Pd-Zn, Ni-Zn and Cu-Zn -brass NPs. I also provide preliminary evidence that this approach can be easily extended to the synthesis of ternary NPs as well.In the case of Ni-Ga intermetallics, I tried to understand the role of Ga in controlling acetylene semi-hydrogenation. I observed a reversal in the ethylene selectivity trend with Ga content between 85 C and 160 C which is unexpected based on published DFT calculations. Previously, it has been suggested that Ga simply acts as a spacer for isolating Ni/Pd active sites much like Zn in the case of Ni-Zn catalysts (studied by Charles Spanjers from our research group). However, Ga has a higher affinity to hydrogen compared to Zn and I attempted to investigate if Ga may have a weak but non-negligible catalytic contribution towards hydrogenation which has typically not been previously considered in any detail in the case of acetylene semi-hydrogenation on metallic catalysts. Additionally, I provide understanding behind the high acetylene semi-hydrogenation selectivity of Ni5Zn21 catalyst which was originally reported by Spanjers et al. (J. Catal. 2014). This involved identifying the site occupation factor of Ni and Zn in the -brass lattice for materials having greater than 15.4 at% Ni. It is determined that (unlike Pd-Zn -brass) exactly single atom Ni1 active sites are exposed on the surface irrespective of the Ni concentration (even though Ni3 clusters are present in the bulk) which explains the suppression of over-hydrogenation and oligomerization on this catalyst, resulting in the high ethylene selectivity reported by Spanjers et al.