Manganese Promoted CO Hydrogenation Catalysts: A Study of Metal Promoter Interaction Effects.

摘要

The aim of the work described in this thesis is to investigate the rational design of promoted metal catalysts for CO hydrogenation reactions. The main body of this work focuses on the use of simple techniques and common elemental precursors to improve the interactions in between a promoter and active metal. One of the many ways of achieving this is through the use of Strong Electrostatic Adsorption (SEA). Special attention to the surface charging parameters of mixed oxide as a function of solution pH can create a driving force to selectively adsorb a precursor complex onto a single phase of a binary mixture.;The use of promoters is ubiquitous in CO hydrogenation reactions to increase active metal's activities as well as the selectivity towards desired products. Although, the precise active site of promoters in reactions and how they interact with active metals to react the reactants require many more studies, it is agreed that a key design objective is to increase the metal-promoter interactions. This work demonstrates a procedure to achieve this with Mn promoter catalysts.;In Chapter 1, we present an introduction and a brief review of CO hydrogenation catalysts for both the conversion of syngas to alcohols (particularly ethanol) and, additionally the conversion of syngas to long-chained hydrocarbons in a typical Fischer- Tropsch (FT) reaction. In Chapter 2, the fundamentals of the Transmission Electron Microscope (TEM) and its applications on catalysis related projects are presented. Chapter 3 gives a detailed discussion for rational design to selectively adsorb a [MnO4]- promoter precursor over a Rh2O3/SiO2 supported catalyst. Various techniques (ICP, STEM-EELS, XANES, EXAFS, TPR) were utilized to study the intrinsic properties of the catalysts and how the different catalysts preparation method could affect the metalpromoter interactions. The alcohol (ethanol) synthesis reactivity measurement also gave us key insight to the role of the promoter as a function of metal-promoter interactions. In Chapter 4, we focus on the rational design of the selective adsorption of a [MnO4]- precursor over a Co3O4/TiO2 supported catalyst, in this study, catalysts with three metalpromoter interactions (namely Mn monolayer coverage on Co, Mn partial coverage on Co and least Mn interaction with Co) were made and the different metal-promoter interactions were visualized by the STEM-EELS analysis, the F-T reactivity results demonstrated that the catalyst which has the strongest Mn-Co interaction (Mn monolayer coverage on Co) has the highest selectivity towards C5+ hydrocarbons (desired product). In Chapter 5, we continue discussing the CO hydrogenation for alcohol synthesis (especially ethanol). However in this chapter, we studied the metal-promoter interaction effects over Mn promoted Rh catalysts supported on multi-walled carbon nanotubes (CNTs) by just using the normal impregnation (DI, dry impregnation) catalysts preparation method. The enhanced Mn interaction with Rh particles was achieved by increasing the Mn loading (2 wt% Mn vs 1 wt% Mn loading), and the enhancement was visualized and quantified by STEM-EELS analysis due to the virtue of CNTs support (low Z number of C). Once again, in the reactivity results, the catalyst with stronger Mn- Rh interactions exhibited higher selectivity towards ethanol.;This thesis ends with main conclusion that the key of rational catalysts design is to enhance the metal-promoter interactions, which can be achieved by either selectively adsorbing promoter onto the active metal or simply increasing the promoter loading by impregnation method. By focusing the intrinsic principles of catalyst preparation, this concept of stronger metal-promoter interactions can be applied to a wide range of catalytic materials to help define the promoter's precise roles in various catalytic reactions.