The dissertation herein reports heterogeneous catalysis studies conducted on two different projects, (1) Hydrodechlorination of trichloroethylene and (2) Ethanol steam reforming. The former is associated with the process for trichloroethylene waste treatment and groundwater remediation technology whereas the latter pertains to hydrogen production. The presented work involves not only investigations on the catalytic activity, but also fundamental studies to understand how and why a catalyst works for a particular reaction. This dissertation is composed of two major parts.
(1) Part I (Chapter 1-4): Trichloroethylene (TCE) is a chlorinated hydrocarbon solvent which has been widely used as a vapor degreaser for metal cleaning. TCE contains high level of toxicity, also known as a carcinogen. Recently, contamination of groundwater due to untreated TCE is a rapidly rising environmental issue, affecting the drinking water quality. The current waste treatment process for removal of TCE are mostly based on adsorption and extraction techniques. However, these processes do not provide a complete destruction of the TCE chemical structure; hence, it requires an additional incinerator to convert TCE into non harmful products.
Hydrodechlorination (HDC) of TCE is a catalytic chemical reaction where TCE is converted to ethane and hydrochloric acid using hydrogen as a reducing agent. The treatment is a single step process, also can be performed in situ in groundwater. The most extensively used catalysts for this reaction are based on Pd metal. The main purpose of this study is to investigate the catalytic activity and stability of Pd supported on swellable organically modified silica (SOMS) for HDC of TCE. SOMS is a highly hydrophobic and adsorptive material. The material was recently developed and was reported in the literature. An interesting characteristic of SOMS is “swelling”, i.e., the volume of SOMS expands while adsorbing organics. The expansion of SOMS leads to generation of new pores thereby increasing its pore volume and surface area. When SOMS is treated with heat, contraction of SOMS occurs resulting a decrease in volume to its original size. Another important characteristic is its high hydrophobicity.
It was deduced that Pd/SOMS showed promising catalytic activity compared to commercial Pd/Al2O3 in liquid phase HDC of TCE. This was attributed to the adsorptive and swelling properties of Pd/SOMS and its hydrophobicity, which helped to concentrate the TCE reactants in the vicinity of the active Pd sites. The increase in concentration near the active sites resulted in better kinetics for HDC reaction obtaining high TCE conversion. Furthermore, the strong hydrophobicity of SOMS helped to secure the Pd sites from ionic poisons such as sulfur and chlorine containing groups without losing its catalytic activity, e.g., when Pd/SOMS and Pd/Al2O3 was treated with 1 M HCl, Pd/SOMS retained its catalytic performance whereas Pd/Al2O3 was completely degraded because of Pd leaching under HDC of TCE conditions.
On the other hand, better catalytic activity was observed over Pd/Al2O3 compared to Pd/SOMS for gas phase HDC of TCE. It was found that the swelling of SOMS in gas phase, in other words, the expansion of the catalyst due to TCE vapors was not enough so that most of the embedded Pd sites were not accessible to the TCE reactants. To resolve this issue, Pd was impregnated on a pre-calcined SOMS which was treated under inert condition at high temperatures (H-SOMS). With respect to Pd/H-SOMS, the Pd accessibility in gas phase to the TCE molecules were improved drastically. A significant increase in Pd dispersion as well as decreased in Pd particle size was observed. When tested for HDC of TCE activities, higher TCE conversions were obtained over Pd/H-SOMS catalyst compared to Pd/SOMS.
(1) Part II (Chapter 5-10): Hydrogen has been considered as the next alternative and renewable energy carrier that can be utilized for many industrial applications. The production of energy from hydrogen is typically conducted through a fuel cell, which only generates water as a product. Among many other hydrogen production processes, ethanol steam reforming (ESR), a catalytic chemical reaction, has gained a lot of attention since it provides a closed carbon loop cycle when ethanol is derived from environmentally friendly sources such as bio-mass. With respect to catalysts, noble metals such as Pt, Pd, Rh and Ru have been tested under ESR conditions and have shown excellent catalytic activities obtaining high hydrogen yield and ethanol conversion. However, noble metals are expensive thereby increase the overall operating cost.
In this dissertation, the catalytic performance and stability of non-noble metals such as Co supported on CeO2 (cerium oxide) was investigated. According to the results obtained in our laboratory during the past decade, Co/CeO2 with 10 wt % cobalt loading, had significant activity for ESR at relatively lower temperatures, around 350 to 600 °C. Some of the findings include 1) high oxygen mobility of CeO2 reduces carbon deposition, 2) transition of Co3O4 to CoO and metallic Co was observed during steam reforming, 3) reducibility of Co and surface activity, morphology and particle size of CeO2 significantly influence the catalytic activity of Co/CeO2 for ESR.
Herein, a detailed kinetic analysis to acquire activation energies of reduction and re-oxidation processes of cobalt was conducted. Moreover, reducibility of CeO2 was studied in two different particle sizes where nano-sized CeO2 contained more Ce3+ reduced sites compared to micro-sized CeO2 both in bulk and surface of the catalyst under ESR conditions. The difference in extent of reduction of CeO2 led to a more basic surface for nano-sized CeO2 resulting better catalytic activity for ESR. When cobalt was present on the catalyst surface (Co/CeO2), the oxidation state of Ce was lower in comparison to a bare CeO2 support under same reaction conditions. This was attributed to the reduction of cobalt taking precedence over the reduction of CeO2. Lastly, the effect of microgravity on synthesis of CeO2 was studied. It was found that CeO2 prepared in microgravity consists lower surface area and pore volume compared to the CeO2 prepared in normal-gravity. A significant difference in particle shape was observed where microgravity-CeO2 was more like rods whereas normal-gravity-CeO2 contained polyhedral particles.