The presented work comprehends a broad spectrum of redox catalysis for various environmental applications, such as i) hydrogen production via water-gas shift reaction, ii) hydrogen purification for fuel cell applications and iii) catalytic after-treatment of lean-burn engines. This dissertation involves, but is not limited to catalyst development, reaction studies and catalyst characterization for the above-mentioned environmental applications, which can be summarized as follows.
(i) Water-gas shift (WGS) remains an essential step in integrated gasification combined cycle (IGCC) for hydrogen production, as it forms a link between the gasification process and fuel cell operations. The current catalysts for WGS application are based on Fe-Cr and Cu/ZnO/Al2O3, as a high temperature (HT-WGS) and low temperature (LT-WGS) catalysts, respectively. This two-stage WGS process is a consequence of several operational drawbacks of the current catalyst formulations including Cr+6 being carcinogenic. Hence the presented WGS project has a two-fold purpose. First, Cr-free Fe-based catalyst development and second, Cu supported catalyst development for WGS that can be operated over a wide temperature range.
In this dissertation, Cr- free Fe-Al-Cu catalyst prepared through “one-step” sol-gel method using propylene oxide as a gelation agent has been reported. Steady state reactions demonstrated that WGS performance of Fe-Al-Cu was superior as compared to commercial Fe-Cr catalyst. The reaction studies along with complementary catalyst characterization indicated that the amount of copper in iron oxide matrix played a crucial role. The optimized ratio of Fe to Cu was found to be five and any further increase in copper loading resulted in copper segregation from the iron oxide matrix. Thus various catalyst characterization techniques were exploited to understand this phenomenon. Furthermore, a detailed study was performed to comprehend the formation of surface species during WGS reaction and to evaluate the reaction mechanism over Fe-Al-Cu.
In the quest of exploring Cu-based catalyst for WGS system, Cu supported over various CeO2 nano-morphologies were investigated. Here, nanoparticles (NP) and nanorods (NR) of CeO2 were prepared through hydrothermal precipitation method and copper was supported on these morphologies using a wet impregnation method. In the current findings, copper was more finely and uniformly dispersed over CeO2 nano-particles compared to nanorods, resulting in better WGS activity compared to particle-based samples. Catalyst characterization indicated finely dispersed copper particles in close interaction with ceria nanoparticles, whereas isolated bulk-like copper species were formed over the ceria nanorods. Finally, the formation of surface species during WGS reaction delineated the redox reaction mechanism over Cu/CeO2.
(ii) Hydrogen produced via WGS reaction may contain up to 1-2% CO in stream which can be poisonous to proton exchange membrane (PEM) fuel cell. Preferential oxidation of carbon monoxide (PROX) is considered as an effective and economical way
to purify the hydrogen stream for PEM fuel cell applications. The major challenge in this process is to selectively oxidize CO with minimum loss of hydrogen. Hence a non-precious metal catalyst such as, cobalt supported over ceria with a special focus on cobalt loading has been utilized. Both, activity and selectivity were found to be a strong function of cobalt content. In addition, CO and hydrogen oxidation kinetics was studied as a function of cobalt loading. The higher activation energy barrier for hydrogen oxidation compared to CO oxidation indicated higher temperature sensitivity for hydrogen oxidation. The cobalt phase was identified, as Co3O4 and it remained stable under PROX atmosphere. Time-on-stream experiment along with various catalyst characterization techniques indicated no significant contribution from lower valency cobalt species. Finally, the formation of surface species during PROX reaction demonstrated conversion of carbonate species to more stable polydentate carbonates and formate type species with increase in reaction temperature.
(iii) Lean-burn natural gas fired engines remains a popular choice in the energy market. Despite emission being greatly reduced, exhaust still contains considerable amount of NOx, CO and hydrocarbons. Hence after-treatment to clean up the exhaust is essential. The selective catalytic reduction using hydrocarbons is considered as a promising alternative for conventional after-treatment technology and is well suited, especially for natural gas lean-burn engines.
For this purpose, a “single-stage” de-NOX system composed of a physical mixture of dual-catalyst bed has been developed. This dual catalyst bed was a physical mixture of reduction (Pd/SZ) and oxidation (CoOx/CeO2) catalysts capable of performing three distinct functions, NO oxidation, NOx reduction, and CO and hydrocarbon oxidation. Here, oxidation catalyst was assumed to play multi-functional role in dual-catalyst bed. These include oxidizing NO or re-oxidizing partially reduced NOx species, CO oxidation and catalyzing the combustion of un-burned hydrocarbons, which have not participated during SCR reaction.
In this dissertation, the role of oxidation catalyst in dual-catalyst bed was addressed. NO oxidation was studied as a function of cobalt loading in CoOx/CeO2 formulation. The dual-catalyst bed was optimized by varying the reduction to oxidation catalyst ratio in order to achieve significantly high NOx conversion during hydrocarbon-SCR. The lower cobalt loading in oxidation catalyst in a dual-bed resulted in higher NOx conversion. This observation was associated with lower hydrocarbon oxidation and hence increased hydrocarbon availability for NOx reduction. Kinetic study along with catalyst characterization confirmed the activation of methane molecule via hydrogen abstraction, consequently participating in either in NOx reduction or directly oxidizing over the oxidation catalyst. Moreover, the effect of water vapor was thoroughly investigated over the optimized dual-catalyst bed. The primary focus of this work was to improve the hydrothermal stability of the dual-catalyst bed by changing the various engine exhaust parameters.