Synthesis and kinetics of supported catalysts for green chemistry
Abstract
In this work, two types of supported catalysts have been studied for; (1) a greener route for the production of propylene oxide, and (2) NOx abatement in diesel exhaust. Propylene oxide (PO) is an important building block used in the production of a large variety of valuable consumer products such as polyurethanes, surfactants, cosmetics, and food emulsifiers. The synthesis and characterization of nanoscale gold supported on modified titanium silicalite-1 (TS-1) catalysts for the vapor-phase epoxidation of propylene to propylene oxide is presented. Deposition precipitation of gold on NH4NO3-treated TS-1 supports resulted in a four-fold increase in Au capture efficiency and produced catalysts with 5-10% conversion of propylene with 75-85% selectivity to PO, at 200°C and a space velocity of 7000 mL h-1 gcat-1. In this system, 10% conversion and 76% selectivity, amounts to a rate of 134 g PO hr-1 kgcat-1. An optimal Au content that is support dependent is observed for these catalysts. Next, model NOx storage/reduction (NSR) type catalysts were utilized to investigate the reaction kinetics of NO oxidation and NOx storage. The reaction was nearly first order in both NO and O2, nearly negative first order in NO2, and the apparent activation energy was 81.8±5 kJ mol-1. A sintered monolith catalyst with a dispersion of 15% compared to 42% on the fresh catalyst, showed a four-fold increase in turn-over-rate (TOR). The sintered catalyst showed a similar Ea (80.9±5 kJ mol-1) and apparent reaction orders for NO (+l) and NO 2 (-1) while the O2 order was lower (0.7±0.04). Titration results using CO show that the oxygen uptake was 1.5 and 2 times the amount of Pt on the surface after reaction and deactivation respectively, for both catalysts. XPS and CO titration data indicate that the higher NO oxidation TOR observed on large Pt particles may be due to their weaker interaction with oxygen when compared to small particles. The dependence of the NO oxidation TOR as a function of the Pt crystallite size was further investigated over a series of Pt/Al2O3 powdered catalysts with dispersions varying from 13-75%. A 15-fold increase in TOR was observed between the most active (30% dispersion) and least active catalyst (75% dispersion). We propose that only particles > 4nm are active for the NO oxidation reaction and those < 4nm get oxidized to an inactive form under reaction conditions. NOx storage on a series of model NSR catalysts (Pt/BaO/Al2O 3) was investigated. The effect of temperature, CO2, H 2O, Pt loading, and Ba loading on NOx storage was examined and a model was proposed to explain the shape of the NOx breakthrough curves. The regeneration of Pt/BaO/Al2O3 NOx traps with H2 was also investigated. The reduction is proposed to occur in a plug flow type mechanism and the chemistry seems to be fast enough to make the process mass transfer limited. The process is consistent with NOx being released from the storage sites and then reduced by H2 (or NH 3) over Pt. The high selectivity of Pt/BaO/A12O3 in forming mostly N2 while being regenerated is achieved by the release of NOx only in the presence of H2 which guarantees the formation of N2 and NH3 and only small amounts of N 2O.
Degree
Ph.D.
Advisors
Delgass, Purdue University.
Subject Area
Chemical engineering
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.