Carbide and nitride catalysts for the water gas shift reaction.

Abstract

This is the first investigation of carbide and nitride catalysts for the water gas shift (WGS) reaction. The goal was to determine how the catalyst structure related to its functional performance, addressing effects of metal and nonmetal formulation, synthesis parameters, pretreatment conditions, and reaction environment. A series of carbides and nitrides containing Group VI and VIII metals were synthesized by carburization or nitridation of oxides. Some had both metals in the bulk structure. Others were made by dispersing metals onto carbide or nitride supports using a novel adsorption process. The WGS kinetics were evaluated (200--350&deg;C) with a feed gas that simulated industrial conditions. Reaction studies were complemented by compositional and microstructural characterization. The character of the Mo₂C catalyst was tracked in detail during passivation, activation, and operation for water gas shift. Synthesis of Mo₂C was investigated to determine effects of precursor, heating/cooling rate, soak time/temperature, pressure, carburizing gas, and humidification. These parameters spanned a response surface that has not been covered in prior studies. This dissertation also describes for the first time the production of high surface area Mo₂C catalyst (80 m<super>2</super>/g) using an isothermal carburizing method. Catalyst metal and nonmetal content were both substantial factors governing activity and selectivity. The catalysts containing molybdenum were more active than analogous materials containing tungsten. Carbides were superior to the nitrides. For the carbides, the WGS activities correlated with density of carbidic carbon at the catalyst surface. Oxygen and carbon content had a strong influence on adsorptive interactions with CO, H₂O, CO₂, and H₂. The addition of transition metals to the Group VI carbides and nitrides resulted in synergistic effects. The WGS rate for Mo₂C in the low temperature range of 200--240&deg;C increased by almost an order of magnitude when platinum was added to its surface. This rate is competitive with existing commercial catalysts. Results of kinetic studies and temperature-programmed desorption were consistent with a bifunctional reaction mechanism. The results of this research provide groundwork for future development of these materials for water gas shift as well as other catalytic reactions.