(779e) Mechanistic Insights on Catalytic Ethanol Steam Reforming

Conversion of bio-derived oils and feedstock with high concentrations of oxygen-containing hydrocarbons into chemicals is a highly desirable objective for environmental and economic sustainability. The production and use of chemicals synthesized from oxygenated feedstocks primarily derived from bio-based sources is projected to grow from $3.6 billion in 2011 to $12.2 billion by 2021. However, no efficient technologies exist for such a conversion because current chemistries rely on catalysts that have been developed for processing natural gas and petroleum fractions with low oxygenate concentrations. To advance progress in chemical synthesis from bio-derived feeds the fundamentals of adsorption and reactivity of oxygenates over metal catalysts, such as Rh, must be better understood. This will enable development of new technologies for oxygenates with promising catalysts formulations.

Ethanol steam reforming (ESR) has been widely explored as a test case in order to identify appropriate catalyst materials for oxygenate reforming as well as to elucidate the reaction scheme governing the process. A primarily classic approach to reaction model development has previously been employed, focusing on measured quantities of the products over a range of test conditions combined with a proposed set of equilibrium reactions to match observed experimental data. Through work of this type, it has been shown that there are myriad competing reactions contributing to the overall mechanistic understanding of ESR. However, in a recent review on catalytic ESR by Hou et al., the authors highlight that there is no agreed upon reaction pathway for the overall ethanol steam reforming process. Here we seek to provide new insights on the reaction pathways involved in steam reforming over a Rh-based catalyst.

The low-temperature ESR reaction mechanism over a supported Rh/Pt catalyst was investigated. An unprecedented level of understanding with respect to the dominant reaction pathways, the contribution of each metal to the product distribution, and the role of the support was achieved. Both the recombination of C-species on the surface of the catalyst as well as preservation of the C-C bond within ethanol are responsible for C₂ product formation. The onset of ethylene, a common byproduct observed after catalyst deactivation, does not occur until incomplete ethanol conversion is observed. In addition, we quantitatively show that 57% of observed ethylene is formed directly through ethanol dehydration. Finally we provide clear evidence that oxygen in the silica-zirconia support likely constitutes 10% of the CO formed during reaction.