Date of Award

Fall 2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

First Advisor

Fabio H. Ribeiro

Committee Chair

Fabio H. Ribeiro

Committee Member 1

W. Nicholas Delgass

Committee Member 2

Arvind Varma

Committee Member 3

Jeffrey T. Miller

Abstract

As economies look to transition away from petroleum for social, economic, and political reasons, biomass will continue to attract attention as a renewable feedstock for the fuels and chemicals industry. In order to turn biomass into end use fuels and chemicals, the oxygen content must be lowered significantly, requiring large hydrogen inputs. For these processes to be completely renewable, the hydrogen must come from biomass or biomass-derived compounds. In this work, catalysts for the aqueous phase reforming (APR) of biomass-derived sugars were characterized by a combination of reaction kinetics, X-ray spectroscopy, electron microscopy, and theoretical computation to determine the active sites and the roles of promoters on APR.

Here, we have studied several Pt-based catalysts for the conversion of glycerol (a model sugar compound) into hydrogen via aqueous phase reforming. Adding the promoters Mo and Co to a carbon supported Pt catalyst increased the reaction rates, but had different effects on selectivity. The addition of Mo increased the rates of hydrogen generation reactions, but also increased the rates of deoxygenation side reactions. In contrast, adding Co increased reaction rates by up to a factor of 4, while maintaining Pt-like selectivity. These kinetic differences manifested themselves in the final gas phase hydrogen selectivity, with PtCo catalysts maintaining selectivity >85% at 70% conversion, and PtMo showing 65% H2 selectivity at similar conversion.

Characterization via X-ray spectroscopy and electron microscopy gave structural clues as to the differences in reaction performance. X-ray absorption spectroscopy (XAS) showed the formation and maintenance of bimetallic PtMo and PtCo catalysts under reaction conditions. However, the XAS also indicated that the Co remains mostly reduced (70-90% Co0 under aqueous conditions), while the Mo remains predominantly oxidized. Theoretical calculations indicate that the oxidized Mo preferentially will occupy the surface of Pt or PtMo alloy particles, and is able catalyze the deoxygenation reactions. The Co does not form the same structures and remains metallic and coordinated to the Pt under aqueous conditions. The difference in structures helps to explain the selectivity differences observed for the two promoters.

Additional studies of PtCo catalysts by XAS and scanning transmission electron microscopy (STEM) revealed how the Co aids in the promotion of the reaction. Via STEM, three different particle configurations were observed for PtCo catalysts: pure Pt, Pt shell/Co core, and well mixed PtCo alloys, with different fractions of each observed for Pt:Co ratios between 1:0.5 and 1:5. Correlations of reaction rates with the fraction of each type of particle revealed that the rates scale with the fraction of well-mixed alloy particles present in the sample. This suggests that the well mixed alloy is responsible for the observed rate promotion. Using the water-gas shift reaction as a probe, it was observed that the CO reaction order was lower for PtCo bimetallic catalysts, which suggests stronger CO binding to the surface. This may have similar effects on the surface energies of the biomass reforming intermediates, which suggests one way in which Co promotes the reaction is by stronger binding of intermediates and increased rates of surface reactions.

In addition to studying glycerol as a probe molecule, other C3 alcohols (1,2- and 1,3-propanediols, 1-propanol) were studied to understand how the position of the alcohol and alkyl groups affected the rates and selectivity of APR. It was observed that regardless of the positions of the functional groups the rates of reaction were similar for all the studied compounds. This suggests that the rate limiting steps for each of these molecules is similar. However, the position of the functional groups did have an effect on the pathway selectivity. Compounds with either a C:O ratio of 1:1, or those with a terminal alkyl fragment (1,2-propanediol, 1-propanol) tended to react along a carbon-carbon bond cleavage pathway (the preferred pathway for hydrogen generation from alcohols) to form H2 and CO2. In contrast, the 1,3-propanediol reacted first through a carbon-oxygen pathway to form 1-propanol (consuming hydrogen), and then along the carbon-carbon cleavage pathway. The consequence of this is that the H2 rates and selectivity for the 1,3-propanediol were lower than for any other alcohol, suggesting that the positioning of the alcohols and alkyl fragments has an impact on the reaction selectivity.

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