Date of Award
8-2018
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemical Engineering
Committee Chair/Advisor
Dr. David A. Bruce
Committee Member
Dr. Rachel B. Getman
Committee Member
Dr. Joseph K. Scott
Committee Member
Dr. Steven J. Stuart
Abstract
In the future, the availability of reliable alternative fuels will be crucial for any country to become energy independent. One such alternative is ethanol as it can be used both as a fuel and as a fuel additive. Most of the ethanol produced in the world today is derived from biomass. The biomass feedstocks and fermentation broths used in ethanol production both contain high amounts of water and therefore, the energy efficiency of the process is lessened by product separation processes (azeotropic separation of water and ethanol) that are non-trivial and highly inefficient (due to the evaporation of water). An alternative route to produce ethanol, which negates the need for costly distillation processes, is via the catalytic conversion of syngas (CO and H2) generated from biomass.
Syngas is a mixture of carbon monoxide and hydrogen, which results from the reforming of natural gas, as well as the gasification of coal, biomass, and solid wastes. In theory, syngas can be readily converted to ethanol using chemical catalysts, but to-date no high efficiency, low-cost catalyst has been found. In this work, sub-nanometer size, bimetallic cobalt-palladium particles are found to be active and selective catalysts for the desired reaction as the particles contain two metals having different CO dissociation capabilities. The reaction mechanism considered for this study includes forty-six reversible reactions, including Fischer-Tropsch reactions. We used Density Functional Theory (DFT) coupled with nudged elastic band methods to determine the activation barrier heights and enthalpy change with reactions for the full reaction pathway needed for ethanol production from syngas. To lessen the computational burden, linear Bronsted-Evans –Polanyi (BEP) relations, for association and dissociation reactions, are developed.
A microkinetic model is built using the reaction information derived from combined DFT and BEP studies, which is used to examine if there is a synergistic effect between Co and Pd favoring the production of ethanol. Coverage dependent sticking coefficients are used to examine the effects of surface coverage on reactivity. It also incorporates diffusion of intermediate species between the sites.
One of the first and important steps in the syngas to ethanol conversion process is carbon monoxide (CO) adsorption on the metal catalyst. Therefore, computational models were developed to help understand CO adsorption energetics as well as surface coverage effects on a Co7Pd6 catalyst. From these initial studies, we determined the adsorption energies of CO on both cobalt and palladium as a function of CO surface coverage (where the number of CO species on the catalyst surface was varied from 1 to 6). Further, we calculated the infrared spectra for adsorbed CO species and key bond lengths (metal–carbonyl carbon and adsorbed CO bond lengths) using DFT. Results from the DFT simulations compared favorably with experimental values.
Separate microkinetic models results on Co, CoPd and Pd sites indicate that ethanol formation happens only on CoPd bimetallic sites indicating the synergetic effect of Co and Pd to make ethanol from syngas. A batch reactor is modeled and 24 ordinary differential equations are solved simultaneously to obtain time evolution of products and intermediates. The pathway for ethanol production is identified as: CO* →HCO*→CH2O*→CH3O*→CH3CO*→CH3CHO*→CH3CH2O*→CH3CH2OH.
Further, the microkinetic model was modified to include diffusion reactions. Ratio of number of sites of cobalt, cobalt-palladium and palladium is altered to study CoxPdy catalysts of different cobalt and palladium ratios.
Recommended Citation
Gundamaraju, Anuradha, "First Principles-Based Microkinetic Modeling of Ethanol from Syngas on Bimetallic Co-Pd Catalysts" (2018). All Dissertations. 2967.
https://open.clemson.edu/all_dissertations/2967