Theoretical and experimental study of bimetallic catalysts in heterogeneous catalysis and electrocatalysis for energy applications

Date
2015
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University of Delaware
Abstract
In this dissertation, non-precious bimetallic heterogeneous catalysts and electrocatalysts with specific functionality are studied for sustainable energy applications. It is demonstrated that through the combination of theoretical and experimental techniques, more active, selective and stable precious-metal-free bimetallic catalysts are identified for biomass-derived oxygenate reactions, reduction of CO2 with ethane, and the hydrogen evolution/oxidation reaction (HER/HOR) in alkaline medium. First of all, the adsorption and reaction of C3 oxygenates on Ni- and Mo-based bimetallic surfaces are investigated via density functional theory (DFT) calculations, temperature programmed desorption (TPD), and high resolution electron energy loss spectroscopy (HREELS). Propanal and 1-propanol are used as probe molecules for biomass-derived oxygenates due to their relatively high vapor pressures which can be introduced into ultrahigh vacuum (UHV) systems easily. The Vienna Ab-initio Simulation Package (VASP) is used in this work mainly to calculate the binding energies and the adsorption geometries of oxygenates and reaction intermediates, and to calculate the vibrational spectra of adsorbates on the catalytic metal surfaces. The trends established from DFT are correlated with the activity and selectivity determined experimentally from TPD, and the bond scission mechanisms are examined through HREEL spectra. On Ni(111), Fe/Ni(111) and Cu/Ni(111) surfaces, DFT calculations predict that the binding energy trend of propanal and 1-propanol is Fe/Ni(111) > Ni(111) > Cu/Ni(111). For propanal, the highest reforming and total activity are observed on Ni(111), while modifying the Ni(111) surface with Fe results in the highest decarbonylation activity to produce gas-phase ethylene. Depositing monolayer (ML) of Cu on Ni(111) leads to the highest total decomposition activity. Different 1-propanol decomposition reaction mechanisms are achieved on the three surfaces studied. Briefly, HREEL spectra show that on Ni(111) 1-propanol undergoes a selective dehydrogenation pathway to produce propanal, while on both bimetallic surfaces 1- propanol favors a non-selective total decomposition pathway to produce CO, H2 and surface hydrocarbons. Next, the bimetallic modification effect of a single layer of Ni and Co on Mo(110) on the bond scission of C3 oxygenates is also studied. DFT results predict that the binding energy trend of propanal and 1-propanol is Mo(110) > Co/Mo(110) > Ni/Mo(110). TPD and HREELS results show that for both molecules, Mo(110) shows a highly selective deoxygenation pathway towards C‒O/C=O bond scission to produce propene, while bimetallic surfaces instead exhibit a higher activity for C‒C and C‒H bond scission. Among the three surfaces, Ni-modification leads to the highest selectivity for decarbonylation to produce ethylene and Co-modification results in the highest selectivity for reforming to produce syngas. The second section of this dissertation is the study of catalytic activation of CO2 using ethane on supported bimetallic catalysts. Steady-states catalytic performance is evaluated using a flow reactor with in-line gas chromatography. CO chemisorption and temperature programmed reduction (TPR) are used to characterize the supported catalysts synthesized from the incipient wetness impregnation (IWI) method. The catalysts evaluated include CoPt/CeO2, CoMo/CeO2, NiMo/CeO2 and FeNi/CeO2, and all bimetallic catalysts achieved an enhanced stability, compared to the corresponding monometallic catalysts. Two distinct types of catalysts, which break the C‒C bond via the dry reforming pathway and which selectively break the C‒H bond via the oxidative dehydrogenation pathway, are identified. It is shown that CoPt/CeO2, CoMo/CeO2 and NiMo/CeO2 favor the dry reforming of ethane to produce synthesis gas, while FeNi/CeO2 is a promising selective oxidative dehydrogenation catalyst to produce ethylene, CO and H2O. DFT energy profiles investigated confirm the two distinct reaction pathways on CoPt(111) and FeNi(111). Lastly, the combined approach of DFT prediction and experimental results is employed to screening and designing highly efficient electrocatalysts in alkaline electrolytes. By determining the activity of HER on a series of monometallic surfaces, it is shown that the HER exchange current density in alkaline solution can be correlated to the calculated hydrogen binding energy (HBE) on the metal surfaces via a volcano type of relationship. Such correlation suggests that the HBE can be used as a descriptor for identifying HER/HOR electrocatalysts in alkaline medium. As a direct extension of this design principle, precious-metal-free bimetallic (NiMo) and multimetallic (CoNiMo) electrocatalysts with enhanced activity are identified.
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