Theoretical and experimental study of bimetallic catalysts in heterogeneous catalysis and electrocatalysis for energy applications
Date
2015
Authors
Journal Title
Journal ISSN
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Publisher
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.