Ethanol synthesis from syngas over Rh-based/SiO2 catalysts: A combined experimental and theoretical modeling study
Catalytic conversion of biomass-derived synthesis gas (CO, CO2 and H2) to ethanol and other ! C 2 + oxygenates has received considerable attention recently due to the strong demands for alternative, renewable energy sources. Combining experimental measurements with first-principles-based kinetic modeling, we investigated the reaction kinetics of ethanol synthesis from CO hydrogenation over silica supported Rh/Mn alloy catalyst. We find that the promoters such as Mn atoms can exist in a binary alloy with Rh and play a critical role in lowering the activation barriers for the C–C bond formation such as CH+CO → CHCO, thus improving catalyst activity and selectivity toward ! C 2 + oxygenates, although the barriers towards methane formation are unaffected. The Rh/Mn alloy nanoparticle model is supported by experimental characterization using X-ray photoelectron spectroscopy, transmission electron microscopy, and X-ray diffraction of practically spent Rh/Mn/SiO2 catalyst material and also by first-principles density functional theory (DFT) calculations. Under the reducing conditions, the binary Rh/Mn alloy is thermodynamically more stable than the mixed metal/metal oxides. Further first-principles studies on the effects of various promoters (Ir, Ga, V, Ti, Sc, Ca, and Li) on the CO+CHx (x = 0~3) coupling reactions that are assumed to be the key reaction steps toward the activity and selectivity of ethanol production indicate that alloying the promoters with the electronegativity difference, Δχ, between the promoter and Rh being 0.7 is the most effective in lowering the C–C bond formation. This conclusion is in excellent accord with the reported catalytic performance of CO hydrogenation over Rh-based catalysts with different promoters. We believe that the electronegativity difference criterion is very useful in improving the catalytic performance using transition metal based catalysts for ethanol synthesis from CO hydrogenation. Finally, the reaction kinetics of CO hydrogenation to ethanol on the Rh/Mn/SiO2 under experimental operating conditions was simulated using kinetic Monte Carlo (KMC) modeling. The simulated reaction kinetics is qualitatively consistent with experimental observations. The ramifications of these findings are discussed and propositions for improving these catalysts are suggested.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 978506
- Report Number(s):
- PNNL-SA-68644; JCTLA5; 33194; BM0101010; TRN: US201010%%142
- Journal Information:
- Journal of Catalysis, 271(2):325-342, Vol. 271, Issue 2; ISSN 0021-9517
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
29 ENERGY PLANNING
POLICY AND ECONOMY
ALLOYS
ATOMS
CATALYSTS
ELECTRONEGATIVITY
ETHANOL
FUNCTIONALS
HYDROGENATION
KINETICS
METHANE
OXIDES
PROMOTERS
REACTION KINETICS
RENEWABLE ENERGY SOURCES
SILICA
SYNTHESIS
SYNTHESIS GAS
TRANSITION ELEMENTS
TRANSMISSION ELECTRON MICROSCOPY
X-RAY DIFFRACTION
X-RAY PHOTOELECTRON SPECTROSCOPY
Syngas
Ethanol
Rhodium nanoparticles
Manganese
Promoters
Density functional theory
Kinetic Monte Carlo
Environmental Molecular Sciences Laboratory