Journal
APPLIED SURFACE SCIENCE
Volume 528, Issue -, Pages -Publisher
ELSEVIER
DOI: 10.1016/j.apsusc.2020.146854
Keywords
Alkali modification; Propylene epoxidation; Reaction mechanism; Copper oxide; Activation barrier
Categories
Funding
- Slovenian Research Agency (ARRS) [P2-0150]
- Slovenian Research Agency [P2-0152, P1-0112]
- ELETTRA [20170045]
- project CALIPSOplus under the EU Framework Programme for Research and Innovation HORIZON 2020 [730872]
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This research focuses on epoxidation of propylene over pristine, Na, Ca and Cs modified CuOx/SiO2 catalysts using O-2. The selectivity of the reaction is analyzed using a combination of catalytic tests, in-situ XAS and DFT calculations. The initially present subnanometer CuO clusters are present in all catalysts which re-disperse/flatten during reaction. During catalytic reaction, the Cu1+ becomes the predominant oxidation state. There is no correlation between propylene oxide (PO) selectivity and copper oxidation state. DFT analysis of the propylene reaction pathway revealed that Na, Cs, and Ca addition decreases the bonding strength of propylene to CuO and decreases the O-2 activation barrier, while simultaneously increase the exothermicity of O-2 dissociation. The Na induced Cu-O bond modification decreases the activation barrier from 0.87 to 0.71 eV for the oxametallacycle (OMC) ring closure (first step in the reaction pathway favoring selectivity towards PO) compared to pristine 5Cu catalyst. At the same time, we observed an increase (from 0.45 to 0.72 eV) of the barrier for the abstraction of allylic hydrogen. The opposite effect is achieved by Ca addition: the activation barrier for OMC ring closure increases to 1.08 eV and that for allylic hydrogen stripping decreases to 0.16 eV.
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