(611g) Surface Stabilization and Subsequent Rupturing of Co in Co-Cu Catalysts By Formation of Adsorbed Di-, Tri-, and Tetracarbonyls
Surface Stabilization and
Subsequent Rupturing of Co in CoCu Catalysts by Formation of Adsorbed Di-,
Tri-, and Tetracarbonyls
Greg Collinge, Norbert Kruse,
The Gene and Linda Voiland
School of Chemical Engineering and Bioengineering
Washington State University,
Long-chain hydrocarbons with
terminal oxygen functionalization (oxygenates) are very desirable chemical
products, evident from the over 6 million tons/year produced in the mega-process
of hydroformylation. In the 1970s, the Institut Francais du Petrole patented a
heterogeneous CoCu catalyst for the production of short-chain alcohols (i.e.
oxygenates), following CO hydrogenation . Recent developments in our group
suggest that both short- and long-chain oxygenates can be produced at both high
selectivity and yield using CoCu-based
(either CoCuNb or CoCuMn)
catalysts in Fischer-Tropsch (FT) synthesis [2-4]. To make this technology
truly robust and to direct future research into highly selective long-chain
oxygenate synthesis, the fundamental role each metal
plays during catalysis needs to be elucidated. However, little is theoretically known
about the interaction of Co and Cu beyond the single-atom impurities in high
and low index crystal facets investigated by Ruban et
al. and later by Nilekar et al., respectively [5, 6].
To this end, our present work uses density functional theory (DFT) calculations
to investigate the segregation tendency of a Cu monolayer on both Co(0001) and Co(10-12) model catalysts (henceforth Cu/Co(0001)
and Cu/Co(10-12)) as a function of CO pressure. Reconstruction is further
investigated by determining the thermodynamic feasibility of CO-induced Co
1. Co rupturing from step sites of Cu/Co(755) as
result of geminal tetracarbonyl formation and stabilization. Here, Co has been
ruptured (right side) from the step site and exists as a subcarbonyl on the
terrace surface. This allows the
adsorbed CO on the Cu terrace sites (left side) to move toward the step and
pump up new step Co, which can then also be subsequently ruptured (center).
Orange spheres are terrace Cu; brown, step Cu; dark blue, subsurface Co; light
blue, surface Co.
We have shown that CO adsorption
causes a reversal in segregation on flat Cu/Co(0001)
wherein Co is effectively pumped to the surface by the presence of the adsorbed
CO . In this work, we have expanded this research to include the stepped Cu/Co(10-12) surface and connected both results to
experimentally relevant conditions through the construction of phase diagrams.
These phase diagrams show that surface Co enrichment is limited at the most
relevant reaction conditions, but is nonetheless highly favorable in the
presence of even a small CO partial pressure. We also address another
phenomenon, which adds another layer of complexity: elucidating how CO induces
not just reconfiguration of the surface, but also the reconstruction of the
surface. This latter investigation is performed using the vicinal Cu/Co(755) surface, and our results suggest that geminal tri-
and tetracarbonyls can favorably rupture Co from the step sites (see Figure 1),
producing mobile subcarbonyls on the surface and providing a mechanistic
explanation for catalyst restructuring.
The next challenge facing the
design of FT catalysts for the production of long-chain oxygenates is the
question of CO dissociation on CoCu as evidenced by X-ray photoelectron
spectroscopy in our group . The results presented suggest that CO bond
cleavage is prohibitively endothermic on both the flat and stepped surfaces,
regardless of coverage. We present preliminary evidence for CO dissociation
driven by carbide formation and discuss implications in relation to mounting
experimental and theoretical evidence [8-13] for carbide formation in certain
catalyst formulations for higher alcohol synthesis.
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