465794 Surface Stabilization and Subsequent Rupturing of Co in Co-Cu Catalysts By Formation of Adsorbed Di-, Tri-, and Tetracarbonyls

Wednesday, November 16, 2016: 5:03 PM
Franciscan B (Hilton San Francisco Union Square)
Greg Collinge1, Norbert Kruse2 and Jean-Sabin McEwen1, (1)The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA, (2)Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA

Surface Stabilization and Subsequent Rupturing of Co in CoCu Catalysts by Formation of Adsorbed Di-, Tri-, and Tetracarbonyls

Greg Collinge, Norbert Kruse, Jean-Sabin McEwen

The Gene and Linda Voiland School of Chemical Engineering and Bioengineering

Washington State University, Pullman, WA

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 [1]. 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 rupturing.

Figure 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 [7]. 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 [3]. 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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