Revisiting alternative pathways in the Fischer–Tropsch process: Accurate density functional theory calculations on “magic” Ru₁₂ clusters

Abstract

Models of the Fischer–Tropsch reaction typically focus on two proposed mechanisms for the initial carbon monoxide dissociation: unassisted dissociation (carbide mechanism), and hydrogen‐assisted dissociation via an adsorbed oxymethylidene (HCO*) intermediate. Much evidence for hydrogen‐assisted dissociation comes from density functional theory calculations modeling ruthenium nanoparticle catalysts as infinite, periodic metal slabs. However, the generalized gradient approximations (GGAs) used in these calculations can make significant errors in reaction barrier heights. How these errors affect the predicted selectivity to unassisted vs. hydrogen‐assisted dissociation is not well understood. We address the problem by considering a different regime, applying GGA and beyond‐GGA approximations to CO dissociation on a “magic” nonmagnetic Ru12 cluster modeling supported nanoparticle catalysts. Both approximations concur that hydrogen‐assisted dissociation is facile on this cluster, providing additional support for its potential role in real catalysts.

Tunable model promoters in DFT simulations of catalysts

“…The unpromoted results are consistent with Ref. . In contrast to the promoters' large effect on the initial CO chemisorption (Fig.…”

“…We recently reviewed the ongoing question over whether CO dissociation is assisted by reaction with adsorbed hydrogen atoms. [41] We found that hydrogen-assisted CO dissociation predominated on the unpromoted Ru 12 model cluster. Here, we use the "edge" reaction mechanisms proposed in Ref.…”

“…CO dissociation is an essential step in the FT process converting synthesis gas (CO and H 2 ) into higher hydrocarbons. We recently reviewed the ongoing question over whether CO dissociation is assisted by reaction with adsorbed hydrogen atoms . We found that hydrogen‐assisted CO dissociation predominated on the unpromoted Ru 12 model cluster.…”

“…). We previously used this cluster to model aspects of the FT reaction . Other model catalysts, including explicit treatments of the catalyst support, will be considered in future work.…”

“…We previously used this cluster to model aspects of the FT reaction. [41] Other model catalysts, including explicit treatments of the catalyst support, will be considered in future work. Computed H atom adsorption energies are around 2.7 eV, consistent with the experimental binding energies of hydrogen to ruthenium surfaces of 2.51-2.90 eV, [42] and with DFT calculations on clean ruthenium surfaces.…”