A 2012 perspective by Lewis, Jewell, Kyriakou and Sykes on cobalt's surface chemistry reviewed metal-supported Co nanoparticles as a platform for investigating the adsorption, diffusion, dissociation and reaction of catalytically relevant molecules, and pointed to the open questions raised by scanning tunneling microscopy (STM) of epitaxial Co nanoparticles on Cu(111): how CO is activated in Fischer–Tropsch synthesis (FTS), how adsorption depends on particle size, shape, layer thickness and support, and how segregated CO and hydrogen phases couple to reaction kinetics. The problem asks for quantitative, atomic-scale connections between the structure of Co nanoparticles (size and shape below roughly 6 nm, layer thickness and electronic structure, low-coordination edge and kink sites, and support identity including Au(111)) and their FTS surface chemistry with CO and H — specifically, whether CO dissociation is direct or H-assisted and on which sites; how CO and H adsorption varies with particle size, shape, and Co layer thickness (quantum-size effects, including whether such effects survive for particles beyond a few nanometres); and whether the interface length between segregated CO and H domains limits FTS kinetics.

Background

Fischer–Tropsch synthesis (FTS) is a heterogeneously catalyzed reaction that converts synthesis gas (CO + H₂) into long-chain hydrocarbons at high pressure (bar range) and 200–350 °C; it supplies on the order of 2% of the world's fuel and cobalt is the preferred catalyst for natural-gas-derived syngas. Two elementary processes anchor the mechanism: dissociative adsorption of H₂ and activation of CO. Two competing CO activation routes are debated: direct (carbide-type) dissociation of adsorbed CO into surface C and O atoms, and hydrogen-assisted pathways in which H first binds to CO to form HCO or COH intermediates that then split. Density functional theory (DFT, a quantum-mechanical method computing energies and barriers from electron density) studies before 2012 disagreed: stepped Co surfaces were computed to strongly favor direct dissociation (Ge and Neurock, 2006), Inderwildi, Jenkins and King (2008) argued for alternative H-assisted routes to higher hydrocarbons, Shetty and van Santen (2010) found the carbide mechanism preferred on open Ru and Co surfaces, and the kinetic and H₂/D₂ isotope analysis of Ojeda, Iglesia, Mavrikakis and co-workers (2010) supported kinetically relevant H-assisted CO activation on Co — a controversy that the 2012 perspective correctly described as unsettled. Because Co single crystals are hard to prepare, the Sykes group developed a model platform: epitaxial (crystallographically registry-matched with the substrate) Co nanoislands grown on Cu(111) and imaged by scanning tunneling microscopy (STM), a probe that maps surface topography and adsorbate structures at atomic resolution. Their STM work revealed ordered hydrogen phases on Co(111)-topped islands, including a high-coverage H-(1×1) phase (an adsorbate overlayer with one H atom per surface Co atom), spillover of H atoms from dissociation sites at island edges onto the surrounding Cu(111) (spillover = migration of an adsorbate from a site where it binds strongly to one where it would not form directly), and — when CO and H are coadsorbed — segregated CO and H phases in which CO exerts two-dimensional pressure that compresses or displaces H. Classical structure–activity knowledge came from supported-catalyst studies: FTS rates per surface Co atom drop sharply for particles smaller than roughly 6 nm (Bezemer et al., 2006), and steady-state isotopic transient kinetic analysis (SSITKA, a method that resolves surface coverages and residence times of intermediates via isotope switching) traced this to higher coverages of strongly bound CO on sub-6-nm particles, ascribed to more low-coordination surface atoms (den Breejen et al., 2009). The perspective also flagged quantum-size effects: in ultrathin metal films, confinement of electrons makes the density of states and hence adsorption energies oscillate with film thickness, as demonstrated for cobalt phthalocyanine on Pb(111) films (Jiang et al., 2008); layer-selective H adsorption on Co/Cu(111) hinted at analogous behavior on Co, but Co islands on Cu(111) are electronically coupled to the metallic substrate, which masks intrinsic (isolated-particle) quantum-size effects, while Co on Au(111) — a support with different lattice mismatch and electronic shift — was chemically almost unexplored. A 2014 review (Beaumont) confirmed that these molecular-level questions constituted the field's open agenda. The unresolved target is a quantitative, atomic-scale relationship linking Co nanoparticle structure to CO and H adsorption, the CO activation pathway, and FTS kinetics.

Problem Statement

Establish a quantitative, atomic-scale relationship between the structure of cobalt nanoparticles and their Fischer–Tropsch surface chemistry with CO and hydrogen, comprising the following linked questions, formulated on the basis of the open-question agenda of the 2012 perspective of Lewis, Jewell, Kyriakou and Sykes: (1) determine, under Fischer–Tropsch (FTS) reaction conditions (bar-range CO/H₂ pressure, 200–350 °C), whether CO activation on cobalt proceeds by direct dissociation or through hydrogen-assisted pathways, on which sites of the particle each pathway operates, and connect the atomically observed segregated CO and H phases to the kinetics of the elementary steps; (2) measure how CO and H adsorption depends on particle size and shape at atomic resolution, especially for very small particles below roughly 6 nm where FTS activity drops sharply; (3) determine the role of coordinatively unsaturated sites (edges, kinks) in CO dissociation or hydrogenation under FTS reaction conditions; (4) establish the quantitative relationship between cobalt layer thickness, electronic structure and adsorption energy — including whether quantum-size effects are significant for particles larger than a few nanometres — where the electronic coupling of Co islands to the Cu(111) substrate masks intrinsic quantum-size effects and comparison with weakly explored supports such as Au(111), with its different lattice mismatch and electronic shift, is one possible route to disentangle them; and (5) determine whether the interface length between segregated CO and H domains is a limiting factor in FTS kinetics, and how it depends on island size and shape. The common objective uniting these components is a structure–reactivity model of the cobalt FTS catalyst surface in which particle geometry, thickness, support and defect structure are tied quantitatively to adsorption energetics, the CO activation pathway, and overall FTS kinetics. The linked questions are independently attackable: a defensible, well-supported answer to any one of them is itself a valid resolution of part of the objective, while the full objective stated here requires resolving the set.

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