Reaction Dynamics of CO Hydrogenation on Iron Catalysts Using ReaxFF Molecular Dynamics Simulation.

The conversion of CO to hydrocarbons using catalysts is a promising route to utilize CO and produce more valuable chemicals in a sustainable manner. Recent studies have shown that iron-based catalysts perform well for the hydrogenation of CO. While the hydrogenation reaction mechanism in the gas phase is straightforward, when catalyzed by iron it has been demonstrated to involve various chemical transformations, and the selectivity and conversion are strongly dependent on the particle size.

Adsorption Mechanism and Characteristic Temperatures of the Monolayer Adsorption of CO on Graphite: The Role of Graphene Dimensions.

Although simulation results for gaseous adsorption on a surface of infinite extent, modeled with periodic conditions at the boundaries of the simulation box, agree with experimental data at high temperatures, simulated isotherms at temperatures below the triple point temperature show unphysical substeps because of the compromise of interactions within the box and interactions between the box and its mirror image boxes. This has been alleviated with surfaces of finite dimensions (Loi, Q. K.

Effects of the adsorbate-gas interface at the pore opening on the lower closure point in gaseous adsorption in porous solids.

The limit to the lower closure point (LCP) observed experimentally in the desorption isotherm of gases in porous solids has been commonly attributed to the homogeneous cavitation of the condensate in cavities. It was proposed recently that the experimental limit to LCP could be described in simulations with the ink-bottle pore, provided that the length of the uniformly sized conduit connecting the closed cavity to the surrounding is shorter than 2 nm, and the evaporation is by way of pore blocking mechanism, rather than homogeneous cavitation. To substantiate this assertion, that deviates from the commonly belief of homogeneous cavitation, we further investigated in this paper with cavities having wedge-like pore opening, that better mimics real solids, and offer further explanation on the limit of the LCP to the pore blocking as the mechanism of evaporation with simulations of argon and nitrogen adsorption over a range of temperatures that are commonly used experimentally.

Effects of a Free Adsorbate Boundary on the Description of an Argon Adsorbed Film on Graphite below the Bulk Triple Point.

Monte Carlo simulations have been carried out to study argon adsorption on graphite at temperatures below the bulk triple point temperature, = 83.8 K. Two models for graphite have been used to investigate the effects of an adsorbate patch with a free boundary on the layering temperatures, the two-dimensional (2D)-triple point and the 2D-critical point for the three adsorbate layers on the surface.

Characterization of non-graphitized carbon blacks: a model with surface crevices.

Experimental isotherms for argon and nitrogen adsorption on two non-graphitized carbon substrates, Carbopack B and Cabot BP280, do not obey Henry's Law in the range of pressures accessible to the most sensitive MKS pressure transducers. At high pressures, close to the bulk coexistence pressure (P0), the isotherms at temperatures below the bulk triple point temperature cross the P0 axis at a finite loading, a behaviour which is interpreted as incomplete wetting. It was found that the adsorbed density at P0 for Cabot BP280 is lower than that for Carbopack B which is, in turn, only slightly lower than that for the highly graphitized Carbopack F, suggesting that there is a long-range effect of the surface structure in non-graphitized carbon blacks, in the accumulation of higher layers, especially for Cabot BP280.

On the canonical isotherms for bulk fluid, surface adsorption and adsorption in pores: A common thread.

Kinetic Monte Carlo simulated isotherms calculated in the canonical ensemble, at temperatures below the critical temperature, for bulk fluid, surface adsorption and adsorption in a confined space, show a van der Waals (vdW) loop with a vertical phase transition between the rarefied and dense spinodal points at the co-existence chemical potential, µ. Microscopic examination of the state points on this loop reveals features that are common to these systems. At state points with chemical potentials greater than μ the microscopic configurations show clusters, which coalesce to form two co-existing phases along the vertical section of the loop (the coexistence line).

Nonwetting/Prewetting/Wetting Transition of Ammonia on Graphite.

Simulations of ammonia adsorption on graphite were carried out over a range of temperatures to investigate the transition from nonwetting to wetting. The process is governed by a subtle interplay between the various interactions in the system and the temperature. At temperatures below the bulk triple point, the system is nonwetting; above the triple point, we observed continuous wetting, preceded by a prewetting region in which the so-called thin-to-thick film transition occurs.

The role of adsorbate size on adsorption of Ne and Xe on graphite.

We have carried out an extensive grand canonical Monte Carlo simulation to investigate the adsorption of neon and xenon on graphite. The adsorbate collision diameters of neon and xenon are smaller and greater respectively, than the commensurate graphite lattice spacing λ=3×3R30 of 0.426 nm.