Revisiting CO Activation on Co Catalysts: Impact of Step and Kink Sites from DFT

Abstract

Fischer–Tropsch synthesis of hydrocarbons from CO and H2 is an established industrial process, during which the C–O bond must break. The preferred mechanism and sites at which CO is activated and hydrocarbon products are formed remains under debate. Density functional theory calculations are used to investigate direct and H-assisted dissociation of CO at kink and step sites on FCC Co(321) and Co(221), at which direct dissociation is demonstrated to be intrinsically preferred at low coverage. The CO dissociation rate is predicted to be higher at defect sites than on extended facets on FCC Co, with dissociation at the kink site having the highest relative rate of all FCC Co site types predicted to be exposed on FCC Co nanocrystals. Nevertheless, the increase in activity is not sufficient to exceed that of highly active HCP Co sites. However, the intrinsically preferred mechanism of CO activation at low coverage, direct rather than H assisted, is concluded to be the same on FCC and HCP Co.

Similarities and Differences for Atomic and Diatomic Molecule Adsorption on the B-5 type sites of the HCP(1016) surfaces of Co, Os, and Ru from DFT Calculation

“…As can be seen in Figure 1 (e) the surface contains 2 unique types of step edge (and hence, step site type) within each surface unit cell-these include a B5A and a B5B type site. [8,26] Asymmetric diatomic adsorbates (CO, etc) were placed over the sites with the more electrondeficient species closer to the surface; symmetric diatomic adsorbates (H 2 , O 2 ) were placed both horizontally and vertically centered over the site(s).…”

“…Cobalt is an active catalyst for reactions such as Fischer-Tropsch (FT) synthesis and Dry Reforming of CH 4 , Osmium a relatively mediocre catalyst for alkene hydroxylation and the Haber-Bosch reaction and a good doping/alloying agent in making intermetallic Oxygen Reduction Reaction (ORR) catalysts, and Ruthenium an active catalyst for FT synthesis, Haber-Bosch and recently in biofuels conversion. [7][8][9][10][11][12][13][14][15][16] Despite the fact these materials have been used and researched for such catalytic applications for decades, only recently has there been a large uptick in research related to understanding exact atomic mechanisms for their catalytic properties via the step and edge sites that their nanoparticles possess. [8,[17][18][19][20][21][22][23][24][25][26][27][28] Previous computational and theory studies of the adsorption energies and catalytic properties of Co,Os, and Ru materials have typically relied on using over-simplified surface models of the step sites that HCP catalyst nanoparticles of these elements possess.…”

“…However, other work has shown that in fact the nanoparticles of real catalysts of these elements have small amounts of (0001) and other low index facets and have large percentages of step and step-related facets and facet-intersections. [8,27,28] The surface energy of various HCP facets was previously examined in small detail in the literature, and among the lowest energy Milled Index HCP surfaces identified could be the HCP (1016)surface. To date, this surface index has not been thoroughly characterized in the literature.…”

“…The reasons for this absence could be compounded by factors including a large calculation cell which makes it expensive to treat via computation/modeling, and the relatively similar Miller Index (1015)which may express higher occurrence via Wulff construction or real surface formation thermodynamics dynamics. [8,[18][19][20]26] The HCP(1016) surface however is nonetheless very interesting and should be used as a computational tool for modeling adsorption and catalysis on stepped HCP surface for the following reasons moderately wide terraces (high step edge density) , multiple stepedge types in a single calculation cell, and low surface formation energy (could be present on certain sized nanoparticles). [26] In the work presented in this manuscript, we present results for comparing and contrasting trends and differences in the adsorption energies of small mono-atomic and diatomic species on the Co, Os, and Ru (1016) surfaces.…”

Accuracy of theoretical catalysis from a model of iron-catalyzed ammonia synthesis

“…Strong double and triple bonds that are often broken during catalysis (e.g., N 2 , CO, O 2 ) pose an additional challenge to DFT, as the error in electron correlation typically scales with the bond strength 22 . Despite this, many theoretical studies of transition metal catalysis routinely use a single popular functional such as RPBE or B3LYP for all steps as the basis for conclusions, with assumptions of accuracy largely extrapolated from previous onestep benchmarks [23][24][25] .…”