Ethylene oxidation by Ag catalysts has been extensively investigated over the past few decades, but many key fundamental issues about this important catalytic system are still unresolved. This overview of the selective oxidation of ethylene to ethylene oxide by Ag catalysts critically examines the experimental and theoretical literature of this complex catalytic system: (i) the surface chemistry of silver catalysts (single crystal, powder/foil, and supported Ag/α-Al2O3), (ii) the role of promoters, (iii) the reaction kinetics, (iv) the reaction mechanism, (v) density functional theory (DFT), and (vi) microkinetic modeling. Only in the past few years have the modern catalysis research tools of in situ/operando spectroscopy and DFT calculations been applied to begin establishing fundamental structure–activity/selectivity relationships. This overview of the ethylene oxidation reaction by Ag catalysts covers what is known and what issues still need to be determined to advance the rational design of this important catalytic system.

mean-f ield microkinetic modeling to point to the role of coordinatively unsaturated sites in catalyzing FA decomposition on Au/SiC.

Hydrodesulfurization is a process to produce ultralow-sulfur diesel fuel. Although promoted molybdenum sulfide (MoS2) catalysts have been used industrially for several decades, the active site requirements for selective hydrodesulfurization of organosulfur compounds with minimal inhibition by organonitrogen constituents of a real gasoil feed has not been resolved. Using molecular binding energy descriptors derived from plane wave density functional theory calculations for comparative adsorption of organosulfur and organonitrogen compounds, we analyzed more than 20 potential sites on unpromoted and Ni- and Co-promoted MoS2. We find that hydrogen sulfide and ammonia are simple descriptors of adsorption of sterically unhindered organosulfur and organonitrogen compounds such as dibenzothiophene and acridine, respectively. Further, organonitrogen compounds in gasoil bind more strongly than organosulfur compounds on all sites except on sites with exposed metal atoms on the corner and sulfur edges of promoted MoS2. Consequently, these sites are proposed as required for maximum-hydrodesulfurization minimum-inhibition catalysis.

A combination of periodic density functional theory (DFT, PW91-GGA) calculations, reaction kinetics experiments, and mean-field microkinetic modeling is used to derive insights on the reaction mechanism and determine the nature of the active site under reaction conditions for the vapor-phase decomposition of formic acid (FA, HCOOH) over Pt/C catalysts. Microkinetic models formulated using DFT energetics derived on the clean Pt(100) and Pt(111) required large parameter adjustments to reproduce the experimentally measured apparent activation energies and reaction orders. Further, these models predicted high surface coverage of adsorbed carbon monoxide (CO*), inconsistent with the environment of the active site in the DFT calculations on the clean surfaces. Consequently, we reperformed DFT calculations for the entire reaction network on partially CO*-covered (4/9 monolayer, ML) Pt(111) and Pt(100). The resultant microkinetic models, with thermochemistry and kinetics explicitly dependent on CO* coverage, were able to reproduce the experimentally determined activation energies and reaction orders, in addition to being self-consistent in CO* coverage. Our results suggest that Pt(100) is likely poisoned by CO* under typical reaction conditions and does not contribute significantly to the experimentally observed reactivity. Instead, we find that Pt(111) better represents the active site for FA decomposition reaction on Pt/C catalysts. The optimized model on 4/9 ML CO*-covered Pt(111) suggests that the reaction occurs via the carboxyl (COOH*) intermediate and that the spectator CO*-assisted pathways play a significant role under reaction conditions. This study underscores the importance of spectator species on the energetics and the mechanism of a catalytic reaction and their key role in developing a model that better addresses the nature of the active site under realistic catalytic reaction conditions.

The supporting information section includes two analyses to demonstrate the comprehensiveness of the reaction networks generated through RING. These are 1. Generation of reactions of 4-octanone on an acid site. The number of reaction rules considered is small enough to manually list possible reactions and products of certain reaction steps and compare it with the output from RING. 2. Generation of all possible instances of homolytic bond scission in different reactants of the homologous series of normal paraffins. The number of possible bonds that can cleave can be calculated based on the size of the paraffin, which can then be compared with the output from RING.All the reactions generated using RING for these analyses are given in the section "Output from RING" at the end of this document. Analysis 1Consider a system with the following reactions 1. Adsorption of a keto group on an acid site (strictly keto, not aldehlyde, acid, ester, etc.) leading to a protonated carbonyl group. 2. Desorption of protonated carbonyl group produced in 1 3. 1,2 Hydride shifts between a C+ and its neighboring carbon atoms 4. Beta scission of carbenium ions involving C-C bond cleavage at the beta-position to the positively charged carbon center 5. keto-enol tautomerism involving the transformation of an enolic group to a keto group 6. Desorption of carbenium ions to form an olefin in the gas phase We use this example as an illustration of the comprehensiveness of the reactions generated using RING. We apply this set of six reaction rules to 4-octanone using RING, which generated 241 reactions based on these rules. Analysis 2Consider the elementary step of homolytic scission of C-C bonds in an alkane. Each C-C bond can be broken to form two radicals, however, not all of these reactions will lead to distinct reactions because a normal paraffin is either symmetric about a central bond (in case of even number of carbons in the molecule) or a central carbon atom (in case of odd number of carbons in the molecule). It can be calculated that the number of distinct homolytic scission reactions is 'n' for a molecule of size '2n' or '2n+1'.We generated reactions pertaining to different paraffins using RING. In each case we observed that RING generated the precise number of reactions as shown below in the section "Output from RING". RemarksThese two analyses, although not mathematical, are simple test cases to demonstrate that RING does indeed generate reactions comprehensively.