Re- and Cs-Copromoted Silver Catalysts for Ethylene Epoxidation: A Theoretical Study

Salaev, M.A.; Salaeva, A.A.; Poleschuk, O. Kh.; Vodyankina, O. V.

doi:10.1134/s0022476619110039

Cited by 6 publications

(3 citation statements)

References 91 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…These authors conjectured that Cs + cations on Ag surfaces facilitated EO desorption and inhibited its secondary reactions. DFT calculations, performed both in the absence [34,35] and presence of Cl adatoms [36,37] on Ag 20 clusters [34,36] and extended Ag(111) surfaces, [35,36] support previously proposed notions that anionic Re-oxo and cationic Cs moieties, when in atomic contact with Ag domains, can tune the electrophilicity of O* to increase C 2 H 4 epoxidation and lower C 2 H 4 , EO, and CH 3 CHO combustion rates.…”

Section: Introductionsupporting

confidence: 80%

Roles of Re and Cs Promoters and Organochlorine Moderators in the Synthesis of Ethylene Oxide on Ag‐based Catalysts

Hwang,

Klaucke,

Lizandara‐Pueyo

et al. 2023

ChemCatChem

View full text Add to dashboard Cite

C2H4−O2 reactions form ethylene oxide (EO) on Ag nanoparticles dispersed on α‐Al2O3 and promoted with alkali and other elements, with Re and Cs most frequently used in practice. Traces of alkyl chlorides (e. g., C2H5Cl) and alkanes in much larger amounts are added to reactants to balance the rate of deposition and removal (and the coverage) of Cl adatoms (Cl*) on Ag surfaces. Such Cl adlayers retain Ag surface ensembles that form O2‐derived intermediates that favor EO synthesis, but typically decrease O2 activation rates. A series of Ag/α‐Al2O3 catalysts (with and without Re or Cs) are used here to examine the role of promoters and Cl moderators through an analysis of the effects of reaction conditions and C2H5Cl levels using convection‐reaction constructs and a mechanistic formalism that considers O2 (as chemisorbed O2*) as the reactant in EO synthesis through one electrophilic O‐atom, with the second O‐atom (O*) acting as a unselective nucleophile that must be scavenged by a sacrificial reductant (C2H4 or EO); such channels are precluded by any intervening O2 dissociation events (that form two O*). O2 consumption rates in C2H4−O2 reactions decrease 10‐fold upon exposing Ag/α‐Al2O3 to C2H5Cl, evincing persistent Cl* species that block active sites and require >104 C2H4 oxidation turnovers to be removed. Activation barriers do not change but relative rates of epoxidation versus combustion increase in the presence of this refractory Cl* adlayer, which reduces the number and shrinks the size of available site ensembles without altering their intrinsic reactivity for O2 activation to form O2* but attenuating rates of its subsequent dissociation. Refractory O* adlayers also form on Ag surfaces, as shown by the accumulation of persistent O* species upon exposing reduced Ag particles to N2O. Inter‐atom repulsive forces weaken O* binding and destabilize N2O decomposition transition states to eventually allow stable N2 and O2 evolution for catalytic N2O decomposition, which occurs at landing ensembles formed at interstices of refractory adlayers. Cl* forms denser adlayers than O*, as evidenced by a 10‐fold decrease not only to C2H4−O2 rates but also to N2O stoichiometric and catalytic rates and O* uptakes. The occasional evolution of O2, which forms larger landing ensembles that more readily form 2O* from O2, occurs with higher frequency from O* than mixed O*/Cl* adlayers, and this effect of Cl* to inhibit (O−O)* dissociation is demonstrated in CO probe reaction studies, which show that highly reactive bound dioxygen species are retained to a greater extent when adlayers comprise both Cl* and O*. Re and Cs do not influence the nature of such adlayers but increase and decrease, respectively, the number of acid sites and selectivity losses via EO combustion. Their respective domains may also block a modest fraction of Ag surfaces without consequences on intrinsic O2 activation rates or EO selectivities. The effects of Re beyond those reported here may emerge upon synthetic protocols that alter Re location and Ag−Re intimacy to enable channels which utilize both O‐atoms in epoxidation.

show abstract

Section: Introductionsupporting

confidence: 80%

Roles of Re and Cs Promoters and Organochlorine Moderators in the Synthesis of Ethylene Oxide on Ag‐based Catalysts

Hwang,

Klaucke,

Lizandara‐Pueyo

et al. 2023

ChemCatChem

View full text Add to dashboard Cite

show abstract

“…Interestingly, when Re and Cs are combined, we observe a strong affinity of Cs* for the O atoms in ReO 4 * (Figure 4; 500−1000 ppm Re and 1500−2000 ppm Cs), as also observed in other computational studies. 19,20 Due to the attractive interactions between ReO 4 * and Cs*, Cs* is forced to spread out evenly on the surface in the same manner as ReO 4 *. In this way, ReO 4 * moieties act as spacers that prevent the agglomeration of Cs*.…”

Section: Computational Workflow To Obtain and Characterizementioning

confidence: 99%

“…The large number of co-adsorbed species makes it challenging to develop realistic, ground-state models of the catalytic system, which are required for further mechanistic studies. Indeed, previous computational studies mostly focused on single adsorbates on pristine Ag(111) surfaces, − and only recently have combinations of promoters been considered. − …”

Section: Introductionmentioning

confidence: 99%

Unraveling the Synergistic Effect of Re and Cs Promoters on Ethylene Epoxidation over Silver Catalysts with Machine Learning-Accelerated First-Principles Simulations

et al. 2022

View full text Add to dashboard Cite

Co-promotion of Ag catalysts by Re and Cs leads to ethylene oxide (EO) selectivity higher than that achievable by either promoter alone. However, the atomistic and electronic mechanisms behind this synergistic co-promotion remain unclear.Here, we shed light on how Re and Cs promote ethylene epoxidation by elucidating realistic catalyst models using machine learning-accelerated first-principles simulations, characterizing their electronic properties, and constructing structure−selectivity relationships. Together with a comprehensive set of experimental data, we show that the synergistic effects of co-promotion are twofold. First, Re, in the form of ReO 4 , increases the dispersion of Cs, thereby maximizing the promotional effects of Cs. Second, combining electron-withdrawing Re and electron-donating Cs balances the amount of charge transferred to the catalyst. Akin to the Sabatier principle, this maximizes EO selectivity by preventing the formation of overly nucleophilic oxygen species that facilitate ethylene combustion and overly electrophilic Ag centers that accelerate EO isomerization. The insights from our work may guide the development of improved promoter combinations, while our methodology may serve as a template for studying other complex, multiply promoted reaction systems.

show abstract

Stability enhancement for silver catalyst in ethylene epoxidation by support treatment

Liu

et al. 2023

Catalysis Communications

View full text Add to dashboard Cite

Re- and Cs-Copromoted Silver Catalysts for Ethylene Epoxidation: A Theoretical Study

Cited by 6 publications

References 91 publications

Roles of Re and Cs Promoters and Organochlorine Moderators in the Synthesis of Ethylene Oxide on Ag‐based Catalysts

Roles of Re and Cs Promoters and Organochlorine Moderators in the Synthesis of Ethylene Oxide on Ag‐based Catalysts

Unraveling the Synergistic Effect of Re and Cs Promoters on Ethylene Epoxidation over Silver Catalysts with Machine Learning-Accelerated First-Principles Simulations

Stability enhancement for silver catalyst in ethylene epoxidation by support treatment

Contact Info

Product

Resources

About