Search

Understanding the detailed mechanism of CO2 electrochemical reduction (CO2ERR) on catalytic surfaces, leading to the formation of carbon one (C1) products, serves as a plausible means for CO2 mitigation. The adsorption of single-atom catalysts (SACs) on MoS2 monolayer surface for CO2 electrochemical reduction reaction (CO2ERR) was studied using density functional theory (DFT) method. Among the selected SACs (Mn, Fe, Co, Ni), manganese (Mn) activated CO2 to the highest extent, with O–C–O bond angle of 142.84°. From our results, single atoms with large size and low atomic number (low d-orbital electrons) will have weak binding to the MoS2 as seen for Mn. Large atoms with lower electronegativity act as better reductants, reducing both the surface and the CO2 molecule adsorbed. Since CO2 activation is a very critical reaction step, we anticipate better performance of Mn-doped MoS2 to its counterparts, i.e. Fe, Ni and Co. In a 2-electron/2-proton reduction process, CO formation will occur at 0 V; however, formic acid will occur at a higher onset potential of − 1.03 V. Methanol will prefer to occur via the CO pathway at − 0.88 V. Selectively CO2 reduction is favoured in the order: CO > methanol > formic acid. The rate-limiting step for methanol production is observed to be the CH2OH formation step (∆G = 0.88 eV). Although the pristine MoS2 monolayer is inactive for catalysing CO2ERR offering CO2 physisorption, the introduction of adatoms serves as a site for CO2 sequestration, activation and electrochemical reduction. [ABSTRACT FROM AUTHOR]

The addition of Tc[O.sub.3]L (L = Cl, [O.sup.-], OC[H.sub.3], C[H.sub.3]) to substituted ketenes along various addition pathways was studied with density functional theory calculations to explore the peri-, chemo-, and regioselectivity of the reactions. In the reactions of Tc[O.sub.3]L with dimethyl ketene, the results show that for L = O- and C[H.sub.3], [1 +1] addition to form a triplet zwitterionic intermediate is the preferred first step; for L = Cl, the [[3 + 2].sub.C=C] addition across the O-Tc-Cl bond is the preferred first step and for L = OC[H.sub.3] the [[3 + 2].sub.C=C] addition across the O-Tc-OC[H.sub.3] bond is the preferred first step. In the reactions of Tc[O.sub.3]Cl with substituted ketenes, [1 +1] addition to form a triplet zwitterionic intermediate is the preferred first step for X = Ph, CN, and Cl; the [[3 + 2].sub.C=C] addition across the O-Tc-O bond of the complex is the preferred first step for X = H, while the [[3 + 2].sub.C=C] addition across the O-Tc-C[H.sub.3] bond is the preferred first step. Reactions involving a change in the oxidation state of metal have high activation barriers, while reactions that do not involve a change in oxidation state have low activation barriers. Reactions of ketenes with Tc[O.sub.3]L complexes have lower activation barriers for the preferred addition pathways than those of the Re[O.sub.3]L complexes reported in the literature. Thus, the Tc[O.sub.3]L complexes may be better catalysts for the activation of the C=C bonds of substituted ketenes than the reported Re[O.sub.3]L complexes. Key words: ketene, technetium, oxidation, computational. Nous avons etudie l'addition des complexes Tc[O.sub.3]L (L = Cl, [O.sup.-], OC[H.sub.3], C[H.sub.3]) sur des cetenes substitues et divers mecanismes par lesquels s'effectuent ces additions a l'aide de calculs de la theorie de la fonctionnelle de la densite en vue d'explorer la periselectivite, la chimioselectivite et la regioselectivite de ces reactions. Dans les reactions des complexes Tc[O.sub.3]L avec ledimethylcetene, les resultats montrent que, dans le cas oUL = O- et C[H.sub.3], l'addition [1 +1] formant un intermediaire zwitterionique a l'etat triplet est l'etape initiale la plus favorable; alors que, dans le cas oU L = Cl, c'est l'addition [[3 + 2].sub.C=C] sur la liaison O-Tc-Cl qui est l'etape initiale la plus favorable et, dans le cas oUL = OC[H.sub.3], l'etape initiale la plus favorable est l'addition [[3 + 2].sub.C=C] sur la liaison O-TcOC[H.sub.3]. En outre, dans les reactions duTc[O.sub.3]Clavecdes cetenes substitues, l'addition [1+1] formant un intermediaire zwitterionique a l'etat triplet est l'etape initiale la plus favorable lorsque X = Ph, CN et Cl; dans le cas oUX = H, c'est l'addition [[3 + 2].sub.C=C] sur la liaison OTc-O du complexe qui est l'etape initiale la plus favorable, alors que sur la liaison O-Tc-C[H.sub.3], c'est l'addition [[3 + 2].sub.C=C] qui est l'etape initiale la plus favorable. Les reactions faisant intervenir un changement de l'etat d'oxydation de l'atome metallique presentent des barrieres d'activation elevees, alors que celles qui ne font intervenir aucun changement d'etat d'oxydation presentent des barrieres d'activation basses. Les mecanismes d'addition favorises dans les reactions entre des cetenes et des complexes Tc[O.sub.3]L presentent des barrieres d'activation moins elevees que celles qui ont ete rapportees dans la litterature pour les memes reactions avec des complexes Re[O.sub.3]L. Les complexes Tc[O.sub.3]L pourraient donc etre de meilleurs catalyseurs que les complexes Re[O.sub.3]L connus pour effectuer l'activation des liaisons C=C de cetenes substitues. [Traduit par la Redaction] Mots-cles: cetene, technetium, oxydation, computationnel., Introduction Ketenes are excellent precursors for catalytic asymmetric reactions, creating chiral centers mainly through addition across their C=C bonds. Ketenes represent one of the keystone reactive intermediates of organic chemistry [...]