Enhanced Electrocatalytic CO2 Reduction Reactivity of S- and N-Doped Fe-Embedded Graphene.

In this work, we studied the reaction mechanisms for CO2 reduction reaction (CRR) on the iron-doped graphene and its coordinating sulfur (S) and nitrogen (N) variants, FeNn S4-n (n=1-4), using density functional theory calculations. Our results revealed that the electronic property and catalytic reactivity of the surfaces can be tuned by varying the N and S atoms ratio. The CRR activities of the mixed surfaces, FeN3 S1 , FeN2 S2 , and FeN1 S3 , were better than FeN4 and FeS4 , where the absolute value of the limiting potential of the mixed surface decreased by 0.3 V. Considering the stability, we suggest FeN3 S surface to be favorable for CRR. For the bare surfaces, we found a positive linear correlation between the magnetic moment and the charge of Fe metal. For these surfaces, the reduction of CO (*CO+(H+ +e- )→*CHO) was important in deciding the limiting potential. We found that the adsorption energy of CO displayed a volcano relationship with the magnetic moment of the Fe atom. The study showed that the change of local coordinating structure around the Fe atom could modify the electronic and magnetic properties of the active Fe center and improve the CRR activity performance.

Oxidation and de-alloying of PtMn particle models: a computational investigation.

We present a computational study of the energetics and mechanisms of oxidation of Pt-Mn systems. We use slab models and simulate the oxidation process over the most stable (111) facet at a given Pt2Mn composition to make the problem computationally affordable, and combine Density-Functional Theory (DFT) with neural network potentials and metadynamics simulations to accelerate the mechanistic search. We find, first, that Mn has a strong tendency to alloy with Pt. This tendency is optimally realized when Pt and Mn are mixed in the bulk, but, at a composition in which the Mn content is high enough such as for Pt2Mn, Mn atoms will also be found in the surface outmost layer. These surface Mn atoms can dissociate O2 and generate MnOx species, transforming the surface-alloyed Mn atoms into MnOx surface oxide structures supported on a metallic framework in which one or more vacancy sites are simultaneously created. The thus-formed vacancies promote the successive steps of the oxidation process: the vacancy sites can be filled by surface oxygen atoms, which can then interact with Mn atoms in deeper layers, or subsurface Mn atoms can intercalate into interstitial sites. Both these steps facilitate the extraction of further bulk Mn atoms into MnOx oxide surface structures, and thus the progress of the oxidation process, with typical rate-determining energy barriers in the range 0.9-1.0 eV.

Understanding the interaction between transition metal doping and ligand atoms of ZnS and ZnO monolayers to promote the CO2 reduction reaction.

Single-atom catalysts (SACs) obtained by doping transition metal (TM) atoms into stable monolayers are a promising way to improve the CO2 reduction reaction (CRR) performance. In this work, we theoretically investigated the effect of ligand atoms around the doped TM (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) in ZnO and ZnS for promoting the CRR performance. We found that the ligand atoms around the TM can influence its oxidation state and the electronic properties of the SACs, thus affecting their CRR activity. Due to the smaller charge transfer between the TM and substrate for TM-ZnS compared to TM-ZnO, the TM binding is weaker for the former. In addition, the more negatively charged oxygen ligand atoms in TM-ZnO interact with reaction intermediates, resulting in CRR products with less electron transfer. Pristine ZnS and ZnO monolayers can produce HCOOH but require a high limiting potential (UL) of about -1.2 V. Doping with TMs can reduce UL compared to the pristine surface. At the same time, the ligand can alter the preferred CRR pathway and product selectivity. We found that Mn-ZnS is selective to the CH4 product with a UL of only -0.29 V, which is a nearly 1 V improvement in the UL compared to ZnS.

Collision-induced dissociation of sodiated glucose, galactose, and mannose, and the identification of anomeric configurations.

Collision-induced dissociation of sodiated α-glucose, ß-glucose, α-galactose, ß-galactose, α-mannose, and ß-mannose was studied using electronic structure calculations and resonance excitation in a low-pressure linear ion trap. We made an extensive search of conformers and transition states in calculations to ensure the transition state with the lowest barrier height for each dissociation channel could be located. The major dissociation channels, in addition to desodiation, are cross-ring dissociation and dehydration. Cross-ring dissociation starts with H atom transfer from the O1 atom to the O0 atom, followed by the cleavage of the C1-O0 bond. Dehydration of the anomer with O1 and O2 atoms in the cis configuration involves the transfer of an H atom from the O2 atom to the O1 atom, followed by the cleavage of the C1-O1 bond. In contrast, dehydration of the anomer with O1 and O2 atoms in the trans configuration mainly occurs through H atom transfer from the O3 or O2 atom to the O1 atom for glucose, from the O3 or O4 atom to the O1 atom for galactose, and from the O4 or O2 atom to the O1 atom for mannose, followed by the cleavage of the C1-O1 bond. The dehydration barrier heights are lower than those of cross-ring dissociation for cis anomers, but higher than those of cross-ring dissociation for trans anomers. The relative barrier heights from calculations are consistent with the experimental measurements of branching ratios. Both computational and experimental results show that the branching ratio of dehydration can be generalized as a simple rule for rapidly identifying the anomeric configurations of these monosaccharides.