New Insight into CO Formation during HCOOH Oxidation on Pt(111): Intermolecular Dehydration of HCOOH Dimers

Density functional theory simulations were performed to investigate CO formation during HCOOH oxidation on the Pt(111) surface in aqueous phase, through the intermolecular dehydrations of various HCOOH dimer models. The formation of CO that is found to poison Pt catalysts proceeds via four major intermolecular dehydration pathways as determined by varying initial HCOOH dimer structures. The computed rate-determining energy barriers of those four pathways are low, suggesting the kinetically and thermodynamically facile formation of intermediates and CO. This work demonstrates that the presence of HCOOH dimers accounts for the easy CO poisoning of Pt-based catalysts, and clarifies the controversy on the intermediates and mechanisms of CO formation found in different HCOOH oxidation experiments

Microscopic Insight into the Activation of O₂ by Au Nanoparticles on ZnO(101) Support

We carry out density functional theory calculations to cast insight on the microscopic mechanism of the activation of O₂ by Au₇ cluster on ZnO(101)-O support. The excellent catalytic activity of Au/ZnO catalyst was ascribed to the distribution of polarized surface charge associated with interface structure. It is found the stoichiometric ZnO(101)-O easily adsorbs and dissociates O₂ to form very stable oxygen-saturated surface. For Au₇ on stoichiometric ZnO(101)-O surface, the two Au atoms neighboring to O could accumulate positive charges, which then upshift the d-band centers toward the Fermi level. These favor the adsorption and dissociation of O₂, providing two Au activation sites. In contrast, for the Au₇ on the oxygen-saturated ZnO(101)-O, all Au atoms become neighboring to O and consequently provide seven activation sites. The workfunction difference between the Au₇ and support induces effective polarized surface charges, substantially promoting O₂ adsorption and dissociation both dynamically and thermodynamically. Further analysis on the effect of different Au positions demonstrates the polarized charge as the microscopic driving force for catalysis. These results would help design of better metal/oxide catalysts by providing important implications for the role of atomic and electronic structures

Theoretical Study of the Reactions of 2-Chlorophenol over the Dehydrated and Hydroxylated Silica Clusters

Silica is the main component of combustion-generated fly ash and is expected to have an important impact on the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in municipal waste incinerators. In this work, we theoretically studied the reactions of 2-chlorinated phenol (2-CP) over the clusters (SiO₂)₃ and (SiO₂)₃O₂H₄, which mimic the dehydrated and hydroxylated silica structures, respectively. The dehydrated cluster is much more active toward the attack of 2-CP to form highly stable 2-chlorophenolate than the hydroxylated silica cluster. The further dissociation of chlorophenolates to form CP radicals (CPRs) is calculated to be very difficult. The calculated energy barrier of the reaction of 2-CP over the dehydrated (SiO₂)₃ cluster and IR data are in good agreement with early experimental observations. On the basis of the calculated results, we propose that the formation of PCDD/Fs from CPs over silica surfaces may not involve CPRs, but be relevant to the further conversion of chlorophenolates over silica surfaces. This mechanism is very different from the corresponding reactions mediated by transition metal oxides. The results presented here may be helpful to understand the chemisorption mechanism of CPs on silica surfaces in real waste combustion

A First-Principle Study of Synergized O₂ Activation and CO Oxidation by Ag Nanoparticles on TiO₂(101) Support

We performed density functional theory (DFT) calculations to investigate the synergized O₂ activation and CO oxidation by Ag₈ cluster on TiO₂(101) support. The excellent catalytic activity of the interfacial Ag atoms in O₂ dissociation is ascribed to the positive polarized charges, upshift of Ag d-band center, and assistance of surface Ti_5c atoms. CO oxidation then takes place via a two-step mechanism coupled with O₂ dissociation: (i) CO + O₂ → CO₂ + O and (ii) CO + O → CO₂. The synergistic effect of CO and O₂ activations reduces the oxidation energy barrier (<i>E</i>_a) of reaction (i), especially for the up-layered Ag atoms not in contact with support. It is found that the coadsorbed CO and O₂ on the up-layered Ag atoms form a metal-stable four-center O–O–CO structure motif substantially promoting CO oxidation. On the oxygen defective Ag₈/TiO₂(101) surface, because of the decreased positive charges and the down-shift of d-band centers in Ag, the metal cluster exhibits low O₂ adsorption and activation abilities. Although the dissociation of O₂ is facilitated by the TiO₂(101) defect sites, the dissociated O atoms would cover the defects so strongly that further CO oxidation would be prohibited unless much extra energy is introduced to recreate oxygen defects

Design of Efficient Catalysts with Double Transition Metal Atoms on C₂N Layer

Heterogeneous catalysis often involves molecular adsorptions to charged catalyst site and reactions triggered by catalyst charges. Here we use first-principles simulations to design oxygen reduction reaction (ORR) catalyst based on double transition metal (TM) atoms stably supported by 2D crystal C₂N. It not only holds characters of low cost and high durability but also effectively accumulates surface polarization charges on TMs and later deliveries to adsorbed O₂ molecule. The Co–Co, Ni–Ni, and Cu–Cu catalysts exhibit high adsorption energies and extremely low dissociation barriers for O₂, as compared with their single-atom counterparts. Co–Co on C₂N presents less than half the value of the reaction barrier of bulk Pt catalysts in the ORR rate-determining steps. These catalytic improvements are well explained by the dependences of charge polarization on various systems, which opens up a new strategy for optimizing TM catalytic performance with the least metal atoms on porous low-dimensional materials