Theoretical study of the crystal plane effect and ion-pair active center for C–H bond activation by Co3O4 nanocrystals

Oxygen Vacancy Promoted O2 Activation over Mesoporous Ni-Co Mixed Oxides for Aromatic Hydrocarbon Oxidation

Whether the oxygen vacancies of heterogeneous catalysts improve their catalytic activity or not has recently been the topic of intense debate in the oxidation of hydrocarbons. We designed an effective strategy to construct mesoporous Ni-Co mixed oxides via a ligand-assisted self-assembly approach. The surface oxygen vacancy concentrations of the mesoporous Ni-Co mixed oxide catalysts were regulated by changing the doping amount of Ni or the reduction method, and the relationship between oxygen vacancies and catalytic activity was studied. Controlled experiments and DFT calculations revealed that oxygen molecules were more favorably adsorbed and activated on oxygen vacancies to form active oxygen species. Increasing the oxygen vacancy concentration within a certain range can effectively enrich the active oxygen species, therefore improving the oxidation rate of ethylbenzene. The optimized mCo₃O₄-0.1NiO catalyst exhibited a remarkable catalytic activity for the solvent-free oxidation of ethylbenzene to acetophenone, typically including 68.0% conversion and 95.4% selectivity (20 mg mCo₃O₄-0.1NiO, 10 mL ethylbenzene, and 0.6 MPa O₂).

Acid etching induced defective Co3O4 as an efficient catalyst for methane combustion reaction

Development of an effective Co₃O₄ material as an advanced non-noble metal catalyst for methane combustion has great economic and environmental significance.

Unravelling the Enigma of Nonoxidative Conversion of Methane on Iron Single-Atom Catalysts

The direct, nonoxidative conversion of methane on a silica-confined single-atom iron catalyst is a landmark discovery in catalysis, but the proposed gas-phase reaction mechanism is still open to discussion. Here, we report a surface reaction mechanism by computational modeling and simulations. The activation of methane occurs at the single iron site, whereas the dissociated methyl disfavors desorption into gas phase under the reactive conditions. In contrast, the dissociated methyl prefers transferring to adjacent carbon sites of the active center (Fe₁ ©SiC₂ ), followed by C-C coupling and hydrogen transfer to produce the main product (ethylene) via a key -CH-CH₂ intermediate. We find a quasi Mars-van Krevelen (quasi-MvK) surface reaction mechanism involving extracting and refilling the surface carbon atoms for the nonoxidative conversion of methane on Fe₁ ©SiO₂ and this surface process is identified to be more plausible than the alternative gas-phase reaction mechanism.

Effects of a Pd/Pt bimetal supported by a γ-Al2O3 surface on methane activation

The catalytic removal of methane (CH₄) in exhaust emissions of natural gas-fueled vehicles is still a major challenge for automotive manufacturers because of the high CH₃-H bond energy and high concentrations of water (H₂O). Density functional theory (DFT) calculations were employed to investigate the adsorption of CH₄ and H₂O, as well as the activation of CH₄, on the surface of a Pd-Pt bimetal supported by γ-Al₂O₃. These are significant factors for catalytic combustion. Pt addition weakened the bonding of the intermediates and increased the availability of electrons on the surface. Besides this, the γ-Al₂O₃ surface and Pt were both beneficial for preventing the aggregation of clusters. CH₄ and H₂O adsorption, as well as the detailed mechanism of CH₄ activation on the Pd-Pt/γ-Al₂O₃ surfaces were simulated. The results showed that a Pt/Pd ratio of three resulted in the best catalytic activity among the different ratios examined in the presence of H₂O.

Well-Defined Materials for Heterogeneous Catalysis: From Nanoparticles to Isolated Single-Atom Sites

The use of well-defined materials in heterogeneous catalysis will open up numerous new opportunities for the development of advanced catalysts to address the global challenges in energy and the environment. This review surveys the roles of nanoparticles and isolated single atom sites in catalytic reactions. In the second section, the effects of size, shape, and metal-support interactions are discussed for nanostructured catalysts. Case studies are summarized to illustrate the dynamics of structure evolution of well-defined nanoparticles under certain reaction conditions. In the third section, we review the syntheses and catalytic applications of isolated single atomic sites anchored on different types of supports. In the final part, we conclude by highlighting the challenges and opportunities of well-defined materials for catalyst development and gaining a fundamental understanding of their active sites.

First principles prediction of CH4 reactivities with Co3O4 nanocatalysts of different morphologies

Co₃O₄ nanocatalysts have been experimentally shown to have excellent performance in catalyzing CH₄ combustion. These nanocatalysts of different morphologies, such as nanoparticle/nanocube, nanorod/nanobelt, and nanoplate/nanosheet, were previously synthesized and characterized to mainly expose the (001), (011), and (112) surfaces, respectively, with distinct reactivities. In this study, rigorous first principles calculations were performed to investigate CH₄ reactivities of the above Co₃O₄ surfaces of different terminations. CH₄ dissociation was predicted to occur at the Co-O pair site on these surfaces. For each surface, the most reactive Co-O pair site was identified based on calculated energy barriers of the different active sites, which should contribute most significantly to the reactivity of that surface. The lowest energy barriers for the (001), (011), and (112) surfaces were predicted to be 0.96, 0.90, and 0.79 eV, respectively, suggesting CH₄ reactivity to increase in that order for the different Co₃O₄ surfaces, consistent with the trend found experimentally for Co₃O₄ nanocatalysts of different morphologies. Direct comparison between the estimated and experimental CH₄ reaction rates per gram of the nanocatalysts at 325 °C further indicate that their relative ratios were well reproduced by considering three main factors: the effective energy barrier for CH₄ dissociation, the surface area of the nanocatalyst, and the number of independent active sites per unit surface area. The important influence of surface area on CH₄ reactivity is also demonstrated by the significant difference in the reactivities of the nanocatalysts when exposing the same facet but with distinct surface areas.

Role of Hydrogen and Oxygen Activation over Pt and Pd-Doped Composites for Catalytic Hydrogen Combustion

Removal of excess amount of hydrogen in a catalytic route is a safety measure to be implemented in fuel cell technologies and in nuclear power plants. Hydrogen and oxygen activation are crucial steps for hydrogen combustion that can be achieved by modifying supports with suitable noble metals. In the present study, Pt- and Pd-substituted Co₃O₄-ZrO₂ (CZ) were synthesized using PEG-assisted sonochemical synthesis. Ionic states of Pt and Pd in CZ supports were analyzed by X-ray photoelectron spectroscopy. Pd and Pt improved H₂ and O₂ activation extensively, which reduced the temperature of 50% conversion (T_50%) to 33 °C compared with the support (CZ). The activation energy of PdCZ catalyst was decreased by more than 2 folds (13.4 ± 1.2 kJ mol^-1) compared with CZ (34.3 ± 2.3 kJ mol^-1). The effect of oxygen vacancies in the reaction mechanism is found to be insignificant with Pt- and Pd-substituted CZ supports. However, oxygen vacancies play an important role when CZ alone was used as catalyst. The importance of hydrogen and oxygen activation as well as the oxygen vacancies in mechanism was studied by H₂-TPD, H₂-TPR, and in situ FTIR spectroscopy.

Observation of Charge Generation and Transfer during CVD Growth of Carbon Nanotubes

Carbon nanotube (CNT) is believed to be the most promising material for next generation IC industries with the prerequisite of chirality specific growth. For various approaches to controlling the chiral indices of CNTs, the key is to deepen the understanding of the catalytic growth mechanism in chemical vapor deposition (CVD). Here we show our discovery that the as-grown CNTs are all negatively charged after Fe-catalyzed CVD process. The extra electrons come from the charge generation and transfer during the growth of CNTs, which indicates that an electrochemical process happens in the surface reaction step. We then designed an in situ measurement equipment, verifying that the CVD growth of CNTs can be regarded as a primary battery system. Furthermore, we found that the variation of the Fermi level in Fe catalysts have a significant impact on the chirality of CNTs when different external electric fields are applied. These findings not only provide a new perspective on the growth of CNTs but also open up new possibilities for controlling the growth of CNTs by electrochemical methods.

Mesoporous Mn- and La-doped cerium oxide/cobalt oxide mixed metal catalysts for methane oxidation

New precious-metal-free mesoporous materials were investigated as catalysts for the complete oxidation of methane to carbon dioxide. Mesoporous cobalt oxide was first synthesized using KIT-6 mesoporous silica as a hard template. After removal of the silica, the cobalt oxide was itself used as a hard template to construct cerium oxide/cobalt oxide composite materials. Furthermore, cerium oxide/cobalt oxide composite materials doped with manganese and lanthanum were also prepared. All of the new composite materials retained the hierarchical long-range order of the original KIT-6 template. Temperature-programmed oxidation measurements showed that these cerium oxide/cobalt oxide and doped cerium oxide/cobalt oxide materials are effective catalysts for the total oxidation of methane, with a light-off temperature (T50%) of ∼400 °C observed for all of the nanostructured materials.