Mingshu Chen

Interfacial Effects in Iron-Nickel Hydroxide–Platinum Nanoparticles Enhance Catalytic Oxidation

Improving Reactions at Interfaces Alloying precious metals such as platinum with more abundant transition metals, such as iron and nickel, can both improve their catalytic reactivity and lower catalyst cost. Chen et al. (p. 495) explored using coatings of iron oxide–hydroxide layers on supported platinum nanoparticles for CO oxidation. The presence of this layer allowed the reaction to run rapidly at room temperature by bringing together different reaction sites on the two metals. The addition of nickel improved catalyst lifetime, and an oxidative transformation created a more complex nanoparticle morphology that increased platinum utilization. An alloy catalyst for room-temperature CO creates sites for O2 activation when the CO2 product is released. Hybrid metal nanoparticles can allow separate reaction steps to occur in close proximity at different metal sites and accelerate catalysis. We synthesized iron-nickel hydroxide–platinum (transition metal-OH-Pt) nanoparticles with diameters below 5 nanometers and showed that they are highly efficient for carbon monoxide (CO) oxidation catalysis at room temperature. We characterized the composition and structure of the transition metal–OH-Pt interface and showed that Ni2+ plays a key role in stabilizing the interface against dehydration. Density functional theory and isotope-labeling experiments revealed that the OH groups at the Fe3+-OH-Pt interfaces readily react with CO adsorbed nearby to directly yield carbon dioxide (CO2) and simultaneously produce coordinatively unsaturated Fe sites for O2 activation. The oxide-supported PtFeNi nanocatalyst rapidly and fully removed CO from humid air without decay in activity for 1 month.

Graphene cover-promoted metal-catalyzed reactions

Significance Carbon deposits have been widely observed on metal surfaces in a variety of catalytic reactions, and the graphitic carbon species are often considered as inhibitors for surface reactions. We demonstrate here that CO adsorption and oxidation can occur on Pt surface covered by monolayer graphene, showing that the space between graphene overlayer and metal surface can act as a two-dimensional (2D) nanoreactor. Inside, CO oxidation happens with lower activation barrier due to the confinement effect of the graphene cover. This finding reminds us to reconsider the role of graphitic carbon in metal-catalyzed surface reactions and further provides a way to design novel catalysts. Graphitic overlayers on metals have commonly been considered as inhibitors for surface reactions due to their chemical inertness and physical blockage of surface active sites. In this work, however, we find that surface reactions, for instance, CO adsorption/desorption and CO oxidation, can take place on Pt(111) surface covered by monolayer graphene sheets. Surface science measurements combined with density functional calculations show that the graphene overlayer weakens the strong interaction between CO and Pt and, consequently, facilitates the CO oxidation with lower apparent activation energy. These results suggest that interfaces between graphitic overlayers and metal surfaces act as 2D confined nanoreactors, in which catalytic reactions are promoted. The finding contrasts with the conventional knowledge that graphitic carbon poisons a catalyst surface but opens up an avenue to enhance catalytic performance through coating of metal catalysts with controlled graphitic covers.

Multiscale structural and electronic control of molybdenum disulfide foam for highly efficient hydrogen production

Hydrogen production through water splitting has been considered as a green, pure and high-efficient technique. As an important half-reaction involved, hydrogen evolution reaction is a complex electrochemical process involving liquid-solid-gas three-phase interface behaviour. Therefore, new concepts and strategies of material design are needed to smooth each pivotal step. Here we report a multiscale structural and electronic control of molybdenum disulfide foam to synergistically promote the hydrogen evolution process. The optimized three-dimensional molybdenum disulfide foam with uniform mesopores, vertically aligned two-dimensional layers and cobalt atoms doping demonstrated a high hydrogen evolution activity and stability. In addition, density functional theory calculations indicate that molybdenum disulfide with moderate cobalt doping content possesses the optimal activity. This study demonstrates the validity of multiscale control in molybdenum disulfide via overall consideration of the mass transport, and the accessibility, quantity and capability of active sites towards electrocatalytic hydrogen evolution, which may also be extended to other energy-related processes.

Hexagonal Boron Nitride Cover on Pt(111): A New Route to Tune Molecule-Metal Interaction and Metal-Catalyzed Reactions

In heterogeneous catalysis molecule-metal interaction is often modulated through structural modifications at the surface or under the surface of the metal catalyst. Here, we suggest an alternative way toward this modulation by placing a two-dimensional (2D) cover on the metal surface. As an illustration, CO adsorption on Pt(111) surface has been studied under 2D hexagonal boron nitride (h-BN) overlayer. Dynamic imaging data from surface electron microscopy and in situ surface spectroscopic results under near ambient pressure conditions confirm that CO molecules readily intercalate monolayer h-BN sheets on Pt(111) in CO atmosphere but desorb from the h-BN/Pt(111) interface even around room temperature in ultrahigh vacuum. The interaction of CO with Pt has been strongly weakened due to the confinement effect of the h-BN cover, and consequently, CO oxidation at the h-BN/Pt(111) interface was enhanced thanks to the alleviated CO poisoning effect.

Mechanistic study of partial oxidation of methane to synthesis gas over supported rhodium and ruthenium catalysts using in situ time-resolved FTIR spectroscopy.

Abstract In situ time-resolved FTIR spectroscopy was used to study the reaction mechanism of partial oxidation of methane to synthesis gas and the interaction of CH 4 /O 2 /He (2/1/45) gas mixture with adsorbed CO species over SiO 2 and γ-Al 2 O 3 supported Rh and Ru catalysts at 500–600°C. It was found that CO is the primary product for the reaction of CH 4 /O 2 /He (2/1/45) gas mixture over H 2 reduced and working state Rh/SiO 2 catalyst. Direct oxidation of methane is the main pathway of synthesis gas formation over Rh/SiO 2 catalyst. CO 2 is the primary product for the reaction of CH 4 /O 2 /He (2/1/45) gas mixture over Ru/γ-Al 2 O 3 and Ru/SiO 2 catalysts. The dominant reaction pathway of CO formation over Ru/γ-Al 2 O 3 and Ru/SiO 2 catalysts is via the reforming reactions of CH 4 with CO 2 and H 2 O. The effect of space velocity on the partial oxidation of methane over SiO 2 and γ-Al 2 O 3 supported Rh and Ru catalysts is consistent with the above mechanisms. It is also found that consecutive oxidation of surface CO species is an important pathway of CO 2 formation during the partial oxidation of methane to synthesis gas over Rh/SiO 2 and Ru/γ-Al 2 O 3 catalysts.

Catalytic performance, structure, surface properties and active oxygen species of the fluoride-containing rare earth (alkaline earth)-based catalysts for the oxidative coupling of methane and oxidative dehydrogenation of light alkanes

Abstract The fluoride-containing catalysts, mostly of the rare earth (alkaline earth)-based system, demonstrate good catalytic performances for the oxidative coupling of methane and oxidative dehydrogenation of ethane, propane as well as iso-butane. The results of structural analysis of the catalysts for oxidative coupling of methane show that the promoting effects of the fluoride in the catalysts may be principally related to the phase–phase interaction between fluoride and oxide. Compared to the corresponding alkaline earth oxide promoted rare earth oxide catalyst system, an alkaline earth fluoride-promoted rare earth oxide catalyst system is less basic and will therefore be favorable to reduce CO2 inhibition in the reaction of methane oxidative coupling. However, there is no simple correlation between the acidity/basicity of a methane oxidative coupling catalyst and its catalytic performance. In the experiments of in situ spectroscopic characterizations carried out at the temperature from 650°C to 800°C, O−2 species was detected over five fluoride-containing rare earth (alkaline earth)-based catalysts for methane oxidative coupling reaction, and the reactions between O−2 species and CH4 to form C2H4 and the corresponding side-products were observed using in situ IR over four catalysts, which suggest that O−2 is probably the active oxygen species for the methane oxidative coupling reaction over the corresponding catalysts.

Promotional Effects of Au in Pd-Au Catalysts for Vinyl Acetate Synthesis

Silica supported Pd-Au bimetallic catalysts are highly selective for the acetoxylation of ethylene to vinyl acetate (VA). In this study we have used model catalysts consisting of planer surfaces and supported nanoparticles to investigate the promotional effects of Au in Pd-Au bimetallic catalysts. Low energy ion scattering spectroscopy, low energy electron diffraction, X-ray photoelectron spectroscopy, infrared reflection adsorption spectroscopy, and temperature-programmed desorption et al, were used to characterize the model systems. The catalytic performance for acetoxylation of ethylene to VA was examined for these model surfaces. In this paper, we summarize the current understanding of the promotional effects of Au in Pd-Au bimetallic catalysts for VA synthesis. The key results are that Au atoms break contiguous Pd atom ensembles at the surface into isolated Pd monomers. The absence of contiguous Pd sites significantly reduces the formation of combustion by-products and suppresses the poison effects of CO, thus enhancing the VA formation selectivity and activity.

Active Surfaces for CO Oxidation on Palladium in the Hyperactive State

Hyperactivity was previously observed for CO oxidation over palladium, rhodium, and platinum surfaces under oxygen-rich conditions, characterized by reaction rates 2-3 orders higher than those observed under stoichiometric reaction conditions [Chen et al. Surf. Sci. 2007, 601, 5326]. In the present study, the formation of large amounts of CO(2) and the depletion of CO at the hyperactive state on both Pd(100) and polycrystalline Pd foil were evidenced by the infrared intensities of the gas phase CO(2) and CO, respectively. The active surfaces at the hyperactive state for palladium were characterized using infrared reflection absorption spectroscopy (IRAS, 450-4000 cm(-1)) under the realistic catalytic reaction condition. Palladium oxide on a Pd(100) surface was reduced eventually by CO at 450 K, and also under CO oxidation conditions at 450 K. In situ IRAS combined with isotopic (18)O(2) revealed that the active surfaces for CO oxidation on Pd(100) and Pd foil are not a palladium oxide at the hyperactive state and under oxygen-rich reaction conditions. The results demonstrate that a chemisorbed oxygen-rich surface of Pd is the active surface corresponding to the hyperactivity for CO oxidation on Pd. In the hyperactive region, the CO(2) formation rate is limited by the mass transfer of CO to the surface.

Surface structure of Cu(001)-c(2 x 2)-Mg: a tensor low energy electron diffraction analysis and a first-principles calculation

A c(2×2) structure formed by adsorption of Mg atoms on Cu(0 0 1) at room temperature was determined by a tensor low-energy electron diffraction (LEED) analysis and a first-principles total-energy calculation. Both studies conclude that in the c(2×2) structure every second Cu atom in the first layer is substituted by Mg. Structural parameters obtained by the LEED analysis and the first-principles calculation are in good agreement and the interlayer distance between Mg and the first Cu layer is determined to be 0.55 and 0.60 A, respectively. The interlayer distance between the first and second Cu layers is contracted by about 5% from the bulk value. The calculation shows that the total adsorption energy at substitutional sites is 0.26 eV per Mg atom larger than that at hollow sites. The reason for large stability of the substitutional c(2×2) structure is examined and discussed theoretically.

High-temperature in situ FTIR spectroscopy study of LaOF and BaF2/LaOF catalysts for methane oxidative coupling

In situ FTIR spectroscopy was used to characterize the oxygen adspecies and its reactivity with CH4 over LaOF and 15 mol% BaF2/LaOF catalysts at OCM temperature (750-800°C). It was found that gas-phase oxygen was activated on the surface of LaOF and 15 mol% BaF2/LaOF, which had been pretreated under vacuum at 750 or 800°C, forming O2- species at high temperature (750-800°C). At 750°C, the adsorbed O2- species can react with pure CH4 accompanied by formation of gas-phase C2H4 and CO2, and there is a good correlation between the rate of disappearance of surface O2and the rate of formation of gas-phase C2H4. The O2- species was also observed over the catalysts under working condition, and it reacted with CH4 in a manner that was consistent with its role in a catalytic cycle. These results suggest that O2- may be the active oxygen species for OCM reaction over these catalysts.