Yi-Fan Han

Biphasic Pd−Au Alloy Catalyst for Low-Temperature CO Oxidation

Low-temperature CO oxidation over a compositional series of Pd-Au nanoalloy catalysts supported on silica fume was studied. Except for the pure metals, these materials invariably showed biphasic separation into palladium- and gold-rich components. Performance was optimal for a catalyst of bulk composition Pd(4)Au(1), a mixture of Pd(90)Au(10) (72.5 at. %) and Pd(31)Au(69) (27.5 at. %), that was remarkably active at 300 K and more stable than a pure Au catalyst. For bulk materials dominated by Pd (Pd:Au = 16:1; 8:1; 4:1), the palladium-rich alloy fraction frequently adopted hollow sphere or annular morphology, while the gold-rich crystals were often multiply twinned. Quantitative powder X-ray diffraction (XRD) showed that under the synthesis conditions used, the Au solubility limit in Pd crystals was approximately 12 at. %, while Pd was more soluble in Au (approximately 31 at. %). This was consistent with X-ray photoelectron spectroscopy (XPS), which revealed that the surfaces of Pd-rich alloys were enriched in gold relative to the bulk composition. In situ Fourier transform infrared spectra collected during CO oxidation contained a new band at 2114 cm(-1) (attributed to linear CO-Au/Au-Pd bonds) and reduced intensity of a band at 2090 cm(-1) (arising from a linear CO-Pd bond) with escalating Au content, indicating that the Pd sites became increasingly obscured by Au. High-resolution electron micrographs (HRTEM) of the Pd-rich alloys revealed atomic scale surface defects consistent with this interpretation. These results demonstrate that gold-containing biphasic Pd nanoalloys may be highly durable alternatives for a range of catalytic reactions.

Iron Oxychloride (FeOCl): An Efficient Fenton-Like Catalyst for Producing Hydroxyl Radicals in Degradation of Organic Contaminants

An iron oxychloride (FeOCl) catalyst was developed for oxidative degradation of persistent organic compounds in aqueous solutions. Exceptionally high activity for the production of hydroxyl radical (OH·) by H2O2 decomposition was achieved, being 2-4 orders of magnitudes greater than that over other Fe-based heterogeneous catalysts. The relationship of catalyst structure and performance has been established by using multitechniques, such as XRD, HRTEM, and EPR. The unique structural configuration of iron atoms and the reducible electronic properties of FeOCl are responsible for the excellent activity. This study paves the way toward the rational design of relevant catalysts for applications, such as wastewater treatment, soil remediation, and other emerging environmental problems.

Hydroxyapatite foam as a catalyst for formaldehyde combustion at room temperature.

The excellent performance of hydroxyapatite, a novel non-precious metal catalyst, for formaldehyde (HCHO) combustion at room temperature is reported. Temperature-programmed surface reaction results indicated that hydroxyl groups bonded with the channel Ca(2+) may be responsible for adsorption/activation of HCHO.

The Formation of PdCx over Pd-Based Catalysts in Vapor-Phase Vinyl Acetate Synthesis: Does a Pd–Au Alloy Catalyst Resist Carbide Formation?

The formation of Pd carbide (PdCx) during the synthesis of vinyl acetate (VA) was investigated over Pd/SiO2 catalysts with two different Pd particle sizes, as well as over a Pd–Au/SiO2 mixed-metal catalyst. XRD data show that PdCx was produced in the pure Pd catalysts after reaction based on the downshift of the Pd(111) and (200) XRD features. The smaller Pd particles showed greater resistance to the formation of PdCx. The XRD and XPS data are consistent with formation of a PdCx species at the surface of the Pd–Au catalyst, however, the primary contributor to the downshift of the Pd(111) feature subsequent to reaction in the mixed-metal catalyst is believed to arise from reaction-induced alloying of Au with Pd. The alloying of Au with Pd is apparently very effective in preventing PdCx formation in Pd-based catalysts for VA synthesis.

Insights into the Oxidation and Decomposition of CO on Au/α-Fe2O3 and on α-Fe2O3 by Coupled TG-FTIR

CO oxidation and decomposition behaviors over nanosized 3% Au/alpha-Fe2O3 catalyst and over the alpha-Fe2O3 support were studied in situ via thermogravimetry coupled to on-line FTIR spectroscopy (TG-FTIR), which was used to obtain temperature-programmed reduction (TPR) curves and evolved gas analysis. The catalyst was prepared by a sonication-assisted Au colloid based method and had a Au particle size in the range of 2-5 nm. Carburization studies of H 2-prereduced samples were also made in CO gas. According to gravimetry, for the 3% Au/alpha-Fe2O3 catalyst, there were three distinct stages of CO interaction with the Au catalyst but only two stages for the catalyst support. At low temperatures (<or=100 degrees C), only the Au catalyst had a rapid weight loss, which confirmed that CO reacted with highly active absorbed oxygen species and/or OH species which were associated with and promoted by the Au nanoparticles. Around 300 degrees C, both the catalyst and support samples experienced the reduction of Fe2O3 to Fe3O4, while above 400 degrees C further reduction to FeO and Fe metal took place. Au played no part in the kinetics of Fe3O4 formation because lattice O mobility was rate-limiting. At higher temperature where Fe3O4 was further reduced to FeO and Fe 0, the initially formed metallic Fe 0 nuclei could decompose CO molecules and release O species. Both this coproduced O species and the lattice oxygen could react with CO molecules. Thus, the CO oxidation was not limited by the mobility of lattice oxygen, and the catalytic function of Au was revealed again. Carburization of metallic Fe, created by prereduction in H 2, revealed a distinct weight gain at 350 degrees C corresponding to Fe 3C formation, as subsequently confirmed by X-ray diffraction (XRD). Sustained carbon deposition ensued at 450 degrees C. In the cases of the 3% Au/gamma-Al 2O 3 and Au/ZrO 2 catalysts prepared by the same method, however, after exposure to CO in the same temperature range, no carbon deposit was observed, indicating that although Au nanoparticles could activate the absorbed oxygen molecules at low temperatures, they were not able to activate the lattice oxygen in the three catalyst supports or to dissociate the CO molecules directly.

Density functional theory study of direct synthesis of H2O2 from H2 and O2 on Pd(111), Pd(100), and Pd(110) surfaces

Abstract The direct synthesis of hydrogen peroxide (H2O2) from hydrogen (H2) and oxygen (O2) on Pd(111), Pd(100), and Pd(110) surfaces was investigated using periodic density functional theory (DFT) calculations. Several elementary steps making up this reaction were postulated and calculated. The Pd(111) surface shows the highest catalytic selectivity for H2O2 among the three surfaces. Open surfaces such as Pd(100) and Pd(110) are not favorable for this reaction because O–O-containing species on these surfaces dissociate easily. The O–O bond energy and the binding energy of O–O-containing surface species are responsible for catalytic selectivity. The higher binding energy of O–O-containing surface species is not favorable for the direct synthesis of H2O2 because the higher binding energy results in lower dissociation barriers.

Probing The Structure Evolution of Iron‐Based Fischer–Tropsch to Produce Olefins by Operando Raman Spectroscopy

By using operando Raman spectroscopy (ORS), we investigated the panoramic structure evolution of an iron oxide (α‐Fe2O3) catalyst, which is used for the production of olefins via Fischer–Tropsch (FTO). During activation in different atmospheres and reaction at 260 °C and 3.0 MPa, α‐Fe2O3 was only partially transformed into γ‐Fe2O3 by H2 pretreatment; meanwhile, a transformation of α‐Fe2O3→γ‐Fe2O3→Fe3O4→Fe5C2 was observed in both CO and syngas (H2/CO). Combining with other techniques such as XRD, TEM, XPS and TPSR, we reveal that assembles of various iron oxides (γ‐Fe2O3, Fe3O4, Fe carbide, and their combinations) are responsible for FTO. Especially, the preliminary relationship of catalyst structure and performance relating to the production of olefins directly from syngas was established. Such a study is critical for further understanding of the FTO reaction and other catalytic reactions.

Degradation of trichloroethylene by hydrodechlorination using formic acid as hydrogen source over supported Pd catalysts

An advanced method for the degradation of trichloroethylene (TCE) over Pd/MCM-41 catalysts through a hydrogen-transfer was investigated. Formic acid (FA) was used instead of gaseous H2 as the hydrogen resource. As a model H-carrier compound, FA has proven to yield less by-products and second-hand pollution during the reaction. Several factors have been studied, including: the property of catalyst supports, Pd loading and size, temperature, initial concentrations of FA and TCE (potential impact on the reaction rates of TCE degradation), and FA decomposition. The intrinsic kinetics for TCE degradation were measured, while the apparent activation energies and the reaction orders with respect to TCE and FA were calculated through power law models. On the basis of kinetics, we assumed a plausible reaction pathway for TCE degradation in which the catalytic degradation of TCE is most likely the rate-determining step for this reaction.

Synthesis of hydrogen peroxide from H2 and O2 in water and ethanol catalyzed by nanoclustered Pd0 on silica: strong selectivity enhancement exerted by the addition of ionic liquids

Hydrogen peroxide (H(2)O(2)) synthesis directly from dioxygen and dihydrogen was carried out using a continuous flow reactor with a Pd catalyst. The effects of ionic liquids on the selectivity to H(2)O(2) were studied on a Pd/SiO(2) catalyst. It was found that the ionic liquid [BMIM][BF(4)] in water or ethanol is quite beneficial to the selectivity to H(2)O(2). Ca. 95% selectivity after 1 h in both solvents and a relatively high selectivity i.e. (about 50% in ethanol and 40% in water) after 5 h reaction have been achieved. On the other hand, a plausible mechanism for the effects of ion liquids on this reaction system was suggested on the basis of the preliminary results.

A Complete Selectivity for the Direct Synthesis of Hydrogen Peroxide over Palladium-Tellurium Catalysts at Ambient Pressure

Highly selective hydrogen peroxide (H2 O2 ) synthesis directly from H2 and O2 is a strongly desired reaction for green processes. Herein a highly efficient palladium-tellurium (Pd-Te/TiO2 ) catalyst with a selectivity of nearly 100 % toward H2 O2 under mild conditions (283 K, 0.1 MPa, and a semi-batch continuous flow reactor) is reported. The size of Pd particles was remarkably reduced from 2.1 nm to 1.4 nm with the addition of Te. The Te-modified Pd surface could significantly weaken the dissociative activation of O2 , leading to the non-dissociative hydrogenation of O2 . Density functional theory calculations illuminated the critical role of Te in the selective hydrogenation of O2 , in that the active sites composed of Pd and Te could significantly restrain side reactions. This work has made significant progress on the development of high-selectivity catalysts for the direct synthesis of H2 O2 at ambient pressure.