Showing papers by "Vladimir Galvita published in 2014"

Delivering a Modifying Element to Metal Nanoparticles via Support: Pt–Ga Alloying during the Reduction of Pt/Mg(Al,Ga)Ox Catalysts and Its Effects on Propane Dehydrogenation

Abstract: The alloying of Pt with Ga delivered from a hydrotalcite-like support was investigated as a strategy to produce bimetallic catalysts for propane dehydrogenation. A series of Pt/Mg(Al,Ga)Ox catalysts (2–3 wt % Pt, Ga/Pt molar ratios between 0 and 10) and a model Pt/Ga2O3 catalyst (4 wt % Pt, Ga/Pt molar ratio of 50) were characterized by means of X-ray diffraction (XRD), transmission electron microscopy, and activity measurements (873 K, Wcat/FC3H8,0 = 25 kgcat·s·mol–1 and PC3H8,0 = 5 kPa at a total pressure of 101.3 kPa). XRD patterns taken during temperature-programmed reduction in 5% H2/He and isothermal reduction/oxidation cycling between 5% H2/He and 20% O2/N2 at 873 K revealed dynamic alloy formation and segregation that depended upon the gas environment and Ga content. Alloying on the Pt/Mg(Al,Ga)Ox catalyst with a Ga/Pt ratio of 2 could not be observed by XRD. For a Ga/Pt ratio of 10, an alloy with a diffraction peak at 40.2° was formed during the initial reduction. After subsequent reduction/oxida...

TAP study of toluene total oxidation over a Co3O4/La-CeO2 catalyst with an application as a washcoat of cordierite honeycomb monoliths

Abstract: The total oxidation of toluene was studied over a Co3O4/La-CeO2 catalyst in a Temporal Analysis of Products (TAP) set-up in the temperature range 713 K to 873 K in the presence and absence of dioxygen. It has been demonstrated that the reaction proceeds via a Mars-van Krevelen mechanism. The reaction rate increased 8.4 times if both toluene and dioxygen were present in the feed. The partial reaction order with respect to O2 diminished from 0.9 to 0.6 with an increase in temperature from 763 to 873 K. Adsorbed oxygen species with a lifetime of ∼8 s have been found on a catalyst fully oxidized by dioxygen. Catalysis of isotopically labeled (18)O2/(12)C6H5(13)CH3 results in the formation of products containing (18)O, which indicates that both lattice and adsorbed oxygen are involved in the total oxidation of toluene. The role of adsorbed oxygen is activation of the C-H bond in toluene. The reaction network of the catalytic total oxidation of toluene consists of the following sequence: adsorption of toluene on the catalyst surface; activation of toluene by dehydrogenation with adsorbed oxygen; oxidation of activated toluene mainly by the lattice oxygen and re-oxidation of the reduced catalyst by dioxygen.

The Role of Different Types of CuO in CuO–CeO2/Al2O3 for Total Oxidation

Abstract: Different types of copper oxide in CuO–CeO2/Al2O3 were investigated structurally and catalytically. Standard preparation lead to the presence of CuO monocrystals and Ce1−xCuxO2−x as evidenced by XRD, HRTEM and EDX. Washing with nitric acid removed the monocrystals, while leaving the Ce1−xCuxO2−x solid solution. The Fourier transformed Cu K patterns for the HNO3-washed catalyst showed a diminished second Cu shell, characteristic for Ce1−xCuxO2−x as well as for X-ray amorphous nano-sized CuAl2O4 spinel phase. XANES analysis and first shell Cu fitting confirmed this CuAl2O4 like structure outside Ce1−xCuxO2−x. By modeling the Ce K EXAFS signal in the washed sample, the amount of Cu in Ce1−xCuxO2−x was determined to be 18 ± 6 %. Total oxidation of CO and C3H8 as model reactions showed little difference in activity between non-washed and washed CuO–CeO2/Al2O3, indicating that CuO monocrystals were hardly active at temperatures below 450 °C. XRD performed in situ during TPR and TPO showed that CeO2 in Ce1−xCuxO2−x added to the redox capacity of both CuO–CeO2/Al2O3 samples as it was partially reduced at 300 °C. At higher temperatures, the CuO monocrystals also participated in reaction, leading to a higher activity for the oxidation of CH4.

Unravelling the Formation of Pt–Ga Alloyed Nanoparticles on Calcined Ga-Modified Hydrotalcites by in Situ XAS

Abstract: The chemical transformations taking place during the formation of catalytic Pt–Ga alloyed nanoparticles supported on calcined Ga-modified hydrotalcite Mg(Ga)(Al)Ox are investigated. The starting point is a Pt(acac)2 precursor impregnated onto a Mg(Ga)(Al)Ox support. An oxidative treatment first yields Pt nanoparticles, while subsequent reduction efficiently delivers Ga from the support framework to Pt, forming Pt–Ga alloyed clusters. Different steps are discerned in this process based on in situ XAS analysis. During oxidative heating to 350 °C, the initially adsorbed Pt(acac)2 precursor molecules decompose and form atomically dispersed Pt4+ species with 5-/6-fold oxygen coordination. A fraction of the formed Pt–O bonds consists of strong anchoring points between Pt4+ species and support oxygen, decreasing the Pt mobility induced by the basic support. Further calcination to 650 °C leads to scission of these Pt–O support bonds, allowing more mobile Pt species to form 3–11 atom Pt fcc nanoparticles with an o...