State of palladium in palladium-aluminosilicate catalysts as studied by XPS and the catalytic activity of the catalysts in the deep oxidation of methane

Palladium catalysts based on Siralox and AS aluminosilicate supports for the deep oxidation of methane were studied. With the use of XRD analysis, it was found that they were heterophase systems consisting of an amorphous aluminosilicate and γ-Al₂O₃ stabilized against agglomeration. It was found that the catalytic activity of palladium-aluminosilicate catalysts in the deep oxidation of methane at 500°C depended on the support precalcination temperature. X-ray photoelectron spectroscopy (XPS) was used to study the states of the AS-30 aluminosilicate support calcined at 600, 800, or 1000°C and palladium supported on it. It was found that the action of an acid impregnation solution of palladium nitrate on the aluminosilicate calcined at 800°C resulted in a structural rearrangement of the aluminosilicate surface. This rearrangement resulted in the stabilization of both palladium oxide and palladium metal particles at surface defects and the incorporation of these particles into the aluminosilicate after catalyst calcination. As a result, an anomalous decrease in catalytic activity was observed in aluminosilicate samples calcined at 800°C. According to XPS data, palladium in the catalyst was stabilized in the following three phases: metal (E _b(Pd 3d _5/2) = 334.8 eV), oxide (E _b(Pd 3d _5/2) = 336.8 eV), and “interaction” (E _b(Pd 3d _5/2) = 335.8 eV) phases. The ratio between these phases depended on support and catalyst calcination temperatures. The interaction phase, which consisted of PdO_x clusters stabilized in the aluminosilicate structure, was responsible for the retention of activity after calcination at high temperatures (800°C). Based on an analysis of XPS data, it was hypothesized that palladium in the interaction phase occurred in a charged state with the formal charge on the Pd atom close to 1 + (δ+ phase).     

Ion bombardment-induced mechanical stress in plasma-enhanced deposited silicon nitride and silicon oxynitride films

We have studied the influence of different deposition conditions on the mechanical stress of silicon nitride and silicon oxynitride layers formed by plasma-enhanced deposition onto silicon substrates. It appears that the mechanical stress of the as-deposited silicon (oxy)nitride layer is a combined effect of the extent of ion bombardment and the deposition temperature on the hydrogen desorption rate. Deposited films show a tensile stress character when the hydrogen desorption rate is thermally controlled, whereas in the case of an ion-bombardement-controlled hydrogen desorption rate the deposited films have a compressive stress. It is also shown that due to annealing at temperatures above the deposition temperature the films are densified as a result of hydrogen desorption and cross-linking.     

Kinetics and mechanism of oxidation of silicon nitride bonded silicon carbide ceramic

Thermogravimetry (TG) has been used to study the oxidation of a commercial silicon nitride bonded silicon carbide (SNBSC) ceramic. The oxidation was studied in air and carbon dioxide atmospheres between 800 and 1300°C. TG/mass spectrometry (MS) shows that the silicon nitride bonding phase oxidises first. The kinetics follow a multi-stage mechanism with diffusion control. Carbon dioxide was found to be a more powerful oxidant than air at temperatures above 1050°C.     

Ni基催化剂上CH~4部分氧化反应的研究

本文研究了NiO/SiO~2, NiO/Al~2O~3, NiO/La~2O~3催化剂的晶相组成及其还原性质。考察了催化剂的活性、选择性、稳定性。讨论了负载型催化剂稳定性较高的原因和发生部分氧化反应时CH~4的活化过程。     

Oxide talum-containing catalysts of ozone decomposition and methane oxidation

The oxidation of manganese ions is shown to occur in ozone decomposition on manganese-containing catalyst, leading to the deactivation of a sample. The structure of the iron-containing catalyst does not change, ensuring its high stability during ozone decomposition in a dry gas flow at room temperature, conducted in the region of inner diffusion with an activation energy of 15.5 ± 0.7 kJ/mol. The presence of manganese cations in the intermediate stages of reduction is found to increase their activity in the reaction of deep methane oxidation.     

Dependence of the rate of methane oxidation on the surface area of the catalysts

Specific rate of methane conversion into C₂-hydrocarbons and deep oxidation products in its interaction with O₂ or N₂O decreases with growing surface area of catalysts that are metal-like compounds of transition elements. This regularity is interpreted in terms of a heterogeneous-homogeneous mechanism.

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C₂ ( CH₄ O₂ N₂O) - . , - .

     

Pd/CeO2甲烷氧化催化剂的构效关系

钯基催化剂是甲烷氧化活最具活性的催化剂.在宏观和纳米尺度上,它们的组成、结构和形态的调整可以显著改变其催化行为和稳定性,对催化剂的整体性能有很大的影响.在已经应用的几种载体和促进剂组合中, Pd/CeO2由于其活性和耐用性以及Pd/Pd O载体之间较强的相互作用而引起了人们的极大关注.这使得人们可在纳米尺度上创建特定的结构,从而对甲烷活化特性产生重大的影响.本文综述了该领域的最新发现,特别是设想如何在纳米尺度上尽可能控制Pd-CeO2相互作用,从而有助于设计更强劲的甲烷氧化催化剂.     

Photocatalytic oxidation of isobutylene at impurity centers on deposited zirconium oxide catalysts

It has been found that illumination in the impurity absorption region of silica-supported zirconium oxides intensifies considerably their ability to catalyze the oxidation of isobutylene to acetone. A correlation has been established between the photocatalytic activity of these compounds and their photoluminescence intensity in vacuum.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 22, No. 5, pp. 635–639, September–October, 1986.     

Structural-chemical activity criteria for catalysts of deep oxidation

Russian Journal of General Chemistry -     

Palladium/graphitic carbon nitride catalyst for selective oxygen transfer in Wacker oxidation

Nanoscaled palladium particles supported on graphitic carbon nitride (Pd⁰/g-C₃N₄) is prepared to improve the oxygen transfer in Wacker oxidation via chemical reduction method. From the analysis of FT-IR, XRD, SEM, TEM, XPS and ICP, Pd⁰ particles are firmly combined with g-C₃N₄ layers, and sub-surface ones occupy most of the components. It is worth mentioning that graphene oxide (GO), which is completely recyclable without further pollution, can be used as a ‘solid weak acid’ taking the place of H₂SO₄ and CF₃COOH. Under the optimization conditions, as many as 46 kinds of olefins are transferred into corresponding products with satisfactory yields, and o-methyl styrene gets the highest yield of 94%. After five times of recycling experiment, the yield of acetophenone only decreases by about 7.0% in the uniform reaction process. In virtue of former research results and molecular electrostatic potential, a possible mechanism is put forward to explain the catalytic process.     

Tap study of the impact of the oxidation state of Pt/PrCeZrO and Pt/GdCeZrO catalysts on their reactivity in the partial/deep oxidation of methane

The temporal analysis of products (TAP) technique coupled with the oxygen TPD was used to elucidate the effects of the Pt-supported fluorite-like doped ceria–zirconia oxide chemical composition and the type of pretreatment on their oxygen bonding strength, mobility, and reactivity as related to catalytic properties in the partial oxidation of methane into synthesis gas. A rapid evolution of hydrogen under CH₄ pulse observed for oxidized catalysts agrees with the direct route of the methane selective oxidation into syngas. This route is favored by the Pt-support interaction and a moderate bonding strength of surface oxygen species along with a high lattice oxygen mobility.     

Influence of the formulation of catalysts deposited on cordierite monoliths for acetic acid oxidation

Monolithic catalysts are prepared by washcoating cordierite monoliths with different sols (Pt/Al₂O₃, Pt/CeO₂, Pt/ZrO₂, Pt/Al₂O₃CeO₂, Pt/Al₂O₃ZrO₂, and Pt/CeO₂ZrO₂). These sols are prepared by a sol–gel process and characterized by specific surface area (S_BET), inductively coupled plasma, hydrogen chemisorption, high-resolution transmission electron microscopy, field emission scanning electron microscopy, oxygen storage capacity, X-ray diffraction, temperature-programmed reduction, CO₂ chemisorption, and the model reaction of 3,3-dimethylbutene isomerization. The catalytic performances of the monolithic catalysts are then evaluated for the acetic acid oxidation. The nature of catalyst coating has been found to influence the adherence with the cordierite monolith and the presence of cerium in the catalyst appears to increase the adherence of the latter. Pt/CeO₂, Pt/Al₂O₃CeO₂, and Pt/CeO₂ZrO₂ are found to be the most reducible catalysts (oxygen storage capacity and temperature-programmed reduction) and to have the lowest acidities (3,3-dimethylbutene isomerization). CO₂ chemisorption shows that these catalysts possess a good basicity. From the relation established between the catalytic activity and the redox and acid–base properties it has been concluded that the reducibility is the key factor for a good catalytic activity although the basicity has a significant influence on the catalytic performance.     

Interaction between creep,oxidation and microporosity in reaction-bonded silicon nitride

Kinetic studies have been performed on primary and secondary (minimal) creep of reaction-bonded silicon nitride in 4-pt-bending tests up to 1500°C. The creep deformation depends strongly on the extent of internal oxidation. In spite of the very marked dependence of the creep rate on material and pretreatment parameters, the stress exponents (n = 1.7–1.8) and activation energies (360–390 kJ/mole) are hardly influenced, suggesting similar creep mechanism. Creep deformation is provided by relative motion and separation of grain boundaries. Oxidation and deformation lead to remarkable changes of the pore size distribution; the creep processes are accompanied by deformation of the pores. The creep rupture strain is very limited in highly creep resistant materials and vice versa. Methods for the determination of oxidation products and oxide profiles along sample cross section have been developed and the chemical changes which the material can undergo during creep are outlined.     

Effect of support and loading on oxidation of methane over platinum catalysts

Catalytic combustion of methane was carried out using platinum catalysts supported on low-and high-surface area alumina (denoted respectively as LSA and HSA) and platinum supported on silica. Methane conversion was the highest for platinum supported on LSA alumina, smaller for Pt/HSA alumina and the smallest for Pt/silica. However, the 3 wt.% Pt/HSA catalyst was found to show the highest selectivity.     

Study of the interaction of hydrogen with palladium-containing catalysts of methane oxidation

The reaction of palladium-containing catalysts PdCl₂/SiO₂ [ratio of the components 0.3:100 (molar)] with H₂ (300°C) after methane oxidation leads to the formation of RH: methane, ethane, propane, n-butane, and isobutane. The sources of RH are PdC_x — products of incomplete oxidation of CH₄ — as well as organic impurities in silica gel. Additions of NaCl (NaCl:SiO₂ = 7:100) triple the yield of RH. Catalysts with additions of H₂SO₄ (H₂SO₄:SiO₂ = 40.100) are not hydrogenated to RH.L. M. Litvinenko Institute of Physico-Organic Chemistry and Coal Tar Chemistry, National Academy of Sciences of Ukraine, 70 R. Lyuksemburg Street, Donetsk 340114, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 3, pp. 177–181, May–June, 1996. Original article submitted July 20, 1995.     

NiO and ZrO2-based catalysts in the reaction of complete methane oxidation

The reaction of complete methane oxidation over deposited catalysts (NiO/ZrO₂ and NiO/YSZ) and binary oxides NiO_ZrO₂ produced by co-precipitation, by the sol gel method, and using a bio-template (NiO content in the samples, 10.1 mol %) is investigated. It is shown that binary oxides cause methane oxidation at lower temperatures than their deposited analogue: the temperature of methane half-transformation is 470, 500, and 520°C for binary oxides, while T ₅₀ = 570°C for NiO/ZrO₂. Major factors affecting the activity of binary oxides in the methane oxidation reaction are determined: the dispersion of the active phase (NiO) and the availability of the second component with high mobility of the lattice oxygen.     

Phase composition of manganese-alumina catalysts for the reactions of deep oxidation

X-ray powder diffraction, transmission electron microscopy and differential dissolution were used for investigation of the phase transformation peculiarities of biphasic manganese-alumina catalysts during their thermal treatment. An effect of interaction between aluminium and manganese oxides depending on the sample preparation procedure was found.     

Catalytic activity of nanosized Co-Cu oxide systems in the deep oxidation of methane

The effect of the composition of nanosized catalysts, produced by thermal decomposition of Co(II) and Cu(II) formates deposited on ZrO₂, SBA-15, and MCM-41, on their activity in the deep oxidation of methane was investigated. It was shown that the activity of the catalyst depends on the nature of the support and on the Co/Cu ratio. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 41, No. 6, pp. 331–335, November–December, 2005.