Catalytic Activity of WO<Subscript>3</Subscript> and MoO<Subscript>3</Subscript> with Pt and Pd Additives in Oxidation of Hydrogen

We have shown that WO₃ and MoO₃ with Pt or Pd additives exhibit high catalytic activity in the reaction of H₂ oxidation. In the temperature range 313 K to 353 K, we have studied the kinetic behavior of the reaction on 0.1 mass % Pt(Pd)/WO₃ and Pt(Pd)/MoO₃ samples. We have established that the kinetics of H₂ oxidation on these catalysts correspond to an Eley - Rideal mechanism. __________ Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 41, No. 5, pp. 313–316, September–October, 2005.     

Properties of the surface layer and catalytic activity of Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> with Pt or Pd additives in the oxidation of hydrogen

The catalytic activity in the oxidation of hydrogen (in the gaseous state in the presence of excess oxygen) has been studied for samples of Pt(Pd)/Ta₂O_5−x, formed by reduction with hydrogen. The samples obtained had greater activity than the traditional catalysts Pt(Pd)/Al₂O₃. According to X-ray diffraction analysis and electron microscopic studies, Ta₂O_5−x becomes amorphous with the formation of more reduced non-stoichiometric oxygen-deficient tantalum oxides with a surface layer of catalyst. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 3, pp. 180–185, May–June, 2008.     

Effect of Pt and Pd additives on the state of the surface layer and catalytic activity of niobium oxides in the oxidation of hydrogen

Catalysts prepared by the hydrogen reduction of Nb₂O₅ in the presence of Pt or Pd have specific surface much greater than for the starting oxide and their catalytic activity in the oxidation of hydrogen is much greater than the activity of Pt/Al₂O₃ or Pd/Al₂O₃. X-ray phase analysis and X-ray photoelectron spectroscopy were used to establish the existence of Nb₂O_5–x nonstoichiometric oxides in the catalyst, which enhances the catalytic activity of the surface. The kinetic behavior of the oxidation of hydrogen on these catalysts is explained in the framework of the Eley–Riedel mechanism.     

Semiconductor Sensors Based on SnO<Subscript>2</Subscript> with Pt Additives and their Catalytic Activity in Oxidation of Methane

We have studied the effect of small additives of Pt on the methane sensitivity of semiconductor adsorption sensors based on SnO₂ (doped with Sb₂O₅) and on the catalytic activity of sensor materials of the same composition in oxidation of methane. We have shown that as the amount of Pt increases, the catalytic activity increases and the sensitivity of the sensors passes through a maximum. The results obtained are explained taking into account the spillover phenomenon.__________Translated from Teoreticheskaya i Eksperimental’naya Khimiya, Vol. 41, No. 3, pp. 176–179, May–June, 2005.     

Influence of Pt and Pd on the catalytic activity of tungsten and molybdenum oxides in the deep oxidation of methane

It has been shown that the phases H_xMO₃ and MO_3−x (M = Mo, W), obtained by reduction of the oxides WO₃ and MoO₃ with hydrogen with supported Pt(Pd) (0.5 mass %), have higher catalytic activity in the deep oxidation of methane than the catalysts Pt/Al₂O₃ and Pd/Al₂O₃ with the same amount of supported metal. At temperatures above 700 K the activity of these catalysts decreases in consequence of the thermal decomposition of the phases H_xMO₃ and MO_3−x and they become similar in activity with Pt(Pd)/Al₂O₃. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 2, pp. 126–129, March–April, 2008.     

Effect of Pt,Pd, and Cs<Superscript>+</Superscript> Additives on the Surface State and Catalytic Activity of WO<Subscript>3</Subscript> in Oxidation of Hydrogen

We have shown that additions of Pt(Pd) and Cs⁺ to WO₃ significantly increase its specific surface area and catalytic activity in H₂ oxidation. After reduction, the promoted specimens contain the phases WO₃, WO_2.9, H_xWO₃; and in the case of Cs⁺ additions, Cs_xWO₃. According to X-ray photoelectron spectroscopy (XPS), the Pt and Pd have an oxidation state close to 0, while tungsten has a +5 oxidation state. The W:O ratio indicates the content of oxygen vacancies in the surface layer. The data are explained taking into account hydrogen spillover from Pt(Pd) to the support.__________Translated from Teoreticheskaya i Eksperimental’naya Khimiya, Vol. 41, No. 2, pp. 126–129, March– April, 2005.     

Oxidation of hydrogen on previously reduced Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> with Pt and Pd additives

X-ray photoelectron spectroscopy (XPS) was used to establish that the surface layer of catalysts obtained by the reduction of Ta₂O₅ with added Pt and Pd by hydrogen contains nonstoichiometric oxides TaO_x, which enhance the activity of the catalyst. A study of the hydrogen oxidation kinetics showed that the kinetic relationships are described satisfactorily by the Eley-Riedel mechanism, according to which the reaction occurs by an interaction of hydrogen from the gas phase with adsorbed oxygen. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 4, pp. 254–258, July–August, 2008.     

Oxidation of CO with WO<Subscript>3</Subscript> and MoO<Subscript>3</Subscript> Promoted with Small Amounts of Platinum,Palladium, and also Cesium Cations

It was shown that addition of Pt(Pd) and Cs⁺ to WO₃ and MoO₃ increases their catalytic activity in the oxidation of CO, at the same time there was a considerable broadening of the low temperature region of the reaction. It was established that formation of an active surface occurs as a result of partial reduction of the molybdenum and tungsten oxides. It is proposed that the reaction occurs at the phase boundary between Pt(Pd) and W and Mo oxides containing oxygen vacancies. __________ Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 41, No. 4, pp. 257–260, July–August, 2005.     

Pd/Co双金属纳米颗粒的制备及催化制氢性能

采用聚乙烯吡咯烷酮(PVP)保护的化学共还原法制备了Pd/Co双金属纳米颗粒, 研究了PVP及还原剂(NaBH₄)的用量、金属盐浓度、金属比例等对Pd/Co双金属纳米颗粒催化NaBH₄制氢性能的影响. 透射电子显微镜(TEM)的结果表明, 所制备的Pd/Co双金属纳米颗粒的平均粒径在1.5-2.8 nm之间. Pd/Co双金属纳米颗粒(BNPs)的催化活性远高于Pd与Co单金属纳米颗粒的活性; 当Pd/Co的理论原子比为1/9时, 双金属纳米颗粒的催化活性最高可达15570 mol·mol^-1·h^-1 (文中纳米颗粒的催化活性均为每摩尔Pd的活性). 密度泛函理论(DFT)的计算结果表明, Pd原子与Co原子之间发生电荷转移, 使得Pd原子带负电而Co原子带正电, 荷电的Pd和Co原子进而成为催化反应的活性中心. 所制备的Pd/Co双金属纳米颗粒具有很好的催化耐久性, 即使重复使用5次后, 该催化剂仍具有较高的催化活性, 且使用后的纳米颗粒催化剂也没有出现团聚现象. 双金属纳米颗粒催化NaBH₄水解反应的活化能约为54 kJ·mol^-1.     

Sensitivity to hydrogen of sensor materials based on SnO2 promoted with 3d metals

The sensitivity to hydrogen and the catalytic activity in the oxidation of hydrogen of sensor materials based on tin dioxide and doped with cobalt, nickel, iron, and copper have been studied. The sensitivity of the sensors and the degree of conversion of hydrogen pass symbatically through a maximum with increasing quantity of each of the dopants. The results are explained by the influence of grain boundaries between tin dioxide and the dopants applied during the course of the oxidation reaction of hydrogen on the sensor material and on the sensitivity of the sensor to H₂. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 2, pp. 121–125, March–April, 2008.     

Catalysis of the oxidation of hydrogen by a platinum-acetylacetone complex anchored on silica

By binding platinum ions with acetylacetone immobilized on a silica surface, we have obtained a metal complex catalyst containing Pt(II). We have also studied its catalytic activity in the model reaction of oxidation of hydrogen. We have shown that over the course of the catalytic process, this catalyst exhibits high catalytic activity at room temperatures and displays the “surface memory” effect with respect to a previous reaction. The activity of the grafted Pt complex is significantly higher than the activity of a supported platinum catalyst. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 34, No. 3, pp. 170–175, May–June, 1998.     

Hydrogen Storage Mediated by Pd and Pt Nanoparticles

The hydrogen storage properties of metal nanoparticles change with particle size. For example, in a palladium–hydrogen system, the hydrogen solubility and equilibrium pressure for the formation of palladium hydride decrease with a decrease in the particle size, whereas hydrogen solubility in nanoparticles of platinum, in which hydrogen cannot be stored in the bulk state, increases. Systematic studies of hydrogen storage in Pd and Pt nanoparticles have clarified the origins of these nanosize effects. We found a novel hydrogen absorption site in the hetero‐interface that forms between the Pd core and Pt shell of the Pd/Pt core/shell‐type bimetallic nanoparticles. It is proposed that the potential formed in the hetero‐interface stabilizes hydrogen atoms rather than interstitials in the Pd core and Pt shells. These results suggest that metal nanoparticles a few nanometers in size can act as a new type of hydrogen storage medium. Based on knowledge of the nanosize effects, we discuss how hydrogen storage media can be designed for improvement of the conditions of hydrogen storage.     

Catalytic activity of the VIII Group metals in the hydrogenation and isomerization of α- and β-pinenes

The kinetic regularities of the liquid-phase hydrogenation and isomerization of α- and β-pinenes over the Pd/C, Ru/C, Rh/C, Pt/C, and Ir/C catalysts were studied at temperatures ranging from 20 to 100 °C and at hydrogen pressures of 1–11 bar using n-octane as the solvent. The hydrogenation and isomerization of α- and β-pinenes occur simultaneously on the Ru/C, Rh/C, Pt/C, and Ir/C catalysts, and the reaction mixture contains the products of double bond hydrogenation, viz., cis- and trans-pinanes. The Ru, Rh, and Pd metals have a higher catalytic activity in β-pinene isomerization than Ir and Pt. Among the VIII Group metals studied, the Pd-based catalyst has the highest catalytic activity in double bond isomerization of α- and β-pinenes. The general scheme of the mechanism of hydrogenation and isomerization of α- and β-pinenes on the Pd/C catalyst was proposed.     

The role of doped Fe on the activity of alumina-supported Pt and Pd diesel exhaust catalysts

Supported Pt and Pd are most commonly used for oxidation catalysts. They have similar and different characteristics for deactivation factors. The catalytic activity of Pt and Pd catalysts supported on ??-Al₂O₃ was studied in the presence and absence of H₂O and SO₂ during CO oxidation under simulated conditions of diesel exhaust gas. Without the addition of H₂O and SO₂ to the feed gas, Pd/Al₂O₃ had a superior catalytic activity compared to Pt/Al₂O₃. The addition of H₂O to the feed gas strongly and negligibly affected the activity of Pd and Pt, respectively, while the addition of SO₂ to the feed gas had a strong poisoning effect on the catalytic activity of both Pt and Pd catalysts. Although being the most active, Pd catalysts exhibited a strong sensitivity to water and sulfur-containing compounds. Fe was added to the Pt and Pd catalysts to introduce sulfur resistance. The addition of Fe enhanced the activity of the catalysts by suppressing the phase transition of Al₂O₃ to Al₂(SO₄)₃ and by hindering metal sintering.     

Palladium versus Platinum: The Metal in the Catalytic Center of a Molecular Photocatalyst Determines the Mechanism of the Hydrogen Production with Visible Light

To develop highly efficient molecular photocatalysts for visible light‐driven hydrogen production, a thorough understanding of the photophysical and chemical processes in the photocatalyst is of vital importance. In this context, in situ X‐ray absorption spectroscopic (XAS) investigations show that the nature of the catalytically active metal center in a (N^N)MCl₂ (M=Pd or Pt) coordination sphere has a significant impact on the mechanism of the hydrogen formation. Pd as the catalytic center showed a substantially altered chemical environment and a formation of metal colloids during catalysis, whereas no changes of the coordination sphere were observed for Pt as catalytic center. The high stability of the Pt center was confirmed by chloride addition and mercury poisoning experiments. Thus, for Pt a fundamentally different catalytic mechanism without the involvement of colloids is confirmed.     

Effect of SnO2 particle size on the hydrogen sensitivity of adsorption–semiconductor sensors with CoxOy/SnO2 active coating

The effect of cobalt additions on the electrical resistance of adsorption–semiconductor sensors based on nanosized SnO₂ in air, their sensitivity to hydrogen, and the catalytic activity of the corresponding sensor materials in the oxidation of H₂ were studied. The extremal nature of the obtained relationships is explained by morphological features of the investigated systems based on nanosized SnO₂.     

Reduction of NO by Propene Over Pt, Pd and Rh-Based ZSM-5 Under Lean-Burn Conditions

Selective catalytic reduction of NO by propene has been investigated over noble metal (Pt, Pd, Rh)-based ZSM-5 catalysts. These samples were tested in a gas mixture system in the presence of excess oxygen, simulating lean-burn exhaust gases. The sequence in activity for NO reduction was Pt > Rh Pd. Regarding the selectivity of the reaction to N₂, an opposite trend was observed: Rh > Pd Pt. The catalytic systems have presented stable operation under isothermal conditions during time-on-stream experiments.     

Platinum nanoparticle size effect on specific catalytic activity in <Emphasis Type="Italic">n</Emphasis>-alkane deep oxidation: Dependence on the chain length of the paraffin

The specific activity of 0.8% Pt/Al₂O₃ catalysts in the deep oxidation of C₁–C₆ n-alkanes increases with an increase in the Pt particle size from 1 to 3–4 nm. Further coarsening of the particles insignificantly changes the specific activity. The size effect was studied for a series of catalysts containing platinum nanoparticles 1 to 11 nm in diameter. The specific catalytic activity variation range depends on the size of the reacting hydrocarbon molecules. As the platinum particle size increases, the specific catalytic activity increases 3–4 times for the oxidation of CH₄ and C₂H₆ and by a factor of 20–30 for the oxidation of n-C₄H₁₀ and n-C₆H₁₄.     

Effect of Pt nanoparticle size on the specific catalytic activity of Pt/SiO<Subscript>2</Subscript> and Pt/TiO<Subscript>2</Subscript> in the total oxidation of methane and <Emphasis Type="Italic">n</Emphasis>-butane

The dependence of the specific catalytic activity (A _sp ) of the catalysts Pt/SiO₂ and Pt/TiO₂ in the total oxidation of CH₄ and n-C₄H₁₀ on the Pt nanoparticle size (in the range from 1 to 4 nm) was studied. The specific catalytic activity increases with an increase in the platinum nanoparticle size, indicating that the total oxidation is a structure-sensitive reaction. The structure sensitivity depends on the size of an oxidized molecule: it increases sharply on going from CH₄ to n-C₄H₁₀. The support also exerts a considerable effect on the A _sp value: in the oxidation of both CH₄ and C₄H₁₀ the specific catalytic activity for the catalysts Pt/TiO₂ is 3–4 times that for Pt/SiO₂.     

Modelling of the impedimetric responses of an aflatoxin B1 immunosensor prepared on an electrosynthetic polyaniline platform

Aflatoxins are a group of mycotoxins that have deleterious effects on humans and are produced during fungal infection of plants or plant products. An electrochemical immunosensor for the determination of aflatoxin B₁ (AFB₁) was developed with AFB₁antibody (AFB₁-Ab) immobilized on Pt electrodes modified with polyaniline (PANi) and polystyrene sulphonic acid (PSSA). Impedimetric analysis shows that the electron transfer resistances of the Pt/PANi–PSSA electrode, the Pt/PANi–PSSA/AFB₁-Ab immunosensor and Pt/PANi–PSSA/AFB₁-Ab incubated in bovine serum albumin (BSA) were 0.458, 720 and 1,066 kΩ, respectively. These results indicate that electrochemical impedance spectroscopy (EIS) is a suitable method for monitoring the change in electron transfer resistance associated with the immobilization of the antibody. Modelling of EIS data gave equivalent circuits which showed that the electron transfer resistance increased from 0.458 kΩ for the Pt/PANi–PSSA electrode to 1,066 kΩ for the Pt/PANi–PSSA/AFB₁-Ab immunosensor, indicating that immobilization of the antibody and incubation in BSA introduced an electron transfer barrier. The AFB₁ immunosensor had a detection limit of 0.1 mg/L and a sensitivity of 869.6 kΩ L/mg.