In this work, we discuss the problem of the afterburning of methane from the exhaust gases of automobile engines fueled by natural gas. In exhaust neutralizers, the PdO/Al2O3 catalyst, the main drawback of which is the reduction of its activity under the action of steam that always present in exhaust gases, is commonly used. To improve the tolerance to steam, a series of PdO-Me x O y /Al2O3 binary catalysts (Me is Co, Cu, Fe, Ni, Mn, or Sn) was prepared and studied. Comparative tests under conditions modeling the methane afterburning process in automobile neutralizers show that Pd catalysts promoted with nickel, cobalt, and tin oxides are more resistant to the inhibiting action of steam. The high crystallinity of supported PdO and its uniform distribution over the surface of modified Al2O3 are indicated as criteria for the stability of catalysts in the presence of steam. Optimization of the concentration of promotors and the preparation method used for their introduction allows the deactivation of Pd catalysts under the action of steam to be almost completely eliminated.

A new technical approach to investigation of heterogeneous catalysts with deposited metallic nanoparticles of the active component is suggested. Small-angle X-ray scattering (SAXS) patterns for powder samples of the support matrices with deposited metal particles (Pt, Pd and others) taken at varying density contrast are shown to provide essential information on the structure and dispersity of the active components and their interaction with the support surface, which allows an integral assessment of the formation of strong chemical binding of deposited metal to the support matrix. The measured structural characteristics are compared with data obtained by other physicochemical methods, in particular, transmission electron microscopy (TEM). The suggested technique is verified using mathematical models and applied to a real heterogeneous catalyst Pt/γ-Al2O3. The proposed method of SAXS data analysis makes it possible to reveal the formation of active sites in heterogeneous catalysts and optimize their preparation procedures.

The possibility of controlling the state of platinum deposited on the support surface via minor changes in the catalyst preparation procedure is demonstrated using a series of highly dispersed Pt/γ-Al2O3 catalysts with different particle size of the active component. Dispersity, local structure and electronic state of supported platinum were examined by a combination of high resolution transmission electron microscopy and X-ray absorption spectroscopy (EXAFS/XANES). It was shown that various platinum species can be obtained on the surface of the support: bulk or surface Pt(II) or Pt(IV) oxides, mixed metal-oxide structures, bulk particles of metallic platinum, and two-dimensional surface Pt0 particles strongly interacting with the support.

Design of a cell for in situ characterization of heterogeneous catalysts by X-ray absorption spectroscopy EXAFS/XANES and X-ray diffraction on synchrotron radiation at high temperatures in a controllable gas medium, which was implemented at the Structural Materials Science end-station installed at KCSRNT, is presented. First results on nanostructural evolution of Pt/γ-Al2O3, Pd/γ-Al2O3 and other catalysts during various treatments — oxidation in oxygen, reduction in H2/N2, and annealing in vacuum — are reported.

The dependence of the specific catalytic activity ( A sp ) of the catalysts Pt/SiO 2 and Pt/TiO 2 in the total oxidation of CH 4 and n -C 4 H 10 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 CH 4 to n -C 4 H 10 . The support also exerts a considerable effect on the A sp value: in the oxidation of both CH 4 and C 4 H 10 the specific catalytic activity for the catalysts Pt/TiO 2 is 3–4 times that for Pt/SiO 2 .

Pt(IV)-nitrate solutions are widely used as precursors for mobile and stationary emission control catalysts for being chlorine-free but are unfit for the preparation of highly dispersed catalysts due to hydrolysis taking place after dilution and/or interaction with a basic carrier surface. The alkalization of the commercial Pt(IV)-nitrate solutions was found not to lead to light-yellow mononuclear [Pt(OH)6]2− solution because the bridged OH ligands stabilizing oligomeric Pt nitrate complexes are not cleaved even after refluxing with excess of alkali. The alkalization results in dilution-resistant polynuclear precursor solutions fit for highly dispersed Pt/Al2O3 catalyst preparation.

A study of monodisperse Pt/γ-Al2O3 catalysts was carried by XAFS. Key factors of the particle size control were the composition of the precursor solutions and the pretreatment of the carrier. XAFS revealed that variation of preparation methods caused formation of three types of Pt particles (metal, oxide, metal-oxide) located on the γ-Al2O3 surface directly affecting the catalytic activity in CH4 complete oxidation. A method of reliable estimation of the phase composition of the active component is developed taking into account the nano-size effects and Pt oxidation state.

Catalytic activity of the size-controlled platinum nanoparticles supported on the acid-pretreated gamma-alumina has been tested in complete methane oxidation under lean conditions. The mean sizes of platinum particles varied from 1.3 to 10 nm with the narrow size distribution (TEM data). It has been found that the reaction under study is strongly size sensitive. The size dependence of the specific catalytic activity is narrow and bell-shaped, with the maximum TOF value observed for the catalysts containing partially oxidized platinum with the mean particle sizes of about 2 nm. The observed strong size sensitivity is shown to originate from the size dependence of the apparent activation energy of the methane oxidation and/or the platinum oxidation state in the catalytically active nanoparticles. (C) 2009 Elsevier Inc. All rights reserved.

Recently great efforts are being devoted to develop new methods of preparation of high-disperse Pd–CeO2 containing nanosystems stabilized on an oxide matrix. A new approach of synthesis consists in using the heterometallic PdII2CeIV2(μ-OOCMe)12(H2O)2 complex as a precursor to anchor Pd nanoparticles on the surface of γ-alumina in direct contact with CeO2. The present work is devoted to a structural study of this disperse Pd–CeO2 containing nanosystem after oxidative or reductive pretreatments in comparison with monometallic alumina-supported samples by XAFS and TEM. A strong interaction between Pd and ceria in the catalyst produced in the studied system affects reducibility of both PdO and CeO2, which in turn results in an increased low-temperature activity in CO oxidation along with a dramatic change of the ignition–extinction curve.

The liquid-phase oxidation of α-pinene with oxygen at 70–90°C is studied in the presence of Pd, Pt, Ru, Rh, and Ir supported on carbon. The conversion of α-pinene and the selectivity of formation of the main reaction products, namely, verbenol ( 1 ), verbenone ( 2 ), and α-pinene oxide ( 3 ), depends on the nature of the metal, on its oxidation state and extent of dispersion, and on the admixtures introduced into the system. In the presence of the Pt catalysts and promoting admixtures of tetrahexylammonium chloride (Hex 4 NCl), the selectivity of formation of the most valuable oxidation products ( 1 + 2 ) reaches 50% at an α-pinene conversion of 20–30%. The fraction of resinlike oxidation products decreases in the presence of the catalysts. The results obtained are discussed in the framework of the radical mechanism of α-pinene oxidation.

The mechanism of the propylene oxidation by Pd(NOn)Cl-2 - m(CH3CN)(2) complexes (n = 2,3; m = 0, 1, 2) in chloroform solutions has been studied by H-1 NMR and IR spectroscopy. The main reaction products are acetone and 2-nitropropylene, with their ratio depending on the equilibrium existing in the reaction solutions between palladium complexes containing NOn ligands bonded to a palladium atom via either an oxygen or a nitrogen atom. Reactivities of the oxygen bonded nitrato and nitrito complexes are significantly higher than that of the nitrogen bonded nitro complex. Various new organopalladium intermediates have been observed and monitored in situ. A reversible insertion of the coordinated propylene into the Pd 0 or Pd-N bonds results in nitrato-, nitrito-and nitropalladation intermediates, which then decompose via a beta-hydrogen elimination. Two isomers of the nitritopalladation intermediate have been detected, i.e., a palladium metallacycle and an open ring complex, with the latter being much more reactive towards the beta-hydrogen elimination than the former. The decomposition of the nitrato- and nitritopalladation intermediates results in the organometallic precursor of acetone, i.e., an acetonylpalladium complex, and then in acetone itself. On the other hand, the nitropalladation intermediate originates 2-nitropropylene. In the presence of dioxygen, which re-oxidizes the nitrosyl groups, the acetone formation becomes a catalytic reaction with respect to both palladium and nitrogen. (C) 2004 Elsevier B.V. All rights reserved.

The review surveys the results of our studies devoted to the design of highly efficient catalysts of hydrolysis of the phosphodiester bonds in RNA. These catalysts contain the imidazole residue in the catalytic domain, one or several bis-quaternized rings of 1,4-diazabicyclo[2.2.2]octane as a polycationic RNA-binding domain, and a lipophilic radical. A versatile approach to artificial ribonucleases of this type was proposed, which allows one to vary not only the number of positive charges in the RNA-binding domain, the structure of the catalytic site, and their mutual arrangement but also the domain structure of the molecule as a whole. Analysis of the catalytic properties of the synthesized constructs makes it possible to optimize the domain structure and the geometry of the molecule ensuring its maximum ribonuclease activity.

Artificial ribonucleases of the ABLkCm series were synthesized. They consist of a lipophilic alkyl radical (Et, n -C 14 H 29 , or n -C 15 H 31 ) А , an “RNA-binding domain” В (bisquaternary salt of 1,4-diazabicyclo[2.2.2]octane), a “catalytic domain” Сm [histamine ( С1 ) or histidine ( С3 ) residue], and a “linker” Lk that joins the “domains” B and Cm [here, k is the number of methylene units (one or three) in the linker]. The effect of the “domain structure” on the catalytic properties of the chemical ribonucleases was analyzed using seven compounds of this series ( ABL1C1 , ABL3C1 , ABL3C3 , AC1 , AB , BL2 , and BL3C3 ). The catalytic activity of the compounds was assessed in the reaction of hydrolysis of the in vitro transcripts of human tRNA Lys and yeast tRNA Asp under physiological conditions. It was shown that only chemical ribonucleases that involve all the fragments of the ABLkCm construct can hydrolyze the substrate tRNA at a high rate (90% of tRNA is hydrolyzed for 10 h at 37°С). The activity of the compounds is largely determined by the presence of a long lipophilic radical linked to 1,4-diazabicyclo[2.2.2]octane and a long linker, which joins the RNA-hydrolyzing and RNA-binding domains. The results indicate an important role of hydrophobic interactions in the acceleration of the RNA hydrolysis reaction.

On the basis of imidazole and bisquaternary salts of 1,4-diazabicyclo[2.2.2]octane, a number of highly effective catalysts of the nDm series (here, n is the number of positive charges at neutral pH values and m is the digital code of the catalytically active fragment: 1, histamine, and 2, histidine methyl ester) were synthesized for the cleavage of the phosphodiester bonds in ribonucleic acids. A general method for the synthesis of chemical ribonucleases was suggested, which helps vary both the number of positive charges in their RNA-binding domain and the catalytic center. By the example of hydrolysis under physiological conditions of the in vitro transcript of tRNA(Lys) from human mitochondria, it was shown that the RNA cleavage rate with the nDm conjugates increases approximately 30-fold along with the increase in the number of positive charges from two to four.

Small mimics of the ribonuclease active centre were synthesized by conjugating imidazole residues to a dicationic compound. These compounds were shown to cleave tRNA under physiological conditions. The compounds provide new probes for the investigation of RNA structure in solution and potential catalytic RNA cleaving groups for antisense oligonucleotide derivatives.

The reaction of methylenecyclobutane (1) with bis(acetonitrile)chloronitropalladium(II) in methylene chloride yields cyclopentanone. Two intermediates - π-olefin complex of palladium and product of β-nitritopalladation of 1 in a Markovnikov fashion - are observed by 1H and 13C NMR. Palladium-catalyzed ring expansion of 1 is suggested to involve heterolysis of the palladium-carbon bond and rearrangement of the resulting cyclobutyl to cyclopentyl cations. The selective formation of ring-expanded and ring-contracted carbonyl products from methylenecyclobutane and 1-methyleyclobutene, respectively, is discussed in terms of proposed mechanism.

Oxidation of 1-methylcyclobutene ( 1 ) by PdCl 2 L 2 and PdCl(NO 2 )L 2 (LCD 3 CN) in methylene chloride was studied with 1 H and 13 C NMR. Reaction of 1 with PdCl 2 L 2 leads to the formation of the stable ring-opened 1-3-η 3 -π-allyl palladium complex ( 2 ). With the addition of an equimolar quantity of PdCl(NO 2 )L 2 to the solution of complex 2 , the allylic oxidation of 2 proceeds slowly via the intermediate formation of another 1-3-η 3 -π-allyl palladium complex ( 3 ). When reacting directly with PdCl(NO 2 )L 2 , 1 undergoes palladium (II)-catalyzed ring contraction to give cyclopropyl methyl ketone. Kinetic and spectral data indicate the formation of an organometallic intermediate, a product of the β-nitritopalladation of the parent olefin 1 .

The stoichiometric oxidation of ethylene by Pd(NOn)mCl2-mL2 complexes (where n=2, 3; m = 0, 1, 2; LCD3CN) in chloroform—acetic acid solution was studied with 1H NMR. This reaction becomes catalytic in the presence of oxygen. The product distribution depends on the nature of the ligands and on the solvent composition. The reaction leads mostly to acetaldehyde, nitroethylene, ethylene glycol monoacetate, yand 1,1-disubstituted ethanes. Kinetics and spectral data indicate the formation of a number of intermediates. The mechanism of ethylene oxidation to 1,1- and 1,2-addition products is discussed.

A detailed mechanism of ethylene oxidation by Pd(NOn)CIL2 complexes (n=2,3; L=CD3CN) in a chloroform-acetic acid nmixture is studied by 1H N M R spectroscopy. The end reaction products are ethyleneglycol monoacetate (EGMA), acetaldehyde, nitroethylene and compounds with the general formula CH3-CHXY (X, Y = OH, OAc, Cl, NO2), whose ratio depends upon the solvent composition. Kinetic and spectral data obtained indicate the formation of a number of intermediates. The structure and routes of decomposition of the intermediates to EGMA and other reaction products are suggested.

Complexes of Pd(NOn)2−mClmL2 (where n = 2, 3; m = 0,1, 2; L= CH3CN, CD3CN) were synthesized and their reactivities towards ethylene and propylene oxidation in a chloroform medium were investigated. The detailed mechanism of ethylene oxidation by dinitro- and mono-nitrato complexes of palladium in chloroform solutions was studied by IR and 1H NMR spectroscopy. The structures and routes of decomposition of the intermediates to reaction end products are proposed.