http://www1.udel.edu/chem/polenova/PDF/Polyoxometalates/POMs_Catalysis_NMR_ChemRev1998.pdf

Catalysis by Heteropoly Acids and Multicomponent Polyoxometalates in Liquid-Phase Reactions

ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

UV-Visible ار راد ن .د TiO2 ( تیفرظ راون مان هب نورتکلا یاراد یژرنا زارت لماش VB و ) رگید زارت ی یژرنا اب ( ییاناسر راون مان هب نورتکلا زا یلاخ و رتلااب VB یم ) .دشاب ت ود نیا نیب یژرنا توافت یژرنا فاکش زار ، پگ دناب هدیمان یم .دوش هک ینامز زا نورتکلا لاقتنا VB هب VB یم ماجنا دریگ ، TiO2 اب ودح یژرنا بذج د ev 2 / 3 ، نورتکلا تفج کی دیلوت یم هرفح .دیامن و نورتکلا هرفح ی نا اب هدش دیلوت یم کرتشم حطس هب لاقت ثعاب دناوت شنکاو ماجنا اه یی ددرگ . TiO2 دربراک ،دراد یدایز یاه هلمج زا یم ناوت اوه یگدولآ هیفصت یارب (CO2) و بآ و ... نآ زا هدافتسا درک .

Advanced Nanoarchitectures for Solar Photocatalytic Applications

Ordered mesoporous materials in catalysis

The open literature concerning chemical and mechanistic aspects of the selective catalytic reduction of NO by ammonia (SCR process) on metal oxide catalysts is reviewed. Catalytic systems based on supported V2O5 (including the industrial TiO2-supported V2O5–WO3 and/or V2O5–MoO3 catalysts) and catalysts containing Fe2O3, CuO, MnOx and CrOx are considered. The results of spectroscopic studies of the adsorbed surface species, adsorption–desorption measurements, flow reactor and kinetic experiments are analyzed. The proposed reaction mechanisms are described and critically discussed. Points of convergence and of disagreement are underlined.

Chemical and mechanistic aspects of the selective catalytic reduction of NOx by ammonia over oxide catalysts: A review

Copper(II) ion-exchanged ZSM-5 zeolites with 100% or more exchange levels were readily prepared by addition of basic compounds such as ammonia and Mg(OH)2 to the mother solution. The catalytic activities of the resulting zeolites for decomposition of nitrogen monoxide are dependent on pH of the solution and the additive.

Novel Preparation Method of Highly Copper Ion-exchanged ZSM-5 Zeolites and Their Catalytic Activities for NO Decomposition

The superoxide ion on the magnesium oxide surface was produced by reacting oxygen with magnesium oxide containing trapped electrons. Four different superoxide species on different sites were identified by NMR spectroscopy and differential thermal analysis. Stoichiometric reactions of the superoxide ion with C/sub 1/-C/sub 4/ alkanes and alkenes at 175/sup 0/C were slower than the corresponding reactions with O(-1) and O/sub 3/(-1) on magnesium oxide. Methane and ethylene reacted only at temperatures above 300/sup 0/C and yielded mainly carbon dioxide. Propylene gave no gaseous products at 175/sup 0/C, but IR spectroscopy revealed that it adsorbed as formate and acetate ions which were converted to carbonates at higher temperatures and desorbed as acetaldehyde and methanol. The 1-butene formed 2-butanol, methanol, acetaldehyde, and acrolein above 300/sup 0/C. The analyses of the oxygen-containing and hydrocarbon products suggested that the initial step of the surface reaction is hydrogen abstraction and that the resulting radicals react with lattice oxygen to form carboxylate ions or with HO/sub 2/(-1) to form alkoxy or epoxide intermediates.

Surface reactions of oxygen ions. 5. Oxidation of alkanes and alkenes by O2- on magnesium oxide

The catalytic activity of alumina for the title reaction has been found to be greatly improved by the loading of copper. The addition of copper resulted in lowering the active temperature region, the higher maximum activity, and the enhancement of the reaction rate. The maximum effect was observed at 0.3 wt% of the loading amount of copper. A similar enhancement was also confirmed on SiO2-Al2O3.

Enhancement of catalytic activity of alumina by copper addition for selective reduction of nitrogen monoxide by ethene in oxidizing atmosphere

Among various metal ion-exchanged zeolites, metal loading alumina, and oxides, iron ion-exchanged mordenite was the most active for the selective reduction of nitrogen monoxide to nitrogen by ethene in the presence of oxygen at the temperature as low as 473 K. The catalytic activities of iron ion-exchanged zeolites depended on the zeolite structure and the iron ion exchange level. The effects of the zeolite structure are in the order of MOR > FER > MFI > Y > LTL at 473 K. The activity of iron ion-exchanged mordenite increased with the increment in the exchange level and levelled off above about 60%.

Iron ion-exchanged zeolite: the most active catalyst at 473 K for selective reduction of nitrogen monoxide by ethene in oxidizing atmosphere

Nanoparticles of iron oxide were encapsulated into the pores of Si-MCM-41 and porous vycor glass (PVG) with pore diameters 2.1–4.5 nm. Transmission electron micrographs (TEM) observation has revealed the formation of iron oxide particles in the pores after an adequate preparation including a suspension treatment in water. Ultraviolet–visible absorption spectra (UV-VIS) of iron oxide loaded on the respective supports were greatly changed with the pore diameters. The bandgaps of iron oxide, evaluated from the absorption edges of the UV-VIS spectra, were significantly widened with decrease in the pore diameters. The dependence of the bandgap on the pore diameter could well be explained by the Brus' equation when the effective mass of an electron and a hole in iron oxide was assumed to be 0.27. It follows that the bandgap was changed by the quantum size effect and could be controlled by the pore size of support.

Masakazu Iwamoto

Papers

Novel Preparation Method of Highly Copper Ion-exchanged ZSM-5 Zeolites and Their Catalytic Activities for NO Decomposition

Surface reactions of oxygen ions. 5. Oxidation of alkanes and alkenes by O2- on magnesium oxide

Enhancement of catalytic activity of alumina by copper addition for selective reduction of nitrogen monoxide by ethene in oxidizing atmosphere

Iron ion-exchanged zeolite: the most active catalyst at 473 K for selective reduction of nitrogen monoxide by ethene in oxidizing atmosphere

Control of bandgap of iron oxide through its encapsulation into SiO2-based mesoporous materials