Low-temperature destruction of carbon tetrachloride over lanthanide oxide-based catalysts: from destructive adsorption to a catalytic reaction cycle

The catalytic destruction of carbon tetrachloride in the presence of steam, CCl(4) + 2 H(2)O-->4 HCl + CO(2), was investigated at 200-350 degrees C over a series of lanthanide (La, Ce, Pr and Nd) and alkaline-earth metal (Mg, Ca, Sr and Ba) oxide-based catalysts with kinetic experiments, Raman spectroscopy, X-ray photoelectron spectroscopy, IR spectroscopy, X-ray diffraction, and DFT calculations. This new catalytic reaction was achieved by combining destructive adsorption of CCl(4) on a basic oxide surface and concurrent dechlorination of the resulting partially chlorinated solid by steam. The combination of the two noncatalytic reactions into a catalytic cycle provided a rare opportunity in heterogeneous catalysis for studying the nature and extent of surface participation in the overall reaction chemistry. The reaction is proposed to proceed over a terminal lattice oxygen site with stepwise donation of chlorine atoms from the hydrocarbon to the surface and formation of the gas-phase intermediate COCl(2), which is readily readsorbed at the catalyst surface to form CO(2). In a second step, the active catalyst surface is regenerated by steam with formation of gas-phase HCl. Depending on the reaction conditions, the catalytic material was found to transform dynamically from the metal oxide state to the metal oxide chloride or metal chloride state due to the bulk diffusion of oxygen and chlorine atoms. A catalyst obtained from a 10 wt % La(2)O(3)/Al(2)O(3) precursor exhibited the highest destruction rate: 0.289 g CCl(4) h(-1) g(-1) catalyst at 350 degrees C, which is higher than that of any other reported catalyst system.     

Role of the Support in Catalysis: Activation of a Mononuclear Ruthenium Complex for Ethene Dimerization by Chemisorption on Dealuminated Zeolite Y

A set of supported ruthenium complexes with systematically varied ratios of chemisorbed to physisorbed species was formed by contacting cis‐[Ru(acac)₂(C₂H₄)₂] ( I ; acac=C₅H₇O₂^?) with dealuminated zeolite Y. Extended X‐ray absorption fine structure (EXAFS) spectra used to characterize the samples confirmed the systematic variation in the loadings of the two supported species and demonstrated that removal of bidentate acac ligands from I accompanied chemisorption to form [Ru(acac)(C₂H₄)₂]⁺ attached through two Ru? O bonds to the Al sites of the zeolite. A high degree of uniformity in the chemisorbed species was demonstrated by sharp bands in the infrared (IR) spectrum characteristic of ruthenium dicarbonyls that formed when CO reacted with the anchored complex. When the ruthenium loading exceeded 1.0 wt % (Ru/Al≈1:6), the additional adsorbed species were simply physisorbed. Ethene ligands on the chemisorbed species reacted to form butenes when the temperature was raised to approximately 393 K; acac ligands remained bonded to Ru. In contrast, ethene ligands on the physisorbed complex simply desorbed under the same conditions. The chemisorption activated the ruthenium complex and facilitated dimerization of the ethene, which occurred catalytically. IR and EXAFS spectra of the supported samples indicate that 1) Ru centers in the chemisorbed species are more electron deficient than those in the physisorbed species and 2) Ru–ethene bonds in the chemisorbed species are less symmetric than those in the physisorbed species, which implies the presence of a preferred configuration for the catalytic dimerization.     

Understanding the Solvent Effect on the Catalytic Oxidation of 1,4‐Butanediol in Methanol over Au/TiO2 Catalyst: NMR Diffusion and Relaxation Studies

The effect of water on the catalytic oxidation of 1,4‐butanediol in methanol over Au/TiO₂ has been investigated by catalytic reaction studies and NMR diffusion and relaxation studies. The addition of water to the dry catalytic system led to a decrease of both conversion and selectivity towards dimethyl succinate. Pulsed‐field gradient (PFG)‐NMR spectroscopy was used to assess the effect of water addition on the effective self‐diffusivity of the reactant within the catalyst. NMR relaxation studies were also carried out to probe the strength of surface interaction of the reactant in the absence and presence of water. PFG‐NMR studies revealed that the addition of water to the initial system, although increasing the dilution of the system, leads to a significant decrease of effective diffusion rate of the reactant within the catalyst. From T₁ and T₂ relaxation measurements it was possible to infer the strength of surface interaction of the reactant with the catalyst surface. The addition of water was found to inhibit the adsorption of the reactant over the catalyst surface, with the T₁/T₂ ratio of 1,4‐butanediol decreasing significantly when water was added. The results overall suggest that both the decrease of diffusion rate and adsorption strength of the reactant within the catalyst, due to water addition, limits the access of reactant molecules to the catalytic sites, which results in a decrease of reaction rate and conversion.     

Anchoring Fe Ions to Amorphous and Crystalline Oxides: A Means To Tune the Degree of Fe Coordination

We report on an IR spectroscopic study on the room‐temperature adsorption of NO on different iron(II )‐containing siliceous matrices. Fe²⁺ hosted inside the channels of MFI‐type zeolites (Fe‐ZSM‐5 and Al‐free Fe‐silicalite) exhibits pronounced coordinative unsaturation, as witnessed by the capability to form, at 300 K, [Fe²⁺(NO)], [Fe²⁺(NO)₂] and [Fe²⁺(NO)₃] complexes with increasing NO equilibrium pressure. Fe²⁺ hosted on amorphous supports (high surface area SiO₂ and MCM‐41) sinks more deeply into the surface of the siliceous support and thus exhibits less pronounced coordinative unsaturation: only [Fe²⁺(NO)₂] complexes were observed, even at the highest investigated NO equilibrium pressures. Activation at higher temperature (1073 K) of the Al‐free Fe‐silicalite sample resulted in the appearance of Fe²⁺ species similar to those observed on SiO₂ and MCM‐41, and this suggests that local (since not detectable by X‐ray diffraction) amorphisation of the environment around Fe²⁺ anchoring sites occurs. The fact that this behaviour is not observed on the Fe‐ZSM‐5 sample activated at the same temperature suggests that framework Al species (and their negatively charged oxygen environment) have an important role in anchoring extraframework Fe²⁺ species. Such an anchoring phenomenon will prevent a random migration of iron species, with subsequent aggregation and loss of coordinative unsaturation. These observations can thus explain the higher catalytic activity of the Fe‐ZSM‐5 system in one‐step benzene to phenol conversion when compared with the parent, Al‐free, Fe‐silicalite system with similar Fe content. The nature of the support and the activation temperature can therefore be used as effective means to tune the degree of Fe coordination.     

The state of metals in the Pt/Al2O3 and (Pt-Cu)/Al2O3 catalysts as indicated by IR spectroscopy with isotope dilution of 12C16O with 13C18O molecules

Adsorption of ¹³C¹⁸O+¹²C¹⁶O mixtures on the Pt(2.9%)/γ-Al₂O₃, (Pt(2.6%)+Cu(2.7%))/γ-Al₂O₃, and (Pt(2.6%)+Cu(5.1%))/γ-Al₂O₃ catalysts was studied by FTIR spectroscopy. On the metallic Pt surface at coverages close to saturation, CO is adsorbed both strongly and weakly to form linear species for which the vibrational frequencies of the isolated ¹³C¹⁸O molecules adsorbed on Pt are ∼1940 and ∼1970 cm⁻¹, respectively. The redistribution of intensities of the high-and low-frequency absorption bands in the spectra of adsorbed ¹³C¹⁸O indicates that these linear forms are present on the adjacent metal sites. The weak adsorption is responsible for the fast isotope exchange between the gaseous CO and CO molecules adsorbed on metal. The Pt-Cu alloys, in which the electronic state of the surface Pt atoms characteristic of monometallic Pt remains unchanged, are formed on the surface of the reduced Pt-Cu bimetallic catalysts. The decrease in the vibrational frequencies of the isolated C=O bonds in the isolated Pt-CO complexes suggests that the CO molecules adsorbed on the Cu atoms affect the electronic properties of Pt. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 831–836, May, 2007.     

Monitoring Surface Processes During Heterogeneous Asymmetric Hydrogenation of Ketones on a Chirally Modified Platinum Catalyst by Operando Spectroscopy

Surface processes occurring at the catalytic chiral surface of a cinchona‐modified Pt catalyst during the asymmetric hydrogenation of activated ketones have been monitored for the first time using operando ATR‐IR spectroscopy. Fundamental information about this catalytic system could be gained, including the chiral modification process of the catalyst, the surface interaction of reactant ketone with preadsorbed chiral modifier, the role of hydrogen as well as the influence of the product enantiomers in the catalytic cycle. The formation of a diastereomeric transient surface complex between ketone and chiral modifier was found to be related to the ketone consumption. Among the studied activated ketones, a correlation between stereoselection and the strength of the intermolecular hydrogen bond was identified. Dissociated hydrogen from the catalytic surface is found to play a crucial role in the formation of the diastereomeric surface complex.     

Selective modification of the acid-base properties of ceria by supported Au

Au supported on CeO(2) prepared by deposition-precipitation with urea leads to a basic catalyst. Au acts in two ways as surface modifier. First, Au selectively interacts with Ce(4+) cations by either blocking access to or reducing Ce(4+) to Ce(3+). Second, the resulting Au atoms (presumably as Au(+) ions) act as soft, weak Lewis acid sites stabilizing carbanion intermediates and enhancing hydride abstraction in the dehydrogenation of alcohols. In consequence, the thus-synthesized basic catalyst catalyzes the dehydrogenation of propan-2-ol to acetone with high efficiency and without notable deactivation. Additionally, the dehydration pathway of propan-2-ol is eliminated, as Au also quantitatively blocks access to strongly acidic Ce(4+) ions or reduces them to Ce(3+).     

Solvent‐Driven Reductive Activation of CO2 by Bismuth: Switching from Metalloformate Complexes to Oxalate Products

In this work, we investigated how the reductive activation of CO₂ with an atomic bismuth model catalyst changes under aprotic solvation. IR photodissociation spectroscopy of mass‐selected [Bi(CO₂)_n]^? cluster ions was used to follow the structural evolution of the core ion with increasing cluster size. We interpreted the IR spectra by comparison with density‐functional‐theory calculations. The results show that CO₂ binds to a bismuth atom in the presence of an excess electron to form a metalloformate ion, BiCOO^?. Solvation with additional CO₂ molecules leads to the stabilization of a bismuth(I) oxalate complex and results in a core ion switch.     

Chirally modified platinum generated by adsorption of cinchonidine ether derivatives: towards uncovering the chiral sites

The adsorption behavior of O-methyl and O-trimethylsilyl derivatives of cinchonidine (CD), employed as chiral modifiers for heterogeneous enantioselective hydrogenations on supported Pt catalysts, has been investigated by using attenuated total reflection infrared spectroscopy (ATR-IR) and density functional theory (DFT) electronic structure calculations. The ATR-IR spectroscopic investigation provided detailed insight of the adsorbed modifiers under conditions close to those employed during catalytic processes, and electronic structure calculations were used as a complement to the experiments to uncover the implications of conformational changes in generating the topology of the surface chiral site. The structural investigation of the adsorbed modifiers revealed a relationship between the spatial positions of the ether substituents and the enantiodifferentiation induced by the modified catalyst observed in the hydrogenation of alpha-activated ketones. Experiments and calculations corroborate a model, according to which the addition of a bulky ether group to CD reshapes the chiral sites, thus generating catalytic chiral surfaces with different and, in some cases (e.g. hydrogenation of ketopantolactone), even opposite enantioselective properties to those obtained with CD without altering the absolute configuration of the modifier. The study also confirms that active surface conformations of cinchona modifiers are markedly different from those existing in vacuum and in solution, thus underlying the necessity of investigating the surface-modifier interaction in order to understand enantioselectivity.     

Chemical routes for production of transition-metal phosphides on the nanoscale: implications for advanced magnetic and catalytic materials

Nanoparticulate transition-metal phosphides remain an unexplored, though emerging area of interest on the materials landscape, due principally to their promising magnetic and catalytic properties. This review describes synthetic strategies for the formation of both supported and unsupported transition-metal phosphide nanoparticles, provides a summary of their relevant magnetic and catalytic properties, and indicates new directions for exploration.