Mechanism of CO + N2O reaction via transient CO3(2-) species over crystalline Fe-substituted lanthanum titanates.

Some newer mechanistic aspects investigated by in situ Fourier transform infrared (FTIR) in conjunction with catalytic activity under similar conditions over crystalline lanthanum titanates as a function of Fe substitution at the B-site for the CO + N(2)O reaction are reported for the first time in the present communication. La(2)Ti(2(1-x))Fe(2x)O(7-delta) (0.0 < or = x < or = 1.0) was synthesized by gel combustion where Fe(3+) substitution effectively enhanced the conversion rates for N(2)O reduction as compared to the pristine La(2)Ti(2)O(7) (LTOGC). Among all samples, maximum conversion over La(2)Ti(0.8)Fe(1.2)O(7-delta) [LF(0.6)GC] catalyst was observed. In situ FTIR results reveal that substitution-induced anionic vacancies/defects provide additional sites on the surface of LF(0.6)GC for CO chemisorptions, whereas a perfect stoichiometric lattice like LTOGC is devoid of such sites. Surface-adsorbed CO reacts with surface lattice oxygen in the case of nonstoichiometric LF(0.6)GC to produce carbonates (M-CO(3)(2-)) at a much lower temperature. The reaction proceeds via carbonate formation, leaving the catalytic surface oxygen deficient in LF(0.6)GC, and therefore facilitates the reduction of preadsorbed, N(2)O [N(2)O(g) + * --> N(2) + *-O) by easily adsorbing the oxygen species (*-O) generated in N(2)O reduction, which is subsequently driven away by adsorbed/gas phase CO, whereas in the case of LTOGC, progress of the reaction was sluggish in the absence of transient carbonate species. Dissociative chemisorptions of N(2)O are not facilitated on stoichiometric oxygen excess titanate, as there is no vacancy in the surface to accommodate another oxygen atom. The redox mechanism via CO(3)(2-) species is proposed for CO + N(2)O reaction over La(2)Ti(2(1-x))Fe(2x)O(7-delta), as against the associative mechanism observed in the unsubstituted sample, La(2)Ti(2)O(7), as suggested by in situ FTIR in conjunction with catalytic activity results.

Heterogeneous photocatalytic degradation of methanol over uranyl-anchored nanoporous MCM-41 and MCM-48.

The vapor-phase photodegradation of methanol to carbon dioxide was carried out over uranyl-anchored nanoporous MCM-41 and MCM-48 hosts (designated as UO2(2+)/MCM-41 and UO2(2+)/MCM-48, respectively) under simulated light and ambient conditions. Preliminary results indicate that the photoactivity of the latter is considerably decreased as compared to the former due to the presence of a smaller fraction of photoactive uranyl (UO2(2+)) ions in UO2(2+)/MCM-48.

Fourier transform infrared and quasielectron neutron scattering studies on the binding modes of methanol molecules in the confined spaces of HMCM-41 and HZSM-5: role of pore structure and surface acid sites.

Quasielastic neutron scattering (QENS) and Fourier transform infrared spectroscopic studies were carried out on methanol molecules adsorbed in HMCM-41 and HZSM-5 molecular sieves to monitor the effect of pore structure on their occluded state under the conditions of ambient temperature and 5-250 mbar pressures. The QENS results have shown that the pore geometry of the host matrix and the dipolar character of the adsorbate are together responsible for the binding state of guest molecules in the confining medium. Thus, neither translational nor free rotational motion was noticed for methanol molecules adsorbed in HZSM-5, in contrast to benzene and cyclohexane molecules of almost similar size that are reported to undergo a rotational motion under the identical conditions of loading (Phys. Chem. Chem. Phys. 2001, 3, 4449; 2003, 5, 3066). In the case of HMCM-41, a translational motion of occluded methanol molecules was clearly observed with a diffusion constant D approximately 1.5 x 10(-5) cm2 s(-1), as compared to a value of D approximately 2.6 x 10(-5) cm2 s(-1) for its liquid state. These results indicate that the adsorbed methanol experiences a considerable extent of supercooling due to capillary condensation in zeolitic pores, giving rise to formation of a metastable state even at room temperature. In HZSM-5, entrapped methanol exists in an almost solidlike state, whereas in HMCM-41, its density lies between that of the solid and the liquid phases. Infrared spectroscopic study conducted using deuterium-labeled adsorbate and host matrixes have given evidence for different kinds of interactions between the methanol molecules and the host matrix, depending upon the loading. For small loadings the internal hydroxy groups within the pore system get perturbed first, giving rise to formation of the methoxy groups. Multilayer adsorption and capillary condensation of methanol occur for a loading of 0.05 mmol per gram and above, within the pore system and also at the external surface, giving rise to a highly compressed state due to strong intermolecular bonding. At the same time, a considerable amount of exchange occurred between the hydroxy groups of the adsorbed methanol and those of the host matrix. Such exchange of hydroxy groups may play an important role in the catalytic properties of the porous aluminosilicates.