Masaaki Haneda

Alkali metal-doped cobalt oxide catalysts for NO decomposition

The effect of addition of alkali metals was investigated on the catalytic activity of cobalt oxide (Co3O4) for the direct decomposition of NO. Although Co3O4 was totally inactive, NO decomposition over Co3O4 was significantly promoted by the addition of alkali metals (Li, Na, K, Rb and Cs). Potassium was found to be the most effective additive. The addition of alkali metals increased not only the surface area but also the specific activity per unit surface area. This suggests that catalytically active sites are created on the Co3O4 surface as a result of interaction with alkali metals. The interface between alkali metals and Co3O4 is clearly essential in the NO decomposition reaction. Characterization of the catalyst using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) and desorption of O2 (O2-TPD) revealed that traces of added alkali metals weaken the CoO bond strength and promote oxygen desorption from Co3O4, resulting in the formation of cobalt species with a slightly lower valence state. We conclude that these effects, caused by addition of alkali metals, are responsible for the activity enhancement of Co3O4.

Remarkable promoting effect of rhodium on the catalytic performance of Ag/Al2O3 for the selective reduction of NO with decane

Abstract The addition of small amounts of rhodium enhanced the activity of Ag/Al 2 O 3 catalyst for the selective reduction of NO with decane at low temperatures. The Rh-promoted Ag/Al 2 O 3 showed its high performance even in the presence of low concentrations of SO 2 . Based on the catalytic activity for elementary reactions, it was suggested that the role of added rhodium is to enhance the reaction between NO x and decane-derived species, leading to NO reduction. Catalyst characterization by UV-Vis spectroscopy indicated that the major silver species on Rh-promoted Ag/Al 2 O 3 is Ag n δ + clusters, which would be responsible for the high activity. FT-IR measurements revealed that the formation rate of isocyanate species, which is a major reaction intermediate, is higher on Rh-promoted Ag/Al 2 O 3 .

Preparation and reaction mechanistic characterization of sol–gel indium/alumina catalysts developed for NOx reduction by propene in lean conditions

The impregnation and sol–gel preparation methods were investigated to develop high activity catalysts and understand the significance of the indium–aluminium interaction on alumina‐supported indium catalysts in NOx reduction with propene. Active In/alumina catalysts with a very high surface area (270 m2/g) and thermal stability were prepared in controlled conditions by sol–gel processing. When Al isopropoxide and In nitrate in ethyl glycol were used as precursors in aqua media, indium atoms were incorporated evenly distributed as a thermally stable form in the aluminium oxide lattice structure. In wet impregnation it was beneficial to use a certain excess of aqueous In solution (volumes of solution : pores = 2 : 1) to have the highest NOx reduction activity. The catalyst containing dispersed Al on In oxide (58 wt% In, phase‐equilibrium preparation method) showed activity at lower temperatures than any other In–Al oxide catalyst or pure In2O3. The adsorption of different reaction intermediates on alumina and stable In2O3 sites were detected by FTIR studies. In/alumina catalysts have active sites to oxidize NO to NO2, partially oxidize HC, form the actual reductant which contains N–H or N–C bonding and react with NO to dinitrogen. The cooperation with indium and aluminium was evident even in the mechanical mixture of sol–gel prepared alumina (301 m2/g) and In2O3 powders (27 m2/g), where the probability for molecular‐scale intimate contact between indium and aluminium sites was very low (particle size 10–250 μm). Short‐lived gaseous intermediates and surface migration are the possible reasons for the high catalytic activities on the two physically separated active sites both necessary for the reaction sequence.

Surface reactivity of prereduced rare earth oxides with nitric oxide: New approach for NO decomposition

Direct decomposition of NO to N2 by rare earth oxides prereduced with H2 was studied. CeO2 and Pr6O11 prereduced at 773 K, in which many oxygen anion vacancies exist, decomposed NO to N2 at 673 K. The addition of Pt accelerated the dissociation of NO to N2. The amount of NO decomposed to N2 and N2O by Pt supported on rare earth oxides was strongly related to the oxygen mobility and to the amount of oxygen anion vacancies in the surface/subsurface. Particularly, for Pt/CeO2, oxygen diffusion on the surface but not in the bulk was the most important factor determining the NO decomposition activity.

CeO2–ZrO2 binary oxides for NOx removal by sorption

Removal of NOx (NO in the presence of O2) by CeO2–ZrO2 binary oxides was investigated. CeO2–ZrO2 prepared by a modified sol–gel method [CZ(SG)] showed high NOx sorption capacity, while CeO2–ZrO2 prepared by a coprecipitation method [CZ(CP)] was not effective. In both cases, NOx was found to be removed by adsorption on the surface but not by absorption into the bulk. In situ diffuse reflectance Fourier transform infrared measurements demonstrated that NOx is adsorbed as several types of nitrates. X-ray diffraction measurements revealed the formation of a complete solid solution, Ce0.5Zr0.5O2, for CZ(SG) and the presence of separate phases of CeO2 and ZrO2 for CZ(CP). Homogeneous mixing of Ce ions and Zr ions in the solid solution was considered to be one of the important factors for the high NOx sorption capacity of CZ(SG). The high NOx sorption capacity was also accounted for by the presence of a large amount of basic sites and high catalytic activity for NO oxidation to NO2, which is the first step in the NOx removal process.

Effect of surface structure of supported palladium catalysts on the activity for direct decomposition of nitrogen monoxide

Abstract The active sites for direct decomposition of NO on supported palladium catalysts were investigated by a comparative study with single-crystal palladium surfaces. In the study of powder catalysts, the activity of Pd/Al 2 O 3 was found to be significantly affected by the type of palladium precursor. Pd/Al 2 O 3 catalysts prepared from palladium nitrate showed the highest activity, expressed in terms of turnover frequency (TOF), for NO decomposition to N 2 . In studies using single-crystal model catalysts reported previously [I. Nakamura, T. Fujitani, and H. Hamada, Surf. Sci. 514 (2002) 409], the adsorption, dissociation, and desorption behavior of NO was closely related to the surface structures, the stepped surface palladium being active for dissociation of NO. NO adsorption by FT-IR spectroscopy confirmed the presence of the stepped palladium sites on Pd/Al 2 O 3 catalysts, and the specific activity (TOF step ) for NO decomposition to N 2 , calculated on the basis of number of step sites estimated from the amount of chemisorbed NO, was not changed for all the Pd/Al 2 O 3 catalysts. These results suggest that direct decomposition of NO mainly proceeds on the step sites of palladium. This is in agreement with the results obtained for single-crystal model catalysts. The present study shows that the activity of powder catalyst can be well interpreted by its surface structure, indicating the usefulness of surface science techniques.

Rh-post-doped Ag/Al2O3 as a highly active catalyst for the selective reduction of NO with decane

The effect of an impregnation preparation procedure on the catalytic performance of Rh-promoted Ag/Al2O3 was investigated for the selective reduction of NO with decane. Rh/Ag/Al2O3, which was prepared by impregnation of Rh on Ag/Al2O3, showed higher NO reduction activity compared with Ag/Rh/Al2O3 prepared using the reverse procedure. UV–Vis measurement confirmed that Agnδ+ clusters, which are probable active species, are more abundant on Rh/Ag/Al2O3 than on Ag/Rh/Al2O3. An FT-IR study indicated that the rate of formation of isocyanate species as a reaction intermediate is faster on Rh/Ag/Al2O3.

Reaction properties of NO and CO over an Ir(211) surface

The adsorption and thermal reactivity of NO and CO over an Ir(211) surface were studied using x-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and temperature-programed desorption. NO adsorbed on the atop site of the (111) terrace and the bridge site of the (100) step at 273K. In contrast, CO adsorbed only on the atop site at 273K, initially on the (111) terrace and then on the (100) step. Both atop NO and bridge NO decomposed to N2 through the recombination of atomic nitrogen, indicating that the Ir(211) surface provides high NO dissociation activity. When NO and CO were coadsorbed, the preadsorption of atop CO on the terrace sites selectively inhibited the adsorption of atop NO on the terrace sites, while preadsorption of atop CO on the step sites significantly inhibited the adsorption of bridge NO on the step sites. These results indicate that NO may be selectively reduced by CO in the presence of O2 when Ir(211) is used as the catalyst.

Ir/SiO2 as a highly active catalyst for the selective reduction of NO with CO in the presence of O2 and SO2

Coexisting SO2 considerably enhanced the catalytic activity of Ir/SiO2 for NO reduction with CO in the presence of O2 because of the formation of a cis-type coordinated species of NO and CO to one iridium atom ([formula: see text]), a possible reaction intermediate leading to N2 formation.

Roles of surface nitrogen oxides in propene activation and NO reduction on Ag/Al2O3

The activation of propene in selective catalytic reduction (SCR) of NO on 4% Ag/Al2O3 has been studied by in situ infrared (IR) spectroscopy. Distinctive propene activation products were detected in the SCR of NO, depending on the nature of surface oxygen and nitrogen oxide species on Ag/Al2O3. C3H6 was oxidized to acetate species in an O2 + C3H6 atmosphere on Ag/Al2O3 above 573 K. The addition of NO to the C3H6 + O2 feed gas suppressed the formation of acetate species but increased the proportion of acrylate species. Acrylate species were further confirmed to be formed preferentially from C3H6 oxidation without the O2 atmosphere on Ag/Al2O3 or nitrate-adsorbed Ag/Al2O3. On the other hand, adsorption of NO led to the formation of nitrito species on Ag/Al2O3, but the nitrito was barely oxidized to nitrate species unless there was an O2 atmosphere at 473–673 K. Thus, the oxidation of propene to acetate species, or the formation of nitrate from nitrito, is attributed to two competitive electrophilic reactions. The formation of nitrate from nitrito species decreased electrophilic oxygen species that oxidized propene to acetate. Nevertheless, the first dehydrogenation of propene to form acrylate species on nitrate-adsorbed Ag/Al2O3 is a nucleophilic reaction, as it is on Ag/Al2O3. Furthermore, there was no decrease in reaction activity for formation of acrylate species on nitrate-adsorbed Ag/Al2O3 compared to Ag/Al2O3. This led to the total reaction occurring easily through the propene nucleophilic oxidation branch because the presence of the adsorbed nitrogen oxides changed selectively the formation rates of the surface reductants. IR spectra data further demonstrate that acrylate and acetate species, as the surface reductants, reacted with nitrate to generate isocyanate intermediates in the SCR of NO. The effect of structures of different reductants on NO reduction is discussed.