Kiyohide Yoshida

Surface isocyanate intermediate formed during the catalytic reduction of nitrogen oxide in the presence of oxygen and propylene

IR spectroscopic measurements have revealed that an IR band ascribable to adsorbed isocyanate species grows up when alumina-supported Cu-Cs oxide catalyst is exposed to a mixture of NO, O2 and C3H6 at room temperature and subsequently heated to 400 °C in vacuum. The species produces N2, CO2 and CO in the ratio of ca. 2:1:1 in the presence of NO at 350°C. Alumina and alumina-supported Cu oxide catalyst are less active for the formation of isocyanate species.

Possible role of isocyanate species in NOx reduction by hydrocarbons over copper-containing catalysts

The behavior of an isocyanate intermediate (-NCO) formed during NOx reduction has been studied on alumina-supported CuCs oxide catalyst in the presence of oxygen and hydrocarbons (propene, acetylene, propane and n-heptane) using infrared spectroscopy. While a reaction involving NO, O2 and acetylene needs some heat treatment to produce the isocyanate species on the catalyst, no heat treatment is required in the NO/O2/propene or n-heptane system. No isocyanate intermediate is formed in a NO/O2/propane system by an ordinary procedure. Adsorbed water on the catalyst surface is found to suppress the formation of the isocyanate species. This inhibition effect is smaller in the acetylene or n-heptane containing system than in the propene containing system. The role of isocyanate species is discussed with reference to results for the practical reduction of NOx.

Formation and reactivity of isocyanate (NCO) species on Ag/Al2O3

The formation of the reactivity of isocyanate species have been studied over Ag/Al2O3 by IR spectroscopy and mass spectrometry. Adsorbed CxHyNOz and NO3− species are produced by reaction among NO, O2 and C3H6 at room temperature. Thermal decomposition of adsorbed CxHyNOz species leads to the formation of two types of NCO species (NCO on Ag and NCO on Al2O3) above 423 K. These NCO species are thermally stable in vacuum at 673 K, while adsorbed NO3− species decompose completely. The NCO species are highly reactive toward NO+O2 at room temperature, being converted into N2, CO2, CO and a small amount of N2O. The NCO species are less active in NO or O2 alone than in the mixture of NO and O2. Thus, excess oxygen added in the NO reduction by C3H6 plays an important role in the formation of adsorbed CxHyNOz species and in the reaction of adsorbed NCO with NO. It is suggested that the formation of adsorbed CxHyNOz and adsorbed NCO is essential for the progress of the NO reduction with C3H6 in the presence of O2 under the present experimental conditions.

Characterization of highly active silver catalyst for NOx reduction in lean-burning engine exhaust

A new Ag/Al2O3 catalyst for removing NOx in lean exhaust gas was developed. Oxidized Ag/Al2O3 catalyst is highly active for reduction of NOx with ethanol and propene, whereas reduced Ag/Al2O3 catalyst is less active for these reactions. Selectivity to N2 is also high on the oxidized Ag/Al2O3 compared to that on the reduced Ag/Al2O3. XRD and SEM studies of these two types of Ag catalysts suggest that oxidation induces an interaction between Ag and the support, where the particles are grown in large size. In contrast, the metallic Ag particles are finely dispersed by the reduction process. Although dispersion of Ag particles is decreased by the oxidation process, the catalytic activity is increased. This suggests that the Ag-alumina sites created in the high temperature oxidizing environment are active in catalytic reduction of NOx.

Effect of SO2 on NOx reduction by ethanol over Ag/Al2O3 catalyst

A new Ag/Al2O3 catalyst for removing NOx in diesel engine exhaust gas was developed. The influence of SO2 on the reduction of lean NOx by ethanol over the Ag/Al2O3 catalyst was evaluated in simulated diesel exhaust and characterized using TPD, XRD, XPS, SEM and BET measurements. The Ag/Al2O3 catalyst was highly active for the reduction of NOx with ethanol in the presence of SO2 although the reduction of NOx is suppressed at lower temperatures. The activity for NOx reduction is high even on the Ag/Al2O3 catalyst exposed to a SO2 (200 ppm)/O2 (10%)/H2O (10%) flow for 20 h at 723 K and comparable to that on the fresh Ag/Al2O3 catalyst. No crystallized Ag metal and Ag compounds were formed by the SO2/O2/H2O exposure. On the other hand, crystallized Ag2SO4 was easily formed when the Ag/Al2O3 catalyst was exposed to a SO2 (200 ppm)/O2 (10%)/NO (800 ppm)/H2O (10%) flow for 10 h at 723 K. XRD, SEM and XPS studies showed that the formation of crystallized Ag2SO4 results in growing of Ag particles in larger size and lowering the surface content of Ag particles. In addition, the specific surface area of the Ag/Al2O3 catalyst decreases from 221 to 193 m2/g. Although the dispersion of Ag particles was decreased by the formation of Ag2SO4, the activity for the reduction of lean NOx was, remarkably, not affected. This suggests that the Ag–alumina sites created by the Ag2SO4 formation are still active for the lean catalytic reduction of NOx.

Infrared study of catalytic reduction of lean NOx with alcohols over alumina-supported silver catalyst

Two intense IR absorption bands due to surface isocyanate (-NCO) species have been observed at 2262 and 2232 cm−1 when an alumina-supported silver catalyst is exposed to a mixture of NO, O2 and ethanol at 150°C and subsequently heated to > 300°C in vacuum. The intensity of the isocyanate band is hardly affected by the water existing in the mixture. Methanol is less reactive than ethanol for the formation of isocyanate species. The reaction mechanism of catalytic reduction of lean NOx with alcohols is discussed based on these IR spectroscopic findings.

Activity enhancement of copper-containing oxide catalysts by addition of cesium in the reduction of nitric oxide

Catalytic reduction of nitric oxide in the presence of propylene and oxygen over alumina and copper-containing oxide catalysts has been studied. The optimum temperature for this reaction is dependent upon the composition of the catalysts: ≈ 640 K on Cu-Cs/Al2O3, ≈ 680 K on Cu/Al2O3, and ≈ 780 K on Al2O3. IR spectroscopic measurements show that an isocyanate (−NCO) intermediate formed on Cu-Cs/Al2O3 is more reactive with NO to give N2 than the intermediate produced on Al2O3 and Cu/Al2O3. Electron donation from Cs to Cu may activate the intermediate.

Promoting effects of metals for NO reduction over potassium doped carbon

Reduction of NO to N2O or N2 was studied over activated carbons using thermal desorption technique. The addition of other metals such as Zn, Cu, Fe, Ni, Sn, Mn or Ce, to potassium doped carbon remarkably enhanced direct NO reduction or NO reduction by carbon. The high activity might be ascribed to the synergism between potassium-metal on the carbon surface.

Synthesis of (H3O)TiNbO5·0.26H2O ia hydronium (H3O+) ion-exchange reaction and its photocatalytic activity forH2 evolution from aqueous methanol solution

(H3O)TiNbO5·0.26H2O powder was prepared from layered titanoniobate, KTiNbO5, using an ion-exchange reaction in acidic aqueous solution. X-ray diffraction, thermogravimetry–differential thermal analysis, energy-dispersive X-ray analysis and infrared spectral data gave the first experimental evidence that the so-called proton-exchanged form HTiNbO5 involves hydrated water or hydrated hydronium ion. The intercalation of water or hydronium ion in KTiNbO5 was not achieved by the conventional high-temperature solid-state reaction method but by the low-temperature polymerizable complex (PC) method. While KTiNbO5ia the PC route, combined with Pt, showed only little improvement of the photocatalytic activity with respect to H2 evolution from aqueous methanol solution, the water or hydronium ion intercalated species, (H3O)TiNbO5·0.26H2O, with Pt loading, showed much higher activities than the others species studied by an order of magnitude.

Photocatalytic conversion of NOx on AgCl/Al2O3 catalyst

Abstract The rate of NO conversion under UV illumination was evaluated over Ag/Al2O3 and AgCl/Al2O3 catalysts at room temperature. The AgCl/Al2O3 catalyst is highly active for the conversion of NOx. The conversion is enhanced in the presence of O2 and further enhanced when oxygen coexists with hydrocarbons. Diffuse reflectance spectra of AgCl/Al2O3 and Ag/Al2O3 show an absorption band at ∼250 nm, and a weak and broad band at ∼230 nm, respectively. The high photocatalytic NOx conversion is achieved over the AgCl/Al2O3 catalyst. The conversion level of NOx is maintained above 60% over 5 h in the presence of O2 and hydrocarbons under UV-irradiation. The absorption band at ∼250 nm is ascribed to the band gap energy of crystallized AgCl particles on Al2O3. These results suggest that high photocatalytic NOx conversion proceeds on crystallized AgCl particles formed on Al2O3.