Tatsuro Horiuchi

Suppression of carbon deposition in the CO2-reforming of CH4 by adding basic metal oxides to a Ni/Al2O3 catalyst

The effect of the oxides of Na, K, Mg, and Ca on CO2-reforming of CH4 over Ni/Al2O3 catalysts was studied from the viewpoint of carbon deposition. These added oxides markedly suppressed the carbon deposition during CO2-reforming, also in the absence of CO2. It was concluded that the suppression of carbon deposition resulted from the decrease in the ability of Ni catalyst for CH4 decomposition. It was also observed that the added basic metal oxides changed the reaction order from negative to positive with respect to the partial pressure of CH4 and from positive to negative with respect to the partial pressure of CO2. The observed reaction kinetics suggested that the surface of the Ni catalyst with basic metal oxides was abundant in adsorbed CO2, while the surface without them was abundant in adsorbed CH4. The abundance in adsorbed CO2 on the Ni metal surface was considered an unfavorable condition for CH4 decomposition and as a result, carbon deposition was further suppressed during CO2-reforming on Ni/Al2O3 catalysts with basic metal oxides.

A novel alumina catalyst support with high thermal stability derived from silica-modified alumina aerogel

A new alumina catalyst support with high thermal stability was synthesized. The high thermal stability was achieved through the synergetic effect of silica addition and the ultra-low bulk density (aerogel). The amount of silica was varied from 2.5 to 10 wt% and 5 wt% was found to be most effective for suppressing phase transformation; the θ phase remained even after heating at 1400°C for 1 h. The surface areas of the present alumina with 5 wt% silica were 86 and 36 m2/g after heating at 1300 and at 1400°C, respectively.

Catalysis of NiO–Al2O3 aerogels for the CO2-reforming of CHF4

Uniform and monolithic NiO–Al2O3 aerogels were prepared from cyclic nickel glycoxide, (CH2O)2Ni, and boehmite sol, AlOOH, and the catalyst performance of the aerogels for the CO2-reforming of methane was investigated. The NiO–Al2O3 aerogels showed higher activity than impregnation NiO/Al2O3 catalysts, while the aerogels exhibited much less activity for coking than the impregnation catalysts. Less deactivation was also observed on the aerogel catalysts than on the impregnation catalysts in the continuous-flow reaction. The Ni was uniformly incorporated throughout alumina where both the metal and the support exist in the aerogel form, i.e., Ni–O–Al bond was considered to be formed in the aerogels. As a result, fine Ni particles appeared after H2 reduction throughout the alumina support with high dispersion, which brought about not only higher activity but also much less activity for coking on the aerogels. Retardation of catalyst deactivation was ascribed to the suppression of both coking and sintering of Ni particles on the aerogels.

Kinetics, intermediates and mechanism for the CO2-reforming of methane on supported nickel catalysts

The nature and the reactivity of an intermediate hydrocarbon species, CHx, ads, have been determined for the CO2-reforming of CH4 over supported Ni catalysts by using pulse surface reaction rate analysis (PSRA). When a small amount of CH4 pulse was injected into flowing gas mixture of CO2 and He, it was immediately adsorbed on the catalyst and then gradually reacted with CO2 to CO and H2. Analysis of the dynamic behaviour of the produced CO enabled us to determine the first-order rate constant for the reaction between adsorbed CH4 and CO2. It was found from the dynamics of the H2 produced that two steps were responsible for H2 production: the dissociative adsorption of CH4 to produce (4 –x)/2 H2 and CHx, ads and the simultaneous production of x/2 H2 together with CO from the CHx, ads species. Separation of the amount of H2 between these two steps led us to determine the average number of hydrogen atoms involved in the hydrogen-deficient adsorbed hydrocarbon species, i.e. 2.7 for Ni/MgO, 2.4 for Ni/Al2O3, 1.9 for Ni/TiO2 and 1.0 for Ni/SiO2. The reactivity of the intermediate species was not correlated with the number of hydrogen atoms involved. The highest activity on Ni/TiO2 and almost the same activity on Ni/MgO, Ni/Al2O3 and Ni/SiO2 were observed both for the pulse reaction and for the steady-state reaction, suggesting that the reaction of CHx, ads with CO2(or Oads) is the rate-determining step in CO2-reforming.

Highly hydrogen-deficient hydrocarbon species for the CO2-reforming of CH4 on Co/Al2O3 catalyst

The dynamics of produced CO and H2, measured by pulse surface reaction rate analysis (PSRA), revealed that the intermediate hydrocarbon species for the CO2-reforming of CH4 was highly hydrogen-deficient (CH0.75) on supported Co/Al2O3 catalyst. It was also found that the species was more reactive than the less hydrogen-deficient one (CH2.4) on Ni/Al2O3 catalyst.

Improvement of thermal stability of alumina by addition of zirconia

To maintain a large surface area at elevated temperatures, zirconia was added to transition alumina. The addition of a small amount of zirconia resulted in a marked suppression of phase transformation from θ- to α-alumina. After heating at 1200°C, ZrO2‐containing alumina exhibited a large surface area of 50 m2/g. UV‐VIS and XRD measurements indicated that zirconia existed in a high dispersion state after calcining at 800°C. XPS measurement also showed that zirconia existed as monolayer. Zirconia monolayers are concluded to cover the alumina surface and the interaction between them may be the cause for the suppression of phase transformation and also for the maintenance of the large surface area at elevated temperatures. The interaction remains up to 1200°C, therefore, θ phase remained at 1200°C.

High surface area alumina aerogel at elevated temperatures

In order to obtain an alumina with a high surface area at elevated temperatures, a suppression of the θ–α phase transformation of alumina by decreasing the bulk density has been attempted. The abundance of the lattice defects and dislocations in the neck region would be a favourable site for nucleation of the α-phase. Therefore the decrease of the neck region, that is, the decrease in bulk density would lead to a suppression of the θ–α phase transformation. In order to decrease the bulk density, the alumina aerogel was prepared from boehmite sol. The bulk density was varied from 0.06 to 1.1 g cm–3 by decreasing the ζ-potential of the boehmite particles during ageing. The phase transformation and the surface area of alumina from aerogels with various bulk densities were examined at elevated temperatures. The phase-transformation temperature increased with decreasing bulk density. While the alumina with its bulk density of 1.1 g cm–3 transformed to the α-phase at 1443 K, alumina aerogel with a bulk density of 0.06 g cm–3 transformed to the α-phase at 1573 K. The shift of the transformation to a higher temperature can be understood in terms of the decrease in the neck region. As a result of the suppression of phase transformation, a large surface area was maintained at elevated temperatures. After heating at 1473 K for 5 h, the alumina aerogel, with a bulk density of 0.06 g cm–3, was composed of the θ-phase and its surface area was 90 m2 g–1. After completion of the phase transformation, i.e. at 1673 K for 5 h, alumina aerogel with a bulk density of 0.06 g cm–3 still exhibited a surface area of 10 m2 g–1, larger than that of the conventional α-phase.

On the promotional effect of Sn in Co–Sn/Al2O3 catalyst for NO selective reduction

NO reduction with propylene over Co/Al2O3 and Co–Sn/Al2O3 catalysts has been investigated. For the Co/Al2O3 catalyst, a calcination temperature exceeding 800 °C led to a decrease of NO conversion. Calcination of the Co/Al2O3 catalyst at 1000 °C resulted in the formation of α-Al2O3 and Co3O4. The presence of 20% water vapor showed a significant shift for the maximum NO reduction temperature from 450 to 600 °C over Co/Al2O3. It has been found that modification of 6 wt% Co/Al2O3 with 2 wt% Sn significantly enhanced the catalyst thermal stability and improved the inhibitory effect of water on NO conversion and reaction temperature. The promotional effect of Sn on the catalyst thermal stability was attributed to the suppression of the phase transformation from highly dispersed Co2+ species on γ-Al2O3 to α-Al2O3 and Co3O4. The smaller influence of water vapor on NO reduction conversion and temperature over Co–Sn/Al2O3, compared to Co/Al2O3, was attributed to the dispersion effect of Sn species on Co2+ species as well as the involvement of Sn species in NO reduction at a relatively lower temperature. The synergetic effect between the octahedral Co2+ species and γ-alumina plays a significant role in the catalysis of NO selective reduction by C3H6.

Catalyst performance of MoS2 and WS2 for the CO2-reforming of CH4 Suppression of carbon deposition

Abstract The catalyst performance of MoS 2 and WS 2 was verified for the CO 2 -reforming of CH 4 through the comparison with that of Ni SiO 2 . No catalyst deactivation was observed on the disulfides, although the rate was much smaller than that on Ni. It was found that the disulfides exhibited a poor ability for the direct decomposition of CH 4 . The poor ability did not lead to carbon deposition during reforming reaction, which assured a stable activity. It was also observed that the rate equation was completely different between disulfide catalysts and Ni catalyst. The reaction order was positive with respect to the CH 4 partial pressure and negative with respect to the CO 2 partial pressure on the former catalysts, while negative with respect to CH 4 partial pressure and positive with respect to the CO 2 partial pressure on the latter. The observed reaction kinetics suggested that the surface of the disulfide catalyst was abundant in adsorbed CO 2 , while the surface of Ni catalyst was abundant in adsorbed CH 4 . The abundance in adsorbed CO 2 was considered an unfavorable condition for the direct decomposition of CH 4 and as a result, carbon deposition was suppressed during CO 2 -reforming on disulfide catalyst. Sulfur tolerance was found to be an additional performance of the disulfide catalyst.

Catalytic reduction of NO by hydrocarbons over a mechanical mixture of spinel Ni–Ga oxide and manganese oxide

NO reduction with propylene over Mn2O3, spinel Ni–Ga oxide and their mechanical mixtures has been investigated. Mn2O3 has no activity to NO reduction, but has a high activity for NO oxidation to NO2. Spinel Ni–Ga oxide showed an apparent activity to NO reduction only at temperatures above 400°C. Mixing of Mn2O3 to the Ni–Ga oxide resulted in a significant enhancement of NO reduction in the temperature range of 250–450°C. The optimal Mn2O3 content in the mixture catalyst was about 10–20 wt%. It is suggested that the synergetic effect of Mn2O3 and Ni–Ga oxide plays an important role in the catalysis of NO reduction. The Ni–Ga oxide and Mn2O3 mixture catalyst is superior to Pt/Al2O3 and Cu-ZSM-5 by showing a higher NO reduction conversion, resistance to water and negligible harmful by-product formation. Other lower hydrocarbons C2H4, C2H6 and C3H8 also give a maximum NO reduction conversion as high as 50%. The difference from using C3H6 is that the temperature at the maximum NO reduction is higher than it is with C3H6.