Kenzi Tamaru

Photocatalytic decomposition of water vapour on an NiO–SrTiO3 catalyst

The photodecomposition of water vapour proceeds steadily for more than 100 h on NiO–SrTiO3 powder and stops immediately when the water vapour is removed.

The mechanism of the reaction between NOx and NH3 on V2O5 in the presence of oxygen

In order to elucidate the mechanism of the reaction between nitric oxide and ammonia on a V2O5 catalyst, the elementary steps of the reaction were separately studied, and the reactivities of the adsorbates were examined by means of ir, XPS, mass spectrometry, and kinetic studies under reaction conditions. The role of oxygen, whose presence is essential for the reaction to occur, was examined. Nitric oxide was not adsorbed on the V2O5 surface in the absence of oxygen but was adsorbed as NO2(ad) in the presence of ambient oxygen. Ammonia was adsorbed on V2O5 in the form of NH4+(ad), which gives a band at 1413 cm−1 in the ir spectrum. These two adsorbates, NO2(ad) and NH4+(ad), reacted readily to form N2. Accordingly, a new mechanism was proposed for the role of oxygen in the reaction; namely, that the reaction proceeds via the two adsorbates, NO2(ad) and NH4+(ad), which react on the catalyst surface through a Langmuir-Hinshelwood mechanism to form N2 and H2O.

Photocatalytic decomposition of liquid water on a NiOSrTiO3 catalyst

Abstract The photocatalytic decomposition of liquid water on a NiOSrTiO3 catalyst was studied. The activity was increased considerably by an improvement of pretreatment conditions and using concentrated NaOH solution.

Mechanism of formation of C2-oxygenated compounds from CO + H2 reaction over SiO2-supported Rh catalysts

Abstract The mechanism of acetaldehyde and ethanol formation from the CO + H 2 reaction below atmospheric pressure has been investigated by combining infrared spectroscopic measurement and 13 CO and C 18 O isotopic tracer studies with reaction kinetics. The rates of acetaldehyde and ethanol formation are markedly dependent on the nature of metal precursors employed. The addition of sodium cations depresses the total catalytic activity, while the selectivity for ethanol is increased by the addition of manganese cations. From the behavior of surface species under reaction conditions, it is concluded that acetaldehyde is formed through the following two steps: (i) CO insertion into C 1 species which are reaction intermediates for not only hydrocarbons but also for the methyl group in acetaldehyde, and (ii) subsequent formation of acetate ions whose one oxygen atom is supplied from the support, finally producing acetaldehyde. Differences in 18 O distribution in acetaldehyde and ethanol during the C 18 O + H 2 reaction indicate that ethanol is not produced via direct hydrogenation of acetaldehyde.

Reaction intermediates in methyl alcohol decomposition on ZnO

The decomposition of methyl alcohol on ZnO was studied using infra-red spectroscopy during the course of the reaction. The concentrations and reactivities of the chemisorbed and gas phase species were measured as well as the overall reaction rate under various non-stationary conditions. When CD3OD vapour was introduced over ZnO, methoxide ion and formate ion were observed, and D2, CO2 and CO were evolved into the gas phase. When the CD3OD in the ambient gas was removed by a dry ice-methyl alcohol trap during the course of the reaction, the evolution of D2 and CO2 stopped, while the evolution of CO continued unchanged. At 240°C, the decomposition rate of the surface formate ion was in reasonable agreement with the rate of production of carbon monoxide. On releasing the trapped methanol, the rates of formation of CO2 and D2 increased again, surface methoxide reappeared, and the concentration of the surface formate ion decreased correspondingly. These results lead to the conclusion that CO was produced mainly by the decomposition of formate ion, and D2 and CO2 came from the reaction between CD3OD and DCOO(a).

Selective hydrogenation of carbon monoxide on palladium catalysts

Novel palladium catalysts prepared from complexes of the type M2PdCl4(M = alkali metal) have been studied which produce methanol selectively from CO + H2 mixtures even below atmospheric pressure. The catalytic activity for methanol formation depends sharply on the alkali-metal cation in the following order: Li > Na K > Rb > Cs. It also depends on the support, exhibiting higher activities with more acidic supports such as alumina and silica–alumina than silica. Different behaviour for methanol and methane formation suggests that different reaction sites operate in each case. Infrared spectroscopic techniques were applied to elucidate the mechanism of the overall reaction, and the behaviour of formate ion on the catalyst, which could be observed under the reaction conditions, was examined.

Dynamic technique to elucidate the reaction intermediate in surface catalysis. Water-gas shift reaction

The mechanism of the water-gas shift reaction on ZnO and MgO was studied by means of infra-red spectroscopy during the course of the reaction. When a mixture of carbon dioxide and hydrogen was introduced over ZnO, formate ion was observed. The rate of decomposition (dehydration) of the surface formate ion was measured at the reaction temperature (230°C) as a function of its concentration, and compared with the rate of the overall reaction on ZnO at the same coverage of the surface formate is the reaction intermediate of the water-gas shift reaction on ZnO and its decomposition is the rate-determining step. When a mixture of carbon monoxide and water vapour was passed over MgO, the formation of surface formate ion was similarly confirmed. It was, however, different from ZnO, in that it was difficult to detect the surface formate ion from a mixture of carbon dioxide and hydrogen. The comparison of the rate of the dehydrogenation decomposition of the surface formate ion with that of the overall reactions at the same formate coverage leads to conclusion that the surface formate ion is the reaction intermediate of the water-gas shift reaction on MgO and the rate determining step is the dehydrogenation of the formate ion.

Molecular-Sieve Type Sorption on Alkali Graphite Intercalation Compounds

The sorption of H2, D2, N2, CH4 and rare gases was studied on alkali graphites C8M and C24M (M = K, Rb, Cs) at 196-63 K. The C24M compounds sorbed gases to various degrees, while all the C8M compounds were non-sorptive. The systematic study of the sorption isotherms for all the compound-gas systems revealed the dependence of the types of the isotherms and that of heats of sorption upon the size of the gas molecules and that of the intercalated alkali metals. Sorption from gas mixtures was also studied for several selected systems, to manifest the actual molecular-sieving action of C24K. The structural study for the system C24K-D2 by means of neutron diffraction technique confirmed that the sorption is due to the occlusion of the gas molecules between the intercalated carbon layers of graphite, and also suggested that the compounds swell up’ by sorption. Based on these results, the relation between the structure of the compounds and the sorption characteristics was discussed.

Dynamic treatment of chemisorbed species by means of infra-red technique. Mechanism of decomposition of formic acid over alumina and silica

A new dynamic approach to study chemisorbed species during the course of catalytic reaction was applied to the catalytic dehydration of formic acid over alumina and silica gel. Formic acid is chemisorbed on alumina surface and dissociates to formate ion and a proton. Formate ions on the alumina do not decompose directly to the reaction products and a fraction desorbs rapidly by exchange with formic acid. The rate of the reaction over the alumina is proportional to the number of protons from the dissociative adsorption of formic acid on the alumina surface, and also to the partial pressure of formic acid. Consequently, the decomposition proceeds between formic acid molecules and the surface protons supplied from the dissociative adsorption of formic acid, i.e., protons from formic acid behave as he reaction sites on the catalyst surface, while the formate ion on the catalyst surface does not behave as the reaction intermediate. For the formic acid dehydration on a silica gel, the adsorption of formic acid on silica gel is non-dissociative, and the rate of the decomposition is proportional to the pressure of formic acid and is independent of the amount of formic acid adsorbed. The silica gel treated with methanol to methoxylate its surface OH to OCH3 had no catalytic activity for the decomposition, which suggests that the surface OH is the reaction site for the decomposition. It is, accordingly, concluded that the decomposition of formic acid on the dehydrating metal oxide catalysts proceeds via the protonic sites on the surface, as with sulphuric acid in the liquid phase.

Mechanism of catalytic reduction of NO by H2 or CO on a Pd foil; Role of chemisorbed nitrogen on Pd

Abstract The adsorption of NO and its reaction with H 2 over polycrystalline Pd were investigated using flash desorption technique and ultraviolet photoelectron spectroscopy under 10 −5 Pa pressure range of reactants and surface temperatures between 300 and 900 K. NO was adsorbed dissociatively onto the Pd surface above 500 K, and the heat of dissociative adsorption was ca. 126 kJ/mol. Atomic nitrogen was observed to accumulate on the Pd surface during the NO-H 2 reaction, whose desorption rate exhibited second order kinetics and is expressed as follows: V d = 10 −9.8 ± 0.3 exp (−67( kJ / mol )/ RT ) (cm 2 /atom·s). Hydrogenation of the adsorbed nitrogen proceeded rapidly at 485 K. It was confirmed from these results that formation of N 2 and NH 3 in the NO-H 2 reaction proceeds through this atomically adsorbed nitrogen. Pd-N bond energy and enthalpies of some intermediate states of the NO-H 2 reaction were estimated.