Study of the NOχ reaction with reducing gases on Fe/ZrO2 catalyst

The Fe/ZrO₂ catalyst (1% Fe by weight) shows a strong adsorption capacity toward the nitric oxide (at room temperature the ratio NOFe is ca. 0.5) as a consequence of the formation of a highly dispersed iron phase after reduction at 500–773 K. Nitric oxide is adsorbed mainly as nitrosyl species on the reduced surface where the Fe²⁺ sites are prevailing, but it is easily oxidised by oxygen forming nitrito and nitrato species adsorbed on the support. However, in the presence of a reducing gas such as hydrogen, carbon monoxide, propane and ammonia at 473–573 K the Fe-nitrosyl species react producing nitrogen, nitrous oxide, carbon dioxide and water, as detected by FTIR and mass spectrometers. The results show that nitric oxide reduction is more facile with hydrogen containing molecules than with CO, probably due the co-operation of spillover effects. Experiments carried out with the same gases in the presence of oxygen show, however, a reduced dissociative activity of the surface iron sites toward the species NO_χ formed by NO oxidation and therefore the reactivity is shifted to higher temperatures.     

Activation and characterization of Fe---Mn---SO4/ZrO2 catalysts

Changes in Fe---Mn---SO₄²⁻/ZrO₂ catalyst formulations during activation have been observed. In air or an inert gas, the added salt, such as iron and/or manganese nitrate, decomposes over a temperature range of about 200–400°C to produce nitric oxide, oxygen and iron and/or manganese oxide. The crystallization of zirconia occurs at 450°C; when the sample contains sulfate the exothermic event occurs at a temperature that is about 200°C higher. Heating in the presence of hydrogen causes the evolution of nitric oxide to occur over a narrow temperature range and at a lower temperature than when the sample is heated in helium or air. It appears that the nitrate ions associated with Fe, Mn and Zr decompose to produce nitric oxide, and presumably water, at different temperatures when the sample is heated in the presence of hydrogen. Heating samples of sulfated zirconia containing iron and/or manganese in hydrogen causes sulfur evolution at a lower temperature, and a significant fraction of it in the form of H₂S.     

Reduction of NOx in C3H6/air mixtures over Cu/Al2O3 catalysts

The reduction of nitrogen oxides by propene in the presence of air under net oxidising conditions has been studied for two Cu/alumina catalysts of low (1%) and high (5%) copper loadings in a flow microreactor and by DRIFT. The reaction was studied in the range 473–773 K using mixtures of 2.5% NO, 1% C₃H₆ and 10% O₂ with a balance of nitrogen or helium, using samples which were pretreated in air at 673 K and also over samples which had been pre-exposed to SO₂ at 473 K. The surface species present under reaction conditions have been identified and the sensitivity of their adsorption sites in the two different loaded catalysts to SO₂ pre-treatment has been investigated. SO₂ adsorption enhanced NO adsorption at 473 K in the absence of oxygen and, in reaction, enhanced formation of NCO species leading to increased levels of adsorbed CO as a decomposition product.     

Nitrous oxide formation in low temperature selective catalytic reduction of nitrogen oxides with V2O5/TiO2 catalysts

Nitric oxide and nitric dioxide compounds (NO_x) present in stack gases from nitric acid plants are usually eliminated by selective catalytic reduction (SCR) with ammonia. In this process, small quantities of nitrous oxide (N₂O) are produced. This undesirable molecule has a high greenhouse gas potential and a long lifetime in the atmosphere, where it can contribute to stratospheric ozone depletion. The influence of catalyst composition and some operating variables were evaluated in terms of N₂O formation, using V₂O₅/TiO₂ catalysts. High vanadia catalyst loading, nitric oxide inlet concentration and reaction temperature increase the generation of this undesirable compound. The results suggest that adsorbed ammonia not only reacts with NO via SCR, but also with small quantities of oxygen activated by the presence of NO. The mechanism proposed for N₂O generation at low temperature is based on the formation of surface V–ON species which may be produced by the partial oxidation of dissociatively adsorbed ammonia species with NO + O₂ (eventually NO₂). When these active sites are in close proximity they can interact to form an N₂O molecule. This mechanism seems to be affected by changes in the active site density produced by increasing the catalyst vanadia loading.     

Low temperature SCR of NO with NH3 over carbon nanotubes supported vanadium oxides

A novel multiwalled carbon nanotube (CNTs) supported vanadium catalyst was prepared. The structure of catalyst prepared was characterized by TEM, BET, FTIR, XRD and temperature-programmed desorption (TPD) methods. The results indicated that vanadium particles were highly dispersed on the wall of carbon nanotubes. The V₂O₅/CNT catalysts showed good activities in the SCR of NO with a temperature range of 373–523 K. The Lewis acid sites on the surface of V₂O₅/CNT are the active sites for the selective catalytic reduction (SCR) of NO with NH₃ at low temperatures. It was suggested that the reaction path might involve the adsorbed NH₃ species reacted with NO from gaseous phase and as well as the adsorbed NO₂ species. The diameter of CNTs showed positive effect on the activities of the catalysts. Under the reaction conditions of 463 K, 0.1 Mpa, NH₃/NO = 1, GHSV = 35,000 h⁻¹, and V₂O₅ loading of 2.35 wt%, the outer diameter of CNTs of 60–100 nm, the NO conversion was 92%.     

Mechanistic insights into the formation of N2O and N2 in NO reduction by NH3 over a polycrystalline platinum catalyst

The reaction pathways of N₂ and N₂O formation in the direct decomposition and reduction of NO by NH₃ were investigated over a polycrystalline Pt catalyst between 323 and 973 K by transient experiments using the temporal analysis of products (TAP-2) reactor. The interaction between nitric oxide and ammonia was studied in the sequential pulse mode applying ¹⁵NO. Differently labelled nitrogen and nitrous oxide molecules were detected. In both, direct NO decomposition and NH₃–NO interaction, N₂O formation was most marked between 573 and 673 K, whereas N₂ formation dominated at higher temperatures. An unusual interruption of nitrogen formation in the ¹⁵NO pulse at 473 K was caused by an inhibiting effect of adsorbed NO species. The detailed analysis of the product distribution at this temperature clearly indicates different reaction pathways leading to the product formation. Nitrogen formation occurs via recombination of nitrogen atoms formed by dissociation of nitric oxide or/and complete dehydrogenation of ammonia. N₂O is formed via recombination of adsorbed NO molecules. Additionally, both products are formed via interactions between adsorbed ammonia fragments and nitric oxide.     

Micro-kinetic analysis of direct N2O decomposition over steam-activated Fe-silicalite from transient experiments in the TAP reactor

Mechanistic and kinetic aspects of the direct decomposition of N₂O over steam-activated Fe-silicalite were investigated by transient experiments in vacuum (N₂O peak pressure of ca. 10 Pa) using the temporal analysis of products (TAP) reactor in the temperature range of 773–848 K. The transient responses of N₂O, N₂, and O₂ obtained upon N₂O decomposition were fitted to different micro-kinetic models. Through model discrimination it was concluded that both free iron sites and iron sites with adsorbed mono-atomic oxygen (*O) species are active for N₂O decomposition. Oxygen formation occurs via decomposition of bi-atomic (*O₂) oxygen species adsorbed over the iron site. This bi-atomic oxygen species originates from another bi-atomic oxygen species (O*O), which is initially formed via interaction of N₂O with iron site possessing mono-atomic oxygen species (*O). Based on our modeling, the recombination of two mono-atomic oxygen (*O) species or direct O₂ formation via reaction of N₂O with *O can be excluded as potential reaction pathways yielding gas-phase O₂. The simulation results predict that the overall rate of N₂O decomposition is controlled by regeneration of free iron sites via a multi-step oxygen formation at least below 700 K.     

Role of active oxygen transients in selective catalytic reduction of N2O with CH4 over Fe-zeolite catalysts

Sharp NO and O₂ desorption peaks, which were caused by the decomposition of nitro and nitrate species over Fe species, were observed in the range of 520–673 K in temperature-programmed desorption (TPD) from Fe-MFI after H₂ treatment at 773 K or high-temperature (HT) treatment at 1073 K followed by N₂O treatment. The amounts of O₂ and NO desorption were dependent on the pretreatment pressure of N₂O in the H₂ and N₂O treatment. The adsorbed species could be regenerated by the H₂ and N₂O treatment after TPD, and might be considered to be active oxygen species in selective catalytic reduction (SCR) of N₂O with CH₄. However, the reaction rate of CH₄ activation by the adsorbed species formed after the H₂ and N₂O or the HT and N₂O treatment was not so high as that of the CH₄ + N₂O reaction over the catalyst after O₂ treatment. The simultaneous presence of CH₄ and N₂O is essential for the high activity of the reaction, which suggests that nascent oxygen species formed by N₂O dissociation can activate CH₄ in the SCR of N₂O with CH₄.     

Decomposition of nitric oxide over perovskite oxide catalysts: effect of CO2, H2O and CH4

Direct nitric oxide decomposition over perovskites is fairly slow and complex, its mechanism changing dramatically with temperature. Previous kinetic study for three representative compositions (La_0.87Sr_0.13Mn_0.2Ni_0.8O_3−δ, La_0.66Sr_0.34Ni_0.3Co_0.7O_3−δ and La_0.8Sr_0.2Cu_0.15Fe_0.85O_3−δ) has shown that depending on the temperature range, the inhibition effect of oxygen either increases or decreases with temperature. This paper deals with the effect of CO₂, H₂O and CH₄ on the nitric oxide decomposition over the same perovskites studied at a steady-state in a plug-flow reactor with 1 g catalyst and total flowrates of 50 or 100 ml/min of 2 or 5% NO. The effect of carbon dioxide (0.5–10%) was evaluated between 873 and 923 K, whereas that of H₂O vapor (1.6 or 2.5%) from 723 to 923 K. Both CO₂ and H₂O inhibit the NO decomposition, but inhibition by CO₂ is considerably stronger. For all three catalysts, these effects increase with temperature. Kinetic parameters for the inhibiting effects of CO₂ and H₂O over the three perovskites were determined. Addition of methane to the feed (NO/CH₄=4) increases conversion of NO to N₂ about two to four times, depending on the initial NO concentration and on temperature. This, however, is still much too low for practical applications. Furthermore, the rates of methane oxidation by nitric oxide over perovskites are substantially slower than those of methane oxidation by oxygen. Thus, perovskites do not seem to be suitable for catalytic selective NO reduction with methane.     

Selective reduction of nitric oxide with propene over platinum-group based catalysts: Studies of surface species and catalytic activity

The reduction of nitric oxide by propene in the presence of oxygen over platinum-group metals supported on TiO₂, ZnO, ZrO₂, and Al₂O₃ has been investigated by combined diffuse reflectance FT-IR spectroscopy and catalytic activity studies under flow reaction conditions at 523–673 K and atmospheric pressure. The catalytic activity for the selective reduction of nitric oxide and the intensity of the IR bands due to reaction species depended strongly on the nature of the support, type of supported metal, reaction time and temperature. The main surface species detectable by IR were adsorbed hydrocarbons (2900–3080 cm⁻¹), isocyanate (2180, and 2232–2254 cm⁻¹), cyanide (2125 cm⁻¹), nitrosonium (1901 cm⁻¹), CO₂ (2343–2357 cm⁻¹), CO (2058 cm⁻¹) and carbonate (1300–1650 cm⁻¹) species. In the case of rhodium containing catalysts, when supported on Al₂O₃, they exhibited both the highest concentration of surface species and the highest activity for nitric oxide reduction and selectivity to nitrogen. The catalytic activity and the IR intensities of the nitrosonium and isocyanate bands increased with reaction temperature, reached their maximum between 570 and 620 K, and then decreased at higher temperatures. The IR band intensities due to nitrogen containing surface species were found to be strongly correlated to the activity for nitric oxide conversion and only slightly related to the selectivity to dinitrogen.     

Kinetics of propylene epoxidation using H2 and O2 over a gold/mesoporous titanosilicate catalyst

The oxidation of propylene to propylene oxide (PO) with hydrogen–oxygen mixtures was studied on gold supported on the mesoporous titanium silicate, Ti-TUD. The catalyst gave stable activity at low conversions of propylene (<6%) and high selectivity to PO (>95%). Kinetic data were fit to a power-rate law and gave the following expression: r_PO = k(H₂)^0.54(O₂)^0.24(C₃H₆)^0.36. The fractional orders in hydrogen, oxygen, and propylene indicated that these reactants interacted with the catalyst to form species that led to the final PO product. The catalyst likely operated by the commonly accepted mechanism of hydrogen peroxide production on gold sites, and epoxidation on titanium centers. Carbon dioxide was formed primarily from further oxidation of PO rather than the oxidation of propylene, while water was produced from the reaction of hydrogen and oxygen.     

Effect of oxygen concentration on the NOx reduction with ammonia over V2O5–WO3/TiO2 catalyst

The catalytic reduction of NO_x in the typical operation temperatures and oxygen concentrations of diesel engines has been studied in the presence of V3W9Ti in a tubular flow reactor. The results have shown that the selective catalytic reduction is strongly affected by the oxygen concentration in low temperature range (150–275 °C). At higher temperatures, the reaction becomes independent of the O₂ concentration. The rate of the selective catalytic reduction of NO with ammonia may be considerably enhanced by converting part of the NO into NO₂. DRIFT measurements have shown that NH₃ and NO₂ are adsorbed on the catalyst surface on the contrary of NO. The experiments have shown that the decrease in N₂ selectivity of the SCR reaction is mainly due to the SCO of ammonia and to the formation of nitrous oxide.     

Study of the reaction of NOx and soot on Fe2O3 catalyst in excess of O2

This study addresses the catalytic reaction of NO_x and soot into N₂ and CO₂ under O₂-rich conditions. To elucidate the mechanism of the soot/NO_x/O₂ reaction and particularly the role of the catalyst -Fe₂O₃ is used as model sample. Furthermore, a series of examinations is also made with pure soot for reference purposes. Temperature programmed oxidation and transient experiments in which the soot/O₂ and soot/NO reaction are temporally separated show that the NO reduction occurs on the soot surface without direct participation of the Fe₂O₃ catalyst. The first reaction step is the formation of CC(O) groups that is mainly associated with the attack of oxygen on the soot surface. The decomposition of these complexes leads to active carbon sites on which NO is adsorbed. Furthermore, the oxidation of soot by oxygen provides a specific configuration of active carbon sites with suitable atomic orbital orientation that enables the chemisorption and dissociation of NO as well as the recombination of two adjacent N atoms to evolve N₂. Moreover, carbothermal reaction, high resolution transmission electron microscopy and isotopic studies result in a mechanistic model that describes the role of the Fe₂O₃ catalyst. This model includes the dissociative adsorption of O₂ on the iron oxide, surface migration of the oxygen to the contact points of soot and catalyst and then final transfer of O to the soot. Moreover, our experimental data suggest that the contact between both solids is maintained up to high conversion levels thus resulting in continuous oxygen transfer from catalyst to soot. As no coordinative interaction of soot and Fe₂O₃ catalyst is evidenced by diffuse reflectance infrared Fourier transform spectroscopy a van der Waals type interaction is supposed.     

Nb2O5-supported WO3: a comparative study with WO3/Al2O3

WO₃/Nb₂O₅-supported samples prepared by impregnation are characterised by X-ray diffraction (XRD), Raman spectroscopy and X-ray absorption spectroscopy (XAS) at the W–L₃ absorption edge, as well as temperature programmed reduction (TPR) and FT-IR monitoring of pyridine adsorption. Results are compared with those obtained for WO₃/Al₂O₃ samples prepared in the same conditions, showing that niobia is able to disperse tungsta better than alumina does. Formation of a crystalline WO₃ needs larger tungsten contents on niobia than on alumina, since tungsten solution into niobia is easier than into alumina. Raman and XAS spectra recorded under ambient conditions suggest that similar WO_x species are formed on both supports at tungsten contents 0.5–1 theoretical monolayers; however, TPR results for the low tungsten loaded samples indicate that, when reduction starts (always at temperatures higher than 700 K under H₂/Ar flow) there is a larger concentration of tetrahedral [WO₄] species on alumina, than on niobia. Samples with low tungsten loading have been tested in isopropanol decomposition and ethylene oxidation, following both processes by FT-IR of adsorbed species up to 673 K. Results show that adsorption of ethylene on WO₃/Nb₂O₅ yields acetaldehyde and acetate at 473 K, while this adsorption is non-reactive either on the supports or on WO₃/Al₂O₃. Isopropanol adsorbs dissociatively on both supports, leading to acetone and propene formation on tungsta–niobia, but only propene on tungsta–alumina, probably due to the larger reducibility of the tungsten-containing phases.     

Pt–Ba–Al2O3 for NOx storage and reduction: Characterization of the dispersed species

Pt–Ba–Al₂O₃ active and selective for NO_x storage and selective reduction to N₂ has been prepared and tested. Characterization of the parent Al₂O₃, Pt–Al₂O₃ and Ba–Al₂O₃ materials, as well as of Pt–Ba–Al₂O₃ catalyst in the oxidized, reduced and sulphated state has been performed by FT-IR spectroscopy of low-temperature adsorbed carbon monoxide and of adsorbed acetonitrile. XRD, TEM and XPS analyses have also been performed. Evidence for the predominance of Ba species, which are highly dispersed on the alumina support surface, and may be carbonated or sulphated, has been provided. Competitive interaction of Pt and Ba species with the surface sites of alumina has also been found.     

Design of a novel bifunctional catalyst IrFe/Al2O3 for preferential CO oxidation

In this study, a novel bifunctional catalyst IrFe/Al₂O₃, which is very active and selective for preferential oxidation of CO under H₂-rich atmosphere, has been developed. When the molar ratio of Fe/Ir was 5/1, the IrFe/Al₂O₃ catalyst performed best, with CO conversion of 68% and oxygen selectivity towards CO₂ formation of 86.8% attained at 100 °C. It has also been found that the impregnation sequence of Ir and Fe species on the Al₂O₃ support had a remarkable effect on the catalytic performance; the activity decreased following the order of IrFe/Al₂O₃ > co-IrFe/Al₂O₃ > FeIr/Al₂O₃. The three catalysts were characterized by XRD, H₂-TPR, FT-IR and microcalorimetry. The results demonstrated that when Ir was supported on the pre-formed Fe/Al₂O₃, the resulting structure (IrFe/Al₂O₃) allowed more metallic Ir sites exposed on the surface and accessible for CO adsorption, while did not interfere with the O₂ activation on the FeO_x species. Thus, a bifunctional catalytic mechanism has been proposed where CO adsorbed on Ir sites and O₂ adsorbed on FeO_x sites; the reaction may take place at the interface of Ir and FeO_x or via a spill-over process.     

Decomposition of CH4 over supported Ir catalysts

The decomposition of methane and its conversion into higher hydrocarbons have been investigated on supported Ir catalysts. The effects of temperature, flow rate and support materials have been examined. The interaction of CH₄ with iridium has been observed at as low a temperature as 473 K. As a result, hydrogen, a small amount of ethane and surface carbonaceous species were produced. With increase of the temperature, the extent of the decomposition significantly increased. At 773 K, the initial conversion varied between 2.0–5.0%, which decreased to low values in a short reaction time. Taking into account the dispersion of Ir, the most effective sample in the decomposition of CH₄ was Ir/MgO. By means of Fourier transform infrared spectroscopy adsorbed CH₃ was identified as a reaction intermediate of methane decomposition. Temperature programmed reactions revealed that the reactivity of surface carbon produced in the decomposition of CH₄ depends on the nature of the support. Hydrogenation of the most reactive carbonaceous species led to the production of aliphatic hydrocarbons up to six carbon atoms.     

Development of a transient kinetic model for the CO oxidation by O2 over a Pt/Rh/CeO2/γ-Al2O3 three-way catalyst

A transient kinetic model was developed for the CO oxidation by O₂ over a Pt/Rh/CeO₂/γ-Al₂O₃ three-way catalyst. The experiments which were modelled consisted of periodically switching between a feed stream containing 0.5 mol% CO in helium and a feed stream containing 0.5 mol% O₂ in helium, with a frequency from 0.1 to 0.25 Hz, in the temperature range 393–433 K. These temperatures are representative for cold start conditions. The transient experiments yield information about the reaction mechanism. A transient kinetic model based on elementary reaction steps was developed which describes the experimental data in the above mentioned range of experimental conditions adequately. The kinetic model consists of two monofunctional and one bifunctional contribution. The first monofunctional reaction path comprises competitive adsorption of CO and O₂ on the noble metal surface followed by a surface reaction. The second monofunctional reaction path consists of CO adsorption on an oxygen atom adsorbed on the noble metal surface, followed by a reaction to CO₂. The bifunctional reaction path involves a reaction between CO adsorbed on the noble metal surface and oxygen from ceria at the noble metal/ceria interface. Also, reversible adsorption of carbon dioxide on the support is taken into account. The kinetic parameters, i.e. preexponential factors and activation energies for the different elementary reaction steps, and the oxygen storage capacity were estimated using multi-response non-linear regression analysis of the oxygen, carbon monoxide and carbon dioxide outlet concentrations.     

The effect of hydrogen on the selective catalytic reduction of NO in excess oxygen over Ag/Al2O3

The selective catalytic reduction (SCR) of nitrogen oxides (NO_x) by propane in the presence of H₂ on sol–gel prepared Ag/Al₂O₃ catalysts (0.5–5 wt.% Ag) was investigated. It was confirmed that hydrocarbon-assisted SCR of NO_x is remarkably enhanced by co-feeding hydrogen to a lean exhaust gas mixture (λ>1), attaining considerable activity within a wide temperature window (470–825 K). The samples had marginal activity at 575 K without co-fed H₂, but achieved up to 60% NO_x conversion in the presence of H₂ at a space velocity of 30,000 h⁻¹. NO₂ as NO_x feed component is not converted to N₂ by C₃H₈ to a substantial extent under lean conditions. This points to an activation route of NO through direct conversion to adsorbed nitrite/nitrate or to a dissociation of NO over Ag⁰, formed through short-term reduction by H₂. The nature of Ag species was characterized by X-ray diffraction, temperature-programmed reduction, pulse thermoanalytical measurements, electron microscopy and FTIR spectroscopy. It could be shown that Ag₂O nano-sized clusters are predominantly present on all samples, whereas formation of silver aluminate could not be confirmed. Nano-sized Ag₂O clusters can reversibly be reduced/reoxidized by H₂. A silver loading higher than 2 wt.% leads to a part of Ag₂O particles, which are thermally decomposed during calcination at 800 K or higher. The catalytic role of this metallic silver is still unclear. Formal kinetic analysis of catalytic data revealed that the activation energy of the overall reaction is significantly lowered in the presence of H₂. The presence of water does not change the activation energy. It is concluded that hydrogen reduces the nano-sized Ag₂O clusters to Ag⁰ on a short-term scale. Zero-valent silver promotes a dissociation pathway of NO_x conversion. The fact that more oxidized ad-species (nitrite/nitrate) are observed in the presence of H₂ is attributed to a dissociative activation of gas-phase oxygen on Ag⁰.     

Methane steam reforming over Ni/Ce–ZrO2 catalyst: Influences of Ce–ZrO2 support on reactivity, resistance toward carbon formation, and intrinsic reaction kinetics

Ni/Ce–ZrO₂ showed good methane steam reforming performance in term of stability toward the deactivation by carbon deposition. It was first observed that the catalyst with Ce/Zr ratio of 3/1 showed the best activity among Ni/Ce–ZrO₂ samples with the Ce/Zr ratios of 1/0, 1/1, 1/3, and 3/1. Temperature-programmed oxidation (TPO) experiments indicated the excellent resistance toward carbon formation for this catalyst, compared to conventional Ni/Al₂O₃; the requirement of inlet H₂O/CH₄ to operate without the formation of carbon species is much lower. These benefits are related to the high oxygen storage capacity (OSC) of Ce–ZrO₂. During the steam reforming process, in addition to the reactions on Ni surface (*), the redox reactions between the gaseous components present in the system and the lattice oxygen (O_x) on Ce–ZrO₂ surface also take place. Among these reactions, the redox reactions between the high carbon formation potential compounds (CH₄, CH_x-*n and CO) and the lattice oxygen (O_x) can prevent the formation of carbon species from the methane decomposition and Boudard reactions, even at low inlet H₂O/CH₄ ratio (1.0/1.0).

Regarding the intrinsic kinetic studies in the present work, the reaction order in methane over Ni/Ce–ZrO₂ was observed to be approximately 1.0 in all conditions. The dependence of steam on the rate was non-monotonic, whereas addition of oxygen as an autothermal reforming promoted the rate but reduced CO and H₂ production selectivities. The addition of a small amount of hydrogen increased the conversion of methane, however, this positive effect became less pronounced and the methane conversion was eventually inhibited when high hydrogen concentration was added. Ni/Ce–ZrO₂ showed significantly stronger negative impact of hydrogen than Ni/Al₂O₃. The redox mechanism on ceria proposed by Otsuka et al. [K. Otsuka, T. Ushiyama, I. Yamanaka, Chem. Lett. (1993) 1517; K. Otsuka, M. Hatano, A. Morikawa, J. Catal. 79 (1983) 493; K. Otsuka, M. Hatano, A. Morikawa, Inorg. Chim. Acta 109 (1985) 193] can explain this high inhibition.