Conversion of N2O to N2 on TiO2(1 1 0)

In this study, we examine the interaction of N₂O with TiO₂(1 1 0) in an effort to better understand the conversion of NO_x species to N₂ over TiO₂-based catalysts. The TiO₂(1 1 0) surface was chosen as a model system because this material is commonly used as a support and because oxygen vacancies on this surface are perhaps the best available models for the role of electronic defects in catalysis. Annealing TiO₂(1 1 0) in vacuum at high temperature (above about 800 K) generates oxygen vacancy sites that are associated with reduced surface cations (Ti³⁺ sites) and that are easily quantified using temperature programmed desorption (TPD) of water. Using TPD, X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), we found that the majority of N₂O molecules adsorbed at 90 K on TiO₂(1 1 0) are weakly held and desorb from the surface at 130 K. However, a small fraction of the N₂O molecules exposed to TiO₂(1 1 0) at 90 K decompose to N₂ via one of two channels, both of which are vacancy-mediated. One channel occurs at 90 K, and results in N₂ ejection from the surface and vacancy oxidation. We propose that this channel involves N₂O molecules bound at vacancies with the O-end of the molecule in the vacancy. The second channel results from an adsorbed state of N₂O that decomposes at 170 K to liberate N₂ in the gas phase and deposit oxygen adatoms at non-defect Ti⁴⁺ sites. The presence of these O adatoms is clearly evident in subsequent water TPD measurements. We propose that this channel involves N₂O molecules that are bound at vacancies with the N-end of the molecule in the vacancy, which permits the O-end of the molecule to interact with an adjacent Ti⁴⁺ site. The partitioning between these two channels is roughly 1:1 for adsorption at 90 K, but neither is observed to occur for moderate N₂O exposures at temperatures above 200 K. EELS data indicate that vacancies readily transfer charge to N₂O at 90 K, and this charge transfer facilitates N₂O decomposition. Based on these results, it appears that the decomposition of N₂O to N₂ requires trapping of the molecule at vacancies and that the lifetime of the N₂O–vacancy interaction may be key to the conversion of N₂O to N₂.     

Water vapor sensitivity of nanosized La(Co, Mn, Fe)1−x(Cu, Pd)xO3 perovskites during NO reduction by C3H6 in the presence of oxygen

A series of La(Co, Mn, Fe)_1−x(Cu, Pd)_xO₃ perovskites having high specific surface areas and nanosized crystal domains was prepared by reactive grinding. The solids were characterized by N₂ adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature programmed desorption (TPD) of O₂, NO + O₂, C₃H₆, in the absence or presence of 5% H₂O, Fourier transform infrared (FTIR) spectroscopy, as well as activity tests towards NO reduction by propene under the conditions of 3000 ppm NO, 3000 ppm C₃H₆, 1% O₂, 0 or 10% H₂O, and 50,000 h⁻¹ space velocity. The objective was to investigate the influence of H₂O addition on catalytic behavior. A good performance (100% NO conversion, 77% N₂ yield, and 90% C₃H₆ conversion) was achieved at 600 °C over LaFe_0.8Cu_0.2O₃ under a dry feed stream. With the exposure of LaFe_0.8Cu_0.2O₃ to a humid atmosphere containing 10% water vapor, the catalytic activity was slightly decreased yielding 91% NO conversion, 51% N₂ yield, and 86% C₃H₆ conversion. A competitive adsorption between H₂O vapor with O₂ and NO molecules at anion vacancies over LaFe_0.8Cu_0.2O₃ was found by means of TPD studies here. A deactivation mechanism was therefore proposed involving the occupation of available active sites by water vapor, resulting in an inhibition of catalytic activity in C₃H₆ + NO + O₂ reaction. This H₂O deactivation was also verified to be strictly reversible by removing steam from the feed.     

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.     

Catalytic decomposition of N2O over supported Rh catalysts: effects of supports and Rh dispersion

The study of catalytic decomposition of nitrous oxide to nitrogen and oxygen over Rh catalysts supported on various supports (USY, NaY, Al₂O₃, ZrO₂, FSM-16, CeO₂, La₂O₃) showed that the activities of Rh/Al₂O₃ and Rh/USY (ultrastable Y zeolite) catalysts were comparable to or higher than the other catalysts reported in the literatures. The catalytic activity of N₂O decomposition was sensitive not only to the Rh dispersion but also to the preparation variables such as the Rh precursors and the supports used. A pulsed N₂O experiment over a Rh/USY catalyst suggested that the catalytic N₂O decomposition occurs on oxygen-covered surface and that O₂ may be freed on collision of N₂O molecules with the adsorbed oxygen atoms.     

High thermal conductive β-SiC for selective oxidation of H2S: A new support for exothermal reactions

This study aims at synthesizing a new by substituting 1 atom% Pd²⁺ in ionic state in TiO₂ in the form of Ti_0.99Pd_0.01O_1.99 with oxide-ion vacancy. The catalyst was synthesized by solution combustion method and was characterized by XRD and XPS. The catalytic activity was investigated by performing CO oxidation, hydrocarbon oxidation and NO reduction. A reaction mechanism for CO oxidation by O₂ and NO reduction by CO was proposed. The model based on CO adsorption on Pd²⁺ and dissociative chemisorption of O₂ in the oxide-ion vacancy for CO oxidation reaction fitted the experimental for CO oxidation. For NO reduction in presence of CO, the model based on competitive adsorption of NO and CO on Pd²⁺, NO chemisorption and dissociation on oxide-ion vacancy fitted the experimental data. The rate parameters obtained from the model indicated that the reactions were much faster over this catalyst compared to other catalysts reported in the literature. The selectivity of N₂, defined as the ratio of the formation of N₂ and formation of N₂ and N₂O, was very high compared to other catalysts and 100% selectivity was reached at temperature of 350 °C and above. As the N₂O + CO reaction is an intermediate reaction for NO + CO reaction, it was also studied as an isolated reaction and the rate of the isolated reaction was less than that of intermediate reaction.     

The origin of N2O formation in the selective catalytic reduction of NOx by NH3 in O2 rich atmosphere on Cu-faujasite catalysts

The selective catalytic reduction (SCR) of NO_x (NO + NO₂) by NH₃ in O₂ rich atmosphere has been studied on Cu-FAU catalysts with Cu nominal exchange degree from 25 to 195%. NO₂ promotes the NO conversion at NO/NO₂ = 1 and low Cu content. This is in agreement with next-nearest-neighbor (NNN) Cu ions as the most active sites and with N_xO_y adsorbed species formed between NO and NO₂ as a key intermediate. Special attention was paid to the origin of N₂O formation. CuO aggregates form 40–50% of N₂O at ca. 550 K and become inactive for the SCR above 650 K. NNN Cu ions located within the sodalite cages are active for N₂O formation above 600 K. This formation is greatly enhanced when NO₂ is present in the feed, and originated from the interaction between NO (or NO₂) and NH₃. The introduction of selected co-cations, e.g. Ba, reduces very significantly this N₂O formation.     

The bifunctional reaction pathway and dual kinetic regimes in NOx SCR by methane over cobalt mordenite catalysts

The pathway for selective reduction of NO_x by methane over Co mordenite cataysts has been studied by comparing the rates of the individual reactions (NO oxidation, CH₄ oxidation, NO₂ reduction) with that of the combined reaction (NO + O₂ + CH₄). Co⁽⁺²⁾ was exchanged into H-MOR and Na-MOR to give catalysts with different metal loading and number of support protons. Additionally, exchanged Co⁽⁺²⁾ ions were precipitated with NaOH to produce dispersed cobalt oxide on Na-MOR. The NO oxidation rate is the same for ion exchanged Co⁽⁺²⁾ ions in H-MOR and Na-MOR, but the rate of Co⁽⁺²⁾ ions is much lower than that of cobalt oxide. NO oxidation equilibrium is obtained only for those catalysts with high metal loading, cobalt oxide or run at low GHSV. Under the conditions of selective catalytic reduction, methane oxidation by O₂ is low for all catalysts. The turnover frequency of Co on Na-MOR, however, is higher than that on H-MOR. The rate of NO₂ reduction to N₂ is directly proportional to the number of support acid sites and independent of the amount of Co. Comparison of the rates and selectivities for the individual reactions with the combined reaction of NO + O₂ + CH₄ indicates that there are two types of catalysts. For the first, the NO oxidation is in equilibrium and the rate determining step is reduction of NO₂. For these catalysts, the rate (and selectivity) for formation of N₂ is identical from NO + O₂ + CH₄ and NO₂ + CH₄. These catalysts have high metal loading and few acid sites. Nevertheless, the rate of N₂ formation increases with increasing number of protons. For the second type of catalyst, NO oxidation is not in equilibrium and is the rate limiting step. For these catalysts the rate of N₂ formation increases with increasing metal loading. Neither catalyst type, however, is optimized for the maximum formation of N₂. By using a mixture of catalysts, one with high NO oxidation activity and one with a large number of Brønsted acid sites, the rate of N₂ is greater than the weighted sum of the individual catalysts. The current results support the proposal that the pathway for selective catalytic reduction is bifunctional where metal sites affect NO oxidation, while support protons catalyze the formation of N₂.     

Effect of different oxygen species originating from the dissociation of O2, N2O and NO on the selectivity of the Pt-catalysed NH3 oxidation

An effect of oxygen species formed from O₂, N₂O and NO on the selectivity of the catalytic oxidation of ammonia was studied over a polycrystalline Pt catalyst using the temporal analysis of products (TAP) reactor. The transient experiments were performed in the temperature range between 773 and 1073 K in a sequential pulse mode with a time interval of 0.2 s between the pulses of the oxidant (O₂, N₂O and NO) and NH₃. In contrast to adsorbed oxygen species formed from NO, those from O₂ and N₂O reacted with ammonia yielding NO. It is suggested that the difference between these oxidising agents may be related to the different active sites for dissociation of O₂, N₂O and NO, where oxygen species of various Pt-O strength are formed. Weaker bound oxygen species, which are active for NO formation, originate from O₂ and N₂O rather than from NO. These species may be of bi-atomic nature.     

Mechanism of N2O decomposition over a Rh black catalyst studied by a tracer method: The reaction of N2O with O(a)

N₂O decomposition on an unsupported Rh catalyst has been studied using tracer technique in order to reveal the reaction mechanism. N₂¹⁶O was pulsed onto ¹⁸O/oxidized Rh catalyst at 220°C and desorbed O₂ molecules (m/e=32,34,36) were monitored by means of mass spectrometer. The ¹⁸O fraction in the desorbed dioxygen was the same value as that on the surface oxygen. The result shows that the O₂ molecules desorb via Langmuir–Hinshelwood mechanism, i.e., the desorption of dioxygen through the recombination of adsorbed oxygen. On the other hand, TPD measurements in He showed that desorption of oxygen from the Rh black catalyst occurred at the higher temperatures. Therefore, reaction-assisted desorption of oxygen during N₂O decomposition reaction at the low temperature was proposed.     

Comparison of catalytic activity and selectivity of Pd/(OSC + Al2O3) and (Pd + OSC)/Al2O3 catalysts

The effect of the Pd addition method into the fresh Pd/(OSC + Al₂O₃) and (Pd + OSC)/Al₂O₃ catalysts (OSC material = Ce_xZr_1−xO₂ mixed oxides) was investigated in this study. The CO + NO and CO + NO + O₂ model reactions were studied over fresh and aged catalysts. The differences in the fresh catalysts were insignificant compared to the aged catalysts. During the CO + NO reaction, only small differences were observed in the behaviour of the fresh catalysts. The light-off temperature of CO was about 20 °C lower for the fresh Pd/(OSC + Al₂O₃) catalyst than for the fresh (Pd + OSC)/Al₂O₃ catalyst during the CO + NO + O₂ reaction. For the aged catalysts lower NO reduction and CO oxidation activities were observed, as expected. Pd on OSC-containing alumina was more active than Pd on OSC material after the agings. The activity decline is due to a decrease in the number of active sites on the surface, which was observed as a larger Pd particle size for aged catalysts than for fresh catalysts. In addition, the oxygen storage capacity of the aged Pd/(OSC + Al₂O₃) catalyst was higher than that of the (Pd + OSC)/Al₂O₃ catalyst.     

Catalytic reduction of N2O over steam-activated FeZSM-5 zeolite: Comparison of CH4, CO, and their mixtures as reductants with or without excess O2

The catalytic reduction of N₂O by CH₄, CO, and their mixtures has been comparatively investigated over steam-activated FeZSM-5 zeolite. The influence of the molar feed ratio between N₂O and the reducing agents, the gas-hourly space velocity, and the presence of O₂ on the catalytic performance were studied in the temperature range of 475–850 K. The CH₄ is more efficient than CO for N₂O reduction, achieving the same degree of conversion at significantly lower temperatures. The apparent activation energy for N₂O reduction by CH₄ was very similar to that of direct N₂O decomposition (140 kJ mol⁻¹), being much lower for the N₂O reduction by CO (60 kJ mol⁻¹). This suggests that the reactions have a markedly different mechanism. Addition of CO using equimolar mixtures in the ternary N₂O + CH₄ + CO system did not affect the N₂O conversion with respect to the binary N₂O + CH₄ system, indicating that CO does not interfere in the low-temperature reduction of N₂O by CH₄. In the ternary system, CO contributed to N₂O reduction when methane was the limiting reactant. The conversion and selectivity of the reactions of N₂O with CH₄, CO, and their mixtures were not altered upon adding excess O₂ in the feed.     

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.     

Selective reduction of nitrogen oxides with various organic substances on precious metal catalysts under a high GHSV condition

Performances of Pt and Rh catalysts for the selective reduction of NO were investigated using various reducing agents under high gas hourly space velocity (GHSV). Higher activities were attained when olefins were used for both Pt and Rh catalysts, and when oxygen containing substances such as methyl tert.-butyl ether (MTBE) were used in the case of Rh catalysts. Selectivity toward N₂ for the NO converted was from 7% to 38% for Pt catalysts and from 45% to 67% for Rh catalysts. A comparison of N₂O decomposition rates on these catalysts has shown that gaseous N₂O is not a principal intermediate in the pathway for N₂ formation. The high selectivity toward N₂ for Rh/Al₂O₃ at low GHSV conditions is attributed to an additional catalytic effect of A1₂O₃, which can reduce NO₂ with propylene at temperatures as low as 350°C.     

Partial oxidation of methane by O2 and N2O to syngas over yttrium-stabilized ZrO2

Catalytic partial oxidation of methane to synthesis gas over ZrO₂ and yttrium-stabilized zirconia (YSZ) is studied using O₂ and N₂O as oxidants. ZrO₂ is much more active than YSZ in oxidation of methane with N₂O. In contrast, YSZ is significantly more active than ZrO₂ when O₂ is used as an oxidant. The presence of O₂ does not influence the rate of N₂O decomposition over ZrO₂ and YSZ, while the presence of H₂O in the system decreases N₂O conversion significantly. O₂ and N₂O are activated at different active sites. Y-induced oxygen vacancies are active for O₂ activation, whereas oxygen co-ordinatively unsaturated Zr cations (Zr-CUS) located at corners, edges, steps and kinks are responsible for N₂O activation. These sites are also capable of dissociating H₂O, resulting in competition between H₂O and N₂O. As compared with N₂O, molecular O₂ is easier to be activated over YSZ and ZrO₂.     

Catalytic decomposition of N2O over CeO2 promoted Co3O4 spinel catalyst

A series of CeO₂ promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N₂O). Addition of CeO₂ to Co₃O₄ led to an improvement in the catalytic activity for N₂O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N₂O conversion could be attained over the CoCe0.05 catalyst below 400 °C even in the presence of O₂, H₂O or NO. Methods of XRD, FE-SEM, BET, XPS, H₂-TPR and O₂-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO₂ could increase the surface area of Co₃O₄, and then improve the reduction of Co³⁺ to Co²⁺ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N₂O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO₂, are responsible for the enhancement of catalytic activity of Co₃O₄.     

Potential rare-earth modified CeO2 catalysts for soot oxidation: Part III. Effect of dopant loading and calcination temperature on catalytic activity with O2 and NO + O2

CeO₂ and CeReO_x_y catalysts are prepared by the calcination at different temperatures (y = 500–1000 °C) and having a different composition (Re = La³⁺ or Pr^3+/4+_, 0–90 wt.%). The catalysts are characterised by XRD, H₂-TPR, Raman, and BET surface area. The soot oxidation is studied with O₂ and NO + O₂ in the tight and loose contact conditions, respectively. CeO₂ sinters between 800–900 °C due to a grain growth, leading to an increased crystallite size and a decreased BET surface area. La³⁺ or Pr^3+/4+ hinders the grain growth of CeO₂ and, thereby, improving the surface catalytic properties. Using O₂ as an oxidant, an improved soot oxidation is observed over CeLaO_x_y and CePrO_x_y in the whole dopant weight loading and calcination temperature range studied, compared with CeO₂. Using NO + O₂, the soot conversion decreased over CeLaO_x_y catalysts calcined below 800 °C compared with the soot oxidation over CeO₂_y. CePrO_x_y, on the other hand, showed a superior soot oxidation activity in the whole composition and calcination temperature range using NO + O₂. The improvement in the soot oxidation activity over the various catalysts with O₂ can be explained based on an improvement in the external surface area. The superior soot oxidation activity of CePrO_x_y with NO + O₂ is explained by the changes in the redox properties of the catalyst as well as surface area. CePrO_x_y, having 50 wt.% of dopant, is found to be the best catalyst due to synergism between cerium and praseodymium compared to pure components. NO into NO₂ oxidation activity, that determines soot oxidation activity, is improved over all CePrO_x catalysts.     

Catalytic reduction of NO by propene over LaCo1−xCuxO3 perovskites synthesized by reactive grinding

One series of LaCo_1−xCu_xO₃ perovskites with high specific surface area was prepared by the new method designated as reactive grinding. These solids were characterized by N₂ adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), H₂-temperature programmed reduction (TPR), O₂-temperature programmed desorption (TPD), NO + O₂-TPD, C₃H₆-TPD, NO + O₂-temperature programmed surface reaction (TPSR) under C₃H₆/He flow as well as catalytic reduction of NO activity tests. The catalytic performance of unsubstituted sample is poor with a maximum conversion to N₂ of 19% at 500 °C at a space velocity of 55,000 h⁻¹ (3000 ppm NO, 3000 ppm C₃H₆, 1% O₂ in helium) but it is improved by incorporation of Cu into the lattice. A maximal N₂ yield of 46% was observed over LaCo_0.8Cu_0.2O₃ under the same conditions. Not only the abundance of -oxygen but also the mobility of β-oxygen of lanthanum cobaltite was remarkably enhanced by Cu substitution according to O₂-TPD and H₂-TPR studies. The better performance of Cu-substituted samples is likely to correspond to the essential nature of Cu and facility to form nitrate species in NO transformation conditions. In the absence of O₂, the reduction of NO by C₃H₆ was performed over LaCo_0.8Cu_0.2O₃, leading to a maximal conversion to N₂ of 73% accompanied with the appearance of some organo nitrogen compounds (identified as mainly C₃H₇NO₂). Subsequently, a mechanism involving the formation of an organic nitro intermediate, which further converts into N₂, CO₂ and H₂O via isocyanate, was proposed. Gaseous oxygen acts rather as an inhibitor in the reaction of NO and C₃H₆ over highly oxidative LaCo_0.8Cu_0.2O₃ due to the heavily unselective combustion of C₃H₆ by O₂.     

Catalytic reduction of NOx with H2/CO/CH4 over PdMOR catalysts

Conversion of NO_x with reducing agents H₂, CO and CH₄, with and without O₂, H₂O, and CO₂ were studied with catalysts based on MOR zeolite loaded with palladium and cerium. The catalysts reached high NO_x to N₂ conversion with H₂ and CO (>90% conversion and N₂ selectivity) range under lean conditions. The formation of N₂O is absent in the presence of both H₂ and CO together with oxygen in the feed, which will be the case in lean engine exhaust. PdMOR shows synergic co-operation between H₂ and CO at 450–500 K. The positive effect of cerium is significant in the case of H₂ and CH₄ reducing agent but is less obvious with H₂/CO mixture and under lean conditions. Cerium lowers the reducibility of Pd species in the zeolite micropores. The catalysts showed excellent stability at temperatures up to 673 K in a feed with 2500 ppm CH₄, 500 ppm NO, 5% O₂, 10% H₂O (0–1% H₂), N₂ balance but deactivation is noticed at higher temperatures. Combining results of the present study with those of previous studies it shows that the PdMOR-based catalysts are good catalysts for NO_x reduction with H₂, CO, hydrocarbons, alcohols and aldehydes under lean conditions at temperatures up to 673 K.     

Two conversion maxima at 373 and 573 K in the reduction of nitrogen monoxide with hydrogen over Pd/TiO2 catalyst

Supporting Pt and Pd catalysts have been examined for the reduction of NO with H₂ in the presence of oxygen and moisture. All catalysts showed a conversion maximum in the NO reduction at around 373 K. An additional conversion maximum was found to appear at around 573 K over several metal oxides supporting Pd, and Pd/TiO₂ gave the highest conversion at around 573 K among the catalysts tested. In the reaction at 373 K, NO might be reduced directly by H₂ both on Pt and Pd catalysts to give N₂ and N₂O. At the conversion maximum of the Pd/TiO₂ catalyst at 575 K, however, in situ generated NO₂ seems to react with H₂.     

Catalytic reduction of NO by CO over NiO/CeO2 catalyst in stoichiometric NO/CO and NO/CO/O2 reaction

A new catalyst composed of nickel oxide and cerium oxide was studied with respect to its activity for NO reduction by CO under stoichiometric conditions in the absence as well as the presence of oxygen. Activity measurements of the NO/CO reaction were also conducted over NiO/γ-Al₂O₃, NiO/TiO₂, and NiO/CeO₂ catalysts for comparison purposes. The results showed that the conversion of NO and CO are dependent on the nature of supports, and the catalysts decreased in activity in the order of NiO/CeO₂ > NiO/γ-Al₂O₃ > NiO/TiO₂. Three kinds of CeO₂ were prepared and used as support for NiO. They are the CeO₂ prepared by (i) homogeneous precipitation (HP), (ii) precipitation (PC), and (iii) direct decomposition (DP) method. We found that the NiO/CeO₂(HP) catalyst was the most active, and complete conversion of NO and CO occurred at 210 °C at a space velocity of 120,000 h⁻¹. Based on the results of surface analysis, a reaction model for NO/CO interaction over NiO/CeO₂ has been proposed: (i) CO reduces surface oxygen to create vacant sites; (ii) on the vacant sites, NO dissociates to produce N₂; and (iii) the oxygen originated from NO dissociation is removed by CO.