Highly Active CNT-Promoted Cu–ZnO–Al2O3 Catalyst for Methanol Synthesis from H2/CO/CO2

With types of in-house-synthesized multi-walled carbon nanotubes (CNTs) and the nitrates of the corresponding metallic components, highly active CNT-promoted Cu–ZnO–Al₂O₃ catalysts, symbolized as Cu _i Zn _j Al _k -x%CNTs, were prepared by the co-precipitation method. Their catalytic performance for methanol synthesis from H₂/CO/CO₂ was studied and compared with the corresponding CNT-free co-precipitated catalyst, Cu _i Zn _j Al _k . It was shown experimentally that appropriate incorporation of a minor amount of the CNTs into the Cu _i Zn _j Al _k could significantly increase the catalyst activity for methanol synthesis. Under the reaction conditions of 493 K, 5.0 MPa, H_2/CO/CO₂/N₂ = 62/30/5/3 (v/v), GHSV = 8000 h^-1, the observed CO conversion and methanol formation rate over a co-precipitated catalyst of Cu₆Zn₃Al₁-12.5%CNTs reached 36.8% and 0.291 mol CH₃OH s^-1 (m²-surf. Cu)^-1, which was about 44 and 25% higher than those (25.5% and 0.233 mol CH₃OH s^-1 (m²-surf. Cu)^-1) over the corresponding CNT-free co-precipitated catalyst, Cu₆Zn₃Al₁. Addition of a minor amount (10–15 wt%) of the CNTs to the Cu₆Zn₃Al₁ catalyst was found to considerably increase specific surface area, especially Cu surface area of the catalyst. H₂-TPD measurements revealed that the CNTs and the pre-reduced CNT-promoted catalyst systems could reversibly adsorb and store a considerably greater amount of hydrogen under atmospheric pressure at temperatures ranging from room temperature to 573 K. This unique feature would be beneficial for generating microenvironments with higher stationary-state concentration of active hydrogen adspecies on the surface of the functioning catalyst, especially at the interphasial active sites since the highly conductive CNTs might promote hydrogen spillover from the Cu sites to the Cu/Zn interphasial active sites, and thus be favorable for increasing the rate of the CO hydrogenation reactions. Alternatively, the operation temperature for methanol synthesis over the CNT-promoted catalysts can be 15–20 degrees lower than that over the corresponding CNT-free contrast system. This would contribute considerably to an increase in equilibrium CO conversion and CH₃OH yield. The results of the present work indicated that the CNTs could serve as an excellent promoter.     

Dehydrogenation of cyclohexanol on Cu–ZnO/SiO2 catalysts: The role of copper species

For the dehydrogenation of cyclohexanol a series of Cu–ZnO/SiO₂ catalysts with various Cu to ZnO molar ratios was prepared using the impregnation method, with the loading of copper fixed at 9.5 at.%. The catalysts were characterized by XPS, H₂–N₂O titration, BET, H₂-TPR, NH₃-TPD and XRD techniques. The results indicate that the addition of ZnO can improve the dispersion of copper species on reduced Cu–ZnO/SiO₂ (CZS) catalysts. Cu⁰ and Cu⁺ species were found on the reduced CZS catalysts surface, and the amount of Cu⁺ increased with the content of ZnO increasing. The addition of ZnO increased the acidity of the CZS catalysts. However, only Cu⁰ species can be found on the reduced Cu/SiO₂ (CS) catalyst surface. According to the reaction results, we found that the selectivity to phenol was related to the amount of Cu⁺ species, the Cu⁺ species should be the active sites for the production of phenol, the Cu⁰ is responsible for cyclohexanol dehydrogenation to cyclohexanone.     

CO2 reforming of CH4 over Ni/Mg–Al oxide catalysts prepared by solid phase crystallization method from Mg–Al hydrotalcite-like precursors

Ni supported catalysts were prepared by the solid phase crystallization (spc) method starting from hydrotalcite (HT) anionic clay based on [Mg₆Al₂(OH)₁₆CO₃ ^2–]H₂O as the precursor. The precursors were prepared by the co-precipitation method from nitrates of the metal components, and then thermally decomposed, in situ reduced to form Ni supported catalysts (spc-Ni/Mg–Al) and used for the CO₂ reforming of CH₄ to synthesis gas. Ni²⁺ can well replace the Mg²⁺ site in the hydrotalcite, resulting in the formation of highly dispersed Ni metal particles on spc-Ni/Mg–Al. The spc-catalyst thus prepared showed higher activity than those prepared by the conventional impregnation (imp) method such as Ni/-Al₂O₃ and Ni/MgO. When Ni was supported by impregnation of Mg–Al mixed oxide prepared from Mg–Al HT, the activity of imp-Ni/Mg–Al thus prepared was not so low as those of Ni/-Al₂O₃ and Ni/MgO but close to that of spc-Ni/Mg–Al. The relatively high activity of imp-Ni/Mg–Al may be due to the regeneration of the Mg–Al HT phase from the mixed oxide during the preparation, resulting in an occurring of the incorporation of Ni²⁺ in the Mg²⁺ site in the HT as seen in the spc-method. Such an effect may give rise to the formation of highly dispersed Ni metal species and afford high activity on the imp-Ni/Mg–Al.     

C9-aldehyde hydrogenation over nickel/kieselguhr catalysts in trickle bed reactor

The objective of the present study was to select the optimal catalyst and operating conditions for the manufacture of C₉-alcohol, using C₉-aldehyde and hydrogen, in a trickle bed reactor. When CaO, Ce₂O₃ or MgO was added as a promoter to the Ni/kieselguhr catalyst, the BET and Ni surface areas were increased. In the reaction for the manufacture of C₉-alcohol, using C₉-aldehyde and hydrogen in a batch reactor, a Ni–MgO/kieselguhr catalyst showed the highest activity. In addition, the catalyst using Na₂CO₃ as a precipitant showed the highest activity. According to the result of an experiment to find the optimal reaction conditions for C₉-alcohol synthesis, using C₉-aldehyde and hydrogen in a trickle bed reactor loaded with Ni–MgO/kieselguhr catalyst, the highest yield of C₉-alcohol was 91.5 wt% at 130 °C, 400 psi and WHSV = 3. The C₉-aldehyde hydrogenation performance of the Ni–MgO/kieselguhr catalyst was similar to that of a Cu/ZnO/Al₂O₃ catalyst, but superior to that of Cu–Ni–Cr–Na/Al₂O₃ and Ni–Mo/Al₂O₃ catalysts. In a long-term catalysis test, the Ni–MgO/kieselguhr catalyst showed higher stability than the Cu/ZnO/Al₂O₃ catalyst.     

Influence of magnesium and aluminium ions on the copper a.c. deposition into aluminium anodic oxide film nanotubes

The influence of Mg²⁺ and Al³⁺ ions on a.c. deposition of copper nanowires into aluminium anodic oxide film (AOF) nanotubes has been studied using cyclic voltammetry and d.c. plasma emission spectrometry. From the analysis of copper quantities deposited into the Al AOF nanotubes (m _Cu), 0.02 M MgSO₄ concentration was found to be optimal for Cu(II) solutions. Moreover, it was shown that Mg²⁺ and Al³⁺ ions not only prevent the breakdown of the barrier layer of AOF, but change the rate of copper deposition and modify the shape of the m _Cu against pH plots depending on the a.c. voltage applied. From the analysis of the quantities of magnesium (m _Mg) incorporated into the Al AOF nanotubes, presumably in the form of Mg(OH)₂, the m _Mg against pH dependences were determined in MgSO₄ and MgSO₄ + CuSO₄ solutions. An increase in m _Mg from 30 g dm^–2 to 1 mg dm^–2 at pH 1.5 and from 6 g dm^–2 to 16 g dm^–2 at pH 7.0 was found under the same a.c. treatment conditions from MgSO₄ solutions without and with Cu²⁺ ions, respectively, indicating the incorporation of Mg(OH)₂ into the Al AOF nanotubes to be lower up to about one hundred times in the case of Cu deposition. Based on the experimental results, it was suggested that incorporation of the Mg(OH)₂ particles into the Al AOF nanotubes occurred simultaneously with growing copper nanowires under a.c. bias is insignificant, if the pH of the CuSO₄ + MgSO₄ solution is 2.5.     

Thermal and chemical stability of Cu–Zn–Cr-LDHs prepared by accelerated carbonation

Layered double hydroxides Cu_xZn_6 − xCr₂(OH)₁₆(CO₃)·4H₂O with different molar ratios of Cu/Zn/Cr were synthesized by accelerated carbonation. The products were characterized by XRD, SEM, FT-IR and TG-DTG-DSC-MS. The chemical stability was tested by the modified Toxicity Characteristic Leaching Procedure (TCLP). The results showed that the products were the mixture of Cu_xZn_6 − xCr₂(OH)₁₆(CO₃)·4H₂O and (CuZn)₂(CO₃)(OH)₂, with similar thermal behavior. All products were chemically stable with reduced leaching at pH > 6 (Cu²⁺, Zn²⁺) or > 5 (Cr³⁺).     

Development of New Cu0–ZnII/Al2O3 Catalyst Supported on Copper Metallic Foam for the Production of Hydrogen by Methanol Steam Reforming

Copper metallic foam with thermal conductive properties, manufactured by S.C.P.S., has been investigated as a support for catalysts to improve thermal exchange inside the reactor for the endothermic steam reforming of methanol. Thus, we have developed a procedure for the in situ preparation of a Cu⁰–Zn^II/Al₂O₃ catalyst onto the copper metallic foam. The foam-based Cu⁰–Zn^II/Al₂O₃ catalyst shows an activity three times as high as commercial catalysts for a conversion of 74% of methanol into hydrogen.     

Adsorption and antibacterial effect of copper-exchanged montmorillonite on Escherichia coli K88

Calcium montmorillonite (Ca-MMT), sodium montmorillonite (Na-MMT) and acid-activated montmorillonite (AAM), and their Cu²⁺-exchanged montmorillonites (Cu-MMT), Cu*Ca-MMT, Cu*Na-MMT and Cu*AAM, were used to study the antibacterial activity on Escherichia coli K₈₈. AAM, Na-MMT and Ca-MMT showed some ability to reduce bacterial plate counts by 37.4%, 13.4% and 14.2%, respectively. Exchanging the montmorillonite with Cu²⁺ enhanced the antibacterial activity. The Cu*AAM, Cu*Na-MMT and Cu*Ca-MMT reduced the bacterial plate counts by 98.6%, 97.5% and 95.6%. Attempts were made to study the desorption of Cu²⁺ by washing with sterile physiological saline solution for 24 h. The washing solutions did not show a significant reduction of the bacterial counts, while the washed Cu-MMT retained their full antibacterial activity. Results from time-depending studies showed that the reduction of the bacterial counts by Cu-MMT increased during 24 h. The ranking of antibacterial activity of the three Cu-MMT was as Cu*AAM > Cu*Na-MMT > Cu*Ca-MMT. E. coli thrived optimally in a pH range from 5 to 7. Beyond this range, the bacterial counts decreased as the pH reduced the viability of the bacteria. The ranking of antibacterial activity of Cu-MMT was not affected by pH. The mechanism by which bacterial counts are reduced may involve the enhanced affinity of Cu-MMT for E. coli K₈₈ and the antibacterial activity of Cu²⁺.     

Gold supported on Mg, Al and Cu containing mixed oxides: Relation between surface properties and behavior in catalytic aerobic oxidation of 1-phenylethanol

Aerobic oxidation of 1-phenylethanol was investigated over Au deposited on flame-derived Mg–Al and Cu–Mg–Al mixed oxides with different metal ratios. A maximum in acetophenone (1-phenyl-ethanone) yield was observed for catalysts based on both Cu–Mg–Al and Mg–Al mixed oxides depending on their composition. Special attention was given to the elucidation of the role of surface basicity and the influence of the preparation route on the particle size of Au. Adsorption of CO₂ from the liquid phase combined with in situ ATR-IR and modulation excitation spectroscopy (MES) was applied to investigate differences in the surface properties of the mixed oxides as a function of the composition. Monodentate and bidentate carbonates were identified, the former being dominant on supports with high Cu contents. In order to obtain a rough quantification of the surface basicity, the retroaldolisation of 4-hydroxy-4-methyl-2-pentanone (diacetone alcohol, DAA) was chosen as a probe reaction indicating that a ratio Mg/Al = 3 results in optimal surface basicity. Moreover, the addition of Cu only lead to a partial loss in retroaldolisation activity, indicating that also the copper sites form basic centers on the surface, however, slightly weaker ones than the corresponding Mg sites. The preparation routes applied (adsorption of colloid, deposition precipitation, and impregnation) lead to different gold particle sizes characterized by mean diameters of ≈2, ≈9 and ≈30 nm, respectively. Catalytic tests using Au/Cu₁Mg₂Al₁O_x catalysts with different mean gold particle size hint towards a particle size dependence of the aerobic oxidation of 1-phenylethanol, showing higher activity for the catalyst containing gold particles of ca. 9 nm compared to those with 2 and 30 nm particles, respectively.     

An EXAFS study of Cu/ZnO and Cu/ZnO-Al2O3 methanol synthesis catalysts

Cu K-absorption edge and EXAFS measurements on binary Cu/ZnO and ternary Cu/ ZnO-Al₂O₃ catalysts of varying compositions on reduction with hydrogen at 523 K, show the presence of Cu microclusters and a species of Cu¹⁺ dissolved in ZnO apart from metallic Cu and Cu₂O. The proportions of different phases critically depend on the heating rate especially for catalysts of higher Cu content. Accordingly, hydrogen reduction with a heating rate of 10 K/min predominantly yields the metal species (>50%), while a slower heating rate of 0.8 K/min enhances the proportion of the Cu¹⁺ species ( 60%). Reduced Cu/ZnO-Al₂O₃ catalysts show the presence of metallic Cu (upto 20%) mostly in the form of microclusters and Cu¹⁺ in ZnO as the major phase ( 60%). The addition of alumina to the Cu/ZnO catalyst seems to favour the formation of Cu¹⁺/ZnO species.     

Cation composition during recrystallization of layered double hydroxides from mixed (Mg, Al) oxides

Changes in cation composition and M²⁺/M³⁺ ratio during hydrotalcite regeneration were studied. Regenerated hydrotalcites were obtained by recrystallization of mixed (Mg, Al) oxides in solutions of divalent (Mg, Zn, Co, Ni, Cu) or trivalent (Al, Fe) cations.Heating Mg–Al–CO₃ hydrotalcites with Mg/Al=2, 3 and 3.7, at 600 °C for 2 h yielded periclase-like mixed (Mg, Al) oxides (HT-P). The hydrotalcite structure was restored by dispersing oxides 48 h in water or aqueous solutions of different cations.The presence of Mg²⁺, Zn²⁺, Ni²⁺, Co²⁺, Cu²⁺ salts or of low soluble hydromagnesite increased the M²⁺/Al ratio, reaching a maximum value of 3.8. An incorporation of Zn²⁺, Ni²⁺, Co²⁺ and Cu²⁺ cations in the newly formed hydrotalcite was detected, while Mg²⁺ remained in solution. In the presence of soluble Al salts or freshly precipitated Al(OH)₃, the M²⁺/Al ratio approximated the minimal possible value of 2.The Mg/Al ratio of a hydrotalcite crystallized from a mixture of two HT-P samples with different Mg/Al ratios is equal to the weighted average value.The results obtained support the conception of the dissolution–crystallization mechanism of hydrotalcite regeneration from mixed (Mg, Al) oxides contrary to the widely accepted concept of topotactic processes.     

Effects of the preparation methods on the performance of the Cu–Cr–Fe/γ-Al2O3 catalysts for the synthesis of 2-methylpiperazine

Co-precipitation, impregnation and ultrasonic sol–gel (USG) methods have been used to prepare Cu–Cr–Fe/γ-Al₂O₃ catalysts, which were further used to synthesize 2-methylpiperazine. The catalysts were characterized by XRD, XPS, TG/DSC, BET, TPR, AAS and TEM. It is found that preparation method can greatly impact the catalytic performance of the catalysts, the Cu–Cr–Fe/γ-Al₂O₃ catalyst prepared by the ultrasonic sol–gel method proved to be the most active and stable for this reaction. The dispersion and stabilization of Cu⁰ in the reduced catalysts are attributed to the existence of CuCr₂O₄ and Fe₂O₃. A surprising copper migration was detected by XPS analysis for the Cu–Cr–Fe/γ-Al₂O₃-USG catalyst after the calcination process, which may be crucial to the high activity and stability of this catalyst.     

Microsilica or MgO grain size: Which one mostly affects the in situ spinel refractory castable expansion?

Microsilica is commonly added to alumina–magnesia castables to counterbalance the in situ spinel expansion. This effect is attained by the generation of a low-melting temperature phase, which also affects the expansive reaction kinetics. Additionally, the MgAl₂O₄ formation depends on the grain size of the reactants. The use of coarse magnesia grains results in lower Mg²⁺ dissolution and could lead, at 1500 °C, to forsterite development (Mg₂SiO₄). For finer MgO, silica was detected at the edge of the spinel grains. Considering these aspects, this work evaluated the effect of microsilica content for different magnesia grain sizes (<45 or <100 μm). Due to a faster spinel formation for the fine MgO source, microsilica counterbalanced the MgAl₂O₄ expansion. Conversely, for the coarser MgO, silica increased the Mg²⁺ dissolution, speeding up the spinel formation and expansion. Therefore, microsilica presented opposite roles, pointing out that it does not always counterbalance the spinel expansion. This work also indicated the need for a systemic approach for the expanding design of alumina–magnesia refractory castables.     

An optimum NiO content in the CO2 reforming of CH4 with NiO/MgO solid solution catalysts

Reduced NiO/MgO, with a NiO content in the range 9.2–28.6 wt%, was found to be a highly effective catalyst for the CO₂ reforming of CH₄ to CO and H₂ (at 790°C, atmospheric pressure and a space velocity of 60000 cm³g^–1h^–1). For smaller or higher NiO contents, the yield was smaller, being negligible for 4.9 wt%. In contrast to the other reforming catalysts, the new catalyst has high stability, since in the optimum NiO range the CO yield remained unchanged at 95% for 120 h without any carbon deposition. The formation of a solid solution between NiO and MgO, which was demonstrated by both X-ray diffraction and temperature-programmed reduction, is most likely responsible for the high selectivity and stability in a large range of compositions of NiO/MgO.     

Effect of Pretreatment Atmosphere on CuO/TiO2 Activities in NO + CO Reaction

Using TiO₂ as carrier, CuO/TiO₂ catalysts with different CuO loading were prepared by the impregnation method. The catalytic activities in NO+CO reaction were examined with a micro-reactor gas chromatography reaction system and the methods of TPR, XPS and NO-TPD. It was found that the catalytic activities were affected by pretreatment atmosphere, i.e. H₂ atmosphere > reduction–reoxidation > 10%CO/He > reaction gas (fresh sample). NO decomposition was better by low-valence Cu species than by high-valence Cu species, i.e. Cu⁰>Cu⁺>Cu²⁺. The XPS results indicated that Cu species on CuO/TiO₂ were Cu⁰, Cu⁺, normal Cu²⁺(Cu²⁺(I)) and chain-structured Cu²⁺(Cu²⁺(II)) as –Cu–O–Ti–O–. The activities of Cu²⁺(II) were much higher than that of Cu²⁺(I), but both species were very unstable in the reaction atmosphere and easily reduced by CO, which accounted for the variable activities of fresh catalysts with increasing reaction temperature. In NO+CO reaction, the redox process was a cycle of Cu⁺–Cu²⁺(I) at low reaction temperature but was a cycle of Cu⁰–Cu⁺ at high reaction temperature. As shown by NO-TPD, high catalytic activities could be attributed to the following factors, e.g. oxygen caves on the catalyst’s surface after pretreatment with H₂ and reduction–reoxidation, formation of Cu⁰ after pretreatment with H₂, and increment of Cu species dispersion and formation of Cu²⁺(II) after pretreatment with reduction–reoxidation.     

Effect of thermal treatment on states of molybdenum in Mo/H–ZSM-5 catalyst for methane dehydroaromatization: ESR and UV–VIS study

Valence and coordination states of molybdenum ions formed upon thermal treatment of Mo/H–ZSM-5 catalyst for methane dehydroaromatization in Ar and Ar/CH₄ media at 573–973 K have been studied by ESR and UV–VIS spectroscopy. For comparison, the characteristic ESR spectra of thermolyzed bulk ammonium heptamolybdate have been studied and analyzed in detail. The nature of earlier observed Mo⁵⁺ ions has been verified, and new paramagnetic states of molybdenum in Mo/H–ZSM-5 catalysts have been detected: Mo³⁺ ions, and Mo⁵⁺ ions in tetrahedral coordination with delocalization of unpaired electron to Al and H or Al and N atoms.     

A Cu/Zn/Al/Zr Fibrous Catalyst that is an Improved CO<Subscript>2</Subscript> Hydrogenation to Methanol Catalyst

A series of Cu/Zn/Al/Zr CO₂ hydrogenation to methanol catalysts containing different ratios of Al/Zr were prepared using a co-precipitation procedure. SEM, TEM, and XRD characterization showed that all the catalysts comprised crystallites in a fibrous structure and their Cu/Zn crystallite dispersions were better than that of a commercial (COM) catalyst. It is suggested that the high dispersion and stability of the Cu/Zn crystallites due to the fibrous structure enhanced CO₂ hydrogenation, and the added Zr component further improved the catalyst. A 5% Zr addition gave a methanol space time yield 80% higher than that on the COM catalyst.     

A highly efficient Cu/ZnO/Al2O3 catalyst via gel-coprecipitation of oxalate precursors for low-temperature steam reforming of methanol

The impact of preparation methods on the structure and catalytic behavior of Cu/ZnO/Al₂O₃ catalysts for H₂ production from steam reforming of methanol (SRM) has been reported. The results show that the nanostructured Cu/ZnO/Al₂O₃ catalyst obtained by a novel gel-coprecipitation of oxalate precursors has a high specific surface area and high component dispersion, exhibiting much higher activity in the SRM reaction as compared to the catalysts prepared by conventional coprecipitation techniques. It is suggested that the superior catalytic performance of the oxalate gel-coprecipitation-derived Cu/ZnO/Al₂O₃ catalyst could be attributed to the generation of “catalytically active” copper material with a much higher metallic copper specific surface as well as a stronger Cu–Zn interaction due to an easier incorporation of zinc species into CuC₂O₄ · x H₂O precursors as a consequence of isomorphous substitution between copper and zinc in the oxalate gel-precursors.     

The difference in the active sites for CO2 and CO hydrogenations on Cu/ZnO-based methanol synthesis catalysts

The effect of Zn in copper catalysts on the activities for both CO₂ and CO hydrogenations has been examined using a physical mixture of Cu/SiO₂+ZnO/SiO₂ and a Zn-containing Cu/SiO₂ catalyst or (Zn)Cu/SiO₂. Reduction of the physical mixture with H₂ at 573–723 K results in an increase in the yield of methanol produced by the CO₂ hydrogenation, while no such a promotion was observed for the CO hydrogenation, indicating that the active site is different for the CO₂ and CO hydrogenations. However, the methanol yield by CO hydrogenation is significantly increased by the oxidation treatment of the (Zn)Cu/SiO₂ catalyst. Thus it is concluded that the Cu–Zn site is active for the CO₂ hydrogenation as previously reported, while the Cu–O–Zn site is active for the CO hydrogenation.     

Electrical properties of V2O5–WO3/TiO2 EUROCAT catalysts evidence for redox process in selective catalytic reduction (SCR) deNOx reaction

Electrical conductivity measurements on EUROCAT V₂O₅–WO₃/TiO₂ catalyst and on its precursor without vanadia were performed at 300°C under pure oxygen to characterize the samples, under NO and under NH₃ to determine the mode of reactivity of these reactants and under two reaction mixtures ((i) 2000 ppm NO + 2000 ppm NH₃ without O₂, and (ii) 2000 ppm NO + 2000 ppm NH₃ + 500 ppm O₂) to put in evidence redox processes in SCR deNO_x reaction.It was first demonstrated that titania support contains certain amounts of dissolved W⁶⁺ and V⁵⁺ ions, whose dissolution in the lattice of titania creates an n-type doping effect. Electrical conductivity revealed that the so-called reference pure titania monolith was highly doped by heterovalent cations whose valency was higher than +4. Subsequent chemical analyses revealed that so-called pure titania reference catalyst was actually the WO₃/TiO₂ precursor of V₂O₅–WO₃/TiO₂ EUROCAT catalyst. It contained an average amount of 0.37 at.% W⁶⁺dissolved in titania, i.e. 1.07 × 10²⁰ W⁶⁺ cations dissolved/cm³ of titania. For the fresh catalyst, the mean amounts of W⁶⁺ and V⁵⁺ ions dissolved in titania were found to be equal to 1.07 × 10²⁰ and 4.47 × 10²⁰ cm⁻³, respectively. For the used catalyst, the mean amounts of W⁶⁺ and V⁵⁺ ions dissolved were found to be equal to 1.07 × 10²⁰ and 7.42 × 10²⁰ cm⁻³, respectively. Since fresh and used catalysts have similar compositions and similar catalytic behaviours, the only manifestation of ageing was a supplementary progressive dissolution of 2.9 × 10²⁰ additional V⁵⁺ cations in titania.After a prompt removal of oxygen, it appeared that NO alone has an electron acceptor character, linked to its possible ionosorption as NO⁻ and to the filling of anionic vacancies, mostly present on vanadia. Ammonia had a strong reducing behaviour with the formation of singly ionized vacancies. A subsequent introduction of NO indicated a donor character of this molecule, in opposition to its first adsorption. This was ascribed to its reaction with previously adsorbed ammonia strongly bound to acidic sites. Under NO + NH₃ reaction mixture in the absence of oxygen, the increase of electrical conductivity was ascribed to the formation of anionic vacancies, mainly on vanadia, created by dehydroxylation and dehydration of the surface. These anionic vacancies were initially subsequently filled by the oxygen atom of NO. N^o atoms, resulting from the dissociation of NO and from ammonia dehydrogenation, recombined into dinitrogen molecules. The reaction corresponded to

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. In the presence of oxygen, NO did not exhibit anymore its electron acceptor character, since the filling of anionic vacancies was performed by oxygen from the gas phase. NO reacted directly with ammonia strongly bound on acidic sites. A tentative redox mechanism was proposed for both cases.