Influence of Hydrocarbon Chemisorption on the NOx Adspecies over Supported Noble-Metal Catalysts

In situ and time-resolved DRIFT methods were used to monitor the change in NO _x adspecies on Pt(1%)–TiO₂ and Rh(1%)–TiO₂ catalysts during interaction with propene with the aim to determine whether or not propene chemisorption and interaction with the catalyst induces a change in the nature of the NO _x adspecies prior to their reduction. The nature of NO _x adspecies produced by interaction of the NO + O₂/He feed with the catalyst is different on Pt- and Rh–TiO₂ (in the Pt–TiO₂ catalyst the IR more intense adspecies are nitrate, while in the Rh–TiO₂ catalyst nitrosyl species are the IR more intense), but modification of the nature of the adspecies prior to their conversion is observed in both cases. The interpretation of the data provides indication about the nature of the reactive NO _x species and the presence of multiple pathways in the mechanism of their conversion.     

Role and importance of oxidized nitrogen oxide adspecies on the mechanism and dynamics of reaction over copper-based catalysts

The formation of nitrate and NO₂ adspecies over Cu/MFI and copper-on-alumina catalysts and their role in the mechanism of reaction is discussed on the basis of FT-IR results and catalytic tests in unsteady-state conditions. Three specific cases are discussed: (i) reduction of NO by propane/O₂ over Cu/MFI, (ii) conversion of NO by NH₃/O₂ over copper-on-alumina catalysts and (iii) oxygen-promoted reduction of NO in the absence of reductants over Cu/MFI. The formation of nitrate species leads to self-deactivation, but Cu²⁺-NO₂ like adspecies are suggested to be a key intermediate in the reduction of NO to N₂ in all three cases examined.     

Analysis of oxidation states of vanadium in vanadia–titania catalysts by the IR spectra of adsorbed NO

Adsorption of NO on vanadia–titania samples pre-subjected to different reduction treatments has been studied by FTIR spectroscopy. When the NO adsorption is performed at 85 K on oxidized samples, antisymmetric NONO species, typical for V⁵⁺ sites, are detected and characterized by bands at 1779 and 1686 cm⁻¹. At ambient temperature, however, adsorption is negligible and only with time reactive adsorption occurs producing NO⁺ (2120 cm⁻¹), nitro/nitrato species (bands in the 1650–1100 cm⁻¹ region) and weakly adsorbed NO (broad band at 1915 cm⁻¹). Adsorption of NO at ambient temperature on reduced samples results in the formation of two types of species: (i) V⁴⁺(NO)₂ dinitrosyls characterized by ν_s(NO) and ν_as(NO) at 1903–1880 and 1769–1753 cm⁻¹, respectively, and (ii) V³⁺(NO)₂ complexes, which give rise to ν_s(NO) at 1834–1822 cm⁻¹ and ν_as(NO) at 1697–1685 cm⁻¹. At low temperature the dinitrosyls are transformed into species in which more than one (NO)₂ dimer is attached to one cationic site. Addition of O₂ to NO, preadsorbed on reduced vanadia–titania samples, results in a fast oxidation of the V³⁺(NO)₂ species, whereas the V⁴⁺(NO)₂ complexes are more stable and do not disappear completely in the presence of oxygen. The results obtained suggest that NO is a convenient probe molecule for the analysis of the oxidation state of vanadium in vanadia–titania catalysts. To prevent oxidation of reduced vanadium sites, low equilibrium pressures of NO and registration of the IR spectrum soon after the NO admission are recommended. This revised version was published online in August 2006 with corrections to the Cover Date.     

Can isotopic transient and formal steady-state kinetics lead to a converging surface chemistry analysis? Case study on the selective reduction of NO by CH4 over Co-ZSM-5 catalysts

The surface coverage analysis derived from two formal steady-state kinetic models is compared to values directly obtained from steady-state isotopic transient analysis (SSITKA) for the selective catalytic reduction (SCR) of NO by CH₄ over Co-ZSM-5 catalysts. It is shown that the most abundant reacting intermediates are NO _x adspecies, though no clear differentiation between the various adspecies identified by DRIFT spectroscopy was achieved. Less numerous carbon containing adspecies were identified and quantified in the reacting system, essentially as methoxy species. Nitromethane-like intermediates remained undetectable due to a very rapid transformation into N₂ and CO₂. On the basis of these converging kinetic analyses related to each elementary step of the SCR process, a microkinetic model can be derived, which allows describing transient operation, in view of a non steady-state application.     

Effect of water on the reduction of NOx with propane on Fe‐ZSM‐5. An FTIR mechanistic study

Adsorption of NO on Fe‐ZSM‐5 leads to formation of Feⁿ⁺–NO (n = 2 or 3) species (1880 cm^-1), Fe²⁺(NO)₂ complexes (1920 and 1835 cm^-1) and NO⁺ (2133 cm^-1). Water strongly suppresses the formation of NO⁺ and Feⁿ⁺(NO)₂ and more slightly the formation of Feⁿ⁺ –NO. Introduction of oxygen to NO converts the nitrosyls into surface nitrates (1620 and 1575 cm^-1) and this process is almost unaffected by water. The nitrates are thermally stable up to ca. 300^°C, but readily interact with propane at 200^°C, thus forming surface C–H–N–O deposit (bands in the 1700–1300 cm^-1 region). Here again, water does not hinder the process. The C–H–N–O deposit is relatively inert (it does not interact with NO or NO + O₂ at ambient temperature) but, at temperatures higher than 250 °C, it is decomposed to NCO^- species (bands at 2215 (Fe–NCO) and 2256 cm^-1 (Al–NCO)). In the presence of water, however, the Fe–NCO species only are formed. At ambient temperature the NCO^- species are inert towards NO and O₂, but easily react with a NO + O₂ mixture. The mechanism of the selective catalytic reduction of nitrogen oxides on Fe‐ZSM‐5 and the effect of water on the process are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

On the reactivity of K2O‐, CaO‐ and P2O5‐doped nickel molybdate catalysts in a periodic‐flow reactor

The propane oxydehydrogenation with monolayer lattice oxygen of undoped and K₂O–, CaO– and P₂O₅–NiMoO₄ was investigated by using a periodic‐flow reactor (PFR). The influence of the nature and the extent of the promoter has been emphasized relative to the doped catalysts with respect to pure NiMoO₄ phases. It was observed that calcium and potassium promoters satisfactorily enhance propylene selectivity, and phosphorus promoter specifically increases the total activity while maintaining the propylene selectivity. Evidence found by thermogravimetric (TG) analyses (oxygen depletion rate) has shown a dependence on lattice oxygen mobility due to the presence of promoters. This dependence has been correlated to the propane conversion while the propylene selectivity was attributed to the acid–base properties. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic Conversion of NO and C3H6 in Exhaust Gases Over Silver Catalysts Under Stoichiometric or Excess Oxygen

The role of Ag in simultaneously catalyzing NO reduction and C₃H₆ oxidation was shown to be strongly dependent on the redox properties of its local environment. Under an atmosphere of 1,000 ppm NO, 3,000 ppm C₃H₆, and 1% O₂ and a GHSV of 30,000 h⁻¹, a perovskite La_0.88Ag_0.12FeO₃ prepared by reactive grinding is active giving a complete NO conversion and 92% C₃H₆ conversion at 500 °C. These values are much higher than the NO conversion of 55% and C₃H₆ conversion of 45% obtained over a 3 wt.% Ag/Al₂O₃ catalyst under the same conditions. Under an excess of oxygen (10% O₂) a good SCR performance with a plateau of N₂ yield above 97% over a wide temperature window of 350–500 °C along with C₃H₆ conversion of 90% at 500 °C was observed over Ag/Al₂O₃, while minor N₂ yields (∼10% at 250–350 °C) and high C₃H₆ conversions (reaching ∼100% at 450 °C) were obtained over La_0.88Ag_0.12FeO₃. Abundant molecular oxygen is desorbed from Ag substituted perovskite after 10% O₂ adsorption as verified by O₂- temperature programmed desorption (TPD). This reflects the strongly oxidative properties of La_0.88Ag_0.12FeO₃, which lead to a satisfactory NO reduction at 1% O₂ due to the ease of nitrate formation but to a significant C₃H₆ combustion above that value. The formation of nitrate species over the less oxidizing Ag/Al₂O₃ was accelerated under an excess of oxygen resulting in an excellent lean NO reduction behavior. The redox properties of silver catalysts could be adjusted via mixing perovskite with alumina for an optimal elimination of both NO and C₃H₆ over the whole range of oxygen concentration between 0 to 10%.     

Reaction pathways for the oxygenates formation from propane and oxygen over potassium‐modified Fe/SiO2 catalysts

The partial oxidation of propane has been compared on Fe/SiO₂ and alkali‐modified Fe/SiO₂ catalysts. Addition of K⁺ to the catalyst can appreciably enhance the conversion of propane along with a high selectivity toward oxygenates. Adjacency of Fe and K on the silica surface seems to be important for the high oxygenate yield. Through the study on the reaction pathways, two types of intermediates, one derived from propene and the other from propan‐2‐ol, were postulated. The former is related to the acrolein formation and the latter to the acetone formation. Contribution of alkali‐catalyzed aldol condensation is also discussed for the formation of oxygenates with higher carbon numbers. This revised version was published online in July 2006 with corrections to the Cover Date.     

A pathway of NO–H2–O2 reaction over Pt/ZrO2 through ammonium nitrate

The relationship between the product selectivity for the NO–H₂–O₂ reaction and characteristics of the catalyst, Pt/ZrO₂, was investigated. From the results of activity tests and characterizations, such as CO adsorption and TEM, the catalysts with high Pt dispersions showed high NH₃ selectivities, and those with significantly agglomerated Pt particles by high temperature calcination exhibited higher N₂ formation. The effect of the reduction and oxidation pretreatment was also investigated. The reduced Pt sites promoted the formation of NH₃. The pretreatment condition influenced not only on the amount of accumulated nitrogen-containing species during the reaction but also on the decomposition of ammonium nitrate, which was suspected to be an accumulated species. The decomposition of ammonium nitrate would be involved in the NO–H₂–O₂ reaction.     

Low-temperature selective catalytic reduction of NO with NH3 based on MnOx-CeOx/ACFN

MnO _x -CeO _x /ACFN were prepared by the impregnation method and used as catalyst for selective catalytic reduction of NO with NH3 at 80°C-150°C. The catalyst was characterized by N₂-BET, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). The fraction of the mesopore and the oxygen functional groups on the surface of activated carbon fiber (ACF) increased after the treatment with nitric acid, which was favorable to improve the catalytic activities of MnO _x -CeO _x /ACFN. The experimental results show that the conversion of NO is nearly 100% in the range 100°C-150°C under the optimal preparation conditions of MnO _x -CeO _x /ACFN. In addition, the effects of a series of performance parameters, including initial NH₃ concentration, NO concentration and O₂ concentration, on the conversion of NO were studied. __________ Translated from Chemical Industry and Engineering Progress, 2007, 27(1): 87–91 [译自: 化工进展]     

Highly Efficient Fe-silicalite Zeolite in Direct Propane Ammoxidation with N2O and O2

A novel process for the direct ammoxidation of propane over steam-activated Fe-silicalite at 723–823 K is reported. Yields of acrylonitrile (ACN) and acetonitrile (AcCN) below 5% were obtained using N₂O or O₂ as the oxidant. Co-feeding N₂O and O₂ boosts the performance of Fe-silicalite compared to the individual oxidants, leading to AcCN yields of 14% and ACN yields of 11% (propane conversions of 40% and products selectivity of 25–30%). The beneficial effect of O₂ on the propane ammoxidation with N₂O contrasts with other N₂O-mediated selective oxidations over iron-containing zeolites (e.g. hydroxylation of benzene and oxidative dehydrogenation of propane), where a small amount of O₂ in the feed dramatically reduces the selectivity to the desired product. It is shown that the productivity of ACN and especially AcCN, expressed as mol product h⁻¹ kg_cat⁻¹, is significantly higher over Fe-silicalite than over active propane ammoxidation catalysts reported in the literature. Our results open new perspectives to improve the performance of alkane ammoxidation catalysts.     

Catalytic performance of La0.66Sr0.34Co0.2Fe0.8O3 perovskite in propane combustion: Effect of preparation and specific surface area

Several compositions in a system of La_1-x SrxCo_1-y FeyO_3-δ perovskites are known as very good electronic and ionic conductors, as well as excellent catalysts for hydrocarbon oxidation. In this study La_0.66Sr_0.34Co_0.2Fe_0.8O₃ was selected as possibly the optimum composition. To assess the effect of preparation and calcination conditions on the activity in propane combustion, a series of different samples was prepared by a method based on slurry of reactive component precursors processed by freeze-drying. Three different materials were used as source of iron and the samples were aged at successively higher temperatures (1,153–1,343 K). The specific surface areas varied between 5.9 and 1 m²/g, depending on iron precursor and/or aging. The activity was determined in an integral U-shape reactor, typically for 1 and 2 vol% propane in air, with 1 g catalyst and 200 or 100 ml/min flowrate. Kinetics determined on the basis of a wider range of concentrations (1–4.3 vol% propane; 10 vol%-pure oxygen) for a selected, the least aged sample indicated that the propane catalytic combustion is best represented by a Mars van Krevelen model with 0.5 order in oxygen and the two kinetic constants having E _app of 83 and 81 kJ/mol, respectively. For the aged samples, the pseudofirst order E _app varied from 85 to 98 kJ/mol.     

Oxidation and reduction effects of propane–oxygen on Pd–chlorine/alumina catalysts

Pd–chloride precursor salt was used to prepare Pd/Al₂O₃ catalysts. TPSR measurements showed three distinct reactions for the oxidation of propane on palladium surface under excess of hydrocarbon: complete oxidation, steam reforming and propane hydrogenolysis. Propane oxidation on palladium catalysts was related to the Pd²⁺ sites observed on Pd/Al₂O₃ through infrared of adsorbed carbon monoxide. In fresh catalysts reduced by H₂, the IR spectra showed the linear and bridge adsorbed CO species on the Pd⁰ surface. After propane reaction, a new band at 2130 cm^-1 related to CO adsorption on Pd²⁺ species was noted. Carbon monoxide species adsorbed on Pd⁰ were also observed in all samples after reaction. Our results suggest surface ratios of Pd⁰/PdO during the propane oxidation. On the other hand, time on stream conversions of the complete oxidation of propane were affected by either the water generated during the reaction or added as a reactant at 10 vol%. The water generated by the reaction helped to eliminate chlorine residues in the form of oxychloride species leading to an increasing of the activity. However, the presence of water into the reaction mixture caused a strong decreasing of the activity. The inhibition mechanism of propane oxidation in the presence of water consisted in the dissociative adsorption of water on palladium sites with the possible formation of palladium hydroxide (Pd–OH) at the surface, diminishing the number of active surface sites. Dynamic fluctuations into the reaction conditions supported the idea that a pseudo‐equilibrium adsorption–desorption of water was reached. After water removal or increasing in the reaction temperature the equilibrium was shifted to the direction of OH–Pd decomposition. This behavior suggests that the inhibitory effect of water is a reversible phenomenon, being a function of the amount of water and the reaction temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

The mechanism for NOx storage

The mechanisms for storing of NO_x in platinum–barium–alumina catalysts during lean–rich transients are investigated. Oxidation of NO to NO₂ is found to be an important step. NO₂ is found to be important for oxidation of the catalyst or of nitrites to form nitrates. NO_x is then stored in the form of surface nitrates. FTIR studies show no formation of bulk nitrates in these experiments. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Effect of nitric oxide on the photocatalytic oxidation of small hydrocarbons over titania

The catalytic UV photo-oxidation of NO in the absence and presence of ethane, ethene, propane, propene, and n-butane over TiO₂ in the presence of excess oxygen was studied in the temperature range 21–150 °C. It was confirmed in our system that in the absence of hydrocarbon NO was photocatalytically oxidised by oxygen to NO₂ over TiO₂ and was strongly absorbed. Both NO and hydrocarbon could be simultaneously photo-converted with the conversion varying considerably with both NO and hydrocarbon concentration and the nature of the hydrocarbon. In some instances the presence of NO in the feed gas enhanced hydrocarbon oxidation via reactions involving NO₂ that is a powerful oxidant. The extent of this effect depended on the relative strengths of adsorption on TiO₂ of the reactants and products. To reduce surface coverage of hydrocarbon most reactions were run at 150 °C, and it was shown that at this temperature NOx adsorbed on titania could be reduced by photogenerated hydrocarbon surface species to N₂O and N₂ under these conditions. The formation of N₂ was confirmed using ¹⁵NO with helium as carrier gas. By contrast, at room temperature in the presence of propene NO was converted to NO₂.     

NO reduction over La2O3 using methanol

Nitric oxide (NO) reduction by methanol was studied over La₂O₃ in the presence and absence of oxygen. In the absence of O₂, CH₃OH reduced NO to both N₂O and N₂, with selectivity to dinitrogen formation decreasing from around 85% at 623 K to 50–70% at 723 K. With 1% O₂ in the feed, rates were 4–8 times higher, but the selectivity to N₂ dropped from 50% at 623 K to 10% at 723 K. The specific activities with La₂O₃ for this reaction were higher than those for other reductants; for example, at 773 K with hydrogen a specific activity of 35 _μmol NO/s m² was obtained whereas that for methanol was 600 μmol NO/s m². The Arrhenius plots were linear under differential reaction conditions, and the apparent activation energy was consistently near 14 kcal/mol with CH₃OH. Linear partial pressure dependencies based on a power rate law were obtained and showed a near‐zero order in CH₃OH and a near‐first order in H₂. In the absence of O₂, a Langmuir–Hinshelwood type model assuming a surface reaction between adsorbed CH₃OH and adsorbed NO as the slow step satisfactorily fitted the data, and the model invoking two types of sites provided the best fit and gave thermodynamically consistent rate constants. In the presence of O₂ a homogeneous gas‐phase reaction between O₂, NO, and CH₃OH occurred to yield methyl nitrite. This reaction converted more than 30% of the methanol at 300 K and continued to occur up to temperatures where methanol was fully oxidized. Quantitative kinetic studies of the heterogeneous reaction with O₂ present were significantly complicated by this homogeneous reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enhanced photocatalytic degradation of tetramethylammonium on silica-loaded titania

Photocatalytic degradation (PCD) of tetramethylammonium (TMA) in water was studied using both pure TiO₂ and silica-loaded TiO₂ (Si–TiO₂). Use of Si–TiO₂ catalyst prepared from commercial TiO₂ powder by a simple method developed in this work enhanced the PCD rate of TMA considerably. The Si/Ti atomic ratio of 18% was found to be an optimum in photoactivity and the calcined sample was more efficient than the uncalcined one. Several factors were noted to be responsible for the higher photoefficiency of Si–TiO₂ catalyst. Si–TiO₂ calcined at 700 °C did not show any sign of change in the crystalline structure from that of uncalcined pure TiO₂. The increased thermal stability of Si–TiO₂ enabled the bulk defects to be removed at high temperatures without forming the inactive rutile phase, which may partly contribute to the higher photoactivity. The most outstanding characteristics of Si–TiO₂ is its surface charge modification. Loading silica on to a titania surface made the surface charge highly negative, which was confirmed by zeta potential measurements. The enhanced electrostatic attraction of cationic TMA onto the negatively charged Si–TiO₂ surface seems to be the main reason for the enhanced photoactivity of Si–TiO₂. As a result of this surface charge change, the TMA PCD rate with Si–TiO₂ exhibited a maximum around pH 7 whereas the PCD with pure TiO₂ was minimized at pH 7. The X-ray photoelectron spectroscopic analysis showed the formation of SiO_x on the TiO₂ surface but the diffuse reflectance UV spectra indicated no significant difference in the band gap transition between pure TiO₂ and Si–TiO₂. In addition, the diffuse reflectance IR spectra showed the presence of more surface OH groups on Si–TiO₂ than on pure TiO₂, which may also contribute to the higher photoactivity of Si–TiO₂ through generating more OH radicals upon UV illumination.     

Effect of sulfur dioxide on nitric oxide adsorption and decomposition on Cu-containing micro- and mesoporous molecular sieves

NO decomposition was studied at different temperatures on copper-exchanged ZSM-5, AlMCM-41 and NbMCM-41 molecular sieves. Cu-ZSM-5 zeolites presented the highest activity. SO₂ poisoning was also performed and Cu–NbMCM-41 was found to be more resistant. IR results of NO and SO₂ coadsorption either at room temperature or at 573 K show evidence that sulfate formation occurred at 573 K and partially prevented NO adsorption on Cu²⁺ in square planar structure in Cu-ZSM-5. Sulfation of Cu–NbMCM-41 was quite low due to niobium incorporated into the lattice. By contrast, niobium present in the extra-lattice position in CuNb-ZSM-5 and CuNb–AlMCM-41 did not protect the catalyst from sulfation. H₂-TPR results suggested that sulfates were formed on copper sites. IR spectra after treatment under SO₂ + O₂ at 673 K indicated that sulfated species were covalently bonded, their structure varying according to the nature of the support. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evolution of Ti–Sn-rutile-supported V2O5–WO3 catalyst during its use in nitric oxide reduction by ammonia

This paper concerns the relation between surface structure of crystalline vanadia-like active species on vanadia–tungsta catalyst and their activity in the selective reduction of NO by ammonia to nitrogen. The investigations were performed for Ti–Sn-rutile-supported isopropoxy-derived catalyst. The SCR activity and surface species structure were determined for the freshly prepared catalyst, for the catalyst previously used in NO reduction by ammonia (320 ppm NO, 335 ppm NH₃ and 2.35 vol% O₂) at 573 K as well as for the catalyst previously annealed at 573 K in helium stream containing 2.35 vol% O₂. The crystalline islands, exposing main V₂O₅ surface, with some tungsten atoms substituted for V-ones, were found, with XPS and FT Raman spectroscopy, to be present at the surface of the freshly prepared catalyst. A profound evolution of the active species during the catalyst use at 573 K was observed. Dissociative water adsorption on V⁵⁺OW⁶⁺ sites is discussed as mainly responsible for the catalyst activity at 473 K and that on both V⁵⁺OW⁶⁺ and V⁴⁺OW⁶⁺ sites as determining the activity at 523 K. This revised version was published online in August 2006 with corrections to the Cover Date.