Ceria–zirconia-supported rhodium catalyst for NOx reduction from coal combustion flue gases

The catalytic activities of ceria–zirconia mixed oxides Ce_xZr_1−xO₂ (x = 0.17, 0.62 and 0.8) rhodium catalysts were determined by isothermal steady-state experiments using a representative mixture of exhaust gases of coal combustion. Results show that all supports are active in deNO_x reaction in the presence of the mentioned gas mixture. However, their catalytic activity varies with the content of cerium and goes through a maximum for x = 0.62, leading to 27% NO_x consumption. The effect of rhodium on Ce_0.62Zr_0.38O₂ considerably improves the catalytic activity during the deNO_x process assisted by hydrocarbons. The rhodium addition decreases by about 34 °C the temperature of NO_x consumption, which goes up to 57%. A mechanism of hydrocarbon (HC) assisted reduction of NO is proposed on ceria–zirconia-supported rhodium catalysts. This mechanism is divided in three catalytic cycles involving (i) the oxidation of NO into NO₂, (ii) the reaction of NO₂ and the hydrocarbons leading to RNO_x species and C_xH_yO_z, and finally (iii) the decomposition of NO assisted by these latter C_xH_yO_z species.     

Structural sensitivity of NO decomposition over a V-O-W/Ti(Sn)O2 catalyst

NO decomposition at 673 K as a function of contact times over a V-O-W/Ti(Sn)O₂ catalyst obtained by sol–gel method and pretreated at 673 K in helium stream was investigated and compared with that over a Cu-ZSM-5 catalyst. Feed containing 4% NO in a helium stream was used in both the cases.

The V-O-W/Ti(Sn)O₂ catalyst showed higher activity as well as higher selectivity to dinitrogen than the Cu-ZSM-5 catalyst over the whole range of used contact times (0.375–15 g s cm⁻³).

The highest activity of the V-O-W/Ti(Sn)O₂ catalyst, especially at higher normalised contact times (τ/τ_max > 0.25), was shown to result from vanadia-like surface layer formation with high tungsten content. It was also shown that the decrease in activity as contact time decreased is connected with tungsta monolayer formation on the V-O-W hybrid crystallites composed of tungsta, V-W oxide bronze and vanadia.     

The influence of the Cr–Al foil texture on morphology of adhesive Al2O3 layers in monolithic environmental catalysts

The FeCrAl-alloy foils of 0.1 and 0.05 mm thickness, being the supports of metallic monolithic environmental catalysts, were investigated by XRD and TEM to determine the reasons of the differences in morphology of the alumina adhesive layers grown during oxidation. Alumina adhesive layers of stable thickness and outgrowing scale-like crystallites were observed on 0.1 mm foil and alumina layers of different thickness with lower population of outgrowing scale-like crystallites on 0.05 mm foil. αFe was found to be the only phase seen by XRD in both kinds of the foil. The density of dislocations on the surface of 0.1 mm foil was found to be of one order of magnitude higher than this on the surface of the 0.05 mm one. Both kinds of the foil reveal different texturing investigated with X-ray. Different texturing connected with various face development results in various possibilities of growing Al₂O₃ layers. However, the high dislocation density is responsible for the high population of scale-like crystallites on 0.1 mm foil.     

DRIFT study of the interaction of NO and O2 with the surface of Ce0.62Zr0.38O2 as deNOx catalyst

Gas–solid interactions and surface intermediates evolution after NO adsorption onto calcined Ce_0.62Zr_0.38O₂ were investigated. The results of adsorption and temperature-programmed desorption of NO were explained using diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) coupled with temperature-programmed experiments in environmental cell. Surface NO-containing species such as nitrites and nitrates were identified during evolution of NO on the surface of Ce_0.62Zr_0.38O₂ solid solution at low and high temperature. The ceria–zirconia solid solution was found to be active in deNO_x reaction in the presence of a “toluene, propene and propane” mixture.     

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.     

The effect of the Rh–Al, Pt–Al and Pt–Rh–Al surface alloys on NO conversion to N2 on alumina supported Rh, Pt and Pt–Rh catalysts

NO conversion to N₂ in the presence of methane and oxygen over 0.03 at.%Rh/Al₂O₃, 0.51 at.%Pt/Al₂O₃ and 0.34 at.%Pt–0.03 at.%Rh/Al₂O₃ catalysts was investigated.

δ-Alumina and precious metal–aluminum alloy phases were revealed by XRD and HRTEM in the catalysts.

The results of the catalytic activity investigations, with temperature-programmed as well as steady-state methods, showed that NO decomposition occurs at a reasonable rate on the alloy surfaces at temperatures up to 623 K whereas some CH₄ deNO_x takes place on δ-alumina above this temperature. A mechanism for the NO decomposition is proposed herein. It is based on NO adsorption on the precious metal atoms followed by the transfer of electrons from alloy to antibonding π orbitals of NO_(ads.) molecules. The CH₄ deNO_x was shown to occur according to an earlier proposed mechanism, via methane oxidation by NO_2(ads.) to oxygenates and then NO reduction by oxygenates to N₂.     

Evolution of surface vanadia-like species on unsupported V–O–W catalyst for NO decomposition in the course of redox-treatment

The evolution of surface species on unsupported V–O–W catalyst (V:W = 2:9) as a result of its oxidizing and reducing thermal treatments was investigated. The catalyst was prepared by annealing an oxalate precursor at 773 K in air for 1 h and then subjected to further oxidizing or reducing thermal treatments. It was revealed using XRD that the freshly prepared catalyst contains mainly the crystallites of the tetragonal phase of V–W oxide bronze which transforms into monoclinic WO₃ during further annealing due to surface vanadium segregation. XPS results showed that in the surface nanolayers of the freshly prepared catalyst vanadium occurs mostly as vanadium suboxides species. Raman spectroscopy results revealed mostly crystalline vanadia-like species as well as monolayer vanadia-like species on the freshly prepared catalyst. Essential changes in the structure of the vanadia-like species as a result of the catalyst thermal treatments were observed. The thermal treatment in the oxidizing atmosphere caused an increase in the content of the crystalline vanadia-derived surface species—that did not contain tungsten in their structure. Monolayer species with relatively high tungsten content were formed during the catalyst thermal treatment in reducing conditions.     

Competition between NO reduction and NO decomposition over reduced V–W–O catalysts

The SCR of NO and NO decomposition were investigated over a V–W–O/Ti(Sn)O₂ catalyst on a Cr–Al steel monolith. The conversions of NO and NH₃ over the reduced and oxidised catalysts were determined. The higher conversion of NO than of NH₃ was observed in SCR over the reduced catalyst and very close conversions of both substrates were found over the oxidised one. The increase of the pre-reduction temperature was found to cause an increase in catalyst activity and its stability in direct NO decomposition. The surface tungsten cations substituted for vanadium ones in vanadia-like active species are considered to be responsible for the direct NO decomposition. The results of DFT calculations for the 10-pyramidal clusters: V₁₀O₃₁H₁₂ (V–V) and V₉WO₃₁H₁₂ (V–W) modelling (0 0 1) surfaces of vanadia and WO₃–V₂O₅ solid solution (s.s.) active species, respectively, show that preferable conditions for NO adsorption exist on W sites of s.s. species and that reduction causes an increase in their ability for electron back donation to the adsorbed molecule. Electron back donation is believed to be responsible for the electron structure reorganisation in the adsorbed NO molecule resulting in its decomposition. The high selectivity of NO decomposition to dinitrogen was considered to be connected with the formation of the tungsten nitrosyl complexes solely via the W–N bond.     

Kinetic investigation of the NO decomposition over V–O–W/Ti(Sn)O2 catalyst

A kinetic study of the NO decomposition over V–O–W/Ti(Sn)O₂ catalyst carried out in a tubular fixed-bed reactor operating under atmospheric pressure at different temperatures and at various space times is presented. Assuming that NO decomposition occurs as a result of electron transfer from the metal active site to antibonding π NO orbital, several kinetic models were derived and applied to describe the kinetics of reaction. The best agreement between the experimental data and theoretical prediction was achieved with the model assuming adsorption of NO on the active sites as the rate-determining step. Finally, it was concluded that V–O–W/Ti(Sn)O₂ catalyst has promising activity for the NO removal in O₂ presence from the effluent gases of the different sources of emission.