C.U. Ingemar Odenbrand

The role of K2O in the selective reduction of NO with NH3 over a V2O5(WO3)/TiO2 commercial selective catalytic reduction catalyst

To elucidate the nature of the acid sites of the V2O5(WO3)/TiO2 catalyst upon K2O addition and its relation to the selective reduction of NO with NH3, measurements were made by means of infrared and Raman spectroscopy, NH3 chemisorption, and NO reduction measurements as a function of the K2O loading. The catalytic activity was found to decrease rapidly with the K2O loading, irrespective of the similar textural properties of all samples. Addition of K2O modified the vanadium species on the catalyst surface. For large additions of K2O, the potassium partially reacted with V2O5 to form KVO3. The amount of NH3 chemisorbed on the catalyst was observed to decrease with both the loading of K2O and the temperature. The adsorption of NH3 on both Bronsted and Lewis acid sites was confirmed. The strength and the number of Bronsted acid sites decrease largely with the loading of K2O in parallel with the decrease of the SCR activity, suggesting that the SCR reaction involves NH3 adsorption on the Bronsted acid sires. At low surface coverage of NH3, the isosteric heat of NH3 chemisorption was determined to be 370 kJ/mol for 0 wt.% K2O addition. With increasing K2O amount, the heat of adsorption decreased and was 150 kJ/mol for the catalyst with higher amounts of K2O addition. The results obtained imply that potassium disturbs the formation of the active ammonia intermediates, NH4+, resulting in deactivation of the catalyst

Catalytic combustion of soot deposits from diesel engines

Abstract The catalytic combustion of diesel soot under the influence of metal oxides and metals has been studied. The catalytically active materials, dispersed on a carrier of γ-alumina, have been studied in order to simulate the conditions in a catalytic particulate trap. A mixture of soot and catalyst was placed in a flow reactor and a gas mixture containing 6% oxygen, 7% water and balance of nitrogen was led through it at a flow-rate of 90 l/h. The combustion rate was determined between 573 and 698 K. The results showed that V 2 O 5 is the most active catalyst for the combustion of diesel soot whereas oxides of copper, manganese and chromium as well as the metals silver and platinum are active in the combustion of hydrocarbons that desorb from the diesel soot upon heating. Catalytically active materials sensitive to sulphur poisoning are deactivated immediately during the experiments as large amounts of SO 2 are released from the diesel soot when heated. The results indicate that V 2 O 5 is not sensitive to sulphur poisoning. This study shows that a catalyst for the combustion of diesel soot should be based on V 2 O 5 .

Regeneration of commercial SCR catalysts by washing and sulphation: effect of sulphate groups on the activity

The use of bio-fuels is becoming more important because of the environmental benefits associated with these fuels. Deactivation of SCR catalysts applied in bio-fuel plants is a major problem due to the high potassium content of bio-fuels and therefore, great potential lies in finding regeneration processes that can be used commercially. Exposing the catalyst surface to sulphate groups generated by SO2 or H2SO4 is a promising way to rejuvenate the initial activity of the catalyst. The chemical stability of the sulphate groups was investigated by the interaction of the SCR reactant gases with the sulphate-promoted catalysts. Sulphate ions deposited on the surface of the TiO2/V2O5/WO3 were thermally stable at 420 degreesC. The introduced sulphate groups were chemically unstable when the catalyst was treated with the SCR reactants at 400 degreesC, but were chemically stable when the catalyst was exposed for the SCR reactants at 350 degreesC. Sulphation after water treatment provided more chemically stable surface sulphate groups at 400 degreesC. The presence of sulphate groups enhanced the number and the strength of the surface acid sites. The amount of ammonia bound to the Bronsted acid sites decreased with the potassium content of the catalyst while the amount of ammonia adsorbed on the Lewis acid sites was almost unaffected. Since potassium both retarded the redox potential of the surface vanadia species and decreased the amount of ammonia bound to the Bronsted acid sites, it is important to wash the strongly deactivated catalyst before sulphation

Regeneration of commercial TiO2-V2O5-WO3SCR catalysts used in bio fuel plants

Deactivation of SCR catalysts applied in bio fuel plants is a major problem due to higher amounts of potassium in bio fuel compared to other fuels. Regeneration of deactivated catalysts seems to be a promising way for minimising the total cost of bio fuel plants and great potential lies in finding regeneration processes that can be used commercially. The first applied regeneration method was washing of two different commercially aged catalysts in different aqueous solutions. The other method was washing with water followed by sulphation at different temperatures. Sulphation with SO2 resulted in higher activation without affecting the amount of potassium accumulated on the surface indicating the role of surface sulphate groups. Since potassium both decreases the catalytic activity for NO-reduction and retards the redox potential of the surface vanadium species during the sulphation procedure, it is necessary to wash the heavily deactivated catalyst before sulphation. Washing with water or diluted sulphuric acid could not restore the vanadium groups to the initial condition. Washing with 0.5 M H2SO4 was the most effective regeneration method and the bio-modified catalyst regained 92% of its initial activity, while the corresponding value for the conventional catalyst was 111%

Catalytic Reduction of Nitrogen Oxides on Mordenites: Some Aspects on the Mechanism

Abstract The emission of nitrogen oxides is a global environmental problem. The ultimate solution would be a catalytic decomposition of NO to N2 and O2. Presently no success has been achieved in developing a suitable catalyst. A working technology to eliminate nitrogen oxides from stationary sources is the Selective Catalytic Reduction (SCR) of nitrogen oxides with ammonia. Since 1983 we have been working with SCR and then initially using V2O5 catalysts. We have found that the reaction rate for equimolar mixtures of NO and NO2 is much higher than that for each gas alone. Since the NOx emissions in flue gases consist of 95% No it is necessary to convert 45% to NO2 to take advantage of the increased activity. The idea was to combine a catalyst for the oxidation of No to NO2 and a catalyst for the reduction step. The chosen catalyst, Nortons Zeolon 900 H, is a good oxidation catalyst for NO and a fairly good reduction catalyst. To enhance the oxidation activity, the zeolite was exchanged with transition metal ions. In contradiction to our expectations the result was a decrease in the activity. However the activity in the reduction of NO to water and nitrogen was greatly enhanced. This is an interesting coupling between the oxidation and the reduction activity, and a link between mordenite and V2O5. V2O5 is also a very good reduction catalyst and a very poor oxidation catalyst for NO. Both the oxidation and reduction activities are depending on the aluminium content in the H-mordenite. The metallic ion is bonded to the zeolite framework on the site where strong Lewis acids are formed on dehydroxylation. The decrease in the oxidation activity is caused by this decreased formation of strong Lewis acids. The ionization of NO-NO2 mixtures to NO+ and NO2− species attached to the surface can explain their catalytic behaviour. In a similar way NO2 alone forms an ion pair with itself. Eventually the same thing applies to mixtures of NO and O2.

Surface acidity of silica-titania mixed oxides

Abstract A study of the acidity of coprecipitated SiO 2 TiO 2 oxides is presented. The amount of acidity has been determined by ammonia adsorption at 150°C. The acidity was also characterized by TPD of adsorbed ammonia and by infrared spectroscopy of various adsorbed probes, such as pivalonitrile, pyridine, ammonia, and n-butylamine. From the quantitative measurements of adsorption of ammonia and from TPD it was concluded that the SiO 2 TiO 2 mixture can be regarded as a mechanical mixture of silica and titania. However, the IR investigation showed that Ti enters in small amounts into the silica framework. This results in formation of very strong Lewis acid sites, caused by incomplete tetrahedral coordination of Ti 4+ ions exposed on the surface.

Catalytic Reduction of Nitrogen Oxides, 1. The reduction of NO

Abstract The selective catalytic reduction of NO with NH3 has been studied over V205/SiO2-TiO2 catalyst. The influence of side reactions has bee

Effect of water vapor on the selectivity in the reduction of nitric oxide with ammonia over vanadia supported on silica-titania

The effect of the presence of water vapor on the reduction of nitric oxide with ammonia was investigated using vanadia on SiO2-TiO2 as a catalyst. The selectivity towards nitrogen is greatly increased upon addition of water. Repeated use at temperatures of 825 K causes sintering of the catalyst and loss of activity as well as selectivity towards nitrogen. The formation of nitrous oxide is preferentially taking place on large crystals of presumably reduced vanadia. Removal of these crystals results in a more selective catalyst.

An infrared and electrical conductance study of V2O5/SIO2-TIO2 catalysts active for the reduction of NO by NH3

Abstract A series of V 2 O 5 SiO 2 TiO 2 catalysts (vanadia content 2–30 wt%) was evaluated for the selective reduction of NO by NH 3 . Activities at 200 °C determined on a per gram vanadia basis were nearly equal for catalysts containing 5–20% vanadia. The 10% catalyst exhibited the highest activity at 350 °C. Characterization of the catalysts with FTIR and XRD showed that the vanadia was highly dispersed on the carrier as an amorphous phase for all catalysts with 20% or less vanadia. Electrical conductance measurements were made to study the dispersion of the vanadia on the support and the effect of different gases on the degree of vanadia reduction. Conductances for the catalysts in 1.5% O 2 Ar carrier gas increased with increasing vanadia content for catalysts with 15% or more vanadia indicating a decreasing distance between V(IV) centers. Exposure of the catalysts to NH 3 in the carrier gas resulted in reversible increases in conductance for all vanadia concentrations. Exposure of the catalysts to NO resulted in reversible conductance increases for the 15, 20, and 30% catalysts. Exposure of the catalysts to NH 3 + NO resulted in conductance changes which indicated a reaction at 350 °C between adsorbed, laterally mobile NH 3 and gaseous NO for all catalysts with the most effective reaction occurring on the 10% catalyst. At 200 °C, the conductance measurements indicated a reaction between strongly bound NH 3 , which exhibited little lateral movement, and gaseous NO.

Surface acid property and its relation to SCR activity of phosphorus added to commercial V2O5(WO3)/TiO2 catalyst

To examine the influence of phosphorus on the commercial V2O5(WO3)/TiO2 SCR catalyst, measurements were carried out by means of infrared and Raman spectroscopy, XPS, and NO reduction measurement as a function of phosphorus loading. Phosphorus added to the catalyst was found to disperse well over the catalyst without a significant agglomeration up to 5 wt% P2O5 addition. The number of the hydroxyl groups bonded to the vanadium and titanium species decreased readily with increasing amount of phosphorus. Correspondingly, the hydroxyl groups bonded to the phosphorus species were formed. NH3 adsorbed on both hydroxyl groups bonded to vanadium and phosphorus as ammonium ions, implying that the P–OH groups formed are also responsible for the Brønsted acidity. The NO reduction activity was found to be decreased with increasing amount of phosphorus; however, the influence of phosphorus was relatively small irrespective of the large amount of phosphorus addition. The deactivation might be caused by the change in the nature of the surface hydroxyl groups as Brønsted acid sites. Phosphorus species might partially wrap the surface V=O and W=O groups, which might also contribute to the deactivation.