Jan Brandin

Characterisation of Silica-Titania Mixed Oxides

Abstract A series of coprecipitated silica-titanias containing between 0 and I00 mol% titania were characterized by various methods. These materials are often used as supports for catalysts in the reduction of NO x and are shown to be micro-, meso-, and macroporous. All textural quantities decrease with the addition of TiO 2 to SiO 2 . The macroporosity, of vital importance in the highly diffusion-controlled NO x reduction, reaches a maximum value at 50 mol% TiO 2 . X-ray diffraction, FT-IR, and XPS studies showed that the materials, calcined at 723 K, consisted of a SiO 2 -TiO 2 glass phase, an amorphous silica phase, and an anatase phase. At I0 mol% TiO 2 , rutile was observed, by XRD, in addition to anatase. XPS data indicate the presence of a TiO 2 phase in all Ti-containing samples, a SiO 2 phase at and below 50 mol% TiO 2 , and a SiO 2 -TiO 2 phase at and above 75 mol% TiO 2 . The quantitative XPS analysis indicates a heterogeneous distribution of phases with an increased surface concentration of Si phases. Similar results were obtained by the FT-IR studies, which additionally indicate the presence of surface free silanol groups in all samples and detect tetrahedrally substituted Ti 4+ in the SiO 2 phase.

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.

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.

Selective catalytic reduction of NOx over acid-leached mordenite catalysts

Abstract Selective catalytic reductions of NO, NO 2 and mixtures of NO and NO 2 over mordeninte catalysts were studied. The activity of mordenite catalysts with different Si/Al ratios, obtained through acid leaching, decreased with the Al content of the mordenite. The change activity with temperature and acid leaching together with the changes in contents of Fe and Al indicate that Lewis acids are active sites. These Lewis acids could be either Fe ions or Lewis acids formed on dehydroxylation of Broensted acid sites. Activities of NO reduction on leached mordenites were correlated to the amount of adsorbed NO + measured by IR. The activity in the reduction of NO x revealed a maximal conversion at a NO 2 /NO x ratio of 0.5, indicating that the oxidation of NO or the decomposition of NO 2 are the rate limiting step in the overall reduction.

Deactivation of oxidation and SCR catalysts used in flue gas cleaning by exposure to aerosols of high- and low melting point salts, potassium salts and zinc chloride

For the purpose of this deactivation study, Pt- and vanadia supported catalysts were used. The catalysts have been exposed to aerosol particles of inorganic salts, with high- or low melting points. The average diameter of the generated salt particle was kept constant at around 70 nm. The aerosol particle penetration depth for the samples exposed to potassium salt, was 1 μm as measured by scanning electron microscopy (SEM). The corresponding depth for zinc chloride salt (ZnCl2) was 5 μm. In order to validate the dependency of the catalytic decay rate to exposure temperature, Pt/wire-mesh catalyst was treated with potassium chloride at two temperatures, namely 300 and 500 °C. Pt/supported catalyst was also treated with ZnCl2 salt at 190 and 300 °C. The extent of decay was tested in the oxidation of CO for particle treated Pt/wire-mesh samples. The degree of the deactivation for the aerosol particle deactivated vanadia supported catalysts were also examined in the reduction of NOx. When the Pt/wire-mesh catalyst have been exposed to the poisons aerosol particles at higher temperature lead to the strongest deactivation in the CO oxidation. The Pt-supported catalysts that were treated with aerosol particles from potassium carbonate and potassium sulphate revealed a minor deactivation in the CO oxidation reaction. No significant deactivation was observed for the salt treated vanadia supported monolith samples used in selective catalytic reduction (SCR). A slight pronunced deactivation effect appeared when the vanadia supported wire-mesh catalysts were salt treated. Generally, the obtained results in this study do not indicate any correlation between the salt melting point and the degree of catalytic decay. The obtained results indicate that the exposure temperature during the deactivation procedure is the most critical parameter. Also, the higher the exposing temperature the stronger deactivated sample is produced.

Catalytic Reduction of Nitrogen Oxides, 2. The reduction of NO2

Abstract The selective catalytic reduction of NO2 with NH3 has been studied over a V2O5/SiO-TiO2 catalyst. The activity for the main reaction was measured between 420 and 670 K. Also reported are activities for the decomposition of NO2 to NO and O2 and the influence of O2 in that reaction. The reaction system NO2-O2-NH3 in N2 has been investigated in detail and activities in single as well as composed reaction media are reported.

Dealuminated mordenites as catalyst in the oxidation and decomposition of nitric oxide and in the decomposition of nitrogen dioxide: characterization and activities

Abstract Dealuminated mordenites were investigated in order to illustrate the effect of the aluminium content on catalytic and physicochemical characteristics. Chemical and physical characterizations of the catalysts were performed by means of X-ray diffraction, chemical analysis, adsorption- and desorption studies and IR-measurements. The catalysts were tested in the oxidation of NO and in the decomposition of NO 2 and NO. Activities for the mordenites in both the oxidation of NO and the decomposition of NO 2 were strongly dependent on the aluminium content of the catalyst. The highest activities were obtained for the original unleached catalyst. No direct decomposition of No to N 2 and O 2 was observed in the temperature range from 420 to 690 K. Adsorbed amounts of No and NH 3 showed a regular decrease with the amount of aluminium in the catalyst. The activities in the oxidation of NO and the decomposition of NO 2 could be correlated to the amount of NO + adsorbed on the catalyst and which was detected by IR.

Pore Condensation in Glycerol Dehydration

Pore condensation followed by polymerization is proposed as an explanatory model of several observations reported in the literature regarding the dehydration of glycerol to acrolein. The major conclusion is that glycerol pore condensation in the micro- and mesopores, followed by polymerization in the pores, play a role in catalyst deactivation.

Effect of Promoters on V2O5/SiO2 Catalysts Active for the Selective Reduction of NO

Abstract Vanadia catalysts supported on SiO 2 containing 3.5 wt% TiO 2 were promoted by Fe and Cu oxides. At 180°C the unpromoted and Fe-promoted catalysts exhibited the highest activities for the selective reduction of NO by NH 3 . The Fe-promoted catalyst maintained activity in the presence of SO 2 during 120 h on-stream. Electrical conductance and mass spectrometric measurements indicated that at 200°C the only reactions occurring were the formation of N 2 and H 2 O from both the NO + NH 3 reaction and the self-reaction of NH3. Additional reactions forming NO and N 2 O from NH 3 were observed at 350°C. Poisoning of the catalysts by SO 2 was followed by electrical conductance as a decrease in the V(IV) concentration formed from the NO + NH 3 reaction.