Accelerated phosphorous poisoning of automotive SCR-catalyst

Transcription

1 Accelerated phosphorous poisoning of automotive SCR-catalyst Karl Henrik Bergquist Department of Chemical Engineering, Lund Institute of Technology, Lund, Sweden Abstract A study of an accelerated phosphorus poisoning method for an automotive SCR-catalyst and the phosphorus influence on catalyst activity has been carried out. The catalyst that was deactivated was a V 2 O 5 -WO 3 -TiO 2 SCR-catalyst. The deactivation was made with phosphorus doped fuel in an engine rig for hours. Parts from the deactivated catalyst were tested for remaining activity and characterized by BET, ICP, XRD and TPD. The activity tests showed that phosphorus has an impact on the activity and that it is dependent on the phosphorus concentration. Phosphorus decreases the activity trough selective poisoning and an increased inner mass transport resistance. The characterization showed that phosphorus is present through the whole length of the catalyst. An increase in phosphorus decreases the specific surface and increases the average pore diameter. The phosphorus also affects the acid sites on the catalyst. The phosphorus did note change any of the crystalline phases detected by XRD. However, phosphorus could be part of the formation of a new crystalline phase in the two parts closest to the inlet of the catalyst. Introduction NO x, defined as NO plus NO 2, is formed during combustions. It is either formed through the reaction of N 2 with O 2 from the air or in the reaction between O 2 and N in the substance that is combusted. NO x contributes to the formation of photochemical fog, depletion of the ozone layer and acid rain. To reduce the emission of NO x from the transport sector, Selective Catalytic Reduction (SCR) has been developed for automotive applications i,ii. A problem during developing of these catalysts has been the long time of validation. To get a picture of how the catalyst ages and deactivates, road aging and engine rig aging tests of up to 5 hours can be needed. The method of accelerated aging can be used to decrease these test times, down to only 1 hours. Accelerated aging consists of two parts, thermal deactivation and poisoning. These parts can be carried out at the same time or separately iii, iv, v. ZDDP is an effective lubricant and is used as additive in engine oil. The compounds in ZDDP can deposit on the catalyst in various forms and cause deactivation. Phosphorus, which is a part of ZDDP, is a well known catalyst poison iii. Phosphorus leads to at decrease in catalytic activity. The higher the phosphorus content, the lower the catalytic activity, probably because it induces a change in the active catalyst from reducible V 2 O 5 to non reducible phosphates vi. That phosphorus changes the nature of acid sites on the catalyst by forming P-OH acid sites with lower acid strength then the acid sites formed as V-OH is well known 6. Phosphorus also changes the textural properties of the catalyst surface by forming different components such as VOPO leading to a decrease in specific surface and an increase in the average pore diameter vii. The object of this work was to investigate the possibility of perform accelerated poisoning with phosphorus doped fuel and to investigate the influence of phosphorus on the activity. Experimental Catalyst and deactivation The catalyst that was deactivated through accelerated poisoning was a 2 litres SCRcatalyst developed by Haldor Topsoe. The catalyst is based on a fibre reinforced ceramic structure with high cell density. The active metals are vanadium oxides and tungsten oxides on a carrier of titanium oxide. The catalyst was deactivated during hours in an engine rig, using fuel doped with tributyl phosphate (TBP). From the deactivated catalyst a length wise sample was taken and divided into 9 parts. Part 1 being the inlet to part 9 being the outlet 1

2 Analytical methods Brunauer, Emmett and Teller (BET) The surface area analysis was performed on a six station automatic BET equipment, Micrometrics ASAP 2. Before measurements, degassing was carried out at 35 o C for 16 hours to a residual pressure under 5 mtorr. X-ray powder diffraction (XRD) XRD characterization was performed on a Seifert XRD 3 TT diffractometer using Nifiltered Cu Kα 1 radiation (wavelength 1.56 Å). Each sample was grinded into a fine powder and pressed into a disk fitted in a sample holder. The scanning speed was.5 degrees per minute and the scanned area was from 2.5 o to o. Temperature programmed desorption (TPD) Temperature programmed desorption was performed on a Micromeritics TPD/TPR 29. Each sample was divided into smaller bits to fit the sample holder. The bits were ranging from mm down to.3 mm. The total mass of each sample was.5 gram. Inductively Coupled Plasma (ICP) ICP analysis was provided by Topsoe. Catalytic activity test Monolith samples form the deactivated catalyst were evaluated in a laboratory reactor using a simulated exhaust gas mixture containing 5 ppm NO, ppm NH 3, 1 % O 2, 5 % H 2 O and He as balance gas at a space velocity of 3 h - 1. Conversions of NO were obtained at 2, 25, 3, and 5 o C. Results BET surface area and pore size The BET results shows that there is a linear relationship between the BET surface area and the phosphorus concentration, see figure 1. The amount of phosphorus has a great impact on the BET surface area. The fresh part has a surface area of 78. m 2 per gram and it decreases linerly to part 1, which has a BET area of 39.6 m 2 per gram. In part 9, which has the smallest concentration of phosphorus, the BET surface has decreased with 29.5 percent. For part 1, the decrease is 5 percent. B E T, m 2 / g y = x R 2 = Figure 1. BET surface vs. weight percent phosphorus wt% P Figure 2 shows that the concentration of phosphorus also has an impact on the pore size. The change in average pore diameter is probably caused by phosphorus forming substances that closes the smaller pores, leading to an increase in the average pore diameter. Pore size, A wt% P Figure 2. Average pores size plotted against the concentration of phosphorus XRD Figure 3 shows the results from the XRD measurements. The only detectable peaks are from the TiO 2 synthetic anatase, which is used as carrier. The peaks that represent TiO 2 is at 2θ= 25.6, 38.3, 8.2, 5.6, 55.5, 63.2, 69.1, 71, Other elements such as V 2 O 5 and WO 3 are of too low concentrations to be noticeable in the XRD spectrum. Measuring the peak width for each parts peak at half the peak height at 2θ = 25.3 o, gave no difference in width among the parts. Since there is no change in peak width, the size of the TiO 2 crystallites on the surface has not been affected by the coverage of phosphorus. Spectra from sample 1and 2 has a peak at 2θ = 7. that is not present in any other part tested. What this peak represents is hard to tell, due to that no other peaks are noticeable that could belong to the unknown substance. However, 2

3 since there is a difference between each sample concerning the phosphorus concentration, an unknown formation of a phosphorus species is highly likely Theta Part fresh Figure 3 XRD spectra from the samples tested TPD Table 1 shows the amount of acid sites per gram of sample. The amount of acid sites is dependent of the concentration of phosphorus. The fresh part and part 9 has almost the same amount of acid sites. However, as the concentration increases, the amount of acid sites decreases, leading to a decrease in capability of adsorbing NH 3. Part µmole/g Fresh 238 Table 1. Acid sites in each sample tested Table 2 shows the number of acid sites per BET surface area. For the deactivated parts the number of acid sites per BET surface does not change upon an increase in the phosphorus concentration. The phosphorus must therefore primarily selectively poison the active sites and secondarily block the pores of the catalyst. The phosphorus has also contributed to an increase in the number of acid sites per BET surface in comparison with the fresh part. Part µmol/m 2 BET Fresh 3.5 Table 2. Number of acid sites/m 2 BET surface area Part 9 Part 7 Part 5 Part 3 Part 1 Figure shows the distribution between strong and weak acid. As the figure shows the phosphorus on the surface, does affect the distribution of the different acidity sites on the surface of the catalyst. At higher phosphorus concentration the weaker acid sites dominate. Nevertheless, as concentration of phosphorus decreases the fraction of strong acid sites increases. fraction weak/strong sites wt % P Strong Weak Figure. Fraction weak/strong sites vs. concentration of phosphorus. ICP In figure 5, we can see the results from the ICP. The highest phosphorus concentration is in part 1 with 3.72 wt% and the concentration decreases lengthwise down to part 9 which has the lowest concentration of 2.33 wt % phosphorus. The results give a 6 percent difference of phosphorus concentration along the catalyst. These results show that % of the added phosphorus in the fuel has been collected on the catalyst. During deactivation of the catalyst, the running conditions must have been almost optimal for phosphorus collection on the catalyst. And since no other poisons were present to compete for the active sites, the catalyst could collect as much phosphorus as possible. Wt % P Part Figure 5. Distribution of phosphorus in the deactivated catalysts. Catalytic activity test The poisoned parts show a decrease in activity in comparison with the fresh part. At 3 o C, were normally very good activity is reached, part 1, which has a phosphorus concentration of 3.71 wt

4 %, has a NO conversion of 5 % while the fresh part shows a NO conversion of almost 8 %. Due to the stoichiometrics between NO and NH 3 this is the highest conversion that can be reached. As the phosphorus concentration in the parts tested decreases, the activity increases. Part 9, which has the smallest phosphorus concentration of 2.33 wt %, the NO conversion reaches 76 % at 3 o C. In figure 6 the conversion for all the parts, including the fresh part, is plotted against the reaction temperature. Since the fresh part, in this temperature range, has a very good conversion towards NO, the curve for the fresh part does not show the s-shape as in figure 6 for a fresh catalyst. In comparison with the fresh part the deactivate parts conversion maximum has moved to the right. This is normally due to loss in active sites, associated with selective poisoning or sintering. However, during deactivation the temperature in the catalyst was held at temperatures where sintering should not occur. Therefore, the change in temperature for maximum conversion is due to selective poisoning of the active sites. The higher the phosphorus concentration the more to the right the maximum of conversion is moved, indicating that the phosphorus concentration affects the active sites by selective poisoning. The slopes in figure 6 has not only moved to the right in the temperature range, the deactivated parts can also no longer reach as high a conversion as the fresh part does. The slopes have flattened out, leading to a reduced capability of conversion. According to ii catalyst deactivation associated with increased pore diffusion resistance leads to a lower conversion, with changes in the slope in the pore diffusion controlled region. The catalyst may not be affected at low temperatures but at medium temperatures, limitation is evident. But since the slopes both changed in shape and moved to the right a combination of selective poisoning and increased pore diffusion, commonly known as masking, is decreasing the activity of the deactivated parts. Conversion of NO, % Temp, C Figure 6. Conversion vs. reaction temperature. Part 1 Part 3 Part 5 Part 7 Part 9 Figure 7 shows the activation energy as a function of the phosphorus percentage. The activation energy is calculated from the three lowest temperatures. The activation energy for the part fresh is not included in this figure due to the fact that in the temperature interval used, the activation energy for the part fresh is limited by mass transport. As the figure shows phosphorus concentration affects the activation energy by changing the active sites, by coverage or selective poisoning. In addition, there is a linear relationship between the activation energy and the concentration of phosphorus. At the lowest phosphorus concentration, an activation energy of 38 kj per mole is registered and at the highest phosphorus concentrations, an activation energy of 6 kj per mole is registered. Ea, kj/mole Wt % P Figure 7. Activation energy plotted against phosphorus concentration. Conclusion The way of accelerated poisoning used here showed very good results. After only hours of deactivation a strong deactivation was obtained. This result shows that it is possible to achieve an accelerated aging, through doping of the diesel fuel, representing thousands of hours of road deactivation. Concentration analysis of phosphorus in the catalyst showed that the concentration varied from the inlet to the outlet. The highest concentration was at the inlet of the catalyst, with 3.72 weight percent and the lowest at the Fresh

5 outlet with 2.33 weight percent. These concentrations correspond to a total collection efficiency of weight percent of the phosphorus added to the fuel. This is probably the result of the constant running conditions and lack of other poisons to compete for the active sites used. The activity test in the laboratory confirmed that the activity decreases with an increased amount of phosphorus caused by a decrease in the number of active sites on the catalyst surface through selective poisoning and pore occlusion leading to increase in inner mass transport resistance and loss of active surface. The combination of these two effects leads to the deactivation caused by the phosphorus. The phosphorus also affected both the amount and the distribution of the acid sites. With an increase in concentration of phosphorus, the amount of acid sites decreases. The distribution between strong and weak acid sites is also dependent of the phosphorus concentration. With an increase of phosphorus, the BET surface area decreases. At a phosphorus concentration of 3.77 wt% the total surface area has decreased by 5 %. This decrease in BET surface area shows that the phosphorus species formed on the surface of the catalyst efficiently covers the surface, also leading to pore blocking. This is conformed by the increased in average pore radius as the concentration increases. The phosphorus has not affected the crystalline substances detectable with XRD on the catalyst surface. No changes on the surface are shown on the surface for part 3-9. On part 1 and 2 a new unknown substance has formed. The two peaks visible could originate from an unknown phosphorous substance on the surface. Nevertheless, since there are no other peaks that could reveal the unknown substance, it is hard to tell what substance the peaks represent. iv D. S. Lafyatis, R. Petrow, C. Bennet, The Effects of Oil-Derived Poisons on Three-Way Catalyst Performance., SAE Paper # v D. J. Ball, A. G. Mohammed, W.A. Schmidt, Application of Accelerated Rapid Aging Test (RAT) Schedules with Poisons: The Effects of Oil Derived Poisons, Thermal Degradation and Catalyst Volume on FTP Emissions, SAE Paper # vi H. Kamata, K. Takahashi, C.U.I. Odenbrand, Surface acid property and its relation to SCR activity of phosphorus added to commercial V 2 O 5 (WO 3 )/TiO 2 catalyst., Catalysis letters 53 (1998) vii J. Zhu, B. Rebenstorf, S.L.T. Andersson, Influence of Phosphorus on the Catalytic Properties of V 2 O 5 /TiO 2 Catalysts for Toluene Oxidation, J. Chem. Soc., Faraday Trans 1, 1989, 85(11), References i G.S Madia, Ph.D.Thesis 22, Measure to enhance the NOx conversion in urea-scr systems for automotive applications, Swiss Federal Institute of Technology. ii Dieselnet iii B.G.Bunting, K.More, S.Lewis, Phosphorus Poisoning and Phosphorus Exhaust Chemistry with Diesel Oxidation Catalysis., SAE Paper #