Kinetic analysis using the model of Dumesic et al

The model of Dumesic et al., (1996) used a rigorous approach and in its complete form features 6 reaction steps to cover both the acid and redox cycles in V₂O₅-WO₃/TiO₂ catalyst behaviour for ammonia SCR. Model parameters in this work were ascertained from an intrinsic kinetic study but were validated against industrial operating conditions, not too dissimilar to the feed and temperature conditions used in the current work.

Three of these steps, which involve product water removal, water inhibition and redox site re-oxidation (V³⁺  V=O) are omitted from this analysis. This is due to high feed concentrations of water and oxygen the current work, which are relatively unchanged at the reactor exit. The 3 remaining reactions considered are shown below (Eq. (5.15-17)):

M

188

Eq. (5.17) describes ammonia adsorption onto an acid site (M) and is a fast, equilibrated term (KNH3). Eq. (5.18) describes ammonia activation via a reactive site (S) and is slow and reversible (rate terms: k2 and k-2). Eq. (5.19) describes the interaction of NO with activated ammonia to produce products such as N₂. The above sequence is mathematically described below (Topsøe et al., 1995):

3 activation energy was employed, based on the values presented in Dumesic et al., (1996) for an V₂O₅-WO₃/TiO₂ catalyst operating under industrially relevant conditions. These values and confidence intervals are shown in Table 5.5 below:

Table 5.5: Kinetic parameters quoted in Dumesic et al., (1996) used in this study.

Parameter ΔHads or Ea (kJ mol^-1)

When fitting these data, the ΔHads and Ea values for each of the parameters were fixed, initially using the centre points in Table 5.5. This left 4 fitting parameters, the pre-exponential factors for each of the Ki terms, which were re-parameterised using a Tbase of 348 K. Initial model testing over all data sets provided a good fit (R² > 0.99) but in all cases the model was over parameterised. Using the parameter reduction method described in Chapter 3, terms K₁ and K₂ yielded insignificant contributions and were removed from the expression.

This is not surprising, due to the wide confidence intervals on the lumped energy values on

189

these parameters. Ultimately, this reduced the kinetic expression to the following equation which was used for the rest of the fitting process:

3

The two parameter model therefore describes two steps, ammonia adsorption (KNH3) and the forwards term for ammonia activation (k2). It is noted that these parameters can also be estimated separately, without the use of lumped terms. No NO reduction term (k3) is present, which agrees with the low Ea values estimated using the model of Lintz and Turek (1992). Previous apparent kinetics analysis revealed the apparent reaction order in NH3 to be approximately zero, suggesting that NH₃ strongly binds to the catalyst surface. The prevalence of the K_NH3 and k₂ in the final kinetic model support this as both pertain to chemisorbed NH₃ (in two different conformations).

The activation energies used in Eq. (5.21) required only minor changes to the values quoted by Dumesic et al., (1996) to optimise the fitting procedure. The adjustments carried out on k₂ were less than 1 kJ mol^-1 for both catalysts. This is well within the confidence range quoted in Table 5.5. R² values for all fits were > 0.99.

Parameter estimation results using Eq. (5.21) are shown in Figure 5.10. SCR1 and SCR2 are distinctly different from one another in their fresh form. SCR1 has a significantly higher KNH3 pre-exponential factor than SCR2 suggesting that it has a greater population of active sites for ammonia adsorption. Meanwhile, for k2, the reverse is true where SCR2 has a much greater population of sites for ammonia activation than SCR1. Given that ammonia activation has previously been cited as a slow step in the catalytic cycle, particularly in comparison to ammonia adsorption (Lintz and Turek, 1992; Dumesic et al., 1996; Efstatiou and Fliatoura, 1995; Forzatti, 2001) this may explain the better performance of SCR2 initially.

For 873 K ageing times of 50, 100 and 200 hr, both catalysts have largely similar k₂ values within parameter confidence intervals. Evolution of the K_NH3 differs for both catalysts.

The population of ammonia adsorption sites for SCR2 quickly equilibrates over the initial 50 hours ageing and then remains roughly constant. Meanwhile, KNH3 site populations for SCR1

190

decrease over 50 – 100 hours ageing before increasing again towards a similar value to SCR2 at 200 hours ageing.

Figure 5.10: Parameter estimates for re-parameterised pre-exponential factors (Tbase = 348K) of K_NH3 (Graph A) and k₂ (Graph B) for SCR1 and SCR2 catalysts. Ageing

conditions were 873 K. N.B.: Lines are to guide the eye.

191

Further analysis was carried out using 823 K ageing data for comparison (Figure 5.11). There are some important similarities and differences to 873 K garnered from this:

Figure 5.11: Comparison of parametric evolution (Tbase = 348K) of ANH3 (graph A) and A₂ (graph B) for SCR1 and SCR2 at different ageing temperatures against ageing time (open symbols: 823 K, closed symbols 873 K). N.B.: Lines are to guide the eye; error

bars are omitted for clarity.

192

 Initial evolution of both KNH3 and k2 for SCR2 over the first 50 hours is ageing temperature dependent; both parameters evolve more significantly under 873 K ageing.

 A SCR1 catalyst aged for 1000 hours at 823 K has very similar kinetic parameters to one aged for 200 hours at 873 K. Evolution of the catalyst is similar for both temperatures of the first 50 hours but appears to be a function of temperature over the longer periods of ageing.

 Similarly, SCR2 catalysts after 200 hours 873 K ageing or 1000 hours 823 K ageing have similar kinetic parameters. Longer term evolution of this catalyst appears to be ageing temperature dependent.

 Pre-exponential factors for KNH3 were also checked for thermodynamic consistency.

In this term, the pre-exponential represents e^ΔSads/R. (-ΔSads) must remain positive and not exceed the entropy of the gas, S⁰ at a given temperature and pressure. For the estimated parameters, (-ΔSads) was found to range between 46.2 and 52.4 J mol^-1 K^-1 which is lower than the entropy of ammonia at 100 ppm in the tested temperature range (222 and 242 J mol^-1 K^-1 for 453 and 773 K respectively (Barin, 1989).

Overall, the model of Dumesic et al., (1996) suggests that a fresh SCR2 catalyst has a high population of ammonia activation sites. These decay with ageing time whilst ammonia adsorption site population increases, thus NO_x conversion decreases as a result. Meanwhile SCR1 undergoes an initial ‘activation’ period whereby ammonia adsorption site population increases, as does NO_x conversion. Following this, SCR1 undergoes long term performance decay in a similar manner to SCR2.

The literature has discussed the importance of the ammonia activation step in context of the V₂O₅-WO₃/TiO₂ de-NO_x mechanism. Some works discuss this as involving interaction between an acid site (V⁵⁺-ONH₄) with a redox site (V=O) (Topsøe et al., 1995; Dumesic et al., 1996) and others discuss NH₃ adsorption (and storage) taking place on Ti sites with subsequent activation by reactive V sites (again such as V=O) (Lietti et al., 1998; Nova et al., 2001).

Based on these arguments, it is possible that an improved calcination procedure (SCR2) can generate an increased quantity of reactive V sites, or indeed an improved surface

193

distribution between NH3 adsorption/storage sites and reactive sites. Performance of SCR2 does decay with ageing time which may be a function of a shift in distribution of these sites or perhaps a change in physical-chemical structure of the reactive V sites. Interestingly, initial thermal ageing of SCR1 increases its quantity of reactive sites for ammonia activation and suggests that a certain level of thermal effects alone can develop a more active catalyst, as has been developed to a greater extent with the calcinations process for SCR2. The product performance data applied in this section has provided a working model for study, which will be probed in the subsequent section using TPD techniques. As the steady state kinetic model has suggested two different populations of chemisorbed NH₃ (adsorption and

‘activated’ sites), TPD will act as an initial approach to probe quantities of these adsorbed states.

The de-NOx performance sensitivities seen as a function of calcination and ageing time and temperature are in line with observations from a previous work (Kompio, 2010). A formulation comprising 5 wt. % WO3 and 1.5 wt. % V2O5 showed a non-linear progression of de-NOx performance as a function of calcination temperature (873 or 1023 K) and time (5 – 10000 min). de-NOx performance maxima were seen following a 15 min or 4 h calcination at 1023 K or a week or 4 weeks for a 873 K calcination.