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Adsorption and desorption studies using NO and NH3, typically comprising spectroscopic and/or desorption techniques, represent an alternative route to extract detailed mechanistic information on supported V₂O₅ systems. Whilst extrapolation of predictive models from these systems to real working conditions is cautioned due to the clean conditions of operation (Busca et al., 1998), they provide an excellent complement to steady state model findings.

Transient response methods have been developed in great detail by the group of Tronconi, Forzatti and co-workers at Politecnico di Milano, Italy (Lietti et al., 1997); their work is based upon an approach originally described by Kobayashi and Kobayashi (1972) and Kobayashi (1982). Such methods, typically using a micro-reactor flow rig with mass spectrometer analysis, utilise abrupt step changes in gas composition and observe the response in catalyst behaviour. Figure 5.3 shows the evolution of V2O5-WO3/TiO2 catalyst behaviour when exposed to either a NH3 or NO step change. In the case of NH3 there is a clear uptake by the catalyst (A1) showing a strong surface adsorption effect. When the NH3 is cut from the feed, some of the adsorbed NH3 desorbs (A2) although a strongly bound proportion remains which can be removed via temperature programme desorption (TPD) (Srnak et al., 1992; Lietti and Forzatti, 1994). In Figure 5.3B, there is very little adsorption of NO which would support steady state kinetics findings.

The experimental approach in Figure 5.3 can be further applied to mixed gases (e.g.

NH₃ + NO + O₂) and targeted feed step changes can provide further response information (Nova et al., 2000). Also a step change response can be substituted for a ramped concentration response (concentration programmed reaction (CPR)) (Nova et al., 2001).

Data generated from these experiments can lead to the development of adsorption, desorption and reaction models for these catalysts. It has been suggested from these models that NH₃ adsorbs on V₂O₅/TiO₂ by a non-activated process (E_a = 0 kJ mol^-1) (Lietti et al., 1997). Regarding the desorption step, Temkin-type kinetics have provided better descriptions of data than their Langmuir-type counterpart, suggesting there are a range of adsorption sites on the catalyst surface (Tronconi et al., 1996; Lietti et al., 1997). Ed0

values of ~100 kJ mol^-1 are typically estimated (Srnak et al., 1992; Lietti et al., 1998).

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In some cases, a dual site modelling approach has been successful, particularly for NH3 adsorption on Fe-Zeolite catalysts at low temperatures (Colombo et al., 2012). Such models utilised a Temkin description for weak acid sites and a Langmuir description for strong acid sites. Similarly, an NO reaction step has been incorporated (Lietti et al., 1998), typically yielding an E_a of ~60 kJ mol^-1. A modified θ kinetics approach (an Eley-Rideal description utilising a critical (θNH3*

) coverage value) provided the most successful description in these cases, suggesting that the NO reduction rate is independent of surface coverage above this value. θ_NH3^* was estimated to be similar to surface V₂O₅ coverage for both V₂O₅-WO₃/TiO₂ and V₂O₅ /TiO₂ formulations.

Figure 5.3: Dynamic adsorption-desorption of NH₃ (Graph A) and NO (Graph B) on a model V₂O₅-WO₃/TiO₂ catalyst at 493 K (carrier gas: 1% O₂ / He at 120 ml min^-1 (STP)).

Dashed line: ideal step, Dotted Line: model fit (taken from Lietti et al., 1998) Fourier transform infra-red (FTIR), TPD and combined FTIR-TPD techniques have also been employed to understand the interaction of NH₃ and NO with supported V₂O₅

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catalysts. Topsøe et al., (1995) analysed FTIR spectra of TiO2 and 0.6 – 6 % V2O5/TiO2

under NH3 + O2 and NH3 + NO + O2 conditions. No spectra peaks were found which related to NO adsorption whilst clear adsorbed NH3 species were identified. For V2O5/TiO2, a number of sites were identified, namely OH groups, Lewis acid and Brønsted acid. V⁵⁺-OH sites were strongly linked to the NH₃ SCR reaction. These findings were further developed in a study of reduced V2O5-WO3/TiO2 and WO3/TiO2 catalysts for the standard and fast SCR reactions to explain importance of Vspecies (Tronconi et al., 2007).

In general, Lewis-bonded ammonia and ammonium ions bonded at Brønsted acid sites have been seen to prevail on V₂O₅/TiO₂ in many further studies via FTIR (Lietti et al., 1996;

Went et al., 1992a,b) and are shown in Figure 5.4. Differentiation of Ti⁴⁺ and vanadyl Lewis-acid sites is not possible using this method. To circumvent this, kinetic analysis of NH₃ adsorption and desorption methods can provide further direction to FTIR findings. Further characterisation using x-ray photoelectron spectroscopy (XPS), temperature programme reduction (TPR) and solid state ⁵¹V nuclear magnetic resonance (NMR) can also be employed (Bond and Tahir, 1991).

Figure 5.4: Proposed structures for ammonia adsorbed on V2O5/TiO2: a) Lewis-bonded NH3 at Ti sites; b) H-bonded NH3 at oxide sites; c)

Lewis-bonded NH3 at vanadyl sites; d) ammonium ions bonded at V Brønsted acid sites (taken from Busca et al., 1998)

172 5.1.3 Chapter Objectives

In this chapter, a challenge is presented to understand the fundamental differences in reactor performance between two commercial V2O5-WO3/TiO2 catalysts, which have undergone different calcination procedures prior to reactor use and show different de-NO_x performance in their fresh form. The three aims to achieve this and in the process develop new methodologies to further understand V2O5-WO3/TiO2 catalyst behaviour are thus:

 Combine an ‘apparent kinetics’ approach, based on integral operation commercial catalyst testing data, with a focused NH3 and NO adsorption-desorption type study to examine and understand the fundamental differences in two different V2O5 -WO3/TiO2 catalyst formulations.

 Understand the extent at which predominantly differential mode kinetic models from literature can be successfully applied to integral operation commercial catalyst testing data to elucidate the nature and magnitude of surface sites on the catalysts under scrutiny.

 Utilise methodologies to elucidate the nature and magnitude of storage and reaction sites on V2O5-WO3/TiO2 catalysts and allow comparison between different formulations.

5.2 Materials and methods

5.2.1 Sample preparation and properties

A V2O5-WO3/TiO2 catalyst (V2O5 = 1.7% w/w, WO3 = 9% w/w, TiO2 anatase form) was employed exclusively in this study and was supplied by Johnson Matthey Catalysts¹². Samples of this catalyst were received in extruded monolithic form with a cell density of 300 cells / in² and a specific surface area of 2200 m²/m³. Two distinct fresh versions of this catalyst are utilised in this study, SCR1 and SCR, which differ based on their final calcination procedure before use. SCR 2 underwent a calcination procedure in air at a temperature 30 K higher than SCR1 and for 50% longer time.

12 All catalysts tested in this study supplied courtesy of Dr. Jőrg Műnch of Johnson Matthey Catalysts, Redwitz, Germany.

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SCR1 and SCR2 catalysts were tested in both fresh and aged form. The ageing procedure for both catalysts was carried out in an oven with no flow at either 823 or 873 K for up to 1000 h. SCR1 and SCR 2 had BET surface areas of 69 and 56 m²g^-1 respectively when fresh and 39 and 38 m² g^-1 following 200 h thermal ageing at 873 K¹³.

All V₂O₅-WO₃/TiO₂ catalysts tested for steady state reaction studies (section 5.2.2) were retained in monolithic form (d = 1 in², l = 140 mm) and for adsorption-desorption-titration studies (section 5.2.3) were ground and sieved into powder form (<106, 106 – 180, 180 – 250, 250 – 355 μm sieve fractions).