NH 3 temperature programmed desorption (TPD) studies

NH3 TPD studies were performed in a flow U-tube micro-reactor setup (Altimira Instruments). The methodology employed in this section is based on previous approaches by

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Lietti and co-workers (Lietti et al., 1997; Lietti et al., 1998). The feed gases supplied to the micro-reactor comprised 200 ppm NH3 / He, 5 % O2 / He and He (BOC Beta Standard). Flow rates of chosen feed gases were measured and controlled by two mass-flow controllers with 50 and 100 ml min^-1 flow rate limits respectively. Gas flows through the two mass-flow controllers were then mixed into a single stream prior to the reactor at a tube T-junction of the two feed gases.

The reactor comprised a U-shaped quartz tube (3 mm ID) which was filled with 1.5 cm height quartz wool, 100 mg V₂O₅-WO₃/TiO₂ catalyst and 1.5 cm height quartz wool on the up-flow side of the tube. During operation, the U-tube was contained within an electric furnace driven by a proportional-integral-derivative (PID) controller. A K-type thermocouple was directly positioned in the catalyst bed during operation to act as a temperature control.

The temperature of the furnace was also measured during operation.

Reactor exit stream analysis was carried out using a mass spectrometer (Ametek Process Instruments: Proline-Dycor) which was directly connected downstream of the U-tube reactor. During operation, eight different mass-to-charge (m/e) ratios were used to monitor the gas phase components, namely: 4 (He), 17 (NH3), 18 (H2O), 28 (N2), 30 (NO), 32 (O2), 44 (N2O) and 46 (NO2). Quantitative analysis was enabled using response factors measured experimentally from blank tube experiments with the supplied gases, which carried an analysis certification from BOC. Further blank checks were carried out to elucidate the interference of H2O on m/e 17. Relevant interference factors were applied to the data to reconcile gas-phase compositions.

Table 5.3 shows the typical reactor feed conditions employed in the process of obtaining the TPD profiles. It is noted that these experiments were carried out as a final step of a longer experiment which also comprises isothermal ammonia adsorption and desorption steps. Unfortunately, unlike in some literature setups (Lietti et al., 1997), the gas composition changes between each step involve a slower, non-stepped response owing to the distance between the reactor and inlet feed valve. Analysis of these steps (and incorporation into kinetic models) is currently in progress due to the additional non-linearity encountered and is not incorporated into this chapter. In future work this will assist in firstly, closing the NH3

balance in the experiment and secondly, providing a physically consistent initial condition for simulation of the subsequent NH3 TPD.

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To achieve the required space time and gas compositions, the flow of O2 / He was set to 25 ml min^-1 and 200 ppm NH3 / He to 100 ml min^-1. Space time under these conditions was 48000 h^-1. In this chapter, the program described in Table 5.3 involved isothermal ammonia adsorption and desorption steps at 523 K, for all catalysts employed. Blank (no catalyst) versions of this program were also employed and a calibration for NH₃ response on the mass spectrometer was also produced. The calibration response was tested at a number of different temperatures. An inert tracer (such as Ar) was not used in this chapter, but should be incorporated in future studies as an internal standard during transient response steps.

Table 5.3: Typical experimental program used in NH3 adsorption-desorption experiments in this study

Prog.

Stage

Total Flow Rate

(ml min^-1)

Overall gas composition Notes

1 125 1% O2 / He Ramp to 523 K from room temp.

(15 K min^-1), Hold for 120 min 2 125 160 ppm NH3 / 1% O2 / He Maintain 523 K, Hold for 90 min

3 125 1% O2 / He Maintain 523 K, Hold for 90 min

4 125 He Cool to 323 K, Hold for 5 min

5 125 He Ramp to 1273 K (15 K min^-1)

Transport criteria were checked at a temperature of 1023 K for these experiments with two distinct particle sizes in Table 5.4. This temperature marks the high-end of ammonia evolution during the TPD step. Calculations were made assuming 250 ppm ammonia desorbs, which was the maximum amount observed during the experiments. In common with the monolith experiments, gas-solid mass transport efficiencies approached unity. When assessing diffusion limitations, particle size was found to be important: mass transport efficiencies were compromised with larger particles (250 – 355 μm) under conditions where higher amounts of NH3 are desorbed. To ensure such transport effects are avoided, a 106 –

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180 μm size fraction was selected to maintain a >99% transport efficiency throughout the TPD experiments.

Table 5.4: Estimated mass transport criteria and efficiency values for a 0.1 g V₂O₅ -WO3/TiO2 catalyst powder fixed bed during step 5 in Table 5.3 at 1023 K kinetic models tested within this work contain multiple parameters including some which are non-linear (e.g. activation energies in the Arrhenius equation). The Levenberg-Marquardt procedure, an indirect method for constrained optimisation of parameters, is appropriate for this problem (Marquardt, 1963). For the steady state study described in section 5.3.2 and 5.3.3, reaction rates connecting the values of the input and response variables are linked by a single reaction (Eq. (5.1)), hence parameter estimation can be carried out explicitly:

u

15 Athena Visual Studio 14.2, Stewart & Associates Engineering Inc.

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Where yu is a model response, ξu are the process settings (input variables), θj are model parameters and εu is error, the latter of which comprises model and experimental error.

Sensitivities of model parameters can subsequently be derived as followed:

) model parameters under estimation (van der Waal, 2000).

For the temperature programmed desorption modelling described in section 5.3.4, gas phase concentration and surface coverage response variables were solved implicitly using a set of differential equations:





dt f

dy  (5.8)

Subsequently, a direct decoupled method is used to estimate parametric sensitivities (Caracotsios and Stewart, 1985):

In Eq. (5.8) it can be seen that defining sensitivities as a function of time allows them to be solved alongside the main system differential equations, improving solver efficiency and performance.

To minimise cross-correlation between activation energy (E_a) and pre-exponential factor (A_i) parameters, a re-parameterised Arrhenius equation was used:



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Where base temperature is Tbase (K) and Ai is the value of the rate constant ki at Tbase. The fitting process can be further improved by solving Ai as an exponential term and lumping fitted value, E_a with constants T_base and ideal gas constant, R (J K^-1 mol^-1) to give fitting parameter E_a,lump. This brings the values of A_i and E_a,lump into the same order of magnitude (typically 1 – 10) further reducing cross-correlation in this expression:



The above procedure was also applied to Van’t Hoff adsorption terms (Ki) featuring a heat of adsorption term (ΔHads) and associated Ai value.

For the TPD studies, data were modelled using a heterogeneous, one-dimensional plug-flow dynamic reactor model, with the assumptions of a catalyst bed that is isothermal and isobaric (as checked in Section 5.2.4). The model is derived following an unsteady-state differential mass balance for gaseous and adsorbed ammonia and is based on a number of previous works (Kobayashi and Kobayashi, 1972, Lietti et al., 1997, Colombo et al., 2012):

d surface (assuming ammonia has a molecular diameter of 3.6 Å), ra and rd denote the rates of ammonia adsorption and desorption respectively, CNH3 denotes the gas phase concentration of ammonia, ν denotes the interstitial gas velocity through the catalyst bed and Ω denotes catalyst ammonia storage capacity.

181 5.3 Results and discussion

5.3.1 Catalyst characterisation

XRD analysis of fresh and aged samples revealed only small structural differences, showing predominantly TiO₂-anatase phase in all cases (Figure 5.5). The strong reflection at 25.2° (a characteristic TiO2-anatase peak (Shi et al., 2011, Madia et al., 2002)), shows differences in height and peak area across the four samples, suggesting some variation in crystallite size.

Figure 5.5: XRD analysis of fresh and aged SCR1 and SCR 2 catalyst powders. All reflections are assigned to TiO2-anatase

The full-width at half maximum (FWHM) for the fresh samples is broader than that of the two aged samples. As a result, calculation of the TiO2-anatase average crystallite size in each sample was carried out using the Scherrer Equation (Klug and Alexander, 1974):

B shape

K





 

 cos (5.15)

Where τ is the mean size of ordered crystal domains (nm), Kshape is a shape factor (-), λ is the X-ray wave-length (nm), β is the line broadening at FWHM (-) and θB is the Bragg angle (-). Use of Eq. (5.13) shows a crystallite size of 24.1 and 27 nm for the fresh SCR1 and

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SCR2 catalysts respectively, whilst the aged samples are 34.3 and 34.7 nm respectively. The greater crystallite size in both aged samples is expected, given their extensive thermal ageing.

Such effects have been shown in previous ageing studies (Madia et al., 2002). The crystallite size of SCR2 is also greater than SCR1 in the fresh form, which may be driven by the higher calcinations temperature employed.

On initial inspection of Figure 5.6A, Raman spectroscopy shows the presence of TiO2-anatase, with spectrum exhibited bands at 397, 516 and 639 cm^-1 respectively (Madia et al., 2002). However, it is important to scrutinise each spectrum between 750 and 1050 cm^-1 (see Figure 5.6B. In all samples, a shouldered peak is observed at 980 cm^-1 and a weakly intense region is observed at ~800 cm^-1. The latter could not be confidently assigned to SCR1 (fresh).

Figure 5.6: Raman spectroscopy analysis of fresh and aged SCR1 and SCR2 catalyst powders. A) Overall spectra, B) Zoomed spectra between 750 and 1050 cm^-1. (■) denotes TiO2-anatase, (▲) W-O-W stretching vibration, (♦) M=O stretching vibration, where M

denotes metal.

Previous literature has attributed the ~800 cm^-1band as an overlapping of second-order anatase features with the W-O-W stretching of octahedrally co-ordinated W units in a similar environment to that of bulk WO3 (Madia et al., 2002, Alemany et al., 1995). Such works have suggested the presence of this band is consistent with ageing. This would support the lack of such a band on the SCR1 (fresh) spectrum, however the presence of this peak in SCR2 (fresh) suggests only a short, higher temperature exposure may be needed to generate it.

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Went et al., (1992) assign the ~800 cm^-1 band to polyvanadate species, albeit in V2O5/TiO2

catalysts. In such works it is observed with V2O5 loadings between 1.3 and 9.8% and is exacerbated by increased reduction of the catalyst in H2 at higher temperatures. An alternative argument may therefore suggest that this band relates to V³⁺=O groups.

It has previously been suggested that the small peaks at 980 cm^-1 relate to the growth of metavanadate and/or metatungstate chains by polymerisation of VOx and WOx groups (Madia et al., 2002). These may be apparent in all samples as a function of the higher temperature calcination and/or ageing procedures. Overall it appears the Raman spectra for all samples are relatively similar barring some small differences in SCR1 (fresh).