Ammonia Decomposition Enhancement by Cs-Promoted Fe/Al 2 O 3 Catalysts
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Ammonia Decomposition Enhancement by Cs‑Promoted Fe/Al2O3 Catalysts Luke A. Parker1,2 · James H. Carter1 · Nicholas F. Dummer1 · Nia Richards1 · David J. Morgan1 · Stanislaw E. Golunski1 · Graham J. Hutchings1 Received: 9 January 2020 / Accepted: 1 May 2020 © The Author(s) 2020
Abstract A range of Cs-doped Fe/Al2O3 catalysts were prepared for the ammonia decomposition reaction. Through time on-line studies it was shown that at all loadings of Cs investigated the activity of the Fe/Al2O3 catalysts was enhanced, with the optimum Cs:Fe being ca. 1. Initially, the rate of NH3 decomposition was low, typically 1 is due to the growth of a CsOH layer that blocks active sites. This is in agreement with the results observed by Hill et al. for Cs-promoted Ru catalysts. Using the conversion at 500 °C after 24 h the surface normalized activity was calculated and is also shown in Table 1. Again this shows that activity initially increases with increasing Cs concentration, however, when normalised for surface area the 1Cs catalyst ( 105.4 mmolNH3 m−2 h−1 ) is ca. 20% more active than the 0.5Cs catalyst ( 82.0 mmolNH3 m−2 h−1 ) and this was not evident from time-on-line data. When normalized for the lower surface area, the 2Cs catalyst ( 75.5 mmolNH3 m−2 h−1 ) is still one of the least active catalysts, again demonstrating that Table 1 Post reaction BET surface area and surface normalised activity at 24 h T.O.L. for Cs-promoted Fe/Al2O3 catalysts
Fig. 2 XRD patterns of Cs-promoted Fe catalysts. Al2O3 reflections are indicated by filled circle and the reflection labelled with a filled square is due to the sample holder
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Catalyst
Surface Area (m2/g)
Conversion after Activity 24 h (%) ( NH3 m−2 h−1 mmol )
Al2O3 0.1Cs 0.25Cs 0.5Cs 1Cs 2Cs
124 122 129 144 112 70
0 14 59 67 67 30
n/a 20.2 80.6 82.0 105.4 75.5
Ammonia Decomposition Enhancement by Cs‑Promoted Fe/Al2O3 Catalysts
the Cs concentration must be carefully balanced between enhancing activity without blocking active sites. X-Ray photoelectron spectroscopy was used to investigate the state of both Fe and Cs in the samples. Figure 3 illustrates Cs 3d and Fe 2p/3 regions of the XPS spectrum. The large peak at 724.0 eV is attributed to CsOH which confirms the assumption from XRD data that the majority of the Cs is present as an amorphous hydroxide phase. The intensity of the peak increases as the Cs concentration increases, as expected. It is important to note that although care was taken to minimise contact of post-reaction samples with air, this does occur for a brief period during XPS sample mounting and may have an effect on sample speciation. The broad peak between 705 and 715 eV is due to two Fe species; FeO (708.7 eV) and Fe2O3 (710.7 eV) are shown in the highlighted section of Fig. 3. As XPS is a surface sensitive technique, this data can also give information on the composition of the reactive surface of the catalysts. As the Cs concentration increases the Fe signal decreases in intensity. This is further illustrated by the ratio of Cs
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