| 1. |
- Adams, Emma, 1989, et al.
(författare)
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Structure-function relationship for alumina supported platinum during formation of ammonia from nitrogen oxide and hydrogen in presence of oxygen
- 2016
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Ingår i: Physical Chemistry Chemical Physics. - 1463-9084 .- 1463-9076. ; 18:16, s. 10850-10855
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Tidskriftsartikel (refereegranskat)abstract
- We study the structure-function relationship of alumina supported platinum during forma- tion of ammonia from nitrogen oxide and dihydrogen by employing in situ X-ray absorption and Fourier transformed infrared spectroscopy. Particular focus is directed towards the effect of increased levels of oxygen on the reaction as a model system for emerging technologies for passive selective catalytic reduction of nitrogen oxides. The suppressed formation of ammo- nia observed as the feed becomes net-oxidizing is accompanied by a considerable increase in the oxidation state of platinum as well as enhanced formation of surface nitrates and loss of NH-containing surface species. In the presence of (excess) oxygen, the ammonia formation is proposed to be limited by the weak interaction between nitrogen oxide and the oxidized platinum surface. This leads to slow dissociation rate of nitrogen oxide and thus low abun- dance of atomic nitrogen surface species that can react with adsorbed hydrogen atoms. In this case the consumption of hydrogen through the competing water formation reaction and decomposition/oxidation of ammonia are of less importance for the net ammonia formation.
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| 2. |
- Adams, Emma, 1989, et al.
(författare)
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Ammonia formation from nitric oxide over Pd-based catalysts in multicomponent feed gas compositions
- 2017
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Ingår i: Catalysis Communications. - : Elsevier BV. - 1566-7367. ; 95, s. 26-30
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Tidskriftsartikel (refereegranskat)abstract
- Using multicomponent gas feeds, the formation of ammonia from nitric oxide over Pd/Al2O3 and Pd/Ce/Al2O3 model catalysts was compared to that of a commercially-available three-way catalyst. Steady- state oxygen-sweep experiments show that the highest activity for ammonia formation is obtained over the Pd/Ce/Al2O3 catalyst whilst the lowest activity is observed for the three-way catalyst. The latter is consid- ered to be linked to the incorporation of rhodium, a component that promotes the NOx reduction selectivity towards N2. Also, lean/rich cycling experiments were carried out to simulate the cycling conditions that passive-SCR after treatment systems depend upon. High activity is again seen over the Pd/Ce/Al2O3 cata- lyst during short periodic switches. For the three-way catalyst, longer periodic switches are required for the onset of ammonia formation due to the high oxygen storage capacity of this sample as compared to the other two. Hence, a future direction of investigation could be to develop materials with equivalent water- gas shift properties of ceria, but with reduced oxygen storage capacity so as to provide hydrogen for reaction without incurring a significant delay in ammonia formation.
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| 3. |
- Adams, Emma, 1989, et al.
(författare)
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Ammonia formation over Pd/Al2O3 modified with cerium and barium
- 2016
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Ingår i: Catalysis Today. - : Elsevier BV. - 0920-5861. ; 267, s. 210-216
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Tidskriftsartikel (refereegranskat)abstract
- We report experimental results for ammonia formation from nitric oxide and either a direct source of hydrogen or from a mixture of carbon monoxide and water over palladium based catalysts. Specifically, the addition of barium or cerium into an alumina supported palladium sample was studied. Static and transient flow reactor experiments were performed in order to identify the effects of temperature and the presence of oxygen on the activity for ammonia formation. Modification of Pd/Al2O3 with cerium proved to be beneficial for the activity due mainly to its enhancement of the water-gas-shift reaction, thus providing a higher availability of hydrogen for ammonia formation, but also because it remains active in the presence of slightly oxidizing global conditions when hydrogen is provided directly to the feed. Although the modification of Pd/Al2O3 with barium did not affect the ammonia formation during static conditions, the activity during lean/rich cycling increased. This is important for applications of passive selective catalytic reduction.
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| 4. |
- Adams, Emma, 1989, et al.
(författare)
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Ammonia formation over supported platinum and palladium catalysts
- 2015
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 165, s. 10-19
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Tidskriftsartikel (refereegranskat)abstract
- We report experimental results for the formation of ammonia from nitric oxide and hydrogen, and from nitric oxide, water and carbon monoxide over silica, alumina and titania supported platinum and palladium catalysts. Temperature programmed reaction experiments in gas flow reactor show a considerable formation of ammonia in the temperature range 200-450 C, which is suppressed by the presence of excess oxygen. However, oxygen sweep experiments show that for the titania supported catalysts minor amounts of oxygen promotes the ammonia formation at low temperatures. In situ DRIFT spectroscopy measurements indicate that cyanate species on the support play an important role in the ammonia formation mechanism. This work shows that aluminasupported palladium is a promising system for passive selective catalytic reduction applications, exhibiting low-temperature activity during the water-gas-shift assisted ammonia formation reaction. Conversely, titania supported samples are less active for ammonia formation as a result of the poor thermal stability of the titania support.
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| 5. |
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| 6. |
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| 7. |
- Adams, Emma, 1989, et al.
(författare)
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Passive SCR: The effect of H2 to NO ratio on the formation of NH3 over alumina supported platinum and palladium catalysts
- 2016
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Ingår i: Topics in Catalysis. - : Springer Science and Business Media LLC. - 1572-9028 .- 1022-5528. ; 59:10-12, s. 970-975
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the relationship between the H2:NO ratio and NH3 formation over alumina supported Pt and Pd catalysts. By kinetic studies and in situ infrared spectroscopy, we report that NH3 formation is not only sensitive to the catalyst formulation but equally dependent on the feed gas composition and tem- perature. Specifically, we identify that hydrogen plays an important role in the dissociation of NO at low tem- perature. We also show that the support material itself plays a vital role in the ammonia formation mechanism due to the redox behaviour of NO adsorption at low temperature. This was unexpected as the noble metal is generally considered to be the active phase for the reaction of NO and H2.
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| 8. |
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| 9. |
- Adams, Emma, 1989, et al.
(författare)
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The chemistry of the palladium phase in Pd/Ce/Al2O3 during ammonia formation
- 2016
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The formation of NH3 from NO is a crucial reaction for passive-selective catalytic reduction applications. In this application, an in situ supply of NH3 is formed in oxygen-lean conditions, which can subsequently be used to reduce NO to N2 in the presence of excess O2 [1]. On the fundamental level, producing NH3 from NO through heterogeneous catalytic processes involves fascinating surface chemistry that is not yet well understood. In the presence of stoichiometric and excess oxygen concentrations, NH3 formation is suppressed. However, we have previously shown that Pd/Ce/Al2O3 is a promising formulation, able to temporarily facilitate the formation of ammonia under globally slightly-oxidizing but locally rich conditions [2].This work uses in situ X-ray absorption spectroscopy (XAS) to identify how the oxidation state of the catalytically active phase is affected during transient pulses of O2 in the presence of either pure NO or a combination of NO, CO and H2.
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| 10. |
- Adams, Emma, 1989, et al.
(författare)
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Water–gas-shift assisted ammonia formation over Pd/Ce/alumina
- 2018
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Ingår i: Catalysis Today. - : Elsevier BV. - 0920-5861. ; 307, s. 169-174
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Tidskriftsartikel (refereegranskat)abstract
- We report experimental results for the water–gas-shift assisted formation of ammonia from nitric oxide over alumina supported palladium, modified with various amounts (0–38 wt.%) of cerium. Static and transient flow reactor experiments were performed in order to identify the effects of temperature and the presence of oxygen on the activity for ammonia formation over the catalysts. The water–gas-shift reaction is enhanced with increasing cerium content, thus providing more hydrogen for ammonia formation. During transient operation at temperatures above 350 °C, a slight delay in ammonia formation is observed over the sample containing the highest cerium loading due to its (relatively) high dynamic oxygen storage capacity. At temperatures below 175 °C, N2O formation is preferred over NH3 formation. This work highlights the importance of balancing the effects that ceria influences when choosing the ceria loading for ammonia formation in passive-SCR applications.
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