Selective Catalytic Reduction of NO_x with H₂ for Cleaning Exhausts of Hydrogen Engines: Impact of H₂O, O₂, and NO/H₂ Ratio

Abstract

A key concept for hydrogen-fueled internal combustion engines is reducing NO x emissions with direct H 2 -SCR. Besides platinum as an active noble metal component, palladium-based catalysts are attractive for NO reduction with high selectivity. However, reliability of the catalytic activity in the presence of water with persistently low side product formation remains challenging. Therefore, a monolithic 1%Pd/5%V 2 O 5 /20%TiO 2 -Al 2 O 3 model catalyst is studied extensively under different conditions, showing that NO x conversion remains high even in the presence of 5% water, maintaining over 65% selectivity toward N 2 . Additionally, the catalyst remained active in a long-term experiment over 12 h in the absence and presence of water with an NO conversion over 45%. Finally, steady-state measurements and a kinetic analysis reveal overall concentration dependencies alongside trends of the reduction reaction competing with the hydrogen combustion reaction.

The role of hydrogen for future internal combustion engines

“…In this case, selective catalytic reduction can be achieved on active catalytic substrates loaded with Pt and supported by g-Al 2 O 3 , SiO 2 , TiO 2 , which have been demonstrated to be suitable in the low temperature range, typical of hydrogen-fuelled engines running in lean/stoichiometric conditions. [64][65][66][67] Moreover, in the literature there are some examples of the combination of different catalysts, in particular Pt and Pd based catalysts supported on CeO 2 and MgO, 65 which provide a good solution for the whole 100-400°C temperature range.…”

“…However, in some cases, the addition of water vapour can lead to a positive effect on the catalytic reactions in terms of NO conversion, for example with a Pt/Mg-Ce-O catalyst in the low 100-200°C range; the N 2 selectivity appears to be only affected at temperatures higher than 200°C. [64][65][66] Generally, a higher hydrogen concentration in the exhaust gas results in an increase of both catalyst activity and N 2 selectivity. On the other hand, this high H 2 concentration could favour the NH 3 production mechanism, so that N 2 selectivity is reduced.…”

“…On the other hand, this high H 2 concentration could favour the NH 3 production mechanism, so that N 2 selectivity is reduced. 64 In other papers 66,67 it is remarked that also a dependency on temperature has to be considered, when looking at the concentration of H 2 in the feed gas. In fact, in the 170°C-220°C range, the high conversion of NO resulting from a higher H 2 concentration, is accompanied by a significant ammonia formation.…”

Der Wasserstoffmotor – Chancen und Herausforderungen auf dem Weg zu einer dekarbonisierten Mobilität

“…Abb. 8a zeigt beispielhaft eine fu ¨r die H 2 -SCR typische hu ¨gelartige NO-Konversionskurve: U ¨ber einem Pd/V 2 O 5 /TiO 2 -Al 2 O 3 Katalysator fu ¨hren steigende Temperaturen zuna ¨chst zu einem ausgepra ¨gten Anstieg des NO-Umsatzes, bevor dieser nach Durchlaufen eines Maximums bei etwa 220 °C wieder zuru ¨ckgeht [86]. Eine A ¨nderung der Katalysatormaterialien vera ¨ndert zwar das Umsatzmaximum sowie die Produktselektivita ¨ten, der hu ¨gelartige Trend der NO x -Umsatzkurve bleibt jedoch meist unabha ¨ngig von der Katalysatorzusammensetzung erhalten.…”

“…Der Einfluss des fu ¨r Wasserstoffmotorabgase typischen großen Wassergehalts auf das Katalysatorverhalten ist dagegen differenzierter zu bewerten. Wa ¨hrend Wasser auf Pt/Mg-Ce-O oder Pd/V 2 O 5 /TiO 2 -Al 2 O 3 in Abha ¨ngigkeit der Temperatur entweder nur sehr geringe oder sogar positive Einflu ¨sse hinsichtlich Aktivita ¨t, Selektivita ¨t und Stabilita ¨t zeigte [79,86], beobachteten Zhao et al [84] eine wasserinduzierte Verringerung des NO-Umsatzes u ¨ber Pt/HZSM5 bei Temperaturen u ¨ber 120 °C. Zusammenfassend la ¨sst sich folglich festhalten, dass der komplexe Einfluss von Wasser, der stark von der Temperatur, der Abgaszusammensetzung selbst und den verwendeten Katalysatormaterialien abha ¨ngt, fu ¨r eine Applikation der H 2 -SCR-Technologie in Serie weitergehend systematisch untersucht werden muss.…”