Elucidating NH3 formation during NO reduction by CO on Pt–BaO/Al2O3 in excess water

a b s t r a c tWe 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 alumina supported 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.

Section: Discussionmentioning

confidence: 94%

“…If the criterion on the H 2 /NO ratio is not fulfilled, selectivity to other nitrogen-containing products has been observed [11,15,16], for example N 2 O and N 2 by the two reactions:…”

Section: Introductionmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 97%

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Ammonia formation over supported platinum and palladium catalysts

Adams

Skoglundh

Folić

et al. 2015

“…A parallel, indirect pathway of NO x reduction by CO and hydrocarbons under rich conditions involves water gas shift and steam reforming reactions where hydrogen formed from CO and hydrocarbons in the presence of water acts as the NO x reducing agent [24,25].…”

Section: And N 2 Omentioning

confidence: 99%

Dynamics of N2 and N2O peaks during and after the regeneration of lean NO trap

Mráček

Kočí

Marek

et al. 2015

a b s t r a c tThe dynamics and selectivity of N 2 and N 2 O formation during and after the regeneration of a commercial NO x storage catalyst containing Pt, Pd, Rh, Ba on Ce/Zr, Mg/Al and Al oxides was studied with high-speed FTIR and SpaciMS analyzers. The lean/rich cycling experiments (60 s/5 s and 60 s/3 s) were performed in the temperature range 200-400 • C, using H 2 , CO, and C 3 H 6 individually for the reduction of adsorbed NO x . Isotopically labeled 15 NO was employed in combination with Ar carrier gas in order to quantify the N 2 product by mass spectrometry. N 2 and N 2 O products were formed concurrently. The primary peaks appeared immediately after the rich-phase inception, and tailed off with breakthrough of the reductant front (accompanied by NH 3 product). Secondary N 2 and N 2 O peaks appeared at the rich-tolean transition as a result of reactions between surface-deposited reductants/intermediates (CO, HC, NH 3 , -NCO) and residual stored NO x . At 200-300 • C, up to 30% of N 2 and 50% of N 2 O products originated from the secondary peaks. The N 2 O/N 2 selectivity ratio as well as the magnitude of secondary peaks decreased with temperature and duration of the rich phase. Among the three reductants, propene generated secondary N 2 peak up to the highest temperature. The primary N 2 peak exhibited a broadened shoulder aligned with movement of reduction front from the zone where both NO x and oxygen were stored to the NO x -free zone where only oxygen storage capacity was saturated. N 2 formed in the NO x -free zone originated from reaction of NH 3 with stored oxygen, while N 2 O formation in this zone was very low.

“…Parallel indirect pathways of NO x reduction by CO and hydrocarbons involves water gas shift and steam reforming reactions where hydrogen formed from CO and hydrocarbons in the presence of water acts as the NO x reducing agent [10,21].…”

Section: Introductionmentioning

confidence: 99%

New operation strategy for driving the selectivity of NO reduction to N2, NH3 or N2O during lean/rich cycling of a lean NO trap catalyst

Mráček

Kočí

Choi

et al. 2016

a b s t r a c tPeriodical regeneration of NO x storage catalyst (also known as lean NO x trap) by short rich pulses of CO, H 2 and hydrocarbons is necessary for the reduction of nitrogen oxides adsorbed on the catalyst surface. Ideally, the stored NO x is converted into N 2 , but N 2 O and NH 3 by-products can be formed as well, particularly at low-intermediate temperatures. The N 2 and N 2 O products are formed concurrently in two peaks. The primary peaks appear immediately after the rich-phase inception, and tail off with the breakthrough of the reductant front accompanied by NH 3 product. The secondary N 2 and N 2 O peaks then appear at the rich-to-lean transition as a result of reactions between surface-deposited reductants/intermediates (CO, HC, NH 3 , NCO) and residual stored NO x under increasingly lean conditions. Based on these mechanistic insights, we propose and demonstrate a novel strategy for driving the selectivity of the secondary peaks towards desired products. It is based on a transition phase of neutral or slightly lean (nearly stoichiometric) character inserted between the rich and the fully lean phase. This strategy allows more complete regeneration of the catalyst with higher N 2 yield and without the undesired formation of a secondary N 2 O peak. Furthermore, NH 3 can be formed during this slightly lean transition phase without any CO or hydrocarbons breakthrough. Such ammonia formation is desirable in the exhaust gas aftertreatment systems combining LNT with passive SCR technology.