Reaction pathway of the reduction by CO under dry conditions of NOx species stored onto PtBa/Al2O3 Lean NOx Trap catalysts (original) (raw)

Reduction of stored NOx on Pt/Al2O3 and Pt/BaO/Al2O3 catalysts with H2 and CO

Journal of Catalysis, 2006

In situ Fourier transform infrared spectroscopy, coupled with mass spectrometry and time-resolved X-ray diffraction, were used to study the efficiency of nitrate reduction with CO and H 2 on Pt/Al 2 O 3 and Pt/BaO/Al 2 O 3 NO x storage reduction (NSR) catalysts. Surface nitrates were generated by NO 2 adsorption, and their reduction efficiencies were examined on the catalysts together with the analysis of the gas-phase composition in the presence of the two different reductants. H 2 was found to be a more effective reducing agent than CO. In particular, the reduction of surface nitrates proceeds very efficiently with H 2 even at low temperatures (∼420 K). During reduction with CO, isocyanates form and adsorb on the oxide components of the catalyst; however, these surface isocyanates readily react with water to form CO 2 and ammonia. The NH 3 thus formed in turn reacts with stored NO x at higher temperatures (>473 K) to produce N 2 . In the absence of H 2 O, the NCO species are stable to high temperatures and are removed from the catalyst only when they react with NO x thermal decomposition products to form N 2 and CO 2 . The results of this study point to a complex reaction mechanism involving the removal of surface oxygen atoms from Pt particles by either H 2 or CO, the direct reduction of stored NO x with H 2 (low-temperature NO x reduction), the formation and subsequent hydrolysis of NCO species, and the direct reaction of NCO with decomposing NO x (high-temperature NO x reduction).

n-Heptane As a Reducing Agent in the NOx Removal over a Pt–Ba/Al2O3 NSR Catalyst

ACS Catalysis, 2014

The reduction with n-heptane of gaseous NO and NO x stored over a model PtBa/Al 2 O 3 catalyst is investigated in this paper by transient microreactor flow experiments coupled with in situ FT-IR spectroscopy for the analysis of the surface species. It is found that n-heptane is an effective reductant of both gaseous NO and stored NO x at temperatures above 200 and 250°C, respectively. The reduction leads to the selective formation of N 2 above 300°C , whereas significant amounts of other species (N 2 O) are also formed at lower temperatures. A reaction pathway for the reduction of stored NO x is suggested where n-heptane initially reduces the oxygen-covered Pt sites. This leads to the activation/ destabilization of the stored NO x , leading to the NO x release. Released NO x dissociate over Pt sites into N-and O-adatoms: the O-adatoms are scavenged by the hydrocarbon leading to CO x and H 2 O, whereas N-adatoms may recombine with undissociated NO molecules, with other N-adatoms, or with Had species to give N 2 O, N 2 , and NH 3 , respectively. The Pt oxidation state (or the oxygen coverage) drives the selectivity of the process. Under wet conditions the hydrocarbon molecule is involved in the SR reaction and WGS reactions as well, leading to the formation of H 2 that participates in the reduction of stored NO x. This route is however of minor importance over the selected catalyst. Finally isocyanate species are also detected under conditions favoring the formation of CO, that may participate in the reduction of the stored NO x .

Catalytic abatement of NO x : Chemical and mechanistic aspects

Catalysis Today, 2005

The chemical and mechanistic aspects of the selective catalytic reduction (SCR) of NO by ammonia and by methane have been investigated. In the classical NH 3 -SCR process, operating near 600 K over vanadia-titania based catalysts, ammonia is activated by coordination over Lewis acid sites and reacts with gas phase or weakly adsorbed NO. The same mechanism occurs for the low-temperature (400-500 K) NH 3 -SCR over Mn and Fe based catalysts. On the contrary, low-temperature NH 3 -SCR over protonic zeolites implies the activation of ammonia as ammonium ions and the previous oxidation of part of NO to NO 2 . The CH 4 -SCR over Co-zeolite catalysts is supposed to imply the activation of NO x in the form of an adsorbed oxidized species, that reacts with the reductant, CH 4 , from the gas phase or activated by adsorption into the zeolite channels. The other catalytic denoxing technologies, like the NO decomposition over Cu-zeolite based catalysts, the N 2 O decomposition over noble or transition metal based catalysts, the storage-reduction on Ba-aluminate based catalysts and the reduction by CO over noble metal based three way catalysts, imply a strong adsorption and activation of NO x over the surface, although activation of hydrocarbons and CO over the noble metals can also be helpful in the last two cases.

The NOx storage-reduction on PtK/Al2O3 Lean NOx Trap catalyst

Journal of Catalysis, 2010

The nature of stored NO x and mechanistic aspects of the reduction of NO x stored over a model PtAK/Al 2 O 3 catalyst sample are investigated in this paper, and a comparison with a model PtABa/Al 2 O 3 catalyst is also made. It is found that at 350°C on both the catalysts the storage proceeds with the initial formation of nitrites, followed by the oxidation of nitrites to nitrates. A parallel pathway involving the direct formation of nitrates species is also apparent; at saturation, only nitrates are present on the catalyst surface over both PtAK/Al 2 O 3 and PtABa/Al 2 O 3. However, whereas bidentate nitrates are present in remarkable amounts on PtAK/Al 2 O 3 , along with ionic nitrates, only very small amounts of bidentate nitrates were observed on PtABa/Al 2 O 3. Under nearly isothermal conditions, the reduction of the stored NO x with H 2 occurs via an in series twosteps Pt-catalysed molecular process involving the formation of ammonia as an intermediate, like for the PtABa/Al 2 O 3 catalyst sample. However, higher N 2 selectivity is observed in the case of the PtAK/Al 2 O 3 catalyst due to the similar reactivity of the H 2 + nitrate and NH 3 + nitrate reactions. Accordingly ammonia, once formed, readily reacts with surface nitrates to give N 2 , and this drives the selectivity of the reduction process to N 2. Notably, a strong inhibition of H 2 on the reactivity of NH 3 towards nitrates is also pointed out, due to a competition of H 2 and NH 3 for the activation at the Pt sites. Finally, the effect of water and CO 2 on the reduction process is also addressed. Water shows a slight promotion effect on the reduction of the nitrates by H 2 , and no significant effect on the reduction by ammonia, whereas CO 2 has a strong inhibition effect due to poisoning of Pt by CO formed upon CO 2 hydrogenation.

Reaction Pathways in the Selective Catalytic Reduction Process with NO and NO2 at Low Temperatures

Industrial & Engineering Chemistry Research, 2001

The low-temperature behavior of the selective catalytic reduction (SCR) process with feed gases containing both NO and NO 2 was investigated. The two main reactions are 4NH 3 + 2NO + 2NO 2 f 4N 2 + 6H 2 O and 2NH 3 + 2NO 2 f NH 4 NO 3 + N 2 + H 2 O. The "fast SCR reaction" exhibits a reaction rate at least 10 times higher than that of the well-known standard SCR reaction with pure NO and dominates at temperatures above 200°C. At lower temperatures, the "ammonium nitrate route" becomes increasingly important. Under extreme conditions, e.g., a powder catalyst at T ≈ 140°C, the ammonium nitrate route may be responsible for the whole NO x conversion observed. This reaction leads to the formation of ammonium nitrate within the pores of the catalyst and a temporary deactivation. For a typical monolithic sample, the lower threshold temperature at which no degradation of catalyst activity with time is observed is around 180°C. The ammonium nitrate route is interesting from a standpoint of general DeNO x mechanisms: This reaction combines the features typical to the SCR catalyst with the features of the NO x storage-reduction catalyst, i.e., NO x adsorption to a basic site.

A simplified simulation of the reaction mechanism of NOx formation and non-catalytic reduction

Combustion Science and Technology, 2018

During fossil fuel combustion, pollutants, such as NO x , SO 2 , CO, CO 2 , organic compounds and fly ash are produced. Taking into consideration that emission limits have been becoming stricter it is crucial to apply technologies that reduce pollutant formation. This work focuses on NO x formation and its consequent emission reduction via SNCR technology. A mathematical model based on the kinetic description of NO x production and its non-catalytic reduction for a boiler operating under specified conditions was developed. A large number of chemical reactions take place during NO x formation and reduction inside the boiler reduction zone. In this paper various important reactions that have significant influence on the SNCR process were selected. Based on the selected reactions a simplified SNCR reaction mechanism was assembled and converted into a numerical model. The model was applied for a denitrification process taking place in the temperature range 850-1050°C. Urea was used as reducing agent. Input gas contained NO in the order of 10-5 molar fraction. Other components of input gas were 6.6 mole% water vapour, 13 mole% CO 2 , 4 mole% O 2 , 0.3 mole% CO, 0.05 mole% H 2 and the balance being N 2. Residence time was 0-2 s. The developed model makes possible to define the reducing zone in different types of boiler while using various reducing agents as well as to predict the degree of denitrification. As a result it is possible to optimize SNCR for any given boiler. The results obtained from model calculations demonstrated that the developed reaction mechanism of NO x formation and non-catalytic reduction can be applied.

A model for the catalytic reduction of NO with CO and N desorption

Physica A: Statistical Mechanics and its Applications, 2017

h i g h l i g h t s • Individual CO and N desorption are added to the Yaldram-Khan model. • The system become reactive when N desorption is added. • The system has enormous fluctuations and a long-lasting non-steady reactive state when individual CO desorption is included. • Spreading of N structures happens through the process.