Over the last several years, nitrogen oxide(s) (NOx) storage/reduction (NSR) catalysts, also referred to as NOx adsorbers or lean NOx traps, have been developed as an aftertreatment technology to reduce NOx emissions from lean‐burn power sources. NSR operation is cyclic: during the lean part of the cycle, NOx are trapped on the catalyst; intermittent rich excursions are used to reduce the NOx to N2 and restore the original catalyst surface; and lean operation then resumes. This review will describe the work carried out in characterizing, developing, and understanding this catalyst technology for application in mobile exhaust‐gas aftertreatment. The discussion will first encompass the reaction process fundamentals, which include five general steps involved in NOx reduction to N2 on NSR catalysts; NO oxidation, NO2 and NO sorption leading to nitrite and nitrate species, reductant evolution, NOx release, and finally NOx reduction to N2. Major unresolved issues and questions are listed at the end of ...

Overview of the Fundamental Reactions and Degradation Mechanisms of NOx Storage/Reduction Catalysts

Catalytic NOx Abatement Systems for Mobile Sources: From Three-Way to Lean Burn after-Treatment Technologies Pascal Granger* and Vasile I. Parvulescu* Unit e de Catalyse et de Chimie du Solide, UMR CNRS 8181, University of Lille 1, 59655 Villeneuve d’Ascq, France Department of Organic Chemistry, Biochemistry and Catalysis, University of Bucharest, Romania, 412 Regina Elisabeta Boulevard, Bucharest 030016, Romania

Catalytic NOx Abatement Systems for Mobile Sources: From Three-Way to Lean Burn after-Treatment Technologies

4.1. Noble Metals 4058 4.1.1. Platinum 4059 4.1.2. Palladium 4060 4.1.3. Bimetallic Systems 4060 4.2. Storage Components 4061 4.2.1. Earth Alkaline Metals 4061 4.2.2. Alkali Metals 4065 4.3. Supports 4065 4.3.1. Single Oxides 4065 4.3.2. Mixed Oxides 4067 4.4. Promoters 4069 4.5. Preparation Methods 4071 4.6. Influence of Remote Control 4073 5. Influence of Exhaust Gas Composition 4074 5.1. Reducing Gas 4074 5.2. Water 4077 5.3. Carbon Dioxide 4077 5.4. Impact of Soot 4079 6. Theoretical and Surface Science Studies 4079 6.1. Theoretical Studies 4079 6.2. Studies on Well-Defined Model Catalysts 4082 7. Reactor Configuration 4082 8. Conclusions and Outlook 4086 9. Acknowledgments 4087 10. References 4087

NOx Storage−Reduction Catalysis: From Mechanism and Materials Properties to Storage−Reduction Performance

The catalytic removal of nitrogen oxide (NOx) under lean‐burn conditions is one of the most important targets in catalysis research. Some lean‐NOx control technologies such as the direct decomposition of NOx, NOx storage‐reduction (NSR), and selective catalytic reduction (SCR) using different reducing agents (diesel soot, NH3, or hydrocarbon) are described. The reaction mechanism of NSR, which is the most promising technology, together with some novel NSR catalysts is discussed. Some mechanisms of SCR of NOx by hydrocarbon (HC‐SCR) were classified into two categories: one is the adsorption/dissociation mechanism, and the other is the oxidation‐reduction mechanism. Based on the discussion of the reaction mechanism, the influence of some factors (catalyst support, metal loading, calcination temperature, catalyst preparation method, oxygen, reducing agents, water, and sulfur) on the activity of HC‐SCR catalyst is discussed. It seems that Ag/Al2O3 catalyst offers the most promising for SCR of NOx by hydrocarb...

Recent Advances in Catalytic DeNOX Science and Technology

The diesel oxidation catalyst (DOC) has been part of diesel exhaust systems since regulations were introduced to limit the amount of harmful emissions released to the environment from diesel engines. The DOC primary functions are oxidation of CO, unburned hydrocarbons, and NO, while active hydrocarbon oxidation can also be used to generate exotherms required for downstream components. This review will describe work that has attempted to understand the reactions, both desired and undesired, that occur over the catalyst. First, the history, configuration, and components of the DOC will be discussed, followed by in-depth coverage of the fundamental reactions that occur over a DOC, including reaction mechanisms, reaction inhibition, and other reactivity effects. Finally, DOC deactivation mechanisms and their effects on the DOC are described. While there is a lot of research literature regarding Pt- and Pd-based catalysts for many different reaction schemes, this review tries to highlight work most relevant to...

Diesel Oxidation Catalysts

NOx adsorption study over Pt–Ba/alumina catalysts: FT-IR and pulse experiments

The reactivity of selected alkaline (Na, K, Cs) and alkaline-earth (Ca, Ba, Mg) oxide catalysts in the oxidation of a model soot sample (Printex U) is investigated in this study. With the aim of obtaining information about the intrinsic reactivity of these elements, samples have been prepared in which the active elements have been directly deposited on the soot (“full contact” conditions). In this way the reactivity of the catalytic elements is not controlled by the type of contact with the soot. It is found that the combustion of soot is greatly enhanced by the presence of either alkaline or alkaline-earth oxides, with Cs and Mg exhibiting the highest and the lowest activity, respectively. Notably, the reactivity in the soot combustion is found to nicely correlate with the electropositivity of the investigated metal ions, in line with literature indications pointing out the relevance of the electron-donor characteristics of the active elements. The correlation between electropositivity and activity in the soot combustion is not apparent in the absence of a close contact, i.e. under loose contact conditions. Under these conditions it is suggested that the catalytic activity is governed by the mobility of the surface species, which favors the soot–catalyst contact. Accordingly the poor performances observed in the case of the alkaline-earth metal oxides suggest for these elements a poor mobility of the active surface species, as opposite to alkaline oxide based catalysts showing a high reactivity even under loose contact conditions.

Intrinsic reactivity of alkaline and alkaline-earth metal oxide catalysts for oxidation of soot

On the dynamic behavior of “NOx-storage/reduction” Pt–Ba/Al2O3 catalyst

Adsorbents and catalyst supports produced by loading different amounts of potassium acetate on alumina followed by calcination are characterized using IR spectroscopy and TPD. CO, CO2 and NOx adsorption is studied. Two different ranges of potassium loading on alumina are found, namely light and heavy doping corresponding to 1% and more than 3% K wt/wt, respectively. Light doping results in the weak adsorption of CO2 as bicarbonate species and NO2 as bidentate nitrates. Heavy doping results in the adsorption of CO2 as bidentate carbonates and NO2 as polydentate nitrate species. Three families of exposed K+ ions are observed at increasing loading.

Lidia Castoldi

Papers

NOx adsorption study over Pt–Ba/alumina catalysts: FT-IR and pulse experiments

Intrinsic reactivity of alkaline and alkaline-earth metal oxide catalysts for oxidation of soot

On the dynamic behavior of “NOx-storage/reduction” Pt–Ba/Al2O3 catalyst

Basic catalysis and catalysis assisted by basicity: FT-IR and TPD characterization of potassium-doped alumina

Soot combustion: Reactivity of alkaline and alkaline earth metal oxides in full contact with soot