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

Plasma catalysis is gaining increasing interest for various gas conversion applications, such as CO2 conversion into value-added chemicals and fuels, CH4 activation into hydrogen, higher hydrocarbons or oxygenates, and NH3 synthesis Other applications are already more established, such as for air pollution control, eg volatile organic compound remediation, particulate matter and NOx removal In addition, plasma is also very promising for catalyst synthesis and treatment Plasma catalysis clearly has benefits over 'conventional' catalysis, as outlined in the Introduction However, a better insight into the underlying physical and chemical processes is crucial This can be obtained by experiments applying diagnostics, studying both the chemical processes at the catalyst surface and the physicochemical mechanisms of plasma-catalyst interactions, as well as by computer modeling The key challenge is to design cost-effective, highly active and stable catalysts tailored to the plasma environment Therefore, insight from thermal catalysis as well as electro- and photocatalysis is crucial All these aspects are covered in this Roadmap paper, written by specialists in their field, presenting the state-of-the-art, the current and future challenges, as well as the advances in science and technology needed to meet these challenges

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The 2020 plasma catalysis roadmap

This Perspective illustrates how the presence of internal and external electric fields can affect catalytic activity and selectivity, with a focus on heterogeneous catalysts. Specifically, experimental investigations of the electric field influence on catalyst selectivity in pulsed field mass desorption microscopes, scanning tunneling microscopes, probe–bed–probe reactors, continuous-circuit reactors, and capacitor reactors are described. Through these examples, we show how the electric field, whether externally applied or intrinsically present, can affect the behavior of a wide number of materials relevant to catalysis. We review some of the theoretical methods that have been used to elucidate the influence of external electric fields on catalytic reactions, as well as the application of such methods to selective methane activation. In doing so, we illustrate the breadth of possibilities in field-assisted catalysis.

Elucidating the Roles of Electric Fields in Catalysis: A Perspective

This paper describes the decomposition of gas-phase benzene using a plasma-driven catalyst (PDC) reactor packed with 1.0 wt.% Ag/TiO2 catalysts. The decomposition of benzene preferentially produced CO2 and CO, and formic acid as minor one. Carbon balance based on these products was satisfactory at around 100%. For the concentration lower than 110 ppm, the PDC reactor successfully decomposed benzene with specific input energy of around 130 J/l, where the formation of nitrogen oxides was small. The plasma-induced catalytic activity appeared only during plasma application and disappeared as the plasma was turned off. Thermal catalysis showed that temperature was not an important parameter in the decomposition of benzene using the PDC reactor. Comparison of the effects of dilution gases (Ar, N2) on the benzene decomposition revealed that the contribution of UV light from the plasma to the activation of Ag/TiO2 catalyst was negligible. The contribution of catalytic reaction became dominant as increasing specific input energy. The removed amount of benzene showed zero-order to the initial concentration of benzene and determined mostly by specific input energy to the PDC reactor.

Decomposition of gas-phase benzene using plasma-driven catalyst (PDC) reactor packed with Ag/TiO2 catalyst

Selective catalytic reduction of NO by hydrocarbons (HC-SCR) is one of the promising technologies for removal of NO in exhausts containing excess oxygen, such as diesel and lean burn gasoline engines. Supported Ag catalysts, especially Ag/Al2O3, are thought to be the promising candidates for use in diesel exhausts, as confirmed by several reports on engine bench tests. The HC-SCR performance of supported Ag catalysts is very sensitive to the reaction conditions, especially the type of hydrocarbons and the addition of H2. The control of reaction conditions would be key for practical use. The current research of supported Ag catalysts is reviewed from the viewpoints of practical use and the reaction mechanism, i.e., the reaction scheme, the role of surface adsorbed species, and the structure of active Ag species.

Selective catalytic reduction of NO over supported silver catalysts—practical and mechanistic aspects

The oxidative potential of a non-thermal plasma (NTP) in engine off-gases with excess oxygen results in an effective conversion of NO to NO2 that can be converted synergistically to molecular nitrogen with appropriate catalysts by selective catalytic reduction of NO2 with hydrocarbons (HC-SCR). The hydrocarbon added has two essential functions: first, it assists the gas-phase oxidation of NO to NO2 by the electric discharge in excess oxygen and, secondly, it reacts with NO2 in the hydrocarbon SCR. Besides CO2 and CO, significant amounts of formaldehyde and acetaldehyde are formed in the plasma-initiated gas-phase reaction. These and other by-products are involved together with the remaining propene in the subsequent catalytic reaction. The combination of SCR and cold plasma enhances the overall reaction and allows an effective removal of NOx at relatively low temperatures. Certain modifications of Al2O3 and ZrO2 have been found to be effective as catalysts in this reaction. With an energy effort of ca. 30 eV per NO-molecule, a temperature of 300 °C and a space velocity of 20,000 h−1 at the catalyst, it is possible to reduce 500 ppm NO in excess oxygen by more than half. The synergistic combination of NTP and HC-SCR has been verified under real conditions with exhaust gas from a diesel engine.

Plasma-enhanced HC-SCR of NOx in the presence of excess oxygen

Reducing organic and inorganic pollutants in exhaust gases and waste air comprises feeding the exhaust gas stream onto an adsorber/catalyst monitored for its load, separating the exhaust gas stream from the adsorber when the charge state is below the maximum charge of the exhaust gas stream, and connecting to a circulating system. The circulating system consists of the adsorber/catalyst, a plasma generator, optionally a gas heater, and a measuring system for measuring the pollutant content of the circulating gas. After the gas has passed through the circulating system several times, the system is opened to indicate the absence of pollutants, the purified exhaust gas ejected and a new cycle of charging, purifying and ejection connected.

Reducing organic and inorganic pollutants in exhaust gases and waste air comprises feeding exhaust gas stream onto adsorber/catalyst, separating exhaust gas stream from adsorber and connecting to circulating system

An ozone dissociation catalyst is used to convert ozone, resulting from direct or indirect plasma treatment of pollutant-containing waste air, into atomic oxygen for oxidation of residual pollutants and their dissociation products. An ozone dissociation catalyst is positioned in the waste air stream from direct or indirect plasma treatment of pollutant-containing waste air, in order to concentrate the pollutants and their reaction products on the catalyst surface and to convert the ozone resulting from plasma treatment into atomic oxygen for oxidation of residual pollutants and their dissociation products.

Catalyst for plasma-chemical waste air cleaning

A cobalt-aluminum mixed oxide catalyst characterized by non- uniform distribution of cations, in or between the crystallite structures, where the selected Co /Al ratio depends on the degree of non-uniformity; for increasing concentration variation in the catalyst the lowest Co/Al ratio is required For oxidation of chlorinated organic compounds, the total Co/Al atomic ratio should not exceed a fixed value, and should be even lower for increasing concentration gradients in the catalyst An independent claim is included for a two stage oxidation sequence employing different stage ozone concentrations

Catalytic oxidation of low concentration air pollutants by ozone, with low energy requirement, uses mixed cobalt-aluminum oxides with heterogeneous crystallite structures

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Klaus-Peter Francke

Papers

Plasma-enhanced HC-SCR of NOx in the presence of excess oxygen

Reducing organic and inorganic pollutants in exhaust gases and waste air comprises feeding exhaust gas stream onto adsorber/catalyst, separating exhaust gas stream from adsorber and connecting to circulating system

Catalyst for plasma-chemical waste air cleaning

Catalytic oxidation of low concentration air pollutants by ozone, with low energy requirement, uses mixed cobalt-aluminum oxides with heterogeneous crystallite structures

1 kW Pulse Corona Reactor for Voc Destruction