During the last few decades, concerns have grown on the negative effects that diesel particulate matter has on health. Because of this, particulate emissions were subjected to restrictions and various emission-reduction technologies were developed. It is ironic that some of these technologies led to reductions in the legislated total particulate mass while neglecting the number of particles. Focusing on the mass is not necessarily correct, because it might well be that not the mass but the number of particles and the characteristics of them (size, composition) have a higher impact on health. To eliminate the threat of diesel particulate matter, essentially absolute filtration in combination with the oxidation of all emitted hydrocarbons is what will be required. After two decades of development, the first filters will soon be introduced on a large scale. Many different problems had to be overcome; it was especially important that the filter was robust and its regeneration was controllable. The key technol...

Science and technology of catalytic diesel particulate filters

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

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

The mechanisms of plasma enhanced selective catalytic reduction of NOx on a V2O5-WO3/TiO2-catalyst were investigated for temperatures between 100 °C and 200 °C by applying various analytical methods. In experiments with synthetic gas mixtures containing Ar instead of N2 as a carrier gas the formation of N2 as a main product and thus catalytic reduction of NOx in reactions with NH3 has been proven using mass spectroscopy. More detailed information on the reaction kinetics of NO-formation and removal induced by non-thermal plasmas has been obtained from experiments in gas mixtures containing isotopically marked 15NO. FTIR-absorption spectroscopy was applied to measure the concentrations of 14Nand 15N-containing molecules. Particles formed by the combined plasmaand catalytic treatment of Diesel exhaust with NH3 as reducing agent were analyzed by energy dispersive X-ray scattering. The spectra indicate, that mainly sulfur containing particles were formed. Formation of NH4NO3 plays a minor role. Non-thermal plasma treatment did not show a strong influence on catalytic removal of unburned hydrocarbons, but increased CO-emission was observed. In all experiments plasma induced conversion of NO to NO2 increased NOx-reduction rates.

Plasma Enhanced Selective Catalytic Reduction: Kinetics of NOx-Removal and Byproduct Formation

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

Plasma-supported conversion of nitrogen oxides in the purification of exhaust gas comprises plasma-chemical treating the exhaust gas stream charged with nitrogen oxides in a reactor filled with a catalyst to reduce the nitrogen oxides to nitrogen. The catalyst contains a nitrogen compound with nitrogen in the negative oxidation state. The catalyst can be regenerated.

Rolf Rudolph

Papers

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

Plasma Enhanced Selective Catalytic Reduction: Kinetics of NOx-Removal and Byproduct Formation

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

Plasma-supported conversion of nitrogen oxides in the purification of exhaust gas