HC-SCR of NOx over Ag/alumina: a combination of heterogeneous and homogeneous radical reactions?

Matrix isolation combined with EPR and FT-IR technique at low temperature has been used to study the gas phase species involved in HC-SCR over a highly active Ag/alumina catalyst. A combination of heterogeneous and homogeneous (radical) reactions is proposed to take place over a Ag/alumina catalyst during HC-SCR. Radicals of low molecular weight were trapped in a growing argon matrix behind the Ag/alumina catalyst. In the same matrix, cyanogen isocyanate was detected and is suggested to be a key intermediate for the formation of amines and ammonia via the hydrolysis of isocyanate species.     

Preparation and characterisation of Ag/alumina catalysts for the removal of NO x emissions under oxygen rich conditions

Topics in Catalysis - A highly active silver/alumina catalyst for continuous reduction of NO to nitrogen with octane under lean conditions was prepared. The effect of impregnation time and...     

Decomposition of ozone on Ag/SiO2 catalyst for abatement of waste gases emissions

A silica-supported Ag system made by the incipient wetness impregnation method was investigated in the reaction of heterogeneous catalytic decomposition of ozone. It was established that the catalytic ozone decomposition on Ag/SiO₂ proceeded in the temperature interval −40 °C to 25 °C as a first order reaction with activation energy of 65 kJ/mol (pre-exponential factor 5.0 × 10¹⁴ s⁻¹). Based on the results from the instrumental methods (SEM, XRD, XPS, EPR, TPD) it can be concluded that in presence of ozone the silver is oxidized to a complicated mixture of Ag₂O₃ and AgO. Due to the high activity and stability of the Ag/SiO₂ catalyst, it is promising for neutralization of waste gases containing ozone.     

Exceptional activity for NO x reduction at low temperatures using combinations of hydrogen and higher hydrocarbons on Ag/Al2O3 catalysts

Topics in Catalysis - The effect of the addition of hydrogen on the SCR of NO x with a hydrocarbon reaction was investigated. It was found that hydrogen had a remarkable effect on the temperature...     

Selective catalytic reduction of NO x over Ag/Al2O3 using various bio-diesels as reducing agents

The effect of bio-diesel compounds (vegetable methyl and ethyl laurate and hexadecane) as reducing agents on the selective catalytic reduction of NO _x over a 2 wt.% Ag/Al₂O₃ was investigated. These components were found to have a two-fold effect on the SCR over Ag/Al₂O₃. First, the reduction activity below 400 °C was higher with bio-diesel than with n-octane, which is a representative compound for fossil fuels. This effect is attributed to the presence of the ester group in these molecules. However, the conversion above 400 °C decreased sharply and was considerable lower than with n-octane. The most interesting observation was found when the reduction efficiency of bio-diesel components was tested in the presence of hydrogen. The well known low temperature boosting effect of hydrogen was visible not only at lower temperatures, but also above 400 °C. Mechanistically the observation is extremely interesting and indicates that hydrogen effect cannot directly be connected to reduction of surface nitrates, which can be operative only at low temperature domain.     

The relationship between the structure of monoalkyl branched saturated fatty acids and some physical properties

The gel point and the refractive index of monoalkyl branched saturated fatty acids were determined and related to the structure of the acids. A three-factor central composite face-centered design was constructed where the three variables were the length of the main chain, the branching position and the length of the side chain. Second-order models were calculated in which the two properties were related to the structure. In order to confirm the validity of the models, two additional fatty acids were analyzed. The gel point was found to be proportional to the length of the main chain and the square of the branching position whereas the refractive index was proportional to the length of both the main and the side chain and also to the square of the main chain.     

Analysis of the state and size of silver on alumina in effective removal of NOx from oxygen rich exhaust gas

Ag/alumina catalysts with different silver contents for octane-SCR were prepared by impregnation and incipient wetness methods. Activity tests revealed that the decisive factor for high activity is not only a high dispersion of silver, but also the ability of the system to redisperse clustered silver. Determination of dispersion by TEM/HAADF and O2-chemisorption experiments resulted in values close to each other even if the results were not directly comparable. This is suggested to be due to not complete silver reduction below 700 degrees C and the samples being very heterogeneous in terms of particle size, e.g., having a bimodal size distribution. Small charged Agsigman+ clusters containing 2-8 silver atoms highly prevailed in the samples containing <2 wt% Ag and exhibiting high octane-SCR activity. In highly loaded Ag/alumina samples or those reduced and reoxidized at high temperature (>400 degrees C), large metallic particles are stabilized, resulting in poor conversion of NOx to N2.     

The relationship between the structure of monoalkyl branched saturated triacylglycerols and some physical properties

Three different physical properties, the gel point (“solidification point”), the refractive index and the density, were determined and related to the structure of the branched triacylglycerols. A four-factor central composite face-centered design was constructed where the four variables were the length of the main chain, the branching position, the length of the side chain, and the number of branched fatty acyl groups attached to the glycerol backbone. Second-order models were calculated in which the three physical properties were related to the structure. Four additional branched triacylglycerols were analyzed in order to confirm the validity of each model. Contour plots are shown in order to visualize the prediction equations which were obtained.     

A Highly Active Ag/Alumina Catalytic Converter for Continuous HC-SCR During Lean-Burn Conditions: From Laboratory to Full-Scale Vehicle Tests

A common-rail diesel vehicle was equipped with a full-scale Ag/alumina catalytic converter. The converter consisted of several Ag/alumina bricks, with free space in between each brick to fulfil important gas phase reactions. An oxidation catalyst was placed at the end of the converter to remove formed CO and unburned HC. High conversion levels of NO _x , around 60%, were recorded at several speeds and loads using additional HC (diesel) injection corresponding to 2–5% fuel penalty.     

Hydrogen as a remedy for the detrimental effect of aromatic and cyclic compounds on the HC-SCR over Ag/alumina

The effect of hydrogen on the selective catalytic reduction of NO_x with n-octane, methylcyclohexane and toluene over a 2 wt.% Ag/Al₂O₃ was investigated. Diesel fuels contain, in addition to straight hydrocarbons, varying amounts of cyclic and aromatic compounds, which have a detrimental effect on the Ag/Al₂O₃ catalyst activity. The results showed that the NO to N₂ conversion was significantly promoted, independent of the nature (straight, cyclic or aromatic) of the hydrocarbon, in the presence of 1 vol.% H₂. The role of hydrogen is connected to a faster oxidation of the hydrocarbons and to enhanced formation of amines, which react with activated NO to form N₂.