• Nie Znaleziono Wyników

Catalytic activity and stability

X- ray Photoelectron Spectroscopy (XPS)

4.5. Catalytic activity and stability

The application of catalysts is only viable if the catalyst has long-term stability in the reaction conditions and is beneficial to the course of the reaction.

Thus, both the stability and activity of the potential catalyst must be investigated to determine if the material can be a viable replacement to currently used systems.

Temperature-Programmed Reduction (TPR)

An important parameter concerning catalyst stability is its reducibility. In oxidation reactions, if lattice oxygen takes part in the reaction or desorbs the oxidation state of the catalyst matrix must be lowered to compensate. Thus, the temperature ease with which the catalyst or active phase is reduced is important. This is especially crucial for manganese oxides, where the oxidation state may change drastically (from +2 to +7). This is beneficial for reactions utilizing the Mars-van Krevelen mechanism, in which lattice oxygen takes part in the oxidation reaction after which the vacancies are replenished by gaseous oxygen, which closes the redox cycle.

Temperature-programmed reduction can be used to determine at which temperatures a material undergoes reduction. The method also gives insight into the oxidation states of the elements present in the sample, as reduction may occur in characteristic ranges or reduction profiles. This method is particularly useful in determining the oxidation state of noble metal promoters on catalytic surfaces.

Temperature-programmed reduction was used in Paper VIII to determine the oxidation state of the manganese-based catalyst as well as to follow the change to the reduction profile incurred by palladium promotion.

Thermogravimetric Analysis/Differential Thermal Analysis (TGA/DTA) Thermogravimetric Analysis (TGA) in conjunction with Differential Thermal Analysis (DTA) can be used to study the catalysts stability in the reaction conditions.

The first method follows changes in mass while heating the sample. This can provide information concerning any desorption that may occur or the loss of bulk oxygen. The latter determines if phase transformations occur, and if so, if they are endo- or exothermic. Characterization using TGA/DTA can further be enhanced by coupling the apparatus with a mass spectrometer in order to determine the nature of the desorbing individua. TGA/DTA can also be used to follow the catalytic reaction, when a catalyst is mixed with the reagents and heated. This can allow for the determination of the energy needed to initiate and proceed with a reaction and can confirm results of temperature-programmed oxidation. TGA/DTA was utilized in Paper III to this end, while in Paper VI it was used to follow

27 the desorption of H2O and O2 from the bulk of the catalyst, aiding in identifying the reaction mechanism.

Temperature-Programmed Oxidation (TPO)

The final, yet basic parameter of a catalyst is its activity in the investigated reaction.

All previous characterization is used to determine the source of the activity or viability to apply the material in real life conditions. To gauge the activity of the tested materials in the desired reactions of soot and VOC oxidation, the active phases were subjected to temperature-programmed oxidation reaction tests.

When testing in the lab it is necessary to produce conditions as similar to those, in which the reaction will really take place while maintaining reproducible and comparable results. In soot oxidation, this proves to be difficult, as soot is gathered by a filter coated with an active phase, a process which is also dependent on the conditions. At times the soot will come into contact with the catalyst with a greater surface, while at others it will be loosely combined.

For testing purposes in soot oxidation, two modes have been devised: tight contact, in which the catalyst and soot have many contact points, which describes a situation in which the soot particle is firmly pressed to the catalyst, and loose contact, in which the soot is gently mixed or shaken with the soot. The former test typically gives more reproducible results (with the soot and catalyst ground together thoroughly), while the latter better represents real world scenarios, in which the soot is trapped on the catalyst while the exhaust passes through the filter walls (with the soot/catalyst mixture shaken in a vial). In the following works both methods of contact were applied, with 50 mg of the catalyst and soot mixed together in a 10:1 ratio.

Not only the mixing of soot and catalyst is important, but also the gas feed, which should also resemble real-world conditions. In Diesel engines, there is often a presence of NO in the gas feed, which can be used in soot oxidation through the oxidation of NO to NO2. The latter molecule is a better oxidant than oxygen and can lower the temperature of soot combustion greatly. Thus, reactions were performed under two gas mixtures: 5% O2 in He and 3.75% O2 + 0.25% NO in He flowing at 60 ml/min. To determine activity a suitable parameter must also be chosen. This can range from the temperature at which the reaction is initiated, the temperature at which 10 or 50% of the soot has been converted or the temperature at maximum soot oxidation. To monitor the progress of the reaction, a quadrupole mass spectrometer was used, monitoring substrates

28 and potential products (m/z = 18 (H2O), 28 (CO/N2), 30 (NO), 32 (O2), 44 (CO2) and 46 (NO2)) while the catalyst/soot mixture was heated at a rate of 10°C/min from room temperature to 800°C. Thus, in the following works conversion curves based on the concentration of CO2 were used to determine the temperature of 50% soot conversion (T50%) as the parameter specifying the catalytic activity of the investigated material in soot oxidation.

For VOC oxidation the testing scenario is somewhat simpler, as the catalyst does not need to be premixed with the gaseous VOCs. Instead, gas mixtures of 5% O2 + 0.4% CH4 and 5% O2 + 0.1% C3H8 were passed over 50 mg of the tested catalyst at a rate of 100 ml/min to test catalytic activity in model VOCs: methane and propane. Similarly, the substrates and products were followed through FT-IR to determine catalytic activity. The reaction temperature was increased in incremental steps from room temperature to 500°C (propane) and 600°C (methane), allowing for the temperature to stabilize every 50°C in order to measure the catalytic activity at steady state. The catalytic activities were determined by comparing the conversion of the VOC to CO2.

Catalytic oxidation of soot was investigated in all Papers bar IV, with additional investigation concerning NO presence in Papers VI, VII and VIII. The catalytic oxidation of model VOCs methane and propane were investigated in Papers VI and VII.

29