Methane Oxidation on Palladium and Platinum Catalysts - A SSITKA Study

The catalytic process of total methane oxidation presents itself as an extremely favorable alternative to flame combustion, as it helps to reduce the production of NO_x, CO emissions and non-oxidized hydrocarbons into the Earth’s atmosphere.

Some of the most dynamic catalytic materials for total methane oxidation are supported palladium and platinum catalysts. Showing great selectivity and activity, these materials demonstrate adequate levels of resistance to heat and mechanical damage.

On the other hand, there are still unanswered questions that need to be addressed before these catalysts become widely used in the industry. One such issues includes the reaction mechanism of total methane oxidation over palladium and platinum catalysts. This study employed the Steady State Isotopic Transient Kinetic Analysis method (SSITKA) to explore this potential obstacle further.

Using SSITKA to Analyze Heterogeneous Catalytic Reactions

This method makes it possible to obtain useful new data relating to the mechanisms of a number of heterogeneous catalytic reactions, as well as other matters of comparable importance, including: surface concentrations of reagents, intermediates and products of the catalytic reaction, the quantity of active centers on the surface of the catalyst, and the average surface life-span.

To successfully carry out the SSITKA study, access to accurate and high-quality equipment was essential. In a study like this, it is especially important to have a system of dosing and quick switching of reagent streams, as well as a mass spectrometer that allows users to analyze miniscule variations in the concentration of isotopes.

For this study, the switches between reaction streams including ¹²CH₄/Ar/O₂/He and ¹³CH₄/Kr/O₂/He were carried out, as well as those between ¹⁶O2/Ar/CH₄/He and ¹⁸O₂/Kr/CH₄/He. Fig. 1 illustrates the examples of the findings for the Pd-Pt/Al₂O₃ catalyst.

Figure 1. Effect of the switching between reaction streams including ¹⁶O₂/Ar/CH₄/He and ¹⁸O₂/Kr/CH₄/He (X is the conversion of methane).

Results

Following a thorough analysis based on these results (full details of which can be found in the reference paper below), it has been suggested that two different types of active centers (α – more active, but less abundant, and β – less active, but more abundant) are present on the catalyst surface, and the process of methane oxidation takes place concurrently, according to two different reaction mechanisms (by Mars-van Krevelen and Langmuir-Hinshelwood).

The degree of their participation in methane oxidation varies depending on the reaction temperature. Furthermore, the process of methane oxidation takes place not only simultaneously - according to two different reaction mechanisms - but also with differing rates of reaction, as determined by the variety of active centers.

Reference

Rotko, A. Machocki, G. Słowik (2014) “The mechanism of the CH₄/O₂ reaction on the Pd-Pt/γ-Al₂O₃ catalyst: A SSITKA study” Applied Catalysis B: Environmental, 160-161 298-306

This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.

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