Thermal Stability of Single-Crystalline IrO2(110) Layers: Spectroscopic and Adsorption Studies

Thermal Stability of Single-Crystalline IrO2(110) Layers: Spectroscopic and Adsorption Studies


Catalysis is an enabling technology and a main driver of our modern economy. Catalysis is a key in the development of sustainable energy systems and production lines of bulk and fine chemicals with low environmental impact. Altogether, the catalysis-based industry contributes to more than a third of the global gross domestic product (GDP). Estimates are that about 90 % of all commercially produced chemical products involve catalysts at some stage in their manufacturing process.

Equally important is heterogeneous catalysis for the reduction of greenhouse gas emissions. Methane, for instance, is an effective greenhouse gas whose comparative impact is 25 times greater than CO2. Therefore, the after-treatment of exhaust gases of methane-driven engines, for instance in the propulsion from ferries in the Scandinavian countries, needs the catalytic removal of slipped methane. Unfortunately, methane is the most difficult hydrocarbon to oxidize catalytically, and in general, a relatively high temperature is needed for the reaction to proceed with an acceptable rate. Recently, oxidized Ir(100) has been found surprisingly active in the low-temperature activation of methane.1 The active phase has been assigned to an IrO2(110) layer as previously predicted by Wang et al. on the basis of density functional theory calculations.2

This was the starting point of the present project. The main objective was to explore the chemical nature of the catalytic active phase of iridium-based methane activation, says Herbert Over, Professor in Surface Chemistry and Model Catalysis at the Justus-Liebig University Giessen. To obtain unambiguous information, we developed a dedicated IrO2(110) model catalyst, a single-crystalline IrO2(110) film grown on the RuO2(110)/Ru(0001) template, and used probe molecules (CO and water) which specifically interact with the active sites of IrO2(110). The interaction of CO and water with the active surface sites has been studied with high-resolution core-level shift spectroscopy (HRCLS) at the FlexPES beamline.

This new soft x-ray beamline is operational since spring 2020 and particularly well suited for studying first stages of catalytic reactions at very low gas pressures or ultra-high vacuum (UVH), says Alexei Preobrajenski, the FlexPES beamline manager. A combination of “convenient” energy range, variable spot size on sample, UHV conditions, high-class detectors for HRCLS and x-ray absorption, versatile sample treatment capabilities and user-friendly control systems make this beamline a powerful modern instrument for studies of a wide variety of on-surface processes, including catalytic reactions.

The model experiments at the FlexPES beamline provided surprising microscopic insights. Firstly, freshly prepared IrO2(110) films are not active at all, but rather these surfaces are poisoned by adsorbed oxygen so that CO cannot adsorb. Secondly, heating the sample to 300 °C under ultra-high vacuum (UHV) conditions leads to the removal of the blocking oxygen species and liberates the active sites. Besides this activation step of the catalyst, the heat treatment leads to the growth of monoatomic Ir metal islands on the IrO2(110) surface. This turns the IrO2(110) surface into a bi-functional surface, exposing active sites both on the oxide and the metal surface. We suggest that this bi-functionality is at the heart of the low-temperature activity of oxidized Ir(100) to methane. With HRCLS we can show that this surface reduction process is reversible to a large extent and the coverage of Ir islands on IrO2(110) depends sensitively on the specific reaction environment. In other words, the catalyst surface acts highly flexible and can adjust itself to the specific reaction conditions.

[1] Liang, Z.; Li, T.; Kim, M.; Asthagiri, A.; Weaver, J. F. Low-Temperature Activation of Methane on the IrO2(110) Surface. Science 2017, 356, 299-303.
[2] Wang, C.-C.; Siao, S. S.; Jiang, J.-C. C-H-Bond Activation of Methane via s-d Interaction on the IrO2(110) Surface: Density Functional Theory Study. J. Phys. Chem. C 2012, 116, 6367-6370.

Header image: Scanning tunnelling microscopy images of freshly prepared IrO2(110) and after heating the surface to 300°C (500-600 K): red areas a mono-atomic high iridium islands. With HRCLS of Ir4f the formation of Ir islands be studied.

Read the article here:

Marcel J. S. Abb, Tim Weber, Daniel Langsdorf, Volkmar Koller, Sabrina M. Gericke, Sebastian Pfaff, Michael Busch, Johan Zetterberg, Alexei Preobrajenski, Henrik Grönbeck, Edvin Lundgren, and Herbert Over
Thermal Stability of Single-Crystalline IrO2(110) Layers: Spectroscopic and Adsorption Studies

Catalysis is the process of increasing the rate of a chemical reaction by adding a multifunctional material – a catalyst – which is not consumed in the catalytic reaction and can continue to act repeatedly. If the activity is high, only very small amounts of catalyst are required to increase the reaction rate.

In general, chemical reactions occur faster in the presence of a catalyst because the catalyst provides an alternative reaction pathway with a lower activation energy than the non-catalysed reaction. In a catalytic reaction, the catalyst usually reacts with the reactants to form temporary intermediates, which are able to generate the desired product via a surface reaction. The resulting product leaves the catalysts and regenerates the original catalyst in a cyclic process.

Catalysts may be classified as either homogeneous or heterogeneous. A homogeneous catalyst is one whose molecules are dispersed in the same phase (usually gaseous or liquid) as the reactant’s molecules. A heterogeneous catalyst is one whose molecules are not in the same phase as the reactant’s, which are typically gases or liquids that are adsorbed onto the surface of the solid catalyst.

To be able to manufacture better and more efficient catalysts, it is crucial to understand the underlying catalytic processes on the microscopic level, employing structurally simple model system such as single crystalline metal oxide films. The best catalytic material would be one which is long-term stable, i.e. keep a high catalytic ability for a long time, and active, i.e. a small amount of the material would give high catalytic conversion, in the environment where it is being used.

The most commonly known catalytic process is probably the exhaust emission control in cars. Here, carbon monoxide, hydrocarbons and other toxic substances are transformed via catalysis into less harmful substances such as carbon dioxide, water and molecular nitrogen.