The Quest to Reliable Structure-Property Relationships in Methanol Steam Reforming

Methanol steam reforming, that is, the generation of hydrogen and carbon dioxide from methanol and water is to date one of the most promising heterogeneously catalyzed chemical reactions for on-board hydrogen storage and use. Its particular advantage is mainly based on the high hydrogen-to-carbon ratio (3:1), if the reaction is led highly CO2-selective. To achieve this goal, at the moment a range of also technologically used and already well-characterized catalyst systems are exploited, including copper-zinc oxide catalysts or intermetallic compounds on palladium basis. However, the used catalysts suffer from the severe drawbacks of either being prone to serious sintering upon catalyst activation or even during reaction, or, in the case of palladium-based intermetallic compounds, being structurally highly dynamic systems. This typically hampers the determination of the catalytically active centers leading to high CO2 selectivity. The copper-zirconium oxide catalysts discussed in the project The Quest to Reliable Structure-Property Relationships in Methanol Steam Reforming exhibit not only a high CO2 selectivity with at the same time multiple activities compared to used materials, but as a particular advantage also offer also an increased structural and chemical stability. This is mainly due to the inherent chemical inactivity of zirconium oxide, which simplifies the determination of the associated catalytically active and selective sites. To reach the set goal, new grounds of catalyst preparation and synthesis need to be broken as well as dedicated structural and chemical characterization techniques capable of resolving structure, morphology and chemistry during reaction applied to optimize the chemistry of the two components copper and zirconium oxide. Only by this knowledge-based approach, a new catalyst material with enhanced activity and selectivity with at the same time structurally stable catalytic center finally results, enabling setting-up reliable structure-property relationships for methanol steam reforming.

The methanol steam reforming reaction, i.e. the generation of hydrogen and carbon dioxide from methanol and water, is a promising and viable way especially also for mobile hydrogen storage applications. The core advantage is the high hydrogen-to-carbon dioxide ratio that is achieved upon steering the reaction towards high carbon dioxide selectivity. The catalyst systems on copper-zirconium dioxide basis, tackled in the project "The quest to reliable structure-property relationships in the methanol steam reforming reaction", because of the structurally more lenient properties of zirconium dioxide, exhibit a higher chemical and structural stability in comparison to up to now used catalysts, with at the same time comparable selectivity and higher activity. This generally rendes the determination and characterization of the catalytically active catalyst sites much easier. This concept applies to copper-zirconium dioxide catalysts and interfaces synthesized directly in the same way as for interfaces accessed through directed decomposition of copper-zirconium intermetallic compound precursor structures in the methanol steam reforming mixture. Especially the surface chemical variation of the zirconium dioxide allows switching the selectivity of the resulting copper-zirconium dioxide interface. In due course, the establishment of reliable structure-property relationships is much facilitated. Through apparative coupling of surface chemical analysis and measurements of the methanol steam reforming activity and selectivity, we were able to gain direct insight into the molecular mechanism of the catalytically active center in the operating state. Extension of the project concept to similar and comparable copper-indium/indium oxide materials revealed strong differences in catalytic behavior, which are governed by the methanol steam reforming performance of indium oxide itself. Indium oxide is itself a highly carbon-dioxide selective methanol steam reforming catalyst. We opted to use indium oxide as a more dynamic material to prove the general concept of the importance of the copper-oxide interface. In fact, such interfaces cannot be readily accessed through decomposition of copper-indium intermetallic compound precursor structures. However, this enabled us to directly compare the stability of such intermetallic compounds to an intermetallic compound-oxide interface, which is accessed through partial decomposition. In summary, especially the copper-zirconium dioxide system allowed us to derive a number of positive and negative aspects in catalysts synthesis, which need to be followed or best avoided to prepare promising metal-oxide methanol steam reforming catalysts. A more knowledge-based catalyst synthesis strategy is, hence, developed.