Transition metals are active toward catalysing a wide variety of reactions; however, it is sometimes difficult to optimize their performance using a single element. Introducing a second metal can significantly enhance a catalyst’s reactivity and selectivity. These improvements are explained by two main effects: the ligand effect (electronic interactions between the metals) and the ensemble effect (the specific arrangement of surface atoms).
Figure 1 (left). TPD Analysis of a Pt/Cu surface.
Furthermore, the atomic structure of the surface can influence both reaction selectivity and reactivity. Water plays a crucial role in catalysis, serving as a critical reactant in key reactions like the water-gas shift reaction or as an essential medium for processes in aqueous solutions, such as electrochemical and photocatalytic reactions. The reactivity and selectivity of catalysts in these reactions can be affected by the presence of low-coordination sites (like steps, kinks, or defects) on the surface.
This research has investigated the role of low-coordination sites on the adsorption characteristics of water on Cu and Pt/Cu surfaces. A stepped Cu(211) surface was used to model the low coordination defect sites. These step sites were decorated by Pt. A combination of infrared reflection absorption spectroscopy (IRRAS) and temperature-programmed desorption (TPD) was used to study how the surface structure and atomic composition can change the adsorption behavior of water on a Pt/Cu(211) surface.
The findings reveal that the step edges on the pure Cu(211) surface are highly chemically active. These steps not only bind water molecules more strongly but also promote their partial dissociation, leading to the formation of hydroxyl (OH) groups. Partial dissociation of the first layer of water on Cu step sites and formation of adsorbed OH groups strongly influence the adsorption strength of the molecular water at step edges. Water molecules are stabilized by hydrogen bonding to pre-formed OH groups at the step edges. These OH groups and water molecules in the first layer of adsorbed water can assemble into chain-like structures. Introducing Pt to decorate Cu step sites fundamentally alters this behaviour and reduces the dissociative adsorption of water on the surface. On the fully Pt-decorated step sites Cu(211) surface, water no longer splits into OH groups; instead, it adsorbs molecularly. The presence of Pt atoms at step edges disrupts the formation of the stable OH-water chain structures that were present on pure Cu(211). Pt also affects the structure of water multilayers, making it easier for them to form crystalline ice, a process that does not occur on the pure Cu(211) surface under the same conditions. This suggests Pt changes the long-range structure of the water adlayer.
Here, we show that the atomic-level structure of a surface, particularly its step edges, is a decisive factor in water interaction with Cu catalyst surfaces. By controlling the structure and composition of bimetallic catalysts, one can potentially modify a surface that optimally activates reactants while resisting poisoning, leading to more efficient and durable catalytic processes.
Project Summary by: Sarp Kaya, Associate Professor, Department of Chemistry, Koç University, Istanbul, Türkiye.
Paper Reference: Mohammadpour, A. and Kaya, S. (2024) ‘Probing water adsorption characteristics of Pt step-edge decorated Cu(211) surface.’ Surface Science. Elsevier BV, 748, October, p. 122537. DOI: 10.1016/j.susc.2024.122537.
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