Surface Fermi Energy and Activity of Atomically Layered Pt/Ru Catalyst

The development of bimetallic catalysts has played a significant role in the development of technologies such as catalytic reforming, fuel cell electrocatalysis, hydrodesulfurization, partial alkene oxidation, etc. The interest in the development of bimetallic catalysis for direct methanol fuel cells (DMFCs) has been over a four-decade-old exercise. Platinum has generally been alloyed with a plethora of other metals such as tin, cobalt, ruthenium etc. in order to attain better methanol deprotonation and reduced carbon monoxide poisoning of the catalyst. Here, to better understand the role of ruthenium in a bilayered Pt/Ru catalyst, we demonstrate its effect on the surface Fermi energy level and catalytic activity.

Using the atomic layer deposition (ALD) process, a number of samples were prepared and the effect of atomic layering of platinum on ruthenium on the surface catalytic activity was studied. The thickness of Pt was monitored by varying the number of Pt ALD cycles and catalytic activity was determined by cyclic voltammetry. In addition, copper underpotential deposition (Cu-UPD) was used to estimate the fractional coverage of Pt on the surface of Ru at different Pt ALD cycles. This is key to the understanding since it ensures that the studied surface involves only one metal. We found that for 10 ALD cycles of Pt, the surface of Ru was completely coated with Pt. Work function of the catalyst surface was estimated by Kelvin probe. A large variation in work function values for the sample with 5 Pt ALD layers suggests that there is significant islanding of Pt on Ru corroborating well with Cu-UPD results.

Our results show that by manipulating Fermi energy of Pt catalysts, optimum activity can be obtained. In addition, it is suggested that the common 50/50% alloying of metals is an unnecessary enterprise and comparable activity can be achieved by careful engineering of catalysts involving lower Pt loading.