Structuring and catalytic activity of palladium thin layers deposited on the Ni(110) surface

Small amounts of palladium atoms, ranging between 0.02 and 3 monolayers in thickness, were deposited onto a nickel (110) surface and studied by scanning tunneling microscopy, first in the as-deposited state and then after annealing at a temperature that allowed only surface diffusion to occur. Two different types of behavior were observed. At low submonolayer coverages palladium adatoms form elongated islands along [110] on terraces, and palladium and nickel atoms alloy within the surface layer near the steps. At higher coverages very specific chain-like nanostructures form along the [001] direction, i.e., perpendicular to the [110] direction bf open rows. In the latter case, the surface morphology differs strongly from that of the initial singular (110) surface; this surface restructuring is attributed to the surface stress that develops when palladium atoms are deposited on the nickel surface. These two kinds of surface morphology can be analyzed with respect to the catalytic performance, which was tested for the 1,3-butadiene hydrogenation reaction. For low palladium deposits the reactivity is comparable to that measured for reference surfaces. such as pure Pd(110). However, the catalytic performance of palladium deposits near or above half a monolayer is improved by more than one order of magnitude. We propose to correlate such enhanced catalytic properties to the presence of very unusual surface sites created, along with the chain-like nanostructures, at strained surfaces. (C) 1999 Elsevier Science B.V. All rights reserved.