Activity of Pd$_{\rm n}$ (n = 1-5) Clusters on Alumina Film on Ni$_3$Al(111) for CO Oxidation: A Molecular Beam Study

TL;DR

This study investigates the catalytic activity of Pd clusters of varying sizes on alumina films for CO oxidation, revealing size-dependent activity and comparable performance to larger clusters and surfaces.

Contribution

It provides quantitative analysis of the activity of Pd$_{n}$ clusters (n=1-5) supported on alumina, highlighting size effects and activity thresholds.

Findings

Single Pd atoms are inactive at 473 K.

Clusters grow and become active at 533 K.

Activity per atom remains constant for n=2-5 at 533 K.

Abstract

Single atom catalyst (SAC) is a vivid new area of research in catalysis. However, the activity in CO oxidation of isolated Pt or Pd atoms, generally supported on an oxide powder, is still controversial. Furthermore, the steady state activity of few atoms clusters is still not yet quantitatively known. In this work we study, by molecular beam reactive scattering (MBRS), the activity of Pd$_{\rm n}$ ($n$ = 1-5) clusters, grown on a nanostructured alumina films on a Ni$_3$Al(111) surface. It is shown that the single atoms are not active at 473 K but they diffuse and coalesce at 533K forming larger clusters. The activity of a cluster is proportional to the number of atoms it contains (n=2-5). At 533 K, the activity per (surface-) atom, which is the turnover frequency (TOF), is constant. Its value is close to those obtained for large clusters of 181$\pm$13 atoms and Pd (111) extended surfaces, in the same experimental conditions.