Subnanometer cobalt oxide clusters as selective low temperature oxidative dehydrogenation catalysts

Sungsik Lee,Glen A Ferguson,Randall E Winans,Detre Teschner,Avik Halder,Stefan Vajda,Jeffrey Greeley,Sönke Seifert,Robert Schlögl,Vasiliki Papaefthimiou,Larry A Curtiss

doi:10.1038/s41467-019-08819-5

Abstract

The discovery of more efficient, economical, and selective catalysts for oxidative dehydrogenation is of immense economic importance. However, the temperatures required for this reaction are typically high, often exceeding 400 °C. Herein, we report the discovery of subnanometer sized cobalt oxide clusters for oxidative dehydrogenation of cyclohexane that are active at lower temperatures than reported catalysts, while they can also eliminate the combustion channel. These results found for the two cluster sizes suggest other subnanometer size (CoO)x clusters will also be active at low temperatures. The high activity of the cobalt clusters can be understood on the basis of density functional studies that reveal highly active under-coordinated cobalt atoms in the clusters and show that the oxidized nature of the clusters substantially decreases the binding energy of the cyclohexene species which desorb from the cluster at low temperature.

Highlights

The discovery of more efficient, economical, and selective catalysts for oxidative dehydrogenation is of immense economic importance
Positively charged cobalt clusters were produced in a cluster beam and using a mass spectrometer, the Co4 or Co27 clusters were filtered out and deposited at 0.1 atomic monolayer coverage on a ~3 monolayer equivalent (ML) thick alumina film prepared by atomic layer deposition on the top of a doped Si chip
A comparison of performance of the subnanometer cobalt clusters with other reported cyclohexane oxidative dehydrogenation (ODH) catalysts is listed in Supplementary Table 3, showing excellent performance of the subnanometer clusters

Summary

Introduction

The discovery of more efficient, economical, and selective catalysts for oxidative dehydrogenation is of immense economic importance. Product selectivity can be altered by changing the reaction conditions, in this case the oxygen to cyclohexane ratio This dramatic lowering of the temperature can be understood on the basis of density functional studies, which indicate that the oxidized nature of the Co clusters substantially decreases the binding energy of the alkene product, i.e. potential poisoning of the catalyst, in comparison with their metallic counterparts. The efficacy of sub-nanometer cobalt clusters for oxidative dehydrogenation at low temperatures is important since cobalt would be very attractive in a variety of industrially important oxidative processes that often utilize precious metals as catalysts and require high temperatures[21,22,23] Such developments would have practical implications ranging from more energy efficient and environmentally friendly strategies for chemical synthesis to the replacement of current petrochemical feedstocks by inexpensive abundant small alkanes

Methods

Results

Conclusion