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NanoEngineering of Model Catalysts Based on Supported, Size-selected Nanoclusters

Final Report Summary - CLUSTERCAT (NanoEngineering of Model Catalysts Based on Supported, Size-selected Nanoclusters)

Model heterogeneous catalysts incorporating catalyst engineering and nanoscience open up new avenues for designing a more efficient class of catalyst materials. This field has stimulated many research endeavours to exploit the size-dependent properties of materials in the nanoscale regime. Hence, modern nanocluster-based model catalysts could be key to develop new catalysts with distinct selectivity and reactivity. The stability and chemical performance of such model catalysts, however, may differ significantly under realistic reaction conditions.

In this project we aimed to conduct modern model catalysis under practical reaction conditions (i.e. high temperatures and pressures). To create and analyse catalyst materials, size-selected cluster deposition technology and scanning transmission electron microscopy (STEM) were employed. In fact, the former method enabled us to control size, shape and composition of the clusters with atomic precision. Pre- and post-reaction analysis using STEM provided crucial information on the (3D) atomic structure, stability and possible modification of the catalyst materials (i.e. cluster size/shape) during the course of chemical reactions. Having a constructive feedback loop between nanofabrication, pre- and post reaction analysis, and catalytic measurements was essential to avoid the uncertainties associated with the reaction conditions. In this work, for the first time chemical performance of powder-supported size-selected nanoclusters was explored using high-pressure chemical reactors.

The scientific objectives of the project were envisioned to be achieved through six main tasks: 1) synthesis of the model catalysts, 2) producing nanocluster-based powders, 3) three-dimensional structural analysis, 4) analysis of the model catalyst performance under realistic reaction conditions, 5) investigating stability and structural transformations of the nanoclusters, and 6) elucidating size- and structure-dependent reactivity and selectivity.

A state-of-the-art radio frequency magnetron sputtering cluster beam source was employed to precisely control the cluster size using a unique time-of-flight mass filter and to immobilise clusters on the surface. Immobilisation of nanoclusters was achieved successfully through two different approaches: a) by soft-landing clusters at predefined surface defects and b) by cluster deposition with sufficient kinetic energy to create a defect on the surface upon landing (self-pinning). Producing nanocluster-based powders from the two dimensional planar samples was performed through a novel approach using dicing blade technology in a well-defined dicing process. Aberration corrected STEM coupled to a high-angle annular dark field (HAADF) detector was used to investigate the 3D atomic structure and stability of the samples both prior to and after catalytic measurements.

Initially, the optimum choices of dicing parameters (i.e. blade, cutting pitch, cutting depth and cutting speed) for the efficient transformation (planar to powder form) and release of the top surface layer of the graphite tape were determined using blank graphite tapes (with no clusters).

To synthesis stable arrays of clusters, both pinning approaches were examined to determine the value of impact energy or density of Ar+ ion bombardment necessary for anchoring Pd clusters of various sizes onto graphite. Then graphite supported, deposited size-selected Pd clusters (N= selected between 55 and 400 atoms) were transformed to powder samples by cutting using optimum parameters.

The catalytic performance (reactivity, selectivity and durability) of such nanocluster-based catalysts was then explored in various oxidation and hydrogenation reactions under realistic reaction conditions. One of the main issues to be addressed here was to overcome the quantity gap by creating sufficient amount of active metal in the catalyst materials. Catalytic testing was undertaken in collaboration with Johnson Matthey on a versatile rig with high-pressure reactor vessel coupled to a gas chromatograph system. As a first simple demonstration of reactivity of the nanocluster powders, the CO oxidation reaction was investigated over graphite-supported size-selected Pd clusters (N= selected between 55 and 400 atoms) clusters. We found that under atmospheric reaction conditions, CO conversion starts at rather high temperatures, T > 280ºC, where the Pd400 gave higher conversion rates than the smaller nanoclusters. These results also served to indicate the good thermal stability and size-dependent activity of these model systems even with such low loadings (sub-microgram regimes).

Next, we studied the vapour phase hydrogenation of 1-pentyne under practical conditions over the same kind of size-selected nanocluster powder. Larger size-selectd Pd clusters (N=55 to 400) clusters revealed better thermal stability and catalytic performance (i.e. higher activity and about 90% selectivity to pentene formation) over prolonged periods (up to 20 h) in the chemical reactor.

The stability and atomic structure of the supported nanoclusters were investigated by STEM before and after catalytic measurements. Clusters were deposited onto graphite-coated molybdenum TEM grids and inserted onto the reactor tube along with the cluster powders to facilitate this analysis. Electron microscopy analysis showed the clusters possessed a good structural stability and formed near-spherical shapes on graphite. In general, larger clusters indicated better stability against sintering but only modest sintering was even seen for the smallest cluster size (Pd55).

To explore chemical performance of cluster powders in liquid phase, hydrogenation of nitrobenzene to aniline over cluster powders ( size-selected between 55 to 400) at high pressures and temperatures was conducted. No reactivity was detected using two different polar (Ethanol) and non-polar (Heptane) solvents at different temperatures (40-80 ºC) and pressures (3-8 bar). It seems that catalytically active sites on Pd clusters are somehow blocked/poisoned in solution, hindering the reactivity. The low amount of active metal available for catalysis could also lead to limited reactivity in liquid phase. We therefore tested commercial Pd/C catalysts with 1% metal loading, though such catalysts have a broad size distribution. To tackle the quantity gap we suggest enhancing the production rate of the cluster-based catalysts. This requires further advanced technological developments in the cluster beam apparatus. In a parallel approach, one also could explore synthesising narrow size-distribution catalyst materials via colloidal chemistry. Such developments lie beyond the timeframe of this project.

In collaboration with Argonne National Laboratory, the catalytic oxidation of Cyclohexane to COx over a focused range of cluster sizes was also studied. Size-selected Pd clusters (N= 10 to 120) were immobilised (self-pinned) onto highly ordered pyrolytic graphite (HOPG). By combining scanning tunnelling microscopy (both before and after chemical reactions) and temperature programmed reaction experiments the stability, size effects and reaction conditions were studied. Clusters were stable against sintering at temperatures up 300 ºC and under an oxygen atmosphere. We also observed significant increase in reactivity with decreasing cluster size.

In conclusion, we have created novel, well-defined model catalyst powders which present arrays of size-selected Pd clusters. We observed size-dependent reactivity and selectivity under realistic reaction conditions. The novel approach employed to synthesis such cluster-based catalysts presents promising applications in design of new/improved catalysts with tunable chemical properties. Furthermore, we successfully combined the cluster deposition, micro-dicing technology with heterogeneous catalysis and electron microscopy to demonstrate the correlation of the catalytic performance with the atomic structures of the model catalyst materials.