Abstract
The generation of beams of atomic clusters in the gas phase and their subsequent deposition (in vacuum) onto suitable catalyst supports, possibly after an intermediate mass filtering step, represents a new and attractive approach for the preparation of model catalyst particles. Compared with the colloidal route to the production of pre-formed catalytic nanoparticles, the nanocluster beam approach offers several advantages: the clusters produced in the beam have no ligands, their size can be selected to arbitrarily high precision by the mass filter, and metal particles containing challenging combinations of metals can be readily produced. However, until now the cluster approach has been held back by the extremely low rates of metal particle production, of the order of 1 microgram per hour. This is more than sufficient for surface science studies but several orders of magnitude below what is desirable even for research-level reaction studies under realistic conditions. In this paper we describe solutions to this scaling problem, specifically, the development of two new generations of cluster beam sources, which suggest that cluster beam yields of grams per hour may ultimately be feasible. Moreover, we illustrate the effectiveness of model catalysts prepared by cluster beam deposition onto agitated powders in the selective hydrogenation of 1-pentyne (a gas phase reaction) and 3-hexyn-1-ol (a liquid phase reaction). Our results for elemental Pd and binary PdSn and PdTi cluster catalysts demonstrate favourable combinations of yield and selectivity compared with reference materials synthesised by conventional methods.
Highlights
Catalysis has always been “nanotechnology”, in the sense that catalyst particles have nanometre dimensions, but the increased level of material control whichPaper nanotechnology offers is an obvious attraction for those who seek to understand and improve the function of catalysts
The potential advantages of the cluster beam approach are several: (i) the size of the catalyst particle can be selected even to single atom precision;[4] (ii) the interaction between the metal cluster and the support can sometimes be tuned by the energy of the impacting particle;[5] (iii) immobilised clusters can show robust behaviour against sintering at elevated temperatures and pressures;[6,7] (iv) the “metal-to-metal” processing produces limited effluent and avoids the cost of ligand molecules; (v) binary and ternary nanoclusters can be prepared in addition to elemental clusters.[8,9,10]
We will show that the cluster beam approach is not con ned to ultra high vacuum surface science experiments but instead enables model catalyst studies under realistic reaction conditions and, that clusters can be deposited onto industrial catalyst powders and not just planar supports
Summary
Paper nanotechnology offers is an obvious attraction for those who seek to understand and improve the function of catalysts. Despite signi cant progress in the synthesis of nanoparticles, with high levels of control of their shape, size and composition (core–shell, homogenous alloy, etc.), the adoption of nanoparticle-based catalysts is not as yet widespread In large part this is because pre-formed nanoparticles typically require stabilisers such as polymers or ligands which interfere with the catalytic activity, for example, by blocking the active site with a donor atom such as sulphur or nitrogen. Drying, calcining and hydrogen reduction of catalysts are all energy intensive and there are economic and environmental drivers to avoid such processes Another interesting aspect of catalyst synthesis using cluster beam deposition is the opportunity to make new materials which cannot be produced by conventional methods such as impregnation, deposition or precipitation.
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