Size selected clusters and particles: from physical chemistry and chemical physics to catalysis

When changing the size of metals from single atoms to monodispersed subnanometer clusters and well defined nanoparticles, their properties may change dramatically, as the types and fractions of their atoms gradually shift from being dominantly under-coordinated to being organized in crystalline facets. Taking into account one- to three-dimensional particle shapes, structural fluxionality, the attachment of particles to a support where interfaces form and the fact that changes in the electronic properties of the particles may take place, there is a further modification to the intrinsic size- and composition dependent properties in the supported particles which add to the complexity of the system.

The ability to make materials with desired properties, such as catalytic and magnetic ones, requires a fundamental understanding of the relationship between structure and function. Overall, most studies are executed individually and address a single problem; thus there is evident need for a larger overarching aim that addresses the complete complexity of the system. Physical vs. chemical synthesis, gas phase studies vs. supported particles, single size vs. size distribution, etc.; such a comprehensive scope cannot be accomplished by a single research group as illustrated by all the papers in this issue. Although there is no single system in this issue that takes into account the complete complexity, recent developments make it in principle possible to virtually completely bridge a model approach to realistic systems. Improved collaboration between research groups with completely different backgrounds is essential for future developments. A list of papers describing the many different competences may be a good starting point.

Unraveling the complexity into individual components is still a rather common approach. Combining results from such individual studies to yield an overarching picture is however not yet done sufficiently, if at all. Recent developments in experimental and theoretical approaches make it possible to interconnect studies of model and realistic systems. The papers in this special issue are illustrative of the feasibility of such an approach by emphasizing a broad spectrum of elements available for the fundamental understanding of a system from its simplified and often isolated components in the gas phase or under ultra-high vacuum conditions, to the complete process under functioning complex conditions. Examples cover matter, starting with ultra-small subnanometer sized clusters that consist of only a handful of atoms, up to particles nanometers in size made of thousands of atoms. Investigated properties comprise structural, electronic, magnetic, reactive and catalytic propensities.

Table 1 lists the papers that appear in the printed version of this themed issue, grouped by topic. Additional papers may be available in the online version of this special issue (http://pubs.rsc.org/en/journals/articlecollectionlanding?sercode=cp&themeid=5c095e50-7ece-4f0b-9595-4fae14589c3c). The collection of papers in this special issue underlines that a close coupling of experimental synthetic and characterization methods with theory is appropriate and essential to form a highly complementary multidisciplinary approach towards the design of new materials over several length scales: from free single atoms and (size-selected) clusters and nanoparticles to particles attached to a support which is fabricated by physical and chemical methods. This approach may yield very similar materials to be compared.

We would like to thank all the authors who have contributed to this themed issue, and the editorial team of PCCP for their assistance. JAvB likes to thank ETH Zurich and PSI Villigen for financial support. SV acknowledges support by the U.S. Department of Energy, BESMaterials Sciences, under Contract DE-AC-02-06CH11357, with UChicago Argonne, LLC, the operator of Argonne National Laboratory.

Table 1 Papers appearing in the printed version of this themed issue, grouped by topic

Theme	Paper	Topic of the paper	DOI	Approach
(A) Clusters prepared by physical methods
Reactivity of and catalysis by gas phase clusters	X.-N. Wu et al.	Gas phase chemistry of [Zn(OH)]^+/C3H8, oxidative dehydrogenation, hydrogen atom abstraction	10.1039/c4cp02139h	Experiment & theory
	S. M. Lang et al.	Water activation on Rux^+ (x = 2–5) and RuxOy^+ (x = 2–5, y = 1–2) clusters, size- and composition-dependent activity	10.1039/c4cp02366h	Experiment & theory
	S. Hirayabashi et al.	Cun^+ clusters (5 ≤ n ≤ 16), CO oxidation with N2O	10.1039/c4cp01554a	Experiment
	C. P. McNary et al.	Fen^+ clusters (4 ≤ n ≤ 17), Fe cluster–CO bond energies as a function of cluster size and binding site	10.1039/c4cp02040e	Experiment
Metal atom mimics in gas phase clusters	K. Vetter et al.	Cun, Cun−1H and Agn, Agn−1H clusters (2 ≤ n ≤ 5), photoelectron spectroscopy, measured and calculated vertical detachment energies	10.1039/c3cp53561d	Experiment & theory
Size and composition selective cluster deposition and characterization	F. Masini et al.	PtxY nanoparticles, transmission electron microscopy, X-ray photoelectron spectroscopy, ion scattering spectroscopy	10.1039/c4cp02144d	Experiment
Well defined cluster synthesis by metal vapor condensation	M. Marsault et al.	Regular arrays of Pd and PdAu clusters on alumina films, scanning tunneling microscopy, grazing incidence small angle X-ray scattering, low energy electron diffraction	10.1039/c4cp02200a	Experiment
Morphology and chemical states of supported clusters	A. Beniya et al.	Ptn clusters (7 ≤ n ≤ 20) on Al2O3/NiAl(110), scanning tunneling microscopy, infrared spectroscopy, temperature-programmed desorption	10.1039/c4cp01767f	Experiment
Size-selected cluster deposition and reactivity	Y. Luo et al.	Aun clusters (6 ≤ n ≤ 8), parallel spatially separated cluster deposition, reaction with oxygen and CO, X-ray photoemission electron microscopy	10.1039/c4cp00931b	Experiment
	H. Yasumatsu et al.	Pt30 cluster on Si(111), CO oxidation, temperature-programmed desorption	10.1039/c4cp02221a	Experiment
	F. Sloan Roberts et al.	Ptn clusters (n = 1, 2, 4, 7, 10, 14, 18) on alumina film grown on Re(0001), CO oxidation, electronic structure, X-ray photoemission electron microscopy, temperature-programmed desorption	10.1039/c4cp02083a	Experiment
Magnetic and electronic properties of supported clusters	C. A. F. Vaz et al.	8–22 nm Fe nanoparticles, evolution of magnetic and electronic properties with size in an oxidizing environment, in situ X-ray photoemission electron microscopy, in situ X-ray absorption spectroscopy	10.1039/c4cp02725f	Experiment
Size and support effects on the electronic properties of supported clusters	B. H. Mao et al.	Ag3 and Ag15 clusters on thin alumina and titania films, near ambient pressure, X-ray photoemission spectroscopy, CO, O2	10.1039/c4cp02325k	Experiment
(B) Computational studies
Reactivity of and catalysis by gas phase clusters	M. Boronat et al.	Ag surfaces, clusters and nanoparticles, propene epoxidation, O2vs. O2 + H2	10.1039/c4cp02198c	Theory
	P. C. Jennings et al.	Octahedral Pt116, O2 dissociation, active sites	10.1039/c4cp02147a	Theory
Reactivity of and catalysis by supported clusters	L. Sementa et al.	Ag3/MgO(100), oxidation of NO and CO, support and coverage effects	10.1039/c4cp02135e	Theory
Reactivity of and catalysis on extended surfaces	L. M. Molina et al.	Partially oxidized Ag surfaces, propene epoxidation	10.1039/c4cp02103g	Theory
Electrochemistry by clusters	C. Liu et al.	CO2 reduction on Cu4, Fe4, Ni4, and Pt4 clusters, graphene	10.1039/c4cp02690j	Theory
Structures and magnetic properties of free clusters	A. Erlebach et al.	(Fe2O3)n clusters, structures, magnetic properties, comparison with bulk	10.1039/c4cp02099e	Theory
Structures and electronic properties of free and supported clusters	C. Rajesh et al.	Aun clusters (n = 1–7 and 10), gas phase and α-Al2O3(0001) supported	10.1039/c4cp02137a	Theory
	L. Shen et al.	Pt5 and Pt4Zn clusters, effect of doping	10.1039/c4cp01877j	Theory
Chemical ordering in mixed clusters	D. Bochicchio et al.	Ag–Pd nanoparticles, chemical ordering, subsurface structure	10.1039/c4cp02143f	Theory
(C) Clusters prepared by chemical methods
Controlled particle synthesis	Q. Liu et al.	Synthesis of Ru and Pt nanoparticles of 1–8 nm size by electrostatic absorption and post treatment	10.1039/c4cp02714k	Experiment
	R. B. Duarte et al.	Synthesis of atomically dispersed Rh, promoters, electron microscopy, X-ray absorption spectroscopy	10.1039/c4cp02596b	Experiment
Chemisorption on clusters	M. Keppeler et al.	Bridging H-atoms on a Pt13 cluster supported on an LTL zeolite; infrared spectroscopy, low energy electronic excitations	10.1039/c4cp02052a	Experiment
Fluorescence probes	N. Vilar-Vidal et al.	Cu13 clusters, fluorescence detection and elimination of Pb ions	10.1039/c4cp02148g	Experiment
Cluster fluxionality/surface restructuring in supported clusters	C. S. Spanjers et al.	1 nm Pd clusters on a silica support, surface sensitive differential X-ray absorption spectroscopy	10.1039/c4cp02146k	Experiment
Catalysis by supported clusters and nanoparticles	L. Delannoy et al.	1–2 nm Cu, Au, AuCu clusters on TiO2, selective hydrogenation of butadiene, electron microscopy, X-ray photoemission spectroscopy	10.1039/c4cp02141j
Photocatalysis by supported mixed clusters	W. Jones et al.	~3 nm doped/core shell AuPd particles, H2 production, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, microscopy	10.1039/c4cp04693e	Experiment

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