Session B03: Supported Nano-Clusters II: Tuning Reactivity Through Cluster Size and Alloy Formation

Old Questions, New Paradigms: Tuning Nanocatalytic Reactivity and Selectivity

Identification and tuning the factors governing reactivity and selectivity have been central productive challenges in catalysis. Attempts were made to uncover organizing concepts, such as the Sabatier principle in the form of volcano plots, seeking correlations between characteristic properties through use of catalytic descriptors, obtained, almost exclusively, from surface science studies of extended catalytic systems. The advent of nanocatalysis brought key assumptions of the above approaches under scrutiny, and critical assessments of the adequacy of the above-noted framework – via systematic theoretical and experimental studies of reactivity, spectroscopy, and microscopic reaction pathways of size-selected supported nanocatalysts and trapped gas-phase clusters – resulted in a paradigm shift. This led to selection of “catalytic tuning knobs” appropriate for the nanoscale, including: non-crystallographic nanocluster structures and their isomers, nanocluster dynamic fluxionality, enhanced nanocluster-support interactions, local work-function and nanocluster charging dependencies on the support-film composition, nanoscale thickness and underlying metal (i.e. a-silica on Pt vs Mo), and atom-by-atom variation of the atomic coordination and the nanocatalyst discrete electronic structure, requiring statistical distribution analysis. Here, size-specific catalytic tuning is illustrated in: structure-sensitive ethylene hydrogenation catalyzed by supported Pt_n (8 < n < 20) clusters [1]; CO oxidation catalyzed by supported gold Au_n (7 < n < 20) [2]; selective methane activation by trapped small gas-phase gold clusters, and temperature-tunable cycles of gold-catalyzed methane conversion to ethylene or formaldehyde [3].
[1] Nat. Commun. 10389 (2016); ACS Catal. 7, 6738 (2017) [2] J. Phys. Chem. A 103, 9573 (1999); Angew. Chem. 42, 1297 (2003); JACS 131, 538 (2009) [3] Angew. Chem. 49, 980 (2010); JPC A 115, 6788 (2011); Angew. Chem. 56, 13406 (2017)
  

Clusters in Action

In this presentation, we highlight several examples of clusters in action. We first present results on the activation of methane on free tantalum and tantalumoxide clusters and show that intrinsic cluster reactivities behave in a non-predictable way with reaction paths strongly dependent on cluster size and composition. Ab initio calculations confirm the existence of a size-dependent rate-determining step measured by kinetic and isotope exchange experiments. In a second part of the talk, strategies for optimizing cluster stability against ripening will be presented, where both, the binding of the clusters to the surface and that of the individual atoms, must be controlled. Such tuning of the interactions may be achieved through the judicious selection of surfaces with laterally modulated wettability. As a third example of clusters in action, results on the hydrogenation of ethylene on Pt_n clusters are presented. Surprisingly we observed an onset of reactivity going from Pt₉ to Pt₁₀ and Pt₁₃ being the most active cluster size. The origin of the size dependent reactivity is the local charging of the reactive atoms in the cluster. We present strategies of how to tune the reactivity through the design of the local environment of the clusters. Finally, we discuss the reaction mechanisms of the hydrogen evolution during methanol photoreforming on small Pt clusters on rutile TiO₂ single crystals. It is shown that the metal clusters on the semiconductor in first place act as an active site for the formation of molecular hydrogen. The found reaction mechanism suggests that some of the general concepts in photocatalysis must be revised and extended.   

Kinetic and Mechanistic Studies of Room-Temperature Methane Activation Mediated by Free Tantalum Clusters

The selective transformation of methane, the most abundant natural gas, is an important task that requires C-H bond activation. Unraveling molecular-level mechanistics of such complex processes is facilitated in gas-phase studies of metal clusters, which serve as model catalysts. In our setup, the combination of a laser vaporization cluster source, a cryogenic ring electrode ion trap and a reflectron time-of-flight mass spectrometer is used to analyze reactions of size-selected metal clusters under well-defined multi-collision conditions. Tantalum clusters bring about dihydrogen elimination from methane and the cluster size drastically affects the reaction efficiency and final products. In addition to these size effects, changes in the reaction mechanism as well as potential C-C and C-O bond formation will be discussed.   

Methanol Partial Oxidation Reaction Mechanism on γ-Al2O3(001)-supported Au Clusters: A First-principles Density Functional Theory Investigation

Following the discovery that gold is a viable catalyst in oxidation reactions [1], much research has focused on its use in the partial oxidation of alcohols. Several studies have demonstrated the good catalytic activity of gold nanoparticles supported on partially reducible metal oxides while the mechanism involving irreducible oxide supports has not been explored to the same extent. In this study, we investigate the reaction mechanism of the partial oxidation of methanol on Au clusters supported on the γ-Al₂O₃(001) surface. We aim to understand how cluster size and charge transfer affect the reaction mechanism. After a survey of structural and electronic properties of supported Au_n (n=1,4) clusters and the energetics of adsorption of methanol (CH₃OH) on both Au and interface sites, we present a comparative analysis of the computed activation barrier of the CH₃OH → CH₃O + H reaction on this system against those characteristic of traditional catalysts. This work is supported by the Scientific and Technological Research Council of Turkey (TUBITAK) within the 1001 program, Grant No: 113F099.

[1] M. Haruta et al. Chem. Lett., 405-408 (1987)   

Graphene-supported metal clusters: A fs two photon photoemission and scanning tunneling microscopy study

During the last decade, the growth of graphene on metal single crystal surfaces has attracted considerable attention. This substrate system provides an ideal template for the ordered preparation of regular metal cluster arrays. Our new experimental setup enables time-resolved measurements due to a femtosecond laser system on the one hand and surface analysis via scanning tunnelling microscopy on the other hand. In this contribution the bound unoccupied electronic state structure of an Ir(111)/graphene surface covered by differently sized and spaced Ir clusters was investigated by means of femtosecond two-photon photoemission spectroscopy. The cluster lattice was found to affect the image potential states of the substrate to an extent, which is surprisingly large compared to the graphene area covered by Ir clusters. This effect can be related to the influence of the cluster lattice on the screening of the image state electron trapped in front of the surface. Furthermore, lateral confinement effects could be observed due to the decreasing extension of bare graphene areas.
  

Size, Structure, Support, and Alloying Effects on Cluster Chemistry

Size-selected model catalysts provide a useful tool for probing catalytic mechanisms, allowing observation of size-dependent correlations between cluster physical and chemical properties. Cluster deposition also allows preparation of model catalysts where the size and coverage of catalytic sites can be varied independently of each other, and of support structure. Size-selected clusters on well characterized supports are also amenable to detailed theory. Alloying catalytic nanoparticles provides an important tool for tuning adsorbate binding and catalytic properties, but extending mass-selected cluster deposition to bimetallic clusters is challenging. This talk will briefly describe an approach in which deposited size-selected clusters of a pure element (e.g. Pt) act as seeds for selective deposition of a second element (e.g. B or Sn). The activity of the clusters for several reactions under both gas-surface and electrochemical reaction conditions will be discussed.   

Determination of Local Structure Descriptors from X-ray Absorption Near Edge Structure in Bimetallic Nano-Alloys Using an Ab-Inito-Trained Neural Network

X-ray absorption spectroscopy (XAS) is a common method for the probing of nanoparticles (NPs). In conventional approaches, three-dimensional geometry is extracted from extended X-ray absorption fine structure (EXAFS) while the other portion of the XAS spectrum, the X-ray absorption near edge structure (XANES), is often neglected. XANES is sensitive to structure and is less perturbed by disorder, compared to EXAFS, but there's a lack of methodology preventing the quantitative determination of structure from XANES spectra. We have recently developed a method to fill this gap: an artificial neural network (NN) trained on ab-inito site-specific XANES calculations [1]. The NN was successful in the quantitative analysis of supported Pt NPs, motivating its application to bimetallic systems. Here we present a viable neural-network-based method for the extraction of local structure descriptors from experimental XANES measured in AuPd nano-alloys. By training the NN with a sufficiently diverse theoretical set to simulate the complexity of a bimetallic, one with a range of shapes, sizes, compositions, and lattice constants, we obtain results comparable to conventional EXAFS analysis.

[1] J. Timoshenko et al., J. Phys. Chem. Lett. 8 (20), 5091-5098. 2017   

Prolate-shaped Gold Cluster Series with Unique Optical Properties: Au6 as a New Building Unit

Subnano-sized gold clusters bearing ligands have been an attracting research topic, because they are expected to exhibit nuclearity- and structure-dependent physicochemical properties. During our systematic study on the gold clusters ligated with bidentate diphosphines, we found that a series of prolate-shaped Au_n clusters (n = 6, 7, 8, 11) have a common Au₆ unit, and exhibit the lowest-energy intense absorption bands in the visible region [1-3]. These bands, which have never been found in the absorption spectra of the conventional spherical gold clusters ligated with monophosphines, were all assigned to HOMO-LUMO intraband transitions. Furthermore, the correlation between the Au₆-based structures and the HOMO-LUMO gaps was revealed from density functional theory calculation. In addition, optical tunabilities of these clusters were also demonstrated through ligand modifications around the gold cores [4,5]. [1] J. Am. Chem. Soc. 136, 12892 (2014). [2] Chem. Commun. 48, 7559 (2012). [3] Angew. Chem. Int. Ed. 50, 7442 (2011). [4] Nat. Commun. 8, 576 (2017). [5] J. Phys. Chem. Lett. 7, 4267 (2016).
  

Selective Adsorption of H2 Molecule on N-doped ZnO Nano-ribbons: Ab-initio Investigation.

Density functional theory combined with the non-equilibrium Green’s function formalism is used to study the adsorption and gas-sensing properties of H₂ gas molecule on pristine and doped ZnO nano-ribbons (NRs). Substitutional doping of oxygen site with C, N and F have been tested versus adsorption of H₂ molecule and other molecules (e.g., N₂, O₂, H₂O, H₂S). The results of relaxation show chemisorption to occur only on C- and N-doped samples. While all these molecules exhibit chemisorption on C-doped ZnO-NR, only H₂ and O₂ molecules are chemisorbed on N-doped ZnO-NRs. The chemisorption of O₂ is associated with the breaking of one π-bond and thus the desorption is possible. Whereas, the chemisorption of H₂ is associated with a complete dissociation and introduce donor states into the gap (i.e., it plays a role of n-type dopant) and consequently enhancing the conductivity. These characteristics made N-doped ZnO-NRs have high sensitivity and selectivity towards the detection of H₂ gas. Furthermore, the calculated IV-curves have paved the way for estimating the sensitivity and consolidating our results. Since the change of conductance is one of the main outputs of sensors, our findings will be useful in developing Hydrogen-based solid-state sensors.