Size Of Metal Clusters Research Articles

Atomic Palladium

Polyaniline (PANI) is a conducting polymer that has been extensively studied and characterized and has been used in sensors, catalysis and other electrochemical applications1. Incorporating metal clusters into the polymer matrix creates a composite material that has improved activity over the polymer itself. The properties of the metal clusters are size-dependent, where inert metals can even become active when they are scaled down from bulk to nanometer or sub-nanometer sizes2. An important factor with nanocluster creation is controlling the size of the final metal cluster, which is typically done synthetically with capping agents. However, taking advantage of the properties and structure of PANI can allow for electrochemically controlled growth of metal clusters at the imine sites while PANI is acting as an aggregation barrier, thus eliminating the need for capping agents3. Cluster formation begins with metal anions, such as AuX4 - and PdX4 2- (X = halide), that have an affinity for protonated imine sites of the PANI; these anions can be spontaneously reduced to neutral metal atoms by the emeraldine salt form of PANI. Previous work used this process with an attempt to control cluster sizes in a “top-down” manner by controlling time and concentration variables4. In this work, a controlled deposition and “bottom-up” approach of one metal atom per imine site is explored. The controlled process is completed by holding the PANI at a potential where it is in the oxidized state until excess metal anions are rinsed away and the potential is dropped and the metal is reduced3. A flow-cell connected to a flow-injection system allows for uninterrupted solution and potential changes. Our recent interest is on understanding the PANI-metal interaction and how to tune the interaction5 to build atomic-sized clusters. The most recent work involves the study of the intercalation of Pd in PANI. PdN (N=0-7) clusters are inserted into a polyaniline matrix through this controlled process using the PdCl4 2- anion. Our recent findings show that the speciation of the metal anion is of critical importance for a specific PANI-metal anion interaction. The mechanism is believed to be dependent on the Bronsted-like interactions and/or charge transfer Results have been confirmed using Raman spectroscopy, XPS, and electrochemical methods. In this presentation we will explain the fundamental steps of the formation of the atomic Pd clusters in PANI and how that Pd-PANI interaction is affected and/or tuned by controlling various conditions such as concentration, potential, and degree of PANI protonation. (1) Huang, W.-S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem. Soc. Faraday Trans. 1 1986, 82 (8), 2385. (2) Schwartz, I.; Jonke, A. P.; Josowicz, M.; Janata, J. Catal. Letters 2012, 142 (11), 1344. (3) Jonke, A. P.; Josowicz, M.; Janata, J.; Engelhard, M. H. J. Electrochem. Soc. 2010, 157 (10), P83. (4) Saheb, A.; Smith, J. A.; Josowicz, M.; Janata, J.; Baer, D. R.; Engelhard, M. H. J. Electroanal. Chem. 2008, 621 (2), 238. (5) Harbottle, A. M.; Hira, S. M.; Josowicz, M.; Janata, J. Langmuir 2016, 32 (33), 8315.

Interpretation of rate optima vs reaction parameters in steady state catalytic kinetics: Molecular aspects beyond concentration dependences

Steady state catalytic reactions often exhibit rate maxima not only as a function of concentration, but also of other reaction parameters such as solvent polarity, Hammett substitution constants or metal cluster size. Analysis of several generic schemes operative in homogeneous and heterogeneous catalysis and comprise linear steps was made showing that kinetic expressions after incorporating dependence of rate constants on abovementioned reaction parameters can be used to model experimental data. Rate minima in steady state reactions depending on substituent constants cannot be easily explained by single route conventional mechanisms even after considering nonlinear steps. Such cases are conventionally explained by a switch in reaction mechanisms. An alternative developed in this study assumes presence of mechanistically similar parallel routes involving at least two different catalytically active species. The advanced model based on two step sequence is capable of explaining concave upward Hammett plots.

(Invited) Regioselective Bis-Prato Addition on TNT-EMF (M3N@Ih -C80, M = Y, Gd) Controlled By Endohedral Metal Cluster

Aiming the development of stable and controlled way of the chemical functionalization of fullerenes, there have been many reactions so far reported. The Prato reaction1 (1,3-dipolar cycloaddition reaction) has been used by many groups as one of the efficient chemical conversion methods in the preparation of functional fullerene derivatives. This method usually provides sufficiently stable fullerene derivatives in reasonable synthetic yields. Addressing the preparation of biocompatible fullerene derivatives, we are using this Prato reaction for the controlled functionalization of C60, C70 and TNT-EMF2 (M3N@Ih -C80 or M3N@C80). C60 usually produces the mono-Prato adduct selectively on the [6,6]-junction. However, as reported by the groups of Dorn3 and Echegoyen,4 M3N@C80 provides the mono-Prato adduct on both [6,6]- and [5,6]-junctions as a results of [6,6]-to-[5,6] isomerization via a [1,5]-sigmatropic rearrangement. The ratio of the thermodynamic regioisomers ([6,6]-mono and [5,6]-mono) was dependent on the size of metal cluster (M3N) inside C80 5 and was not affected by the exohedral functional groups outside C80 cage. 6 Although the bis-Prato addition of C60 proceeded uncontrolled manner, interestingly, bis-Prato addition of M3N@C80 proceeded in a regioselective manner especially in the presence of larger size of endohedoral metal clusters such as Y3N and Gd3N.7 Thermal treatment of bis-Prato adduct of Y3N@C80 caused random isomerization, but the bis-Prato adduct of Gd3N@C80 stayed rather stable presumably by the effect of larger endohedral metal cluster. (1) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115, 9798. (2) Stevenson, S.; Rice, G.; Glass, T.; Harich, K.; Cromer, F.; Jordan, M. R.; Craft, J.; Hadju, E.; Bible, R.; Olmstead, M. M.; Maitra, K.; Fisher, A. J.; Balch, A. L.; Dorn, H. C. Nature 401, 898. (3) Cai, T.; Slebodnick, C.; Xu, S.; Harich, L.; Glass, T. E.; Chancellor, C.l Fettinger, J. C.; Olmstead, M. M.; Balch, A. L.; Gibson, H. W.; Dorn, H. C. J. Am. Chem. Soc. 2006, 128, 6486. (4) Cardona, C. M.; Eliott, B.; Echegoyen, L. J. Am. Chem. Soc. 2006, 128, 6480. (5) Aroua, S.; Yamakoshi, Y. J. Am. Chem. Soc. 2012, 134, 20242. (6) Aroua, S.; Garcia-Borras, M.; Osuna, S.; Yamakoshi, Y. Chem. Eur. J. 2014, 20, 14032. (7) Aroua, S.; Garcia-Borràs, M.; Bölter M. F.; Osuna, S.; Yamakoshi, Y. J. Am. Chem. Soc. 2015, 137, 58. Figure 1

The influence of metal nanoparticles on electrical properties of carbon nanotubes

First, sputtering deposition of Au, Ag, Pt and Pd resulted in changes to the electrical resistance of carbon nanotube (CNT) films due to electron localization followed by creation of percolation pathways. Metal-CNT composites created this way showed different reaction of electrical resistance to temperature as well as thermal stability. Furthermore, their surface properties show much higher affinity towards aqueous media than as-made CNT films. Finally, we have developed an easy way of tailoring the size of metal clusters on the surface of CNT films depending on the employed deposition and heating conditions. Electrothermal sintering yielded metal clusters of controlled size that showed big potential for the application in catalyst-assisted chemical transformations.

Size Dependent Bond Length of Metallic Clusters by Considering Bond Number

In this study, size-dependent bond length of metallic clusters is established by introducing bond number. This model, free of any adjustable parameters, can be utilized to predict the change rule of bond length with size. If the atomic structure of a cluster is known, the size and shape-dependent bond number are obtained. The cubooctahedral structure is taken for simplicity to describe the shape and geometric characteristics of metallic clusters. It is found that the bond length decreases with the decreased size of metallic clusters, which is due to the structure relaxation and enhanced single bond energy. The theoretical predictions are consistent with the evidences of the simulations for Au and Ag clusters. This confirms the validity of taking cubooctahedron structure, even if the simulated Au and Ag clusters are not cuboctahedron ones. This can be expected to other metallic clusters even with other atomic structures.

Cluster Size Dependent Kinetics: Analysis of Different Reaction Mechanisms

Quantitative description for turnover frequency dependence on the metal cluster size is discussed for competitive Langmuir-Hinshelwood mechanism showing that the apparent reaction orders depend on the cluster size. Position of the maximum rate of the turnover frequency for a two-step sequence is a function on temperature and only in a special case, maximum of the turnover frequency is temperature independent on reaction temperature. For the same reaction mechanism the impact of internal diffusion limitations on structure sensitivity is discussed. Graphical Abstract

Essential factors for control of the equilibrium in the reversible rearrangement of M₃N@I(h)-C₈₀ fulleropyrrolidines: exohedral functional groups versus endohedral metal clusters.

The effects of exohedral moieties and endohedral metal clusters on the isomerization of M3N@I(h)-C80 products from the Prato reaction through [1,5]-sigmatropic rearrangement were systematically investigated by using three types of fulleropyrrolidine derivatives and four different endohedral metal clusters. As a result, all types of derivatives provided the same ratios of the isomers for a given trimetallic nitride template (TNT) as the thermodynamic products, thus indicating that the size of the endohedral metal clusters inside C80 was the single essential factor in determining the equilibrium between the [6,6]-isomer (kinetic product) and the [5,6]-isomer. In all the derivatives, the [6,6]- and [5,6]-Prato adducts with larger metal clusters, such as Y3N and Gd3N, were equally stable, which is in good agreement with DFT calculations. The reaction rate of the rearrangement was dependent on both the substituent of exohedral functional groups and the endohedral metal-cluster size. Further DFT calculations and (13)C NMR spectroscopic studies were employed to rationalize the equilibrium in the rearrangement between the [6,6]- and [5,6]-fulleropyrrolidines.

Cu cluster deposition on ZnO[formula omitted]: Morphology and growth mode predicted from molecular dynamics simulations

Clusters of Cu on ZnO surfaces are established catalyst systems produced by vapor deposition of Cu that consolidates into discrete clusters. Factors such as the geometry of the ZnO surface, the nature of the interface between Cu and ZnO, and the size and distribution of metal clusters strongly control the reactivity and stability of the catalyst. Here, we report on the development and use of charge-optimized many-body (COMB) potentials to model the deposition and subsequent evolution of Cu clusters on ZnO101¯0 using classical molecular dynamics (MD) simulations. The simulations predict that the deposited Cu spreads to form two-dimensional (2D) structures until the coverage is above 0.4 monolayers (ML). Thereafter, the 2D structures grow thicker with increasing Cu coverage and finally form three-dimensional (3D) clusters. The predictions are compared to published experimental data and help to elucidate aspects of the growth that are currently the subject of controversy in the literature. In addition, the influence of such factors as incident cluster deposition energy, ZnO surface temperature, and different supports on the surface morphology of the Cu clusters are examined. The results are expected to provide useful guidance for the improved production of dispersed Cu clusters on ZnO surfaces.

고품질 금속 이온 첨가 MCM-41 분자체 촉매의 제법, 특성화 및 응용 반응

콜로이드 실리카와 가용성 실리카를 이용하여 나트륨이 첨가되지 않은 다양한 금속이온 첨가 MCM-41 촉매를 제조하였다. 전이금속 이온인 <TEX>$V^{5+}$</TEX>, <TEX>$Co^{2+}$</TEX> 및 <TEX>$Ni^{2+}$</TEX>이 MCM-41에 첨가되었을 경우 기공벽 내의 실리콘 이온과 등방치환을 하여 실리카 기공벽 내에서 독립된 단일 활성점을 형성하여 우수한 환원 및 활성 내구성을 보였다. 수소 승온 환원법을 이용하여 Co-MCM-41 촉매의 기공 곡률 반경효과에 대해 검토해 본 결과, 적절한 환원 처리와 기공 크기 및 pH 조절에 따라 코발트 금속입자의 크기를 1nm 이하의 범위에서 조절할 수 있었으며, 이 미세 금속 입자들은 표면 금속이온들과의 결합으로 인해 상당한 고온 안정성이 있음을 발견하였다. 완전 환원 후에도 비정형 실리카의 부분 덮힘으로 인해 금속 입자들의 표면 이동 및 뭉침 현상이 현저히 저하되는 것을 볼 수 있었다. 이들 촉매의 반응 예로 금속 입자 크기에 민감한 단일층 탄소 나노튜브의 합성을 Co-MCM-41을 이용하여 실시하였고, 금속 입자의 안정성 시험반응으로 Co 및 Ni-MCM-41을 이용한 CO 메탄화 반응, V-MCM-41을 이용한 메탄올 및 메탄의 부분 산화반응 및 기공곡률 반경이 촉매활성에 미치는 영향 등을 살펴보았다. Various metal ions (transition and base metals) incorporated MCM-41 catalysts can be synthesized using colloidal and soluble silica with non-sodium involved process. Transition metal ion-typically <TEX>$V^{5+}$</TEX>, <TEX>$Co^{2+}$</TEX>, and <TEX>$Ni^{2+}$</TEX>-incorporated MCM-41 catalysts were synthesized by isomorphous substitution of Si ions in the framework. Each incorporated metal ion created a single species in the silica framework, single-site solid catalyst, showing a substantial stability in reduction and catalytic activity. Radius of pore curvature effect was investigated with Co-MCM-41 by temperature programmed reduction (TPR). The size of metallic Co clusters, sub-nanometer, could be controlled by a proper reduction treatment of Co-MCM-41 having different pore size and the initial pH adjustment of the Co-MCM-41 synthesis solution. These small metallic clusters showed a high stability under a harsh reaction condition without serious migration, resulting from a direct anchoring of small metallic clusters to the partially or unreduced metal ions on the surface. After a complete reduction, partial occlusion of the metallic cluster surface by amorphous silica stabilized the particles against aggregations. As a probe reaction of particle size sensitivity, carbon single wall nanotubes (SWNT) were synthesized using Co-MCM-41. A metallic cluster stability test was performed by CO methanation using Co- and Ni-MCM-41. Methanol and methane partial oxidations were carried out with V-MCM-41, and the radius of pore curvature effect on the catalytic activity was investigated.

Surface-enhanced Raman scattering and density functional theory study of 1,4-benzenedithiol and its silver complexes

This paper experimentally and theoretically investigated Raman and surface-enhanced Raman scattering (SERS) of 1,4-benzenedithiol (1,4-BDT). Density functional theory methods were used to study Raman scattering spectra of isolated 1,4-BDT and 1,4-BDT-Agn (n=2,4,6) complexes with B3LYP/6-311+g(d)(C,H,S)/Lanl2dz(Ag) basis set. A full assignment of the Raman spectrum of 1,4-BDT has been made based on the DFT analysis. The calculated data showed good agreement with experimental observations. The adsorption sites, metal cluster size, and HOMO–LUMO energies are discussed to give insight in the SERS mechanisms for 1,4-BDT molecules.

Nanoparticles for Heterogeneous Catalysis: New Mechanistic Insights

Metallic nanoparticles finely dispersed over oxide supports have found use as heterogeneous catalysts in many industries including chemical manufacturing, energy-related applications and environmental remediation. The compositional and structural complexity of such nanosized systems offers many degrees of freedom for tuning their catalytic properties. However, fully rational design of heterogeneous catalysts based on an atomic-level understanding of surface processes remains an unattained goal in catalysis research. Researchers have used surface science methods and metal single crystals to explore elementary processes in heterogeneous catalysis. In this Account, we use more realistic materials that capture part of the complexity inherent to industrial catalysts. We assess the impacts on the overall catalytic performance of characteristics such as finite particle size, particle structure, particle chemical composition, flexibility of atoms in clusters, and metal-support interactions. To prepare these materials, we grew thin oxide films on metal single crystals under ultrahigh vacuum conditions and used these films as supports for metallic nanoparticles. We present four case studies on specifically designed materials with properties that expand our atomic-level understanding of surface chemistry. Specifically, we address (1) the effect of dopants in the oxide support on the growth of metal nanoclusters; (2) the effects of size and structural flexibility of metal clusters on the binding energy of gas-phase adsorbates and their catalytic activity; (3) the role of surface modifiers, such as carbon, on catalytic activity and selectivity; and (4) the structural and compositional changes of the active surface as a result of strong metal-support interaction. Using these examples, we demonstrate how studies of complex nanostructured materials can help revealing atomic processes at the solid-gas interface of heterogeneous catalysts. Among our findings is that doping of oxide materials opens promising routes to alter the morphology and electronic properties of supported metal particles and to induce the direct dissociation and reaction of molecules bound to the oxide surface. Also, the small size and atomic flexibility of metal clusters can have an important influence on gas adsorption and catalytic performance.

Cu/boehmite: A highly active catalyst for hydrogenolysis of glycerol to 1,2-propanediol

Small Cu clusters (~ 1 nm) were highly dispersed over boehmite via an aqueous chemical reduction method. In comparison with Cu/γ-Al2O3, Cu/SiO2 and Ru/C catalyst, Cu/boehmite catalyst showed the highest conversion and selectivity of 1,2-propanediol in the hydrogenolysis of glycerol. The good conversion and selectivity are ascribed to the small size of Cu metal clusters and the Lewis acid sites of boehmite, which provides high surface concentrations of active metal sites without CC bond cleavage activity and promotes the dehydration of glycerol to acetol as the intermediate of 1,2-propanediol, respectively.

Interactions and Reactions of Transition Metal Clusters with the Interior of Single-Walled Carbon Nanotubes Imaged at the Atomic Scale

Clusters of transition metals, W, Re, and Os, upon encapsulation within a single-walled carbon nanotube (SWNT) exhibit marked differences in their affinity and reactivity with the SWNT, as revealed by low-voltage aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM). Activated by an 80 keV electron beam, W reacts only weakly with the SWNT, Re creates localized defects on the sidewall, and Os reacts readily causing extensive defect formation and constriction of the SWNT sidewall followed by total rupture of the tubular structure. AC-HRTEM imaging at the atomic level of structural transformations caused by metal-carbon bonding of π- and σ-character demonstrates what a crucial role these types of bonds have in governing the interactions between the transition metal clusters and the SWNT. The observed order of reactivity W < Re < Os is independent of the metal cluster size, shape, or orientation, and is related to the metal to nanotube bonding energy and the amount of electronic density transferred between metal and SWNT, both of which increase along the triad W, Re, Os, as predicted by first-principles density functional theory calculations. By selecting the appropriate energy of the electron beam, the metal-nanotube interactions can be controlled (activated or precluded). At an electron energy as low as 20 keV, no visible transformations in the nanotube in the vicinity of Os-clusters are observed.

Atomic force microscopy of nanostructured gold films after their irradiation with a powerful pulsed microwave radiation

The possibility of using microwave radiation in combination with metallic nanoobjects for cancer therapy has been studied. The results of optical and atomic force investigations of nanostructured metallic films used for testing the methods of such a therapy have been presented. The size of metal clusters separated from the film by microwave pulses has been estimated. Possible mechanisms for the destruction of cancer cells by nanostructures affected by microwave radiation have been considered.

Industrial Ziegler-Type Hydrogenation Catalysts Made from Co(neodecanoate)2 or Ni(2-ethylhexanoate)2 and AlEt3: Evidence for Nanoclusters and Sub-Nanocluster or Larger Ziegler-Nanocluster Based Catalysis

Ziegler-type hydrogenation catalysts are important for industrial processes, namely, the large-scale selective hydrogenation of styrenic block copolymers. Ziegler-type hydrogenation catalysts are composed of a group 8-10 transition metal precatalyst plus an alkylaluminum cocatalyst (and they are not the same as Ziegler-Natta polymerization catalysts). However, for ∼50 years two unsettled issues central to Ziegler-type hydrogenation catalysis are the nature of the metal species present after catalyst synthesis, and whether the species primarily responsible for catalytic hydrogenation activity are homogeneous (e.g., monometallic complexes) or heterogeneous (e.g., Ziegler nanoclusters defined as metal nanoclusters made from combination of Ziegler-type hydrogenation catalyst precursors). A critical review of the existing literature (Alley et al. J. Mol. Catal. A: Chem. 2010, 315, 1-27) and a recently published study using an Ir model system (Alley et al. Inorg. Chem. 2010, 49, 8131-8147) help to guide the present investigation of Ziegler-type hydrogenation catalysts made from the industrially favored precursors Co(neodecanoate)(2) or Ni(2-ethylhexanoate)(2), plus AlEt(3). The approach and methods used herein parallel those used in the study of the Ir model system. Specifically, a combination of Z-contrast scanning transmission electron microscopy (STEM), matrix assisted laser desorption ionization mass spectrometry (MALDI MS), and X-ray absorption fine structure (XAFS) spectroscopy are used to characterize the transition metal species both before and after hydrogenation. Kinetic studies including Hg(0) poisoning experiments are utilized to test which species are the most active catalysts. The main findings are that, both before and after catalytic cyclohexene hydrogenation, the species present comprise a broad distribution of metal cluster sizes from subnanometer to nanometer scale particles, with estimated mean cluster diameters of about 1 nm for both Co and Ni. The XAFS results also imply that the catalyst solutions are a mixture of the metal clusters described above, plus unreduced metal ions. The kinetics-based Hg(0) poisoning evidence suggests that Co and Ni Ziegler nanoclusters (i.e., M(≥4)) are the most active Ziegler-type hydrogenation catalysts in these industrial systems. Overall, the novelty and primary conclusions of this study are as follows: (i) this study examines Co- and Ni-based catalysts made from the actual industrial precursor materials, catalysts that are notoriously problematic regarding their characterization; (ii) the Z-contrast STEM results reported herein represent, to our knowledge, the best microscopic analysis of the industrial Co and Ni Ziegler-type hydrogenation catalysts; (iii) this study is the first explicit application of an established method, using multiple analytical methods and kinetics-based studies, for distinguishing homogeneous from heterogeneous catalysis in these Ziegler-type systems; and (iv) this study parallels the successful study of an Ir model Ziegler catalyst system, thereby benefiting from a comparison to those previously unavailable findings, although the greater M-M bond energy, and tendency to agglomerate, of Ir versus Ni or Co are important differences to be noted. Overall, the main result of this work is that it provides the leading hypothesis going forward to try to refute in future work, namely, that sub, M(≥4) to larger, M(n) Ziegler nanoclusters are the dominant, industrial, Co- and Ni- plus AlR(3) catalysts in Ziegler-type hydrogenation systems.

Microstructural design for fabrication of strain sensor utilizing tungsten-doped amorphous carbon coatings

Tungsten-containing diamond-like carbon (W-DLC) coatings were fabricated with an aim of producing new strain sensor applications. W-DLC coatings with various metal concentrations were prepared by chemical vapor deposition and DC magnetron co-sputtering of a tungsten metal target. The electrical resistance was shown to be proportional to the value of compression and tensile strain. The expression Δ R/ R = Kε, was applied to the experimental results in order to determine the gauge factor of each coating, and this gauge factor varied according to the metal concentration. Transmission electron microscope was employed for structural analyses and it was shown that the structure of the W-DLC coatings consisted of nanometer size grains dispersed into an amorphous carbon host matrix. The electrical properties of the W-DLC coatings were simulated using a model based on a composite insulator-metal cluster structure. These results were in agreement with the experimental ones, and a high gauge factor can be obtained with low metal concentration or increase in size of metal cluster.

ChemInform Abstract: Kinetic Analysis of Cluster Size Dependent Activity and Selectivity

AbstractReview: 47 refs.

On quantitative description of metal particles size effect in catalytic kinetics

Quantitative description for turnover frequency dependence on the metal cluster size for a two-step catalytic cycle was performed based on a thermodynamic approach, which accounts for changes of the chemical potential upon adsorption depending on the metal cluster size. Such analysis revealed a possibility for turnover frequency to exhibit a maximum. A very good correspondence between calculated and experimental data in hydrogenation and decarboxylation reactions over palladium was achieved.

Comparative study of the photodeposition of Pt, Au and Pd on pre-sulphated TiO2 for the photocatalytic decomposition of phenol

A comparative study of the photodeposition of Pt, Au and Pd under the same experimental conditions onto pre-sulphated and non-sulphated TiO2 was performed. Morphological and surface characterisation of the samples as well as photocatalytic activity for phenol photooxidation was studied. The influence of sulphate pre-treatment on the deposits size and dispersion onto the TiO2 surface, and photodeposition yields with the different metals were also analysed. The photocatalytic activity of the doped materials was then investigated, observing that catalytic behaviour can be correlated to physical characteristics of the samples determined by (XRD) X-ray diffraction, (XPS) X-ray photoelectron spectroscopy, (XRF) X-ray fluorescence spectrometry and (TEM) transmission electron microscopy.Sulphate pre-treatment was found to influence both the level of dispersion and the size of metal clusters on the TiO2 surface. Sulphation and metallisation of samples was found to produce a synergistic enhancement in photoactivity for the degradation of phenol. The photoactivity of the catalysts with respect to the doped metal species was ordered Pt>Pd>Au.

Kinetic analysis of cluster size dependent activity and selectivity

The impact of nanoparticle size effects in heterogeneous catalytic kinetics over supported metal catalysts is discussed taking into account different activities of edges and terraces. Kinetic equations for the Eley–Rideal mechanism as well as for the two-step sequence were derived, analyzed and compared with experimental data for ethene hydrogenation. Quantitative description for selectivity in a consecutive reaction of pyrrole hydrogenation/ring opening as function of the metal cluster size is provided demonstrating good correspondence between theory and experiments.