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Highlights: Mechanistic understandings of metal-free carbon thermocatalysis and electrocatalysis from the viewpoint of model method are summarized. Active sites and reaction mechanisms are discussed with a focus on in-situ techniques and 2D structure–activity relationships. The real contribution of each alien species, defect and edge configuration to catalytic reactions are systematically highlighted at a molecular level. Metal-free carbon, as the most representative heterogeneous metal-free catalysts, have received considerable interests in electro- and thermo-catalytic reactions due to their impressive performance and sustainability. Over the past decade, well-designed carbon catalysts with tunable structures and heteroatom groups coupled with various characterization techniques have proposed numerous reaction mechanisms. However, active sites, key intermediate species, precise structure–activity relationships and dynamic evolution processes of carbon catalysts are still rife with controversies due to the monotony and limitation of used experimental methods. In this Review, we summarize the extensive efforts on model catalysts since the 2000s, particularly in the past decade, to overcome the influences of material and structure limitations in metal-free carbon catalysis. Using both nanomolecule model and bulk model, the real contribution of each alien species, defect and edge configuration to a series of fundamentally important reactions, such as thermocatalytic reactions, electrocatalytic reactions, were systematically studied. Combined with in situ techniques, isotope labeling and size control, the detailed reaction mechanisms, the precise 2D structure–activity relationships and the rate-determining steps were revealed at a molecular level. Furthermore, the outlook of model carbon catalysis has also been proposed in this work. [ABSTRACT FROM AUTHOR]

Colloidal synthesis of metal nanocrystals (NC) offers control over size, crystal structure and shape of nanoparticles, making it a promising method to synthesize model catalysts to investigate structure-performance relationships. Here, we investigated the synthesis of disk-shaped Co-NC, their deposition on a support and performance in the Fischer–Tropsch (FT) synthesis under industrially relevant conditions. From the NC synthesis, either spheres only or a mixture of disk-shaped and spherical Co-NC was obtained. The disks had an average diameter of 15 nm, a thickness of 4 nm and consisted of hcp Co exposing (0001) on the base planes. The spheres were 11 nm on average and consisted of ε-Co. After mild oxidation, the CoO-NC were deposited on SiO2 with numerically 66% of the NC being disk-shaped. After reduction, the catalyst with spherical plus disk-shaped Co-NC had 50% lower intrinsic activity for FT synthesis (20 bar, 220 °C, H2/CO = 2 v/v) than the catalyst with spherical NC only, while C5+-selectivity was similar. Surprisingly, the Co-NC morphology was unchanged after catalysis. Using XPS it was established that nitrogen-containing ligands were largely removed and in situ XRD revealed that both catalysts consisted of 65% hcp Co and 21 or 32% fcc Co during FT. Furthermore, 3–5 nm polycrystalline domains were observed. Through exclusion of several phenomena, we tentatively conclude that the high fraction of (0001) facets in disk-shaped Co-NC decrease FT activity and, although very challenging to pursue, that metal nanoparticle shape effects can be studied at industrially relevant conditions. [ABSTRACT FROM AUTHOR]

The growth, electronic structure and stability of Sm on ordered CeO2(111) thin films grown on Cu(111) were investigated by means of X-ray photoelectron spectroscopy (XPS), low energy electron diffraction, and scanning tunneling microscopy (STM). Metallic samarium was deposited on the CeO2(111) surface by thermal evaporation under ultrahigh vacuum conditions at room temperature. The XPS data suggest that metallic Sm is oxidized to Sm3+ upon the deposition of Sm on CeO2, accompanied by the reduction of Ce4+ to Ce3+. With increasing the Sm coverage, the concentration of Ce3+ increases monotonically. After depositing 6 ML of Sm, only Ce3+ is observed within the detection depth of XPS. The STM results indicate that Sm exhibits a two-dimensional growth on the CeO2(111) surface at low coverages. Annealing to higher temperatures leads to the agglomeration of Sm particles and concurrent diffusion of Sm into the ceria film. These results illustrate that Sm can modify both the electronic and structural properties of ceria. [ABSTRACT FROM AUTHOR]

Electronic adjustment is one of the most commonly used strategies to improve the catalytic performance of heterogeneous catalysts. We prepared hexagonal ultrathin Pd nanosheets with edges modified by gold nanoparticles (Au@Pd nanosheets) using galvanic replacement method. By virtue of the electronic interactions between the Pd nanosheets and Au nanoparticles, the Au@Pd nanosheets exhibited excellent catalytic performances in the carbonylation of iodobenzene by carbon monoxide. The novel nanocomposites could be applied as model catalysts to explore electronic effects in catalysis. [ABSTRACT FROM AUTHOR]

There is an ongoing debate on the structure of titania supported vanadia used for catalytic partial oxidation reactions as well as for abatement of nitrogen oxide (NO) by ammonia selective catalytic reduction (NH-SCR). To gain further insight into the essential features of the catalyst composition of the surface we introduce a new class of vanadia model catalysts based on monodispersed silica particles as a platform. The evaluation of the influence of different catalyst components (TiO, WO) and different vanadia synthesis approaches (incipient wetness, ion-exchange) is facilitated by the hierarchical structure of the catalysts as well as the controlled synthesis approach. Catalysts tested under industrial ammonia SCR conditions show a strong dependence on the catalyst composition. In particular, the presence of crystalline titania and tungsta leads to a strong increase in NO conversion. Detailed catalyst characterization by X-ray diffraction and by visible Raman, UV Raman, UV-Vis and X-ray photoelectron spectroscopies provides insight into the bulk structure, the surface composition as well as the vanadia surface structure of the catalysts. The potential of UV resonance Raman spectroscopy for structural analysis of vanadia model catalysts for SCR is highlighted. Differences in catalytic activity are discussed in light of the structural results and the available literature. [ABSTRACT FROM AUTHOR]

From studies on supported model catalysts, we know that the reactivity of catalysts depends on the size and shape of the metal nanoparticles but also for some reactions it can depend on the distance between them. We discuss recent methods to prepare regular arrays of supported model catalysts. One of these methods is based on the growth of regular arrays of metal clusters on a template (graphene or h-BN monolayers or ultrathin oxide film on a metal single crystal) and allows the study of size effects down to clusters of a few atoms. A second method uses self-assembled micelles from diblock co-polymers, and produces hexagonal arrays of clusters on large area flat supports which are well suited to study effects of the size (1-10 nm) and of the density of clusters. A third method using colloidal synthesis is able to produce in large quantity metal nanoparticles with a very narrow size distribution and a single shape which spontaneously self-organize in regular arrays on a large area. This method is very promising for studying the effect of the shape of metal particles on catalytic activity and selectivity. We discuss also some other methods like electron beam or nanosphere lithography which are less promising for the fabrication of model catalysts. Finally we make some remarks about the likely evolution of studies on supported model catalysts. Graphical Abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Experimental evidence for support effect of carbon are summarized for Pt or PtRu catalysts supported by carbon nanotubes and graphene as well as for model catalysts of Pt/HOPG (highly oriented pyrolytic graphite). The electronic modification of Pt catalysts by the support effect is explained by a π-d interaction at the interface between graphitic surface and Pt. The interface interaction is related to non-bonding localized states of carbon, which are observed by scanning tunneling spectroscopy near defects or the doped-nitrogen atom on HOPG. The localized states propagate to a few nanometer regions behaving as acid and basic sites. [ABSTRACT FROM AUTHOR]

Oxides are widely used in heterogeneous catalysis, thus fundamental understanding of structure-activity relationships of oxide catalysts is of great importance for the improvement and design of efficient catalysts. Recently the synthesis of oxide nanocrystals with uniform and well-defined structures has achieved great progress, and these oxide nanocrystals consist of a novel type of model catalysts for oxide catalysts. Employing oxide nanocrystal model catalysts, crystal plane-dependent surface reactivity and catalytic property of oxide catalysts have been successfully revealed. The recent progress is reviewed in this article and two systems are highlighted. One is cuprous oxide whose crystal planes affect the surface reactivity and catalytic property dominantly by crystal plane-dependent surface composition and structure; the other is ceria whose crystal planes affect the surface reactivity and catalytic property dominantly by crystal plane-dependent concentration and type of oxygen vacancy. [ABSTRACT FROM AUTHOR]

Today there is a need for inexpensive and reliable catalysts due to the increasing demand for 'green' catalytic processes of high conversion and selectivity. We now report on the performance of palladium nanoparticles supported on various carbon allotropes in the model reaction of cyclohexene hydrogenation. Palladium catalysts supported on pristine and oxidatively functionalized multiwalled carbon nanotubes and activated carbon were synthesized and characterized by transmission electron microscopy, X-ray diffraction, temperature programmed desorption and nitrogen adsorption measurements. Catalyst based on multiwalled carbon nanotube proved to be more active and less prone to deactivation than the activated carbon supported one. This finding could be interpreted on the basis of morphological differences between the supports. [ABSTRACT FROM AUTHOR]

A tremendous development of nanotechnology enabled us to prepare precisely structure-controlled nanocrystals (NCs). Recently, such structure controlled NCs have been applied as model catalysts with controlled active sites to evaluate reaction mechanisms. Here, we review our recent works on preparation of shape controlled Pt NCs, such as cube and wire, and on application as model catalysts for olefin hydrogenation to assess active Pt sites. A hydrogen reduction method of Pt(II) ion was adopted in an aqueous phase to prepare Pt cube in the presence of shape forming agent (NaI) and organic protective agent (sodium polyacrylate or sodium succinate). We succeed to prepare Pt cube with high size and shape selectivity, followed by very small Pt cube of less than 3 nm. Such simultaneous control of both size and shape of Pt NCs has been successfully achieved by the sophisticated tuning of multiple conditions for the growth kinetics of Pt nuclei during hydrogen reduction of PtCl in aq. N- , N-dimethylformamide (DMF) solution. The key strategy is to produce small Pt nuclei and to avoid the excessive growth of Pt nuclei, in conjunction with face selective adsorption of anionic protective and shape-forming agents by the control of solvent system. For the preparation of Pt nanowires with high anisotropy, template-assisted methods have been usually used. We have developed facile liquid phase preparation method at room temperature in air without any template. The key factors of the method are: firstly, to use aqueous organic solvent system to control solvent polarity; secondly, to use co-solvent DMF as shape forming agent; thirdly, to use excess NaBH as a stabilizer of Pt nuclei to prevent formation of conventional particles. Single-crystalline Pt nanowires of 2 nm diameter and more than micron length were easily obtained by the reduction of Pt(IV) with excess NaBH in water-DMF-toluene solution (1:8:5 volume ratio) in a short reaction period of 3 h. Then, we have applied shape-controlled Pt NCs protected with PAA prepared by our original methods, such as cube, tetrahedron and wire, as model catalysts for olefin ( cis- and trans-stilbenes) hydrogenation in ethanol to evaluate the active facet of Pt catalysts; e.g., Pt(111) or Pt(100). The estimated TOF values for the hydrogenation of cis- and trans-stilbene decreased in the order: cube > cuboctahedron > tetrahedron ~ nanowire. This tendency indicates that Pt(100) show high activity compared with Pt(111). The result is compatible with measurement of XPS and Raman spectra, suggesting strong adsorption of reactant on Pt(100). Hydrogenation of olefins (1-hexene and cyclohexene) has been carried out over Pt cubes with different sizes (8.2-10.1 nm) as catalysts to get information about active sites of Pt catalysts, e.g., flat facet or edge/corner atoms. The results of similar TOF values among Pt cubes with different sizes imply that the active sites are flat facets. [ABSTRACT FROM AUTHOR]

We present X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD) investigations of ordered chromium oxide ultrathin films prepared on a Pd(111) single-crystal surface. The films where grown by thermal evaporation of Cr under an oxygen atmosphere and sample temperature of 600 K. The ordered films produced are strongly dependent on the film thickness and annealing treatment. Films with thickness below 5 Å, produced at low coverages, display a p(2 × 2) structure relative to Pd(111). Thicker films (thickness > 10 Å) always have a $$ \left( {\sqrt 3 \times \sqrt 3 } \right)R30^{ \circ } $$ structure. The photoemission measurements were done using conventional X-ray sources as well as synchrotron radiation taken at the Laboratório Nacional de Luz Síncrotron. The XPD data were interpreted through the use of a multiple scattering calculation approach combined with a genetic algorithm for surface structure optimization. Combining the information from XPS, low energy electron diffraction and XPD measurements we have determined the surface structure of the $$ \left( {\sqrt 3 \times \sqrt 3 } \right)R30^{ \circ } $$ phase. Elsewhere, we report on the structure of the p(2 × 2) phase. [ABSTRACT FROM AUTHOR]

CO oxidation has been performed on Co3O4 nanobelts and nanocubes as model catalysts. The Co3O4 nanobelts which have a predominance of exposed {011} planes are more active than Co3O4 nanocubes with exposed {001} planes. Temperature programmed reduction of CO shows that Co3O4 nanobelts have stronger reducing properties than Co3O4 nanocubes. The essence of shape and crystal plane effect is revealed by the fact that turnover frequency of Co3+ sites of {011} planes on Co3O4 nanobelts is far higher than that of {001} planes on Co3O4 nanocubes. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) was used to investigate the effects of water on CO oxidation catalyzed by TiO2 supported Au. The introduction of water into the reactant mixture caused two effects: (1) a decrease in the CO coverage; and (2) the appearance of a carbonate species. Since both effects inhibit CO oxidation, the promotional effects of water found by previous researchers can only be rationalized by its effects on molecular oxygen adsorption and activation. Although this argument is not confirmed by the PM-IRAS data alone, this conclusion is consistent with previous studies. It is further inferred from this study that carbonate species first form on TiO2 sites and then migrate to Au sites. Surface hydroxyl species may play the same promotional role as adsorbed water. [ABSTRACT FROM AUTHOR]

A systematic series of model methanol synthesis catalysts was prepared by sequential impregnation of a mesoporous silica material (5 nm average pore size) with an organometallic ZnO precursor which is liquid at room temperature, followed by the infiltration with an aqueous Cu nitrate solution. These catalysts, which contained 14–20 wt.% Cu and 1–5 wt.% Zn, were characterized by N2O reactive chemisorption, by EXAFS and by measuring their methanol synthesis activities. It was observed that the formation of confined, nanocrystalline ZnO prior to copper infiltration is of major importance for the development of catalyst activity. Severe reduction of properly prepared catalysts (10% CO/H2, 673 K, 15 min) leads to the emergence of a new feature in the ZnK EXAFS spectrum which was assigned to a Cu neighbour by combined evidence from the ZnK EXAFS and XANES regions. The zinc oxide component was partially reduced as well, but Zn(0) was not formed to any significant extent. Catalysts which developed this Cu–Zn2+ interaction under severe reduction were superior in terms of methanol synthesis rate per m2 Cu surface area to a sample which did not exhibit this feature. [ABSTRACT FROM AUTHOR]

Sintering is one of the most important causes for loss of catalytic activity when catalysts are subjected to elevated temperatures, as in automotive exhaust or during catalytic combustion. Herein we report a novel form of model catalyst for sintering studies at elevated temperatures. These model catalysts based on disks of SiO2 (quartz) and Al2O3 (sapphire) allow sintering temperatures of > 900 °C to be studied. High resolution scanning electron microscopy (SEM) was used for the study of these catalysts. Using modern in-lens backscatter electron detection operating at low voltages, high contrast images can be obtained of the metal catalyst on these model oxide supports. Model catalysts make it possible to observe the same area of the sample, before and after sintering, which is essential to gain mechanistic insights into the sintering phenomena. Our results show that while Ostwald ripening is the dominant mechanism for Pd sintering at 900 °C, we see evidence for the migration of particles as large as 50 nm on the alumina surface. [ABSTRACT FROM AUTHOR]

Flat model catalysts give access to fundamental aspects of olefin polymerization over heterogeneous catalysts. They are especially suited for the investigation on the early stages of polymer film growth employing scanning probe and electron microscopy. The polymerization centers are confined to a well defined two dimensional plane that remains constant during the polymerization. Using the Phillips (CrO x /SiO2) catalyst as an example we demonstrate how this approach can be used do visualize the lateral distribution of polymerization activity in the initial stages of polymer growth. [ABSTRACT FROM AUTHOR]

The interaction of CO with structurally well-defined, planar Au/TiO2 model catalysts at elevated pressures (up to 50 mbar) was studied in-situ by polarization-modulated infrared reflection absorption spectroscopy and ex-situ by X-ray photoelectron spectroscopy performed before and after CO exposure. The results indicate a CO-induced partial reduction of the oxide surface, which is evidenced by a low frequency C–O vibration at 2060 cm−1, combined with a spreading of the Au nanoparticles due to a modification of the Au-oxide interface energy. In a 2:1 CO:O2 atmosphere, TiO2 support reduction was not observed, and a pre-reduced surface was re-oxidized. The consequences of these results for the understanding of the CO oxidation mechanism on Au/TiO2 (model) catalysts are discussed. [ABSTRACT FROM AUTHOR]

The partial and total oxidation of ethene was studied on clean and oxidized polycrystalline Pd in a UHV high-pressure cell. The model catalyst was prepared under UHV conditions and characterized by thermal desorption and Auger and Raman spectroscopy. Starting from either a Pd or PdO surface, reaction to CO2 is the predominant pathway, and acetic acid is the main byproduct between 453 and 523 K. The route to partially oxidized products, in particular acetic acid, is more favoured on the PdO surface, while on Pd metal acetic acid is formed at lower temperatures than on PdO. On stoichiometric PdO activation for both partial and total oxidation occurs during an induction period, until under working conditions Pd metal, PdO, ‘PdC x’ and isolated defects are coexistent on the surface. The presence of an extended metal surface favours the follow-up oxidation of byproducts, while the presence of PdO stabilizes acetic acid far beyond 473 K. [ABSTRACT FROM AUTHOR]

Planar “inverse catalyst” model systems, i.e. noble metal single crystal surfaces decorated by oxide nanostructures, have been used to visualise surface processes in the STM with atomic level precision. The processes chosen for their prototypical relevance in heterogeneous catalysis systems comprise the oxidation-reduction of surface oxide phases, the effects of the metal-oxide phase boundary in promoting the CO oxidation reaction and the O2+H2 water reaction, and the mass transport processes of oxide materials over metal surfaces via the diffusion of oxide clusters. [ABSTRACT FROM AUTHOR]

Pd/CeO2/Ta/Si model catalysts were prepared by spin coating and sputter deposition method, and characterized by means of AFM, SEM and in situ XPS, especially focusing on the redox properties of Ce and Pd elements. Compared with thin CeO2 films (about 2.2nm), the thicker ones (about 22nm) maintained Ce4+ oxidation state even after treatment with H2 up to 500°C while the presence of Pd facilitated the reduction of ceria. The reduction of ceria brought about following that of PdO, which was explained by the spillover of hydride in Pd to CeO2 originating from hydrogen adsorption on the Pd surface. Compared with the sputter deposition method, spin coating produced the smaller size of Pd particles, thus leading to formation of the stable PdO species against hydrogen. Based on these results, a schematic model of Pd/CeO2/Ta/Si was suggested and it might be assumed that spin coating method provided with an environment similar to the conventional impregnation. [ABSTRACT FROM AUTHOR]

Thin films of magnesium vanadates oriented to expose a single crystalline face, could potentially serve as ideal models for high surface area magnesium vanadate catalysts for oxidative dehydrogenation. The growth of oriented films of one particular magnesium vanadate phase, the orthovanadate (Mg3(VO4)2), has been achieved by rf sputter deposition of the orthovanadate onto Au(111) surfaces. X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy have been used to investigate the structure and composition of the films. The orthorhombic orthovanadate grows epitaxially with the (021) plane oriented parallel to the surface. By varying oxygen flow rates during deposition the stoichiometry of the films can be varied from fully oxidized to highly oxygen deficient. At very low oxygen flow rates or in the complete absence of oxygen, a reduced Mg3V2O6 phase is formed. This reduced phase has a cubic structure and grows with the (100) plane parallel to the surface. [ABSTRACT FROM AUTHOR]

Evolution of microstructure and morphology of Co3O4 particles in the model systems during reduction in hydrogen was studied by transmission electron microscopy methods. Based on SAED and HRTEM results we found that the degree of reduction of Co3O4 strongly depends on the particle size and morphology, which are determined by the pretreatment conditions. Preferential epitaxial growth of CoO and Co phases on Co3O4 during reduction was deduced from HRTEM images. [ABSTRACT FROM AUTHOR]

By alternating the supply of propylene and ethylene monomers, alternating polypropylene and polyethylene films were produced on one model Ziegler–Natta catalyst. This way, we could show that the active polymerization sites remain under the growing polymer films and thus the monomer molecules must transport through the growing polymer to the active sites. [ABSTRACT FROM AUTHOR]

A planar CrO x /SiO2/Si(100) model for the Phillips ethylene polymerization catalyst has been prepared by spincoat impregnation from an aqueous solution of CrO3. The model catalyst polymerizes ethylene from the gas phase at 160°C with a constant activity and forms a 400 nm thick layer of polyethylene in 1 h. The superior definition of the polymer films produced on this catalyst and the control over the distribution of active sites on its flat surface make this model catalyst an ideal substrate for kinetic studies on catalytic polymerization and for morphologic studies of the polymer product by scanning force microscopy. At extremely low catalyst loading we observe isolated polymer islands formed on single chromium sites. The work also opens attractive opportunities for future studies of nascent morphology of catalytically formed polymers. [ABSTRACT FROM AUTHOR]

Two types of model catalysts are compared: thin film catalysts consisting of polyhedral noble metal nanocrystals (Rh and Pt) supported by reducible and non reducible oxides, and their inverted pendants, submonolayers of titania and vanadia deposited under UHV conditions on the respective metal surfaces (Pd and Rh(111) and Rh (polycrystalline)). The structure and composition of the inverse catalysts were examined in situ by LEED and AES and the nanoparticles were characterized by HRTEM. The activity of thin film and inverse catalysts was studied in a series of reactions, such as the ring opening of methylcyclopentane and methylcyclobutane, the dissociation of CO and the CO methanation. Reaction conditions comprise atmospheric pressure but also molecular beam experiments. The reaction rates are related to the oxidation state of the supporting oxide, to the free metal surface area and to the number of sites at the interface between metal and support. [ABSTRACT FROM AUTHOR]

Nanostructured 4 inch Si wafers were prepared as new supports for model catalysts. A single wafer exhibits 109–1010 pits on an otherwise flat silicon oxide surface. To produce these nanostructures, oxidized wafers were covered with photoresist material and exposed to laser interference patterns. Using hydrofluoric acid, wet chemical etching of the SiO2 through the structured resist resulted in a pitted surface. The etched pits with diameters of 200–400 nm and depths between 50 and 70 nm were loaded with metal particles by evaporation (palladium, silver) or by means of spin coating (copper). Optimizing the methods enabled exact deposition of single metal clusters in the pits. The resulting model catalysts are remarkably stable against sintering, a major problem of conventional model catalysts when exposed to elevated temperatures and oxidizing gas atmosphere. The topography and chemical composition as well as their changes, induced by the reaction conditions applied, including stability and chemical behavior of the nanostructured systems, were investigated by means of AFM, SEM, temperature programmed methods and XPS. To determine their usefulness in catalysis, specially designed reactors were developed for catalytic investigations. [ABSTRACT FROM AUTHOR]

A model Pd/Fe2O3 catalyst prepared by the vacuum technique has been studied in the carbon monoxide oxidation in the temperature range of 300–550 K at reagent pressures P(CO)=16 Torr, P(O2)= 4 Torr. It has been shown that the activity of the fresh catalysts is determined by palladium. According to the XPS data, the reduction with carbon monoxide results in the formation of Fe2+ (formally Fe3O4) and appearance of the catalytic activity in this reaction at low temperatures (350 K). High low-temperature activity of the catalyst is supposed to be connected with the reaction between oxygen adsorbed on the reduced sites of the support (Fe2+) and CO adsorbed on palladium (COads) at the metal–oxide interface. [ABSTRACT FROM AUTHOR]

Ziegler–Natta heterogeneous catalytic systems are extensively used to polymerize ethylene and propylene. Some industrial catalysts consist of TiCl4 chemisorbed on activated MgCl2 and subsequently reduced and alkylated by reaction with an aluminum alkyl (generally AlEt3). Lewis bases are added to the catalytic systems to control the enantio-selectivity for the production of isotactic polypropylene. Our aim is to clarify the chemical composition of the active centers by modern surface science methods. Model catalysts are prepared in the form of ultra-thin films by gas-phase deposition on a gold foil in ultrahigh vacuum. Under these conditions, MgCl2 films grow to controlled thickness via a layer-by-layer mechanism, as revealed by AES and XPS. TiCl4 can be deposited on these films near room temperature by both electron irradiation-induced and metallic magnesium-induced chemical vapor deposition. Angle-resolved XPS studies indicate that these films consist of a few layers of TiCl2 with one monolayer of TiCl4 chemisorbed on its surface. The exposure of these titanium chloride films to the co-catalyst AlEt3 produces an active model Ziegler–Natta catalyst. XPS analysis reveals the presence of TiCl2Et on the catalyst surface: this is believed to be the active site. Prolonged reaction with the co-catalyst reduces the titanium sites to TiClEtn (n = 1 and/or 2). High molecular weight polyethylene and polypropylene are synthesized on these catalysts, as shown by Raman spectroscopy. Highly isotactic polypropylene is produced without need for stereo-regulating Lewis bases. [ABSTRACT FROM AUTHOR]

Metal clusters arranged on nanostructured oxidized silicon wafers are presented as new model catalyst systems. A photoresist layer spun on top of a wafer was patterned by laser interference exposure. The grid obtained after removing the exposed parts of the resist is used as an etching mask. Hollows with diameters of 300 nm and depths between 50 and 60 nm were etched into the oxide layer using wet chemical methods. Two methods were applied to deposit metal clusters (Pd or Cu) in a defined way within the hollows. The particles ranged from 10 to 50 nm in height and from 80 to 200 nm in diameter. The model catalyst systems were characterized by atomic force microscopy and X-ray photoelectron spectroscopy. The method presented here allows us to produce 4 inch wafers that are covered completely by nanometer-sized structures in a reasonable period of time. [ABSTRACT FROM AUTHOR]

The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number. [ABSTRACT FROM AUTHOR]