Pt single sites are highly attractive due to their high atom economy and can be generated on CeO2 by an oxidative high-temperature treatment. However, their location and activity are strongly debated. Furthermore, reaction-driven structural dynamics have not been addressed so far. Here, we were able to evidence Ptinduced CeO2 surface restructuring, locate Pt single sites on CeO2 and track the variation of the active state under reaction conditions using a complementary approach of density functional theory calculations, in situ infrared spectroscopy, operando high-energy-resolution fluorescence detected X-ray absorption spectroscopy and catalytic CO (as well as C3H6 and CH4) oxidation. We find that the onset of CO oxidation is linked to the migration of Pt single sites from four-fold hollow sites to form small clusters containing few Pt atoms. This demonstrates that operando studies on single sites are essential to assess their fate and the resulting catalytic properties. a promise as they lower the noble metal content significantly as all atoms are potentially active species. [5][6][7][8][9] Exploiting the strong noble metal support interaction between Pt and CeO2, metallic Pt particles can be formed orin contrast to weakly interacting supports like Al2O3redispersed, with tremendous impact on the catalyst activity. [10][11][12][13] The preparation of SAC has been demonstrated for Pt, which can be atomically dispersed when using CeO2 as a support through an oxidizing treatment at 800 °C. 14 However, the exact structure of the single sites, their reactivity and, particularly, their state and dynamics during reaction are still unknown and heavily debated. 4,15,16 The location of Pt single sites is claimed to range from surface adsorbates on {111} ceria steps, 17,18 {111}, 19 {110} 20,21 or {100} 6,22,23 ceria facets to surface 21,24,25 or bulk Ce substitutes [26][27][28] forming Ce1-XPt 2+ XO2-Y-composites. During change of the gas atmosphere and of the temperature, the structure of the single sites may strongly change resulting in a new and more active state. For example, after a high temperature treatment strong Pt-O-Ce bonds are reported to over-stabilize the single sites which are thus less active. 29 During the catalytic oxidation, oxygen is suggested to be provided by the support, while the reactant e.g. CO is adsorbed directly on Pt, 22,25 similar to Pt nanoparticles on CeO2. 30 Bera et al. correlated the intensity of the Pt-O-Ce bond observed by Extended X-ray Absorption Fine Structure (EXAFS) measurements with the catalytic activity for Ce1-XPt 2+ XO2-Y, 31 and Nie et al. 24demonstrated that the catalytic activation of a Pt single atom catalyst can be increased by steam treatment. It is suggested that this treatment leads to the formation of Ce1-XPt 2+ XO2-YH-OH species that are catalytically more active than Ce1-XPt 2+ XO2-Y. 24 In contrast, other studies report an increase in catalytic activity after a reductive treatment at temperatures below 300 °C. 9,32-34 Importantly, such

Pd/Al2O3 and Pd/CeO2 catalysts were investigated for methane oxidation at conditions typical for the exhaust of lean burn gas engines. The results show that catalyst prereduction significantly increases the catalytic activity during the light-off irrespective of the gas composition. Operando X-ray absorption spectroscopy revealed a fully reduced catalyst state after the reductive pretreatment, which is reoxidized with increasing temperature in a lean reaction mixture, resulting in bulk PdO formation at 350 °C. The correlation of catalytic activity with oxidation state during light-off tests led to the conclusion that PdO is a mandatory species for methane oxidation. We attribute the increased conversion after prereduction to the slight sintering of Pd particles and higher reactivity of the formed PdO surface species. Additionally, the H2O inhibition effect was found to be retarded under dry conditions due to the relatively slow palladium reoxidation. The results presented are in particular relevant for the activity of methane oxidation catalysts at low temperature and under dynamic conditions.

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We successfully explored, for the first time, the use of the w/o inverse miniemulsion route to prepare surfactant-functionalised nanocrystalline ZnO colloids. The adopted route exploits the micelles as nanoreactors for the precipitation of the desired oxide in a confined space. Triton X-100 (TritX-), sodium dodecyl sulfate (SDS-) and polyvinylpyrrolidone (PVP-) coated ZnO crystalline nanoparticles (NPs) have been obtained at room temperature (RT) with no need for post-treatment, by precipitation of zinc chloride with ammonium or sodium hydroxide into w/o inverse micelles. Their hydrodynamic diameter, evaluated by Dynamic Light Scattering (DLS), is about 35 nm. X-Ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES), Fourier Transform Infrared (FT-IR) spectroscopy and Thermogravimetric Analysis (TGA) have been used to characterize powders separated by miniemulsions. The NP inorganic core is constituted of wurtzite ZnO, with a high degree of crystallinity, as determined by XRD. XRD data and TEM images revealed the formation, in the case of ZnO TritX , of anisotropic plate-like crystallites, with an average diameter of 72 nm and a thickness of 15-20 nm. The RT photo-luminescent (PL) spectrum of ZnO PVP NPs shows a strong UV emission band, attributed to the free exciton recombination, with a relevant tail in the Vis region due to the presence of structural defects. The morphology of these systems, investigated by SEM, corresponds to a homogeneous dispersion of globular sponge structures in a compact and fibrous matrix.

Metallocene injection into a metal combustion flame has been used for trapping transition metal ions inside MgO nanocrystals. Vacuum annealing changes the properties of resulting nonequilibrium solids toward thermodynamic equilibrium and provides means to control impurity localization and, as a result, the nanomaterials’ functional properties. By combining structure characterization (X-ray diffraction and transmission electron microscopy) with X-ray absorption spectroscopy and Mössbauer measurements, we tracked valence state and local chemical environment changes of Fe3+ ions inside vapor phase synthesized MgO nanocrystals. At a concentration of (1.5 ± 0.2) at. % Fe about (1400 ± 200) Fe3+ ions are effectively diluted within 12 nm sized nanocubes, where they form complexes between Fe3+ ions and Mg2+ vacancies. Increase of the iron concentration produces additional effects: enhanced ion diffusion and particle coarsening at elevated temperatures, clustering of Fe3+–Mg2+ vacancy complexes and, after annealing to T = 1173 K, the nucleation of a magnesioferrite phase that can be detected by X-ray diffraction for 4 at. % samples. At 3 at. % Fe, corresponding impurity ions induce surface energy changes that have a substantial impact on particle shape. With regard to the functional properties associated with transition metal ions in insulating MgO host lattices, the here presented insights underline that annealing-induced reorganization of oxide nanoparticles provides important parameters to control distribution and localization of impurity ions.

During the CO oxidation over metallic Pt clusters and Pt nanoparticles in Pt/CeO 2 catalysts, we found that the Pt surface concentration is a key descriptor for the reaction rate. By increasing the surface noble metal concentration (SNMC) of a Pt/CeO 2 catalyst by a factor of ∼4, while keeping the weight hourly space velocity constant, the ignition temperature of CO oxidation was decreased by ∼200 °C in the as-prepared state. Moreover, the stability was enhanced at higher SNMC. Complementary characterization and theoretical calculations unraveled that the origin of this improved reaction rate at higher Pt surface concentrations can be traced back to the formation of larger oxidized Pt-clusters and the SNMC-dependent aggregation rate of highly dispersed Pt species. The Pt diffusion barriers for cluster formation were found to decrease with increasing SNMC, promoting more facile agglomeration of active, metallic Pt particles. In contrast, when Pt particle formation was forced with a reductive pretreatment, the influence of the SNMC was temporarily diminished, and all catalysts showed a similar CO oxidation activity. The work shows the general relevance of the proximity influence in the formation and stabilization of active centers in heterogeneous catalysis.

The controlled, room-temperature synthesis of M-doped (M = Cu-II, Mn-II, Sm-III, Gd-III, or Tb-II) ZnS nanostructures (with an average crystallite size of 5-10 nm) in the confined space of miniemulsion droplets is reported herein and discussed. The adopted synthetic route is a colloidal method, ideally suited to easily achieve small particle size and narrow size distribution. The synthesized functional nanostructures crystallize in the sphalerite lattice, as determined by powder X-ray diffraction. In addition to structural characterization, several complementary techniques, such as X-ray photoelectron spectroscopy, thermogravimetric analysis, differential scanning calorimetry, micro-Raman spectroscopy, inductively coupled plasma mass spectrometry, and Fourier transform infrared spectroscopy were used to determine chemical and physical properties as well as the microstructural features of our products. As far as the morphological aspects of the obtained samples are concerned, they were studied by means of scanning and transmission electron microscopies. Finally, the local structure around dopant ions was unravelled by means of X-ray absorption spectroscopy, in particular through extended X-ray absorption fine structure measurements carried out at the Zn, S, and dopant K or L-3 edges. This information has been complemented with the investigation of the photoluminescence properties

ZnS nanosystems are being extensively studied for their possible use in a wide range of technological applications. Recently, the gradual oxidation of ZnS to ZnO was exploited to tune their structural, electronic, and functional properties. However, the inherent complexity and size dependence of the ZnS oxidation phenomena resulted in a very fragmented description of the process. In this work, different-sized nanosystems were obtained through two different low temperature wet chemistry routes, namely, hydrothermal and inverse miniemulsion approaches. These protocols were used to obtain ZnS samples consisting of 21 and 7 nm crystallites, respectively, to be used as reference material. The obtained samples were then calcinated at different temperatures, ranging from 400 to 800 °C toward the complete oxidation of ZnO, passing through the coexistence of the two phases (ZnS/ZnO). A thorough comparison of the effects of thermal handling on ZnS structural, chemical, and functional evolution was carried out by TEM, XRD, XAS, XPS, Raman, FT-IR, and UV–Vis. Finally, the photocatalytic activity in the H2 evolution reaction was also compared for selected ZnS and ZnS/ZnO samples. A correlation between size and the oxidation process was observed, as the smaller nanosystems showed the formation of ZnO at lower temperature, or in a larger amount in the case of the ZnS and ZnO co-presence. A difference in the underlying mechanism of the reaction was also evidenced. Despite the ZnS/ZnO mixed samples being characterized by an increased light absorption in the visible range, their photocatalytic activity was found to be much lower.