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In this article we shed light on newly emerging perspectives to characterize and understand the interplay of diffusive mass transport and surface catalytic processes in pores of gas phase metal catalysts. As a case study, nanoporous gold, as an interesting example exhibiting a well-defined pore structure and a high activity for total and partial oxidation reactions is considered. PFG NMR (pulsed field gradient nuclear magnetic resonance) measurements allowed here for a quantitative evaluation of gas diffusivities within the material. STEM (scanning transmission electron microscopy) tomography furthermore provided additional insight into the structural details of the pore system, helping to judge which of its features are most decisive for slowing down mass transport. Based on the quantitative knowledge about the diffusion coefficients inside a porous catalyst, it becomes possible to disentangle mass transport contributions form the measured reaction kinetics and to determine the kinetic rate constant of the underlying catalytic surface reaction. In addition, predictions can be made for an improved effectiveness of the catalyst, i.e., optimized conversion rates. This approach will be discussed at the example of low-temperature CO oxidation, efficiently catalysed by npAu at 30 °C. The case study shall reveal that novel porous materials exhibiting well-defined micro- and mesoscopic features and sufficient catalytic activity, in combination with modern techniques to evaluate diffusive transport, offer interesting new opportunities for an integral understanding of catalytic processes. Graphical Abstract

Synthesis of chlorine-free, rare earth oxide aerogels from the lanthanide series was achieved using a modified epoxide-assisted sol-gel method. An ethanolic solution of the hydrated metal nitrate, propylene oxide, and ammonium carbonate was found to gel upon heating to 333 K. Critical point drying of the wet gel in CO2 yielded monolithic aerogels. Most of the aerogels were amorphous as-prepared, but became nano-crystalline after calcination at 923 K in air. The aerogels had high surface areas (up to 150 m2/g), low densities (40–225 mg/cm3), and were photoluminescent.

Ni and Co containing precursor derived ceramics (ceramers) were prepared from a polysiloxane based preparation route. All catalysts were characterised by BET, XRD and TEM as well as by water and heptane adsorption and tested for CO2 methanation and Fischer–Tropsch synthesis. Different pyrolysis temperatures between 400 and 600 °C were used to get catalysts with different surface hydrophilicities. With increasing synthesis temperature less organic groups remain on the surface, resulting in a more hydrophilic catalyst. For all Ni containing ceramers, well dispersed particles in the range of 3 nm were formed and comparable surface areas were found. The catalysts with the lowest tendency towards water adsorption showed the highest activity for CO2 methanation. In contrast to the Ni catalysts, for the Co containing ceramers particle formation was dependent on the pyrolysis temperature. While no metallic particles were formed at 400 °C, small particles in the range of ~5 nm were obtained, using a pyrolysis temperature of 500 °C. Increasing the pyrolysis temperature to 600 °C, the particle size increased to ~10 nm. First tests for CO2 methanation and Fischer–Tropsch reaction were successfully carried out and the catalysts with the less hydrophilic surface showed higher activity and a higher selectivity towards C5+-products.

A new sol–gel synthesis route for alumina–samaria mixed aero- and xerogel catalysts based on the so-called epoxide addition method and the use of these systems as catalysts for the oxidative coupling of methane (OCM) is reported. As precursors simple chloride or nitrate salts can be used. The mesoporous materials are X-ray amorphous even after calcination to 800 °C and show an intimate mixing of Al and Sm on the nanoscale. In the case of the xerogels derived from chlorides, C2 yields comparable to pure samaria can be achieved under OCM reaction conditions with 100 % O2 conversion. Even at lower O2 conversions the activity of the xerogel is competitive with a pure samaria reference catalyst taking the lower samaria content of 20 % into account. Accordingly, the approach is suitable to reduce the costs associated with the rare earth oxide. In addition to the preparation of aerogel and xerogel particles, the presented synthesis also allows the fabrication of xerogel films which can be coated on a suitable (monolithic) support. First results of such films are presented.

Lanthanide oxide-based aerogels were synthesized employing the so-called epoxide addition sol–gel method already successfully applied for main- and transition metal oxide aerogels. Using chlorides and nitrates as precursors, our aim was to test the transferability of this robust sol–gel methodology to the entire lanthanide series. By adding the proton scavenging organic epoxide, propylene oxide, to hydrated lanthanide trichloride dissolved in ethanol or methanol, uniform monolithic alcogels were obtained. Subsequent processing in supercritical CO2 resulted in monolithic aerogels. No gelling process could be induced by using nitrates, in contrast to previous results with iron oxide or alumina aerogels. All materials were characterized by nitrogen adsorption/desorption analysis, transmission electron microscopy, and powder X-ray diffraction. With the exception of cerium, for which fractions of crystalline CeO2 were found already in the as-prepared material, XRD analysis revealed that the other materials were mainly amorphous. Subsequent heat treatment of the aerogels above 650 °C resulted in nanocrystalline phases for all aerogel materials. However, except for ceria, more detailed TEM and XRD studies provided evidence that crystalline oxychloride phases are formed in addition to fractions of oxide phases. The trends and possible explanations are discussed in this contribution. A sol–gel methodology using organic epoxides as proton scavengers has been used to synthesize oxide aerogels of the entire lanthanide series. As precursors, nitrates and chlorides were used but only in the latter case, gelation set in. After supercritical drying, aerogels with high surface areas and mesoporous, nanoparticulate structures were obtained. Further characterization revealed that cerium plays a special role, since it contained crystalline CeO2 fractions already before calcination. After calcination it was the only material consisting of a pure oxide phase. The syntheses with the other lanthanides resulted in mixtures with varying fractions of crystalline oxychlorides in addition to an oxide phase. The trends and possible explanations are discussed in this contribution.A sol–gel methodology using organic epoxides as proton scavengers has been used to synthesize oxide aerogels of the entire lanthanide series. As precursors, nitrates and chlorides were used but only in the latter case, gelation set in. After supercritical drying, aerogels with high surface areas and mesoporous, nanoparticulate structures were obtained. Further characterization revealed that cerium plays a special role, since it contained crystalline CeO2 fractions already before calcination. After calcination it was the only material consisting of a pure oxide phase. The syntheses with the other lanthanides resulted in mixtures with varying fractions of crystalline oxychlorides in addition to an oxide phase. The trends and possible explanations are discussed in this contribution.

Copper, nickel and copper–nickel nanoparticles were prepared by solution combustion method for use in direct write printing. Structural (X-ray diffraction) and morphological (transmission electron microscope) investigations showed that pure metal (Cu and Ni) and CuNi alloy particles with face-centred cubic crystal structure were formed. Atomic absorption spectrometer studies confirmed that the nanoparticle compositions corresponded to the initial Cu/Ni molar ratios selected for synthesis. Particle size and morphology were significantly influenced by composition, with high Cu content coinciding with small, spherical particles as opposed to larger, irregular shapes observed at high Ni concentrations. X-ray photoelectron spectroscopy measurements revealed that after the reduction process the surface of the alloy nanoparticles was partially oxidised in air and the amount of metallic surface species decreased, while the concentration of oxidic surface species and hydroxides increased with increasing Cu concentration (i.e. decreasing particle size). Dispersions of CuNi nanoparticles have been deposited by use of AerosolJet® and sintered under reducing atmosphere at 300–800 °C in order to prepare conductive structures. Resistivity measurements and microscopical studies (SEM-FIB) of printed and sintered CuNi structures showed that the sintering properties of nanoparticles were dependent on their chemical composition.

Although actuation in biological systems is exclusively powered by chemical energy, this concept has not been realized in man-made actuator technologies, as these rely on generating heat or electricity first. Here, we demonstrate that surface-chemistry-driven actuation can be realized in high-surface-area materials such as nanoporous gold. For example, we achieve reversible strain amplitudes of the order of a few tenths of a per cent by alternating exposure of nanoporous Au to ozone and carbon monoxide. The effect can be explained by adsorbate-induced changes of the surface stress, and can be used to convert chemical energy directly into a mechanical response, thus opening the door to surface-chemistry-driven actuator and sensor technologies.

CO adsorption on gold particles deposited on well-ordered alumina and iron oxide films was studied with temperature-programmed desorption. Scanning tunneling microscopy was used to provide correlative structural characterization. The results show that the adsorption of CO on gold exhibits a size effect in that small particles adsorb CO more strongly. For a given particle size (∼3 nm), CO desorption temperature (at ∼170 K) is essentially independent of the supports studied. Therefore, support effects seen in CO oxidation on real catalytic systems must arise from the interaction of oxygen rather than CO with these catalysts.

Alumina-supported Pd model catalysts were prepared by Pd evaporation onto a thin alumina film grown on a NiAl(110) substrate. Adsorption and co-adsorption of ethene, CO and hydrogen on Pd/Al2O3/NiAl(110) covered by carbon species, formed by ethene dehydrogenation at ∼550 K, was studied by temperature programmed desorption (TPD). TPD results show that carbon deposits do not prevent adsorption but inhibit dehydrogenation of di-σ bonded ethene. Carbon species suppress CO adsorption in the highly coordinated sites and also suppress the formation of hydrogen ad-atoms on the surface. The ethene hydrogenation reaction performed by co-adsorption of hydrogen and ethene is inhibited by the presence of carbon deposits. The inhibition is independent of particle size studied (1-3 nm). The effects are rationalized in terms of a site-blocking behavior of carbon species occupying highly coordinated sites on the Pd surface.

Using periodic self-consistent density functional calculations it is shown that the barrier for CO dissociation is ≃120 kJ/mol lower on the stepped Rh(211) surface than on the close-packed Rh(111) surface. The stepped surface binds molecular CO and the dissociation products more strongly than the flat surface, but the effect is considerably weaker than the effect of surface structure on the dissociation barrier. Our findings are compared with available experimental data, and the consequences for CO activation in methanation and Fischer–Tropsch reactions are discussed.

Utilizing a combination of molecular beam techniques and scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions we have studied the interaction of oxygen with an alumina-supported Pd model catalyst as well as the influence of the oxygen pretreatment on the kinetics of the CO oxidation reaction. The Pd particles were deposited by metal evaporation in UHV onto a well-ordered alumina film prepared on a NiAl(110) single crystal. The particle density, morphology and structure are determined by STM both immediately after preparation and after oxygen adsorption and CO oxidation. The oxygen sticking coefficient and uptake in the temperature regime between 100 and 500 K and the kinetics of the CO oxidation reaction are quantitatively probed by molecular beam techniques. It is found that starting at temperatures below 300 K the Pd particles rapidly incorporate large amounts of oxygen, finally reaching stoichiometries of PdO>0.5. STM shows, that neither the overall particle shape nor the dispersion is affected by the oxygen and CO treatment. Only after saturation of the bulk oxygen reservoir are stable CO oxidation conditions obtained. In the low-temperature regime (

The preparation of model systems based on thin epitaxial oxide films and oxide single crystals is discussed. A variety of surface sensitive techniques has been applied to study the geometric and electronic properties of these systems. The findings are correlated with adsorption and reaction of probe molecules on the surfaces. Metal vapor deposition under controlled conditions leads to the formation of metal aggregates with narrow size distributions. Their properties have been characterized, establishing that we can begin to bridge the materials gap between catalysis and surface science. While mainly performed under UHV conditions, adsorption measurements can be pushed to ambient conditions using non-linear optical techniques such as sum frequency generation. Results for systems with deposited metal aggregates will be discussed.

In order to investigate how the presence of surface hydroxyl groups on oxide surfaces affects the interaction with the supported metal, we have modified a well-ordered alumina film on NiAl(110) by Al deposition and subsequent exposure to water. This procedure yields a hydroxylated alumina surface as revealed by infrared and high-resolution electron energy loss spectroscopy. By means of scanning tunneling microscopy, we have studied the growth of rhodium on the modified film at 300 K. Clear differences in the particle distribution and density are observed in comparison to the clean substrate. While, in the latter case, decoration of domain boundaries as typical defects of the oxide film governs the growth mode, a more isotropic island distribution and a drastically increased particle density is found on the hydroxylated surface. From infrared data, it can be deduced that the growth is connected with the consumption of the hydroxyl groups due to the interaction between the metal deposit and the hydroxylated areas. This finding is in line with photoemission results published earlier.

Nanoporous gold is a material with many possible applications e.g. in catalysts, sensors and electrode materials. We studied the functionalization of the nanoporous gold with TiO2 particles. Aiming at the low temperature oxidation of CO, the nanoporous gold can be coated with TiO2 in order to enhance catalytic activity. Structure and distribution of the TiO2 on the gold surface are important structural features, which were investigated by transmission electron microscopy. The preparation of the porous gold was tested with focused ion beam - preparation, conventional Ar+ ion beam preparation of nanoporous gold embedded in epoxy and ultramicrotome preparation of nanoporous gold embedded in epoxy. Considering the beam damage on the structure and the contamination of the surface, ultramicrotome preparation turned out to be the best solution. It was shown, that the gold ligaments are abundantly covered by approximately 5 nm TiO2 particles. The determination of the largest lattice fringe distance in high resolution mode revealed that the crystalline nanoparticles consist of the anatase phase. The spatial Ti distribution was measured with energy filtered transmission electron microscopy. Scanning transmission electron microscopy tomography was applied to reconstruct the three-dimensional structure of the gold coated with TiO2 particles.