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The influence of the support basicity, according to the Lewis and Bronsted definition, was investigated for the Ru catalyzed CO2 methanation in the temperature range from 200 °C to 400 °C. Due to the structure-sensitivity of the reaction, a novel building block approach was used to ensure a constant Ru particle size, while varying the support material. In this way, differences in the catalytic behaviour could be directly related to support effects. Eight oxides – the rare earth metal oxides Gd2O3, Sm2O3 and Y2O3 (REOs) as well as TiO2, ZrO2, Al2O3, MgO and SiO2 as a non-basic oxide – were chosen to cover different types and combinations of basic surface sites on the support, such as Bronsted basic hydroxyl groups, Lewis basic oxygen atoms and oxygen vacancies. Above 310 °C, the REO supported catalysts showed the highest methane formation rates. The consumption of carbonate species formed upon CO2 adsorption on all three types of basic sites indicated their catalytic involvement in the high temperature regime. Below 310 °C, TiO2 and – to a lesser extent – ZrO2 excelled the other supports. For ZrO2 the enhanced performance could be related to the sole presence of Lewis basic oxygen vacancies, acting as additional CO2 adsorption and activation sites on the support. On contrary, in case of TiO2 they seemed not to be directly but only indirectly involved by facilitating the conversion on the Ru particles on the basis of a favourable electronic metal–support interaction. The inferior catalytic results obtained with the other supports were in accord with the absence of basic sites or a spectator role of the carbonates formed – except for Al2O3 which stood out probably due to Bronsted basic OH-groups formed under reaction conditions. Overall, the study reveals that basic supports can noticeably contribute to the catalytic turnover by opening new support-related pathways in addition to the Ru-related pathway evidenced in all cases and/or by promoting the latter. Their impact is dependent on the type, density and strength of basic sites available and varies with temperature.

The state-of-the-art approach to stabilize nanoparticles (NPs) for applications in heterogeneous catalysis is to support them on inert inorganic material, which is limited to low loadings of the catalytic component. However, applications such as catalytic gas sensing require a high density of catalytically active sites at a low total heat capacity. To offer an alternative to the supporting of NPs, the stabilization of catalytic NPs with organic ligands in solid state is presented. Therefore, the preparation strategy, consisting of NP synthesis and subsequent functionalization with mono- and bifunctional ligands, is introduced. The molecular linkage of Pt NPs with bifunctional amine ligands (ligand-linking) results in three-dimensional porous networks with ligand-free surface sites. The catalytic properties of ligand-stabilized NPs were investigated in a thermoelectric hydrogen sensor. In addition to an enhanced activity, the stability of the NPs can be significantly improved by ligand-linking. One reason may be that the bifunctional ligand is anchored on the NPs by two head groups. The criteria for ligand structures to enable a successful NP stabilization were identified. para-Phenylenediamine (PDA) combines the criteria and, consequently, by linking of Pt NPs with PDA a constant catalytic activity over more than 20 h on stream was achieved. Thus, ligand-stabilized NPs are presented as a novel catalyst for catalytic gas sensing.

Based on a sol–gel coating method, a series of nanoporous gold (npAu) catalysts functionalized with titania–ceria mixed oxides were prepared. Metal-oxides with different composition were formed inside the mesoporous material (ligaments and pores ∼45 nm) after thermal treatment at over 200 °C for 2 h. The water-gas shift (WGS) reaction (H2O + CO → H2 + CO2) was studied in a continuous flow reactor at ambient pressure using these Ce–TiOx/npAu catalytic materials. Formation of CO2 was observed at temperatures between 200 °C and 450 °C. The addition of CeO2 to TiO2 resulted in an strongly increased activity; the sample (with the molar ratio of Ce : Ti = 1 : 2 abbreviated as Ce1Ti2Ox/npAu) shows the highest activity which was nearly twice as high as the activity of all other samples at 300 °C. The loss of activity after 2 catalytic runs was only about 10% at 450 °C for the Ce1Ti2Ox/npAu sample and no coarsening was observed. Raman spectroscopic characterization of the materials indicates a dynamic correlation between the crystallization (oxygen storage) of the metal-oxides under oxidizing and reducing conditions.

Cobalt-based catalysts are often promoted with noble metals to improve the reducibility of the catalyst and provide a high number of metallic Co sites. The high cost of such noble metals requires new synthetic strategies enabling the use of such promoters at as low concentrations as possible. In this article, we present platinum-promoted Co–Al2O3 catalysts with very small concentrations of platinum (between 0.03 and 0.43 wt%) synthesized by double flame spray pyrolysis (DFSP) as a very versatile preparation technique. Catalysts with Pt contents as low as 0.03 wt% Pt lead to a significant improvement in the reducibility of Co3O4 and to high catalytic activity for the CO2 methanation reaction compared to non-promoted Co–Al2O3. Upon further increasing the Pt content up to 0.43 wt%, only a slight improvement in catalyst reduction and catalytic activity is observed. All prepared catalysts were characterised using XRD, BET, TPR, TEM and EDX followed by catalytic tests for CO2 methanation. Furthermore, two different preparation schemes were used for DFSP, where platinum was combusted either with Co or with the Al precursor solution in one flame, which results in catalysts with a tight chemical contact between Pt and Co3O4 or Pt and Al2O3, respectively. Based on TPR and catalytic tests it could be demonstrated that the deposition of platinum on one or the other oxidic phase has no influence on the reducibility and catalytic performance. The conversion and reducibility were similar for both preparation schemes, an observation which can be explained by H2 spillover during catalyst reduction and catalytic reaction.

Sol–gel chemistry offers versatile new ways to prepare catalysts with tunable compositions and in different forms of application. In particular the option to obtain catalytic coatings is of interest for microreactors or the deposition of catalysts on monolithic supports. The present study explored a sol–gel approach for Fe and Co Fischer–Tropsch catalysts based on the so-called epoxide addition method. As a support and structural promoter, respectively, alumina was added. In the case of cobalt, the role of different precursors was studied, whereas in the case of iron, varying Fe–Al2O3 ratios were investigated. In both cases, ambient drying resulted in xerogels with high specific surface areas. The performance of the xerogels as catalysts for the Fischer–Tropsch (FT) reaction was studied in a fixed bed reactor and compared to cobalt- and iron-based reference catalysts synthesized by established impregnation and precipitation methods, respectively. All catalysts were carefully characterized in the as-prepared state as well as after catalysis with respect to their reducibility, stability and the distribution of the FT active component. Our study proves the suitability of the applied sol–gel technique to prepare highly active alumina-promoted Fe as well as alumina-supported Co FT catalysts and provides insight into the structure–performance relationships of these systems.

Although monolithic catalysts offer distinct advantages such as low pressure drops and easy catalyst separation compared to tube bundle reactors filled with pellets and are for instance widely used for automobile exhaust treatment, the coating of the monolithic structures with the catalytically active phase is often complex and recipes need to be established empirically. In contrast to traditional methods, such as dip-coating or impregnation of the monolith, where the active nanoparticles are formed during the preparation on an oxidic washcoat usually used and deposited simultaneously or in a previous step, we demonstrate in this study that alternatively preformed colloidally synthesized nanoparticles can be employed to obtain homogeneous coatings with or without a washcoat. In this way, one can take advantage of the far-reaching possibilities of colloidal methods to control the structure and size of the nanoparticles and also to tune and optimize their binding to the monolithic surface. For cases where beneficial metal–support interactions between the nanoparticles and a washcoat improve the catalytic properties we demonstrate that colloidally prepared nanoparticles can be directly mixed with a washcoat slurry and successfully deposited on monolithic supports. Turnover frequencies comparable to the corresponding powder catalysts could be reached. In a second approach, we present here a facile method to directly coat three different monolithic supports (cordierite honeycomb, Al2O3 foam and Nickel fleece) with preformed Pt nanoparticles in the presence and absence of organic ligands. In order to realize high metal loadings, the beneficial influence of a ligand “double-layer” (coating of nanoparticles and the support by organic ligands) enhancing the adhesion between the Pt nanoparticles and the underlying monolithic support will be discussed. In the case of the metallic Ni substrate, this approach furthermore allows to circumvent alloy formation and nanoparticle diffusion into the metallic substrate. This can greatly increase long-term stability of systems coated directly onto metallic substrates without an additional oxidic washcoat.