Methanol as a green and renewable resource can be used to generate hydrogen by reforming, i.e., its catalytic oxidation with water. In combination with a fuel cell this hydrogen can be converted into electrical energy, a favorable concept, in particular for mobile applications. Its realization requires the development of novel types of structured catalysts, applicable in small scale reactor designs. Here, three different types of such catalysts were investigated for the steam reforming of methanol (SRM). Oxides such as TiO2 and CeO2 and mixtures thereof (Ce1Ti2Ox) were deposited inside a bulk nanoporous gold (npAu) material using wet chemical impregnation procedures. Transmission electron and scanning electron microscopy reveal oxide nanoparticles (1–2 nm in size) abundantly covering the strongly curved surface of the nanoporous gold host (ligaments and pores on the order of 40 nm in size). These catalysts were investigated in a laboratory scaled flow reactor. First conversion of methanol was detected at 200 °C. The measured turn over frequency at 300 °C of the CeOx/npAu catalyst was 0.06 s−1. Parallel investigation by in situ infrared spectroscopy (DRIFTS) reveals that the activation of water and the formation of OHads are the key to the activity/selectivity of the catalysts. While all catalysts generate sufficient OHads to prevent complete dehydrogenation of methanol to CO, only the most active catalysts (e.g., CeOx/npAu) show direct reaction with formic acid and its decomposition to CO2 and H2. The combination of flow reactor studies and in operando DRIFTS, thus, opens the door to further development of this type of catalyst.

•Aerobic oxidation of methanol and ethanol in liquid and gas phase already at 40 °C.•High activity due to low coordinated Au surface sites (∼25%).•In the absence of a support molecular transformations in line with surface chemistry of Au.•Molecular transformations can be anticipated based on model experiments.The aerobic oxidation of methanol and ethanol in liquid phase is investigated using unsupported nanoporous gold (npAu) catalysts and compared to gas phase experiments. We pursue the question if this material is still suited as a type of model catalyst even under much more complex conditions in liquid phase. Indeed, npAu is an active catalyst for both liquid and gas phase oxidation of methanol and ethanol with O2 under mild conditions (starting at 40 °C). While in case of methanol solely the self coupling product (methyl formate) is detected, the more facile formation of the acetaldehyde in case of ethanol results in the formation of the free aldehyde (acetaldehyde) and the coupling product (ethyl acetate) in similar amounts. To further probe the surface chemistry, cross coupling reactions between both alcohols, as well as the acetaldehyde are performed. As observed also in gas phase experiments, the formation of the surface bonded aldehyde by β-H elimination of the alcoxy species is rate determining.Unsupported nanoporous gold provides an extended and strongly curved surface suitable studying the surface chemistry and reactivity under applied conditions. While the selectivity of the catalysed reaction is determined by the β-H elimination the large fraction of low coordinated Au atoms provide reactivity.Download high-res image (48KB)Download full-size image

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.

•Homogeneous discharge product deposition in macropores.•Restricted oxygen diffusion limits the discharge product formation in mesopores.•Direct oxygen feed enhances the lithium carbonate and lithium oxide formation.•LiF forms during the discharge reaction due to electrolyte and binder degradation.During the discharge of an aprotic Li/O2 battery solid products assemble inside of the gas diffusion electrodes (GDE). The distribution of these in dependence of pore size of the GDE is investigated by X-ray Photoelectron Spectroscopy (XPS). Depth profiling of the electrolyte facing side of the cathode reveal that macroporous electrodes are able to store discharge products homogeneously in the pore structure. Mesoporous GDE, however, develop a concentration gradient with large amounts of discharge product at the electrode/electrolyte interface and low amount in the bulk of the electrode. The investigation of the cross-section of these GDE reveals that most of the discharge products form near the oxygen facing side of the GDE. Here, the chemical compositions differ strongly from those at the electrolyte facing side. The high oxygen concentration and the limited lithium supply lead to the formation of lithium carbonate, lithium oxide and lithium fluoride. Also a thin layer of discharge product blocking further oxygen supply into the GDE through macroporous cracks is formed at the gas/electrolyte interface.

Catalysis is one of the key technologies for the 21st century for achieving the required sustainability of chemical processes. Critical improvements are based on the development of new catalysts and catalytic concepts. In this context, gold holds great promise because it is more active and selective than other precious metal catalysts at low temperatures. However, gold becomes only chemically and catalytically active when it is nanostructured.Since the 1970s and 1980s, the first type of gold catalysts that chemists studied were small nanoparticles on oxidic supports. With the later onset of nanotechnology, a variety of nanostructured materials not requiring a support or organic stabilizers became available within about the last 10 years. Among these are gold nanofoams generated by combustion of gold compounds, nanotube membranes prepared by electroless deposition of gold inside a template, and corrosion-derived nanoporous gold. Even though these materials are macroscopic in their geometric dimensions (e.g., disks, cubes, and membranes with dimensions of millimeters), they are comprised of gold nanostructures, for example, in the form of ligaments as small as 15 nm in diameter (nanoporous gold, npAu). The nanostructure brings about a high surface to volume ratio and a large fraction of low coordinated surface atoms.In this Account, we discuss how unsupported materials are active catalysts for aerobic oxidation reaction in gas phase (oxidation of CO and primary alcohols), as well as liquid phase oxidation and reduction reactions. It turns out that the bonding and activation of molecular oxygen for gas phase oxidations strongly profits from trace amounts of an ad-metal residue such as silver. It is noteworthy that these catalysts still exhibit the special gold type chemistry, characterized by activity at very low temperatures and high selectivity for partial oxidations. For example, we can oxidize CO over these unsupported catalysts (npAu, nanotubes, and powder) at temperatures well below water’s freezing point (−30 °C) and with turnover frequencies up to 0.5 s–1 (at 30 °C). Yet, we can anticipate the surface chemistry of these unsupported and extended gold surfaces based on model experiments under UHV conditions. We have demonstrated this for the selective oxidation of primary alcohols at low temperatures employing npAu catalysts.Chemists have paid growing interest to oxidation and reduction reactions in liquid phase catalysis, most suitable for synthetic organic chemistry. Early work on the aerobic oxidation of d-glucose in 2008 using Raney type npAu already showed the potential of this type of catalyst for liquid phase reactions. Since then, researchers have investigated further oxidation reactions (silanes to silanols) and reduction reactions of alkynes, as well as C–C coupling reactions ([4 + 2] benzannulation) and azo compound decomposition, with likely several more reactions to be reported in the next years. The advantage of this unsupported skeletal type of catalyst is its recyclability and retrievability without leaching of gold into the reaction medium, owing to its monolithic structure. Even though these materials contain nanoscopic structures, they are macroscopic in their geometric dimensions and pose no threat to the environment or health as discussed for other nanomaterials.

Inspired by model studies under ultrahigh vacuum (UHV) conditions, inverse monolithic gold/ceria catalysts are prepared using thermal decomposition of a cerium nitrate precursor on a nanoporous gold (npAu) substrate. Cerium oxide deposits throughout the porous gold material (pores and ligaments 30–40 nm) are formed. npAu disks and coatings were prepared with loadings of about 3 to 10 atom % of ceria. The composite material was tested for the water–gas shift (WGS) reaction (H2O + CO → H2 + CO2) in a continuous flow reactor at ambient pressure conditions. Formation of CO2 was observed at temperatures as low as 135 °C with excellent stability and reproducibility up to temperatures of 535 °C. The considerably increased thermal stability of the material can be linked to the presence of metal oxide deposits on the nanosized gold ligaments. The loss of activity after about 15 h of catalytic conversion with heating to 535 °C was only about 10%. Photoemission spectroscopy indicates a defect (Ce3+) concentration of about 70% on the surface of the cerium oxide deposits, prior to and after WGS reaction. Raman spectroscopic characterization of the material revealed that the bulk of the oxide is reoxidized during reaction.

Nanostructured materials are governed by their surface chemical properties. This is strikingly reflected by np-Au. This material can be generated by corrosion of bulk Ag–Au alloys. Based on a self-organisation process, a 3 dimensional sponge like gold structure evolves with ligaments in the range of only a few tens of nanometers. Due to its continuous porosity, the material can be penetrated by gases which then adsorb and interact with the surface. In this perspective we will review potential applications of np-Au resulting from this effect, namely heterogeneous gas phase catalysis, surface chemistry driven actuation, and adsorbate controlled stability of the nanostructure. We will summarize the current knowledge about the low temperature oxidation of CO as well as the highly selective oxidation of methanol. Furthermore, we will address the question how surface chemistry can influence the material properties itself. In particular, we will deal with (a) the actuation of np-Au by the reversible oxidation of its surface using ozone and (b) the adsorbate controlled coarsening of ligaments, using annealing experiments under ozone or inert gas atmosphere.

Understanding the role of surface chemistry in the stability of nanostructured noble-metal materials is important for many technological applications but experimentally difficult to access and thus little understood. To develop a fundamental understanding of the effect of surface chemistry on both the formation and stabilization of self-organized gold nanostructures, we performed a series of controlled-environment annealing experiments on nanoporous gold (np-Au) and ion-bombarded Au(111) single-crystal surfaces. The annealing experiments on np-Au in ambient ozone were carried out to study the effect of adsorbed oxygen under dynamic conditions, whereas the ion-bombarded Au single-crystal surfaces were used as a model system to obtain atomic-scale information. Our results show that adsorbed oxygen stabilizes nanoscale gold structures at low temperatures whereas oxygen-induced mobilization of Au surface atoms seems to accelerate the coarsening under dynamic equilibrium conditions at higher temperatures.

Methanol as a green and renewable resource can be used to generate hydrogen by reforming, i.e., its catalytic oxidation with water. In combination with a fuel cell this hydrogen can be converted into electrical energy, a favorable concept, in particular for mobile applications. Its realization requires the development of novel types of structured catalysts, applicable in small scale reactor designs. Here, three different types of such catalysts were investigated for the steam reforming of methanol (SRM). Oxides such as TiO2 and CeO2 and mixtures thereof (Ce1Ti2Ox) were deposited inside a bulk nanoporous gold (npAu) material using wet chemical impregnation procedures. Transmission electron and scanning electron microscopy reveal oxide nanoparticles (1–2 nm in size) abundantly covering the strongly curved surface of the nanoporous gold host (ligaments and pores on the order of 40 nm in size). These catalysts were investigated in a laboratory scaled flow reactor. First conversion of methanol was detected at 200 °C. The measured turn over frequency at 300 °C of the CeOx/npAu catalyst was 0.06 s−1. Parallel investigation by in situ infrared spectroscopy (DRIFTS) reveals that the activation of water and the formation of OHads are the key to the activity/selectivity of the catalysts. While all catalysts generate sufficient OHads to prevent complete dehydrogenation of methanol to CO, only the most active catalysts (e.g., CeOx/npAu) show direct reaction with formic acid and its decomposition to CO2 and H2. The combination of flow reactor studies and in operando DRIFTS, thus, opens the door to further development of this type of catalyst.

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.

Nanostructured materials are governed by their surface chemical properties. This is strikingly reflected by np-Au. This material can be generated by corrosion of bulk Ag–Au alloys. Based on a self-organisation process, a 3 dimensional sponge like gold structure evolves with ligaments in the range of only a few tens of nanometers. Due to its continuous porosity, the material can be penetrated by gases which then adsorb and interact with the surface. In this perspective we will review potential applications of np-Au resulting from this effect, namely heterogeneous gas phase catalysis, surface chemistry driven actuation, and adsorbate controlled stability of the nanostructure. We will summarize the current knowledge about the low temperature oxidation of CO as well as the highly selective oxidation of methanol. Furthermore, we will address the question how surface chemistry can influence the material properties itself. In particular, we will deal with (a) the actuation of np-Au by the reversible oxidation of its surface using ozone and (b) the adsorbate controlled coarsening of ligaments, using annealing experiments under ozone or inert gas atmosphere.