In-operando FTIR study of ligand-linked Pt nanoparticle networks employed as catalysts in hydrogen gas micro sensors.

Microporous networks of Pt nanoparticles (NP) interlinked by aromatic diamines have recently shown prospects of application as hydrogen combustion catalysts in H2 gas microsensors. In particular with respect to long-term sensor performance, they outperformed plain Pt NP as catalysts. In this paper, electron microscopy and Fourier transform infrared (FTIR) spectroscopy data on the stability of p-phenylene diamine (PDA) and of the PDA-linked Pt NP network structure during catalyst activation and long-term sensor operation at elevated temperature (up to 120-180 °C) will be presented. For the first time, all data were collected directly from microsensor catalysts, and FTIR was performed in operando, i.e., during activation and sensor operation. While the data confirm high long-term catalyst activity far superior to that of plain Pt NP over 5 days of testing, they reveal that PDA fully decomposed during long-term sensor operation and that the network of discrete Pt nanoparticles changed to a sponge-like Pt nanostructure already during catalyst activation. These findings are at variance with previous work which assumed that stability of the PDA-linked Pt NP network is prerequisite for catalyst stability and performance.

New Perspectives for Evaluating the Mass Transport in Porous Catalysts and Unfolding Macro- and Microkinetics.

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.

Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry.

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

Catalytic activity of 1D chains of gold oxide on a stepped gold surface from density functional theory.

The rich surface chemistry of gold at the nanoscale has made it an important catalyst for low-temperature applications. Recent studies point to the possible role of self-organized structures formed by chemisorbed O atoms on the surface of gold catalysts for their catalytic activity and/or deactivation. In this study, we investigate the reactivity of a double O chain running along a step on a Au(221) surface with oxygen vacancies as a prototypical model of a 1D surface gold oxide. We compare CO and O2 adsorption on such a chain with the oxygen-free Au(221) surface model. A systematic study of the reactivity of the double chain with O vacancies was done with respect to the regular Au(221) surface using CO as a probe. The CO oxidation was investigated assuming dissociative and associative mechanisms. Remarkably, O2 adsorbs stronger on the double oxygen vacancy than on the regular Au(221) surface, and its dissociation barrier reduces significantly from 1.84 eV to 0.87 eV, whereas the CO adsorption energy is similar on these surfaces. Calculations suggest that CO oxidation should occur more efficiently on the double O vacancy than on the regular Au(221) surface due to stronger adsorption of O2 and a low activation barrier for O2 + CO surface reaction.

Synthesis and Characterization of Ligand-Linked Pt Nanoparticles: Tunable, Three-Dimensional, Porous Networks for Catalytic Hydrogen Sensing.

Porous networks of Pt nanoparticles interlinked by bifunctional organic ligands have shown high potential as catalysts in micro-machined hydrogen gas sensors. By varying the ligand among p-phenylenediamine, benzidine, 4,4''-diamino-p-terphenyl, 1,5-diaminonaphthalene, and trans-1,4-diaminocyclohexane, new variants of such networks were synthesized. Inter-particle distances within the networks, determined via transmission electron microscopy tomography, varied from 0.8 to 1.4 nm in accordance with the nominal length of the respective ligand. While stable structures with intact and coordinatively bonded diamines were formed with all ligands, aromatic diamines showed superior thermal stability. The networks exhibited mesoporous structures depending on ligand and synthesis strategy and performed well as catalysts in hydrogen gas microsensors. They demonstrate the possibility of deliberately tuning micro- and mesoporosity and thereby transport properties and steric demands by choice of the right ligand also for other applications in heterogeneous catalysis.

Assessment of PBE+U and HSE06 methods and determination of optimal parameter U for the structural and energetic properties of rare earth oxides.

Rare earth oxides are attracting increasing interest as a relatively unexplored group of materials with potential applications in heterogeneous catalysis and electrocatalysis; therefore, a credible and universal computational approach is needed for modeling their reactivity. In this work, we systematically assessed the performance of the PBE+U method against the results of the hybrid HSE06 method with respect to the description of structural parameters and energetic properties of the selected hexagonal lanthanide sesquioxides and the cubic fluorite-type cerium dioxide. In addition, we evaluated the performance of PBE+U in describing the electronic structure and adsorption properties of the CeO2(111) and Nd2O3(0001) surfaces. The HSE06 method reproduces rather well the lattice parameters and selected energetic properties with respect to the experimental values. The PBE+U method is able to reproduce the results of HSE06 or the experimental values only if the U parameter is selected from an appropriate range of values. The U value around 3 eV gives the best description of the lattice parameters of most bulk oxides. 2 eV-3 eV is also found to be the optimal range of U for the reaction energies of bulk La2O3, Ce2O3, Nd2O3, Er2O3, and Ho2O3. U = 1 eV gives the best results for Pr2O3, Pm2O3, Eu2O3, Tm2O3, and Lu2O3, whereas Gd2O3 could not be accurately described by the PBE+U method. The U values (∼3 eV) found optimal for most bulk oxides also work well in the calculations of adsorption of small molecules on Nd2O3(0001) and CeO2(111), although larger U values are required to obtain sufficient localization of 4f electrons.

Design and Fabrication Challenges of a Highly Sensitive Thermoelectric-Based Hydrogen Gas Sensor.

This paper presents a highly sensitive thermoelectric sensor for catalytic combustible gas detection. The sensor contains two low-stress (+176 MPa) membranes of a combination of stoichiometric and silicon-rich silicon nitride that makes them chemically and thermally stable. The complete fabrication process with details, especially the challenges and their solutions, is discussed elaborately. In addition, a comprehensive evaluation of design criteria and a comparative analysis of different sensor designs are performed with respect to the homogeneity of the temperature field on the membrane, power consumption, and thermal sensitivity. Evaluating the respective tradeoffs, the best design is selected. The selected sensor has a linear thermal characteristic with a sensitivity of 6.54 mV/K. Additionally, the temperature profile on the membrane is quite homogeneous (20% root mean standard deviation), which is important for the stability of the catalytic layer. Most importantly, the sensor with a ligand (p-Phenylenediamine (PDA))-linked platinum nanoparticles catalyst shows exceptionally high response to hydrogen gas, i.e., 752 mV at 2% concentration.

Ligand-Linked Nanoparticles-Based Hydrogen Gas Sensor with Excellent Homogeneous Temperature Field and a Comparative Stability Evaluation of Different Ligand-Linked Catalysts.

This paper presents a thermoelectric gas microsensor with improved stability where platinum nanoparticles linked by bifunctional ligands are used as a catalyst. The sensor design provides a homogeneous temperature field over the membrane, an important factor for the long-term stability of the catalyst. A comprehensive study of heat transfer from the chip is performed to evaluate the convection heat loss coefficient and to understand its effect on the homogeneity of the temperature field in a real-time situation. The effect of highly heat-conductive thermopiles is also analyzed by comparing the temperature distribution and power consumption with a thermoresistive sensor of the same dimensions and materials. Despite the thermopiles, the thermoelectric sensor gives better temperature homogeneity and consumes 23% less power than the thermoresistive sensor for 90 °C average temperature on the membrane. A comparative stability analysis among ligand-linked nanoparticles with 5 different ligands and unprotected nanoparticles was done through 3 consecutive 24 h tests under 1.5% continuous hydrogen gas flow. The sensors give very stable output, almost no degradation, through 72 h (3 × 24 h) tests for 3 different ligand-linked nanoparticles. The sensor design provides superb stability to the catalyst: Even catalysts of unprotected nanoparticles withstood more than 24 h and the sensor signal degradation is only 20%.

Nanoporous gold functionalized with praseodymia-titania mixed oxides as a stable catalyst for the water-gas shift reaction.

Dealloyed nanoporous metals hold great promise in the field of heterogeneous catalysis; however their tendency to coarsen at elevated temperatures or under catalytic reaction conditions sometimes limit further applications. Here, we report on a highly stable nanoporous gold catalyst (npAu) functionalized with praseodymia-titania mixed oxides as synthesized by a sol-gel method. Specifically, we used aberration-corrected transmission electron microscopy to study the morphology and the interface between the oxide deposits and the npAu substrate at the atomic level. Based on electron energy loss spectroscopy (EELS), it is concluded that Pr-TiOx mixed oxides form a solid solution. Flow reactor tests reveal that the Pr-TiOx functionalized nanoporous gold is not only highly active but also very stable for the water gas shift reaction in a large temperature range (180-400 °C). Our results demonstrate the potential of engineering the compositions of oxides coatings on npAu for advanced functional systems.

Independent control over residual silver content of nanoporous gold by galvanodynamically controlled dealloying.

A new procedure was developed and characterized for the galvanodynamically controlled dealloying (GCD) of AuxAg100-x alloys to obtain nanoporous gold (npAu) mainly as an unsupported catalyst material for partial oxidation of alcohols. Such catalysts require residual Ag content of less than 1 at%. GCD was compared to the preparation of npAu by potentiostatically controlled deallyoing (PCD) and free corrosion (FC). The main advantage of GCD is the ability to obtain npAu with a predetermined residual Ag content including residual Ag contents below 1 at% while retarding the coarsening of the ligaments. For PCD and FC, there is a strong increase of ligament size with decreasing residual Ag content because the longer times required for dealloying unavoidably lead to coarsening of the npAu structure. On the other hand, GCD also prevented too high initial current density that leads to cracking of the samples and prevents formation of mechanically stable monoliths. GCD tolerates different compositions of the starting alloy for AuxAg100-x within the tested composition range (20 at% ≤ xAu ≤ 30 at%). The samples obtained by GCD were tested for methanol and ethanol oxidation and showed favorable characteristics for partial oxidation of methanol to methyl formate and of ethanol to ethyl acetate.

Coatings of active and heat-resistant cobalt-aluminium xerogel catalysts.

The application of catalytically coated metallic foams in catalytic processes has a high potential for exothermic catalytic reactions such as CO2 methanation or Fischer-Tropsch synthesis due to good heat conductivity, improved turbulent flow properties and high catalyst efficiencies. But the preparation of homogenous catalyst coats without pore blocking is challenging with conventional wash coating techniques. Here, we report on a stable and additive free colloidal CoAlOOH suspension (sol) for the preparation of catalytically active Co/Al2O3 xerogel catalysts and coatings. Powders with 18wt% Co3O4 prepared from this additive free synthesis route show a catalytic activity in Fischer-Tropsch synthesis and CO2 methanation which is similar to a catalyst prepared by incipient wetness impregnation (IWI) after activating the material under flowing hydrogen at 430°C. Yet, the xerogel catalyst exhibits a much higher thermal stability as compared to the IWI catalyst, as demonstrated in catalytic tests after different heat agings between 430°C and 580°C. It was also found that the addition of polyethylene glycol (PEG) to the sol influences the catalytic properties of the formed xerogels negatively. Only non-reducible cobalt spinels were formed from a CoAlOOH sol with 20wt% PEG. Metallic foams with pores sizes between 450 and 1200µm were coated with the additive free CoAlOOH sol, which resulted in homogenous xerogel layers. First catalytic tests of the coated metal foams (1200µm) showed good performance in CO2 methanation.

On the suppression of background signals originating from NMR hardware components. Application to zero echo time imaging and relaxation time analysis.

Modern NMR imaging systems used for biomedical research are equipped with B0 gradient systems with strong maximum gradient strength and short switching time enabling (1)H NMR measurements of samples with very short transverse relaxation times. However, background signal originating from non-optimized RF coils may hamper experiments with ultrashort delays between RF excitation and signal reception. We demonstrate that two simple means, outer volume suppression and the use of shaped B0 fields produced by higher-order shim coils, allow a considerable suppression of disturbing background signals. Thus, the quality of NMR images acquired at ultrashort or zero echo time is improved and systematic errors in quantitative data evaluation are avoided. Fields of application comprise MRI with ultrashort echo time or relaxation time analysis, for both biomedical research and characterizing porous media filled with liquids or gases.

Influence of Water on Chemical Vapor Deposition of Ni and Co thin films from ethanol solutions of acetylacetonate precursors.

In chemical vapor deposition experiments with pulsed spray evaporation (PSE-CVD) of liquid solutions of Ni and Co acetylacetonate in ethanol as precursors, the influence of water in the feedstock on the composition and growth kinetics of deposited Ni and Co metal films was systematically studied. Varying the water concentration in the precursor solutions, beneficial as well as detrimental effects of water on the metal film growth, strongly depending on the concentration of water and the ß-diketonate in the precursor, were identified. For 2.5 mM Ni(acac)2 precursor solutions, addition of 0.5 vol% water improves growth of a metallic Ni film and reduces carbon contamination, while addition of 1.0 vol% water and more leads to significant oxidation of deposited Ni. By tuning the concentration of both, Ni(acac)2 and water in the precursor solution, the fraction of Ni metal and Ni oxide in the film or the film morphology can be adjusted. In the case of Co(acac)2, even smallest amounts of water promote complete oxidation of the deposited film. All deposited films were analyzed with respect to chemical composition quasi in situ by XPS, their morphology was evaluated after deposition by SEM.

Controlling the physics and chemistry of binary and ternary praseodymium and cerium oxide systems.

Rare earth praseodymium and cerium oxides have attracted intense research interest in the last few decades, due to their intriguing chemical and physical characteristics. An understanding of the correlation between structure and properties, in particular the surface chemistry, is urgently required for their application in microelectronics, catalysis, optics and other fields. Such an understanding is, however, hampered by the complexity of rare earth oxide materials and experimental methods for their characterisation. Here, we report recent progress in studying high-quality, single crystalline, praseodymium and cerium oxide films as well as ternary alloys grown on Si(111) substrates. Using these well-defined systems and based on a systematic multi-technique surface science approach, the corresponding physical and chemical properties, such as the surface structure, the surface morphology, the bulk-surface interaction and the oxygen storage/release capability, are explored in detail. We show that specifically the crystalline structure and the oxygen stoichiometry of the oxide thin films can be well controlled by the film preparation method. This work leads to a comprehensive understanding of the properties of rare earth oxides and highlights the applications of these versatile materials. Furthermore, methanol adsorption studies are performed on binary and ternary rare earth oxide thin films, demonstrating the feasibility of employing such systems for model catalytic studies. Specifically for ceria systems, we find considerable stability against normal environmental conditions so that they can be considered as a "materials bridge" between surface science models and real catalysts.

Influence of Sn content on the hydrogenation of crotonaldehyde catalysed by colloidally prepared PtSn nanoparticles.

Bimetallic PtSn nanoparticles (NPs) of well-defined size and metal composition were prepared by means of colloidal methods. The mean particle diameter was about 2 nm for all samples irrespective of the Pt/Sn-ratio, which enables a systematic study of the influence of the composition on the catalytic properties while excluding particle size effects. The hydrogenation of crotonaldehyde was investigated as a reaction for which chemoselectivity is known to be a challenging task. Already very low atomic Sn contents (≈10%) were found to lead to a significantly improved activity which may be attributed to an electronic effect of Sn on Pt. For further increasing tin contents the activity decreased gradually. This trend was accompanied by a steady increase in selectivity towards the desired product (crotylalcohol). The results show that the highest crotylalcohol time yields can be obtained by using catalysts with an atomic Sn content of approximately 23%. In contrast, maximum crotylalcohol selectivities are achieved by using catalysts with a high tin content (>50%).

Methanol Adsorption and Reaction on Samaria Thin Films on Pt(111).

We investigated the adsorption and reaction of methanol on continuous and discontinuous films of samarium oxide (SmOx) grown on Pt(111) in ultrahigh vacuum. The methanol decomposition was studied by temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS), while structural changes of the oxide surface were monitored by low-energy electron diffraction (LEED). Methanol dehydrogenates to adsorbed methoxy species on both the continuous and discontinuous SmOx films, eventually leading to the desorption of CO and H2 which desorbs at temperatures in the range 400-600 K. Small quantities of CO2 are also detected mainly on as-prepared Sm2O3 thin films, but the production of CO2 is limited during repeated TPD runs. The discontinuous film exhibits the highest reactivity compared to the continuous film and the Pt(111) substrate. The reactivity of methanol on reduced and reoxidized films was also investigated, revealing how SmOx structures influence the chemical behavior. Over repeated TPD experiments, a SmOx structural/chemical equilibrium condition is found which can be approached either from oxidized or reduced films. We also observed hydrogen absence in TPD which indicates that hydrogen is stored either in SmOx films or as OH groups on the SmOx surfaces.

Design of a compact ultrahigh vacuum-compatible setup for the analysis of chemical vapor deposition processes.

Optimizing thin film deposition techniques requires contamination-free transfer from the reactor into an ultrahigh vacuum (UHV) chamber for surface science analysis. A very compact, multifunctional Chemical Vapor Deposition (CVD) reactor for direct attachment to any typical UHV system for thin film analysis was designed and built. Besides compactness, fast, easy, and at the same time ultimately clean sample transfer between reactor and UHV was a major goal. It was achieved by a combination of sample manipulation parts, sample heater, and a shutter mechanism designed to fit all into a NW38 Conflat six-ways cross. The present reactor design is versatile to be employed for all commonly employed variants of CVD, including Atomic Layer Deposition. A demonstration of the functionality of the system is provided. First results of the setup (attached to an Omicron Multiprobe x-ray photoelectron spectroscopy system) on the temperature dependence of Pulsed Spray Evaporation-CVD of Ni films from Ni acetylacetonate as the precursor demonstrate the reactor performance and illustrate the importance of clean sample transfer without breaking vacuum in order to obtain unambiguous results on the quality of CVD-grown thin Ni films. The widely applicable design holds promise for future systematic studies of the fundamental processes during chemical vapor deposition or atomic layer deposition.

Ligand-stabilized Pt nanoparticles (NPs) as novel materials for catalytic gas sensing: influence of the ligand on important catalytic properties.

Different mono- and bifunctional amine ligands have been used to stabilize Pt NPs for catalytic H2 gas sensing. Depending on the chemical structure and properties of the ligand, the catalysts show different overall sensor performances, activation periods, and long-term stabilities. These sensor characteristics are put into relation with chemical processes like cleaning of the surface, degradation processes of the ligands and nanoparticle (NP) sintering. It has been found that during activation free adsorption sites are formed primarily due to desorption of synthetic residues. Furthermore, partial desorption of the ligands followed by their degradation may occur. For monoamines the latter process results in destabilization of the NPs followed by catalyst deactivation through particle sintering. The use of bifunctional ligands that link individual NPs shows significantly enhanced stabilities which can be related to the reduction of the ligand desorption rates and degradation. Besides the functionality of the ligands it was observed that the chemical nature of their hydrocarbon skeleton affects the catalyst stability: aromatic substructures remain intact upon H2 oxidation, while alkyl fragments undergo oxidation and decomposition. The advantages of bifunctionality and an aromatic hydrocarbon skeleton can be combined by the use of para-phenylenediamine (PDA) as a linking ligand. Networks formed by this ligand were indeed found to be stable under the applied catalytic conditions for more than 24 h.

Stabilizing catalytically active nanoparticles by ligand linking: toward three-dimensional networks with high catalytic surface area.

A general approach for the linking of Pt nanoparticles (NPs) with bifunctional amine ligands (organic molecules with two amine groups) is presented that allows for the preparation of NP catalysts without inorganic supports and high densities of the catalytically active metal. Advantage was taken of the use of "unprotected" NPs, which enables us to prepare different ligand-functionalized NPs from the same particle batch and thus to relate changes of the resulting material properties exclusively to the influence of the ligand. Three bifunctional ligands with similar functional groups (amines) but different hydrocarbon skeletons were used and compared to monofunctional ligands of similar molecular structures (alkyl and aryl amines) showing significantly different material properties. Monofunctional molecules with minor steric demand cover almost completely the NP surface and lead to two-dimensional assembling of the NPs. In contrast, the use of bifunctional amine ligands leads to the formation of porous, three-dimensional NP networks (ligand-linked NPs) with a high density of ligand free surface atoms, thus enabling for the application as catalytic materials. The stabilizing effect of bifunctional ligands serves as an alternative to the use of inorganic support materials and enables for catalytic applications of ligand-linked NP networks.

Sol-gel preparation of alumina stabilized rare earth areo- and xerogels and their use as oxidation catalysts.

A new sol-gel synthesis route for rare earth (Ce and Pr) alumina hybrid aero- and xerogels is presented which is based on the so-called epoxide addition method. The resulting materials are characterized by TEM, XRD and nitrogen adsorption. The results reveal a different crystallization behavior for the praseodymia/alumina and the ceria/alumina gel. Whereas the first remains amorphous until 875°C, small ceria domains form already after preparation in the second case which grow with increasing calcination temperature. The use of the calcined gels as CO oxidation catalysts was studied in a quartz tube (lab) reactor and in a (slit) microreactor and compared to reference catalysts consisting of the pure rare earth oxides. The Ce/Al hybrid gels exhibit a good catalytic activity and a thermal stability against sintering which was superior to the investigated reference catalyst. In contrast, the Pr/Al hybrid gels show lower CO oxidation activity which, due to the formation of PrAlO3, decreased with increasing calcination temperature.