Inverse opal structures are of interest for various applications, as they exhibit high surface areas in conjunction with unique structure-specific properties such as the possibility to create photonic band gaps, e.g., for photocatalytic applications. An established synthetic pathway to prepare these nanostructures is to infiltrate the voids of a template comprised of close-packed spheres with a metal oxide and to remove the template subsequently by pyrolysis. To this end, polymer spheres are typically used which are produced by a water-based emulsion polymerization process. In this work, we present an improved and extended approach of that kind in case of PMMA spheresfeaturing narrow size distributions and mean diameters that can be varied over a large range between 170 up to 800 nm by properly adjusting the synthesis temperature and the ionic-strength of the water phase. By using reflux conditions, advanced experimental techniques requiring protective gas atmospheres are dispensable and comparatively short synthesis times can be realized. Time-resolved experiments reveal a two-step growth process occurring at temperatures below ∼400 K. It consists of a first phase, during which initial particles are formed, followed by a time-delayed second phase, where their diameter increases by roughly a factor of 2, most likely due to coalescence processes. At higher temperatures, both processes increasingly overlap so that only a single growth phase is observed.

Nanoporous gold (npAu) has garnered significant attention as a highly active and selective oxidation catalyst, particularly at low temperatures. While most studies have focused on npAu derived from AuAg alloys, the incorporation of other less noble metals (LNMs), such as Cu, was shown to be a viable opportunity for tuning and enhancing its catalytic properties. The interplay between multiple LNMs present at the same time remains underexplored though. While trimetallic systems like npAu(CuAg) offer the chance for broadening the catalytic scope of npAu even further, a key challenge in utilizing them lies in understanding whether synergistic interactions or a combination of effects exerted by the individual LNMs determine their catalytic performance. To address this, we developed a reliable and reproducible method for synthesizing npAu(CuAg) and compared the trimetallic catalysts to their bimetallic counterparts npAu(Ag) and npAu(Cu) with respect to their catalytic activity for aerobic CO oxidation. Using controlled dealloying, we synthesized npAu(CuAg) with ∼20 nm ligaments, exhibiting an Ag‐rich surface, and a Cu‐enriched bulk. These trimetallic catalysts showed high conversion over a broad temperature regime ranging from room temerature to 250 °C. Electron microscopy and X‐ray photoelectron spectroscopy revealed that during activation under reaction conditions Cu segregates to the surface at elevated temperatures so that finally Ag as well as Cu sites are available enabling the activation of oxygen – on the former at low temperatures and on the latter in the high temperature region >120 °C. In this way, an overall temperature window for catalytic activity is achieved which encompasses the range of npAu(Ag) as well as that of npAu(Cu). At the same time, the thermal stability of the nanoporous network is improved, demonstrating in essence that the ternary alloy outperforms the binary systems by unifying catalytically relevant features of both LNMs.

Nanoporous gold (npAu) attracted increasing attention over the last 20 years as a highly active and selective oxidation catalyst in particular at low temperatures. Previous research mainly focused on npAu that was fabricated by corrosive dealloying of AuAg parent alloys. Yet, the use of other binary alloys, such as AuCu, promises interesting variations of the catalytic properties, when considering that residual amounts of the less noble metal were shown to be co-catalytically involved. Aiming at providing a platform for systematic studies in this direction for Cu, we not only dealt with strategies for a reliable and reproducible preparation of npAu(Cu) catalysts from AuCu, but also with their potential for CO oxidation in comparison to npAu(Ag). We were able to develop an approach based on thermally quenched Au0.3Cu0.7 alloys, providing distinct synthetic advantages as a starting material for the catalyst fabrication versus the thermodynamically more stable AuCu3 intermetallic compound. Using PCD (potentiostatically controlled dealloying), well-defined pore structures with ligament diameters of similar to 40 nm and variable residual Cu concentrations in the range between similar to 0.6 at % and similar to 1.2 at % could be straightforwardly obtained. After activating such catalysts at 150 degrees C, they reproducibly showed catalytic activity for aerobic CO oxidation in a broad temperature window between 40 degrees C and 250 degrees C. As opposed to npAu(Ag), the activity increased with decreasing residual Cu content, outperforming the former at temperatures above similar to 60 degrees C not only with respect to CO2 formation rates but also with respect to thermal stability. Based on X-ray photoelectron spectroscopic and transmission electron microscopic results, it was possible to conclude that Cu segregates to the surface and, with rising Cu bulk content, increasingly occurs in form of Cu2+ species at the surface. While the latter are expected to be catalytically inactive, Cu and Cu+ species are likely candidates for the activation of oxygen being not possible on pure Au. Nanoporous gold has emerged as a highly active oxidation catalyst, especially at low temperatures. While in the past it was predominantly synthesized from AuAg alloys, this study explores AuCu as starting material. Since residues of the less noble metal are co-catalytically involved, deviating properties are expected and were indeed observed. image

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 degrees 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.

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.

The thermal stability of microporous networks of Pt nanoparticles interlinked by p-Phenylenediamine (PDA) was investigated by temperature-programmed desorption spectroscopy (TPD) and transmission electron microscopy (TEM). The detection of aniline, benzene, and NH3 as major desorbing fragments in TPD revealed cleavage of the C-N bonds in PDA as a major route of thermally induced decomposition, probably via hydrogenolysis assisted by dehydrogenation reactions of PDA running alongside. Varying the Pt:PDA ratio in the networks demonstrated that the Pt nanoparticles catalytically promote the decomposition of their PDA links. A quantitative analysis of the TPD spectra indicated that the thermal stability of PDA strongly correlates with the number of bonds (0, 1 or 2) formed by the amino groups of an individual PDA molecule with Pt. Only the most stable PDA species appears to be of relevance for the structural stability of the nanoparticle network, as heating experiments with in situ TEM showed. The results have strong implications for the application of PDA-linked Pt nanoparticles as heterogeneous catalysts in hydrogen gas microsensors where the choice of operating temperature has to balance maximization of signal-to-noise ratio and maintenance of structural stability. An anealing step up to 450-500 K after synthesis of the catalyst is suggested in order to optimize its catalytic activity by removing PDA species that are not essential for structural stabilty.

Since the first studies reporting on its surprising catalytic properties, nanoporous gold (npAu) has emerged as a novel and ever since intensively investigated type of Au based catalyst. To judge its genuine catalytic potential and to be able to optimize its use in applications, it is mandatory, however, to quantify the influence of mass transport in the porous structure on the observed catalytic rates, i.e., to study the interplay between diffusion and reaction. To this end, we used pulsed field gradient (PFG) NMR for the first time to directly determine the diffusivities of reaction gases in a nanoporous metal - in this case for CO and CO2 as species involved in low temperature CO oxidation efficiently catalyzed by npAu. By comparing the diffusion coefficients within the 20 nm pores of the material with the values in the bulk gas phase, the tortuosity of npAu's pore system was assessable as the central geometrical parameter describing the extent to which diffusive transport in the pore system is slowed down. This knowledge allowed us in the following to disentangle the contributions of mass transport and the kinetics of the sur-face reaction (microkinetics). In particular, we were able to determine the rate constant and turnover fre-quency for low-temperature CO oxidation without previous ambiguities arising from potential transport limitations and to compare the results with other reported values. Based on the results, it was further-more possible to predict optimized dimensions of the catalyst, resulting in minimized or even suppressed diffusion limitations. These predictions could be successfully verified, using np-Au platelets with lateral dimensions in the range of a few hundred microns. In this way, the catalytic conversion could be ramped up by 50 % and an activity level advanced which reflected the microkinetic potential of np-Au.(c) 2022 Published by Elsevier Inc.

There is considerable motivation in the catalysis community and chemical industry to envision a future where rational catalyst design and targeted chemical process optimization become standard. Achieving this goal for heterogeneous catalysis requires a cultural shift centered around effective research data management. The core elements of modern catalysis research are synthesis, characterization, and testing, while all can be elevated by effective collection, correlation, interoperation, and exploitation of data between disciplines and stakeholders. Here, first steps are made towards a holistic picture of an industrial Ni/Al 2 O 3 reference catalyst for CO 2 methanation. A range of conventional and advanced characterization tools are applied to probe metal particle size and pore characteristics of the support, selected as crucial parameters for catalyst performance. Challenges are shown with respect to current reporting of characterization data and metadata, which ultimately influences the development and reliability of digital twins in catalysis research. Furthermore, the cooperation and combined expertise of diverse research groups from different fields is recognized as essential to deliver meaningful progress towards the digital future of catalysis research.

Heterogeneous catalysts based on gold have attracted considerable attention over the last 30 years. In spite of its chemical inertness, the research on gold catalysis has proven that in oxidation reactions, for instance, high activities and, in particular, selectivities can be achieved. While mostly Au nanoparticles supported on suitable oxides have been focused on in the literature, more recently, nanoporous gold (npAu) was found to be an alternative that shows similarly good catalytic performance, even though the characteristic structural length (diameter of ligaments vs particle diameter) is an order of magnitude larger. To date, however, no recipe has been reported to reliably activate npAu for total oxidations, such as CO oxidation. Here, we present such an activation procedure that allows obtaining high conversion levels on the time scale of an hour, independently of the prehistory of the samples. It consists of one or several steps where the catalyst is subjected to a thermal treatment in the reaction mixture (CO + O-2) at 300 degrees C for 1 min only. Previously, it was assumed that such a high-temperature treatment would result in coarsening of the nanoporous structure and thus in an intolerable loss of surface area. According to our results, however, short annealing steps hardly alter the ligament size but effectively remove residuals from the preparation and/or modify the surface chemistry so that a catalytically active state is obtained quickly. Reproducibly, rates can be achieved, which are in good agreement with values previously reported in the literature and are stable for at least several days on stream. Our results open the way for a practical use of npAu as well as for further studies on the mechanistic origin of its catalytic activity.

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

The impact of different co-catalytically acting promoters (Pt, ZrOx and SmOx) during COx methanation was investigated on alumina supported Co-based model catalysts. To obtain samples with identical structure and morphology independent of the presence and the type of promoter added, double flame spray pyrolysis was employed for the synthesis, which, in contrast to classical catalyst preparation techniques, allows controlling and separating the particle formation processes of the different catalyst components. In this way, differently promoted and unpromoted catalysts with identical size distributions and dispersions could be synthesized to study co-catalytic effects in isolation. For CO2 methanation, all promoters led to improved methanation yields and long-term activities within the whole temperature range as compared to the unpromoted catalyst. Among them, zirconia and platinum performed best. In case of CO methanation, a beneficial influence of the studied promoters could also be verified - with Pt showing the best results. Yet, all catalysts deactivated rapidly above 310 degrees C, limiting their usability to lower temperatures.

The influence of the support basicity, according to the Lewis and Brønsted definition, was investigated for the CO2 methanation over isostructural Ru catalysts.

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.

To probe particle-support interactions and their mechanistic role for catalytic CO oxidation on nanoporous gold (npAu) coated with ceria nanoparticles, we carried out ab initio molecular dynamics (AIMD) simulations and standard density functional theory (static DFT) computations. To this end, we studied ceria clusters (Ce10O20/19) supported on a Au(321) surface exhibiting a high density of steps and kinks. Our theoretical model represents the structurally inverse situation compared to more commonly studied ceria-supported Au nanoparticle systems. In agreement with previous results for Au(111), we find that reduced (Ce10O20/19) as well as stoichiometric (Ce10O20) ceria nanoparticles transfer electrons to the Au(321) support. This charge transfer (particularly strong in the case of Ce10O19) reflecting a strong chemical interaction between ceria and Au is probably responsible for the stabilization of np-Au against thermal coarsening experimentally observed upon deposition of oxide nanoparticles. The adsorption energies of the ceria cluster on Au(321) are more negative than on the Au(111) surface by around similar to 0.5 eV. AIMD simulations were employed to study the mechanism of catalytic CO oxidation with O-2 for the ceria/Au(321) system. We found that a CO molecule adsorbed near the ceria/gold perimeter interface can extract a Au atom from the surface in the form of a mobile linear Au-CO complex, which results in a very low activation energy when this species reacts with lattice O to CO2. The released bare Au adatom subsequently attaches to a step edge of the gold surface, leading to a dynamic restructuring of the Au support. Next, an activated O-2 molecule adsorbed at a perimeter site between ceria and Au reacts with a second CO molecule to CO2 and an adsorbed O atom, which eventually fills the vacancy site created in the first half of the cycle. As compared to ceria particles supported on Au(111), the reactivity is enhanced as a new low-energy mechanism is enabled, revealing the positive impact of the stepped structure of Au(321).

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Oxidation reactions catalyzed by Au nanoparticles supported on reducible oxides have been widely studied both experimentally and theoretically, whereas inverse catalysts , in which oxide nanoparticles are supported on metal surfaces, received considerably less attention. In both systems catalytic activity at metal – oxide interfaces can arise not only from each material contributing its functionality, but also from their interactions creating properties beyond the sum of individual components. Inverse catalysts may retain the synergy between the metal and oxide functionalities, while offering further specific advantages, e.g. a possibility to have better control over interfacial sites or to yield improved stability, activity, and selectivity. Our work provides the mechanism of O atom/vacancy diffusion-assisted Mars-van-Krevelen CO oxidation on gold-supported ceria nanoparticle through state-of-the-art ab initio molecular dynamic simulation studies.

A catalytic hydrogen gas sensor of superior sensitivity, selectivity, resolution and dynamic response was developed with catalysts composed of ligand-stabilized N nanoparticles. We have characterized different catalysts utilizing the bi-functional ligands trans-1,4-diaminocyclohexane (DACH), 1,5-diaminonaphthalene (DAN), 4,4 ''-diamino-p-terphenyl (DATER), benzidine (BEN) and p-phenylene diamine (PDA) for hydrogen gas sensing. A comprehensive evaluation by comparison, with respect to both, structural aspects (TEM and SEM) and gas sensing performance of the 5 types of ligand-linked N nanoparticles and non-stabilized N nanoparticles was conducted to select the optimized catalysts. From this investigation, DATER-linked N nanoparticles appear immensely promising: the sensor is selective to hydrogen and shows extremely high sensitivity, around 400 mV/1% vol., but exhibits no cross-sensitivity to methane and ethane. Likewise, sensors with DATER- and DAN-linked N nanoparticles can detect down to 0.001 % (10 ppm) alongside 650 ms average response time (t(90)).

The effects of low-valence dopants on the catalytic properties of samarium oxide xerogel catalysts were investigated in the oxidative coupling of methane (OCM). More specifically, very low concentrations (0.1 and 1.0 % by mol) of transition metal (Ag, Ni, and Cu) and traditional alkali metal (Li and K) dopants were investigated. At these low loadings, it was shown that transition metal dopants have potential to improve the activity and selectivity over an undoped Sm2O3 xerogel, but these dopants can only outperform alkali metal dopants under certain conditions. Even at a concentration of 0.1 mol %, the dopants significantly increased the number of basic sites compared with the pure Sm2O3 xerogel. However, no trend is evident between the number or strength of the basic sites and the activity or selectivity in the methane coupling reaction. The XRD data reveal a lattice expansion upon addition of the low valence dopants, which is consistent with substitutional doping and the formation of oxygen vacancies due to charge compensation. In most cases the majority of the dopant stayed in the lattice during reaction. The dopants were also shown to influence the Sm2O3 structure, and the dopants that were more effective in suppressing the transformation from cubic to monoclinic Sm2O3 in general resulted in the more active and selective catalysts. While the Ag- and Ni-dopants could outperform the alkali metal doped catalysts in narrow temperature ranges, the best performing catalysts were still the K-doped Sm2O3 catalysts, as the 1.0 % K catalyst exhibited the highest activity at the lowest temperature (500 ?C) and the 0.1 % K-doped catalyst was the most stable during extended operation. These results indicate that transition metal dopants, at low concentrations, can positively affect the activity and selectivity of a methane coupling catalyst, such as Sm2O3, and suggests that there may be benefits to other OCM catalyst systems from traditionally non-selective dopants, as long as the concentrations are kept very low and stability issues are addressed.