We provide experimental evidence that is inconsistent with often proposed Langmuir-Hinshelwood (LH) mechanistic hypotheses for water-promoted CO oxidation over Au-Fe2O3. Passing CO and H2O, but no O-2, over Au-gamma-Fe2O3 at 25 degrees C, we observe significant CO2 production, inconsistent with LH mechanistic hypotheses. Experiments with (H2O)-O-18 further show that previous LH mechanistic proposals cannot account for water-promoted CO oxidation over Au-gamma-Fe2O3. Guided by density functional theory, we instead postulate a water-promoted Mars-van Krevelen (w-MvK) reaction. Our proposed w-MvK mechanism is consistent both with observed CO2 production in the absence of O-2 and with CO oxidation in the presence of (H2O)-O-18 and O-16(2). In contrast, for Au-TiO2, our data is consistent with previous LH mechanistic hypotheses.

Anisotropic cellulose nanofiber (CNF) foams represent the state-of-the-art in renewable insulation. These foams consist of large (diameter >10 mu m) uniaxially aligned macropores with mesoporous pore-walls and aligned CNF. The foams show anisotropic thermal conduction, where heat transports more efficiently in the axial direction (along the aligned CNF and macropores) than in the radial direction (perpendicular to the aligned CNF and macropores). Here we explore the impact on axial and radial thermal conductivity upon depositing a thin film of reduced graphene oxide (rGO) on the macropore walls in anisotropic CNF foams. To obtain rGO films on the foam walls we developed liquid-phase self-assembly to deposit rGO in a layer-by-layer fashion. Using electron and ion microscopy, we thoroughly characterized the resulting rGO-CNF foams and confirmed the successful deposition of rGO. These hierarchical rGO-CNF foams show lower radial thermal conductivity (lambda(r)) across a wide range of relative humidity compared to CNF control foams. Our work therefore demonstrates a potential method for improved thermal insulation in anisotropic CNF foams and introduces versatile self-assembly for postmodification of such foams.

Oxide-derived metals are produced by reducing an oxide precursor. These materials, including gold, have shown improved catalytic performance over many native metals. The origin of this improvement for gold is not yet understood. In this study, operando non-resonant sum frequency generation (SFG) and ex situ high-pressure X-ray photoelectron spectroscopy (HP-XPS) have been employed to investigate electrochemically-formed oxide-derived gold (OD-Au) from polycrystalline gold surfaces. A range of different oxidizing conditions were used to form OD-Au in acidic aqueous medium (H₃PO₄, pH = 1). Our electrochemical data after OD-Au is generated suggest that the surface is metallic gold, however SFG signal variations indicate the presence of subsurface gold oxide remnants between the metallic gold surface layer and bulk gold. The HP-XPS results suggest that this subsurface gold oxide could be in the form of Au₂O₃ or Au(OH)₃. Furthermore, the SFG measurements show that with reducing electrochemical treatments the original gold metallic state can be restored, meaning the subsurface gold oxide is released. This work demonstrates that remnants of gold oxide persist beneath the topmost gold layer when the OD-Au is created, potentially facilitating the understanding of the improved catalytic properties of OD-Au.

Monodispersed metal and semiconductor nanocrystals have attracted great attention in fundamental and applied research due to their tunable size, morphology, and well-defined chemical composition. Utilizing these nanocrystals in a controllable way is highly desirable especially when using them as building blocks for the preparation of nanostructured materials. Their deposition onto oxide materials provide them with wide applicability in many areas, including catalysis. However, so far deposition methods are limited and do not provide control to achieve high particle loadings. This study demonstrates a general approach for the deposition of hydrophobic ligand-stabilized nanocrystals on hydrophilic oxide supports without ligand-exchange. Surface functionalization of the supports with primary amine groups either using an organosilane ((3-aminopropyl)trimethoxysilane) or bonding with aminoalcohols (3-amino-1,2-propanediol) were found to significantly improve the interaction between nanocrystals and supports achieving high loadings (>10 wt. %). The bonding method with aminoalcohols guarantees the opportunity to remove the binding molecules thus allowing clean metal/oxide materials to be obtained, which is of great importance in the preparation of supported nanocrystals for heterogeneous catalysis.

Colloidal transition metal nanocrystals have gained increased interest in several fields because of the fascinating structural, optical, and catalytic properties obtained at the nanoscale. A thorough and systematic understanding of their characteristics could prove beneficial for different applications. Among these characteristics, the optical properties of colloidal nanoparticles are especially of interest due to the intriguing features that emerge at sub-wavelength scales, such as surface plasmon resonance. However, most research in this domain has focused on metals with absorption maxima in the visible light range, while the optical characteristics of other species remain relatively less studied. Therefore, in this work, we report on the optical properties of uniform platinum, palladium, and nickel nanocrystals, which are widely used in fields from sensors to catalysis, yet, with regard to their optical properties, remain largely uninvestigated. In particular, we measure and analyze their extinction coefficients, showing a size-dependence that can be rationalized with simple equations that relate light absorption to the diameter of the particles. The comprehension of this relationship enables a simple, time-efficient, and accurate method to measure the concentrations of metallic nanoparticles in solution with UV-vis spectroscopy, a useful property which facilitates the preparation of materials based on these colloidal nanoparticles for further research.

Supported metal nanoparticles are essential components of high-performing catalysts, and their structures are intensely researched. In comparison, nanoparticle spatial distribution in powder catalysts is conventionally not quantified, and the influence of this collective property on catalyst performance remains poorly investigated. Here, we demonstrate a general colloidal self-assembly method to control uniformity of nanoparticle spatial distribution on common industrial powder supports. We quantify distributions on the nanoscale using image statistics and show that the type of nanospatial distribution determines not only the stability, but also the activity of heterogeneous catalysts. Widely investigated systems (Au-TiO2 for CO oxidation thermocatalysis and Pd-TiO2 for H-2 evolution photocatalysis) were used to showcase the universal importance of nanoparticle spatial organization. Spatially and temporally resolved microkinetic modeling revealed that nonuniformly distributed Au nanoparticles suffer from local depletion of surface oxygen, and therefore lower CO oxidation activity, as compared to uniformly distributed nanoparticles. Nanoparticle spatial distribution also determines the stability of Pd-TiO2 photocatalysts, because nonuniformly distributed nanoparticles sinter while uniformly distributed nanoparticles do not. This work introduces new tools to evaluate and understand catalyst collective (ensemble) properties in powder catalysts, which thereby pave the way to more active and stable heterogeneous catalysts.

To deepen our knowledge of the film formation and the structure–property relations of poly(furfuryl glycidyl ether)-block-poly(ethylene glycol) (PFGE_p-b-PEG_q) macromonomers at the air–water interface, we synthesized PFGE_p-b-PEG_q in six different block lengths. The molar mass of the PFGE_p-b-PEG_q macromonomers varied from ∼2000 g mol⁻¹ to ∼7000 g mol⁻¹ and included a wide range of hydrophilic–lipophilic balance (HLB) values between 3.6 and 13.9. Surface pressure–area (π–A) isotherms of these amphiphilic macromonomers revealed that the block lengths and the molar mass influence the isotherm shape and onset. Smaller, more hydrophobic macromonomers (HLB < 8) showed a steeper surface pressure increase in the liquid condensed phase compared to larger, more hydrophilic macromonomers with HLB > 8. The molecular area for isotherm onsets increased almost linearly with growing molar mass of the macromonomers. Static and dynamic film stability measurements demonstrated limited stability of all macromonomer monolayers at the air–water interface. The more hydrophilic macromonomers PFGE₈-b-PEG₇₉, PFGE₁₈-b-PEG₆₆ and PFGE₁₃-b-PEG₁₁₁ (HLB > 8) showed higher film stability compared to the more hydrophobic macromonomers (HLB < 8). Hysteresis experiments displayed an almost linear increase of the film degradation with rising HLB values of the macromonomers. Due to partial film recovery of our macromonomers, we propose an interplay between a reversible folding and an irreversible submersion mechanism for the macromonomer monolayers at the air–water interface. The molecular structure and the film forming ability of the macromonomers at the air–water interface indicate that they are promising surface functionalization reagents for materials formed from aqueous solutions, such as hydrogels. In this regard, PFGE₁₀-b-PEG₉ is the most promising hydrogel surface functionalization reagent, because it can introduce the highest number of functional groups per surface area.

Catalytic materials are an essential component of the chemical industry. They find applications in everything from fine chemical manufacturing to greenhouse gas mitigation. They are indispensable for developing a sustainable future. Their development has been continuous, from early trial and error efforts to the first fundamental insights gained through surface science, to modern in-situ characterization and computational predictions. The accumulation of knowledge on the working principles of catalytic surfaces allowed designing and producing better systems with improved performance. Even though tremendous progress has been made thanks to surface science techniques, these studies are usually performed under ultra-high vacuum and are therefore limited in their applicability to more relevant industrial conditions. The control over size, shape and composition in colloidal nanocrystals makes them formidable precursors for model heterogeneous catalysts. These model systems enable linking the insights from surface science studies via in-situ and operando studies to realistic catalytic reaction conditions. In this review, colloidal nanocrystals are presented as powerful building blocks for catalytic materials in the quest for fundamental understanding. A review of the principal methods to produce colloidal nanocrystals with a high level of control is reported, complemented by procedures for how to prepare active catalysts from these particles. Examples and guidelines for the catalytic applications of these materials revolve around the three guiding objectives in catalysis science: activity, selectivity and stability. This work will be limited to examples of this colloidal approach in the areas of thermal, electro- and photocatalysis. The exposed approaches can be used and extended to many other areas of catalysis science, thus providing a new avenue to explore fundamentals and applications of catalytic materials.

Depositing a morphologically uniform monolayer film of graphene oxide (GO) single-layer sheets is an important step in the processing of many composites and devices. Conventional Langmuir-Blodgett (LB) deposition is often considered to give the highest degree of morphology control, but film microstructures still vary widely between GO samples. The main challenge is in the sensitive self-assembly of GO samples with different sheet sizes and degrees of oxidation. To overcome this drawback, here, we identify a general method that relies on robust assembly between GO and a cationic surfactant (cationic surfactant-assisted LB). We systematically compared conventional LB and cationic surfactant-assisted LB for three common GO samples of widely different sheet sizes and degrees of oxidation. Although conventional LB may occasionally provide satisfactory film morphology, cationic surfactant-assisted LB is general and allows deposition of films with tunable and uniform morphologies-ranging from close-packed to overlapping single layers-from all three types of GO samples investigated. Because cationic surfactant-assisted LB is robust and general, we expect this method to broaden and facilitate the use of GO in many applications where precise control over film morphology is crucial.

The structure and properties of segmented block copolymer films of aromatic polyimide (PI) and poly(ethylene glycol) (PEG) doped with an ionic liquid are studied for potential polymer electrolyte membrane applications for fuel cells. Poly(amic acid) precursors of PI-PEG copolymers of 4,4-(hexafluoroisopropylidene) diphthalic anhydride, 4,4-(1,3-phenylenedioxy) dianiline, and bis(3-aminopropyl) terminated PEG (M-n approximate to 1500) are synthesized and then thermally imidized in membrane films, followed by swelling in ethylammonium nitrate (EAN) ionic liquid. The small-angle X-ray scattering results from the EAN-doped PI-PEG copolymer films show disordered bicontinuous phase-separated nanostructures described by Teubner-Strey theory, with the interface fractal dimension determined from the Porod equation. Thermal annealing of the EAN-doped membranes at 100-140 degrees C results in increased correlation lengths and smoother interfaces of the bicontinuous nanostructures. Such improved nanostructures lead to the increased ionic conductivity by two to five times with the maximum conductivity of 210 mS cm(-1) at 60 degrees C and 70% RH, much greater (nearly fivefold) than that of Nafion films, while maintaining the mechanical stability possibly up to 140 degrees C. Moreover, the investigation of the disordered bicontinuous phase-separated nanostructure of EAN-doped PI-PEG copolymer membranes is highly relevant to understanding the nanostructures of hydrated Nafion membranes and segmented block copolymers in general.