The dehydration of alcohols is an important class of reactions for the development of fossil-free fuel and chemical industries. Acid catalysts are well known to enhance the reactivity of alcohols following two main pathways of either dehydration to olefins or dehydrogenation to ketones/aldehydes. TiO2 surfaces have been well documented for primary and secondary alcohol dehydration with selectivity ranging from 1-100% towards dehydration products based on process conditions and catalyst structure. In this work we document the effects of various sulfur treatments of TiO2 surfaces which induce higher activity and, more importantly, higher selectivity for alcohol dehydration than untreated surfaces. The increase in activity and >99% dehydration selectivity is coupled with demonstrated stability for several hours on stream at high conversion. Using temperature programmed reaction studies, XPS and FT-IR spectroscopy, we identify Lewis acidic sites correlated with sulfate species on TiO2 surfaces as active sites for the reaction.

Improvement and new technologies for effective poly (lactic acid) (PLA) foaming are still in developing. This study examines the ability of biochar (BC) to assist in PLA foaming through a supercritical CO2 batch process. To determine suitable foaming parameters, process optimization was conducted on neat PLA films. BC particles derived from three different sources – sludge (SLG), pistachio shells (PST) and green waste (GW) were ground, and ball milled to a microscale and then blended with PLA. The PLA/BC blends were extruded into films and then batch foamed. The effect of BC was found to be concentration and source dependent. At 0.25 wt%, BC particles were found to be effective nucleating agents showing an increase up to four orders of magnitude in cell density and 10-fold reduction in the mean pore size compared to the neat PLA foams. An increase in BC content to 0.5 and 1 wt% induced particle aggregation, which resulted in non-homogenous foam densities. BC particles derived from SLG were the most effective nucleating agents due to their narrow size distribution, improved dispersibility and mesoporous surface. This work shows that BC can act as an effective bio-derived nucleating agent for PLA.

The tandem nature of selective alcohol oxidation with in situ-generated oxidants over Au/metal oxide catalysts is demonstrated. Colloidal synthesis is used to show that while Au nanoparticles require an oxide interface to generate the selective oxidant from a mixture of hydrogen and oxygen, dilute Pd/Au alloy nanoparticles are able to generate oxidants even when immobilized on inert carbon supports because of the hydrogen dissociation ability of Pd single atoms in Au. These oxidants may travel through the gas phase to be utilized by a variety of metal oxides to selectively oxidize primary and secondary alcohols to their respective carbonyls.

The structure of enzyme catalytic centers often guides the synthesis of heterogeneous catalysts. These structures are dynamic and provide changes in the chemical environment and redox state of transition metal centers that result in drastic changes in the catalytic behavior. Inspired by these catalytic traits, porous polymers represent a versatile platform for preparing catalysts by anchoring metals to organic functional groups. Following this inspiration, we studied the structural evolution of a series of catalysts containing atomically dispersed Cu ions grafted onto imine-linked porous organic frameworks (IPOFs) during synthesis and reaction. Cu species were found to be bound to the imine or diiminopyridine units of IPOFs and to be active for oxidation of carbon monoxide. Combining X-ray photoelectron spectroscopy (XPS) and in situ X-ray absorption spectroscopy (S) showed that the metal centers could gradually activate oxygen and change their oxidation state. Interestingly, depending on the type of functional group responsible for binding Cu, the active sites either reach a balanced redox turnover between Cu(I) and Cu(II) states or get poisoned by CO and locked at a Cu(I) state during reaction. Furthermore, samples with agglomerated Cu species were found to be redispersed upon contact with the reaction mixture, which promoted a high initial activity not seen in the original samples. Overall, our results show that catalytic sites in porous polymers can have very dynamic structures that are highly dependent on the polymer functional groups as well as reaction conditions and show how this strategy can be used to produce active catalysts with a wide range of possible compositions and chemistries.

Biological enzymes are very effective catalysts which have evolved the capability to carry out chemical reactions at room temperature with high selectivity. Their diverse amino acid functional groups stabilize specific reaction intermediate states and control their transport to and from an active metal site at their center. However, they can only operate under highly specific reaction conditions, and designing a catalyst with similar behavior that can be used in industrial conditions is challenging. Recently, a novel class of catalysts was reported [1] which mimics this enzymatic functionality to produce highly selective, synthetic catalysts. These polymer-nanocrystal hybrid catalysts are produced in a modular fashion in which the first step is the production of a nanoporous polymeric organic framework (POF) to serve as the support structure. This is followed by impregnation of the support with a metal containing solution and a rapid calcination treatment to produce nanocrystals dispersed on the POF surface. Finally, the entire assembly is encapsulated in a second layer of POF material. The versatility of this type of catalyst is that the transport of reactants and products to and from the active metal sites can be controlled by varying the pore-size and thickness of the overlayer. The type of reaction intermediates that form can also be affected by changing the chemistry of the POF.

Functional organic-inorganic hybrid materials with tunable properties are useful across many application areas, ranging from gas storage to electronics, flame retardants, separations, and catalysis. Combining polymers, with a suite of functional groups and conformational flexibility, and inorganic nanoparticles, with tunable surface chemistry and composition, yields hybrids with novel functional properties. Specifically, in catalysis, control of the electronic environment at a metal interface is paramount in determining the catalytic properties. In this contribution, we describe a modular process to prepare porous polymer-nanocrystal (NC) composites in a hierarchical, multilayered synthesis, in which multiple parameters can be accurately tuned: polymer functional groups and the corresponding pore structure, the polymer layer thickness, and the NC size, shape, and composition. This process provides for a variety of controlled materials with high surface area, tunable chemistry, and thermal and chemical stabilities. Furthermore, we demonstrate their utility for shape- and size-selective catalytic conversions both in oxidation and hydrogenation reactions, where they show increased selectivity by orders of magnitude compared to conventional polymer-supported metal catalysts. In light of the high degree of control in the composite structure, this method allows for the design and realization of catalysts for several reactions and reaction environments and for nanomaterials with other applications.

Single atom catalysts have recently attracted interest due to their maximization of the utilization of expensive noble metals as well as their unique catalytic properties. Based on its surface atomic properties, CeO2 is one of the most common supports for stabilizing single metal atoms. Many single atom catalysts are limited in their metal contents by the formation of metal nanoparticles once the catalyst support capacity for single atoms has been exceeded. Currently, there are no direct measurements to determine the capacity of a support to stabilize single atoms. In this work we develop a nanoparticle-based technique that allows for quantification of that capacity by redispersing Ru nanoparticles into single atoms and taking advantage of the different catalytic properties of Ru single atoms and nanoparticles in the CO2 hydrogenation reaction. This method avoids complications in metal loading caused by counterions in incipient wetness impregnation and can eventually be applied to a variety of different metals. Results using this technique follow trends in oxygen vacancy concentration and surface oxygen content and show promise as a new method for quantifying support single atom stabilization capacity.

The need for sustainable chemicals has increased the numbers of published studies about photocatalytic hydrogen production via photoreforming and water-splitting. However, comparison of results, already complicated by non-standardized reaction setups and instrumentation, can be further hampered by variation in degree of powdered catalyst dispersion and temporal instability related to co-catalyst morphology and the reaction setup. Building on guidelines already in the literature, we demonstrate the degree to which these sources of variation can impact apparent photocatalytic rates: in the worst cases, variable dispersion caused up to ~400% variation in activity for the same powdered photocatalyst, photodeposition of a Pt co-catalyst contributed to a lengthy activation period lasting up to two hours, and lamp instability increased quantum yield measurement error by a factor of four. To reduce impacts from these instabilities, we recommend mechanically and chemically tuning the particle sizes in dispersion to increase dispersion stability, avoiding comparisons across samples during the initial activation period, and ensuring lamp output stability on the timescale of the reaction. It is also informative to report particle size distributions, zeta potentials, and time-resolved catalytic activity.

Pd- and Pt-based catalysts are highly studied materials due to their widespread use in emissions control catalysis. However, claims continue to vary regarding the active phase and oxidation state of the metals. Different conclusions have likely been reached due to the heterogeneous nature of such materials containing various metal nanoparticle sizes and compositions, which may each possess unique redox features. In this work, using uniform nanocrystal catalysts, we study the effect of particle size and alloying on redox properties of Pd-based catalysts and show their contribution to methane combustion activity using operando quick extended x-ray absorption fine structure measurements. Results demonstrate that for all studied Pd sizes (3 nm-16 nm), Pd oxidation directly precedes CH4 combustion to CO2, suggesting Pd oxidation as a prerequisite step to methane combustion, and an oxidation pretreatment shows equal or better catalysis than a reduction pretreatment. Results are then extended to uniform alloyed PtxPd1-x nanoparticles, where oxidative pretreatments are shown to enhance low-temperature combustion. In these uniform alloys, we observe a composition-dependent effect with Pt-rich alloys showing the maximum difference between oxidative and reductive pretreatments. In Pt-rich alloys, we initially observe that the presence of Pt maintains Pd in a lower-activity reduced state. However, with time on stream, PdO eventually segregates under oxidizing combustion conditions, leading to a slowly increasing activity. Overall, across particle sizes and alloy compositions, we relate increased catalytic activity to Pd oxidation, thus shedding light on previous contrasting results related to the methane combustion activity of these catalysts.

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.

Effective catalysts stabilize specific transition states and control the transport of species to and from catalytically active sites. Enzymes show these traits thanks to their diverse amino acid functional groups encapsulating metal centres, but are limited in the reaction conditions in which they can operate. Realizing a catalyst with this kinetic and transport control that can be used under demanding industrial conditions is challenging. Here, we show a modular approach for the systematic synthesis of polymer–nanocrystal hybrids, where palladium nanocrystals are encapsulated within tunable microporous polymer layers. The polymer chemistry and morphology control the catalytic performance of the metal sites, affecting the transition state for CO oxidation and controlling the transport of CO2 away from the active site. This approach can be applied to other polymer–nanocrystal compositions and catalytic applications, and is therefore expected to have an impact in many areas of catalysis. Encapsulation is an effective strategy to tune metal-catalysed reactions, although its potential has not been fully explored. Here, design principles and advanced understanding of the reactivity of different polymer-encapsulated Pd nanocrystals are provided using CO oxidation as a benchmark reaction.

Normalizing photocatalytic rates by catalyst mass in heterojunction systems can artificially inflate performance; we outline a method that can be used to correctly understand photocatalytic performance.

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. (C) 2019 Elsevier Ltd. All rights reserved.

Selective oxidations are crucial for the creation of valuable chemical building blocks but often require expensive and unstable stoichiometric oxidants such as hydroperoxides and peracids. To date, many catalysts that contain a single type of active site have not been able to attain the desired level of selectivity for partially oxidized products over total combustion. However, catalysts containing multiple types of active sites have proven to be successful for selective reactions. One category of such catalysts is bimetallic alloys, in which catalytic activity and selectivity can be tuned by modifying the surface composition. Traditional catalyst synthesis methods using impregnation struggle to create catalysts with sufficient control over surface chemistry to accurately tune the ensemble size of the desired active sites. Here we describe the synthesis of colloidal nanocrystals of dilute alloys of palladium and gold. We show that when supported on titania (TiO2), tuning the composition of the Pd/Au nanocrystal surface provides a synergistic effect in the selective oxidation of 2-propanol to acetone in the presence of H2 and O2. In particular, we show that certain Pd/Au surface ratios exhibit activity and selectivity far superior to Pd or Au individually. Through precise structural characterization we demonstrate that isolated atoms of Pd exist in the most active catalysts. The synergy between isolated Pd atoms and Au allows for the formation of reactive oxidizing species, likely hydroperoxide groups, responsible for selective oxidation while limiting oxygen dissociation and, thus, complete combustion. This work opens the way to more efficient utilization of scarce noble metals and new options for catalyzed selective oxidations.

Removing CO from hydrogen streams is an important industrial process. The catalytic preferential oxidation of CO (PrOx) is a promising method for CO removal, leaving the hydrogen concentration unchanged. Here, the effect of size and support on the gold‐catalyzed PrOx reaction using size‐controlled Au nanocrystals (NCs) is investigated. For all supports, Au NC sizes of 2–5 nm show the highest rates, whereas for larger sizes rates drop. Ceria‐supported Au shows by far the best performance. By analyzing the dependency of the reaction rate on the NC diameter, the most active centers for CO oxidation on Au/CeO2 are Au+ corner atoms at the interface with the support, resulting in 2.1 nm Au NCs supported on ceria reaching full O2 conversion and CO selectivity of about 50%. Therefore, it is suggested that increasing the fraction of Au‐ceria interface sites would lead to the best performing materials for this reaction. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3159–3167, 2018

Langmuir-Blodgett deposition is a popular route to produce thin films of graphene oxide for applications such as transparent conductors and biosensors. Unfortunately, film morphologies vary from sample to sample, often with undesirable characteristics such as folded sheets and patchwise depositions. In conventional Langmuir-Blodgett deposition of graphene oxide, alcohol (typically methanol) is used to spread the graphene oxide sheets onto an air-water interface before deposition onto substrates. Here we show that methanol gives rise to Marangoni flow, which fundamentally limits control over Langmuir-Blodgett depositions of graphene oxide. We directly identified the presence of Marangoni flow by using photography, and we evaluated depositions with atomic force microscopy and scanning electron microscopy. The disruptive effect of Marangoni flow was demonstrated by comparing conventional Langmuir-Blodgett depositions to depositions where Marangoni flow was suppressed by a surfactant. Because methanol is the standard spreading solvent for conventional Langmuir-Blodgett deposition of graphene oxide, Marangoni flow is a general problem and may partly explain the wide variety of undesirable film morphologies reported in the literature.