Emission control catalysts are crucial for protecting human health by preventing the release of harmful gases and unburnt fuel into the atmosphere. These catalysts often face deactivation through sintering processes in high-temperature, chemically reactive environments containing multiple gas species. Here, we use in situ environmental scanning transmission electron microscopy to monitor the sintering behavior and transient morphologies of Pt/Al2O3 in various relevant gas environments through controlled experiments. Our results reveal the particle migration and atomic ripening behavior of Pt/Al2O3 at the atomic scale in the presence of water vapor and oxygen, which differs from behaviors observed in single gas environments. We identify an atomic ripening mechanism involving the dissociation and migration of Pt adatom chains from Pt nanoparticles, observed only in combinational gases. These findings provide valuable insights into catalyst degradation behavior in complex gas environments.

Multicomponent catalysts can be designed to synergistically combine reaction intermediates at interfacial active sites, but restructuring makes systematic control and understanding of such dynamics challenging. We here unveil how reducibility and mobility of indium oxide species in Ru-based catalysts crucially control the direct, selective conversion of CO2 to ethanol. When uncontrolled, reduced indium oxide species occupy the Ru surface, leading to deactivation. With the addition of steam as a mild oxidant and using porous polymer layers to control In mobility, Ru-In2O3 interface sites are stabilized, and ethanol can be produced with superior overall selectivity (70%, rest CO). Our work highlights how engineering of bifunctional active ensembles enables cooperativity and synergy at tailored interfaces, which unlocks unprecedented performance in heterogeneous catalysts.

Platinum exhibits desirable catalytic properties, but it is scarce and expensive. Optimizing its use in key applications such as emission control catalysis is important to reduce our reliance on such a rare element. Supported Pt nanoparticles (NPs) used in emission control systems deactivate over time because of particle growth in sintering processes. In this work, we shed light on the stability against sintering of Pt NPs supported on and encapsulated in Al2O3 using a combination of nanocrystal catalysts and atomic layer deposition (ALD) techniques. We find that small amounts of alumina overlayers created by ALD on preformed Pt NPs can stabilize supported Pt catalysts, significantly reducing deactivation caused by sintering, as previously observed by others. Combining theoretical and experimental insights, we correlate this behavior to the decreased propensity of oxidized Pt species to undergo Ostwald ripening phenomena because of the physical barrier imposed by the alumina overlayers. Furthermore, we find that highly stable catalysts can present an abundance of under-coordinated Pt sites after restructuring of both Pt particles and alumina overlayers at a high temperature (800 °C) in C3H6 oxidation conditions. The enhanced stability significantly improves the Pt utilization efficiency after accelerated aging treatments, with encapsulated Pt catalysts reaching reaction rates more than two times greater than those of a control supported Pt catalyst.

Sustainable aviation fuel (SAF) production from biomass and biowaste streams is an attractive option for decarbonizing the aviation sector, one of the most-difficult-to-electrify transportation sectors. Despite ongoing commercialization efforts using ASTM-certified pathways (e.g., lipid conversion, Fischer-Tropsch synthesis), production capacities are still inadequate due to limited feedstock supply and high production costs. New conversion technologies that utilize lignocellulosic feedstocks are needed to meet these challenges and satisfy the rapidly growing market. Combining bio- and chemo-catalytic approaches can leverage advantages from both methods, i.e., high product selectivity via biological conversion, and the capability to build C-C chains more efficiently via chemical catalysis. Herein, conversion routes, catalysis, and processes for such pathways are discussed, while key challenges and meaningful R&D opportunities are identified to guide future research activities in the space. Bio- and chemo-catalytic conversion primarily utilize the carbohydrate fraction of lignocellulose, leaving lignin as a waste product. This makes lignin conversion to SAF critical in order to utilize whole biomass, thereby lowering overall production costs while maximizing carbon efficiencies. Thus, lignin valorization strategies are also reviewed herein with vital research areas identified, such as facile lignin depolymerization approaches, highly integrated conversion systems, novel process configurations, and catalysts for the selective cleavage of aryl C–O bonds. The potential efficiency improvements available via integrated conversion steps, such as combined biological and chemo-catalytic routes, along with the use of different parallel pathways, are identified as key to producing all components of a cost-effective, 100% SAF.

Reductive catalytic fractionation (RCF) is a promising method to extract and depolymerize lignin from biomass, and bench-scale studies have enabled considerable progress in the past decade. RCF experiments are typically conducted in pressurized batch reactors with volumes ranging between 50 and 1000 mL, limiting the throughput of these experiments to one to six reactions per day for an individual researcher. Here, we report a high-throughput RCF (HTP-RCF) method in which batch RCF reactions are conducted in 1 mL wells machined directly into Hastelloy reactor plates. The plate reactors can seal high pressures produced by organic solvents by vertically stacking multiple reactor plates, leading to a compact and modular system capable of performing 240 reactions per experiment. Using this setup, we screened solvent mixtures and catalyst loadings for hydrogen-free RCF using 50 mg poplar and 0.5 mL reaction solvent. The system of 1:1 isopropanol/methanol showed optimal monomer yields and selectivity to 4-propyl substituted monomers, and validation reactions using 75 mL batch reactors produced identical monomer yields. To accommodate the low material loadings, we then developed a workup procedure for parallel filtration, washing, and drying of samples and a H-1 nuclear magnetic resonance spectroscopy method to measure the RCF oil yield without performing liquid-liquid extraction. As a demonstration of this experimental pipeline, 50 unique switchgrass samples were screened in RCF reactions in the HTP-RCF system, revealing a wide range of monomer yields (21-36%), S/G ratios (0.41-0.93), and oil yields (40-75%). These results were successfully validated by repeating RCF reactions in 75 mL batch reactors for a subset of samples. We anticipate that this approach can be used to rapidly screen substrates, catalysts, and reaction conditions in high-pressure batch reactions with higher throughput than standard batch reactors.

Encapsulation of metal nanoparticles within oxide materials has been shown as an effective strategy to improve activity, selectivity, and stability in several catalytic applications. Several approaches have been proposed to encapsulate nanoparticles, such as forming core-shell structures, growing ordered structures (zeolites or metal-organic frameworks) on nanoparticles, or directly depositing support materials on nanoparticles. Here, a general nanocasting method is demonstrated that can produce diverse encapsulated metal@oxide structures with different compositions (Pt, Pd, Rh) and multiple types of oxides (Al2O3, Al2O3-CeO2, ZrO2, ZnZrOx, In2O3, Mn2O3, TiO2) while controlling the size and dispersion of nanoparticles and the porous structure of the oxide. Metal@polymer structures are first prepared, and then the oxide precursor is infiltrated into such structures and the resulting material is calcined to form the metal@oxide structures. Most Pt@oxides catalysts show similar catalytic activity, demonstrating the availability of surface Pt sites in the encapsulated structures. However, the Pt@Mn2O3 sample showed much higher CO oxidation activity, while also being stable under aging conditions. This work demonstrated a robust nanocasting method to synthesize metal@oxide structures, which can be utilized in catalysis to finely tune metal-oxide interfaces.

Platinum group metals (PGMs) are widely used for exhaust emission abatement. Sintering during high-temperature emission control conditions decreases noble metal utilization efficiency. Efficient use of scarce noble metals requires sinter-resistant catalysts. Here, we extend an approach to synthesize catalysts consisting of platinum nanoparticles encapsulated in a mixture of cerium and aluminum oxides (Pt@Al2O3-CeO2). We tested the activity of this catalyst toward carbon monoxide, propene, and propane oxidation, chosen as model oxidation reactions for emission control catalysts. Pt@Al2O3-CeO2 catalysts demonstrated similar activity and stability upon aging as the comparison system without ceria, Pt@Al2O3, while maintaining small Pt nanoparticles and ceria crystallites. Additionally, we studied the influence of various thermal treatments on the carbon monoxide (CO) oxidation activity and determined that a steam treatment can activate the low-temperature CO oxidation activity of Pt@Al2O3-CeO2. Scanning transmission electron microscope-energy-dispersive X-ray spectroscopy (STEM-EDS) analysis revealed that thermal treatments led to the colocation of Pt and CeO2, and temperature-programmed reduction analysis revealed that the steam treatment specifically enhanced CO oxidation activity through surface reduction of the CeO2. In summary, we demonstrate the versatility of this encapsulation approach to generate mixed metal-oxide supports with improved metal-support interactions without hindering the nanoparticle stability.

Stable catalysts are essential to address energy and environmental challenges, especially in harsh environment applications (high temperature, oxidizing atmosphere, steam). In such conditions, supported metal catalysts deactivate due to sintering – a process where initially small nanoparticles grow into larger ones with reduced active surface area. Strategies to stabilize them lead to decreased performance. Here, we report stable catalysts prepared through the encapsulation of platinum particles inside an alumina framework. These catalysts do not sinter at 800 °C in the presence of oxygen and steam, conditions in which conventional catalysts sinter to large extents, while showing similar reaction rates. Extending this approach to Pd/Pt bimetallic catalysts leads to maintained small particle size at temperatures as high as 1,100 °C in air and steam. This strategy can be broadly applied to other metal and metal oxides for applications where sintering is a major cause of materials deactivation.

Sustainable aviation fuel (SAF) is essential to decrease the carbon footprint of the aviation industry. Although many strategies have been developed to provide the linear and branched aliphatic components of SAF, few viable strategies have been demonstrated to supply the aromatic and cycloalkane fraction of the SAF at the necessary scale from bio-based feedstocks. Lignin is the largest natural source of renewable aromatic compounds; however, major challenges in deoxygenation have prevented its use as a feedstock for SAFs. Here, we report a continuous, two-stage catalytic process using molybdenum carbide to deoxygenate lignin from poplar into aromatic hydrocarbons with 87.5% selectivity toward aromatic hydrocarbons at 86% of the theoretical carbon recovery. Tier α fuel property testing indicates that the SAF-range lignin-derived aromatic compounds are likely performance-advantaged across multiple properties relative to conventional jet fuel aromatic compounds. This work demonstrates an effective approach to convert lignin into aromatic SAF blendstocks.

Reductive catalytic fractionation (RCF) is a promising approach to fractionate lignocellulose and convert lignin to a narrow product slate. To guide research towards commercialization, cost and sustainability must be considered. Here we report a techno-economic analysis (TEA), life cycle assessment (LCA), and air emission analysis of the RCF process, wherein biomass carbohydrates are converted to ethanol and the RCF oil is the lignin-derived product. The base-case process, using a feedstock supply of 2000 dry metric tons per day, methanol as a solvent, and H2 gas as a hydrogen source, predicts a minimum selling price (MSP) of crude RCF oil of $1.13 per kg when ethanol is sold at $2.50 per gallon of gasoline-equivalent ($0.66 per liter of gasoline-equivalent). We estimate that the RCF process accounts for 57% of biorefinery installed capital costs, 77% of positive life cycle global warming potential (GWP) (excluding carbon uptake), and 43% of positive cumulative energy demand (CED). Of $563.7 MM total installed capital costs, the RCF area accounts for $323.5 MM, driven by high-pressure reactors. Solvent recycle and water removal via distillation incur a process heat demand equivalent to 73% of the biomass energy content, and accounts for 35% of total operating costs. In contrast, H2 cost and catalyst recycle are relatively minor contributors to operating costs and environmental impacts. In the carbohydrate-rich pulps, polysaccharide retention is predicted not to substantially affect the RCF oil MSP. Analysis of cases using different solvents and hemicellulose as an in situ hydrogen donor reveals that reducing reactor pressure and the use of low vapor pressure solvents could reduce both capital costs and environmental impacts. Processes that reduce the energy demand for solvent separation also improve GWP, CED, and air emissions. Additionally, despite requiring natural gas imports, converting lignin as a biorefinery co-product could significantly reduce non-greenhouse gas air emissions compared to burning lignin. Overall, this study suggests that research should prioritize ways to lower RCF operating pressure to reduce capital expenses associated with high-pressure reactors, minimize solvent loading to reduce reactor size and energy required for solvent recovery, implement condensed-phase separations for solvent recovery, and utilize the entirety of RCF oil to maximize value-added product revenues.

Lignin is an alkyl-aromatic biopolymer that, despite its abundance, is underutilized as a renewable feedstock because of its highly complex structure. An approach to overcome this challenge that has gained prominence in recent years leverages the plasticity and malleability of lignin biosynthesis to tune lignin structure in planta through genetic approaches. An improved understanding of lignin biosynthesis can thus provide fundamental insights critical for the development of effective tailoring and valorization strategies. Although it is widely accepted that lignin monomers and growing chains are oxidized enzymatically into radicals that then undergo kinetically-controlled coupling in planta, direct experimental evidence has been scarce because of the difficulty of exactly replicating in planta lignification conditions. Here, we computationally investigate a set of radical reactions representative of lignin biosynthesis. We show that, contrary to the widely held notion that radical coupling reactions should be barrierless and dynamically controlled, the computed activation energies can be qualitatively consistent with key structural observations made empirically for native lignin in a variety of biomass types. We also rationalize the origins of regioselectivity in coupling reactions through structural and activation strain analyses. Our findings lay the groundwork for more detailed multiscale simulations of the lignification process.

The ratio of syringyl (S) and guaiacyl (G) units in lignin has been regarded as a major factor in determining the maximum monomer yield from lignin depolymerization. This limit arises from the notion that G units are prone to C-C bond formation during lignin biosynthesis, resulting in less ether linkages that generate monomers. This study uses reductive catalytic fractionation (RCF) in flow-through reactors as an analytical tool to depolymerize lignin in poplar with naturally varying S/G ratios, and directly challenges the common conception that the S/G ratio predicts monomer yields. Rather, this work suggests that the plant controls C-O and C-C bond content by regulating monomer transport during lignin biosynthesis. Overall, our results indicate that additional factors beyond the monomeric composition of native lignin are important in developing a fundamental understanding of lignin biosynthesis.

Lignin is an abundantbiopolymer of phenylpropanoid monomers that is critical for plant structure andfunction. Based on the abundance of lignin in the biosphere and interest inlignin valorization, a more comprehensive understanding of lignin biosynthesis isimperative. Here we present an open-source software toolkit, LIGNIN-KMC, thatcombines kinetic Monte Carlo and first-principles calculations of radicalcoupling events, which enables modeling of lignin biosynthesis in silico. Specifically, lignificationis simulated using the Gillespie algorithm with a graph representation of thelignin fragments and with rates derived from density functional theorycalculations of individual fragment couplings. Using this approach, we confirm experimentalfindings regarding the impact of lignification conditions on final polymerstructure and identify new features of fundamental interest to plant cell wallbiosynthesis including (1) the monolignol supply rate under in planta lignification conditionslikely varies as a function of evolutionary stresses; (2) under conditions of lowmonolignol supply rates, increasing the fraction of sinapyl alcohol increasesthe depolymerization yield of monomers upon ether bond cleavage; and (3) byincluding calculated energetics of caffeyl alcohol homopolymers, the modelaccurately predicts C-lignin structures. These examples not only highlight the robustnessof our modeling framework, but also motivate future studies of new lignintypes, unexplored monolignol chemistries, and lignin structure predictions, allwith an overarching aim of developing a more comprehensive, molecularunderstanding of native lignin, which, in turn, can advance the biological and chemistrycommunities interested in this important biopolymer.

Cobalt phosphosulfide (CoPS) has recently emerged as a promising earth-abundant electrocatalyst for the hydrogen evolution reaction (HER). Nonetheless, the influence of crystallographic surface on the HER activity of CoPS and other nonmetallic electrocatalysts remains an important open question in the design of high-performance catalysts. Herein, the HER activities of the (100) and (111) facets of CoPS single crystals were studied using complementary experimental and computational approaches. Natural (111) and polished (100) facets of CoPS single crystals were selectively exposed to reveal that the HER behaviors on these two facets are quite different, with current density-potential curves crossing near 0.35 V vs RHE. Computational analysis can explain this phenomenon in terms of strongly differing H atom adsorption free energies and H-H recombination barriers on the facets, in conjunction with a simple kinetic model. At low potential (0-0.35 V), H adsorption (Volmer step) is rate limiting due to the endergonic adsorption on the (111) facet vs exergonic adsorption on the (100) facet, yielding a faster HER rate for the latter. However, at high potential (> 0.35 V), H-2 recombination/desorption becomes limiting and thus the (111) facet, with lower associated barriers, shows better HER activity. Explicit consideration of both steps and their interplay allows for a comprehensive description of the overpotential-dependence of the HER activity. This integrated study yields additional insight into the factors which govern the facet-dependence of catalytic activity on nonmetallic electrocatalysts and can further improve the design of advanced nanostructured HER catalysts.

Reductive catalytic fractionation (RCF) is an effective active-stabilization strategy to selectively extract and depolymerize lignin into aromatic monomers. Here, the kinetics of RCF were investigated by using flow-through reactors to decouple the two limiting mechanistic steps, namely lignin solvolysis and reduction. When operating in a solvolysis-limited regime, apparent energy barriers of 63 +/- 1 and 64 +/- 2 kJ mol(-1) were measured for the solvent mediated lignin extraction of poplar using particle diameters of 0.5 < d < 1 mm and 0.075 < d < 0.25 mm, respectively. In contrast, when using mechanically stirred batch reactors, apparent barriers of 32 +/- 1 and 39 +/- 3 kJ mo1(-1) were measured for particle diameters of 0.5 < d < 1 mm and 0.075 < d < 0.25 mm, respectively. The difference of activation barriers between flow and batch reactors indicated that lignin extraction under typical RCF conditions in a 100 mL batch reactor stirred at 700 rpm was mass-transfer limited. In the reduction-limited regime, cleavage of the beta-O-4 bond in a model compound exhibited an apparent activation barrier of 168 +/- 14 kJ mol(-1). This study demonstrates RCF occurs by two limiting processes that can be independently controlled. Furthermore, both controlling which process limits RCF and verifying if transport limitations exist, are critical steps to develop a mechanistic understanding of RCF and to design improved catalysts.

Lignin is an abundant biopolymer important for plant function while holding promise as a renewable source of valuable chemicals. Although the lignification process in plant cell walls has been long-studied, a comprehensive, mechanistic understanding on the molecular scale remains elusive. A better understanding of lignification will lead to improved atomistic models of the plant cell wall that could, in turn, inform effective strategies for biomass valorization. Here, using first-principles quantum chemical calculations, we show that a simple model of kinetically-controlled radical coupling broadly rationalizes qualitative experimental observations of lignin structure across a wide variety of biomass types, thus paving the way for predictive, first-principles models of lignification while highlighting the ability of computational chemistry to help illuminate complex biological processes.

Reductive catalytic fractionation (RCF) has emerged as a leading biomass fractionation and lignin valorization strategy. Here, flowthrough reactors were used to investigate RCF of poplar. Most RCF studies to date have been conducted in batch, but a flow-based process enables the acquisition of intrinsic kinetic and mechanistic data essential to accelerate the design, optimization, and scale-up of RCF processes. Time-resolved product distributions and yields obtained from experiments with different catalyst loadings were used to identify and deconvolute events during solvolysis and hydrogenolysis. Multi-bed RCF experiments provided unique insights into catalyst deactivation, showing that leaching, sintering, and surface poisoning are causes for decreased catalyst performance. The onset of catalyst deactivation resulted in higher concentrations of unsaturated lignin intermediates and increased occurrence of repolymerization reactions, producing high-molecular-weight species. Overall, this study demonstrates the concept of flowthrough RCF, which will be vital for realistic scale-up of this promising approach.

The scalable and sustainable production of hydrogen fuel through water splitting demands efficient and robust Earth-abundant catalysts for the hydrogen evolution reaction (HER). Building on promising metal compounds with high HER catalytic activity, such as pyrite structure cobalt disulphide (CoS2), and substituting non-metal elements to tune the hydrogen adsorption free energy could lead to further improvements in catalytic activity. Here we present a combined theoretical and experimental study to establish ternary pyrite-type cobalt phosphosulphide (CoPS) as a high-performance Earth-abundant catalyst for electrochemical and photoelectrochemical hydrogen production. Nanostructured CoPS electrodes achieved a geometrical catalytic current density of 10 mA cm(-2) at overpotentials as low as 48 mV, with outstanding long-term operational stability. Integrated photocathodes of CoPS on n(+)-p-p(+) silicon micropyramids achieved photocurrents up to 35 mA cm(-2) at 0 V versus the reversible hydrogen electrode (RHE), onset photovoltages as high as 450 mV versus RHE, and the most efficient solar-driven hydrogen generation from Earth-abundant systems.