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The performance of Brønsted- and Lewis-acidic La, Nb, and Zr phosphates (LaPO, NbPO, and ZrPO) during the aqueous phase conversion of dihydroxyacetone (DHA) to lactic acid (LA) is investigated using a fixed-bed reactor. Mass-transfer phenomena are thoroughly investigated, and the masstransfer coefficient is deconvoluted from the intrinsic kinetic constant for each catalyst to enable the quantitative assessment of both. NbPO is found to be masstransfer-limited. Despite this limitation, NbPO shows the highest yield of LA at 36%. The reaction over ZrPO is not transport-limited, allowing for a rigorous analysis of intrinsic kinetics. This analysis shows that the conversion of DHA into pyruvaldehyde (PVA) follows a second-order reaction mechanism via a dimeric intermediate, which consolidates previous reports in the literature. Additionally, a correlation between LA production and the carbon missing from the carbon balance (carbon loss) is observed. Finally, NbPO and ZrPO show stable performance up to 10 h on stream at 150 °C. After 15 h of reaction, the PVA yield increases at the expense of LA with NbPO. This is ascribed to the deactivation of the active sites necessary to produce LA, which are different from the sites that produce PVA.This hypothesis is supported by the characterization of the spent catalyst with 13C magic-angle spinning nuclear magnetic resonance and attenuated total reflectance infrared spectroscopy.

Synthesis of ammonia through photo- and electrocatalysis is a rapidly growing field. Titania-based catalysts are widely reported for photocatalytic ammonia synthesis and have also been suggested as electrocatalysts. The addition of transition-metal dopants is one strategy for improving the performance of titania-based catalysts. In this work, we screen d-block transition-metal dopants for surface site stability and evaluate trends in their performance as the active site for the reduction of nitrogen to ammonia on TiO$_2$. We find a linear relationship between the d-band center and formation energy of the dopant site, while the binding energies of N$_2$, N$_2$H, and NH$_2$ all are strongly correlated with the cohesive energies of the dopant metals. The activity of the metal-doped systems shows a volcano type relationship with the NH$_2$ and N$_2$H energies as descriptors. Some metals such as Co, Mo, and V are predicted to slightly improve photo- and electrocatalytic performance, but most metals inhibit the ammonia synthesis reaction. The results provide insight into the role of transition-metal dopants for promoting ammonia synthesis, and the trends are based on unexpected electronic structure factors that may have broader implications for single-atom catalysis and doped oxides.

Understanding surface reactions of biomass-derived oxygenates on metal oxides is important for designing catalysts for valorization of biomass. This work elucidated the effect of different pretreat...

Chemical kinetics and reaction engineering consists of the phenomenological framework for the disentanglement of reaction mechanisms, optimization of reaction performance and the rational design of chemical processes. Here, we utilize feed-forward artificial neural networks as basis functions to solve ordinary differential equations (ODEs) constrained by differential algebraic equations (DAEs) that describe microkinetic models (MKMs). We present an algebraic framework for the mathematical description and classification of reaction networks, types of elementary reaction, and chemical species. Under this framework, we demonstrate that the simultaneous training of neural nets and kinetic model parameters in a regularized multi-objective optimization setting leads to the solution of the inverse problem through the estimation of kinetic parameters from synthetic experimental data. We analyze a set of scenarios to establish the extent to which kinetic parameters can be retrieved from transient kinetic data, and assess the robustness of the methodology with respect to statistical noise. This approach to inverse kinetic ODEs can assist in the elucidation of reaction mechanisms based on transient data., Pre-print for first submission

Zeolites are nanoporous aluminosilicates widely used in catalysis and separations applications. Though generally formed as 3D crystals, new synthesis techniques have given access to 2D zeolite nanosheets with small diffusion path lengths and accelerated molecular diffusion. Since most previous research has focused on bulk zeolite crystals, there is little understanding of the surface adsorption and diffusion mechanisms likely involved at such length scales and their contributions to the permeability and selectivity of different species. To enable the systematic examination of such surface properties, we constructed a database of more than 800,000 computation-ready 2D zeolite nanosheets from the full range of known zeolite structures in the IZA database of zeolite structure types. The nanosheet surfaces cover a wide range of orientations and were created via the principle of minimizing the number of bonds broken during the termination of a unit cell. The database consists of two sets of nanosheets: one set...

Photo-catalytic fixation of nitrogen by titania catalysts at ambient conditions has been reported for decades, yet the active site capable of adsorbing an inert N2 molecule at ambient pressure and the mechanism of dissociating the strong dinitrogen triple bond at room temperature remain unknown. In this work in situ near-ambient-pressure X-ray photo-electron spectroscopy and density functional theory calculations are used to probe the active state of the rutile (110) surface. The experimental results indicate that photon-driven interaction of N2 and TiO2 is observed only if adventitious surface carbon is present, and computational results show a remarkably strong interaction between N2 and carbon substitution (C*) sites that act as surface-bound carbon radicals. A carbon-assisted nitrogen reduction mechanism is proposed and shown to be thermodynamically feasible. The findings provide a molecular-scale explanation for the long-standing mystery of photo-catalytic nitrogen fixation on titania. The results suggest that controlling and characterizing carbon-based active sites may provide a route to engineering more efficient photo(electro)-catalysts and improving experimental reproducibility.

Methanol is a major fuel and chemical feedstock currently produced from syngas, a CO/CO2/H2 mixture. Herein we identify formate binding strength as a key parameter limiting the activity and stability of known catalysts for methanol synthesis in the presence of CO2. We present a molybdenum phosphide catalyst for CO and CO2 reduction to methanol, which through a weaker interaction with formate, can improve the activity and stability of methanol synthesis catalysts in a wide range of CO/CO2/H2 feeds.

Catalysis informatics is a distinct subfield that lies at the intersection of cheminformatics and materials informatics but with distinctive challenges arising from the dynamic, surface-sensitive, and multiscale nature of heterogeneous catalysis. The ideas behind catalysis informatics can be traced back decades, but the field is only recently emerging due to advances in data infrastructure, statistics, machine learning, and computational methods. In this work, we review the field from early works on expert systems and knowledge engines to more recent approaches utilizing machine-learning and uncertainty quantification. The data–information–knowledge hierarchy is introduced and used to classify various developments. The chemical master equation and microkinetic models are proposed as a quantitative representation of catalysis knowledge, which can be used to generate explanative and predictive hypotheses for the understanding and discovery of catalytic materials. We discuss future prospects for the field, i...

Using a combined density functional theory and descriptor based microkinetic model approach, we predict production rate volcanos for higher oxygenate formation on (111) transition-metal surfaces. Despite their lower activity for CO conversion compared to stepped surfaces, (111) transition metal surfaces bring the potential for selectivity toward C2+ oxygenates. The volcano plots can be used to rationalize and predict activity and selectivity trends for transition-metal-based catalysts.

We present a generic analysis of the implications of energetic scaling relations on the possibilities for bifunctional gains at homogeneous bimetallic alloy catalysts. Such catalysts exhibit a large number of interface sites, where second-order reaction steps can involve intermediates adsorbed at different active sites. Using different types of model reaction schemes, we show that such site-coupling reaction steps can provide bifunctional gains that allow for a bimetallic catalyst composed of two individually poor catalyst materials to approach the activity of the optimal monomaterial catalyst. However, bifunctional gains cannot result in activities higher than the activity peak of the monomaterial volcano curve as long as both sites obey similar scaling relations, as is generally the case for bimetallic catalysts. These scaling-relation-imposed limitations could be overcome by combining different classes of materials such as metals and oxides.

Over the last century, the industrialization of agriculture and the consumption of fossil fuels have resulted in a significant imbalance and redistribution in nitrogen-containing resources. This has sparked an interest in developing more sustainable and resilient approaches for producing nitrogen-containing commodities such as fertilizers and fuels. One largely neglected but emerging approach is photocatalytic nitrogen fixation. There is significant evidence that this process occurs spontaneously in terrestrial settings, and it has been demonstrated in numerous engineered systems. Yet many questions still remain unanswered regarding the rates, mechanisms, and impacts of photocatalytically producing fixed nitrogen “out of thin air”. This work reviews the fascinating history of the reaction and examines current progress toward understanding and improving photofixation of nitrogen. This is supplemented by a quantitative review of the thermodynamic considerations and limitations for various reaction mechanism...

Aromatics, including benzene, toluene, and xylenes (BTX), are essential chemical building blocks and are widely used as solvents, fuel additives, and polymers. With the recent development in CO2 capture technologies and the progress made in producing H2 using renewable energy, direct hydrogenation of CO2 to aromatics via heterogeneous catalysis has emerged as a promising pathway to accomplish the production of aromatics with simultaneous utilization of waste CO2. In this review, we focus on recent advances in the nascent field of direct CO2 aromatization, whereby tandem catalysts composed of CO2 hydrogenation and aromatization functionalities are designed and deployed. We review two categories of tandem catalysts: catalysts integrating Fe-based/H-ZSM-5 components following RWGS (reverse water-gas shift of CO2 to CO)-FT (Fischer-Tropsch synthesis of lower olefins)-aromatization pathways, and catalysts combining metal oxide/H-ZSM-5 domains following CO2 to methanol-aromatization pathways. The key parameters that determine the catalytic performance, such as the composition and structure of the Fe-based or metal oxide-based CO2 conversion catalysts, the properties of H-ZSM-5, and the synergy between the two components, are analyzed to provide insights for the design of efficient tandem catalysts for CO2 aromatization. In parallel, thermodynamic analyses, mechanistic studies, and density functional theory (DFT) computations for the relevant reaction routes and pathways are discussed to offer improved understanding of CO2 activation, reaction intermediates, and product formation. Finally, the challenges and prospects for these tandem reactions are addressed to provide suggested paths forward for future research.

A new method for computational catalyst screening that is based on the concept of the degree of rate control (DRC) is introduced. It starts by developing a full mechanism and microkinetic model at the conditions of interest for a reference catalyst (ideally, the best known material) and then determines the degrees of rate control of the species in the mechanism (i.e., all adsorbed intermediates and transition states). It then uses the energies of the few species with the highest DRCs for this reference catalyst as descriptors to estimate the rates on related materials and predict which are most active. The predictions of this method regarding the relative rates of twelve late transition metals for methane steam reforming, using the Rh(2 1 1) surface as the reference catalyst, are compared to the most commonly-used approach for computation catalyst screening, the Norskov–Bligaard (NB) method which uses linear scaling relationships to estimate the energies of all adsorbed intermediates and transition states. It is slightly more accurate than the NB approach when the metals are similar to the reference metal (

We discuss three concepts that have made it possible to develop a quantitative understanding of trends in transition-metal catalysis: scaling relations, activity maps, and the d-band model. Scaling relations are correlations between surface bond energies of different adsorbed species including transition states; they open the possibility of mapping the many parameters determining the rate of a full catalytic reaction onto a few descriptors. The resulting activity map can be viewed as a quantitative implementation of the classical Sabatier principle, which states that there is an optimum “bond strength” defining the best catalyst for a given reaction. In the modern version, the scaling relations determine the relevant “bond strengths” and the fact that these descriptors can be measured or calculated makes it a quantitative theory of catalysis that can be tested experimentally by making specific predictions of new catalysts. The quantitative aspect of the model therefore provides new possibilities in catalyst design. Finally, the d-band model provides an understanding of the scaling relations and variations in catalytic activity in terms of the electronic structure of the transition-metal surface.

Descriptor-based analysis is a powerful tool for understanding the trends across various catalysts. In general, the rate of a reaction over a given catalyst is a function of many parameters—reaction energies, activation barriers, thermodynamic conditions, etc. The high dimensionality of this problem makes it very difficult and expensive to solve completely, and even a full solution would not give much insight into the rational design of new catalysts. The descriptor-based approach seeks to determine a few “descriptors” upon which the other parameters are dependent. By doing this it is possible to reduce the dimensionality of the problem—preferably to 1 or 2 descriptors—thus greatly reducing computational efforts and simultaneously increasing the understanding of trends in catalysis. The “CatMAP” Python module seeks to standardize and automate many of the mathematical routines necessary to move from “descriptor space” to reaction rates for heterogeneous (electro) catalysts. The module is designed to be both flexible and powerful, and is available for free online. A “reaction model” can be fully defined by a configuration file, thus no new programming is necessary to change the complexity or assumptions of a model. Furthermore, various steps in the process of moving from descriptors to reaction rates have been abstracted into separate Python classes, making it easy to change the methods used or add new functionality. This work discusses the structure of the code and presents the underlying algorithms and mathematical expressions both generally and via an example for the CO oxidation reaction.

Assessing calculated DFT properties Density functional theory (DFT) is now widely used to calculate molecular and material properties. DFT's reliability is usually assessed by comparison with experimental values and higher-level theoretical methods. Medford et al. used the BEEFvdW, an exchange-correlation density functional tailored for surface chemistry, and looked at uncertainties with ensembles of functionals. For the specific case of ammonia synthesis catalyzed by transition-metal surfaces, relative rates between different catalysts had lower errors than the absolute rates. Science , this issue p. 197

The activity of heterogeneous catalysts is often limited by a strong correlation between the chemisorption energies of reaction intermediates described by the “scaling relations” among the transition metals. We present electronic structure calculations that suggest that metal carbides do not in general follow the transition-metal scaling relations and tend to exhibit a carbophobic departure relative to the transition metals, meaning they tend to bind carbon-bound species weakly compared to oxygen-bound species. This contrasts with the oxophobic departure exhibited by Pt and Pd. Relative to the parent metals, carbides tend to bind carbon and oxygen more weakly and hydrogen more strongly. The departures are rationalized with the adsorbate–surface valence configuration and the energy of the metal sp-states. We employ these general trends to aid in the understanding of various catalytic properties such as the high activity of iron carbides for Fischer–Tropsch synthesis and Pt-group catalysts for partial oxida...

The Haber–Bosch process for ammonia synthesis has been suggested to be the most important invention of the 20th century, and called the ‘Bellwether reaction in heterogeneous catalysis’. We examine the catalyst requirements for a new low-pressure, low-temperature synthesis process. We show that the absence of such a process for conventional transition metal catalysts can be understood as a consequence of a scaling relation between the activation energy for N 2 dissociation and N adsorption energy found at the surface of these materials. A better catalyst cannot obey this scaling relation. We define the ideal scaling relation characterizing the most active catalyst possible, and show that it is theoretically possible to have a low pressure, low-temperature Haber–Bosch process. The challenge is to find new classes of catalyst materials with properties approaching the ideal, and we discuss the possibility that transition metal compounds have such properties.

The potential of bimetallic Pt–Co catalysts for production of higher alcohols in high pressure CO hydrogenation has been assessed. Two catalysts (Pt3Co/SiO2 and PtCo/SiO2) were tested, and the existing literature on CO hydrogenation over Pt–Co catalysts was reviewed. It is found that the catalysts produce mainly methanol in the Pt-rich composition range and mainly hydrocarbons (and to a modest extent higher alcohols) in the Co-rich composition range. The transition between the two types of behavior occurs in a narrow composition range around a molar Pt:Co ratio of 1:1.

In this work, we examine the thermochemistry of methanol synthesis intermediates using density functional theory (DFT) and analyze the methanol synthesis reaction network using a steady-state micro-kinetic model. The energetics for methanol synthesis over Zn-terminated ZnO (0 0 0 1) are obtained from DFT calculations using the RPBE and BEEF-vdW functionals. The energies obtained from the two functionals are compared and it is determined that the BEEF-vdW functional is more appropriate for the reaction. The BEEF-vdW energetics are used to construct surface phase diagrams as a function of CO, H 2 O, and H 2 chemical potentials. The computed binding energies along with activation barriers from literature are used as inputs for a mean-field micro-kinetic model for methanol synthesis including the CO and CO 2 hydrogenation routes and the water–gas shift reaction. The kinetic model is used to investigate the methanol synthesis rate as a function of temperature and pressure. The results show qualitative agreement with experiment and yield information on the optimal working conditions of ZnO catalysts.

Production of higher alcohols directly from synthesis gas is an attractive chemical process due to the high value of alcohols as fuel blends and the numerous possibilities for production of synthesis gas. Despite years of research the industrial viability of such a process is severely limited due to lack of suitable catalysts. In this work we contribute to an understanding why it has been difficult to find transition-metal higher alcohol catalysts, and point to possible strategies for discovering new active and selective catalysts. Our analysis is based on extensive density functional theory calculations to determine the energetics of ethanol formation on a series of metal (211) surfaces. The energetic information is used to construct a mean-field micro-kinetic model for the formation of ethanol via CHx–CO coupling. The kinetic model is used along with a descriptor-based analysis to gain insight into the fundamental factors determining activity and selectivity on transition-metal surfaces.

Synthesis gas (CO + H2) conversion is a promising route to converting coal, natural gas, or biomass into synthetic liquid fuels. Rhodium has long been studied as it is the only elemental catalyst that has demonstrated selectivity to ethanol and other C2+ oxygenates. However, the fundamentals of syngas conversion over rhodium are still debated. In this work a microkinetic model is developed for conversion of CO and H2 into methane, ethanol, and acetaldehyde on the Rh (211) and (111) surfaces, chosen to describe steps and close-packed facets on catalyst particles. The model is based on DFT calculations using the BEEF-vdW functional. The mean-field kinetic model includes lateral adsorbate-adsorbate interactions, and the BEEF-vdW error estimation ensemble is used to propagate error from the DFT calculations to the predicted rates. The model shows the Rh(211) surface to be ∼6 orders of magnitude more active than the Rh(111) surface, but highly selective toward methane, while the Rh(111) surface is intrinsically selective toward acetaldehyde. A variety of Rh/SiO2 catalysts are synthesized, tested for catalytic oxygenate production, and characterized using TEM. The experimental results indicate that the Rh(111) surface is intrinsically selective toward acetaldehyde, and a strong inverse correlation between catalytic activity and oxygenate selectivity is observed. Furthermore, iron impurities are shown to play a key role in modulating the selectivity of Rh/SiO2 catalysts toward ethanol. The experimental observations are consistent with the structure-sensitivity predicted from theory. This work provides an improved atomic-scale understanding and new insight into the mechanism, active site, and intrinsic selectivity of syngas conversion over rhodium catalysts and may also guide rational design of alloy catalysts made from more abundant elements.

Bifunctional coupling of two different catalytic site types has often been invoked to explain experimentally observed enhanced catalytic activities. We scrutinize such claims with generic scaling-relation-based microkinetic models that allow exploration of the theoretical limits for such a bifunctional gain for several model reactions. For sites at transition-metal surfaces, the universality of the scaling relations between adsorption energies largely prevents any improvements through bifunctionality. Only the consideration of systems that involve the combination of different materials, such as metal particles on oxide supports, offers hope for significant bifunctional gains.

Gold is known to become significantly more catalytically active as its particle size is reduced, and other catalysts are also known to exhibit finite-size effects. To understand the trends related to finite-size effects, we have used density functional theory to study adsorption of representative adsorbates, CO and O, on the late transition metals Co, Ni, Cu, Ir, Pd, Ag, Rh, Pt and Au. We studied adsorption energies and geometries on 13-atom clusters and compared them to the fcc(111) and fcc(211) crystal facets. In all cases, adsorbates were found to bind significantly more strongly to the 13-atom clusters than to the extended surfaces. The binding strength of both adsorbates were found to correlate very strongly with the average coordination number of the metal atoms to which the adsorbate binds, indicating that the finite-size effects in bonding are not specific to gold.

Density functional theory (DFT) and ab initio thermodynamics are applied in order to investigate the most stable surface and subsurface terminations of Mo{sub 2}C(001) as a function of chemical potential and in the presence of syngas. The Mo-terminated (001) surface is then used as a model surface to evaluate the thermochemistry and energetic barriers for key elementary steps in syngas reactions. Adsorption energy scaling relations and Broensted-Evans-Polanyi relationships are established and used to place Mo{sub 2}C into the context of transition metal surfaces. The results indicate that the surface termination is a complex function of reaction conditions and kinetics. It is predicted that the surface will be covered by either C{sub 2}H{sub 2} or O depending on conditions. Comparisons to transition metals indicate that the Mo-terminated Mo{sub 2}C(001) surface exhibits carbon reactivity similar to transition metals such as Ru and Ir, but is significantly more reactive towards oxygen.

The growing demands for reagentless hydrogen peroxide (H2O2) and β-nicotinamide adenine dinucleotide (β-NADH) sensors from food, pharmaceutical, chemical, and biochemical fields have stimulated extensive research interest on nano-engineered Pd. In this paper, Pd/carbon composite nanofibers were prepared by electrodepositing Pd onto electrospun carbon nanofibers to act as a catalyst toward the electrocatalytic redox reactions of H2O2 and β-NADH. The morphology of nano-engineered Pd was controlled by selectively adjusting the electrodeposition time and potential. Scanning electron microscopy and transmission electron microscopy results showed that nanocactus- and nanoflower-like Pd depositions were obtained on the surface of carbon nanofibers. Electrocatalytic analysis demonstrated a high electrocatalytic activity of the composite nanofibers for the redox of H2O2 and oxidation of β-NADH.

RAFFINAGE:INGENIERIE+RHE:YSC:UOL; Research on zeolite-catalyzed methanol-to-hydrocarbons (MTH) conversion has long been concerned with the mechanism of the reaction between methanol and alkenes. Two pathways have been debated: (1) the stepwise, proceeding through a surface-methoxy intermediate and (2) the concerted, in which the alkenes react directly with methanol. This work addresses the debate through micro-kinetic modeling based on density functional theory calculations of both pathways, as well as experiments employing temporal analysis of products to investigate the kinetics of the stepwise pathway for a series of alkenes in HZSM-22 zeolite. The model predicts the stepwise pathway to prevail at typical MTH reaction temperatures, due to a higher entropy loss in the concerted as compared to the stepwise pathway. The entropy difference results from intermediate release of water in the stepwise pathway. These findings lead us to suggest that the stepwise pathway should also be considered when modeling MTH conversion in other zeolites. (c) 2014 Elsevier Inc. All rights reserved.