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Metal phosphide screening for ethane dehydrogenation.

We use high-frequency mobile phone movement data and quick-release administrative data from Georgia to examine how time at home during the COVID-19 pandemic is related to child maltreatment referrals. Findings show that referrals plummeted by 58% relative to previous years, driven by fewer referrals from education personnel. After this initial decline, however, each 15 minutes at home was associated with an increase in referrals of material neglect by 3.5% and supervisory neglect by 1%. Our results describe how children have fared during the initial wave of the pandemic, and the results have long-term implications for child development and well-being.

Immobilization of single metal atoms on a solid host opens numerous possibilities for catalyst designs. If that host is a two-dimensional sheet, sheet curvature becomes a design parameter potentially complementary to host and metal composition. Here, we use a combination of density functional theory calculations and microkinetic modeling to compare the mechanisms and kinetics of formic acid decomposition and formation, chosen for their relevance as a potential hydrogen storage medium, over single Ru atoms anchored to pyridinic nitrogen in a planar graphene flake (RuN4-G) and curved carbon nanotube (RuN4-CNT). Activation barriers are lowered and the predicted turnover frequencies are increased over RuN4-CNT relative to RuN4-CNT. The results highlight the potential of curvature control as a means to achieve high performance and robust catalysts.

Plasma-assisted catalysis is an emerging technology for the atmospheric pressure and low bulk gas temperature synthesis of ammonia from molecular nitrogen and hydrogen. Direct evidence for plasma-i...

The arrangement of framework Al heteroatoms in zeolite lattices influences the energetics of proton siting and thus the distribution of protons among crystallographically-distinct oxygen atoms and void environments, as shown here for H-CHA zeolites of fixed bulk Al content (Si/Al = 15) but varying amounts of proximal Al sites (Al-O-(Si-O)x-Al, x = 1,2) in a six-membered ring (6-MR). Stretching vibrations of Bronsted acidic OH groups give rise to composite infrared (IR) features whose shapes and integrated intensities are invariant with temperature (448–748 K) on H-CHA zeolites containing predominantly 6-MR isolated Al sites. In sharp contrast, IR peak shapes change and integrated OH areas decrease reversibly with increasing temperature on H-CHA zeolites containing 6-MR paired Al sites (quantified by Co2+ titration). Periodic density functional theory (DFT) calculations show that the four distinct OH configurations at an isolated Al site exhibit distinct vibrational frequencies and molar absorptivities, but are isoenergetic (within 10 kJ mol−1) rendering equilibrium proton populations and OH IR spectra insensitive to temperature. In contrast, paired proton configurations differ in energy by up to ~40 kJ mol−1, with higher energy configurations having lower oscillator strengths; as a result, increasing temperatures shift proton populations toward higher energy configurations that result in a decrease in composite OH IR peak areas. First-order rate constants of protolytic propane cracking (748 K, per H+) are ~12× higher on paired than isolated protons, despite similar apparent activation energies, reflecting apparent activation entropies that are less negative at paired proton sites. Propane cracking rates (748 K, per g) decrease linearly with Na+ content upon partial titration of H+ sites in CHA zeolites containing 6-MR isolated sites, but decrease more strongly at low Na+ contents on CHA zeolites containing 6-MR paired sites, consistent with experimental OH IR spectra and DFT calculations evincing the preferential exchange of Na+ at 6-MR paired Al sites. These conclusions about proton proximity, founded on the interrogation of well-defined and characterized CHA zeolite samples by experiment and theory, provide new insights regarding its consequences for the temperature sensitivity of OH IR spectra and protolytic alkane cracking rates in Bronsted acidic zeolites. The finding that proximal proton sites accelerate high-temperature alkane activation turnovers via non-polar transition states represents a mechanistically distinct extension of our prior reports, wherein proximal protons accelerate low-temperature alkanol dehydration turnovers via polar transition states stabilized by hydrogen bonding interactions, heralding the catalytic diversity among zeolites containing different framework Al arrangements.

At low temperatures (

We investigated the influence of P incorporation into a Ni catalyst on ethane dehydrogenation (EDH). Density functional theory calculations on model Ni(111) and Ni2P(001) surfaces reveal that surfa...

We explore the consequences of non-thermal plasma activation on product yields in catalytic ammonia synthesis, a reaction that is equilibrium-limited at elevated temperatures. We employ a minimal microkinetic model that incorporates the influence of plasma activation on N2 dissociation rates to predict NH3 yields into and across the equilibrium-limited regime. NH3 yields are predicted to exceed bulk thermodynamic equilibrium limits on materials that are thermal-rate-limited by N2 dissociation. In all cases, yields revert to bulk equilibrium at temperatures at which thermal reaction rates exceed plasma-activated ones. Beyond-equilibrium NH3 yields are observed in a packed bed dielectric-barrier-discharge reactor and exhibit sensitivity to catalytic material choice in a way consistent with model predictions. The approach and results highlight the opportunity to exploit synergies between non-thermal plasmas and catalysts to affect transformations at conditions inaccessible through thermal routes.

Ammonia oxidation is the heart of the Ostwald process and is important in emissions control. Catalytic behaviors are a function of conditions and are observed to vary across the platinum group metals (PGMs) Pt, Pd, Rh. Here, we combine density functional theory computations and microkinetic modeling to rationalize these dependencies. We compute reactions over model (2 1 1) and (1 1 1) surfaces of PGMs. Binding energies are similar on Pd and Pt and generally greater on Rh, while activation energies vary across all metals. Rates on (2 1 1) surfaces are greater than (1 1 1) surfaces. The stepped Pt is most active and stepped Rh most selective to N2 at ammonia slip conditions, while at Ostwald process conditions, stepped Pd is most active and stepped Pt most selective to NO. Degree of rate and selectivity control analysis provides insights into the reactions limiting performance of PGMs. Both activation barriers and surface coverages influence rates and selectivities.

We combine experiment and theory to investigate the cooperation or competition between organic and inorganic structure-directing agents (SDAs) for occupancy within microporous voids of chabazite (CHA) zeolites and to rationalize the effects of SDA siting on biasing the framework Al arrangement (Al-O(-Si-O)x-Al, x = 1-3) among CHA zeolites of essentially fixed composition (Si/Al = 15). CHA zeolites crystallized using mixtures of TMAda+ and Na+ contain one TMAda+ occluded per cage and Na+ co-occluded in an amount linearly proportional to the number of 6-MR paired Al sites, quantified by Co2+ titration. In contrast, CHA zeolites crystallized using mixtures of TMAda+ and K+ provide evidence that three K+ cations, on average, displace one TMAda+ from occupying a cage and contain predominantly 6-MR isolated Al sites. Moreover, CHA crystallizes from synthesis media containing more than 10-fold higher inorganic-to-organic ratios with K+ than with Na+ before competing crystalline phases form, providing a route to decrease the amount of organic SDA needed to crystallize high-silica CHA. Density functional theory calculations show that differences in the ionic radii of Na+ and K+ determine their preferences for siting in different CHA rings, which influences their energy to co-occlude with TMAda+ and stabilize different Al configurations. Monte Carlo models confirm that energy differences resulting from Na+ or K+ co-occlusion promote the formation of 6-MR and 8-MR paired Al arrangements, respectively. These results highlight opportunities to exploit using mixtures of organic and inorganic SDAs during zeolite crystallization in order to more efficiently use organic SDAs and influence framework Al arrangements.

Background Recent research has shown that the likelihood of children experiencing intergenerational, upward income mobility depends on the community in which they are raised. Whether parents consider their children's economic chances in their parenting decisions, however, is not well understood. Objective To examine the relationship between county-level income mobility–distinct from income inequality and poverty–and child maltreatment. Participants and setting Administrative data from the National Child Abuse and Neglect Data System: Child File for 2406 counties were merged with measures of intergenerational income mobility from Chetty et al. (2014a), including the probability that a child born in the bottom quintile of the national income distribution reaches the top quintile by age thirty. Methods Weighted least squares analyses were used to empirically estimate the relationship between intergenerational income mobility and child maltreatment report rates. Maltreatment reports were also divided into subgroups by age and metropolitan status. Results Counties where children have a greater chance of moving up the income ladder have lower child maltreatment report rates, independent from income inequality and poverty rates. This relationship is consistent across all child ages (0–17). The relationship between upward income mobility and substantiated child maltreatment is also negatively correlated among non-metropolitan counties. Conclusions Children experience a lower risk for maltreatment if they are more likely to move up the income ladder in adulthood. Macroeconomic factors and policies that reduce income inequality and enhance economic mobility are likely to prevent child maltreatment.

Background Although there is evidence of economic disparities in parents' financial and time investments in children, little existing empirical work has considered the disparities in authoritarian parenting, a risk for child maltreatment. Similarly, existing research has largely focused on the role of objective markers of socioeconomic status (SES), although perceived subjective social status (SSS) may be equally powerful in shaping disparities in parenting behaviors. Data This article draws on 30 years of General Social Survey data to examine the association between objective socioeconomic status and subjective social status and parents' endorsement of authoritarian parenting practices. Methods We model the association between parents' SES and SSS and approval of authoritarian parenting practices estimated with odds ratios from logistic regressions and examine parental race as a potential moderator. Results We find that SES and SSS are both associated with increased odds of endorsing authoritarian parenting, that SSS-based disparities are independent of SES, and that white parents' parenting may be more influenced by both SES and SSS than Black parents' parenting. Conclusions This work provides evidence that SES not only drives gaps in parental investments in children, but also gaps in their endorsement of authoritarian parenting. This is important because authoritarian parenting is not only directly associated with adverse outcomes for children, but is also associated with an increased risk for child maltreatment. It also expands the existing literature by showing that subjective measures of social status are important and distinct from objective measures of SES, and that these associations vary by race.

Heterogeneous catalysts coupled with non-thermal plasmas (NTP) are known to achieve reaction yields that exceed the contributions of the individual components. Rationalization of the enhancing potential of catalysts, however, remains challenging because the background contributions from NTP or catalysts are often non-negligible. Here, we first demonstrate platinum (Pt)-catalyzed nitrogen (N2) oxidation in a radio frequency plasma afterglow at conditions at which neither catalyst nor plasma alone produces significant concentrations of nitric oxide (NO). We then develop reactor models based on reduced NTP- and surface-microkinetic mechanisms to identify the features of each that lead to the synergy between NTP and Pt. At experimental conditions, NTP and thermal catalytic NO production are suppressed by radical reactions and high N2 dissociation barrier, respectively. Pt catalyzes NTP-generated radicals and vibrationally excited molecules to produce NO. The model construction further illustrates that the optimization of productivity and energy efficiency involves tuning of plasma species, catalysts properties, and the reactor configurations to couple plasma and catalysts. These results provide unambiguous evidence of synergism between plasma and catalyst, the origins of that synergy for N2 oxidation, and a modeling approach to guide material selection and system optimization., Heterogeneous catalysts coupled with non-thermal plasma are known to achieve higher reaction yields, but disentangling the gain achieved by the individual components remains challenging. Here, NO is produced from N2 and O2 in a plasma-catalytic reactor at conditions at which neither catalyst nor plasma alone is productive, providing unambiguous evidence of plasma-catalyst synergy.

Structure-directing agents (SDAs) influence not only zeolite structures but also the distribution of Al on zeolite lattices. The origins of this Al-directing influence are not well understood. Here, we use first-principles methods to explore the interactions between an N,N,N-trimethyl-1-adamantyl ammonium (TMAda+) OSDA and framework [AlO2]− in the dilute limit within an SSZ-13 lattice. Molecular dynamics and geometric and electronic structures illuminate the rotational freedom of TMAda+ OSDA within an SSZ-13 cage, the primarily electrostatic nature of its interactions with [AlO2]−, the effect of the solvent in screening the electrostatics, and the relative invariance of that interaction within a subset of Al locations within the same SSZ-13 major cavity. These results indicate that the Al distribution patterns observed in relevant SSZ-13 samples are not consequences of the location bias for Al substitution within a single TMAda+–cage complex but likely involve long-range electrostatics among multiple TMAd...

Chemical kinetic modeling in heterogeneous catalysis is advancing in its ability to provide qualitatively or even quantitatively accurate prediction of real-world behavior because of new advances i...

Pd-catalyzed reduction has emerged as a promising treatment strategy to remove the recalcitrant disinfection byproduct N-nitrosodimethylamine (NDMA). However, the reaction pathways remain unexplored, and questions remain about how water solvent influences NDMA reduction mechanisms and selectivity. Here, we compute the energies and barriers of all relevant elementary steps in NDMA reduction by H2 on Pd(111) using density functional theory. We further calculate water-assisted H-shuttling for all hydrogenation reactions explicitly and include water solvation for all elementary reactions implicitly. We parametrize microkinetic models to predict product formation rates and selectivities over a wide range of NDMA concentrations. We show that H2O-mediated H-shuttling lowers the reaction barriers for all hydrogenation reactions involved in NDMA reduction while implicit solvation has negligible impact on the reaction and activation energies. We further conduct batch experiments with SiO2-supported Pd nanoparticles and compare them with the microkinetic models. The predicted rates, selectivity, and apparent activation energy from the model parametrized with both explicit H2O-mediated H-shuttling and implicit solvation correspond well with experimental observations. Models that ignore water as an H-shuttle or solvent fail to recover the experimental rates and apparent activation energy. We identified the rate-determining steps of the reaction and show the reaction flow pathways of the complicated reaction network. Finally, we demonstrate that water-mediated H-shuttling changes the rate-determining steps and reaction flows of elementary reactions.

Plasma-assisted catalysis is the process of electrically activating gases in the plasma-phase at low temperatures and ambient pressure to drive reactions on catalyst surfaces. Plasma-assisted catalytic processes combine conventional heterogeneous surface reactions, homogeneous plasma phase reactions, and coupling between plasma-generated species and the catalyst surface. Herein, we perform kinetically controlled ammonia synthesis measurements in a dielectric barrier discharge (DBD) plasma-assisted catalytic reactor. We decouple contributions due to plasma phase reactions from the overall plasma-assisted catalytic kinetics by performing plasma-only experiments. By varying the gas composition, temperature, and discharge power, we probe how macroscopic reaction conditions affect plasma-assisted ammonia synthesis on three different γ-alumina-supported transition metal catalysts (Ru, Co, and Ni). Our experiments indicate that the overall reaction and plasma-phase reaction are first-order in both N2 and H2. In ...

Nonthermal plasma-driven catalysis is an emerging subfield of heterogeneous catalysis that is particularly promising for the chemical transformation of hard-to-activate molecules (e.g., N2, CO2, CH4). In this Review, we illustrate this promise of plasma-enhanced catalysis, focusing on the ammonia synthesis and methane dry reforming reactions, two reactions that have received wide attention and that illustrate the potential for plasma excitations to mitigate kinetic and thermodynamic obstacles to chemical conversions. We highlight how plasma activation of reactants can provide access to overall reaction rates, conversions, product yields, and/or product distributions unattainable by thermal catalysis at similar temperatures and pressures. Particular emphasis is given to efforts aimed at discerning the underlying mechanisms at play in these systems. We discuss opportunities for and challenges to the advancement of the field.

The effects of sulfur poisoning on Cu-SSZ-13 zeolites, used commercially for the selective catalytic reduction (SCR) of nitrogen oxides (NOX) with ammonia, were studied by exposing model Cu-zeolite powder samples to dry SO2 and O2 streams at 473 and 673 K, and then analyzing the surface intermediates formed using spectroscopic and kinetic assessments. Model Cu-SSZ-13 zeolites were synthesized to contain distinct Cu active site types, predominantly either divalent Cu2+ ions exchanged at proximal framework Al (Z2Cu), or monovalent CuOH+ complexes exchanged at isolated framework Al (ZCuOH). SCR turnover rates (473 K, per Cu) decreased linearly with increasing S content to undetectable values at equimolar S:Cu ratios, consistent with poisoning of each Cu site with one SO2-derived intermediate. Cu and S K-edge X-ray absorption spectroscopy and density functional theory calculations were used to identify the structures and binding energies of different SO2-derived intermediates at Z2Cu and ZCuOH sites, revealing that bisulfates are particularly low in energy, and residual Bronsted protons are liberated at Z2Cu sites as bisulfates are formed. Molecular dynamics simulations also show that Cu sites bound to one HSO4− are immobile, but become liberated from the framework and more mobile when bound to two HSO4−. These findings indicate that Z2Cu sites are more resistant to SO2 poisoning than ZCuOH sites, and are easier to regenerate once poisoned.

Ammonia oxidation is operated at different temperatures over Pt catalysts of different structures to recover different products. In this work, we elucidate the dependency of ammonia oxidation rates and selectivities on both Pt structure and temperature. We perform density functional theory (DFT) computations to compare the reaction and activation energies of elementary reactions on Pt(211) and Pt(111). We develop a microkinetic model parametrized with the DFT results. We show that barriers to product formation are lower on stepped Pt than on terrace, leading to a much higher step rate at low temperature to selectively oxidize ammonia to nitrogen. At high temperature, however, both step and terrace perform comparably in rate to selectively produce nitric oxide. While N2 is always the thermodynamic product, relative N and O coverages interact to make NO the kinetic product at high temperature. The predicted rate and selectivity are consistent with experiments. We further show rate-controlling steps on the t...

Plasma catalysis is an emerging new technology for the electrification and downscaling of NH3 synthesis. Increasing attention is being paid to the optimization of plasma catalysis with respect to the plasma conditions, the catalyst material, and their mutual interaction. In this work we use microkinetic models to study how the total conversion process is impacted by the combination of different plasma conditions and transition metal catalysts. We study how plasma-generated radicals and vibrationally excited N-2 (present in a dielectric barrier discharge plasma) interact with the catalyst and impact the NH3 turnover frequencies (TOFs). Both filamentary and uniform plasmas are studied, based on plasma chemistry models that provided plasma phase speciation and vibrational distribution functions. The Langmuir-Hinshelwood reaction rate coefficients (i.e., adsorption reactions and subsequent reactions among adsorbates) are determined using conventional scaling relations. An additional set of Eley-Rideal reactions (i.e., direct reactions of plasma radicals with adsorbates) was added and a sensitivity analysis on the assumed reaction rate coefficients was performed. We first show the impact of different vibrational distribution functions on the catalytic dissociation of N-2 and subsequent production of NH3, and we gradually include more radical reactions, to illustrate the contribution of these species and their corresponding reaction pathways. Analysis over a large range of catalysts indicates that different transition metals (metals such as Rh, Ni, Pt, and Pd) optimize the NH(3)TOFs depending on the population of the vibrational levels of N-2. At higher concentrations of plasma-generated radicals, the NH3 TOFs become less dependent on the catalyst material, due to radical adsorptions on the more noble catalysts and Eley-Rideal reactions on the less noble catalysts.

In this work, we compare the poisoning (inhibiting) effects of CO and C3H6 on H2 permeation across Pd77Ag23 and Pd foil membranes under identical conditions to investigate the influence of alloying Pd with Ag on its sensitivity to poisoning by these contaminants. H2 flux measurements show that H2 permeation across Pd77Ag23 is significantly more sensitive to CO poisoning, but less sensitive to C3H6 poisoning, than Pd. CO inhibits H2 permeation across both Pd and Pd77Ag23 primarily by adsorbing on the membrane surfaces—exclusively on Pd atoms—and by blocking H2 dissociative adsorption at these sites. Density functional theory calculations show that CO binds more strongly to the Pd3Ag(1 1 1) surface, both bulk-terminated and Pd-terminated, than to the Pd(1 1 1) surface. Thus, H2 permeation across Pd77Ag23 is more sensitive to CO poisoning than Pd because there is a higher coverage of CO and fewer available sites for H2 adsorption than on Pd. C3H6 poisoning of H2 permeation across Pd77Ag23 and Pd is much more complex than CO poisoning and involves decomposition of C3H6 and deposition of at least four different carbonaceous species. In contrast to the CO site blocking mechanism, C3H6 poisoning of Pd and Pd77Ag23 is primarily an electronic effect where the carbonaceous deposits modify the electronic structure of the Pd77Ag23 and Pd surfaces making them less active for H2 dissociation. Thus, even though Pd77Ag23 accumulates ∼ 10 × more carbon during C3H6 exposure than Pd, due to the higher carbon solubility of Pd77Ag23, the Pd77Ag23 surface is more resistant to deactivation from carbon deposits than Pd. This work highlights the importance of benchmarking, which is surprisingly rare in the Pd-alloy membrane literature, but is critical for understanding alloying effects.

Plasma catalysis is gaining increasing interest for various gas conversion applications, such as CO2 conversion into value-added chemicals and fuels, CH4 activation into hydrogen, higher hydrocarbons or oxygenates, and NH3 synthesis. Other applications are already more established, such as for air pollution control, e.g. volatile organic compound remediation, particulate matter and NOx removal. In addition, plasma is also very promising for catalyst synthesis and treatment. Plasma catalysis clearly has benefits over ‘conventional’ catalysis, as outlined in the Introduction. However, a better insight into the underlying physical and chemical processes is crucial. This can be obtained by experiments applying diagnostics, studying both the chemical processes at the catalyst surface and the physicochemical mechanisms of plasma-catalyst interactions, as well as by computer modeling. The key challenge is to design cost-effective, highly active and stable catalysts tailored to the plasma environment. Therefore, insight from thermal catalysis as well as electro- and photocatalysis is crucial. All these aspects are covered in this Roadmap paper, written by specialists in their field, presenting the state-of-the-art, the current and future challenges, as well as the advances in science and technology needed to meet these challenges.

Background The geographic location of birth has implications for low-income children's upward economic mobility, as Chetty, Hendren, Kline, and Saez (2014) found in an examination of millions of income tax records from each county in the US. Additional work indicates that low income children in higher economic mobility counties have higher language scores and fewer behavioral problems (Donnelly et al., 2017). However, the processes by which the geography of opportunity influences parenting are less well-understood. Objective This study examines whether living in higher intergenerational mobility counties is associated with less harsh parenting, material hardship, household violence and substance use, and low child supervision – parenting behaviors that increase the risk for child maltreatment – for low-income families. Data Data come from the Fragile Families and Child Wellbeing Study, a longitudinal birth cohort of low income families in 20 cities in the U.S (N ~, 2841; 76% lower (household income of $41,994 or less) and 24% higher-income) linked to county level data on intergenerational mobility from the Equality of Opportunity Project. Methods We estimate OLS and Linear Probability regressions of the association between (1) exposure to county-level intergenerational mobility and (2) number of waves of exposure to county intergenerational mobility 1 standard deviation above the mean and maternal parenting behaviors. Results A 1 standard deviation increase in county level intergenerational mobility is associated with decreases in harsh parenting, but not indicators of neglect. Longer exposure to high intergenerational mobility areas was associated with decreased maternal harsh parenting and risk for child maltreatment. Conclusions For low-income children, higher intergenerational mobility is associated with decreased risk of harsh parenting, particularly at younger ages, as is longer exposure to high intergenerational mobility areas. That lower-income families are less likely to live in economically mobile geographies may exacerbate inequalities among income groups.

ConspectusCopper-exchanged chabazite (Cu-CHA) zeolites are catalysts used in diesel emissions control for the abatement of nitrogen oxides (NOx) via selective catalytic reduction (SCR) reactions wi...

The combination of catalysis and nonthermal plasma holds promise for enabling difficult chemical conversions. The possible synergy between both depends strongly on the nature of the reactive plasma species and the catalyst material. In this paper, we show how vibrationally excited species and plasma-generated radicals interact with transition metal catalysts and how changing the catalyst material can improve the conversion rates and product selectivity. We developed a microkinetic model to investigate the impact of vibrational excitations and plasma-generated radicals on the nonoxidative coupling of methane over transition metal surfaces. We predict a significant increase in ethylene formation for vibrationally excited methane. Plasma-generated radicals have a stronger impact on the turnover frequencies with high selectivity toward ethylene on noble catalysts and mixed selectivity on non-noble catalysts. In general, we show how the optimal catalyst material depends on the desired products as well as the plasma conditions.

The relative proximity of Al atoms substituted in zeolite lattices is an important parameter that influences both hydrothermal stability and catalytic function, but the underlying chemistry that governs Al site proximity is not well understood. Here, we examine relationships between exchanged countercations and different Al–Al arrangements in a chabazite (SSZ-13) zeolite lattice. We report periodic supercell density functional theory (DFT) calculations for structures and energies of SSZ-13 lattices with systematically enumerated and varied Al–Al proximity, both charge-uncompensated and charge-compensated by either proton pairs (H+/H+) or divalent copper cations (Cu2+). Al–Al interactions are electrostatically repulsive without charge compensation, but the relative energies of certain Al–Al site arrangements change upon compensation by countercations. Al–Al interactions are uniformly attractive when compensated by H+/H+ pairs but are attractive at long and repulsive at short Al–Al distances when compensate...

Metal-exchanged zeolites are common catalysts and adsorbents, but the relationship between their macroscopic composition (Si:Al and M:Al ratios) and microscopic details of exchange site composition and reactivity are difficult to infer. Here we address this general problem for Fe exchange in an SSZ-13 zeolite. We report periodic supercell density functional theory (DFT) calculations for the structures and energies of candidate Fe-exchange sites, including monomeric and dimeric Fe species with formal oxidation states ranging from 2+ to 5+ and charge-compensated by arbitrary combinations of framework Al, oxo, and hydroxyl ligands plus H2O adsorbates. We show that the chemical identity of an Fe-exchange site depends strongly on the number and proximity of framework Al and, through first-principles thermodynamics models, that these sites evolve in distinct ways as a function of external treatment conditions. By placing the results on a common energy reference and combining with simulated Al distributions, we ...

A combination of ab initio calculations and classical molecular dynamics simulations was used to calculate the free energy of reacting an aprotic heterocyclic anion ionic liquid with CO2. The overall reaction was broken into a series of steps using a thermodynamic cycle to calculate the free energy of the gas phase reaction and the free energy contributions of solvation environment effects, which make comparable contributions to the total free energy of reaction. CO2 absorption isotherms that agree reasonably well with experimental data were calculated using a derived expression for the free energy of reaction as a function of temperature, pressure, and the extent of reaction.

Ionic liquids (ILs) have gained considerable attention in recent years as CO2-reactive solvents that could be used to improve the economic efficiency of industrial-scale CO2 separations. Researchers have demonstrated that IL physical and chemical properties can be optimized for a given application through chemical functionalization of both cations and anions. The tunability of ILs presents both a great potential and a significant challenge due to the complex chemistries and the many ways in which ILs can be made to react with CO2. However, computer simulations have demonstrated great potential in understanding the behavior of ILs from the underlying molecular interactions. In the present review, we examine how computer simulations have aided in the design of ILs that chemically bind CO2. We present the historical development of CO2-reactive ILs while highlighting the insights provided by molecular modeling which aided in understanding IL behavior to further experimental findings. We also provide a brief d...

Correlations between the energies of elementary steps limit the rates of thermally catalysed reactions at surfaces. Here, we show how these limitations can be circumvented in ammonia synthesis by coupling catalysts to a non-thermal plasma. We postulate that plasma-induced vibrational excitations in N2 decrease dissociation barriers without influencing subsequent reaction steps. We develop a density-functional-theory-based microkinetic model to incorporate this effect, and parameterize the model using N2 vibrational excitations observed in a dielectric-barrier-discharge plasma. We predict plasma enhancement to be particularly great on metals that bind nitrogen too weakly to be active thermally. Ammonia synthesis rates observed in a dielectric-barrier-discharge plasma reactor are consistent with predicted enhancements and predicted changes in the optimal metal catalyst. The results provide guidance for optimizing catalysts for application with plasmas. Plasma catalysis holds promise for overcoming the limitations of conventional catalysis. Now, a kinetic model for ammonia synthesis is reported to predict optimal catalysts for use with plasmas. Reactor measurements at near-ambient conditions confirm the predicted catalytic rates, which are similar to those obtained in the Haber–Bosch process.

Adsorbate free energies are fundamental quantities in the microkinetic modeling of catalytic reactions. In first-principles modeling, finite-temperature free energies are generally obtained by combining density functional theory energies with standard approximate models, such as the harmonic oscillator, the hindered translator, or the two-dimensional ideal gas. In this work, we calculate accurate free energies directly from first-principles potential energy surfaces combined with exact quantum mechanical solutions for the translational energy states to benchmark the reliability of common approximations. Through a series of case studies of monatomic adsorbates on metal surfaces, we show that no one free energy model performs satisfactorily in all cases. Moreover, even combinations of different approximations sometimes deviate significantly from the free energies calculated by our first-principles approach. Using observations from these case studies, we discuss how a full quantum mechanical approach can be ...

The ability of metal-exchanged zeolites to chemisorb small gas molecules is key to their performance as heterogeneous catalysts and gas-separating agents. Here, we propose and evaluate an ab initio potential of mean force (PMF) method for computing adsorption free energies of representative small molecules to Cu-exchanged sites in SSZ-13 zeolite. We show that Cu ions are mobilized by adsorbates and, as a result, computed free energies are significantly more negative than those obtained from a conventional harmonic oscillator model. PMF-derived free energies are consistent with available experiment and, in many cases, with a dynamics-based quasi-harmonic analysis (QHA). The PMF approach avoids the artificial partitioning of degrees of freedom intrinsic to the QHA. On the basis of the PMF results, we propose a simple correlation to estimate free energies from computed adsorption energies and gas-phase entropies.

Heterogeneous catalysis is essential to industrial chemical processes, from those that transform petroleum into fuels and chemicals to those, like the Haber Bosch process, that create fertilizers to feed the planet. Catalysis science has revealed that the most common heterogeneous catalysts, which include the later members of the d block transition metals, present a tableau of reactivity limited by intrinsic correlations between the various reaction steps that make up a surface catalytic reaction. Coupling of heterogeneous catalysts with non-thermal plasmas offers the potential to break these constraints. While empirical evidence suggests that such combinations can enhance apparent catalytic function relative to plasmas or catalysts alone, the absence of basic models to rationalize this behavior and guide material and plasma selection has limited progress. In this talk I will describe our recent work to bridge this gap, focusing on nitrogen reduction (ammonia synthesis) and nitrogen oxidation (NO synthesis) as two different approaches to realize chemical transformations that are difficult or impossible to achieve through catalytic routes alone.