This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book deals with heterogeneous catalysis and allows several leading practitioners to describe examples of materials and processes in heterogeneous catalysis under investigation by nuclear magnetic resonance (NMR) techniques. Modern solid-state NMR makes it possible to detect signals from distinguishable sites in molecules and materials and to monitor the connectivities, correlations, and dynamics of these sites. Furthermore, NMR spectroscopy is essentially noninvasive and can be carried out in the presence of gases or liquids over a wide range of temperatures and pressures. While the principal use of NMR spectroscopy is to obtain information about the chemical environment of elements in catalysts or species adsorbed on catalysts, the technique can also be used to characterize atomic and molecular motions. The types of information that may be derived from NMR signals include site identification and intersite correlations.
Synthesis of high surface-area colloidal assemblies of calixarene-phosphine-capped nanoporous gold with a remarkably high surface-to-volume ratio is reported.
Previous experimental work has demonstrated that variations in the confinement of <em>n</em>-butane at Brønsted acid sites due to changes in zeolite framework structure strongly affect the apparent and intrinsic enthalpy and entropy of activation for cracking and dehydrogenation. Quantum chemical calculations have provided good estimates of the intrinsic enthalpies and entropies of activation extracted from experimental rate data for MFI, but extending these calculations to less confining zeolites has proven challenging, particularly for activation entropies. Herein, we report our efforts to develop a theoretical model for the cracking and dehydrogenation of n-butane occurring in a series of zeolites containing 10-ring channels and differing in cavity size (TON, FER, -SVR, MFI, MEL, STF, and MWW). Here, we combine a QM/MM approach to calculate intrinsic and apparent activation parameters, with thermal corrections to the apparent barriers obtained from configurational-bias Monte Carlo simulations, to account for configurational contributions due to global motions of the transition state. We obtain good agreement between theory and experiment for all activation parameters for central cracking in all zeolites. For terminal cracking and dehydrogenation, good agreement between theory and experiment is found only at the highest confinements. Experimental activation parameters, especially those for dehydrogenation, tend to increase with decreasing confinement. This trend is not captured by the theoretical calculations, such that deviations between theory and experiment increase as confinement decreases. We propose that, because transition states for dehydrogenation are later than those for cracking, relative movements between the fragments produced in the reaction become increasingly important in the less confining zeolites.
The electrochemical reduction of carbon dioxide (CO<sub>2</sub>R) driven by renewably generated electricity (e.g., solar and wind) offers a promising means for reusing the CO<sub>2</sub> released during the production of cement, steel, and aluminum as well as the production of ammonia and methanol. If CO<sub>2</sub> could be removed from the atmosphere at acceptable costs (i.e., <$100/t of CO<sub>2</sub>), then CO<sub>2</sub>R could be used to produce carbon-containing chemicals and fuels in a fully sustainable manner. Economic considerations dictate that CO<sub>2</sub>R current densities must be in the range of 0.1 to 1 A/cm<sup>2</sup> and selectivity toward the targeted product must be high in order to minimize separation costs. Industrially relevant operating conditions can be achieved by using gas diffusion electrodes (GDEs) to maximize the transport of species to and from the cathode and combining such electrodes with a solid-electrolyte membrane by eliminating the ohmic losses associated with liquid electrolytes. Additionally, high product selectivity can be attained by careful tuning of the microenvironment near the catalyst surface (e.g., the pH, the concentrations of CO<sub>2</sub> and H<sub>2</sub>O, and the identities of the cations in the double layer adjacent to the catalyst surface).We begin this Account with a discussion of our experimental and theoretical work aimed at optimizing catalyst microenvironments for CO<sub>2</sub>R. We first examine the effects of catalyst morphology on the production of multicarbon (C<sub>2+</sub>) products over Cu-based catalysts and then explore the role of mass transfer combined with the kinetics of buffer reactions in the local concentration of CO<sub>2</sub> and pH at the catalyst surface. This is followed by a discussion of the dependence of the local CO<sub>2</sub> concentration and pH on the dynamics of CO<sub>2</sub>R and the formation of specific products over both Cu and Ag catalysts. Next, we explore the impact of electrolyte cation identity on the rate of CO<sub>2</sub>R and the distribution of products. Subsequently, we look at utilizing pulsed electrolysis to tune the local pH and CO<sub>2</sub> concentration at the catalyst surface. The last part of the discussion demonstrates that ionomer-coated catalysts in combination with pulsed electrolysis can enable the attainment of very high (>90%) selectivity to C<sub>2+</sub> products over Cu in an aqueous electrolyte. This part of the Account is then extended to consider the difference in the catalyst-nanoparticle microenvironment, present in the catalyst layer of a membrane electrode assembly (MEA), with respect to that of a planar electrode immersed in an aqueous electrolyte.
Racial disparity in academia is a widely acknowledged problem. The quantitative understanding of racial based systemic inequalities is an important step towards a more equitable research system. However, because of the lack of robust information on authors' race, few large scale analyses have been performed on this topic. Algorithmic approaches offer one solution, using known information about authors, such as their names, to infer their perceived race. As with any other algorithm, the process of racial inference can generate biases if it is not carefully considered. The goal of this article is to assess the extent to which algorithmic bias is introduced using different approaches for name based racial inference. We use information from the U.S. Census and mortgage applications to infer the race of U.S. affiliated authors in the Web of Science. We estimate the effects of using given and family names, thresholds or continuous distributions, and imputation. Our results demonstrate that the validity of name based inference varies by race/ethnicity and that threshold approaches underestimate Black authors and overestimate White authors. We conclude with recommendations to avoid potential biases. This article lays the foundation for more systematic and less biased investigations into racial disparities in science.
Passive NO<sub>x</sub> adsorbers (PNAs) have been proposed for trapping NO<sub>x</sub> present in automotive exhaust during the period of cold start during which the three-way convertor is not yet hot enough to be effective for NO<sub>x</sub> reduction. Pd-exchanged chabazite (Pd/H–CHA) is a good candidate for passive NO<sub>x</sub> adsorption due to its ability to store NO and retain it to high temperatures (>473 K). Previous research suggests that NO adsorbs on both Pd<sup>2+</sup> and Pd<sup>+</sup> cations and that NO desorption from Pd<sup>2+</sup> cations occurs at lower temperatures than from Pd<sup>+</sup> cations. Since experimental evidence shows that Pd exchanges into CHA exclusively as Pd<sup>2+</sup>, it is not clear how these cations are reduced to Pd<sup>+</sup>. In this study we show through experiments and theoretical analysis that Pd<sup>+</sup> cations can form via two processes, each of which involves water adsorbed on Brønsted-acid sites of the zeolite. The first of these processes is 1.5 NO + Pd<sup>2+</sup>Z<sup>–</sup>Z<sup>–</sup> + 0.5 (H<sub>2</sub>O)H<sup>+</sup>Z<sup>–</sup> → (NO)Pd<sup>+</sup>Z<sup>–</sup>H<sup>+</sup>Z<sup>–</sup> + 0.5 NO<sub>2</sub> + 0.5 H<sup>+</sup>Z<sup>–</sup>. Experiments confirm that the ratio of the NO2 formed upon NO adsorption to the NO desorbing from Pd<sup>+</sup> at elevated temperatures corresponds to 0.5. Pd<sup>2+</sup> can also be reduced via the reaction 1.5 CO + Pd<sup>2+</sup>Z<sup>–</sup>Z<sup>–</sup> + 0.5 (H<sub>2</sub>O)H<sup>+</sup>Z<sup>–</sup> → (CO)Pd<sup>+</sup>Z<sup>–</sup>H<sup>+</sup>Z<sup>–</sup> + 0.5 CO<sub>2</sub> + 0.5 H<sup>+</sup>Z<sup>–</sup>. Upon subsequent adsorption of NO, NO fully displaces CO from Pd<sup>+</sup> to form (NO)Pd<sup>+</sup>Z<sup>–</sup>H<sup>+</sup>Z<sup>–</sup>. In this case, the amount of CO<sub>2</sub> formed upon CO adsorption is 0.5 of the NO desorbing at elevated temperatures from Pd<sup>+</sup>. Gibbs free energy calculations for the above processes at various potential ion-exchange sites in the CHA framework indicate that these reactions are thermodynamically feasible. We also find that Pd<sup>+</sup> is not formed in the absence of adsorbed water and is readily reoxidized to Pd<sup>2+</sup> by trace amounts of O<sub>2</sub>.
Here, copper electrodes, prepared by reduction of oxidized metallic copper, have been reported to exhibit higher activity for the electrochemical reduction of CO<sub>2</sub> and better selectivity toward C<sub>2</sub> and C<sub>3</sub> (C<sub>2+</sub>) products than metallic copper that has not been preoxidized. We report here an investigation of the effects of four different preparations of oxide-derived electrocatalysts on their activity and selectivity for CO<sub>2</sub> reduction, with particular attention given to the selectivity to C<sub>2+</sub> products. All catalysts were tested for CO<sub>2</sub> reduction in 0.1 M KHCO<sub>3</sub> and 0.1 M CsHCO<sub>3</sub> at applied voltages in the range from –0.7 to –1.0 V vs RHE. The best performing oxide-derived catalysts show up to ~70% selectivity to C<sub>2+</sub> products and only ~3% selectivity to C<sub>1</sub> products at –1.0 V vs RHE when CsHCO<sub>3</sub> is used as the electrolyte. In contrast, the selectivity to C<sub>2+</sub> products decreases to ~56% for the same catalysts tested in KHCO<sub>3</sub>. By studying all catalysts under identical conditions, the key factors affecting product selectivity could be discerned. These efforts reveal that the surface area of the oxide-derived layer is a critical parameter affecting selectivity. A high selectivity to C<sub>2+</sub> products is attained at an overpotential of –1 V vs RHE by operating at a current density sufficiently high to achieve a moderately high pH near the catalyst surface but not so high as to cause a significant reduction in the local concentration of CO<sub>2</sub>. On the basis of recent theoretical studies, a high pH suppresses the formation of C<sub>1</sub> relative to C<sub>2+</sub> products. At the same time, however, a high local CO<sub>2</sub> concentration is necessary for the formation of C<sub>2+</sub> products.
On the basis of constraints from reported experimental observations and density functional theory simulations, in this paper we propose a mechanism for the reduction of CO<sub>2</sub> to C<sub>2</sub> products on copper electrodes. To model the effects of an applied potential bias on the reactions, calculations are carried out with a variable, fractional number of electrons on the unit cell, which is optimized so that the Fermi level matches the actual chemical potential of electrons (i.e., the applied bias); an implicit electrolyte model allows for compensation of the surface charge so that neutrality is maintained in the overall simulation cell. Our mechanism explains the presence of the seven C<sub>2</sub> species that have been detected in the reaction, as well as other notable experimental observations. Furthermore, our results shed light on the difference in activities toward C<sub>2</sub> products between the (100) and (111) facets of copper. Finally, we compare our methodologies and findings with those in other recent mechanistic studies of the copper-catalyzed CO<sub>2</sub> reduction reaction.
The physical mechanisms which may contribute to the energy and entropy of mixing in oxide systems are identified and discussed. Ionic size, magnetism and electrostatics can all contribute to the configurational energy dependence of transition-metal oxides. While the many sources of substitutional disorder make configurational entropy an essential contribution to the free energy of oxides, electronic and magnetic entropy may be of the same order of magnitude. This is illustrated with some first-principles results on LiCoO2 and LiMnO2.
We have carried out a periodic Kohn-Sham density functional theory investigation of the pathways by which carbon-carbon bonds could be formed during the electrochemical reduction of CO2 on Cu(100) using a model that includes the effects of the electrochemical potential, solvent, and electrolyte. The electrochemical potential was set by relating the applied potential to the Fermi energy and then calculating the number of electrons required by the simulation cell for that specific Fermi energy. The solvent was included as a continuum dielectric, and the electrolyte was described using a linearized Poisson-Boltzmann model. The calculated potential of zero charge for a variety of surfaces agrees with experiment to within a mean average error of 0.09 V, thereby validating the assumptions of the model. Analysis of the mechanism for C-C bond formation revealed that at low-applied potential, C-C bond formation occurs through a CO dimer. However, at high applied potentials, a large activation barrier blocks this pathway; therefore, C-C bond formation occurs through reaction of adsorbed CHO and CO. Rate parameters determined from our calculations were used to simulate the kinetics of ethene formation during the electrochemical reduction of CO over a Cu(100) surface. An excellent match was observed between previously reported measurements of the partial current for ethene formation as a function of applied voltage and the variation in the partial current for C-C bond formation predicted by our microkinetic model. The electrochemical model reported here is simple, fairly easy to implement, and involves only a small increase in computational cost over calculations neglecting the effects of the electrolyte and the applied field. Therefore, it can be used to study the effects of applied potential and electrolyte composition on the energetics of surface reactions for a wide variety of electrochemical reactions.
This work quantifies the performance of gas-diffusion electrodes using multiphysics modeling and provides design guidance.
Racial disparity in academia is a widely acknowledged problem. The quantitative understanding of racial-based systemic inequalities is an important step towards a more equitable research system. However, because of the lack of robust information on authors’ race, few large-scale analyses have been performed on this topic. Algorithmic approaches offer one solution, using known information about authors, such as their names, to infer their perceived race. As with any other algorithm, the process of racial inference can generate biases if it is not carefully considered. The goal of this article is to assess the extent to which algorithmic bias is introduced using different approaches for name-based racial inference. We use information from the U.S. Census and mortgage applications to infer the race of U.S. affiliated authors in the Web of Science. We estimate the effects of using given and family names, thresholds or continuous distributions, and imputation. Our results demonstrate that the validity of name-based inference varies by race/ethnicity and that threshold approaches underestimate Black authors and overestimate White authors. We conclude with recommendations to avoid potential biases. This article lays the foundation for more systematic and less-biased investigations into racial disparities in science.
Read moreThe behavior and role of hydrogen is investigated by using Pt–Ga nano-alloy formation as a probe reaction.
Read moreThe electrochemical reduction of carbon dioxide is sensitive to electrolyte polarization, which causes gradients in pH and the concentration of carbon dioxide to form near the cathode surface. It is desirable to measure the concentration of reaction-relevant species in the immediate vicinity of the cathode because the intrinsic kinetics of carbon dioxide reduction depend on the composition of the local reaction environment. Meeting this objective has proven difficult because conventional analytical methods only sample products from the bulk electrolyte. In this study, we describe the use of differential electrochemical mass spectrometry to measure the concentration of carbon dioxide and reaction products in the immediate vicinity of the cathode surface. This capability is achieved by coating the electrocatalyst directly onto the pervaporation membrane used to transfer volatile species into the mass spectrometer, thereby enabling species to be sampled directly from the electrode-electrolyte interface. This approach has been used to investigate hydrogen evolution and carbon dioxide reduction over Ag and Cu. We find that the measured CO<sub>2</sub> reduction activity of Ag agrees well with what is measured by gas chromatography of the effluent from an H-cell operated with the same catalyst and electrolyte. A distinct advantage of our approach is that it enables observation of the depletion of carbon dioxide near the cathode surface due to reaction with hydroxyl anions evolved at the cathode surface, something that cannot be done using conventional analytical techniques. We also demonstrate that the influence of this relatively slow chemical reaction can be minimized by evaluating electrocatalytic activity during a rapid potential sweep, thereby enabling measurement of the intrinsic kinetics. For CO<sub>2</sub> reduction over Cu, nine products can be observed simultaneously in real time. A notable finding is that the abundance of aldehydes relative to alcohols near the cathode surface is much higher than that observed in the bulk electrolyte. It is also observed that for increasingly cathodic potentials the relative abundance of ethanol increases at the expense of propionaldehyde. These findings suggest that acetaldehyde is a precursor to ethanol and propionaldehyde and that propionaldehyde is a precursor to n-propanol.
Read moreIn the original version of this article, when equations were referenced within the main text, the numbering was offset by 2. For instance, when eq 1 was referenced, it was incorrectly referred to as eq 3. When eq 16 was referenced, it was incorrectly referenced as eq 18, and so on and so forth for all other references to the equations in the main text. The correct equation referencing is shown below for all instances:n.
Read moreThe aim of this study was to investigate the influence of Si/Al ratio on the locations of exchangeable cations in H-MFI and on the monomolecular cracking and dehydrogenation reactions of n-butane. On the basis of UV-visible spectroscopic analysis of Co(II) exchanged into MFI, it was inferred that the fraction of Co(II) (and, by extension, Brønsted protons) located at channel intersections relative to straight and sinusoidal channels increases with increasing Al content. Concurrently, turnover frequencies for all monomolecular reactions, and the selectivities to dehydrogenation versus cracking and to terminal cracking versus central cracking, generally increased. The changes in selectivity with Al content are consistent with the finding that the transition-state geometry for dehydrogenation is bulky and resembles a product state, and should therefore exhibit a stronger preference to occur at channel intersections relative to cracking. Increases in turnover frequencies are attributed partly to increases in intrinsic activation entropies that compensate for concurrent increases in intrinsic activation energies, most strongly for dehydrogenation and terminal cracking, resulting in increased selectivity to these reactions at higher Al content. This interpretation contrasts with the view that intrinsic activation barriers are constant. It is also observed that isobutene inhibits the rate of n-butane dehydrogenation. Theoretical calculations indicate that this effect originates from adsorption of isobutene at the channel intersections. Because cracking reaction rates are not affected by the presence of isobutene, this result suggests that the preference of dehydrogenation to occur at channel intersections is much stronger than the preference for cracking to occur at these locations.
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