Single-atom catalysts (SACs), consisting of individual metal atoms dispersed on a support, attract attention due to their unique reactivity, efficient use of precious metals, and precise chemical ...tunability. Characterization of the metal species is crucial to substantiate structure–function relationships. Authors often useand referees often requireX-ray absorption spectroscopy (XAS) data to prove the absence of clustered metal (or metal oxide) structures after pre-treatment and under in situ or operando conditions. However, there has been no critical assessment of the limitations of XAS in substantiating such conclusive statements, which is particularly important given the potential outsized influence of minority catalyst structures in dictating catalytic activity. In this article, we quantitatively assess the detection limits of XAS to identify metal (or metal oxide) clusters in samples containing predominantly single atoms by modeling the extended X-ray absorption fine structure (EXAFS) of mixtures of structures. We identified that a significant fraction of clusters can coexist with SAC active sites (e.g., ∼10% metallic Pt or ∼40% oxidized Pt clusters in Pt/CeO2 SACs), while eluding detection via EXAFS with any statistical significance. To generalize these conclusions, a descriptor-based screening of bulk metal oxides using a continuous Cauchy wavelet transform was proposed that suggests certain materials for which differentiating atomically dispersed metal species and metal oxide clusters would be infeasible by EXAFS (e.g., ReO x ). Based on this analysis, we suggest best practices for the study of SACs using EXAFS and provide recommendations to ensure that conclusions do not outpace the evidence used to support them. In this rapidly expanding research area, rigorous characterization will lead to greater understanding of the behavior of SACs and ultimately improved catalytic materials.
This article reports world averages of measurements of
b
-hadron,
c
-hadron, and
τ
-lepton properties obtained by the Heavy Flavor Averaging Group using results available through summer 2016. For the ...averaging, common input parameters used in the various analyses are adjusted (rescaled) to common values, and known correlations are taken into account. The averages include branching fractions, lifetimes, neutral meson mixing parameters,
C
P
violation parameters, parameters of semileptonic decays, and Cabbibo–Kobayashi–Maskawa matrix elements.
Developing heterogeneous catalysts involves continuous efforts in understanding and designing active sites with improved performance for various applications. In recent years, the recognition and ...appreciation of the dynamic nature of active sites has permeated the field, leading to renewed efforts in describing dynamic phenomena and exploiting them to go beyond classical barriers of catalyst performance. The challenges in this field are daunting, yet the rewards could lead to new classes of catalysts that outperform traditional systems for critical chemical transformations at the industrial scale. Furthermore, a deeper understanding of the dynamic nature of active sites can lead to fundamental knowledge of practical importance in the field. In this Perspective, we highlight this emerging area of catalysis science using recent examples to motivate the importance of understanding the dynamic nature of heterogeneous catalysts. We predict that this area of study will continue to expand, eventually allowing us to exploit dynamics to control catalytic performance in numerous applications.
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•Tuning bulk reaction environment parameters modulates catalyst local environment.•Effect of pH and cation identity on oxygen reduction reaction (ORR) is studied.•Changes in local pH ...are measured during ORR using an iridium oxide pH sensing probe.•Cation identity provides variable pH-buffering and modulates changes in local pH.•Changes in local pH more directly impact ORR mechanisms and catalytic performance.
Local catalyst environments fluctuate during electrochemical reactions which significantly influences reaction efficiency and selectivity. However, elucidating fundamental relationships between local environment and electrocatalytic performance is challenging due to the difficulty in probing this local region. Here, we study the impact of electrolyte composition on the kinetics and mechanism of the oxygen reduction reaction (ORR) for a carbon catalyst. Using a rotating ring-disk electrode equipped with an iridium oxide pH probe, we monitor changes in local pH during ORR. We confirm that local pH changes significantly for near-neutral bulk pH and find that different cations provide variable pH-buffering, thereby modulating overall magnitude and onset of local pH changes. Overall, local pH more strongly dictates changes in mechanisms and performance, while the cation identity modulates local pH to influence these trends. This work highlights the importance relating local pH to electrocatalytic performance to further improve understanding of complex electrochemical processes.
Single-atom catalysts (SACs), consisting of individual metal atoms dispersed on a support, attract attention due to their unique reactivity, efficient use of precious metals, and precise chemical ...tunability. Characterization of the metal species is crucial to substantiate structure–function relationships. Authors often use—and referees often require—X-ray absorption spectroscopy (XAS) data to prove the absence of clustered metal (or metal oxide) structures after pre-treatment and under in situ or operando conditions. However, there has been no critical assessment of the limitations of XAS in substantiating such conclusive statements, which is particularly important given the potential outsized influence of minority catalyst structures in dictating catalytic activity. Here, in this article, we quantitatively assess the detection limits of XAS to identify metal (or metal oxide) clusters in samples containing predominantly single atoms by modeling the extended X-ray absorption fine structure (EXAFS) of mixtures of structures. We identified that a significant fraction of clusters can coexist with SAC active sites (e.g., ~10% metallic Pt or ~40% oxidized Pt clusters in Pt/CeO2 SACs), while eluding detection via EXAFS with any statistical significance. To generalize these conclusions, a descriptor-based screening of bulk metal oxides using a continuous Cauchy wavelet transform was proposed that suggests certain materials for which differentiating atomically dispersed metal species and metal oxide clusters would be infeasible by EXAFS (e.g., ReOx). Based on this analysis, we suggest best practices for the study of SACs using EXAFS and provide recommendations to ensure that conclusions do not outpace the evidence used to support them. In this rapidly expanding research area, rigorous characterization will lead to greater understanding of the behavior of SACs and ultimately improved catalytic materials.
•Estradiol non-significantly reduced heroin intake with a moderate effect size.•Estradiol significantly reduced remifentanil intake with a large effect size.•Estrogen-based pharmacotherapy may ...represent novel treatment for women with OUD.
Heroin intake decreases significantly during proestrus in normally cycling female rats, and this effect is mediated by endogenous estradiol but not endogenous progesterone. The purpose of this study was to determine whether chronic administration of exogenous estradiol decreases intake of the semi-synthetic opioid, heroin, and the fully synthetic opioid, remifentanil, in intact female rats. Normally cycling female rats were implanted with intravenous catheters and trained to self-administer heroin on a fixed ratio (FR1) schedule of reinforcement. Rats were treated chronically with daily administration of either a low dose of estradiol (0.5 mcg, sc), a high dose of estradiol (5.0 mcg, sc), or vehicle (peanut oil, sc). After two weeks of heroin self-administration training, dose-effect curves were determined for both heroin and remifentanil. Chronic administration of estradiol non-significantly decreased heroin intake and significantly decreased remifentanil intake. Estradiol-induced decreases in remifentanil intake were dose-dependent, characterized by large effect sizes, and greatest in rats treated with the high dose of estradiol. These data indicate that chronic estradiol administration decreases opioid intake in intact female rats with medium to large effect sizes across opioids. These findings suggest that estrogen-based pharmacotherapies may represent a novel treatment approach for women with opioid use disorder.
Rationale
Heroin intake decreases during the proestrus phase of the estrous cycle in female rats. Circulating concentrations of both estradiol and progesterone peak during proestrus, and it is not ...known which of these hormones, or their combination, are responsible for these effects.
Objectives
The purpose of this study was to determine the effects of estradiol, progesterone, and their combination on heroin self-administration in female rats.
Methods
In Experiment 1, the estrous cycle of intact female rats was tracked daily. If a rat was in proestrus, either the estrogen receptor antagonist, raloxifene, the progesterone receptor antagonist, mifepristone, or their combination was administered 30 min prior to a heroin self-administration session. In Experiment 2, separate groups of ovariectomized female rats were treated chronically with exogenous estradiol, progesterone, estradiol + progesterone, or vehicle, and heroin intake was examined over a 100-fold dose range.
Results
In Experiment 1, raloxifene, but not mifepristone, significantly blocked proestrus-associated decreases in heroin intake. In Experiment 2, estrogentreated rats self-administered less heroin than any other group and significantly less heroin than rats treated with progesterone.
Conclusions
These data suggest that (1) estradiol but not progesterone is responsible for proestrus-associated decreases in heroin intake and (2) estradiol decreases heroin intake relative to progesterone. These data differ from those reported previously with stimulants and suggest that estrogen-based pharmacotherapies may be of value to women with opioid use disorder.
Heroin intake decreases during the proestrus phase of the estrous cycle in female, Long-Evans rats. The purpose of this study was to (1) determine if proestrus-associated decreases in heroin intake ...extend across rat strains and (2) determine if proestrus-associated decreases in responding extend to a nondrug reinforcer. Female rats were implanted with intravenous catheters and trained to self-administer heroin. Estrous cycle was tracked daily for the duration of the study. During testing, Lewis, Sprague Dawley, and Long-Evans rats self-administered low (0.0025 mg/kg) and high (0.0075 mg /kg) doses of heroin and then self-administered sugar on fixed ratio (FR1) schedules of reinforcement. Heroin intake decreased significantly during proestrus in all three rat strains under at least one dose condition; however, sugar intake did not decrease during proestrus in any strain. These data suggest that responding maintained by heroin, but not a nondrug reinforcer, significantly decreases during proestrus in female rats and that these effects are consistent across rat strain.
Several methods exist to measure CP violation observables related to the CKM unitarity triangle angle γ using B meson decays. These observables are different for every B meson decay considered, ...although the information they contain on γ is encoded in a similar way for all of them. This paper describes a strategy for a simultaneous measurement of γ using several B meson decays that takes into account possible correlations between them based on the methodologies described in J. Garra Ticó, arXiv:1804.05597. Sensitivity studies demonstrate that the simultaneous analysis of several B meson decay modes results in smaller uncertainties and improved statistical behavior compared to a combination of standalone measurements.
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