Surface Site Density Research Articles

Reactants‐Driven Surface‐Active Sites on a Cu/CeO2 Catalyst for Methanol Synthesis from Syngas

AbstractThis study investigated the dynamic structure of a working Cu/CeO2 catalyst and its catalytic consequences for CH3OH synthesis via CO hydrogenation by kinetics, high‐precise chemical titration, and a series of in situ X‐ray spectroscopies. We have revealed that the apparent rate coefficients for CH3OH synthesis from CO hydrogenation are a single‐valued function of the CO2‐to‐CO ratio in the contacting gas phase. This ratio determines the extent of surface oxidation of Ce2O3 to CeO2, the relative abundance of surface Ce3+ and Ce4+ sites, and in turn, the density of Cu0‐Ce3+ sites pair on the working Cu/CeO2 catalyst, although the chemical state of Cu and elemental distribution of both Cu and Ce were insensitive to the ratio. The reactivity of CH3OH formation and the Cu0‐Ce3+ sites decrease monotonously with an increase in the CO2‐to‐CO ratio. The direct connection of the reactivity and the oxidant‐to‐reductant ratio appears to be general for heterogeneous catalysis, for which the oxidant oxidizes metal atoms. In contrast, the reductant scavenges surface oxygen atoms, thus dictating the density of surface sites and their thermodynamic activities.

Formic Acid Dehydration Using Mechanochemically Prepared TiO2‐Graphite Composites

AbstractCommercial high surface area graphite (HSAG300) and commercial TiO2 were used to produce composite materials through a simple mechanochemical method involving milling and ultrasonic treatments. The acid and basic sites exposed on the surfaces of these materials were characterized by temperature‐programmed desorption (TPD) of ammonia and carbon dioxide. The catalytic materials were tested in the dehydration reaction of formic acid to produce hydrogen‐free CO. While HSAG300 is practically inactive under reaction conditions (continuous gas flow at temperatures in the range of 100–250 °C), all samples containing TiO2 are active, exhibiting high selectivity to CO without significant deactivation at moderate reaction temperatures. It is demonstrated that the presence of graphite in the catalysts enhances the specific catalytic activity of TiO2. Assuming that the dehydration reaction is catalyzed by acid sites on the TiO2 surfaces, a comparative evaluation of the surface sites reveals that the graphite‐TiO2 interactions not only change the density of surface sites but also modify the strength of the acid centers of TiO2. In summary, the interaction of HSAG300 with TiO2 modulates the surface properties of the prepared composite catalysts, decreasing the total number of basic surface sites and increasing the strength of acidic sites compared to bare TiO2.

The Conundrum of "Pair Sites" in Langmuir-Hinshelwood Reaction Kinetics in Heterogeneous Catalysis.

Understanding reaction kinetics is crucial for designing and applying heterogeneous catalytic processes in chemical and energy conversion. Here, we revisit the Langmuir-Hinshelwood (L-H) kinetic model for bimolecular surface reactions, originally formulated for metal catalysts, assuming immobile adsorbates on neighboring pair sites, with the rate varying linearly with the density of surface sites (sites per unit area); r ∝ [*]o 1. Supported metal oxide catalysts, however, offer systematic control over [*]o through variation of the active two-dimensional metal oxide loading in the submonolayer region. Various reactions catalyzed by supported metal oxides are analyzed, such as supported VO x catalysts, including methanol oxidation, oxidative dehydrogenation of propane and ethane, SO2 oxidation to SO3, propene oxidation to acrolein, n-butane oxidation to maleic anhydride, and selective catalytic reduction of nitric oxide with ammonia. The analysis reveals diverse dependencies of reaction rate on [*]o for these surface reactions, with r ∝ [*]o n , where n equals 1 for reactions with a unimolecular rate-determining step and 2 for those with a bimolecular rate-limiting step or exchange of more than 2 electrons. We propose refraining from a priori assumptions about the nature and density of surface sites or adsorbate behavior, advocating instead for data-driven elucidation of kinetics based on the density of surface sites, adsorbate coverage, etc. Additionally, recent studies on catalytic surface mechanisms have shed light on nonadjacent catalytic sites catalyzing surface reactions in contrast to the traditional requirement of adjacent/pair sites. These findings underscore the need for a more nuanced approach in modeling heterogeneous catalysis, especially supported metal oxide catalysts, encouraging reliance on experimental data over idealized assumptions that are often difficult to justify.

Measurement report: A comparison of ground-level ice-nucleating-particle abundance and aerosol properties during autumn at contrasting marine and terrestrial locations

Abstract. Ice-nucleating particles (INPs) are an essential class of aerosols found worldwide that have far-reaching but poorly quantified climate feedback mechanisms through interaction with clouds and impacts on precipitation. These particles can have highly variable physicochemical properties in the atmosphere, and it is crucial to continuously monitor their long-term concentration relative to total ambient aerosol populations at a wide variety of sites to comprehensively understand aerosol–cloud interactions in the atmosphere. Hence, our study applied an in situ forced expansion cooling device to measure ambient INP concentrations and test its automated continuous measurements at atmospheric observatories, where complementary aerosol instruments are heavily equipped. Using collocated aerosol size, number, and composition measurements from these sites, we analyzed the correlation between sources and abundance of INPs in different environments. Toward this aim, we have measured ground-level INP concentrations at two contrasting sites, one in the Southern Great Plains (SGP) region of the United States with a substantial terrestrially influenced aerosol population and one in the Eastern North Atlantic Ocean (ENA) region with a primarily marine-influenced aerosol population. These measurements examined INPs mainly formed through immersion freezing and were performed at a ≤ 12 min resolution and with a wide range of heterogeneous freezing temperatures (Ts above −31 °C) for at least 45 d at each site. The associated INP data analysis was conducted in a consistent manner. We also explored the additional offline characterization of ambient aerosol particle samples from both locations in comparison to in situ data. From our ENA data, on average, INP abundance ranges from ≈ 1 to ≈ 20 L−1 (−30 °C ≤ T ≤ −20 °C) during October–November 2020. Backward air mass trajectories reveal a strong marine influence at ENA with 75.7 % of air masses originating over the Atlantic Ocean and 96.6 % of air masses traveling over open water, but analysis of particle chemistry suggests an additional INP source besides maritime aerosols (e.g., sea spray aerosols) at ENA. In contrast, 90.8 % of air masses at the SGP location originated from the North American continent, and 96.1 % of the time, these air masses traveled over land. As a result, organic-rich SGP aerosols from terrestrial sources exhibited notably high INP abundance from ≈ 1 to ≈ 100 L−1 (−30 °C ≤ T ≤ −15 °C) during October–November 2019. The probability density function of aerosol surface area-scaled immersion freezing efficiency (ice nucleation active surface site density; ns) was assessed for selected freezing temperatures. While the INP concentrations measured at SGP are higher than those of ENA, the ns(T) values of SGP (≈ 105 to ≈ 107 m−2 for −30 °C ≤ T ≤ −15 °C) are reciprocally lower than ENA for approximately 2 orders of magnitude (≈ 107 to ≈ 109 m−2 for −30 °C ≤ T ≤ −15 °C). The observed difference in ns(T) mainly stems from varied available aerosol surface areas, Saer, from two sites (Saer,SGP > Saer,ENA). INP parameterizations were developed as a function of examined freezing temperatures from SGP and ENA for our study periods.

A triple-layer based surface complexation model for oil-brine interface in low-salinity waterflooding: Effect of sulphate interaction with carboxylic and basic groups, pH and temperature

The determination of the electrokinetic properties of oil-brine interface is necessary to understand and evaluate the low-salinity waterflooding (LSWF) effect on enhanced oil recovery (EOR). Among the least understood aspects are the effect of SO42- interaction with the polar oil components, and temperature on the zeta-potential of the oil-brine interface. Therefore, a novel triple-layer based surface complexation model (SCM) was developed to capture the effect of SO42- interaction with both carboxylic and amine sites. One of the key uncertainties in a SCM is the surface site density of the functional groups. In this regard a Monte-Carlo based optimization algorithm for determination of these parameters as a function of pH and brine composition was devised. The SCM was successfully validated with the published zeta-potential data and was further used to evaluate the electrokinetic properties of oil-brine interface in a wide range of temperature and pH and to unravel the reason behind positive oil-brine zeta-potential at high ionic strength and pH, as reported for some crude oils in the literature. A sensitivity analysis was conducted to determine the relative contribution of site densities of carboxylic and amine groups as well as the capacitances to the oil-brine zeta-potential. The results clearly show that the interaction of SO42- ions with both carboxylic and particularly amine groups affects the surface potential significantly, thus should not be underestimated. Presence of SO42- in Na2SO4 and MgSO4 brines results in an incremental negative zeta-potential compared to that in NaCl and MgCl2 brines; confirming its role in changing charge distribution in the Stern layer. If these interactions are ignored, it should be compensated for by artificially adjusting concentration of positively and neutrally charged surface species. One important finding is that, despite the common perceptions, the site density of the oil polar groups is not constant and should be changed as pH and brine composition are changed and linear correlations between acid and base number and site densities are not justifiable. It is also found that zeta-potential tends to shift to neutral values as temperature is elevated due to the increased adsorption of divalent cations to the interface. The findings of this paper can help predict the zeta-potential of oil-brine interface more accurately and explain the observed trends in the experimental data.

Influence of the Neutralization Degree on the Ice Nucleation Ability of Ammoniated Sulfate Particles

AbstractPrevious laboratory measurements suggest that ammonium sulfate crystals (AS, (NH4)2SO4) are efficient ice‐nucleating particles under cirrus conditions. Sulfate particles not completely neutralized by ammonium are less well studied and include two other solids, ammonium bisulfate (AHS, NH4HSO4) and letovicite (LET, (NH4)3H(SO4)2). In this work, we have obtained the first comprehensive data set for the heterogeneous ice nucleation ability of crystallized particles in the AS–LET–AHS system as a function of their degree of neutralization at a temperature of about 220 K. Quantitative data on nucleation onsets, ice‐active fractions, and ice nucleation active surface site densities were derived from expansion cooling experiments in a large cloud chamber and measurements with two continuous flow diffusion chambers. We found a strong dependence of the efficiency and the mode of heterogenous ice nucleation on the degree of neutralization. Ice formation for AS, mixed AS/LET, and LET crystals occurred by the deposition nucleation or pore condensation and freezing mode. The lowest nucleation onset was observed for AS, where 0.1% of the particles became ice‐active at an ice saturation ratio of 1.25. This threshold gradually increased to 1.35 for LET, and abruptly further to 1.45 for mixed LET/AHS crystals, which partially deliquesced and induced ice formation via immersion freezing. Pure AHS crystals did not form due to the inhibition of efflorescence. Our data allow for a more sophisticated treatment of ice formation in the AS–LET–AHS system in future model simulations, which have so far only considered the available data for AS alone.

A novel oil/brine surface complexation model: Capturing the dynamic nature of the interface using IFT

The electrical properties of the oil/brine interface play a principal role in wettability alteration during low salinity/water flooding. There are few existing surface complexation models (SCMs) for the oil/brine interface. Similar to the mineral surface, all models assume that the surface site density of oil/brine is always constant. Assessment of the existing models shows that they ignore the dynamic nature of the crude oil/brine interface and fail to capture experimentally measured ζ potentials appropriately. The current study proposes a novel diffuse layer SCM considering the interfacial concentration of surface carboxylic acid as a function of brine pH, salinity, and composition. The oil/brine interfacial tension (IFT) was utilized to describe the change in the interfacial concentration of > COOH sites vs. pH and ionic strength. The developed model matches the experimental data of the literature far better than previous models successfully. Based on the model, Na+, Cl-, and SO42- cannot be adsorbed on the oil/brine interface, however; the role of these ions in the electrical behavior of crude oil/brine is only to affect the interfacial concentration of > COOH. The novel model surpasses the previous model at high and low pH conditions. At a high pH range, due to the significant transport of acid groups to the interface, the oil surface becomes more negatively charged, which was accurately captured by the proposed model (while the previous models are malfunctioning at high pH conditions). Assuming maximum > COOH density at the whole pH range, previous models use a high > NH density to compensate for the extra negative surface charge due to > COOH dissociation. Therefore, these models overestimate the zeta potential significantly at low pH conditions, where > NH protonation is the dominant reaction. One of the main features of the developed model in this study is to predict a critical salinity for the extremum of zeta potential. All previous models state that salinity reduction must result in a decrease in the zeta potential. These models cannot capture the observed rise in zeta potential with further brine dilution after a specified salinity. However, the presented model of this study could predict the increase in the zeta potential at lower salinities.

Towards building a unified adsorption model for goethite based on direct measurements of crystal face compositions: I. Acidity behavior and As(V) adsorption

We have determined the proton, electrolyte (NaNO3) and arsenate affinity constants on the corresponding surface sites of goethite. An empirical optimization approach was applied using the Charge Distribution Multisite Complexation (CD-MUSIC) model, initially, by simulating an extensive proton adsorption data set obtained from four goethites of specific surface areas (SSAs) spanning from 43 to 89 m2/g. Each goethite showed different crystal face compositions, which were measured previously from independent high resolution electron microscopy observations. Only the capacitance value was left as an additional optimizing parameter. For arsenate adsorption, with the above optimized values fixed, the contributing surface arsenate complexes and their respective affinity constants and charge distributions (CD) were also optimized. A unified set of affinity and CD parameters was obtained, where the only resulting variables among goethites were the capacitance 1, the specific surface area and the crystal face contributions (and thus their reactive surface site densities).It was found that the site density parameter is decisive when data show adsorption maxima, such as in the arsenate adsorption isotherms. The self-consistent modeling approach yielded pKa values of singly-coordinated sites of 8.82, and of triply-coordinated sites of 9.65. Three surface arsenate complexes bound only to singly-coordinated sites were found to describe all experimental data, although only two of them were predominant across the pH and As(V) concentration ranges investigated: a bidentate non protonated (BD) complex, and a protonated monodentate (HMD) one. Charge distribution of surface arsenate complexes was found to significantly influence the optimizations, but increased values of optimized capacitance 1 were found to compensate for previously underestimated surface site density values, to yield almost identical surface affinity and CD parameters.Finally, correlations found between surface site density and capacitance-1 parameters with SSA expanded the applicability of the modeling approach presented, to other goethites with different SSAs than the ones investigated here.

Ultrathin 2D/2D nanohybrid for efficient electrochemical detection of nitrite

An excessive amount of nitrite ions and their long-term accumulation pose a significant threat to both the ecological system and the public health. Thus, it is highly desirable to construct a unique electrode architecture for the precise and efficient quantification of nitrite. This study reports an electrochemical nitrite sensor using an ultrathin 2D/2D hematene/GO nanohybrid with a few nanometers of thickness and face-to-face contact. Due to their high surface-to-volume ratio, which make them extremely susceptible to their interaction with the surrounding environment, and the high density of active surface sites, they exhibit remarkable electrochemical sensing performance towards nitrite. The proposed ultrathin 2D/2D hematene/GO nanohybrid sensor responds linearly with increasing concentrations of nitrite up to 1000 µM and a low detection limit (2 µM). In addition, the sensor's selectivity, stability, and reproducibility qualities are also evaluated. Furthermore, it successfully detected the analyte in tap water, mineral water, and river water samples with relative standard deviations (RSD) less than 5%. Density functional theory (DFT) simulations demonstrate that the hematene/GO nanohybrid efficiently lowers the energy barrier for the electrochemical oxidation of HNO2 to HNO3 and that the synergistic impact of hematene and GO enhances the electrochemical sensing reactivity for the reaction of nitrite detection.

Application of community data to surface complexation modeling framework development: Iron oxide protolysis

This study presents a comprehensive community data-driven surface complexation modeling framework for simulating potentiometric titration of mineral surfaces. Compiled community data for ferrihydrite, goethite, hematite, and magnetite are fit to produce representative protolysis constants that can reproduce potentiometric titration data collected from multiple literature sources. Using this framework, the impact of surface complexation model type and surface site density (SSD) on the fit quality and protolysis constants can be readily evaluated. For example, the non-electrostatic model yielded a poor data fit compared to diffuse double layer model and constant capacitance models due to the absence of known surface charge effects. Regardless of the choice of iron oxide mineral, pKa1 decreased with increasing SSD while the opposite tendency was observed for pKa2. This newly developed framework demonstrates a method to reconcile community data-wide potentiometric titration data using Findable, Accessible, Interoperable, Reusable data principles to produce mineral protolysis constants that improve robustness of surface complexation models for applications in metal sorption and reactive transport modeling. The framework is readily expandable (as community data increase) and extensible (as the number of minerals increase). The framework provides a path forward for developing self-consistent, comprehensive, and updateable surface complexation databases for surface complexation and reactive transport modeling.

Prediction of alkaline earth metal ion adsorption on goethite for various background electrolytes with the CD-MUSIC model

As water scarcity drives the use of more saline water sources, contaminant fate and transport models must capture the impact of high concentrations of alkaline earth metal ions (AEMs) and background electrolytes in these more complex waters. By utilizing macroscopic adsorption data from various electrolyte systems, a Charge Distribution – Multisite Complexation (CD-MUSIC) model, capable of incorporating electrolyte adsorption, was able to accurately simulate the adsorption behavior of alkaline earth metal ions onto goethite. The modeling effort was guided by previous spectroscopic and surface complexation modeling of alkaline earth metal adsorption and built on previous CD-MUSIC modeling that accounted for changes in crystal face contributions to the surface site density as a function of specific surface area. The model was constrained to consider only two dominant surface complex species for each metal ion adsorption reaction. These two species were selected from 44 possible species through objective curve fitting of single-solute macroscopic adsorption data. While most of the alkaline earth metal surface complexes formed outer-sphere complexes at the goethite surface, an inner-sphere species was utilized for Mg2+. With the surface complex species and equilibrium constants obtained from this study, the calibrated model successfully predicted alkaline earth metal ion adsorption over a wide range of solution and surface conditions; the model predictions encompassed a wide range of pH (5–11), solute/solid ratio (1.37 × 10−5– 8.33 × 10−4 mol-solute/g-solid), ionic strengths (0.01 M – 0.7 M), and background electrolytes (Na+, Cs+, Rb+, Cl−, and NO3−) using the same crystal face contribution methodology for site density, capacitance values, and surface acidity constants adopted for proton and cadmium adsorption in previous work (Han and Katz, 2019). Model simulations for a range of background water chemistries demonstrated the potential for Mg2+ to reduce Cd2+ adsorption to goethite in model seawater and oil- and gas-produced waters.

Vapor-Liquid phase transition of associating fluids in a functionalized cylindrical pore: a Monte Carlo study

In this study, we investigate the phase behaviour of associating fluids with two interacting sites inside a functionalised cylindrical pore using Grand Canonical Monte Carlo simulations. The structural behaviour of the coexisting liquid and vapor phases is characterised by the radial density, orientation, and bond formation inside cylindrical pores of different radii. The critical properties of the phase diagrams of the confined fluids were investigated as functions of the associating strengths at different pore sizes. The impact of site strength on the phase diagram is characterised by the critical and structural properties of the coexisting phases inside the cylindrical pore. The organisation of the associating fluids near the pore surface strongly depends on the surface – site interaction with the interfacial fluid molecules that propagate to the bulk regime of the system. Thus, the surface site density is a crucial parameter for manipulating the properties of coexisting phases and critical properties of the phase diagram.

Phosphate adsorption on amorphous aluminum hydroxide-yielded transition aluminas: Insights on fundamental chemistry and making engineering adsorbents

Transition aluminas (TAs) are evolved alumina polymorphs during mineral phase transformation from amorphous to crystalline structure, which have been extensively used as engineering adsorbents or platforms of functional materials for pollutants (e.g. eutrophication nutrient PO43−) mitigation in water remediation. A well-understanding of structure-binding activity of pollutants on transition aluminas is essential for developing promising water remediation materials. In this work, TAs were synthesized through annealing amorphous aluminum (oxy)hydroxide, and a decreased trend of phosphate (PO43−) adsorption was observed, which was not in pace with the changes of specific surface areas and site densities. The structure disorders of TAs deteriorated continuously peaked at 700 °C and TA at 700 °C has the largest extent of structural disorder, but does not present the highest adsorption capacity, which is not consistent with the consensus that oxides' structural disorder facilitate adsorption. Multiple techniques including pair distribution function, selective chemical extraction, 27Al NMR and quantum chemistry calculations were employed to reveal the underlying mechanism differences of phosphate adsorption on TAs. The results indicated the fraction of amorphous contents is the key feature governing PO43− adsorption, regardless of mineral phase, structural disorder, AlO4/AlO6 arrangement, and Gibbs free energy of adsorption. Three strategic measures are suggested on making promising alumina engineering adsorbents, including tuning transition temperature, introducing non-tetrahedral and non-octahedral nanostructured grains, and employing more potential methods for synthesizing amorphous nanocrystalline composite. This work sheds lights on understanding the fundamental chemistry of phosphate on transition alumina and producing prospective alumina remediation materials.

Surface Site Density of Synthetic Goethites and Its Relationship to Atomic Surface Roughness and Crystal Size.

The capacity for crystals to adsorb elements and molecules is a function of the structures of their crystal faces and the relative proportions of those faces. More importantly, this study shows that the surface structure of crystal faces is affected by their surface roughness and is the dominant factor controlling the absorption site density. In a continuation of the study of synthetic goethites with varying single crystal size distributions, two more synthetic goethites with intermediate sizes were analyzed by Brunauer-Emmett-Teller (BET) and atomic-resolution scanning transmission electron microscopy (STEM) to determine the effects of crystal size on their shape, atomic-scale surface roughness, and ultimately on their total surface site density. Results show that surface roughness scales directly with the size [or inversely with the specific surface area (SSA)] of synthetic goethites in the SSA range of 40-75 m2/g. This surface roughness, in turn, increases the total site density over ideal atomically smooth crystals. The total site density of synthetic goethite increases from a combination of decreasing crystal length/width ratio and increasing surface roughness.

Ice-nucleating particles near two major dust source regions

Abstract. Mineral dust and sea spray aerosol represent important sources of ice-nucleating particles (INPs), the minor fraction of aerosol particles able to trigger cloud ice crystal formation and, consequently, influence multiple climate-relevant cloud properties including lifetime, radiative properties and precipitation initiation efficiency. Mineral dust is considered the dominant INP source in many parts of the world due to its ice nucleation efficiency and its sheer abundance, with global emission rates of up to 4700 Tg a−1. However, INPs emitted from the ocean surface in sea spray aerosol frequently dominate INP populations in remote marine environments, including parts of the Southern Ocean where cloud-resolving model simulations have demonstrated that cloud radiative properties are likely strongly controlled by INPs. Here we report INP concentrations measured in aerosol and seawater samples during Air Quality and Climate Change in the Arabian Basin (AQABA), a shipborne campaign that spanned the Red Sea, Gulf of Aden, Arabian Sea, Arabian Gulf and part of the Mediterranean. In aerosol samples collected within a few hundred kilometers of the first and second ranked sources of dust globally, the Sahara and Arabian Peninsula, INP concentrations ranged from 0.2 to 11 L−1 at −20 ∘C with observed ice-active surface site densities (ns) 1–3 orders of magnitude below levels predicted by mineral dust INP parameterizations. Over half of the samples (at least 14 of 26) were collected during dust storms with average dust mass concentrations between 150 and 490 µg m−3 (PM10), as simulated by the Modern-Era Retrospective analysis for Research and Application, version 2 (MERRA-2). The impacts of heat and peroxide treatments indicate that organics dominated the observed ice nucleation (IN) activity at temperatures ≥ −15 ∘C with proteinaceous (heat-labile) INPs frequently observed at high freezing temperatures >−10 ∘C. INP concentrations in seawater samples ranged between 3 and 46 mL−1 at −19 ∘C, demonstrating the relatively low INP source potential of seawater in the region as compared to seawater from multiple other regions reported previously. Overall, our results demonstrate that despite proximity to the Sahara and the Arabian Peninsula and the dominance of mineral dust in the aerosol sampled, existing mineral dust parameterizations alone would not skillfully represent the near-surface ns in the observed temperature regime (−6 to −25 ∘C). Future efforts to develop or improve representations of dust INPs at modest supercooling (≥-15 ∘C) would benefit from a characterization of the specific organic species associated with dust INPs. More generally, an improved understanding of the organic species associated with increased IN activity and their variability across dust source regions would directly inform efforts to determine whether ns-based parameterizations are appropriate for faithful representation of dust INPs in this sensitive temperature regime, whether region-specific parameterizations are required, or whether an alternative to the ns approach is necessary.

Significant continental source of ice-nucleating particles at the tip of Chile's southernmost Patagonia region

Abstract. The sources and abundance of ice-nucleating particles (INPs) that initiate cloud ice formation remain understudied, especially in the Southern Hemisphere. In this study, we present INP measurements taken close to Punta Arenas, Chile, at the southernmost tip of South America from May 2019 to March 2020, during the Dynamics, Aerosol, Cloud, And Precipitation Observations in the Pristine Environment of the Southern Ocean (DACAPO-PESO) campaign. The highest ice nucleation temperature was observed at −3 ∘C, and from this temperature down to ∼-10 ∘C, a sharp increase of INP number concentration (NINP) was observed. Heating of the samples revealed that roughly 90 % and 80 % of INPs are proteinaceous-based biogenic particles at >-10 and −15 ∘C, respectively. The NINP at Punta Arenas is much higher than that in the Southern Ocean, but it is comparable with an agricultural area in Argentina and forestry environment in the US. Ice active surface site density (ns) is much higher than that for marine aerosol in the Southern Ocean, but comparable to English fertile soil dust. Parameterization based on particle number concentration in the size range larger than 500 nm (N>500 nm) from the global average (DeMott et al., 2010) overestimates the measured INP, but the parameterization representing biological particles from a forestry environment (Tobo et al., 2013) yields NINP comparable to this study. No clear seasonal variation of NINP was observed. High precipitation is one of the most important meteorological parameters to enhance the NINP in both cold and warm seasons. A comparison of data from in situ and lidar measurements showed good agreement for concentrations of large aerosol particles (>500 nm) when assuming continental conditions for retrieval of the lidar data, suggesting that these particles were well mixed within the planetary boundary layer (PBL). This corroborates the continental origin of these particles, consistent with the results from our INP source analysis. Overall, we suggest that a high NINP of biogenic INPs originated from terrestrial sources and were added to the marine air masses during the overflow of a maximum of roughly 150 km of land before arriving at the measurement station.

Kinetic Diagnostics and Synthetic Design of Platinum Group Metal-Free Electrocatalysts for the Oxygen Reduction Reaction Using Reactivity Maps and Site Utilization Descriptors.

The experimental development of catalytically ever-more active platinum group metal (PGM)-free materials for the oxygen reduction reaction (ORR) at fuel cell cathodes has been until recently a rather empirical iteration of synthesis and testing. Here, we present how kinetic reactivity maps based on kinetic descriptors of PGM-free single-metal-site ORR electrocatalysts can help to better understand the origin of catalytic reactivity and help to derive rational synthetic guidelines toward improved catalysts. Key in our analysis are the catalytic surface site density (SD) and the catalytic turnover frequency (TOF) in their role as controlling kinetic parameters for the ORR reactivity of PGM-free nitrogen-coordinated single-metal M-site carbon (MNC) catalysts. SD-TOF plots establish two-dimensional reactivity maps. We also consider the ratio between SD and the total number of single-metal sites in the bulk, referred to as the site utilization factor, which we propose as another guiding parameter for optimizing the synthesis of MNC catalysts. Exemplified by two sets of FeNC, CoNC, and SnNC catalysts prepared using two distinctly different N- and C-precursor material classes (Zn-based zeolitic imidazolate frameworks and covalent polyaniline), we comparatively diagnose the intrinsic kinetic ORR parameters as well as structural, morphological, and chemical properties. From there, we derive and discuss possible synthetic guidelines for further improvements. Our approach can be extended to other families of catalysts and may involve kinetic performance data of idealized liquid-electrolyte cells as well as gas diffusion layer-type flow cells.

Complexation mechanism of Pb2+ at the ferrihydrite-water interface: The role of Al-substitution

Ferrihydrite is a poorly crystalline iron (hydr)oxide and highly efficient adsorbent for heavy metals. Al-substitution in ferrihydrite is ubiquitous in nature. However, the effect of Al-substitution on the surface reactivity of ferrihydrite remains unclear due to its low crystallinity. The present study aims to clarify the microstructure and interfacial reaction of Al-substituted ferrihydrite. Al-substitution had little effect on the morphology and surface site density of ferrihydrite, while the presence of ≡AlOH−0.5 sites resulted in higher proton affinity and surface positive charge of ferrihydrite. Besides, the affinity constant of Pb2+ adsorption on the surface of ferrihydrite decreased at higher Al content, which further decreased the adsorption performance of ferrihydrite for Pb2+. The modeling results revealed that bidentate complex was the dominant Pb complexation species on the surface of ferrihydrite, which was less affected by Al-substitution. The present study provides important insights into the effect of Al-substitution on the interfacial reaction at the ferrihydrite-water interface. The obtained parameters may facilitate the future advance of surface complexation model.

Modelling the effects of reservoir parameters and rock mineralogy on wettability during low salinity waterflooding in sandstone reservoirs

Low salinity waterflooding has been reported to yield incremental oil recovery from both field applications and laboratory experiments compared to regular waterflooding. Crude oil-brine-rock (COBR) interactions dictate wettability alteration during low salinity waterflooding in sandstones. In this work, triple-layer surface complexation modelling (TLM) is utilised to simulate the interactions at rock-brine and oil-brine interfaces, while accommodating the crucial role of sandstone mineralogy in surface chemistry. Derjaguin-Landau-Verwey-Overbeek (DLVO) theory is applied to characterise the COBR stability. Moreover, we propose the use of the maximum energy barrier (MEB) parameter, which is calculated from the DLVO theory's interaction potential, as an indicator of reservoir wettability.Correlating the MEB with the experimentally measured contact angles revealed an abrupt increase in contact angles as the MEB drops below the zero-value thereby leading to a less water-wet COBR system. Results analyses showed that the different clays affect the rock-brine zeta potential and wettability distinctly based on their surface site densities and specific surface areas. And the studied clays cause the zeta potential to become more negative in the order: smectite > montmorillonite > illite > chlorite > kaolinite. Subsequently, further investigation employing the developed TLM and MEB revealed that higher amounts of kaolinite will make the reservoir more oil-wet.Lastly, the sensitivity analysis performed on reservoir wettability indicated that the ionic composition is the most important factor to affect rock wettability followed by pH and temperature. Moreover, the presence of CaCl2 salt in the formation water significantly supresses the areas of strong water wettability under varying reservoir conditions compared to NaCl salt.The work conducted in this study presents a novel approach to model the individual and combined effects of sandstone minerals, specifically, quartz, kaolinite, chlorite, illite, montmorillonite and smectite, on the overall sandstone zeta potential behaviour. Furthermore, a new method was proposed to characterise reservoir wettability as a function of the maximum energy barrier which allowed us to obtain valuable insights into the most affecting reservoir parameters on COBR wettability. These findings will have practical implications to efficiently design the low salinity waterflooding processes for sandstone reservoir applications.

The Importance of Surface Oxygen for Lithiation and Morphology Evolution during Calcination of High‐Nickel NMC Cathodes

AbstractNanoscale morphology has a direct impact on the performance of materials for electrochemical energy storage. Despite this importance, little is known about the evolution of primary particle morphology nor its effect on chemical pathways during synthesis. In this study, operando characterization is combined with atomic‐scale and continuum simulations to clarify the relationship between morphology of cathode primary particles and their lithiation during calcination of LiNi0.8Mn0.1Co0.1O2 (NMC‐811). This combined approach reveals a key role for surface oxygen adsorption in facilitating the lithiation reaction by promoting metal diffusion and oxidation, and simultaneously providing surface sites for lithium insertion. Furthermore, oxygen surface termination is shown to increase the activation energy for sintering, leading to smaller primary particle sizes at intermediate temperatures. Smaller particles provide both shorter diffusion lengths for lithium incorporation and increased surface site density for lithium insertion. These insights provide a foundation for more tailored syntheses of cathode materials with optimized performance characteristics.