Deep-sea sediments hold large quantities of critical rare earth-elements and yttrium (REY) sequestered in nanoparticulate biogenic fluorapatite (Ca5(CO3) x (PO4)3-x F1+x ). Understanding their enrichment processes and improving recovery and mineral processing methods require atomic-scale information about their chemical form, but it is difficult to obtain. Here, we use novel high-energy-resolution fluorescence-detected extended X-ray absorption fine structure (HERFD-EXAFS) spectroscopy to elucidate the local structure of gadolinium (Gd) in the highly enriched REY deposit from the Clarion-Clipperton fracture zone (CCFZ) in the Pacific Ocean. Our findings reveal that Gd is neither incorporated into the apatite structure nor precipitated alongside Ce in a Ce-PO4 precipitate. Instead, it is bound at short-range distances to Ca and PO4 in a defective apatite-type bonding environment within an amorphous matrix that encases fluorapatite nanocrystals. Density functional theory (DFT) suggests that Gd and Y, whose atomic fraction is ten times higher than that of Gd, are not dispersed throughout the amorphous matrix, but are likely segregated at medium-range distances. The entrapment of Ce, Gd, and Y within an amorphous matrix explains, at the microscopic level, why REY can be easily recovered through straightforward acid leaching. This is due to the intrinsic instability of disordered atomic structures compared to crystalline phases. This research highlights the complementarity of HERFD-EXAFS and DFT calculations for atomic-scale analysis of trace elements in complex natural matrices. It establishes a basis for their use across diverse terrestrial and marine materials.

The electrocatalytic hydrogenation (ECH) of furfural (FF) to 2-methylfuran (2MF) is a promising route for producing sustainable biofuels and aviation fuels from nonedible biomass. However, achieving high selectivity and Faradaic efficiency (FE) for 2MF remains a significant challenge due to the competing hydrogen evolution reaction and the formation of furfuryl alcohol (FOH). In this study, we demonstrate that electrolyte engineering can greatly enhance the selectivity of FF ECH toward 2MF. Using a copper electrocatalyst at pH 2, we show that a phosphate-based electrolyte boosts 2MF selectivity (72.1%) compared to a sulfate-based electrolyte (54.8%) at -0.5 V vs RHE. Using in situ surface-enhanced Raman spectroscopy (SERS), we demonstrate that H3PO4 can form hydrogen bonds with FF adsorbates, thus acting as a proton relay. This promotes a proton-coupled electron transfer (PCET) mechanism steering selectivity toward 2MF. We further establish a direct correlation between 2MF selectivity and the intensity ratio of Raman bands associated with PCET and FOH intermediates under a wide range of conditions (pH, potential, and electrolyte). Finally, by introducing bromide ions into the phosphate electrolyte, the competing hydrogen atom transfer (HAT) pathway is further suppressed, achieving a record-high Faradaic efficiency of 91.4 +/- 5.7% and a relative selectivity of 89.0 +/- 4.3% for 2MF. Taken together, these results highlight the critical role of electrolyte composition in optimizing the ECH process for sustainable fuel production.

Electrified interfaces are ubiquitous in heterogeneous electrocatalysis. To achieve an atomistic description of their reactivity, grand-canonical density functional theory (GC-DFT) has become one of the most popular and convenient approaches. Moreover, GC-DFT is almost unavoidable to capture non-Nernstian effects. However, the accuracy of the widely used implicit solvation models that are usually used in combination with GC-DFT is limited for metal/water interfaces. We have developed a quantum mechanics (QM)/molecular mechanics (MM) approach, applied separately to the surface and the solvent, respectively, to model the solvent in a more realistic way and maintaining an acceptable computational cost. First, GC-DFT with an implicit solvent is employed to optimize the geometry and determine the atomic charges. This information is then transferred to the MM part, where solvent molecules and electrolyte ions are explicitly added. Following molecular dynamics for the solvent and electrolyte, the average charge distribution is extracted and combined with GC-DFT to retrieve the electrochemical potential. In this work, the hybrid QM/MM scheme with electrostatic embedding is assessed against an experimental model system: the potential-dependent re-orientation of pyridine adsorbed on Au(111). Three adsorption modes of pyridine to Au surface have been reported experimentally: parallel adsorption of the aromatic ring, tilted and perpendicular adsorption via lone-pair electrons of the nitrogen atom. In contrast to implicit solvents, the transition potential is predicted in better agreement with experiment when explicitly taking the solvent into account. While trends are found to be robust across a panel of methods (solvent models and density functional approximations), our benchmarking study demonstrates that both the precise nature of the solvent treatment and, to a minor extent, the choice of the density functional approximation impacts the reorientation potential and the adsorption energy. Hence, quantitative agreement between computation and experiment for the transition potential, potential-dependent coverage and adsorption energy has not been achieved. This illustrates that the subtle competition between pyridine and water adsorption makes pyridine adsorption on Au(111) a very challenging model system.

The enrichment of platinum (Pt) in marine ferromanganese (FeMn) deposits has attracted persistent interest for over half a century; yet the chemical form of Pt remains unclear. Here, we collected Pt-enriched FeMn crusts and nodules from the world’s oceans and used high-energy-resolution X-ray absorption spectroscopy (XAS) to decipher how Pt is sequestered at the atomic level. Platinum occurs in its tetravalent form, resulting from the oxidation of divalent Pt in seawater upon contact with Mn oxides. Tetravalent platinum is precipitated as α-PtO 2 nanoparticles with longer Pt-Pt distances than well-crystallized α-PtO 2 . Density functional theory (DFT) shows that the local structure of Pt is well represented by α-PtO 2 layers topotactically stacked on vernadite phyllomanganates. Evidence of α-PtO 2 nanoparticles challenges previous hypotheses that Pt exists as discrete metallic particles or within phyllomanganate MnO 2 layers replacing Mn. Since α-PtO 2 is the most thermodynamically stable Pt oxide, this Pt form may represent the ultimate sedimentary sink of Pt in oceans.

The electrochemical reduction of CO2 (CO2RR) is a promising strategy for mitigating climate change and producing valuable fuels and chemicals, yet its efficiency and selectivity remain major challenges. This study provides thermodynamic and kinetic insights into CO2RR catalyzed by the sulfur edge of MoS2 under acidic conditions using grand-canonical density functional theory. The limiting potential toward hydrogenated products beyond CO is identified to be -0.47 V vs SHE. In order to assess the activity and selectivity of the MoS2 sulfur edge, various mechanisms are explored, and key activation energies are determined to analyze the selectivity toward oxygenated and deoxygenated C-1 and C-2 products, i.e., C-O bond breaking and C-C coupling. For the latter, CO insertion, CHx dimerization, and CHO dimerization (glyoxal pathway) are investigated. The hydrogen coverage of the MoS2 edge (tuned by hydrogen evolution) significantly affects the kinetics of C-O cleavage and C-C coupling. The first C-O bond scission involving COOH is evaluated at 0.8 eV. For the subsequent C-O bond scissions, hydrogenated oxygenate intermediates exhibit lower C-O cleavage barriers (down to 0.4 eV for CHOH) than nonhydrogenated intermediates (>= 1.0 eV). For C-C coupling, CH2 dimerization emerges as the most favorable pathway with an activation energy of 0.4-0.6 eV. Methanol is identified as the primary C-1 product, while efficient production of multi-carbon products would require an optimal balance between hydrogen evolution, PCET-driven hydrogenation, C-C coupling, and C-O bond scission. The kinetic and thermodynamic results on the S-edge of MoS2 are compared with previous theoretical studies on copper-based catalysts.

Efficient hydrogen evolution reaction (HER) catalysts that reduce the use of noble metals and can be synthesized on a large scale are essential for advancing anion exchange membrane water electrolyzers (AEMWEs) toward commercialization. Herein, we present a composite catalyst in which Ru nanoparticles coexist with Ru single-atom alloys (SAAs) dispersed within Ni nanoparticles (Ru-SAA/Ni), creating a highly active HER electrocatalyst. Using a one-pot and scalable synthesis method, we can tune the material composition from SAA, i.e. materials containing atomically dispersed Ru atoms (with ≤0.4 at% Ru) to composite structures in which SAAs coexist with Ru NPs. Comprehensive characterization using XPS, XAS, and TEM confirms Ru-SAA formation at a low Ru content and composite structures at higher contents. Electrochemical evaluations conducted in a three-electrode setup reveal that Ru-SAA/Ni composites achieve HER performance on par with that of Pt/C. Computational insights suggest that water dissociation is significantly faster at the Ru/Ni interface compared with that on extended surfaces. These active sites are thermodynamically as active as basal planes, preventing the excessive accumulation of reaction intermediates (H*, OH*). All these results highlight the synergistic interaction between Ru SAAs and Ru nanoparticles and their potential for large-scale applications with minimal use of precious metals. Finally, the materials are processed and tested in AEMWEs, achieving 1.85 V at 0.5 A cm-2 with a total noble metal loading of only 0.1 mg cm-2.

Room-temperature phosphoresence (RTP) in purely organic systems has attracted significant attention due to its promising applications in organic electronics, bioprobes or even security solutions. Despite the well-established observation of RTP, a comprehensive materials perspective on the underlying photophysical mechanism is still missing. Here, we investigate an archetypical class of host/guest systems, carbazoles doped with benzoindoles, using periodic (time-dependent) density functional theory. We first confirm that the phosphoresence is due to the benzoindole guest. Second, we demonstrate that the triplet excited state located on the benzoindole guest is significantly lower in energy than charge-separated states between the host and the guest. This strongly suggests that an energy transfer from the host to the guest, rather than charge transfer, is involved in the RTP mechanism. Finally, we use the valence and conduction bandwidth obtained by periodic calculations to estimate the transfer integral of charge carriers. We underline the role in the RTP efficiency of the herringbone organization of the carbazole units. Having elucidated the key properties leading to RTP in this class of systems, we extended the analysis to a newly-designed OMe functionalized host. With this classical functionalization, used to increase the donor character of molecule, we clearly show both computationally and experimentally that the electronic and structural requirements of this host/guest system are preserved and indeed lead to RTP. This corroborates the mechanistic insights and suggests that a design of organic host/guest RTP systems is within reach.

We report for the first time a one-pot catalytic route for the selective hydrogenation of biomass-derived 5-hydroxymethylfurfural (HMF) into 1,2,5-hexanetriol and related value-added products such as 1-hydroxyhexane-2,5-dione (HHD) and 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTHF) using a Ru/TiO2 catalyst. In particular, the influence of the solvent composition (water-dioxane) on the reaction pathways for HMF ring opening and ring reduction products was investigated, revealing that water acts not only as a sustainable solvent but also as a reactive substrate, promoting OH-group rearrangements. Under optimized conditions, BHMTHF was obtained in >99% yield in pure 1,4-dioxane, while HHD and 1,2,5-hexanetriol were produced at 90% and 75% yields, respectively, in pure aqueous media. DFT calculations on Ru(0001) surfaces indicate that water increases the concentration of surface hydroxyl species (OH*), facilitating selective ring-opening reactions. Catalyst characterization showed minor increases in Ru particle size and solvent-dependent metallic Ru availability, influenced by carbon deposition. Nevertheless, the Ru/TiO2 catalyst retained high activity and selectivity over several cycles in both solvents. This study demonstrates a solvent-directed, highly selective strategy for converting HMF into valuable chemicals, integrating experimental and theoretical insights to advance green catalytic processes.

Understanding how critical metals such as platinum (Pt) are enriched in marine environments is essential for advancing knowledge of their geochemical cycles and the formation of oceanic mineral resources. Pt occurs in seawater at picomolar concentrations, yet deep-sea ferromanganese (FeMn) deposits contain up to eight orders of magnitude more Pt. The mechanism behind this enrichment remains unresolved. Although FeMn deposits exhibit high Pt partitioning, their low Pt levels and the co-occurrence of multiple elements pose challenges to investigating Pt oxidation state and uptake processes using traditional spectroscopic methods. In this study, high-energy-resolution X-ray absorption spectroscopy (XAS) was used to elucidate how Pt is sequestered at the atomic scale in FeMn crusts and nodules from the Pacific, Atlantic, and Indian oceans. Pt is present in the tetravalent form, resulting from the oxidation of dissolved divalent Pt in seawater upon interaction with Mn oxides. Tetravalent Pt is precipitated as α-PtO2 nanoparticles with longer Pt-Pt distances than in well-crystallized α-PtO2. Density functional theory calculations indicate that the local structure of Pt is best described by α-PtO2 layers topotactically stacked on vernadite phyllomanganates, similar to (Co,Ni)-asbolane. The Pt content depth profiles are constant during spot analyses by laser ablation-inductively coupled plasma-mass spectrometry, suggesting that the Pt nanoparticles are uniformly dispersed within the ablated volume, rather than occurring as discrete metallic nuggets. These findings reveal a new pathway for Pt immobilization in marine environments and suggest that thermodynamically stable α-PtO2 serves as a long-term sink for oceanic Pt. The advanced capabilities of high-energy-resolution XAS offer new opportunities to deepen understanding of how trace metals are sequestered in structurally disordered and chemically complex natural materials, with potential applications in georesource exploration.

The oxidation of cerium (Ce) from the soluble trivalent state (Ce(III)) to the insoluble tetravalent state (Ce(IV)) on manganese (Mn) oxides critically influences its environmental fate and geochemical cycle, and is also of interest in water treatment. However, a comprehensive mechanistic understanding of how Ce is immobilized upon interaction with Mn oxides in soils and marine sediments is still lacking. The bonding structure of Ce on δ-MnO2, the most abundant Mn oxide, was investigated by X-ray absorption spectroscopy at environmentally relevant pH and Ce concentration, and the oxidation reaction was modeled by atomistic calculation. Ce(III) is adsorbed as a six-coordinate complex at particle edges and a nine-coordinate complex at Mn(IV) vacancy sites of the MnO2 phyllomanganate layer. Ce(III) oxidation is nonspontaneous and requires hydrolysis of the sorption complexes to proceed. Gibbs free energy calculations of possible oxidation pathways show that electron transfers from Ce(III) to Mn(IV) at edge sites, and from Ce(III) to interlayer Mn(III) at vacancy sites, are thermodynamically favorable. Thus, the redox reactivity of δ-MnO2 depends on its crystallographic structure and the Mn valence. Our findings show that Mn(IV) and Mn(III) are kinetically more effective oxidants of Ce(III) than dissolved oxygen, and therefore, that cerium can be immobilized by Mn oxides even under suboxic conditions. The new mechanistic insights from this study improve understanding of the oxidative uptake of Ce by Mn oxides and its relevance to natural and engineered systems.

Deep-sea rare earth elements (REEs) have attracted global interest as terrestrial resources become less available and demand rises sharply. Seafloor REEs are hosted in authigenic carbonate fluorapatite (a-CFA), formed by the precipitation of calcium and phosphate, and in biogenic carbonate fluorapatite (b-CFA), derived from marine organisms’ skeletons. The ease with which REEs can be extracted from pelagic sediments via acid leaching challenges the common view that they are structurally incorporated into CFA, given fluorapatite’s low solubility. This apparent paradox was elucidated by investigating the nanostructure of samarium (Sm), used as a chemical REE probe. Using transmission electron microscopy and X-ray absorption spectroscopy, we found that the CFA crystals are nanosized and that Sm is not incorporated into the CFA lattice but instead primarily resides in an amorphous phase surrounding the a-CFA and b-CFA nanocrystals. At the atomic scale, Sm has a disordered apatitic bonding environment, and atomistic modeling suggests it clusters with other REEs. The facile extractability of REEs from pelagic sediments is attributed to the poor crystallinity of the host matrix at the atomic scale. This study demonstrates that understanding the location and atomic structure of critical elements enables quantitative prediction of their macroscopic properties. This knowledge can help improve recovery processes and inform the design of more efficient remediation strategies that protect the environment and public health. Rare earth elements accumulate at the fluorapatite-seawater interface in a poorly crystalline phase, making them both enriched in marine sediments and easy to extract by mild acid leaching, according to spectroscopy, geochemical and computational modeling analysis.

The oxidation of organic molecules such as 5-hydroxymethylfurfural (HMF) is a promising alternative to water oxidation in electrolysers generating green hydrogen. While the thermodynamics of HMF oxidation is considerably more favorable than water oxidation (difference of similar to 1 V), the onset potentials on Earth-abundant catalysts such as NiOOH are only 0.1 to 0.2 V lower than the ones for water oxidation. One of the reasons is the generation of NiOOH itself, which already requires a minimal potential of 1.4 to 1.5 V vs RHE. Second, the oxidation reaction itself is not very fast, as mainly "chemical" (rather than electrochemical) reaction steps dominate. To understand these well-established experimental observations, we here present a detailed grand-canonical density functional theory-based atomistic mechanism and determine key activation energies as well as their electrochemical potential dependence. We show that the C-H activation of the aldehyde functional groups is a limiting step (activation energy of similar to similar to 1 eV), while the oxidation of the alcohol functional group to the aldehyde can be achieved in a single step with a low activation energy (similar to 0.4 eV). A second limitation is the favorable desorption of carboxylate intermediates, which amounts to a reduced selectivity when the reaction is not driven to completion. Substituting part of the Ni atoms by Cr or Mn lowers the oxidation potential to reach the NiOOH-like phase. The mechanistic investigation of Cr and Mn substituted NiOOH surfaces confirms that the oxidative power of the corresponding Ni NiIII centers is still sufficient to drive HMF oxidation, suggesting that such substituted catalysts should enable lower onset potentials compared to pure Ni catalysts. Thereby, our study contributes to the rationalization of mechanistic insights under realistic conditions and to the design of more efficient nickel oxyhydroxide catalysts.

The electrocatalytic hydrogenation (ECH) of furfural (FF) to 2-methylfuran (2MF) is a promising route for producing sustainable biofuels and aviation fuels from nonedible biomass. However, achieving high selectivity and Faradaic efficiency (FE) for 2MF remains a significant challenge due to the competing hydrogen evolution reaction and the formation of furfuryl alcohol (FOH). In this study, we demonstrate that electrolyte engineering can greatly enhance the selectivity of FF ECH toward 2MF. Using a copper electrocatalyst at pH 2, we show that a phosphate-based electrolyte boosts 2MF selectivity (72.1%) compared to a sulfate-based electrolyte (54.8%) at -0.5 V vs RHE. Using in situ surface-enhanced Raman spectroscopy (SERS), we demonstrate that H3PO4 can form hydrogen bonds with FF adsorbates, thus acting as a proton relay. This promotes a proton-coupled electron transfer (PCET) mechanism steering selectivity toward 2MF. We further establish a direct correlation between 2MF selectivity and the intensity ratio of Raman bands associated with PCET and FOH intermediates under a wide range of conditions (pH, potential, and electrolyte). Finally, by introducing bromide ions into the phosphate electrolyte, the competing hydrogen atom transfer (HAT) pathway is further suppressed, achieving a record-high Faradaic efficiency of 91.4 ± 5.7% and a relative selectivity of 89.0 ± 4.3% for 2MF. Taken together, these results highlight the critical role of electrolyte composition in optimizing the ECH process for sustainable fuel production.

The oxidative enrichment and isotopic fractionation of cerium (Ce) in contact with vernadite (δ-MnO2) serve as a proxy for past redox conditions in both terrestrial and marine environments. However, the molecular processes that govern the scavenging of Ce from the dissolved 3+ to the insoluble 4+ oxidation states remain obscure. Adsorption experiments on synthetic δ-MnO2 suggest that aqueous Ce(III) precipitates as ceric hydroxide (Ce(OH)4), an unknown mineral. Here, the atomic-scale structure of Ce in natural vernadite from ferromanganese crusts collected across the Pacific, Atlantic, and Indian Oceans was examined using advanced high-energy-resolution extended X-ray absorption fine structure spectroscopy. The findings provide direct evidence for the uptake of Ce as mononuclear Ce(IV) complexes at the layer-edge sites (DES complex) and Mn(IV) vacancy sites of vernadite. Density functional theory-based Gibbs free-energy calculations indicate that hydrolysis of the DES complex promotes the oxidation of Ce(III) to Ce(IV). Quantum mechanical calculations predict that the equilibrium 136Ce/140Ce isotope fractionation factor between Ce(III) dissolved in seawater and the Ce(IV) complexes can reach 1.2-1.3 ‰ at 25 °C, indicating that the 136Ce/140Ce ratio has high potential as a new paleoredox proxy. Seawater cerium (III) forms mononuclear complexes in vernadite from global ferromanganese crusts and is oxidized to cerium (IV) during hydrolysis, as shown by atomic-scale analysis and quantum calculations. This leads to strong Ce isotope fractionation.

Nickel oxyhydroxide (NiOOH), featuring redox-active NiIII, is a one of the best non-noble electro- oxidation catalyst in alkaline solution. However, NiOOH is only stable at potentials ≥ 1.5V vs RHE, with Ni(OH)2 being the stable reduced form at lower potentials. The potential of the phase transition from inactive Ni(OH)2 to active NiOOH can be tuned by doping. Lowering the potential for reaching the phase-transition is thought to be beneficial for lowering the overpotential of oxidation reactions catalysed by NiOOH. Here, we investigate which first row transition metals are most plausible for this purpose: First, the doped structure should be more stable than the phase-segregated system and second the potential for reaching the NiOOH-like phase should be lower compared to the pure Ni compound. Substitutional doping of NiOOH is found to be plausible for many dopants, but only V can be incorporated exothermically compared to their pure oxyhydroxides. Furthermore, dopants lead to a substantial lowering in the potential necessary to reach the phase transition. Since catalysis is more a surface than a bulk process, we then investigate the surface state of NiOOH and the impact of substitutional doping on it. To address this question, we apply grand-canonical density functional theory (GC-DFT) in order to explicitly account for the electrochemical potential. We find that the stoichiometric surface (50% hydrogen coverage) is the most stable one over a large range of relevant potentials at pH 14. Oxidizing the surface lowers the hydrogen coverage and occurs at about 1.7 V vs RHE, i.e.,∼0.2 V less positive compared to the potential of the phase transition. At a doping level of 25%, only V and Cr allow to stabilize NiIII at significantly lower potentials compared to pure NiOOH (down to 1.1 V vs RHE) in the bulk. Furthermore, vanadium, chromium and manganese might be suitable choices as these metal centers, which remain in the +III or +IV state at lower potentials compared to Ni, could also act as active sites in electro-oxidation reactions.

Deep-sea mud is rich in rare earth elements and yttrium (REY), with yttrium (Y) exhibiting the highest concentration. REY are found in authigenic (a-CFA) and biogenic (b-CFA) carbonate fluorapatite (CFA, Ca5(CO3) x (PO4)3-x F1+x ). The presence of REY in both CFA types suggests different enrichment processes in abyssal environments, which may be traced through detailed structural analysis of REY's coordination chemistry. The bonding environment of Y in CFA was investigated in 2018 and 2023 using extended X-ray absorption fine structure (EXAFS) spectroscopy at a resolution of 0.15 Å. While these studies offered valuable insights into Y's short-range order, they also presented inconsistencies. Moreover, a resolution of 0.15 Å is insufficient to uncover the intricate local structure of Y in CFA. Here, we present EXAFS data at a resolution of 0.11 Å for Y in a-CFA and b-CFA collected several meters beneath the Pacific Ocean seafloor. Y is predominantly hydrated and bound to Ca and PO4 in an amorphous phase surrounding the a-CFA and b-CFA nanocrystals and is secondarily incorporated into the crystal structure of a-CFA. There is no EXAFS evidence indicating the presence of polynuclear Y precipitate, which contrasts with a recent finding on cerium (Ce), nor supporting the formation of a Y-carbonate complex. The latter two findings are backed by density functional theory, which indicates that Y-Y pair formation is thermodynamically unfavorable and that the predicted Y-C distance is inconsistent with the EXAFS distances. This research highlights the geochemical enrichment of Y in abyssal sediments through the formation of a hydrated yttrium-calcium phosphate phase in a-CFA and b-CFA and Y for Ca substitution in authigenic a-CFA nanocrystals during the coprecipitation of calcium and phosphate.

Fluorapatite (FAp, nominally Ca10(PO4)6F2) has been identified as an important host-material for rare earth elements and yttrium (REY) in marine sediments. REY can be accommodated in either the larger 6+3 coordinated Ca1 site or the smaller 6+1 coordinated Ca2 site, yet little is known about the site preference of REY through the lanthanide series despite its importance for understanding REY enrichment processes in FAp. Theoretical investigations based on density functional theory (DFT) predict that all REY intrinsically prefer the smaller and more ionic Ca2 site. The Ca2 site preference is less pronounced when the excess of positive charge resulting from the REY3+ for Ca2+ substitution is compensated by a coupled Na+ for Ca2+ substitution, instead of the energetically more favorable Si4+ for P5+ coupled substitution. The site preference varies quadratically with the ionic radius of REY and linearly with the sum of their first and second ionization energies. The quadratic shape of the site preference is similar to the shape of Onuma diagrams, which suggests that the local effective elastic constant of the site controls the site preference, rather than the nominal size of the site. Despite being smaller, the Ca2 site has a lower effective elastic constant, and is, therefore, more flexible than the Ca1 site for accommodating larger and smaller trivalent REY cations. Concentration-dependent computations show that REY clustering is thermodynamically favorable, except for Yb and Lu.

Oxygen evolution reaction (OER) is a key process for sustainable energy, although renewable sources require the use of proton exchange membrane electrolyzers, with IrO2-based materials being the gold standard under anodic polarization conditions. However, even for the (110) facet of a single-crystalline IrO2 model electrode, the reaction mechanism is not settled yet due to contradictory reports in literature. In the present manuscript, we disentangle the conflicting results of previous theoretical studies in the density functional theory approximation. We demonstrate that dissimilar reaction mechanisms and limiting steps for the OER over IrO2(110) are obtained for different active surface configurations present on the IrO2 electrode. In contrast to previous studies, we factor Walden-type mechanisms, in which the formation of the product O2 and adsorption of the reactant H2O occur simultaneously, into the analysis of the elementary steps. Combining free-energy diagrams along the reaction coordinate and Bader charge analysis of the active site, we elucidate why mononuclear- or bifunctional-Walden pathways excel the traditional OER mechanisms for the OER over IrO2(110). Our computational methodology to identify the reaction mechanism and limiting step of proton-coupled electron transfer steps is widely applicable to electrochemical processes in the field of energy conversion and storage.

Deep-sea mud is rich in rare-earth elements, primarily found in fluorapatite, a mineral deposit that forms over hundreds of thousands to millions of years through the accumulation of fish remains. After fish die, biogenic apatite captures rare earth elements from seawater on the seafloor and from pore waters during the diagenesis process. The conventional model for rare earth element enrichment suggests that they are incorporated into the bioapatite crystal structure through solid-state diffusion. However, our data reveal that cerium atoms are instead precipitated within an amorphous layer surrounding bioapatite nanocrystals, as shown by high-energy-resolution X-ray absorption spectroscopy and transmission electron microscopy. Computational simulations further support this finding, predicting that cerium atoms cluster on the surface of fluorapatite. These results suggest that the fluorapatite-water interface plays a crucial role in the enrichment of cerium, as well as other rare earth elements, in marine sediments.