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All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Multiscale Electricity Modeling for Evaluating Carbon Capture and Sequestration Technologies (Final Report)

Carbon capture and sequestration (CCS) technologies that can operate with a high degree of operating flexibility could provide necessary electric grid flexibility in a system with high shares of variable renewables. This project examines the deployment and dispatch potential of twelve unique flexible CCS (FLECCS) technologies that encompass post-combustion carbon dioxide (CO 2 ) capture designs, concepts using a storage media to enable energy arbitrage, and hybrid processes that integrate CCS with direct air capture (DAC) for flexibility with net zero or negative CO 2 emissions. FLECCS technology potential is explored with a multi-model, multi-scale framework including the Regional Energy Deployment System (ReEDS) electric sector capacity expansion model (CEM) and the PLEXOS production cost model (PCM). Innovative methods were developed to represent FLECCS technology operating modes, performance, and cost in the two models. ReEDS was then used to simulate nine scenarios for each FLECCS technology, three CO 2 emissions price futures reaching $\$$150, $\$$225, and $\$$300/tCO 2 in 2050; and three scenarios for FLECCS technology deployment favorability relative to competing technologies. For each CO 2 price and reference FLECCS favorability, the 2050 infrastructures from ReEDS model are downscaled and implemented in PLEXOS to examine hourly dispatch under detailed operational constraints that are not included in ReEDS. FLECCS technologies exhibited a wide range of deployment potential ranging from none to several hundred gigawatts of capacity, with outcomes highly sensitive to input cost and performance parameters that are inherently highly uncertain. When deployed, FLECCS tended to displace a combination of wind, solar, and natural gas-based technologies rather than supporting increased renewable deployment. As a result, CO 2 emissions reductions facilitated by FLECCS deployment tended to come with higher overall system costs and electricity prices. When economically competitive, FLECCS technologies can contribute significant flexible generation and firm capacity to the grid, but continued technology development and an expanded analytical scope are necessary to fully understand FLECCS deployment potential its impact on the electric power sector. Follow-on analysis incorporating captured CO 2 tax credit value from the Inflation Reduction Act (IRA) and other potential policy scenarios could be particularly valuable, as this policy can substantially change the relative competitiveness of FLECCS technologies.

03 NATURAL GAS↗
DOE OSTI2020

Materials Data on Ho2CdS4 by Materials Project

CdHo2S4 crystallizes in the tetragonal I-4m2 space group. The structure is three-dimensional. there are two inequivalent Ho3+ sites. In the first Ho3+ site, Ho3+ is bonded to six S2- atoms to form HoS6 octahedra that share corners with four equivalent HoS6 octahedra, edges with four equivalent HoS6 octahedra, and edges with four equivalent CdS6 octahedra. The corner-sharing octahedral tilt angles are 3°. There are two shorter (2.71 Å) and four longer (2.75 Å) Ho–S bond lengths. In the second Ho3+ site, Ho3+ is bonded to six S2- atoms to form HoS6 octahedra that share corners with two equivalent CdS6 octahedra, corners with four equivalent HoS6 octahedra, edges with four equivalent HoS6 octahedra, and edges with four equivalent CdS6 octahedra. The corner-sharing octahedra tilt angles range from 0–2°. There are two shorter (2.68 Å) and four longer (2.75 Å) Ho–S bond lengths. Cd2+ is bonded to six S2- atoms to form CdS6 octahedra that share corners with two equivalent HoS6 octahedra, corners with four equivalent CdS6 octahedra, and edges with eight HoS6 octahedra. The corner-sharing octahedra tilt angles range from 0–2°. There are four shorter (2.75 Å) and two longer (2.82 Å) Cd–S bond lengths. There are two inequivalent S2- sites. In the first S2- site, S2- is bonded in a square co-planar geometry to three Ho3+ and one Cd2+ atom. In the second S2- site, S2- is bonded to three Ho3+ and two equivalent Cd2+ atoms to form a mixture of edge and corner-sharing SHo3Cd2 square pyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Insights into the Mechanism of Neptunium Oxidation to the Heptavalent State

Abstract Neptunium can exist in multiple oxidation states, including the rare and poorly understood heptavalent form. In this work, we monitored the formation of heptavalent neptunium [Np(VII)O 4 (OH) 2 ] 3− during ozonolysis of aqueous M OH ( M =Li, Na, K) solutions using a combined experimental and theoretical approach. All experimental reactions were closely monitored via absorption and vibrational spectroscopy to follow both the oxidation state and the speciation of neptunium guided by the calculated vibrational frequencies for various neptunium species. The mechanism of the reaction partly involves oxidative dissolution of transient Np(VI) oxide/hydroxide solid phases, the identity of which are dependent on the co‐precipitating counter‐cation Li + /Na + /K + . Additional calculations suggest that the most favorable energetic pathway occurs through the reaction of a [Np(V)O 2 (OH) 4 ] 3− with the hydroxide radical to form [Np(VI)O 2 (OH) 4 ] 2− , followed by an additional oxidation with HO⋅ to create [Np(VII)O 4 (OH) 2 ] 3− .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Matrimid substrates with bicontinuous surface and macrovoids in the bulk: A nearly ideal substrate for composite membranes in CO 2 capture

Scalable fabrication of thin-film composite (TFC) membrane for post-combustion carbon capture is often limited by the availability of high-performance nanoporous substrate. Ideally, the substrate should allow for fast gas transport at the selective layer/substrate interface as well as in the bulk of the substrate. In this study, highly permeable Matrimid substrates were prepared via vapor- and nonsolvent-induced phase separations. The phase separation mechanism of the Matrimid®/N-methyl-2-pyrrolidone casting solution was modulated by the addition of LiCl to control the solution thermodynamic stability, and nanoporous Matrimid substrates were formed with a bicontinuous surface and macrovoids in the bulk. This hierarchically optimized structure resulted in a CO 2 permeance of 2.60 × 10 5 GPU (1 GPU = 1 × 10 –6 cm 3 (STP) cm –2 s –1 cmHg –1 = 3.349 × 10 –10 mol m –2 s –1 Pa –1 ), which was ca. 11 times more permeable than a benchmark polyethersulfone substrate with cellular pores. The benefit of using this new substrate was demonstrated by coating a 170-nm amine-containing polymer to form a TFC facilitated transport membrane. The membrane exhibited a CO 2 permeance of 932 GPU at 57°C, which was 72 GPU higher than the counterpart coated on the benchmark substrate. Meanwhile, the CO 2 /N 2 selectivity was remained at 158. This permeance improvement could be well explained by the resistance-in-series model, where the improved permeance was attributed to the reductions in substrate and lateral diffusion mass transfer resistances. The upper bound (UB) analysis indicates that the Matrimid substrate is nearly ideal for the state-of-the-art polymers for CO 2 /N 2 separation. Here, the substrate improvement could also reduce the parasitic energy associated with the membrane process due to the reduced membrane cost and flue gas compression requirement.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Investigation of mechanochemical interactions of hydrogen with earth materials in a subsurface gas storage

The goal of this project is to investigate the molecular interactions of H 2 with earth materials (EMs) that may potentially affect economics and safety of H 2 geological storage (HGS). We investigated (1) the H 2 intercalation into interlayers of phyllosilicates, (2) the competitive adsorption of H 2 /CH 4 onto porous materials, and (3) solubility of H 2 in interfacial and confined hydrocarbons. Our results indicate that (i) H 2 intercalation into hydrated interlayers is thermodynamically unfavorable and H 2 solubility in hydrated clay interlayers is in the same order of magnitude as that in bulk water, (ii) CH 4 outcompetes H 2 in adsorption onto kerogen, due to stronger CH 4 -kerogen interactions than H 2 -kerogen interactions, (iii) H 2 tends to dissolve more in oil than in water, and the introduction of CO 2 as a cushion gas reduces H 2 partitioning near the kaolinite surfaces. The outcomes provide foundational knowledge for preparing the USA for future storage site selection and storage system design, supporting DOE missions in clean and secured energy.

08 HYDROGEN↗
DOE OSTI2020

Crystal structures and comparisons of huntite aluminum borates RE Al 3 (BO 3 ) 4 ( RE = Tb, Dy and Ho)

Three huntite-type aluminoborates of stoichiometry RE Al 3 (BO 3 ) 4 ( RE = Tb, Dy and Ho), namely, terbium/dysprosium/holmium trialuminium tetrakis(borate), were synthesized by slow cooling within a K 2 Mo 3 O 10 flux with spontaneous crystallization. The crystal structures were determined using single-crystal X-ray diffraction (SC-XRD) data. The synthesized borates are isostructural to the huntite [CaMg 3 (CO 3 ) 4 ] structure and crystallized within the trigonal R 32 space group. The structural parameters were compared to literature data of other huntite RE Al 3 (BO 3 ) 4 crystals within the R 32 space group. All three borates fit well into the trends calculated from the literature data. The unit-cell parameters and volumes increase linearly with larger RE cations whereas the densities decrease. All of the crystals studied were refined as inversion twins.

36 MATERIALS SCIENCE↗
DOE OSTI2021

100% Carbon-Free Electricity for Sandia NM and KAFB Using Concentrating Solar Power (CSP) (SAND Report)

This report provides a design study to produce 100% carbon-free electricity for Sandia NM and Kirtland Air Force Base (KAFB) using concentrating solar power (CSP). Annual electricity requirements for both Sandia and KAFB are presented, along with specific load centers that consume a significant and continuous amount of energy. CSP plant designs of 50 MW and 100 MW are then discussed to meet the needs of Sandia NM and the combined electrical needs of both Sandia NM and KAFB. Probabilistic modeling is performed to evaluate inherent uncertainties in performance and cost parameters on total construction costs and the levelized cost of electricity. Total overnight construction costs are expected to range between ~$300M - $400M for the 50 MW CSP plant and between ~$500M - $800M for the 100 MW plant. Annual operations and maintenance (O&M) costs are estimated together with potential offsets in electrical costs and CO 2 emissions. Other considerations such as interconnections, land use and permitting, funding options, and potential agreements and partnerships with Public Service Company of New Mexico (PNM), Western Area Power Administration (WAPA), and other entities are also discussed.

14 SOLAR ENERGY↗
DOE OSTI2021

Persistent Structure and Frustrated Magnetism in High Entropy Rare-Earth Zirconates

The configurational complexity and distinct local atomic environments of high entropy oxides remain largely unexplored, leaving structure-property relationships and the hypothesis that the family offers rich tunability for applications ambiguous. This work investigates the influence of cation size and materials synthesis in determining the resulting structure and magnetic properties of a family of high entropy rare-earth zirconates (HEREZs, nominal composition RE 2 Zr 2 O 7 with RE = rare-earth element combinations including Eu, Gd, Tb, Dy, Ho, La, or Sc). Here, the structural characterization of the series is examined through synchrotron X-ray diffraction and pair distribution function analysis, and electron microscopy, demonstrating average defect-fluorite structures with considerable local disorder, in all samples. The surface morphology and particle sizes are found to vary significantly with preparation method, with irregular micron-sized particles formed by high temperature sintering routes, spherical nanoparticles resulting from chemical co-precipitation methods, and porous nanoparticle agglomerates resulting from polymer steric entrapment synthesis. In agreement with the disordered cation distribution found across all samples, magnetic measurements indicate that all synthesized HEREZs show frustrated magnetic behavior, as seen in a number of single-component RE 2 Zr 2 O 7 pyrochlore oxides. These findings advance the understanding of the local structure of high entropy oxides and demonstrate strategies for designing nanostructured morphologies in the class.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on NaHo(Pd3O4)2 by Materials Project

NaHo(Pd3O4)2 crystallizes in the cubic Pm-3 space group. The structure is three-dimensional. Na1+ is bonded in a body-centered cubic geometry to eight O2- atoms. All Na–O bond lengths are 2.63 Å. Ho3+ is bonded in a body-centered cubic geometry to eight O2- atoms. All Ho–O bond lengths are 2.42 Å. Pd2+ is bonded in a square co-planar geometry to four O2- atoms. All Pd–O bond lengths are 2.06 Å. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded to one Na1+, one Ho3+, and three equivalent Pd2+ atoms to form a mixture of edge and corner-sharing ONaHoPd3 trigonal bipyramids. In the second O2- site, O2- is bonded to one Na1+, one Ho3+, and three equivalent Pd2+ atoms to form a mixture of edge and corner-sharing ONaHoPd3 trigonal bipyramids. All O–Pd bond lengths are 2.06 Å. In the third O2- site, O2- is bonded to one Na1+, one Ho3+, and three equivalent Pd2+ atoms to form a mixture of edge and corner-sharing ONaHoPd3 trigonal bipyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ba2Zn(HO)6 by Materials Project

Ba2Zn(HO)6 crystallizes in the monoclinic P2_1/c space group. The structure is two-dimensional and consists of one Ba2Zn(HO)6 sheet oriented in the (-1, 0, 2) direction. Ba2+ is bonded in a 6-coordinate geometry to two H1+ and four O2- atoms. There are one shorter (2.84 Å) and one longer (2.96 Å) Ba–H bond lengths. There are a spread of Ba–O bond distances ranging from 2.50–2.79 Å. Zn2+ is bonded in a square co-planar geometry to four O2- atoms. There are two shorter (1.82 Å) and two longer (2.57 Å) Zn–O bond lengths. There are three inequivalent H1+ sites. In the first H1+ site, H1+ is bonded in a distorted water-like geometry to one Ba2+ and one H1+ atom. The H–H bond length is 0.76 Å. In the second H1+ site, H1+ is bonded in a single-bond geometry to one H1+ atom. In the third H1+ site, H1+ is bonded in a single-bond geometry to one Ba2+ and one O2- atom. The H–O bond length is 0.99 Å. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded in a single-bond geometry to one Ba2+, one H1+, and one O2- atom. The O–O bond length is 1.50 Å. In the second O2- site, O2- is bonded in a 3-coordinate geometry to one Ba2+, one Zn2+, and one O2- atom. In the third O2- site, O2- is bonded in a trigonal non-coplanar geometry to two equivalent Ba2+ and one Zn2+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Defect Engineering of WO 3 by Rapid Flame Reduction for Efficient Photoelectrochemical Conversion of Methane into Liquid Oxygenates

Photoelectrochemical (PEC) conversion is a promising way to use methane (CH 4 ) as a chemical building block without harsh conditions. However, the PEC conversion of CH 4 to value-added chemicals remains challenging due to the thermodynamically favorable overoxidation of CH 4 . Here, we report WO 3 nanotube (NT) photoelectrocatalysts for PEC CH 4 conversion with high liquid product selectivity through defect engineering. By tuning the flame reduction treatment, we carefully controlled the oxygen vacancies of WO 3 NTs. The optimally reduced WO 3 NTs suppressed overoxidation of CH4 showing a high total C1 liquid selectivity of 69.4% and a production rate of 0.174 μmol cm –2 h –1 . Scanning electrochemical microscopy revealed that oxygen vacancies can restrain the production of hydroxyl radicals, which, in excess, could further oxidize C1 intermediates to CO 2 . Additionally, band diagram analysis and computational studies elucidated that oxygen vacancies thermodynamically suppress overoxidation. In conclusion, this work introduces a strategy for understanding and controlling the selectivity of photoelectrocatalysts for direct conversion of CH 4 to liquids.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Annual carbon sequestration and loss rates under altered hydrology and fire regimes in southeastern USA pocosin peatlands

Peatlands drained for agriculture or forestry are susceptible to the rapid release of greenhouse gases (GHGs) through enhanced microbial decomposition and increased frequency of deep peat fires. We present evidence that rewetting drained subtropical wooded peatlands (STWPs) along the southeastern USA coast, primarily pocosin bogs, could prevent significant carbon (C) losses. To quantify GHG emissions and storage from drained and rewetted pocosin we used eddy covariance techniques, the first such estimates that have been applied to this major bog type, on a private drained (PD) site supplemented by static chamber measurements at PD and Pocosin Lakes National Wildlife Refuge. Net ecosystem exchange measurements showed that the loss was 21.2 Mg CO2 ha –1 year –1 (1 Mg = 10 6 g) in the drained pocosin. Under a rewetted scenario, where the annual mean water table depth (WTD) decreased from 60 to 30 cm, the C loss was projected to fall to 2 Mg CO 2 ha –1 year –1 , a 94% reduction. If the WTD was 20 cm, the peatlands became a net carbon sink (–3.3 Mg CO2 ha –1 year –1 ). Hence, net C reductions could reach 24.5 Mg CO 2 ha –1 year –1 , and when scaled up to the 4000 ha PD site nearly 100,000 Mg CO 2 year –1 of creditable C could be amassed. We conservatively estimate among the 0.75 million ha of southeastern STWPs, between 450 and 770 km 2 could be rewet, reducing annual GHG emissions by 0.96–1.6 Tg (1 Tg = 10 12 g) of CO 2 , through suppressed microbial decomposition and 1.7–2.8 Tg via fire prevention, respectively. Despite covering <0.01% of US land area, rewetting drained pocosin can potentially provide 2.4% of the annual CO 2 nationwide reduction target of 0.18 Pg (1 Pg = 10 15 g). Finally, suggesting pocosin restoration can contribute disproportionately to the US goal of achieving net-zero emission by 2050.

58 GEOSCIENCES↗
DOE OSTI2020

Materials Data on KHoCoSi2O7 by Materials Project

KHoCoSi2O7 crystallizes in the monoclinic C2 space group. The structure is three-dimensional. K1+ is bonded in a 9-coordinate geometry to nine O2- atoms. There are a spread of K–O bond distances ranging from 2.63–3.36 Å. Ho3+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Ho–O bond distances ranging from 2.14–2.55 Å. There are two inequivalent Co2+ sites. In the first Co2+ site, Co2+ is bonded to four O2- atoms to form CoO4 tetrahedra that share corners with four SiO4 tetrahedra. There is two shorter (1.97 Å) and two longer (1.99 Å) Co–O bond length. In the second Co2+ site, Co2+ is bonded to four O2- atoms to form CoO4 tetrahedra that share corners with four SiO4 tetrahedra. There are two shorter (1.99 Å) and two longer (2.02 Å) Co–O bond lengths. There are two inequivalent Si4+ sites. In the first Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share a cornercorner with one SiO4 tetrahedra and corners with two CoO4 tetrahedra. There are a spread of Si–O bond distances ranging from 1.61–1.69 Å. In the second Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share a cornercorner with one SiO4 tetrahedra and corners with two CoO4 tetrahedra. There are a spread of Si–O bond distances ranging from 1.63–1.68 Å. There are eight inequivalent O2- sites. In the first O2- site, O2- is bonded to one K1+, one Ho3+, one Co2+, and one Si4+ atom to form distorted corner-sharing OKHoCoSi tetrahedra. In the second O2- site, O2- is bonded in a 2-coordinate geometry to two equivalent Ho3+ and two equivalent Si4+ atoms. In the third O2- site, O2- is bonded in a 4-coordinate geometry to one K1+, one Ho3+, one Co2+, and one Si4+ atom. In the fourth O2- site, O2- is bonded in a 2-coordinate geometry to two equivalent K1+, one Ho3+, and one Si4+ atom. In the fifth O2- site, O2- is bonded in a 2-coordinate geometry to two equivalent K1+ and two equivalent Si4+ atoms. In the sixth O2- site, O2- is bonded in a 2-coordinate geometry to one K1+, one Ho3+, one Co2+, and one Si4+ atom. In the seventh O2- site, O2- is bonded in a 2-coordinate geometry to two equivalent K1+, one Ho3+, one Co2+, and one Si4+ atom. In the eighth O2- site, O2- is bonded in a 3-coordinate geometry to one K1+, one Ho3+, and one Si4+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Molecular complexes of low-valent ƒ-elements from lanthanum to californium

This review outlines the structure and chemistry of divalent lanthanides and actinides in small molecule systems, focusing on the less accessible divalent lanthanides such as the configurational crossover (Dy 2+ , Nd 2+ ) and non-traditional lanthanides(II) (La 2+ , Ce 2+ , Pr 2+ , Gd 2+ , Tb 2+ , Ho 2+ , Er 2+ , and Lu 2+ ) and the rarely accessible actinides(II). Expansive progress, including recent synthetic firsts in low-oxidation state ƒ-block chemistry, are reviewed and analyzed to build knowledge for future synthetic efforts.

Wineinger, Hannah B. [Colorado School of Mines, Go↗
DOE OSTI2024

Machine learning-accelerated discovery of iron cobalt phosphides as rare-earth-free magnets

Here, the discovery of rare-earth-free permanent magnets has been a goal of scientists for decades. The absence of rare-earth elements will alleviate a pressing concern about the availability of rare-earth elements used in permanent magnets. These magnets are crucial for applications such as wind turbines, electric cars, and memory devices. Rare-earth magnets are special owing to a large magnetic anisotropy energy (K 1 ). In contrast, iron cobalt phosphides hold promise since doping P into cubic FeCo can induce anisotropy, leading to a large coercivity, without introducing rare-earth elements. We present a comprehensive search over the Fe-Co-P ternary space for magnets, utilizing recently developed adaptive machine learning feedback to efficiently screen over 850 000 structures. We focus on machine learning acceleration as a paradigm for materials design. Further adaptive genetic algorithm searches and first-principles calculations aid in the identification of 16 new structures below the known convex hull. Five of them possess high magnetic polarization (J s > 1 T). The structures with desirable magnetic properties center on (Fe,Co) 2⁢ P. This supports conventional wisdom, which focuses on the mixture of the two known end compounds: Fe 2 ⁢P and Co 2 ⁢P. Our work provides guidance for synthesis. We find Fe 7 ⁢CoP 4 shows the most promise (J s = 1.03T and K 1 = 0.83MJ/m 3 ).

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Sr2Zn(HO)6 by Materials Project

Sr2Zn(HO3)2(H2)2 crystallizes in the monoclinic P2_1/c space group. The structure is two-dimensional and consists of four hydrogen molecules and one Sr2Zn(HO3)2 sheet oriented in the (-1, 0, 2) direction. In the Sr2Zn(HO3)2 sheet, Sr2+ is bonded in a 6-coordinate geometry to six O2- atoms. There are a spread of Sr–O bond distances ranging from 2.43–2.60 Å. Zn2+ is bonded in a distorted square co-planar geometry to six O2- atoms. There are a spread of Zn–O bond distances ranging from 1.96–2.57 Å. H1+ is bonded in a single-bond geometry to one O2- atom. The H–O bond length is 0.98 Å. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded in a 4-coordinate geometry to two equivalent Sr2+, one Zn2+, and one O2- atom. The O–O bond length is 1.54 Å. In the second O2- site, O2- is bonded in a distorted single-bond geometry to two equivalent Sr2+, one Zn2+, and one H1+ atom. In the third O2- site, O2- is bonded in a 1-coordinate geometry to two equivalent Sr2+, one Zn2+, and one O2- atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ba2Ho2Fe(CoO4)3 by Materials Project

Ba2Ho2Fe(CoO4)3 crystallizes in the tetragonal P4mm space group. The structure is three-dimensional. there are two inequivalent Ba2+ sites. In the first Ba2+ site, Ba2+ is bonded to twelve O2- atoms to form BaO12 cuboctahedra that share corners with four equivalent BaO12 cuboctahedra, corners with eight HoO12 cuboctahedra, faces with two HoO12 cuboctahedra, faces with four equivalent BaO12 cuboctahedra, faces with four equivalent FeO6 octahedra, and faces with four equivalent CoO6 octahedra. There are a spread of Ba–O bond distances ranging from 2.80–2.97 Å. In the second Ba2+ site, Ba2+ is bonded to twelve O2- atoms to form BaO12 cuboctahedra that share corners with four equivalent BaO12 cuboctahedra, corners with eight HoO12 cuboctahedra, faces with two HoO12 cuboctahedra, faces with four equivalent BaO12 cuboctahedra, and faces with eight CoO6 octahedra. There are a spread of Ba–O bond distances ranging from 2.80–2.92 Å. There are two inequivalent Ho3+ sites. In the first Ho3+ site, Ho3+ is bonded to twelve O2- atoms to form HoO12 cuboctahedra that share corners with four equivalent HoO12 cuboctahedra, corners with eight BaO12 cuboctahedra, faces with two BaO12 cuboctahedra, faces with four equivalent HoO12 cuboctahedra, and faces with eight CoO6 octahedra. There are eight shorter (2.56 Å) and four longer (2.80 Å) Ho–O bond lengths. In the second Ho3+ site, Ho3+ is bonded to twelve O2- atoms to form HoO12 cuboctahedra that share corners with four equivalent HoO12 cuboctahedra, corners with eight BaO12 cuboctahedra, faces with two BaO12 cuboctahedra, faces with four equivalent HoO12 cuboctahedra, faces with four equivalent FeO6 octahedra, and faces with four equivalent CoO6 octahedra. There are a spread of Ho–O bond distances ranging from 2.54–2.80 Å. Fe3+ is bonded to six O2- atoms to form FeO6 octahedra that share corners with two CoO6 octahedra, corners with four equivalent FeO6 octahedra, faces with four equivalent BaO12 cuboctahedra, and faces with four equivalent HoO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 0–12°. There are a spread of Fe–O bond distances ranging from 1.94–2.04 Å. There are three inequivalent Co+3.67+ sites. In the first Co+3.67+ site, Co+3.67+ is bonded to six O2- atoms to form CoO6 octahedra that share corners with six CoO6 octahedra, faces with four equivalent BaO12 cuboctahedra, and faces with four equivalent HoO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 0–11°. There are a spread of Co–O bond distances ranging from 1.84–2.00 Å. In the second Co+3.67+ site, Co+3.67+ is bonded to six O2- atoms to form CoO6 octahedra that share a cornercorner with one FeO6 octahedra, corners with five CoO6 octahedra, faces with four equivalent BaO12 cuboctahedra, and faces with four equivalent HoO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 0–12°. There are a spread of Co–O bond distances ranging from 1.79–1.99 Å. In the third Co+3.67+ site, Co+3.67+ is bonded to six O2- atoms to form CoO6 octahedra that share a cornercorner with one FeO6 octahedra, corners with five CoO6 octahedra, faces with four equivalent BaO12 cuboctahedra, and faces with four equivalent HoO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 0–13°. There are a spread of Co–O bond distances ranging from 1.77–1.99 Å. There are eight inequivalent O2- sites. In the first O2- site, O2- is bonded in a 2-coordinate geometry to two equivalent Ba2+, two equivalent Ho3+, and two equivalent Fe3+ atoms. In the second O2- site, O2- is bonded in a 6-coordinate geometry to two equivalent Ba2+, two equivalent Ho3+, and two equivalent Co+3.67+ atoms. In the third O2- site, O2- is bonded in a 6-coordinate geometry to two equivalent Ba2+, two equivalent Ho3+, and two equivalent Co+3.67+ atoms. In the fourth O2- site, O2- is bonded in a 6-coordinate geometry to two equivalent Ba2+, two equivalent Ho3+, and two equivalent Co+3.67+ atoms. In the fifth O2- site, O2- is bonded to four equivalent Ba2+, one Fe3+, and one Co+3.67+ atom to form a mixture of distorted edge and corner-sharing OBa4FeCo octahedra. The corner-sharing octahedral tilt angles are 1°. In the sixth O2- site, O2- is bonded to four equivalent Ba2+ and two Co+3.67+ atoms to form a mixture of distorted edge and corner-sharing OBa4Co2 octahedra. The corner-sharing octahedral tilt angles are 2°. In the seventh O2- site, O2- is bonded in a linear geometry to four equivalent Ho3+ and two Co+3.67+ atoms. In the eighth O2- site, O2- is bonded in a linear geometry to four equivalent Ho3+, one Fe3+, and one Co+3.67+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Enhanced Selectivity for C 2 H 4 Production from C 2 H 6 on Partially Chlorinated IrO 2 (110) Surfaces

Modifying metal oxide surfaces to limit their oxidizing activity can provide a means of improving catalytic selectivity toward the partial oxidation of light alkanes. Here, in this study, we investigated the oxidation of C 2 H 6 on Cl-modified IrO 2 (110) surfaces using temperature-programmed reaction spectroscopy (TPRS) and first-principles microkinetic modeling. We find that substituting Cl for O in the IrO 2 (110) surface enhances the selectivity for C 2 H 6 conversion to C 2 H 4 during TPRS by suppressing extensive oxidation to CO x products, while also either enhancing C 2 H 4 production or altering it to a lesser extent, depending on the initial C 2 H 6 coverage. The C 2 H 4 selectivity increased with increasing C 2 H 6 and Cl coverage, but reached a limiting value below 50%. The Cl coverage changed negligibly during C 2 H 6 oxidation, and the surface reactivity decreased only marginally for Cl coverages up to 0.5 ML (monolayer). TPRS simulations using a microkinetic model predict C 2 H 4 and CO x product yields as a function of the Cl coverage that agree closely with the experimental results. According to the simulations, C 2 H 6 conversion to C 2 H 4 occurs on Cl-IrO 2 (110) by the hydrogenation of C 2 H 3 * species adsorbed in blocked states, in which neighboring sites are occupied only by unreactive HO and Cl species. The microkinetic modeling shows that H-hopping away from surface HO groups provides a relatively efficient route for C 2 H 3 * to escape blocked configurations and dehydrogenate, and that this process can limit the C 2 H 4 selectivity on Cl-IrO 2 (110) under the conditions studied. Overall, our results demonstrate that Cl-substitution into IrO 2 (110) enhances the selectivity for C 2 H 4 production from C 2 H 6 and provides insights into the reaction mechanism that can guide strategies to further improve the C 2 H 4 selectivity.

IrO2↗