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[Hiroshi Mizoguchi](https://orcid.org/0000-0002-0992-7449), Yuichi Shirako, [Shusaku Shoji](https://orcid.org/0000-0002-8481-2633), [Hideki Abe](https://orcid.org/0000-0002-8392-7586), [Takeshi Fujita](https://orcid.org/0000-0002-2318-0433), [Hideo Hosono](https://orcid.org/0000-0001-9260-6728)

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[Exploring Ortho–Para Hydrogen Conversion Catalysts Based on Surface Electric Field Gradient](https://mdr.nims.go.jp/datasets/753ae947-4a40-45ef-a93b-d290c7e02f1f)

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Exploring Ortho–Para Hydrogen Conversion Catalysts Based on Surface Electric Field GradientExploring Ortho−Para Hydrogen Conversion Catalysts Based onSurface Electric Field GradientHiroshi Mizoguchi,* Yuichi Shirako, Shusaku Shoji, Hideki Abe,* Takeshi Fujita, and Hideo Hosono*Cite This: J. Phys. Chem. Lett. 2026, 17, 3701−3705 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: According to our hypothesis that ortho (O) to para (P) hydrogen conversionis promoted by an inhomogeneous electric field on the surface of the insulating oxide withhigh ionicity, we searched for OP conversion catalysts using lattice energy as an indicator ofhigh ionicity. As a result, we found new oxide catalysts, including SiO2, γ-Al2O3, and CeO2combined with 3d late transition metal cocatalysts. The fraction of para-H2 on 5%Fe-loadedSiO2 powder reached 50% (equilibrium value at 77 K) within 20 min. The activities of thesecatalysts are significantly superior to those of benchmark catalysts, such as Mn3O4. Thecocatalyst nanoparticles of 1−12 nm size dispersed on the oxide catalysts adsorb hydrogenwell without dissociating it. The nuclear spin state of ortho-H2 adsorbed at the asymmetricsite (nonzero electric field gradient) of the oxide surface is thermally excited by nuclearquadrupole interaction, and the OP conversion rate is increased by nuclear spin relaxation.Recently, the demand for liquid hydrogen as an energycarrier for transportation and storage in hydrogeneconomy has increased owing to its potential for highvolumetric and energy storage densities.1 However, there is adifficulty inherent to hydrogen. The homonuclear diatomicmolecule H2 possessing 1H nuclei (I = 1/2), has two nuclearspin isomers of ortho (O; J = 1) and para (P; J = 0). NormalH2 is a mixture of these nuclear spin isomers with an O/P ratioof 3 at room temperature. The equilibrium O/P ratio followsthe Maxwell−Boltzmann distribution and changes significantlywith temperature (Figure S1 in the Supporting Information),whereas p-H2 occupies the rotational ground state of J = 0 andis more stable than o-H2 (J = 1) by an energy difference of 2B(B: rotational constant). However, the ortho to para (OP)conversion does not proceed without the help of a catalyst,despite the downhill reaction.2 Liquid hydrogen obtained fromthe liquefaction process contains a high proportion of o-H2,which causes boil-off, leading to the loss of liquid hydrogen.Hydrogen is the lightest element, resulting in a high rotationalenergy (2B = 15 meV), which is higher than the vaporizationenergy (ΔHvap = 9.4 meV). To overcome this obstacle,catalysts that promote OP conversion before liquefaction arerequired.Many catalysts for OP conversion, including Fe2O3·nH2O, γ-Fe2O3, and Cr2O3, have been proposed so far.3−11 Althoughthe exact origin for the conversion has not yet been elucidated,there are two representative models depending on the type ofsurface. One is an inhomogeneous magnetic field appearing onthe surfaces of antiferromagnetic materials containing magneticions such as Fe or Cr ions, which generate the magneticdipole−dipole or Fermi contact interaction.12,13 The other isan electric field appearing on the surfaces of ionic compoundshaving no magnetic ions. (Stark effect)14 Recently, we haveproposed a working hypothesis, determined through the searchof the catalysts.15 A key discovery is that whereas metallicmaterials are inactive, active catalysts are in most casesinsulators with the ionic bonding characteristic, whose cationshave an ionic radius smaller than the interatomic distance(0.74 Å) of the H2 molecule. Highly charged cations with smallradii on insulating surfaces can generate an electrostatic fieldextending over physisorbed hydrogen, with a gradient shorterthan the internuclear distance, causing hydrogen to behave as amolecule with two distinct nuclei. Here in this letter, we reporton a new high-activity OP conversion catalyst explored on thebasis of this working hypothesis.According to our hypothesis, OP conversion is promoted byan inhomogeneous electric field on the surface of the insulatingoxide with a high ionicity composed of small ions with a largevalence. Considering the large negative charge of anions,oxides are promising candidates as catalysts. Hydrogen hasamphoteric character, and alters its valence state from positive(cationic) to negative (anionic) through electron transfer,depending on its chemical environment, owing to itselectronegativity.16 The H2 molecule dissociates heterolyticallyat room temperature or above on the surfaces of insulatingoxides where the distribution of anions/cations is similar to aReceived: February 2, 2026Revised: March 5, 2026Accepted: March 9, 2026Published: March 12, 2026Letterpubs.acs.org/JPCL© 2026 The Authors. Published byAmerican Chemical Society3701https://doi.org/10.1021/acs.jpclett.6c00357J. Phys. Chem. Lett. 2026, 17, 3701−3705This article is licensed under CC-BY 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on March 26, 2026 at 07:09:04 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hiroshi+Mizoguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yuichi+Shirako"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Shusaku+Shoji"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hideki+Abe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takeshi+Fujita"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hideo+Hosono"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.jpclett.6c00357&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6c00357/suppl_file/jz6c00357_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/jpclcd/17/12?ref=pdfhttps://pubs.acs.org/toc/jpclcd/17/12?ref=pdfhttps://pubs.acs.org/toc/jpclcd/17/12?ref=pdfhttps://pubs.acs.org/toc/jpclcd/17/12?ref=pdfpubs.acs.org/JPCL?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acs.jpclett.6c00357?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/JPCL?ref=pdfhttps://pubs.acs.org/JPCL?ref=pdfhttps://creativecommons.org/licenses/by/4.0/checkerboard pattern,17,18 and we can expect variousmodulations on the surfaces of oxides, caused by theinhomogeneous electric field. Thus, we expect that latticeenergy will serve as an indicator for the search for catalystsbecause it is one of the main factors for stabilizing ioniccrystals. Table S1 summarizes the calculated lattice energy forrepresentative oxides. The primary factors governing latticeenergy are the charge state, the distance between charges, andthe degree of ion packing. Spinel-type oxides have thetendency to have high lattice energy values. In fact, oxidesexhibiting high catalytic activity, such as Mn3O4 and γ-Fe2O3(= Fe2.67O4) are of the spinel type, suggesting that the latticeenergy is a useful indicator for catalyst development.Therefore, we selected SiO2, Al2O3, and CeO2 as thecandidates on the basis of their high lattice energy values.While the Si4+ ion is smaller with higher valence, α-SiO2(quartz type) does not have a very high lattice energy becauseof the loose packing derived from the two coordination of theO2− ion. As for Al2O3, we selected the type with γ-polymorphism, having a lower density (3.64 gcm−3) thanthat with α-polymorphism (4.00 gcm−3). We describe thecrystal structures of these candidates. In amorphous SiO2, SiO4tetrahedra connect to each other through corner sharing toform an amorphous structure with a lower packing feature. γ-Al2O3 (= Al2.67O4) adopts a defect spinel-type structure withplenty of crystallographic voids.19 Figure 1a shows the B2O4sublattice in a normal spinel-type crystal structure with theAB2O4 composition, where A and B cations occupy thetetrahedral and octahedral sites, respectively. This structureconsists of alternating layers of a closed-packed layer of O ions,stacked along the [111] direction in an ABCABC sequence,and Al1, Al2, or Al3 ions occupy the crystallographic cavity sitebetween the layers with an occupancy smaller than 1, resultingin cationic deficiency. Whereas the lattice energy of the γ-phasemust be slightly smaller than that of the α-phase, the cationicvacancy and low atomic density of the γ-phase are expected toenhance catalytic activity, because of the increase of activecenter. Figure 1b shows the crystal structure of CeO2. Ce4+ ioncoordinates with eight O2− ions, and the O2− ion in thetetrahedral symmetry (Td) site coordinates with four Ce4+ ions,as shown in Figure 2a.Figure S3a shows the Raman spectra for CeO2 loaded with 5mol %Ni, as an example. The sharp peaks at 354.4 and 588.4cm−1 are ascribed to J = 0 (p-H2) and J = 1 (o-H2),respectively, and the fraction of p-H2 evaluated from theintensity ratio is 25% before exposure to the catalyst even at 77K. As soon as H2 gas is exposed to the catalyst, the intensityratio of the peaks begins to change, reaching 50% (equilibriumvalue at 77 K). Figure S3b shows the time course of OPconversion on these oxides at 77 K. The data on Mn3O4 orFe2O3 is also shown as a reference15 in Figure S3c. Wecalculated the reaction rate constant (k) from the time coursedata. As an example, Mn3O4 was estimated to have k =10.5(2.3) h−1, and an equilibrium fraction value (50%) wasachieved after ∼30 min. Figure S3 shows that the catalyticactivities of SiO2, Al2O3, and CeO2 are significantly inferior tothat of Mn3O4. We considered that the promotion of low-temperature adsorption of H2 on the catalyst surface that doesnot involve the H2 dissociation process is key for OPconversion. In general, hydrogen adsorption on insulatingoxide surfaces is more difficult than the adsorption of metalliccompounds. The observed low activities of SiO2, Al2O3 andCeO2 appear to originate from the difficulty in hydrogenadsorption. To overcome this difficulty, we loaded a smallamount (5 mol %) of a 3d late transition metal (TM) onsupported oxide catalysts by the impregnation method. It isnoted that these 3d TMs are generally inactive for OPconversion,15 whereas they cause H2 dissociation on thesurface. Figure 3a shows the time course of the catalyticFigure 1. (a) (Al1O)1.36O4 sublattice in γ-Al2O3 (= Al2.68O4 = (Al2T,Al3T)1.32(Al1O)1.36O4).19 AlO and AlT are the Al ions in the octahedraland tetrahedral coordination sites, respectively. In the framework,AlO6 octahedra share edges and corners to form a three-dimensionalnetwork. (b) Crystal structure of CeO2.Figure 2. (a) Local coordination of defective sites in γ-Al2O3 andCeO2. (b) Energy splitting ΔE of nuclear spin levels for a nucleus of I= 1 (q: electric field gradient at the nuclear position).Figure 3. (a) Trends of the OP conversion at 77 K by SiO2, γ-Al2O3,and CeO2-based catalysts. The dotted and dashed lines indicate theequilibrium values at 300 and 77 K, respectively. (b) Rate constants ofOP conversion by representative catalyst materials.The Journal of Physical Chemistry Letters pubs.acs.org/JPCL Letterhttps://doi.org/10.1021/acs.jpclett.6c00357J. Phys. Chem. Lett. 2026, 17, 3701−37053702https://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6c00357/suppl_file/jz6c00357_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6c00357/suppl_file/jz6c00357_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6c00357/suppl_file/jz6c00357_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6c00357/suppl_file/jz6c00357_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6c00357/suppl_file/jz6c00357_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig3&ref=pdfpubs.acs.org/JPCL?ref=pdfhttps://doi.org/10.1021/acs.jpclett.6c00357?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asreaction of these samples, and the obtained rate is summarizedin Table 1, together with the BET surface area. It is obviousthat the addition of TM as a cocatalyst significantly improvedthe catalytic activity: 21.3 (0.8) h−1 for SiO2:Fe, 29.4 (7.5) h−1for Al2O3:Co, 15.2 (2.5) h−1 for CeO2:Fe, and 6.2 (0.9) h−1 forCeO2:Ni. As an example, the fraction of p-H2 on SiO2/Fepowder reached 50% (equilibrium value at 77 K) within 20min. The catalytic activity of TM-supported oxides containingenvironmentally benign elements was superior to those ofreference oxides, including Mn3O4, as shown in Figure 3b. Theorder of the obtained activities (SiO2 ≈ Al2O3 > CeO2)matched moderately those of the lattice energies.We characterized the active catalysts in order to clarify theeffect of the TM cocatalyst. Figure S4 shows the powder XRDpatterns of the catalysts. Although we observe the diffractionpattern originating from γ-Al2O3 and CeO2, there was notmuch information about the diffraction by TM cocatalystsbecause of their small loaded amounts. We also observed themicrostructure by TEM. Figure 4(a) shows an STEM image ofthe SiO2/Fe catalyst, with FeOx particles confirmed by EDSmapping in Figure 4(b). Figure 4(c) shows CoOx nano-particles well dispersed in the Al2O3/Co catalyst. The electrondiffraction of Al2O3/Co indicates the coexistence of Co, CoO,and Co3O4 (not shown). Figure 4(e) shows a TEM image ofthe CeO2/Ni catalyst, with metallic Ni particles confirmed byEDS mapping in Figure 4(d). Figure S5 shows the sizedistribution of TM species for selected catalysts. The sizes ofthese particles are 1−5, 3−12, and 2−5 nm for Fe, Co, and Nispecies, respectively, suggesting that these species showsuperparamagnetism, judging from the size. The oxidationstates estimated from STEM−EDS results were consistent withthose estimated from the chemical shift in XPS spectra, asshown in Figure S6. The valence state of the TM cocatalystdecreased from Fe to Ni in the periodic table, whichcorresponds to the tendency of the workfunction of TMs.20No influence of the basicity of the oxide (supporters) has beenobserved. In the case of CeO2/Ni, the reduction of Ce ion wasconfirmed in Ce 3d XPS, as shown in Figure S6(c). In fact, thecolor of CeO2-based catalysts was changed from cream yellowto dark brown by low-temperature heat treatment under anAr−5%H2 atmosphere, suggesting the formation of Ce3+ ions.Figure S7 shows the H2-TPD profiles of the oxides with andwithout the cocatalyst. Obtained information obtained fromthe curves is summarized in Table S2. For SiO2/Fe, significantH2 desorption was observed at temperatures above 300 °C,from which the composition was determined to be SiO2/Fe0.05/(H2)0.0034. This hydrogen content represents an increaseof more than 15 times compared with the sample without thecocatalyst [SiO2(H2)0.0002], indicating that the Fe cocatalystmarkedly improves hydrogen adsorption.The OP conversion does not proceed without a catalyst,despite the downhill reaction with an energy difference of 2B =15 meV. We discuss the main factor that promotes OPconversion. Since it involves the conversion between nuclearspin isomers, directly stimulating the nuclear spin of the 1Hatom must be effective. This requires modulation of thenuclear spin levels using a magnetic or electric field. In 1953,Reif and Purcell reported the nuclear magnetic resonance of o-H2 dispersed in solid hydrogen in zero magnetic fields.21 Theyobserved the absorption of radio waves with ΔE = 6.8 × 10−10eV. This reminds us of its similarity to the nuclear quadrupoleresonance in zero magnetic fields and the Mossbauer effect,which are applicable to nuclei with I ≥ 1. As a trial, we regardo-H2 (J = 1) as a single nucleus (I = 1). A single nucleus with I= 1 exhibits an ellipse-shaped charge distribution, giving rise toan electric quadrupole, which induces the splitting of nuclearspin levels, depending on the chemical environment [electricfield gradient (EFG): eq] (Figure 2b). The energy splitting(ΔE) is proportional to this eq and increases in low-symmetryenvironments including surfaces. The thermal energy at 77 or25 K is sufficient for excitation of nuclear spin levels.Therefore, it is possible to excite nuclear spins directly, onlywhen o-H2 locates in low-symmetry environments. Thus, wemay expect the relaxation from J = 1 (o-H2) to J = 0 (p-H2),i.e., enhancement of OP conversion. Here, we need to examinethe details of the crystal structure of our oxides because theenergy splitting depends highly on the local environmentsaround the adsorbed o-H2. As an example, the O-site in CeO2with the fluorite-type crystal structure has Td symmetry with eq= 0 (that is, ΔE = 0), as shown in Figure 2a. The equilibriumoxygen partial pressure for the Ce2O3/CeO2 oxidation reactionis ∼1 × 10−90 atm at 573 K, according to the Ellinghamdiagram,22 which is impossible to realize under our conven-tional experimental condition. It is difficult to realize the Odeficiency in CeO2. However, our reagent consists ofnanoparticles with a diameter of ∼10 nm. The formation ofCe3+ ions on the surface is expected because of the largeTable 1. Properties of SiO2, Al2O3, and CeO2-BasedCatalystsCatalyst Reaction rate at 77 K (h−1) Surface area (m2 g−1)SiO2 - 27.45 mol % Fe 21.3(0.8) -γ-AlO1.5 - 1695 mol % Co 29.4(7.5) -5 mol % Ni 9.1(1.3) -CeO2 - 138.15 mol % Fe 15.2(2.5) -5 mol % Ni 6.2(0.9) 117.6Mn3O4 10.5(2.3) 18.9Fe2O3 1.3(0.1) 8.7Figure 4. Scanning TEM (STEM) and energy-dispersive X-rayspectroscopy (EDS) mapping of representative catalysts. (a) Low-magnification STEM image of SiO2/Fe, showing large oxidic particles.(b) High-magnification STEM image and EDS elemental maps (Fe,O, and Si), and merged map, confirming the oxide nature of Fespecies dispersed on SiO2. (c) TEM image of Al2O3/Co, showingparticles of CoOx species. (d) High-magnification STEM image ofCeO2/Ni, EDS elemental maps in square region (Ni, Ce, and O), andmerged map, showing well dispersed metallic Ni particles. (e) TEMlattice image showing CeO2 nanocrystal with a diameter of ∼10 nm,which is in close contact with a metallic Ni particle.The Journal of Physical Chemistry Letters pubs.acs.org/JPCL Letterhttps://doi.org/10.1021/acs.jpclett.6c00357J. Phys. Chem. Lett. 2026, 17, 3701−37053703https://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6c00357/suppl_file/jz6c00357_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6c00357/suppl_file/jz6c00357_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6c00357/suppl_file/jz6c00357_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6c00357/suppl_file/jz6c00357_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6c00357/suppl_file/jz6c00357_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6c00357/suppl_file/jz6c00357_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357?fig=fig4&ref=pdfpubs.acs.org/JPCL?ref=pdfhttps://doi.org/10.1021/acs.jpclett.6c00357?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ascontribution of the surface energy of nanoparticles, which hasbeen confirmed by TEM observation.23 CeO2 is a bandinsulator with a bandgap of ∼4 eV. The conduction bandminimum (CBM) originates primarily not from Ce 5d butfrom 4f states, whereas the Ce4+ ion has the (5d4f)0 electronicconfiguration. Two electrons generated by an O vacancy aretrapped on the 4f levels to form two Ce3+ ions, withoutforming free carriers at the CBM. The symmetry of the Ovacancy site surrounded by two Ce4+ and two Ce3+ ionsdecreases from the Td symmetry and the EFG exhibits itsmaximum value at n = 2 in a local environment surrounded byCe4+4−nCe3+n (n = 0, 1, 2, 3, or 4), according to a point chargemodel.24 The O vacancy with a diameter of 2.42 Å is expectedto accommodate a hydrogen molecule. It is expected that alarge number of such low-symmetry sites will exist on thesurface and near-surface regions of CeO2 nanocrystals.Similarly, γ-Al2O3 (= Al2.67O4) has a lot of cationic deficiencysites, as expected from its chemical composition and lowdensity (Figure 2a), which possibly form an inhomogeneouselectric field on the surface. We found noble OP conversioncatalysts showing highly catalytic activities at 77 K by searchingbased on our working hypothesis related to ionic bondingcharacteristics in insulating oxides. Next, the design andcontrol of surface defects, considering the crystallographicsymmetry in insulating oxides, will be our next focus.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.jpclett.6c00357.Lattice energy, temperature dependence of hydrogenisomers, catalyst evaluation setup, time course ofcatalytic activity, powder XRD, size distribution histo-grams, XPS, and H2-TPD (PDF)■ AUTHOR INFORMATIONCorresponding AuthorsHideo Hosono − Research Center for MaterialsNanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), Tsukuba, Ibaraki 305-0044,Japan; MDX Research Center for Element Strategy,International Research Frontiers Initiative, Institute of ScienceTokyo, Midori-ku, Yokohama 226-8503, Japan;orcid.org/0000-0001-9260-6728; Email: hosono@mces.titech.ac.jpHideki Abe − Center for Green Research on Energy andEnvironmental Materials, National Institute for MaterialsScience (NIMS), Tsukuba, Ibaraki 305-0044, Japan;orcid.org/0000-0002-8392-7586; Email: ABE.Hideki@nims.go.jpHiroshi Mizoguchi − Research Center for MaterialsNanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), Tsukuba, Ibaraki 305-0044,Japan; orcid.org/0000-0002-0992-7449;Email: MIZOGUCHI.Hiroshi@nims.go.jpAuthorsYuichi Shirako − Center for Green Research on Energy andEnvironmental Materials, National Institute for MaterialsScience (NIMS), Tsukuba, Ibaraki 305-0044, JapanShusaku Shoji − Center for Green Research on Energy andEnvironmental Materials, National Institute for MaterialsScience (NIMS), Tsukuba, Ibaraki 305-0044, Japan;orcid.org/0000-0002-8481-2633Takeshi Fujita − Kochi University of Technology, Kami, Kochi782−8502, Japan; orcid.org/0000-0002-2318-0433Complete contact information is available at:https://pubs.acs.org/10.1021/acs.jpclett.6c00357NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work was supported by a Grant-in-Aid for ScientificResearch (nos. 24K21813 and 23K23440) from JSPS; JSTMIRAI Program (no. JPMJMI18A3); JFE 21st CenturyFoundation; and Iketani Science and Technology Foundation.This work was also supported by and “Advanced ResearchInfrastructure for Materials and Nanotechnology in Japan(ARIM)” of the Ministry of Education, Culture, Sports,Science and Technology (MEXT) (nos. JPMXP1225NM5175and JPMXP1225NM5319).■ REFERENCES(1) Al Ghafri, S. Z. S.; Munro, S.; Cardella, U.; Funke, T.;Notardonato, W.; Trusler, J. P. M.; Leachman, J.; Span, R.; Kamiya,S.; Pearce, G.; Swanger, A.; Rodriguez, E. D.; Bajada, P.; Jiao, F.;Peng, K.; Siahvashi, A.; Johns, M. L.; May, E. F. Hydrogenliquefaction: a review of the fundamental physics, engineering practiceand future opportunities. Energy & Environmental Sci. 2022, 15 (7),2690−2731.(2) Fukutani, K.; Sugimoto, T. Physisorption and ortho-paraconversion of molecular hydrogen on solid surfaces. Prog. Surf. Sci.2013, 88 (4), 279−348.(3) Cunningham, C.; Johnston, H. 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