Occupation Of Tetrahedral Sites Research Articles

Narrowband red luminescence of tetrahedral-site Fe3+ In Ca8Mg(SiO4)4Cl2

We have measured the excitation and emission spectra, and lifetime for Fe3+ in the Ca8Mg(SiO4)4Cl2 compound. The spectroscopic data of the ferric ion (Fe3+) is interpreted within the framework of the Tanabe-Sugano crystal field theory for ions with the d5 electronic configuration. Quantitative evaluation of the Racah parameters B and C and the crystal field parameter (10 Dq) is provided. The emission and excitation data are consistent for Fe3+ in a tetrahedral-site occupancy. The red emission is assigned to the 4T1 (4G)→6A1 (6S) crystal field transition. The remarkable feature of the luminescence is the positioning of the emission band; it is centered at 623 nm with a Full Width at Half Maximum of 51 nm. To the best of our knowledge this is one of the highest energy emission for the tetrahedral-site Fe3+ ion. The practical implication of this spectral power distribution is discussed. The temperature dependency of the luminescence lifetime and emission intensity are reported.

Enhancing optical and resistive characteristics of MnFe2O4 spinel ferrite through Ni2+ doping: An advancement in structural refinement

Spinel ferrites are highly notable magnetic materials with vast potential for applications in microwave and high-frequency transformer applications. Manganese ferrites (MnFe2O4) are selected for their excellent optical, magnetic, and resistive properties. This research presents a co-precipitation method for nickel (Ni2+) doping in manganese spinel ferrite (MnFe2O4) to advance structural refinement and subsequently enhance optical and resistive properties. It is confirmed that Ni2+ doping has affected the crystal lattice and possesses a single crystalline phase. The diffraction peak at 36.2° suggests increased Ni2+ ion incorporation, impacting unit cell dimensions, lattice parameters, and phase composition. The Ni2+ doping introduces the strong covalent M − O bonds at octahedral and tetrahedral sites which impacts the grain size to 20∼35 nm and crystallite size to 33.29–21.23 nm. Infrared analysis reveals that the increasing Ni2+ shifts the M − O stretch in the octahedral site to higher frequencies, indicating stronger and more covalent M − O bonding. The band gap reduction from 1.41 eV to 1.28 eV results from Ni2+ and Mn2+ ion substitution in the ferrite lattice, driven by sp-d exchange interactions with their localized electrons. The resistivity of MnFe2O4 is increased from 9.37 × 103 to 9.43 × 103 Ω m with Ni2+ doping. This effect is ascribed to the occupation of both octahedral and tetrahedral lattice sites by Ni2+ ions. This communication conclusively establishes nickel as the optimal dopant for spinel ferrite, showcasing its versatility for a wide range of applications.

Ab initio study of metastable occupation of tetrahedral sites in palladium hydrides and its impact on superconductivity

A recent experimental work on palladium hydrides suggested that metastable structures with hydrogen atoms occupying tetrahedral sites could lead to superconductivity above 50 K, a huge increase compared to the 10 K critical temperature of the stable structure with all hydrogen atoms occupying octahedral sites. By generating many structures with hydrogen atoms randomly occupying the octahedral and tetrahedral sites of the face-centered cubic lattice and calculating their energy at different theoretical levels from first principles, we determine that metastable structures with partial or full occupation of tetrahedral sites are possible, even when the ionic quantum zero-point energy and anharmonicity are included in the calculations. Anharmonicity is crucial in palladium hydrides when hydrogen atoms occupy octahedral sites and, in fact, makes the structure with full octahedral occupation the ground state. Despite the metastable existence of structures with full or partial tetrahedral sites occupation, the superconducting critical temperature is reduced with the number of tetrahedral sites occupied. Our calculations discard that the occupation of tetrahedral sites can increase the critical temperature in palladium hydrides.

Understanding the Effects of Tetrahedral Site Occupancy by the Zn Dopant in Li-NMCs toward High-Voltage Compositional–Structural–Mechanical Stability via Operando and 3D Atom Probe Tomography Studies

Ni-containing "layered"/cation-ordered LiTMO2s (TM = transition metal) suffer from Ni-migration to the Li-layer at the unit cell level, concomitant transformation to a spinel/rock salt structure, hindrance toward Li-transport, and, thus, fading in Li-storage capacity during electrochemical cycling (i.e., repeated delithiation/lithiation), especially upon deep delithiation (i.e., going to high states-of-charge). Against this backdrop, our previously reported work [ACS Appl. Mater. Interfaces 2021, 13, 25836-25849] revealed a new concept toward blocking the Ni-migration pathway by placing Zn2+ (which lacks octahedral site preference) in the tetrahedral site of the Li-layer, which, otherwise, serves as an intermediate site for the Ni-migration to the Li-layer. This, nearly completely, suppressed the Ni-migration, despite being deep delithiated up to a potential of 4.7 V (vs Li/Li+) and, thus, resulted in significant improvement in the high-voltage cyclic stability. In this regard, by way of conducting operando synchrotron X-ray diffraction, operando stress measurements, and 3D atom probe tomography, the present work throws deeper insights into the effects of such Zn-doping toward enhancing the structural-mechanical-compositional integrity of Li-NMCs upon being subjected to deep delithiation. These studies, as reported here, have provided direct lines of evidence toward notable suppression of the variations of lattice parameters of Li-NMCs, including complete prevention of the detrimental "c-axis collapse" at high states-of-charges and concomitant slower-cum-lower electrode stress development, in the presence of the Zn-dopant. Furthermore, the Zn-dopant has been found to also prevent the formation of Ni-enriched regions at the nanoscaled levels in Li-NMCs (i.e., Li/Ni-segregation or "structural densification") even upon being subjected to 100 charge/discharge cycles involving deep delithiation (i.e., up to 4.7 V). Such detailed insights based on direct/real-time lines of evidence, which reveal important correlations between the suppression of Ni-migration and high-voltage compositional-structural-mechanical stability, hold immense significance toward the development of high capacity and stable "layered" Li-TM-oxide based cathode materials for the next-generation Li-ion batteries.

Optimizing metal geometrical structure of transition metal phosphides/P doped g-C3N4 enabling boosted water splitting

Metal geometrical structure engineering of composite catalysts is powerful to boost their photocatalytic performance of hydrogen evolution. Herein, by rationally designing, transition metal phosphides (TMPs) have been loaded onto the P doped g-C3N4 (PCN) to construct catalysts TMPs/PCN. For the first time, the effect of geometrical structure of TMPs on the photocatalytic activity is optimized and a geometrical-site-dependent photocatalytic hydrogen evolution is experimentally determined. Tri-metallic FeCoNiP/PCN, having 59.6% occupancy of tetrahedral cobalt sites, gives hydrogen production rate of 205 μmol/g/h, better than that of mono-metallic (FeP, CoP, NiP), bi-metallic (FeCoP, CoNiP, FeNiP) TMPs/PCN, and even much superior to that of Pt/b-g-C3N4 and Pt/PCN. Trimetallic TMPs/PCN catalysts also show excellent OER performance. In addition to the efficient electron transfer between FeCoNiP and PCN, which can effectively improve the separation of photogenerated carriers, the high occupancy of tetrahedral cobalt sites of FeCoNiP as a co-catalyst promotes the formation of active intermediates, being the key to significantly improve the catalytic performance of ternary FeCoNiP/PCN. The findings in this work, especially about a link of the photocatalytic performance of TMPs/PCN composites and occupancy of tetrahedral cobalt site in co-catalysts TMPs, can open new occasion for creating novel composite catalysts and extending to other applications.

Synthesis of spinel ferrites and application in the photodegradation of Rhodamine in water via pH control assistance

Nanostructured powders of cobalt ferrite with stoichiometry CoxFe3-xO4, where 0.4 ≤ x ≤ 1.2, were synthesized by the wet chemical route from the Co-precipitation method. Single-phase and crystalline nanopowders of spinel type structure with cubic symmetry and space group Fd3‾m were characterized by X-ray diffraction. The structural refinement was carried out by the Rietveld Method showing a transition of normal spinel to an inverse spinel, as a function of Co2+concentration. Both spectra, Infrared Spectroscopy and Diffuse reflectance spectroscopy were used to characterize chemical bonds Co2+ in tetrahedral coordination, for higher cobalt concentration, and band gap analysis, respectively. The photoactivity of powders was evaluated from the photodegradation of the Rhodamine 6G dye under UV light. CoFe2O4 nanoparticles showed the best photocatalytic degradation with 90.9% in an alkaline medium, which obeys a pseudo-first order kinetic reaction, being the maximum apparent rate constant equal to 10.23 × 10−3. Effects of the occupation of tetrahedral and octahedral sites in the spinel structure in the photoactivity parameter are discussed.

Enhancing the Liquefied Petroleum Gas Sensing Sensitivity of Mn-Ferrite with Vanadium Doping

Mn-Ferrite with a nanostructure is a highly valuable material in various technological fields, such as electronics, catalysis, and sensors. The proposed article presents the hydrothermal synthesis of Mn-ferrite doped with V (V) ions. The range of the doping level was from 0.0 to x to 0.20. The fluctuation in tetrahedral and octahedral site occupancies with Fe (III), Mn (II), and V (V) ions was coupled to the variation in unit cell dimensions, saturation magnetization, and LPG sensing sensitivity. The total magnetic moment shows a slow decrease with V-doping up to x = 0.1 (Ms = 51.034 emu/g), then sharply decreases with x = 0.2 (Ms = 34.789 emu/g). The dimension of the unit cell increases as x goes up to x = 0.1, then lowers to x = 0.2. As the level of V (V) ion substitution increases, the microstrain (ε) also begins to rise. The ε of a pure MnFe2O4 sample is 3.4 × 10−5, whereas for MnFe2−1.67 xVxO4 (x = 0.2) it increases to 28.5 × 10−5. The differential in ionic sizes between V (V) and Fe (III) and the generation of cation vacancies contribute to the increase in ε. The latter is created when a V (V) ion replaces 1.6 Fe (III) ions. V-doped MnFe2O4 displays improved gas-sensing ability compared to MnFe2O4 at lower operating temperature. The maximum sensing efficiency was observed for 2 wt% V-doped MnFe2O4 at a 200 °C optimum operating temperature.

A detailed structural analysis, morphological and optical study of Mg-Zn nano ferrite

Mg-Zn nanoferrite with the composition of Mg0.4Zn0.6Fe2O4 was prepared using the citrate precursor process. Detailed structural analysis was performed on Mg-Zn nanoferrite to calculate crystallite size, lattice constant, X-ray density, bulk density, and porosity using XRD data. Also, the strain on the sample was calculated by plotting Williamson – Hall (W-H) plot. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were performed to study morphological and optical properties. The spinel structure was confirmed by XRD with a crystallite size of 38 nm. SEM micrograph of the sample showed almost spherical-shaped nanoparticles. FTIR spectra disclosed two characteristic bands showing tetrahedral (A) and octahedral (B) site occupancies. Under UV–visible light irradiation, photocatalytic degradation of rhodamine B dye and the degradation efficiency is obtained to be 70 % in 60 min. Therefore, it is concluded that Mg0.4Zn0.6Fe2O4 is a promising material for water purification.

Hydrogen localization and cluster formation in α-Zr from first-principles investigations

Alpha-zirconium is a hydrogen host material used in nuclear energy and semiconducting applications due to Zr's excellent properties such as corrosion resistance, low thermal neutrons cross-section, and thermodynamically favorable interstitial sequestering of H atoms. Herein we use density functional theory simulations to systematically analyze the initial stages of H insertion in the Zr matrix, and determine optimal hydrogen localization, distribution, and clustering tendencies from a thermodynamics and kinetics perspective. Our results suggest that H occupation of tetrahedral sites is strongly favored, consistent with previous experimental and theoretical studies. Further, a cooperative effect in H localization is shown where H atoms favor clustering. The H pair is found to be the most stable by occupying tetrahedral sites with pairs at second nearest-neighbor positions (TTn2). Among the four H atom clusters, the most favorable configuration has two TTn2 pairs forming “box” or “zig-zag” shaped-cluster with the pairs separated by 3.03 or 3.25 Å, respectively. Nudged elastic band simulations suggest that the formation of TTn2 is facile and is limited by the small diffusion barrier of the monomer of less than 0.1 eV. Further, the formation routes for the 4H atom clusters are also presented. The current study can serve as a foundation to systematically model H atom clustering in metals and alloys.

Engineering of Cation Occupancy of CoFe2O4 Oxidation Catalysts by Nanosecond, Single‐Pulse Laser Excitation in Water

AbstractDue to the variability of the cation occupancy of octahedral and tetrahedral sites, spinel ferrites and cobaltites are particularly interesting to investigate activity trends in oxidation catalysis. Yet, the preparation of the respective catalyst series remains challenging. We employed pulsed laser defect engineering of CoFe2O4 nanoparticles in water to gradually alter the cation occupancy of octahedral and tetrahedral sites by single laser pulses and study its effect on cinnamyl alcohol oxidation. Three CoFe2O4 catalysts from different synthesis methods resembling different initial site occupancy were chosen as starting materials. The laser‐induced randomization of the cation occupancy was verified by Mössbauer spectroscopy and linearly correlated with the conversion of cinnamyl alcohol while the size and Co : Fe ratio was maintained during laser processing. The study solidifies the importance of octahedral Co3+‐sites and the feasibility of pulsed laser processing for altering the cation occupancy and related crystallographic defect density in oxidation catalysis.

Li5 SnP3 - a Member of the Series Li10+4x Sn2-x P6 for x=0 Comprising the Fast Lithium-Ion Conductors Li8 SnP4 (x=0.5) and Li14 SnP6 (x=1).

The targeted search for suitable solid‐state ionic conductors requires a certain understanding of the conduction mechanism and the correlation of the structures and the resulting properties of the material. Thus, the investigation of various ionic conductors with respect to their structural composition is crucial for the design of next‐generation materials as demanded. We report here on Li5SnP3 which completes with x=0 the series Li10+4x Sn2−x P6 of the fast lithium‐ion conductors α‐ and β‐Li8SnP4 (x=0.5) and Li14SnP6 (x=1). Synthesis, crystal structure determination by single‐crystal and powder X‐ray diffraction methods, as well as 6Li, 31P and 119Sn MAS NMR and temperature‐dependent 7Li NMR spectroscopy together with electrochemical impedance studies are reported. The correlation between the ionic conductivity and the occupation of octahedral and tetrahedral sites in a close‐packed array of P atoms in the series of compounds is discussed. We conclude from this series that in order to receive fast ion conductors a partial occupation of the octahedral vacancies seems to be crucial.

Half-Heusler-like compounds with wide continuous compositions and tunable p- to n-type semiconducting thermoelectrics

Half-Heusler and full-Heusler compounds were considered as independent phases with a natural composition gap. Here we report the discovery of TiRu1+xSb (x = 0.15 ~ 1.0) solid solution with wide homogeneity range and tunable p- to n-type semiconducting thermoelectrics, which bridges the composition gap between half- and full-Heusler phases. At the high-Ru end, strange glass-like thermal transport behavior with unusually low lattice thermal conductivity (~1.65 Wm−1K−1 at 340 K) is observed for TiRu1.8Sb, being the lowest among reported half-Heusler phases. In the composition range of 0.15 < x < 0.50, TiRu1+xSb shows abnormal semiconducting behaviors because tunning Ru composition results in band structure change and carrier-type variation simultaneously, which seemingly correlates with the localized d electrons. This work reveals the possibility of designing fascinating half-Heusler-like materials by manipulating the tetrahedral site occupancy, and also demonstrates the potential of tuning crystal and electronic structures simultaneously to realize intriguing physical properties.

The origin of anomalous hydrogen occupation in high entropy alloys

The hydrogen to metal ratio of high entropy alloys (HEAs) can exceed 2 due to the simultaneous occupation of tetrahedral and octahedral sites by hydrogen. Such anomalous hydrogen occupations originate from the chemical disordering in HEAs.

Magnetism and site occupancy in epitaxial Y-rich yttrium iron garnet films

Y-rich yttrium iron garnet (Y-YIG) thin films were grown on gadolinium gallium garnet substrates using pulsed laser deposition from a ${\mathrm{YFeO}}_{3}$ target. The films are epitaxial on the substrate, and the unit cell of (001)- and (111)-oriented Y-YIG undergoes a tetragonal or rhombohedral distortion, respectively. The Y-YIG has lower room temperature magnetization but higher low-temperature magnetization than bulk YIG, and its Curie temperature was in the range of 330--400 K, depending on the growth conditions. First-principles calculations predict that ${\mathrm{Y}}_{\mathrm{Fe}}$ antisite defects are more stable in octahedral than tetrahedral Fe sites. Temperature-dependent magnetization measurements together with a superexchange dilution model and composition determined from x-ray photoelectron spectroscopy indicate that nonmagnetic species (${\mathrm{Y}}^{3+}$ and vacancies) are predominantly accommodated in the octahedral sites, but there is a significant occupancy of tetrahedral sites.

Low temperature synthesis and influence of Ce3+ substitution on structural and magnetic properties of nanocrystalline cobalt ferrite using citrate precursor sol-gel method

Rare earth element Ce3+ substituted spinel cobalt ferrite, CoFe2-xCexO4 for (x = 0.0, 0.05, 0.10, 0.15, 0.20) is synthesized using an economical sol–gel technique. The X-ray diffraction analysis reveals that all samples have spinel symmetry indexed to Fd3m space group. Fullprof Rietveld analysis is performed to find out lattice parameters that increase monotonically due to induced lattice strains in samples. The crystallite size was determined to be 54.9 nm using Williamson-Hall plots. The morphological analysis of samples has been performed using SEM imaging. FTIR spectrum confirms octahedral and tetrahedral site occupancy by metal–oxygen bonds for all samples. The magnetic properties of samples refine with Ce3+ doping at iron site, with magnetization ranging from 23.59 to 57.68 emu/g and retentivity 17.14 to 24.08 emu/g. The Coercive field values drastically increased from 739 to 1912 Gauss. Thus, we have explored the influence of rare earth Ce3+ (1.14 Å) substitution at the Fe3+ site (0.55 Å) in CoFe2O4 crystal lattice for improvement of its structural and magnetic properties in this paper.

Incorporation of Ti in epitaxial Fe2TiO4 thin films

The titanomagnetites (Fe2−x Ti x O4, x ⩽ 1) are a family of reducible spinel-structure oxides of interest for their favorable magnetic, catalytic, and electrical transport properties. To understand the stability of the system during low temperature deposition, epitaxial thin films of Fe2TiO4 were deposited by molecular beam epitaxy (MBE) on MgO(001) at 250–375 °C. The homogeneous incorporation of Ti, Fe valence state, and film morphology were all found to be strongly dependent on the oxidation conditions at the low substrate temperatures employed. More oxidizing conditions led to phase separation into epitaxial, faceted Fe3O4 and rutile TiO2. Less oxidizing conditions resulted in polycrystalline films that exhibited Ti segregation to the film surface, as well as mixed Fe valence (Fe3+, Fe2+, Fe0). A narrow window of intermediate oxygen partial pressure during deposition yielded nearly homogeneous Ti incorporation and a large fraction of Fe2+. However, these films were poorly crystallized, and no occupation of tetrahedral sites in the spinel lattice by Fe2+ was detected by x-ray magnetic circular dichroism at the Fe L-edge. After vacuum annealing, a small fraction of Fe2+ was found to occupy tetrahedral sites. Comparison of these results with previous work suggests that the low temperature deposition conditions imposed by use of MgO substrates limits the incorporation of Ti into the spinel lattice. This work suggests a path towards obtaining stoichiometric, well-crystallized Fe2TiO4 by MBE by utilizing high substrate temperature and low oxygen partial pressure during deposition on thermally stable substrates.

Impact of Solution Chemistry on Growth and Structural Features of Mo-Substituted Spinel Iron Oxides

The effect of crystallizing solution chemistry on the chemistry of subsequently as-grown materials was investigated for Mo-substituted iron oxides prepared by thermally activated co-precipitation. In the presence of Mo ions, we find that varying the oxidation state of the iron precursor from Fe(II) to Fe(III) causes a progressive loss of atomic long-range order with the stabilization of 2-4 nm particles for the sample prepared with Fe(III). The oxidation state of the Fe precursor also affects the distribution of Fe and Mo cations within the spinel structure. Increasing the Fe precursor oxidation state gives decreased Fe-ion occupation and increased Mo-ion occupation of tetrahedral sites, as revealed by the extended X-ray absorption fine structure. The stabilization of Mo within tetrahedral sites appears to be unexpected, considering the octahedral preferred coordination number of Mo(VI). The analysis of the atomic structure of the sample prepared with Fe(III) indicates a local ordering of vacancies and that the occupation of tetrahedral sites by Mo induces a contraction of the interatomic distances within the polyhedra as compared to Fe atoms. Moreover, the occupancy of Mo into the thermodynamic site preference of a Mo dopant in Fe2O3 assessed by density functional theory calculations points to a stronger preference for Mo substitution at octahedral sites. Hence, we suggest that the synthetized compound is thermodynamically metastable, that is, kinetically trapped. Such a state is suggested to be a consequence of the tetrahedral site occupation by Mo ions. The population of these sites, known to be reactive sites enabling particle growth, is concomitant with the stabilization of very small particles. We confirmed our hypothesis by using a blank experiment without Mo ions, further supporting the impact of tetrahedral Mo ions on the growth of iron oxide nanoparticles. Our findings provide new insights into the relationships between the Fe-chemistry of the crystallizing solution and the structural features of the as-grown Mo-substituted Fe-oxide materials.

Bulk O2 formation and Mg displacement explain O-redox in Na0.67Mn0.72Mg0.28O2

Summary O-redox in compounds with Li on the transition-metal layers (TML) has recently been attributed to the formation of molecular O2 on charge, trapped in the lattice. Here, we show that a similar process occurs for P2-Na0.67[Mn0.72Mg0.28]O2, which contains Mg2+ on the TML. The molecular O2 is identified by high-resolution RIXS and quantified by magnetometry, showing that it equates to the charge passed. This O2 is trapped in voids that are formed by Mg2+ out-of-plane displacement and Mn4+ in-plane disordering and is then reduced on discharge associated with a large voltage hysteresis. In contrast to compounds containing Li+ in the TML, in which the honeycomb ordering and the high-voltage plateau are irreversibly lost after the first cycle, in P2-Na0.67[Mn0.72Mg0.28]O2, the plateau reappears partially on the second charge due to the partial reversibility of Mn in-plane and Mg out-of-plane migration and the local reformation of the honeycomb ordering.

Effects of deuterium content on the thermal stability and deuterium site occupancy of TiZrHfMoNb deuterides

A series of high entropy alloy (HEA) hydrides/deuterides were synthesized and characterized to explore the relationship between thermal stability and the structure of TiZrHfMoNb deuterides. The X-ray diffraction, thermogravimetric analysis, and differential scanning calorimetry results reveal that the lattice constant and cell volume of TiZrHfMoNb hydrides increase with the increasing hydrogen content. The desorption temperature decreases with the increasing hydrogen content, indicating that the increased lattice constant arising from the introduction of deuterium atoms will destabilize the HEA deuterides. A density functional theory calculations demonstrate that the binding energies between hydrogen and metals in BCC TiZrHfMoNb hydrides decline with the increasing hydrogen content, meaning that the thermal stability decreases with the increasing hydrogen content. The HEA deuterides are found to be more stable than their hydrides, implying the remarkable isotopic effect of TiZrHfMoNb HEA. Neutron powder diffraction refinements confirm that deuterium atoms prefer to occupy both of the tetrahedral and octahedral interstitials in BCC structure, while a prior occupation of tetrahedral interstitial sites in FCC structure. The deuterium/hydrogen concentrations and the lattice constants are proposed to be the major factor regulating the thermal stability of HEA deuteride, making them suitable for different application field of hydrogen storage.

Manganese containing copper aluminate catalysts: Genesis of structures and active sites for hydrogenation of aldehydes

Copper aluminate spinel (CuO.CuAl2O4) is the favoured Cr-free substitute for the copper chromite catalyst (CuO.CuCr2O4) in the industrial hydrogenation of aldehydes. New insights in the catalytic mechanism were obtained by systematically studying the structure and activity of these catalysts including effects of manganese as a catalyst component. The hydrogenation of butyraldehyde to butanol was studied as a model reaction and the active structure was characterised using X-ray diffraction, temperature programmed reduction, N2O chemisorption, EXAFS and XANES, including in-situ investigations. The active catalyst is a reduced spinel lattice that is stabilised by protons, with copper metal nanoparticles grown upon its surface. Incorporation of Mn into the spinel lattice has a profound effect on the spinel structure. Mn stabilises the spinel towards reduction of CuII to Cu0 by occupation of tetrahedral sites with Mn cations, but also causes decreased catalytic activity. Structural data, combined with the effect on catalysis, indicate a predominantly interface-based reaction mechanism, involving both the spinel and copper nanoparticle surface in protonation and reduction of the aldehyde. The electron reservoir of the metallic copper particles is regenerated by the dissociative adsorption and oxidation of H2 on the metal surface. The generated protons are stored in the spinel phase, acting as proton reservoir. Cu(I) species located within the spinel and identified by XANES are probably not involved in the catalytic cycle.