Chemistry of actinide centers in heterogeneous catalytic

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Chemistry of actinide centers in heterogeneous catalytic transformations of small molecules Jennifer Leduc, Michael Frank, Lasse Jürgensen, David Graf, Aida Raauf, and Sanjay Mathur ACS Catal., Just Accepted Manuscript • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Catalysis

Electronic configurations, ionic radii, standard reduction potentials under acidic aqueous condition and approximate colors of corresponding ions in aqueous solution of uranium (a) and thorium (b). Variation of metal-based valence orbital energies for the 5fn-16d1 electronic configurations in [AnCp3] complexes (An = Th, Pa, U) exemplified for 6d-5f hybridization. 159x80mm (150 x 150 DPI)

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Reductive oxo-metalation of uranyl complexes with macrocyclic pyrrole ligand. (a) Schematic concept of lewis acid-oxo interactions, (b) functionalization and c) photochemical reduction and of the uranyl (VI) cation. 160x102mm (150 x 150 DPI)


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ACS Catalysis

Molecular uranium (II) compounds with three cyclopentadienyl derivative ligands (a) and qualitative frontier molecular orbital diagram of 5f and 6d orbitals (b). Uranium (II) monoarene-trisaryloxide complex (c) and representations of the highest singly-occupied spin orbitals for mS = 2 electronic configuration from (scalar) DFT calculations with the BP functional (d). 234x145mm (150 x 150 DPI)


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Binary phase diagrams of uranium and oxygen (left), neptunium and oxygen (middle) and plutonium and oxygen (right). 160x55mm (150 x 150 DPI)


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Different binary and ternary uranium systems with carbon (grey), nitrogen (dark blue) and oxygen (red) with binary phases on the axes and ternary phases in the planes. 121x100mm (150 x 150 DPI)


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Overview of uranium and thorium oxide nanomaterial classes. From 0D to 3D nanomaterials. 156x106mm (150 x 150 DPI)


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a) Schematic illustration of the crystal structure of ThO2 (111) grown onto a polycrystalline iridium substrate, b) Lowest energy (111) surface of UO2 with red uranium atoms and yellow oxygen atoms. 160x60mm (150 x 150 DPI)


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Formation of UO2(OH)2 nanotubes and nanowires via tuning of the current density during electrodeposition. 116x70mm (150 x 150 DPI)


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Synthesis of ThO2 nanorods via reverse micellar route using CTAB as surfactant and [Th(C2O4)2] as reactant. 160x53mm (150 x 150 DPI)


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Generation of oxygen vacancies associated with the reduction of the thorium metal center in ThO2 upon doping. 160x50mm (150 x 150 DPI)


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DFT calculations on the potential energy profile of CO oxidation using Au-doped ThO2. 134x54mm (150 x 150 DPI)


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DFT calculated oxygen vacancy formation energies of pristine CeO2 and its metal (M = Ti, Hf, Zr, U, Th) doped analogs. 89x66mm (150 x 150 DPI)


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a) Oxygen-vacancy formation energy as function of the uranium fraction. b) Hydrogen production as a function of the oxygen-vacancy formation energy. 153x60mm (150 x 150 DPI)


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a) Cross-section SEM micrograph of the U3O8//Fe2O3 bilayer. b) Schematic drawing of the PEC device architecture. c) Photoelectrochemical measurements of bare U3O8, a-Fe2O3 and U3O8//Fe2O3 in the dark (dashed lines) and under illumination (solid lines) in 1M NaOH electrolyte. 121x78mm (150 x 150 DPI)


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ACS Catalysis

Localized density of states and band edge alignment at U3O8//Fe2O3 interfaces determined by DFT calculations. The localized density of states is averaged over the lateral directions such that it is resolved in the direction perpendicular to the interface plane. The band edges of the U3O8 and Fe2O3 sides are indicated by horizontal lines. The dark region between these lines corresponds to the electronic band gaps of both materials. In the atomistic structures U ions are shown in blue, Fe ions in brown and O ions in red. The electron energy levels are given with respect to the vacuum and the normal hydrogen electrode (NHE) at pH=0. 107x79mm (150 x 150 DPI)


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Schematic synthetic route to generate actinide-doped graphene from graphite. 140x26mm (138 x 138 DPI)


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Cyclic voltammograms of U3O8 supported on reduced graphene oxide a) in oxygen and nitrogen saturated 0.1 M KOH solutions and b) in a phosphate buffer solution containing 10 mM hydrogen peroxide. 140x54mm (150 x 150 DPI)


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Graphical summary highlighting the challenges of small molecule activation using homogeneous and heterogeneous actinide-containing catalysts. 124x121mm (150 x 150 DPI)


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Activation of CS2 and CO2 using [Th(Cp´´)3], Cp´´ = C5H3(SiMe3)2. 118x88mm (150 x 150 DPI)


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Catalytic conversion of butadiene to 1,4-cis-polybutadiene, using [n-(C3H5)3UX], X = Cl, Br, I. 88x20mm (96 x 96 DPI)


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ACS Catalysis

Proposed catalytic cycle for thorium mediated hydrothiolation of terminal alkynes. 159x136mm (150 x 150 DPI)


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Deacon process to recycle chlorine from hydrogen chloride. 59x11mm (150 x 150 DPI)


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ACS Catalysis

C(sp3)-H fluorination of cyclooctane with N fluorobenzenesulfonimide and the uranyl ion as photocatalyst under visible light irradiation. 88x21mm (96 x 96 DPI)


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Postulated catalytic cycle of the activation of the uranium (III) compound with CO2 and CS2. 160x97mm (150 x 150 DPI)


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The postulated catalytic mechanism for the synthesis of H2 from H2O with an arene-anchored U (III) compound. 107x86mm (150 x 150 DPI)


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Reduction of nitrate ions (1) at low current densities and water (2) at high current densities and reaction of uranyl ions with as generated hydroxide ions to give uranyl hydroxide (3). 118x16mm (150 x 150 DPI)


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Redox mechanism for the catalytic CO oxidation on a ceria-based catalyst. 94x20mm (150 x 150 DPI)


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Process of solar water splitting under the formation of molecular hydrogen and oxygen. 71x15mm (150 x 150 DPI)


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Postulated reaction mechanism for the photocatalytic water oxidation on uranium oxides with UV light. 96x32mm (150 x 150 DPI)


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Activation reactions of methane to form syngas. 68x14mm (150 x 150 DPI)


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Formation of methanol from methane, carbon dioxide and hydrogen. 65x5mm (150 x 150 DPI)


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Formation of methanol and water from carbon dioxide and hydrogen. 62x5mm (150 x 150 DPI)


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Oxidative coupling of methane to give ethane and water. 61x5mm (150 x 150 DPI)


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Synthesis of halogenated synthetic polymers. 60x9mm (150 x 150 DPI)


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Deacon process to recycle chlorine from hydrogen chloride. 57x5mm (150 x 150 DPI)


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Chemistry of actinide centers in heterogeneous catalytic transformations of small molecules Jennifer Leduc, Michael Frank, Lasse Jürgensen, David Graf, Aida Raauf and Sanjay Mathur* Institute of Inorganic Chemistry, University of Cologne, Greinstr. 6, D-50939 Cologne, Germany. * Corresponding author. Email: [email protected] The chemistry of actinide molecules and materials has shown remarkable conceptual advancements in the past decade illustrating their unique reactivity profiles, when compared to lanthanides and transition elements, but also posing some challenging questions on the intriguing stability of low valent states and the significant role of 5f orbitals in bonding and reactivity of actinides. The distinctive electronic flexibility of actinide centers makes them potential catalysts for heterogeneous molecular transformations due to the kinetic lability of their coordination states and facile switching among oxidaton states. Actinide-enable chemical transformations such as the 6-electron reduction of dinitrogen into two reactive ammonia molecules or 4-electron oxidation of water into oxygen under mild conditions are promising pathways in the quest of high efficiency heterogeneous catalysts. This review provides a comprehensive account on actinide-mediated catalytic transformation of small molecules such as CO, CO2, N2, O2, H2O, CH4, HCl and NH3. The emphasis is placed on the emerging phenomena in actinide-based solid catalysts and controlled synthesis of nanostructured actinide materials as pristine and substrate-grown phases. The mechanistic investigations highlight the influence of the 5f electrons in multi-electron transfer reactions and the propensity of actinide centers to achieve higher oxidation states that defines the surface termination in actinide oxides. Finally, the status and perspectives of actinide containing materials beyond the nuclear fuel applications is discussed underlining their exciting chemistry and unexplored potential towards alternative catalytic energy production processes.

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ACS Catalysis

1

Introduction

Actinide compounds are becoming increasingly popular in catalytic transformations mainly due to their large ionic radii and the active participation of 5f orbitals in bonding that leads to new and different chemo- as well as regioselectivity thereby expanding the scope of accessible intermediates and products. In general, f-elements ions exhibit high tendency towards oxidation state shuttling in conjunction with lability of coordination spheres, however characteristic structural, electronic and bond enthalpy relationships markedly differ among actinides. For instance, uranium exhibits a dynamic redox behavior with oxidation states ranging from +II to +VI in solution as well as in the solid-state (Figure 1a), the predominant oxidation state of thorium is +IV (Figure 1b).1 This characteristic behavior of uranium (Z = 92) compared to thorium (Z = 90) correlates with the respective electronic configuration but is also a consequence of the poor shielding of the 5f electrons with increasing Z.

Figure 1: Electronic configurations, ionic radii, standard reduction potentials under acidic aqueous condition and approximate colors of corresponding ions in aqueous solution of uranium (a) and thorium (b).1 (c): Variation of metal-based valence orbital energies for the 5fn-16d1 electronic configurations in [AnCp3] complexes (An = Th, Pa, U) exemplified for 6d-5f hybridization. Reprinted and modified with permission from [2]

The occurrence of a wide range of oxidation states in uranium results from the indirect relativistic effect accompanied by radial extension of the orbitals. In addition, across the actinide series the energy levels of 5f orbitals decrease, whereas the energy of 6d orbitals increase causing an overall reduced difference in the orbital energies, which consequently allows the unique 6d-5f hybridization in uranium (Figure 1c).2 In consequence, the typical color variation of the different oxidation states in aqueous solution helps the experimental chemists to visually follow the reaction pathway. Uranium compounds can be isolated for the entire spectrum of oxidation states, with the most common oxidation states being +VI and +IV. New insights available on metal-ligand bond enthalpies in actinide complexes offer a playground for the design of new catalysts and different types of reactions.3, 4 Since the first report in 1974 on the extraordinary catalytic activity of an organometallic uranium complex ([(η3-allyl)3UCl]),5 demonstrated in the polymerization of butadiene (Scheme 1), a large number of actinide 2 ACS Paragon Plus Environment

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coordination compounds and organoactinides have been developed and studied to explore their potential as new catalysts in homogeneous organic transformations.6-19 [(-C3H5)3UX], X = Cl, Br, I hexane, 15 h, 0 °C

n 1,4 cis: 99%

Scheme 1: Catalytic conversion of butadiene to 1,4-cis-polybutadiene, using [-(C3H5)3UX], X = Cl, Br, I.

This growing interest in the synthesis and application of new actinide compounds is mainly driven by (i) unique reactivity patterns and product distributions not accessible by corresponding transition metal complexes and (ii) the need to understand the deterministic role of f-electrons in governing the reactivity of actinide compounds through systematic structurereactivity studies. Especially catalysis with small molecules are of huge interest both from the view point of fundamentally new insights and potential applications in the domains of pharmaceutical, agricultural and fine chemical industry.20 In this context, Haber first applied uranium and uranium nitride as heterogeneous catalysts in 1909 and demonstrated their high efficiency in the conversion of molecular dihydrogen (H2) and dinitrogen (N2) to ammonia under high pressure and temperature.21 Actinides compounds could play a future key role in relevant global industrial processes such as the Haber-Bosch process for ammonia (NH3) production, due to their unique and diverse physical and chemical properties.22 As there are already several comprehensive and recent review articles on the use of molecular actinide complexes for homogeneous catalysis,7-9, 16, 23 this review exclusively reports on the heterogeneous catalysis of small molecules using actinide (nano)materials. Although there are several reports on transuranium compounds such as neptunium10, 24-26 and plutonium3, 27, investigations on thorium and uranium-containing materials predominate the published reports largely due to their comparable low radioactivity and outstanding catalytic capabilities and hence will be the primary focus of this review. A recent review by Shi et al. on “Inorganic Synthesis of Actinides” covers the synthesis of both actinide coordination compounds and actinide containing materials.28 Several methods for the preparation of uranium oxides (especially U3O8 and UO3) were described in “Designing Heterogeneous Oxidation Catalysts” (1998)29 and “Metal Oxide Catalysis” (2008)30. In 2013, Ismagilov et al. reviewed the preparation of different uranium-containing catalysts, predominantly uranium oxides, via thermal and electrolytic decomposition of salts as well as hydrothermal and sol-gel synthesis.31 Despite the fact that several uranium carbides32 and nitrides33 are known (Figure 2), reports on the evaluation of their catalytic properties are rare leaving room for future studies (especially of ternary uranium oxycarbides, carbonitrides, oxynitrides) in this area.34-44


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ACS Catalysis

Figure 2: Different binary and ternary uranium systems with carbon (grey), nitrogen (dark blue) and oxygen (red) with binary phases on the axes and ternary phases in the planes.

The predominance of actinide oxides for heterogeneous catalysis applications derives from their high abundance, structural diversity as well as chemical and temperature stability.45 Therefore, this review mainly focused on the synthesis and application of actinide oxide based materials. The cubic fluorite-type thorium dioxide ThO2 displays properties similar to cerium dioxide (CeO2) and can be produced as a single-phase material, due to the lack of other thermodynamic stable modifications and stoichiometries. The similarity to ceria is reflected in the formation of oxygen vacancies associated with reduced metal centers (Th3+) present on the surface and created by treatment in reducing gases and under low oxygen partial pressures.46 Whereas plutonium and neptunium exhibit stable oxides over a wide temperature and M:O ratio range, the U-O binary phase diagram displays multiple stoichiometries (e.g. UO2, U3O8 and UO3, Figure 3) and polymorphs potentially suitable for heterogeneous catalysis. A phase selective generation of uranium oxide nanomaterials is difficult due to the tendency of uranium oxides to form hyper-stoichiometric compounds.47 Indeed, the synthesis of stoichiometric UO2 is challenged by their tendency to uptake oxygen resulting in UO2+x stoichiometries (x = up to 0.33) with interstitial oxygen in the cubic fluorite-type lattice. Consequently, the interstitial oxygen dramatically changes the physical properties, for example it decreases the electrical conductivity by several order of magnitudes from 3 · 10-1 to 4 · 10-8 (Ω·cm)-1.48 In addition, UO2 can form oxygen-deficient stoichiometries (UO2-x) that exhibit n-type semiconducting behavior in contrast to p-type conductivity observed in hyperstoichiometric UO2+x phases.49 The higher mixed-valent oxide U3O8 and hexavalent UO3 are reportedly n-type semiconductors.50


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Figure 3: Binary phase diagrams of uranium and oxygen (left)31, neptunium and oxygen (middle)51 and plutonium and oxygen (right).52 Reprinted and modified with permissions from [31, 51, 52]

The catalytic conversion of small molecules, such as CO, CO2, H2O, CH4, NH3 etc., is challenging due to their electronic stability and high bond energies resulting in high activation barriers that demand active and electron rich catalysts capable of fulfilling the electronic requirements (Table 1). Upon comparing heterogeneous and homogenous catalysis, several practical limitations become obvious. For example, most of the reactions performed with molecular actinide catalysts are dominated by highly reactive and difficult to handle low-valent compounds and are rather stoichiometric than catalytic reactions.53, 54 This is due to the tendency of actinide complexes to form thermodynamically stable intermediates upon activation of small molecules, which in turn leads to difficult regeneration of the active catalyst and incomplete release of the end product. In contrast, the heterogeneous catalysts are easy to recycle, scalable and show lower contaminations of (by)products, but the catalytic activity is surface limited that is manifested in lower turnover of the substrate and product diversity. A controlled modification of active catalyst is rather difficult due to not well-defined reaction mechanisms. Table 1: Overview of common target molecules in heterogeneous catalysis and their reaction requirements. Target Molecule

Binding Energy [kJ/mol]

Reaction Pathway

Electronic Requirement

ΔH0 [kJ/mol]

N2

945

N2 + 3 H2 → 2 NH3

+6 e‒

‒46

CO

1077

CO + H2O → CO2 + H2

‒2 e‒

‒41

e‒

‒2 +2 e‒

+242 +242

e‒

H2O

463

H2O → 0.5 O2 + 2 H2O → H2 ‒ 2 e‒

H2O2

463 (O‒H) 139 (O‒O)

2 H2O2 → 2 H2O + O2

+4 e‒

‒108

O2

498

O2 + 2 H2 → 2 H2O

+4 e‒

‒484

CH4

416

CH4 + H2O → CO + H2 CH4 + 0.5 O2 → CO + 2 H2 CH4 + CO2 → 2 CO + 2 H2 CH4 + CO2 + 2 H2 → 2 CH3OH 2 CH4 + O2 → C2H4 + 2 H2O

‒2 e‒ ‒2 e‒ ‒2 e‒ ‒2 e‒ ‒4 e‒

+206 ‒36 +247 +67 ‒282

HCl

432

4 HCl + O2 → 2 Cl2 + 2 H2O

‒4 e‒

‒116


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ACS Catalysis

In order to enhance the conversion activity in heterogeneous catalysis, nanomaterials are preferably used due to their advantageous high surface-to-volume ratio, which has recently triggered enormous interest in the synthesis of uranium, thorium and other actinide oxide nanoparticles (0D), nanowires and nanotubes (1D), thin films (2D) and aerogels (3D, Figure 4). Thus, in the following section, synthetic approaches towards the formation of early actinide nanomaterials are summarized (Table 2). 2

Synthesis of Actinide Oxide Nanomaterials for Application in Heterogeneous Catalysis

2.1

3D Nanomaterials

For the catalytic applications of early actinide materials, typically nanomaterials are incorporated in a supporting host matrix, although only few reports are available on the formation of 3D nanomaterials. Whereas molecular 3D nano-sized networks, such as molecular organic frameworks55-57 or inorganic actinide nanocage clusters58 are relatively well investigated, reports on the formation of 3D porous networks, such as aerogels are rare. Examinations of low-density nanocrystalline UO359 and ThO2 aerogels60 prepared via sol-gel synthesis were published by Reibold et al. For this purpose, either UO2(NO3)2 · 6 H2O or Th(NO3)4 · 4 H2O were dissolved in ethanol with addition of NH4OH and propylene oxide as gelation initiators. Crystalline primary particles of UO3 and ThO2 were obtained in sizes of 5-20 nm in diameter as observed by HRTEM, respectively. The synthesized aerogels exhibited high surface areas (120 m2/g) and pore diameters in the range of micro- and mesoporous features.60 This material class is of high interest for heterogeneous catalysis, as the actinide material functions as both active material and support simultaneously and should be examined further. 2.2

2D Nanomaterials

2.2.1 Solution-based methods Owing to their facile stoichiometric variations, the formation of phase pure uranium oxide layers via solution-based deposition techniques used to be rather challenging. For instance, the growth of uranium oxides onto iron surfaces from uranyl nitrate solutions as reported by Qiu et al. resulted in the formation of amorphous uranium (VI) oxide with incorporated water that could be converted into polycrystalline uranium (IV) oxide layers upon heating in vacuum.61 However, several cracks were generated during the thermal treatment facilitating the reoxidation at the surface of the sample. Electrodeposition from a uranyl nitrate electrolyte solution resulted in the formation of polycrystalline uranium dioxide films with a thickness of 350 nm as investigated by Adamska et al.62 Uranium oxide films doped with B, Al, P, and S and with thicknesses ranging from 150-350 nm were synthesized by Meek et al. via sol-gel processes using uranyl acetate precursor.63 The first synthesis of phase pure, single-crystal like UO2 and U3O8 thin films (hexagonal and orthorhombic) was achieved by Burrell et al. via polymer-assisted deposition in 2007.64 The stabilization of the desired uranium oxides was accomplished by epitaxial growth onto lanthanum aluminum oxide (UO2), r-plane sapphire (orthorhombic U3O8) and c-cut sapphire (hexagonal U3O8) substrates. In contrast, deposition onto yttria-stabilized zirconia (YSZ) substrates led to epitaxial UC2 films synthesized by spin6 ACS Paragon Plus Environment

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coating of a solution of polyethylenimine (PEI) polymer and uranium-EDTA complex followed by heating at 350°C under a H2/ethylene flow. The formation of UC2 was completed by annealing the samples at 1000°C vie carbothermal reduction and stabilized by the YSZ lattice.32, 65 Although thorium exhibits less pronounced valence dynamics, solution-based synthesis often resulted in the formation of oxygen deficient thoria nanomaterials. For instance, ThO2-x and ThO2 thin films were prepared via UV photodecomposition of spin coated [Th(acac)4] on Si(100).66 Thereby, amorphous ThO2 films were generated below annealing temperatures of 550 °C and polycrystalline ThO2 films with a preferred orientation along the (111) plane above 550 °C as shown via XRD analysis. In contrast, as deposited films consisted of substoichiometric ThO2-x as confirmed by high-resolution XPS studies. Photoluminescence spectra of these ThO2-x films revealed a band at λ = 366 nm, which was attributed to charge transfer corresponding to ThIV/ThIII ↔ ThIII/ThIV transitions.67

Figure 4: Overview of uranium and thorium oxide nanomaterial classes. From 0D to 3D nanomaterials. Images reprinted with permission from [59, 60, 68-73].

2.2.2 Gas phase methods Reactive DC magnetron sputtering from a depleted actinide target in an oxygen/argon atmosphere is a commonly used approach to produce actinide oxide thin films, however these films often suffer from the presence of defects or phase impurities. For instance, He et al. reported on the formation of a ~100 nm thick film using reactive sputtering, which consisted of a mixture of -UO3 and U3O8.74 In order to synthesize phase pure and single-crystalline UO2 and U3O8 films, reactive DC magnetron sputtering depositions were performed onto singlecrystalline r-plane sapphire and yttria-stabilized zirconia (YSZ) substrates.75 Argon etching and in-situ annealing of these films led to the formation of urania with the composition UO2.12,76 whereas with a subsequent reduction step using atomic hydrogen, generated by an electron 7 ACS Paragon Plus Environment

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cyclotron resonance plasma source, pure and homogeneous thin films of U2O5 with a thickness of approximately 20-30 monolayers were produced.77 Preferentially oriented (111) UO2 thin films grown with a deposition rate of 6 nm/min, were deposited via sputtering onto Si(111) substrates.70 Thus, both crystallinity and crystallographic orientation are strongly influenced by the choice of substrate in sputtering techniques as confirmed by Elbakhshwan et al. in a systematic study on the deposition of single crystalline UO2 thin films.78 Reactive sputter deposition of thorium metal in an Ar/O2 atmosphere at 150 °C generated mixed ThO/ThO2 films with a thickness of ~100 nm. The presence of the metastable ThO phase was confirmed by time-resolved in-situ dynamic neutron reflectometry coupled with hybrid functional density calculations.79 In contrast, sputtering at 100 °C led to the formation of ThO2-x thin films with a thickness of ~360 nm.80 Using Si(111) substrates, polycrystalline ThO2 films with a preferential orientation along the (111) plane were generated. Their electronic structure was investigated via XPS and UPS spectroscopy as well as reflection electron energy loss spectroscopy (REELS) revealing a band gap of about 5.2 eV, which is in the range of values reported for bulk ThO2 (3.4-5.7 eV).81 Due to defects present in the layer resulting from the DC sputter method, the Fermi level is pinned between the valence and the conduction band. This group also published very recently the synthesis of bimetallic actinide oxide U1-xThxO2 (x = 01) thin films via DC sputter deposition.82 In order to examine the effect of thorium on the oxidation mechanism, detailed cyclovoltammetric, photoelectron spectroscopy (XPS and UPS) studies were performed. These revealed that due to the higher oxygen affinity of thorium and increased stability of ThO2 (ΔfG0(298 K): −1170 kJ mol−1) when compared to UO2 (ΔfG0(298 K): −1031 kJ mol−1),83 only thorium gets completely oxidized during the deposition process.82

Figure 5: a) Schematic illustration of the crystal structure of ThO2 (111) grown onto a polycrystalline iridium substrate, b) Lowest energy (111) surface of UO2 with red uranium atoms and yellow oxygen atoms. Reprinted with permission from [47, 84].

Moreover, ThO2 films with a thickness of ~40 nm and preferred (111) orientation were generated via physical vapor deposition onto iridium substrate (Figure 5a).84 Analogous to CaF2, and directions exhibit the lowest surface energies.85 However, due to a stronger interlayer bonding for , the termination is energetically most favorable analogous to UO2 (Figure 5b).47 8 ACS Paragon Plus Environment

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In order to generate phase pure and crystalline nanomaterials without the presence of undesired defects and with a good adhesion to the substrate, metal-organic chemical vapor deposition processes are typically applied. Chemical vapor deposition of uranium and thorium oxide films using the respective -diketonate chelate complexes was first reported by Shiokawa et al.86 The phase analysis was performed via powder XRD and showed the formation of polycrystalline U3O8, UO2+x (x > 0.25), U4O9 and ThO2, reliant on the growth conditions. Recently, we reported the preparation of three air-stable, volatile uranium (IV) and (VI) heteroarylalkenolates of general formula Ar-CH-C(Rf)-OH (with Ar = aryl, Rf = fluoroalkyl unit).87 For instance, [U(DMOTFP)4] (DMOTFP-H = 1-(4,5-dimethyl-oxazol-2-yl)-3,3,4,4,4-pentafluoro-but-1-en2-ol) was used in the thermal CVD process to obtain a film consisting of a mixture of hexavalent UO3 and mixed penta- and hexavalent U3O8. In contrast, selective generation of U3O8 layers with a thickness of ~75 nm was achieved via plasma-enhanced chemical vapor deposition of the uranyl compound [UO2(DMOTFP)2(DMOTFP-H)] with a subsequent calcination step.88 These films were tested towards their efficiency in photoelectrochemical water splitting to demonstrate that the activity of the heterogeneous catalyst is strongly dependent on the nanoscopic dimensions of the actinide oxide material. 2.3

1D Nanomaterials

Whereas syntheses of 0D actinide oxide nanostructures are well investigated,28 reports on the formation of 1D actinide oxide nanomaterials, such as nanorods, nanowires and nanotubes are rather scarce. This might be due to safety concerns as conventional preparation methods such as electrospinning89 may result in the inhalation of 1D nanostructures and cause severe health issues due to fibrous nature of the materials that poses potential danger of fibrosis similar to asbestos materials. However, due to the unique electrical, magnetic, optical and thermoelectric properties resulting from the high aspect ratio of 1D nanomaterials,90 this material class is of high interest for heterogeneous catalysis, especially for light-induced transformations. Whereas the formation of 1D actinide oxide nanomaterials via gas phase depositions according to the vapor-liquid-solid mechanism have not been reported yet, solution-based syntheses of uranium and thorium oxide nanorods, nanowires and nanotubes were elaborated by several research groups.

Figure 6: Formation of UO2(OH)2 nanotubes and nanowires via tuning of the current density during electrodeposition. Modified and redrawn with permission of [69].


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Rod-shaped UO2 nanostructures with a diameter of 80 nm and a length of 500 nm were formed via electron beam irradiation (7 MeV) of a uranyl nitrate solution in water/propanol mixture. These nanorods decomposed in the presence of molecular oxygen to give [UO2]2+ species but could be stabilized in the presence of nitrogen.91 The oxidation behavior of UO2 nanostructures in the presence of water and oxygen was also investigated by Ma et al.92 They have shown that UO2 nanoparticles without addition of capping agents, such as oleic acid, were slowly oxidized in the presence of water and oxygen to give 1D nanowires composed of ianthinite (U(IV)2(U(VI)O2)4O6(OH)4(H2O)9) after 2 days. Within a week, microdiscs of schoepite ((UO2)8O2(OH)12 ⋅ 12 H2O) were formed. Ianthinite 1D nanowires could also be directly synthesized via hydrothermal decomposition of acetyl acetonate in monoethylene glycol, whereas 0D UO2 nanoparticles were formed in triethylene glycol. It was demonstrated, that the ethylene glycol chain length influenced both the morphology and phase formation of the uranium containing nanomaterial. The influence of the reducing agent was further elaborated by Wang et al. via hydrothermal decomposition of uranyl acetate in different amines.93 Whereas in the presence of ethylenediamine UO2 nanoparticles were generated, U3O8 1D nanorods with a diameter of 80-100 nm and a length of 500-1500 nm were produced using tripropylamine as reducing agent. In contrast, mixed-phases were formed using either triethanolamine or cyclohexylamine. The growth mechanism was ascribed to the fact that tripropylamine attached preferably to specific facets of U3O8 and thereby suppressed the growth in other directions. As these U3O8 nanorods were tested with regard to their efficiency in the catalytic conversion of benzyl alcohol to benzaldehyde and compared to intergrown U3O8 nanoparticles. They found that given similar surface areas, the activity is rather dependent on crystal facets than on the morphology. In numbers both exhibit 100% selectivity but oxidative conversion at intergrown U3O8 porous aggregates is higher (30%) than for U3O8 nanorods (20%) while both suffer from activity loss after 24 h. Thus, future investigations should deal with the growth of 1D actinide oxide nanostructures exhibiting active crystal facets for the respective catalytic conversion. Moreover, Hu et al. have shown that upon addition of octadecene as a cosolvent into a mixture of uranyl acetate dissolved in oleylamine and oleic acid both the phase and morphology were changed from ultrathin U3O8 nanoribbons to ultrathin U3O7 nanowires.94 Increasing the amount of oleylamine even resulted in the formation of UO2 nanoparticles. These nanoparticles were doped with cerium and tested for electrocatalytic water reduction. NO3- + H2O + 2 e2 H 2O + 2 e UO22+ + 2 OH-

NO2- + 2 OH2 OH- + H2 UO2(OH)2

(1) low current densities (2) high current densities (3)

Scheme 2: Reduction of nitrate ions (1) at low current densities and water (2) at high current densities and reaction of uranyl ions with as generated hydroxide ions to give uranyl hydroxide (3).

Wang et al. reported on the growth of both uranyl hydroxide nanowires and nanotubes via electrodeposition using uranyl nitrate in water and a nanoporous membrane coated with a thin layer of gold on one side as hard template.69 At low current densities (-0.2 mA/cm2), the nitrate ions get reduced in the first step generating hydroxide ions and resulting in an increase of the pH value at the working electrode (Scheme 2). In the second step, the uranyl ions react with the hydroxide ions under formation of uranyl hydroxide. As the deposition occurs homogenously inside the membrane channels, nanowires are generated. In contrast, at high current densities 10 ACS Paragon Plus Environment

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(-0.4 mA/cm2) and short deposition times, water electrolysis occurs under the formation of hydroxide ions and molecular hydrogen. The uranyl ions still react with the hydroxide ions under formation of UO2(OH)2, however, due to the hydrogen bubbles inside the membrane, nanotubes are generated (Figure 6). Both uranyl hydroxide nanostructures could be converted into crystalline U3O8 via calcination without changing their original morphology. Heterostructured amorphous silicon oxide clad uranium oxide nanowires were prepared by He et al. via oxidation of U3Si2 pellets using water in an autoclave.95 Thorium oxide nanotubes with a width of 80 nm and a length of < 20 µm were prepared via sol-gel synthesis starting from thorium nitrate in the presence of water and a porous anodic aluminum oxide template.72 In contrast, thorium oxide nanorods with a width of < 20 nm and a length of 2-2.5 µm were prepared via the reverse micelle technique using cetyl trimethyl ammonium bromide (CTAB) as a surfactant (Figure 7).96 Both 1D nanomaterials were tested as hosts for Eu3+ (Tb3+ and Dy3+) and were evaluated with regard to their photoluminescent properties.

Figure 7: Synthesis of ThO2 nanorods via reverse micellar route using CTAB as surfactant and [Th(C2O4)2] as reactant.

2.4

0D Nanomaterials

Due to their application in catalysis (e.g. in selective oxidation reactions), the synthesis and investigation of uranium oxide nanoparticles (with and without supports) have been the focus of numerous investigations. Therefore, only selected publications have been referred in this review to provide a general overview. In 2003, O’Loughlin et al. developed the formation of UO2 nanoparticles via reduction of uranyl acetate using hydroxysulfate green rust.97 The generated nanoparticles were polydispersed spheres with a diameter of 2-9 nm. Wu et al. reported on the synthesis of monodisperse UO2 nanoparticles via thermal decomposition of uranyl acetylacetonate in a mixture of oleic acid, oleylamine and octadecene solutions.68 This reduction-based method generated spherical nanoparticles with a diameter of 5.4 nm that were surface passivated by oleylamine through chelating bidentate interactions as determined by IR spectroscopy. Similarly, PuO2 nanocrystals with a size of ~3.2 nm were synthesized by thermal decomposition of [PuO2(NO3)2] ⋅ 3 H2O in a highly coordinating organic medium.19 The reactivity of actinide precursors (e.g. [U(acac)4], [Th(acac)4]) in organic systems and their influence onto the size and shape of synthesized actinide oxide nanocrystals were investigated 11 ACS Paragon Plus Environment

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by Hudry et al.98 They additionally investigated the synthesis of thorium/uranium mixed oxide nanocrystals by controlled hot co-injection of thorium acetylacetonate and uranyl acetate.99 The hydrothermal decomposition of actinide (IV) oxalates in aqueous media yielded highly crystalline PuO2, UO2 and ThO2 nanocrystals with a size of < 15 nm.100 U3O8 nanoparticles with a size of ~3 nm were prepared via wet impregnation of uranyl acetate in a silica host matrix.101 Cuboids, nanoplates and nanorods of uranium oxide hydroxide hydrate species were synthesized via hydrothermal routes.102 In a first step, uranium oxide hydroxide hydrate nanoplates with diverse morphologies were generated (e.g. hexagonal plates) and subsequently converted into U3O8 via annealing at 700 °C. These nanoplates showed a high catalytic activity (80% conversion, ~100% selectivity) for benzyl alcohol oxidation to benzaldehyde within 8 h of reaction time. A room temperature and surfactant-free synthesis of UO2 nanoparticles was presented by Nenoff et al. in 2011.103 The reduction of uranyl nitrate was achieved using gamma irradiation of a -radiation source (60Co), however the samples had to be exposed to the irradiation for at least 7 d to complete the reduction.

Table 2: Overview of preparation methods and characterization techniques used to investigate different uranium oxide nanomaterials.

Form

Size

Phase

Crystallinity/ Dispersity

Method

Investigations

Ref.

3D Nanomaterials Solution based methods aerogel

5–20 nm

UO3

nanocrystalline

sol-gel

HRTEM, nitrogen adsorption/desorption analyses

59

aerogel

5–20 nm

ThO2

nanocrystalline

sol-gel

HRTEM, nitrogen adsorption/desorption analyses

60

2D Nanomaterials Gas phase deposition techniques film

20-30 monolayers

U2O5

polycrystalline

pulsed dc magnetron sputtering

XPS, PES

77

film

0.7-1.5 m

UO2, U3O7, U3O8

polycrystalline

pulsed dc magnetron sputtering

XRD, SEM, TEM, APT

104

film

150 nm

UO2.12

polycrystalline

XPS, PES

76

film

35-100 nm

UO2, U3O8

single-crystalline

reactive dc magnetron sputtering reactive dc magnetron sputtering

XRD, XRR, RBS, SIMS, XPS, UPS

75

film

180-750 nm

UO2

polycrystalline

reactive dc magnetron sputtering

XRD, SEM, AFM, XPS, UPS

70


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film

5 nm-1 µm

UO2

polycrystalline

reactive dc magnetron sputtering

XRD, XPS, UPS

105

film

150 nm

UO2+x

single crystalline

reactive sputtering onto YSZ (111)

XPS

76

film

370 Å

UO2

single crystalline

XRD, RBS

78

film

15-450 nm

UO2

single-crystalline

reactive gas magnetron sputtering dc magnetron sputtering

ED

106

film

360 nm

(U1 − xThx)O2 (x = 0-1)

reactive DC sputter technique

XRD, XPS, UPS, CV

82

film

360 nm

ThOx (x ≤ 2)

sputter deposition

XRD, XPS, UPS,

80

film

~100 nm

ThO/ThO2

sputter deposition

NR

79

multilayered films

105 nm

mixture of (,)-UO3 and U3O8

polycrystalline

reactive physical vapor deposition

NR, surface-enhanced Raman spectroscopy, XRD, SEM

74

film

40 nm

ThO2

crystalline (preferred (111) orientation)

PVD on polycrystalline Ir

AES, XRD, SEM

84

film

-

UO2, U3O7, U3O8

polycrystalline

CVD

XRD

86

film

350 nm

mixture of UO3 and U3O8

polycrystalline

CVD

XRD, XPS, SEM

87

film

100 nm

U3O8

polycrystalline

PECVD

XRD, XAS, XPS, TAS, SEM

88

polycrystalline (preferential orientation along the ThO2 (111) direction)

Solution based methods multilayered films

450-700 nm

mixture of UO3 and UO2

polycrystalline

from solution onto Fe surfaces

RBS, LEIS, XRD, SEM, XPS, NEXAFS

61

film

100-500 nm

PbS(Th) films deposited on GaAs(100)

crystalline

chemical bath deposition process

XPS, AFM, TEM, XRD

73

film

350 nm

UO2

polycrystalline

electrodeposition

XRD, SEM, AFM, EDX, SIMS

62

film

150-350 nm

UO2 (doped)

polycrystalline

sol-gel

UV-vis

63 64

film

100 nm

phase pure UO2, U3O8 (h,o)

single-crystalline

polymerassisted deposition


XRD, angle-resolved PES

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film

2.9 nm (crystallite size)

ThO2

polycrystalline

photochemical deposition

XPS, XRD, AFM

66

film

85 nm

ThO2

crystalline (higher T), amorphous

photochemical deposition

XPS, XRD, AFM

66

film

25-220 nm

ThO2

crystalline (higher T), amorphous

spray pyrolysis technique

FTIR, XRD, AFM

107

spraypyrolysis

XRD, SEM, EDAX, XPS

108

electron beam irradiation

TEM, SEM, XRD

91

film

Th:SnO2 1D Materials

Solution based synthesis rodshaped nanoparticles

80, 500 nm

UO2

ultrathin nanoribbons

4 nm, 100 nm

U3O8

hydrothermal

TEM, XRD

94

ultrathin nanowires

1 nm, 50-500 nm

U3O7

hydrothermal

TEM, XRD

94

nanorods [001] crystallographic direction, U3O8 nanorods

80-100 nm, 5001500 nm

U3O8

polydisperse

hydrothermal

XRD, SEM; TEM

93

nanotubes, nanowires nanowires

400 nm, 3.5-7 m

U3O8

polydisperse

electrodeposition

SEM, XRD

69

10 nm diameter

UO2.34

Hydrothermal -assisted by silicon oxide

XRD, SEM, TEM, EDS

95

nanowires

10-250 nm, 3-8.5 µm

[email protected]

hydrothermal

XRD, SEM, TEM, EDS

95

ianthinite

hydrothermal polyol reduction

SEM, XPS, (HR-)TEM, XRD

92

nanowires

polydisperse

nanorods

< 20 nm, 2-2.5 µm

ThO2: Eu,Tb, Dy:ThO2

hydrothermal

XRD, TEM, ED, PL, EPR

96

nanotubes

80 nm, < 20 µm

ThO2, Eu:ThO2

sol-gel

TEM, XRD, XPS, PL

72

OD Nanomaterials Solution based synthesis nanoparticles

2-9 nm

UO2

polydisperse

reduction by green rust

TEM, XRD, XANES

97

nanoparticles

3-5 nm

UO2

polydisperse

hydrolysis

TG/DTA; HT-XRD

109

nanoparticles

~15 nm

ThO2, UO2

polydisperse

sol-gel

XRD, BET, SEM, EDX

110


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wet impregnation in silica matrix

XRD, XPS, TEM, UV-vis, IR

101

precipitation

TEM, XRD

111

precipitation

HRTEM, SAXS, XRD

112

Gd:UO2

coprecipitation

TEM, XRD

113

150-300 nm

UO2, UO2.34

hydrothermal

XRD, SEM, BET

114

< 15 nm

ThO2, UO2 PuO2

hydrothermal

TEM, XRD

100

nanoparticles

~60 nm

ThO2

hydrothermal

XRD, SEM, IR, TGA

71

spheres cuboids

30-250 nm 400 nm, 1 m

UO2, U3O8

polydisperse

hydrothermal

XRD, SEM, IR

115

nanoplates

1 m

UO(OH)(H2O)x, U3O8

polydisperse

hydrothermal

TEM, XRD, UV

102

nanoparticles

4.5 nm

UO2, ThO2

single-crystalline, monodisperse

heating-up method

IR, XPS, XRD, TEM

98

nanoparticles

~1.5 nm ~28 nm

Th1–xUxO2

hot-injection

XRD, SEM, SQUID

99

nanospheres

2-10 µm

ThO2

hydrothermal/ thermal

SEM, XRD

116

nanoparticles

5.4 nm

UO2

thermal decomposition

TEM, XRD, IR

68

nanoparticles

10-50 nm

ThO2, UO2


SEM, TEM, XRD, Raman, TG

117

nanoparticles

~3.2 nm

PuO2


XRD, XAFS, XANES, TEM, IR, Raman, UV-vis, SQUID

19

nanoparticles

~6 nm

NpO2, UO2


XRD, SEM

118

nanoparticles

50-100 nm

U3O8

pyrolysis

XRD, SEM, DLS, TEM

119

radiolysis

TEM, XRD, UV-vis

103

nanoparticles

3 nm

U3O8

nanoparticles nanoparticles nanoparticles

~4 nm

UO2+x

~2.5 nm

PuO2

~100 nm

nanoparticles nanoparticles

polydisperse

monodisperse

monodisperse

SEM,

EDX,

Further synthesis methods nanoparticles

5 nm

UO2

polydisperse

nanoparticles

5 nm

UO2

radiolytic reduction

XRD, SEM, DSC

120

nanoparticles

7 nm

Pu/PuO2

sonochemical synthesis

HRTEM, Pu LIII-edge XAS, O K-edge NEXAFS/ STXM, XRD, BET

18


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nanoparticles

~6 nm

UO2

nanoparticles

3-15 nm

UO2, ThO2 and mixed U0.56Th0.44O2

3

nanocrystalline, narrow size distribution

microwaveassisted decomposition

TEM, XRD, UV-vis, O K-edge NEXAFS/ STXM, in-situ XAS

121

photochemically induced precipitation

XRD, HRTEM, SAED

122

Heterogeneous Catalysis: Small Molecule Activation Using Uranium Materials

The application of uranium containing compounds in heterogeneous catalysis was reviewed by Taylor et al. in 200930 and by Ismagilov et al. in 201331, however, a large body of data has become available since then and some of the highlights are summarized in the following account. Whereas Taylor et al. focused on uranium oxides and their applications in oxidation,123-126 reduction127 and steam forming reactions,128 Ismagilov reviewed solidsupported, uranium oxide composites and uranium containing compounds for catalytic reactions in organic synthesis,129-132 syngas production,133, 134 Fischer-Tropsch135 and hydrocracking processes,136 hydrodesulfurization137 and oxidation138 and reduction reactions.139 Due to the thermal stability and resistance to catalytic poisons, such as sulfur, water and chlorine, the application of uranium oxides for purification of waste gases polluting the environment was examined heavily in the past decades.140, 141 As the present review mainly focuses on recent developments in small molecule activation using actinide containing compounds, several interesting publications and break-through observations on catalytic reactions such as the reduction of 4-nitrophenol using urania-palladium-graphene nanohybrids,142 the oxidation of chlorobenzene and benzyl alcohols,93, 102, 124, 126, 143-145 the degradation of Rhodamine B146 or Suzuki-Miyaura cross-coupling reactions of aryl halides147 will not be reviewed herein. Instead, the focus is laid on the catalyst requirements and mechanistic studies on activations of the following small molecules: CO, H2O, H2O2, O2, CH4 and HCl (Table 3).

3.1

Catalytic water-gas shift: CO oxidation

The catalytic water-gas shift reaction (reaction of carbon monoxide and water vapor to form carbon dioxide and hydrogen) using metal promoted transition metal oxides have been wellreported.148 The reaction mechanism was postulated to occur via reaction of CO with transition metal oxide hydroxyl groups generating surface formates as intermediates. Metal supported lanthanide and actinide oxides have also been demonstrated to be remarkable catalysts for CO oxidation at low temperatures in the following order of activity: 149 Au/CeO2 > Au/UO3 > Au/Al2O3 > Au/SiO2. The Au/UO3 system was prepared by thermal decomposition of UO2(NO3)2 · 6 H2O at 300-400 °C with subsequent impregnation in a basic solution of HAuCl4 and revealed a CO conversion activity of 54.6% at a temperature of


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350 °C.149 However, it was postulated that the catalytic activity is mainly derived from metallic gold with a small amount of oxidized gold species.

Figure 8: Generation of oxygen vacancies associated with the reduction of the thorium metal center in ThO2 upon doping.

In order to investigate the influence of f-orbitals on the catalytic CO oxidation mechanism, Ptsupported tetravalent CeO2 catalysts were examined.150, 151 Thereby, evidence for formation of surface formates and a redox mechanism were found with Pt promoting the reduction of surface defects in both mechanisms.151-154 The redox mechanism (Scheme 3) implied the adsorption of CO (1) on transition-metal sites (M*), the reduction of Ce4+ to Ce3+ upon reaction with CO forming CO2 (2) and the reoxidation of Ce3+ via reaction with water to form H2 (3).46, 154 CO + M*

COad

(1)

COad + 2 CeO2

CO2 + Ce2O3 + M*

(2)

H2O + Ce2O3

2 CeO2 + H2

(3)

Scheme 3: Redox mechanism for the catalytic CO oxidation on a ceria-based catalyst.

During the reductive half-reaction, oxygen deficiencies are generated. As for both Au supported transition metal and lanthanide oxides it was found, that reducible oxides are more active than irreducible oxides,155-158 the adsorption of CO on gold adatom (Au1) stoichiometric MO2 and reduced MO2-x (M = Ti, Zr, Ce, Hf) was investigated via DFT calculations.46 In addition, the influence of the f-orbitals was studied in more detail by comparing the results to the 5f orbital thorium oxide analogue. For the Au1/MO2 system, it was found, that a weaker adsorption occurred on Au(0) for M = Zr, Hf, Th than for M = Ce with charge back donation to CO 2π* antibonding orbitals. In contrast, for the Au1/MO2-x system a stronger adsorption for M = Zr, Hf, Th was observed attributed to the quantum primogenic effect, which describes the relatively lower orbital energies and strongly contracted radial distribution of the first-shell atomic orbitals of each angular quantum number (1s, 2p, 3d, 4f) that facilitates a preferred charge transfer from gold to the 4f orbital containing CeO2.159 However, experimental studies on Pt/CeO2 and Pt/ThO2 systems have shown that the metal-promoted actinide catalyst exhibits a higher conversion rate than ceria. This was attributed to the higher surface area (165 m2/g) and thus enhanced active site density of the thoria catalyst.46 In general, the catalysts were prepared via precipitation of their corresponding aqueous nitrate solutions in the presence of urea and ammonia. The platinum (1 wt.%) was incorporated by incipient wetness impregnation of tetraamine platinum(II) nitrate. In order to evaluate the catalytic mechanism of CO oxidation 17 ACS Paragon Plus Environment

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using the Pt/ThO2 catalyst, the kinetic isotope effect was determined (rH2O/rD2O ~ 1.6) and was correlated with the decomposition of formate to be the rate-determining step of the reaction. The role of the platinum was comparable to the Pt/CeO2 system and facilitated the removal of surface oxygen atoms and carbonates to activate the hydroxyl groups. To further elaborate the catalytic CO oxidation mechanism of Au-doped ThO2 and gold adatom supported ThO2 in the presence of molecular oxygen, DFT calculations were performed.160 The CO oxidation on Audoped ThO2 (111) was found to occur in three steps according to the Mars-van Krevelen mechanism: firstly, a lattice oxygen of the ThO2 surface reacts with CO to form CO2 and ThO2-x (Figure 8). In the second step, molecular oxygen is adsorbed at the oxygen vacancy site and in the final step, CO reacts with the adsorbed oxygen resulting in the formation of an OCOO* intermediate, which subsequently decomposes to give CO2 under regeneration of ThO2. Thereby, the decomposition of the OCOO* intermediate is the rate-limiting step (0.58 eV, Figure 9).160

Figure 9: DFT calculations on the potential energy profile of CO oxidation using Au-doped ThO2. Reprinted with permission from [160].

In contrast, in the gold adatom supported ThO2 system it was found that CO oxidation can occur without CO and O2 coadsorption on Au. The molecular oxygen is adsorbed at the thorium site at the surface and CO reacts directly with the activated oxygen to give CO2, which is the ratedetermining step (0.46 eV). This allows for the simultaneous activation of several small molecules on a support. 3.2

Solar thermal and electrocatalytic water reduction

The process of solar thermal water splitting involves two steps: the generation of molecular oxygen via reduction of a metal oxide and its reoxidation by reaction with water under the formation of molecular hydrogen (Scheme 4). MOx

MOx-1 + 0.5 O2

(1)

MOx-1 + H2O

MOx + H2

(2)

Scheme 4: Process of solar water splitting under the formation of molecular hydrogen and oxygen.

As the first step is an endothermic reaction, it requires a relatively high amount of energy. Half or full-shell (d) transition metal oxides, such as iron oxides or zinc oxides typically requiring 18 ACS Paragon Plus Environment

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high reduction temperatures (1600-2300 °C), were intensively studied.161 Ceria has been shown to produce hydrogen at a temperature of ~2000 °C,162 however, when doped with other metals163, 164 this temperature could be decreased up to 600 °C.165 In the case of Zr4+-doped samples, the enhanced activity was ascribed to the lattice stabilization of ceria.166 Upon reduction of Ce4+ to Ce3+ a lattice expansion occurs due to the larger size of Ce3+. The smaller size of Zr4+ was found to compensate this stress. In addition, computational studies have shown that dopants with smaller ionic radii, such as Ti4+, Zr4+, and Hf4+ decrease the formation energy for oxygen vacancy, whereas dopants with larger ionic radii such as Th4+ do not show this effect.167 In order to investigate whether the formation energy for oxygen vacancy is solely dependent on the ionic radius of the dopant ion, the influence of U4+, which exhibits a comparable large ionic radius to Ce4+, was investigated via DFT calculations.163 As the oxygen vacancy formation energy was considerably lower than that of Ce4+ and Th4+ (Figure 10), the effect was demonstrated to be rather due to a charge transfer mechanism as supported by pDOS analyses.

Figure 10: DFT calculated oxygen vacancy formation energies of pristine CeO2 and its metal (M = Ti, Hf, Zr, U, Th) doped analogs. Reprinted and modified with permission from [163].

In order to investigate the influence of uranium dopants on the solar thermal water reduction performance of ceria, mixed oxides of compositions CexU1-xO2 (with x = 0, 0.25, 0.5, 0.75 and 1) were prepared via co-precipitation of cerium(III) and uranyl nitrates.165 Thereby, a linear increase in hydrogen production efficiency was observed with Ce0.75U0.25O2 exhibiting the highest activity of 5.3 · 10-6 moles at a reduction temperature of 600 °C. This corresponds to an order of magnitude enhancement when compared to pristine cerium or uranium oxides. The increase in performance (enhanced hydrogen consumption as well as lower reduction temperatures) was explained by the facilitated oxygen ion transfer from the mixed oxide materials as U4+ cations cause a partial reduction of Ce4+ to Ce3+. The charge transfer mechanism Ce4+(4f0) + U4+(5f2) → Ce3+(4f1) + U5+(5f1) was additionally evaluated by DFT calculations.163 These revealed a linear correlation between oxygen vacancy formation energies (Figure 11b) and the uranium content in the CexU1-xO2 system (Figure 11a), which is in agreement with the experimental linear dependence between the amount of hydrogen produced and the uranium fraction. Thus, the higher the uranium doping content, the lower the amount 19 ACS Paragon Plus Environment

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of oxygen vacancies and the lower the hydrogen production efficiency. This underlines the important role of oxygen vacancies in the solar thermal water reduction mechanism.

Figure 11: a) Oxygen-vacancy formation energy as function of the uranium fraction. b) Hydrogen production as a function of the oxygen-vacancy formation energy. Reprinted and modified with permission from [163].

Hu et al. prepared Ce-doped UOx to investigate the electrocatalytic hydrogen evolution reaction (HER).94 0D, 1D and 2D uranium oxide nanostructures were generated via hydrothermal syntheses using uranyl acetate as starting material and oleylamine and/or oleic acid as coordinating solvents. By tuning the pressure and temperature, anisotropic growth was achieved. Cerium dopants were introduced by codissolving ammonium cerium(IV) nitrate in different molar ratios. It was found, that the cerium dopant significantly influenced the growth mode of the nanostructures.94, 168, 169 For the electrochemical characterization via polarization curves and alternating current impedance spectra, nanoparticles were dispersed in Nafion (sulfonated tetrafluoroethylene-based fluoropolymer-copolymer) and drop coated onto glassy carbon. Whereas pristine uranium oxides did not show any HER electrocatalytic activity, cerium doping in the range of 1-3% significantly improved the performance. The best performance with an onset overpotential of 370 mV versus reversible hydrogen electrode (RHE) was achieved at a doping level of 3%. Higher doping concentrations resulted in more negative onset overpotentials but a higher impedance. As the doping concentration also influences the growth of the nanostructures and thus the surface area, the performance is both dependent on the energy band structure and the surface effect. 3.3

Photoelectrochemical and photocatalytic water oxidation

The photocatalytic water oxidation on uranium mixed oxide with plutonium and thorium was investigated by photoexcitation with UV light (20-50 eV, HeI and HeII radiation) during photoelectron spectroscopy (UPS, XPS).170 Since the measurements had to be performed under vacuum, ice was chosen to ‘fix’ water onto the metal oxide surface and to favor photochemical over thermal reactions. It was observed that the mixed oxide surface was reduced by reaction with water to postulated the reaction mechanism outlined in Scheme 5, which is analogous to the water oxidation reaction using TiO2.171 Upon UV irradiation of the mixed uranium oxide, electrons are excited into the conduction band and the holes remaining in the valence band cleave the metal-oxygen bond (1). The emerging metal cation is hydroxylated in water (2), and subsequently reacts with the M-O· radical (3) to give the metal peroxide. The reaction continues 20 ACS Paragon Plus Environment

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until molecular oxygen is generated (5). The electrons in the conduction band simultaneously reduce the metal oxide (6), thus PuO2 and UO2+x get reduced to Pu2O3 and UO2 as monitored by an increase in the U 5f: O 2p ratio in the HeII valence band spectra. (1) M-O + M+ M-O-M + h+ + + (2) M-OH + H M + H 2O + + (3) M-O-O-M + H M-O + M-OH + h + (4) M-O-O + M M-O-O-M + h+ + + (5) O2 + M-O-M + H M-O-O + h + M-OH n+ 1(n-1)+ (6) M -(OH) + M Mn+-O2--Mn+ + e- + H+ Scheme 5: Postulated reaction mechanism for the photocatalytic water oxidation on uranium oxides with UV light.

We have very recently published the application of heterostructured U3O8//Fe2O3 photoanodes (Figure 12) for photoelectrochemical (PEC) water splitting.88 PEC water splitting setups are used for light absorption and water oxidation (Oxygen Evolution Reaction (OER) formation at the anode) and reduction (Hydrogen Evolution Reaction (HER) at the cathode), respectively.

Figure 12: a) Cross-section SEM micrograph of the U3O8//Fe2O3 bilayer. b) Schematic drawing of the PEC device architecture. c) Photoelectrochemical measurements of bare U3O8, a-Fe2O3 and U3O8//Fe2O3 in the dark (dashed lines) and under illumination (solid lines) in 1M NaOH electrolyte.

An external bias needs to be applied, if the valence band and/or conduction band of the photoelectrode do not straddle the water redox potentials. Besides a suitable band gap to absorb the solar spectrum, the photocatalyst needs to exhibit a sufficient exciton lifetime, electrical conductivity, catalytic efficiency (low over-potentials) and high stability in the electrolyte. In general, uranium oxides are insoluble in water but the solubility strongly increases under oxidizing conditions (Figure 13).172 Therefore, buried layer systems realized in the form of oxide-oxide semiconductor bilayers are suitable for ensuring efficient catalytic activity in OER.


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Figure 13: Schematic representation of uranyl hydroxide formation under oxidative conditions in the electrolyte.

These prerequisites for effective PEC electrodes are augmented by other demands like using abundant materials, which are stable and have a low environmental footprint, that will support future commercial applications.

Figure 14: Localized density of states and band edge alignment at U3O8//Fe2O3 interfaces determined by DFT calculations. The localized density of states is averaged over the lateral directions such that it is resolved in the direction perpendicular to the interface plane. The band edges of the U3O8 and Fe2O3 sides are indicated by horizontal lines. The dark region between these lines corresponds to the electronic band gaps of both materials. In the atomistic structures U ions are shown in blue, Fe ions in brown and O ions in red. The electron energy levels are given with respect to the vacuum and the normal hydrogen electrode (NHE) at pH = 0.

Regarding transition metal oxides, none of the most studied materials like TiO2, Fe2O3 or WO3 fulfill all mentioned properties in the pure form and make nanostructuring, doping and multicomponent assemblies necessary to form a potent PEC system.173-175 Uranium oxides are attractive semiconductors for this application due to their wide range of band gap energies of 1.8-3.2 eV, which implies high photon absorption in the visible part of the solar spectrum.47, 50, 176 In order to ensure a fast diffusion of the holes to the surface of the photoelectrode and to prevent charge recombination with electrons, thin film nanostructures are preferably required, which can be obtained conveniently by chemical vapor deposition (CVD) of suitable volatile metal organic precursors on heated substrates. In U3O8//Fe2O3, the mixed-valent uranium oxide underlayer was prepared via plasma-enhanced chemical vapor deposition of the volatile and air-stable uranium (VI) compound [UO2(DMOTFP)2(DMOTFP-H)]. In conjunction with 22 ACS Paragon Plus Environment

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hematite (-Fe2O3) overlayers, U3O8 thin films were shown to effectively accelerate the hydrogen evolution reaction. This was ascribed to high correlation effects and electronic versatility of U3O8//Fe2O3 bilayers that show an optimal type II band edge alignment as demonstrated by DFT calculations (Figure 14). This indicates good charge separation properties of the interface since both electron depletion from the Fe2O3 surface and hole transfer towards the Fe2O3 surface are favored. Transient and X-ray absorption (TAS and XAS) spectroscopy measurements confirmed the improved charge-transfer resulting in the enhanced photocurrent density (Figure 12c) of the heterostructures (J = 2.42 mA/cm²), when compared to pristine hematite (J = 1.22 mA/cm²). 3.4

Electrocatalytic hydrogen peroxide and oxygen reduction

In 2014, Sofer et al. reported on the reduction of oxygen and hydrogen peroxide using uraniumdoped graphene (Figure 15).177 The hybrid material was synthesized by exposing graphene oxide to the dissolved uranium salt with subsequent thermal exfoliation. A homogeneous distribution of the uranium dopant (1 wt.%) was confirmed by SEM/EDX, TEM, XPS and Raman spectroscopy. However, they found that UO3 and U3O8 (or the uranium carbide) coexisted in the catalyst. For electrochemical measurements, the hybrid was dispersed in DMF and coated onto a GC electrode without the addition of an overlayer. Thus, the cyclic voltammograms in pH = 7 (phosphate buffer) showed the oxidation of doping atoms (peak maximum at 0.15-0.2 V). Electrochemical measurements in 0.1 M KOH saturated with oxygen revealed that the uranium-doped graphene exhibited an oxygen reduction efficiency comparable to a standard Pt/C catalyst.177 Due to the ability of uranium to adopt a wide range of oxidation states, they hypothesized that the dopant serves as the catalytic center and graphene as the high area conductor.

Figure 15: Schematic synthetic route to generate actinide-doped graphene from graphite. Reprinted and modified with permission from [177].

Gao et al. prepared phase pure, polydisperse U3O8 cuboid nanoparticles supported on reduced graphe ne oxide and investigated their electrocatalytic activity for oxygen and hydrogen peroxide reduction (Figure 16).178 In order to investigate the reaction mechanism, electrochemical measurements were performed using a rotating disc electrode loaded with the catalyst and covered with a layer of Nafion in oxygen saturated 0.1 M KOH solution. The Koutecký-Levich plots demonstrated that the oxygen reduction reaction occurs in a fourelectron transfer process with the hybrid material being more stable than the commercial Pt/C catalyst.178 In order to examine whether the oxygen and not the U3O8 itself was reduced, they repeated the electrochemical measurements in N2 saturated alkali solution. As the CV of U3O8/rGO hybrids showed no peaks in the nitrogen saturated solution, the reduction of U3O8 was precluded (Figure 16a). 23 ACS Paragon Plus Environment

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Figure 16: Cyclic voltammograms of U3O8 supported on reduced graphene oxide a) in oxygen and nitrogen saturated 0.1 M KOH solutions and b) in a phosphate buffer solution containing 10 mM hydrogen peroxide. Reprinted and modified with permission from [178].

3.5

Catalytic methane activation

The formation of syngas (H2 + CO) via activation of methane (Scheme 6) can occur either by reaction with water (steam reforming, 1), by reaction with molecular oxygen (partial oxidation, 2) or by reaction with carbon dioxide (dry reforming, 3). These reactions mainly differ in their enthalpy of reaction with the steam reforming (ΔH298K = +205 kJ/mol) and dry reforming reactions (ΔH298K = +247 kJ/mol) being endothermic and hence require a high energy input to proceed.179 In contrast, the partial oxidation of methane (POM, ΔH298K = -38 kJ/mol) is slightly exothermic and thus a suitable process to generate syngas from methane.179 CH4 + H2O CH4 + 0.5 O2 CH4 + CO2

CO + 3 H2 CO + 2 H2 2 CO + 2 H2

(1) (2) (3)

Scheme 6: Activation reactions of methane to form syngas.

Noble metal-based compounds have been demonstrated to be very active for the catalytic syngas formation from methane.180-183 However, aiming for less expensive catalysts with comparable performances, bimetallic cobalt-actinide, copper-actinide and nickel-actinide oxides were prepared and tested for partial oxidation of methane.133, 184 The first generation of catalysts was synthesized by Choudhary et al. via mixing of nickel or cobalt nitrate with uranyl nitrate and subsequent decomposition in air at higher temperatures.185 Thereby, the order of performance was the following: NiO-ThO2 > NiO-UO2 > NiO-ZrO2 with NiO-ThO2 showing the highest conversion of methane (~95%) with a CO and H2 selectivity of > 96% at 800 °C. Alumina supported mixed nickel–uranium catalysts (15% U) were reported by Ismagilov et al. and showed almost 85% conversion of methane and 68% yield for hydrogen production at a reaction temperature of 800 °C.186 These catalysts were prepared by incipient wet impregnation of the γ-Al2O3 support with uranyl nitrate and nickel nitrate and a subsequent annealing step in air. The performance of these catalysts was shown to be influenced by the interaction of active phase and support through both the preparation procedure and the annealing temperature. In order to avoid sintering effects or the formation of inactive binary metal oxide phases, Branco et al. synthesized their bimetallic oxide catalysts in two steps:133 first, the intermetallic compounds ThCu2, ThNi2 and UNi2 were prepared by melting the elements in the corresponding stoichiometric ratios.187 In the second step, controlled oxidation of the intermetallic compounds was induced by heating in air. The catalysts were demonstrated to be 24 ACS Paragon Plus Environment

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active and selective for the conversion of methane to syngas with the nickel-containing catalysts being more active than the copper analogue and with a higher activity when uranium was incorporated. The best performance was observed for the nickel-uranium oxide catalyst (2 NiO · UO3) with a stability of 18 h, a CH4 conversion rate of 80% and selectivity to H2 and CO of 80% at 750 °C. Thus, a performance similar to 5 wt.% Pt/Al2O3 used as the reference catalyst was achieved.188 The catalytic activity was ascribed to the presence of the UO3 phase in the nickel-uranium oxide catalyst, whereas for thorium containing compounds, the active phase was determined to be either CuO or NiO. Both bimetallic copper-lanthanide/copper-actinide oxides derived from the oxidation of intermetallic compounds and the corresponding bimetallic oxides supported on alumina were tested as catalysts for the formation of methanol from carbon dioxide and hydrogen in the presence of methane according to reaction (Scheme 7).184 CH4 + CO2 + 2 H2

2 CH3OH

Scheme 7: Formation of methanol from methane, carbon dioxide and hydrogen.

However, Branco et al. have shown that the addition of methane is detrimental for the formation of methanol, especially in the case of alumina supported catalysts and consequently the reaction was performed solely in the presence of carbon dioxide and molecular hydrogen (Scheme 8,).184 CO2 + 3 H2

CH3OH + H2O

Scheme 8: Formation of methanol and water from carbon dioxide and hydrogen.

In the absence of methane, all copper f-block element oxide catalysts showed a higher activity than the commercial copper catalyst. In particular, the copper-cerium oxide catalyst derived from the intermetallic compound exhibited the highest intrinsic activity of ~900 mLMeOH/m2Cu · h. The high performance was ascribed to the synergetic interaction of copper oxide with the lanthanide oxide phase resulting in an electronic enrichment of the copper.184 Further, the activation of methane for coupling reactions according to (Scheme 9) was investigated using bimetallic copper/nickel-actinide189 and calcium-actinide oxides.190 As oxidant N2O was used, which decomposed to give N2 and O2 in a ratio of 1:0.5, as it allows the generation of selective and mobile surface oxygen species. 2 CH4 + O2

C 2H 4 + 2 H 2O

Scheme 9: Oxidative coupling of methane to give ethane and water.

Bimetallic copper- and nickel-actinide oxides derived from the oxidation of intermetallic compounds were tested for the oxidative coupling of methane.189 It was shown, that the nickelactinide catalysts showed higher activities when compared to the copper analogues due to their acidity. The highest methane conversion (20%) with selectivity towards ethane formation (60%) at 750 °C was achieved using the Ni-U-O oxide catalyst. In contrast, the Ni-Th-O oxide catalyst rather catalyzed the formation of syngas according to reaction (Scheme 12) with a methane conversion rate of 50% and a selectivity of 90% towards the formation of syngas. 25 ACS Paragon Plus Environment

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Bimetallic calcium-actinide oxide catalysts were prepared via sol-gel reaction of calcium and actinide metal nitrates (Ca(NO3)2 · 5 H2O, Th(NO3)4 · 5 H2O, UO2(NO3)2 · 6 H2O) in the presence of citric acid and ethylene glycol according to the Pechini method.191 Thereby, the CaAn-O systems were described as CaO embedded in a matrix of either ThO2 or UO3. The highest activity and selectivity were observed for the bimetallic calcium-thorium oxide catalyst with a CH4 conversion rate of ~28% and an ethane selectivity of ~53% at a temperature of 800 °C. The higher performance of thorium in comparison to uranium was explained by the acid-base properties and the oxide conductivity type (CaO and ThO2, p-type semiconductors; UO3, n-type semiconductor). In general, it was reported that p-type semiconductors exhibit a higher activity towards oxidative coupling of methane.192-194 3.6

Hydrogen chloride activation

Hydrogen chloride formation occurs on a large scale during the industrial production of chlorinated and fluorinated synthetic polymers, such as Teflon or PVC.195 The generation of fluorinated organic compounds requires reaction of organic reactants with molecular chlorine to give hydrogen chloride and the chlorinated product (Scheme 10, 1), which in the second step can react with hydrogen fluoride to produce hydrogen chloride and the fluorinated product (Scheme 10, 2). R-H + Cl2 R-Cl + HF

R-Cl + HCl (1) R-F + HCl (2)

Scheme 10: Synthesis of halogenated synthetic polymers.

In order to recycle chlorine from the hydrogen chloride waste product directly at the production site, the Deacon process was developed (Scheme 11). However, the simple homogenous gas phase reaction of hydrogen chloride and oxygen requires temperatures higher than 800 °C.196 In order to reduce the reaction temperature, catalysts were screened with regard to their activity and stability under these harsh conditions. Thereby, RuO2 based catalysts, in particular RuO2/SiO2/TiO2 and RuO2/SnO2-Al2O3 were reported to be quite stable and to be active at lower reaction temperatures (300 °C).197 4 HCl + O2

2 Cl2 + 2 H2O

Scheme 11: Deacon process to recycle chlorine from hydrogen chloride.

Within the quest of identifying cheaper catalytic materials, different bulk uranium oxides were tested and exhibited activities of 7 · 10-3 mol Cl2 h-1 m-2, 8 · 10-3 mol Cl2 h-1 m-2 and 3.4 · 10-3 mol Cl2 h-1 m-2 for -U3O8, UO2 and -UO3, respectively.45 Whereas UO2 and -UO3 transformed into -U3O8, the latter demonstrated an extraordinary high-temperature stability when compared to transition metal oxides (e.g. CuO and Cr2O3),45 lanthanide oxides (e.g. CeO2)198 and even RuO2.199 In order to find a suitable support for the catalytically active uranium oxide, different carriers (ZrO2, Al2O3, SiO2 and TiO2) were screened.45 At a catalyst loading of 10 wt.% the following activity order was determined: U3O8/ZrO2 > U3O8/SiO2 > U3O8/TiO2 ~ U3O8/Al2O3. Especially in ZrO2 supported -U3O8 catalysts, activation was related to in-situ redispersion of uranium oxide as unraveled by high26 ACS Paragon Plus Environment

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angle annular dark field scanning transmission electron microscopy at different stages of the reaction.200 The HCl conversion efficiency of U3O8/ZrO2 was tested over time and showed an increase from 27 to 31% within the first 20 h, a further increase to 35% within the next 60 h (resulting from the restructuring into well distributed uranium species) and a stable activity for at least 100 h on stream.200 These results substantiate the high potential of uranium oxide materials in heterogeneous catalysis as manifested in the declaration of several patents by Bayer Technology Services.201, 202

Table 3: Summary of uranium containing materials for (small molecule) activation reactions in heterogeneous catalysis. Material

Reaction Type

Substrate

References

Carbon Monoxide Activation Reaction Au1/ThO2

Oxidation

CO, O2

160

Au1/ThO2

Oxidation

CO, O2

203

UOx/γ-Al2O3

Oxidation

CO, O2

204

Bi2UO6

Oxidation

CO, O2

205

Au/ThO2 NP

Water-gas shift reaction

CO, H2O

206

Pt/ThO2


CO, H2O

207

Pt/ThO2


CO, H2O

46

Au/UO3 Au/U3O8


CO, H2O

208

UO2-x

Reductive coupling

CO, H2

209

Water Activation Reaction CexZryU1-(x+y)O2

Solar thermal water reduction

H2O

163

CexZryU1-(x+y)O2

Solar thermal water reduction

H2O

165

UOx:Ce

Electrocatalytic hydrogen evolution reaction

H2O

94

UO2+x

Photocatalytic water oxidation

H2O

170


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U3O8//Fe2O3

Photoelectrocatalytic water oxidation

H2O

88

Hydrogen Peroxide and Oxygen Activation Reaction

U3O8/rGO

Reduction

H2O2

178

U- and Th-doped graphene hybrids

Reduction

H2O2

177

U- and Th-doped graphene hybrids

Reduction

O2

177

Partial oxidation

CH4, O2

141

2 NiO · UO3 2 NiO · ThO2 2 CuO · ThO2

Partial oxidation

CH4, O2

133

U/Al2O3

Partial oxidation

CH4, O2

186

Ni/UO2, Ni/ThO2 Co/UO2, Co/ThO2

Partial oxidation

CH4, O2

185

2 CuO · ThO2 2 CuO · ThO2 · Al2O3

Reforming

CH4, CO2, H2

184

Ni–U/Al2O3

Reforming

CO2, CH4

186

Oxidative coupling

CH4, N2O

190

Oxidative coupling

CH4, N2O

189

Methane Activation Reaction U/Al2O3

UO3 ThO2 “Ca-U-O” “Ca-Th-O” 2 CuO · ThO2 2 NiO · ThO2 2 NiO · UO3

Hydrogen Chloride Activation Reaction U3O8/ZrO2

Oxidation

HCl, O2

200

U3O8/ZrO2

Oxidation

HCl, O2

45

Further Activation Reactions U3O8 (γ-Al2O3)

Reduction

NO

210

UO2(NO3)2/H1-SiO2

Reduction

NO

204

Fe/U

Oxidation

Propane

211


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UO2Sb2O4

Oxidation

Propylene

212

TiO2/UO2

Oxidation

Isobutene

213

UO2

Reductive coupling

Acetaldeyhde

214

α-U3O8

Aldolization

Acetaldeyhde

214

β-UO3

Cyclization

Acetaldeyhde

214

UO2

Oxidation

Acetone

215

U:TiO2, U:Al2O3, U:SiO2

Visible light oxidation

Acetone

216

Uranyl anchored MCM41

Oxidation

Methanol

217

UO2(NO3)2/TiO2

Photooxidation

Ethanol

218

ThO2/NixMnyOz

Aerobic oxidation

Benzyl alcohol

144

U3O8/UO2 NP

Oxidation

Benzyl alcohol

93

Au/U3O8

Oxidation

Benzyl alcohol

126

Au/U3O8

Partial oxidation

Benzyl alcohol

124

(UO2)8O2(OH)12(H2O)12/U3O8

Oxidation

Benzyl alcohol

102


Oxidation

Benzylic alcohols

145

U3O8 NP

Oxidation

Alcohols

119


Photodegradation

Alcohols

219

Urania-palladium-graphene nanohybrids

Reduction

4-Nitrophenol

142

[UO2]2+/mesoporous MCM-41

Photocatalytic oxidation

VOCs

220

M/U3O8/SiO2 M = Cu, Fe, Cr, Co

Oxidative destruction

VOCs

29



VOCs

221



VOCs

29

β-UO3 γ-UO3 α-U3O8

Deep oxidation

VOCs

140

U3O8/SiO2

Oxidation

VOCs

123


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Mesoporous silica material incorporating U3O8 NP

Thermal oxidation

VOCs

222


VCOCs

223

Oxidation

Toluene

210

Au/U3O8

Epoxidation

Styrene

125

Bi2UO6

Oxidation

Benzene

210

Cr, Mn, Co:UOx

Oxidation

Chlorobenzene

143

Au/U3O8

Suzuki-Miyaura cross-coupling

Aryl halides

147

NiO · 3 UO3

Steam reforming

Naphtha

210

TiO2/ThO2

Photocatalytic degradation

Malachite Green

224

U:TiO2

Photocatalytic degradation

Rhodamine B

146

U3O8 U/SiO2 Co/U/SiO2 Cr/U/SiO2 Cu/U/SiO2 Fe/U/SiO2 Ni/U/SiO2 Mn/U/SiO2 UO2WO4 (+ Al2O3) UO2MoO4 U(MoO4)2


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4

Summary

In heterogeneous catalysis, the active material is typically either incorporated in a host material or immobilized on a substrate surface. In this context, the preparation of phase pure actinide materials and the long-term stability of the supporting material are decisive for their potential applications. Preparation of solid-state uranium-based catalysts is challenging due to the oxidative nature of uranium centers that alters the chemical topography creating local heterogeneities and chemical gradients in surface chemistry. In this context, chemical conversion of molecular compounds into well-defined oxide materials is a credible approach to obtain phase-pure catalysts.123 For nanostructured catalysts, the higher surface-to-volume ratio allows pronounced and mostly irreversible oxygen uptake that triggers valence switching, reflected in catalyst poisoning as well as in the change of electronic properties. Moreover, the preparation of nanomaterials typically involves the addition of additives, which generally adhere to uranium oxide materials thereby influence the activity at the catalyst surface. In order to overcome these challenges, new and efficient synthetic approaches are required to control the interface between supporting and active material. The formation of heterostructures, such as core-shell structures could possibly enhance both the catalytic activity and long-term stability of the systems. Furthermore large surface area materials with well-defined microstructure and chemical compositions such as poly-oxometallates (POMs) and metal-organic frameworks (MOFs) are promising alternatives to unify predefined chemical topography with high chemical reactivity and specificity.

Figure 17: Graphical summary highlighting the challenges of small molecule activation using homogeneous and heterogeneous actinide-containing catalysts.


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In summary, this brief review illustrates that the chemistry of actinide-based functionalities in molecules and materials is rapidly unfolding to provide examples that are divergent from the current models. The less understood role of f orbitals remains a trigger for the steadily growing interest in this field. In addition, it offers largely unexplored experimental space (Figure 17) to expand our understanding of chemical binding models, reactivity patterns and electronic structure. The relative high abundance of uranium and thorium calls for concerted efforts in actinide chemistry to demonstrate its implications in catalysis, recycling and reuse of actinides.

5

Acknowledgement

Authors are thankful to the University of Cologne for providing the infrastructural support. J.L. is thankful to Fonds der chemischen Industrie for a PhD fellowship. The financial support in the framework of the DFG priority programs (SPP 1613; “Fuels Produced Regeneratively Through Light-Driven Water Splitting: Clarification of the Elemental Processes Involved and Prospects for Implementation in Technological Concepts” and SPP 1959 "Manipulation of matter controlled by electric and magnetic field: Towards novel synthesis and processing routes of inorganic materials") and the Framework Program of the European Commission (FP7) that funded the European Project SOLAROGENIX (www.solarogenix.eu) are gratefully acknowledged. S.M. acknowledges the DAAD and the German Federal Ministry of Education and Research (BMBF) in the frame of the NANOFLEX (03X0125C) and MOPGA-GRI Initiative.

6

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