Probing Surface Transformations of Lanthanum Nickelate Electrocatalysts during Oxygen Evolution Reaction.

Monitoring the spontaneous reconstruction of the surface of metal oxides under electrocatalytic reaction conditions is critical to identifying the active sites and establishing structure-activity relationships. Here, we report on a self-terminated surface reconstruction of Ruddlesden-Popper lanthanum nickel oxide (La2NiO4+δ) that occurs spontaneously during reaction with alkaline electrolyte species. Using a combination of high-resolution scanning transmission electron microscopy (HR-STEM), surface-sensitive X-ray photoelectron spectroscopy (XPS), and soft X-ray absorption spectroscopy (sXAS), as well as electrochemical techniques, we identify the structure of the reconstructed surface layer as an amorphous (oxy)hydroxide phase that features abundant under-coordinated nickel sites. No further amorphization of the crystalline oxide lattice (beyond the ∼2 nm thick layer formed) was observed during oxygen evolution reaction (OER) cycling experiments. Notably, the formation of the reconstructed surface layer increases the material's oxygen evolution reaction (OER) activity by a factor of 45 when compared to that of the pristine crystalline surface. In contrast, a related perovskite phase, i.e., LaNiO3, did not show noticeable surface reconstruction, and also no increase in its OER activity was observed. This work provides detailed insight into a surface reconstruction behavior dictated by the crystal structure of the parent oxide and highlights the importance of surface dynamics under reaction conditions.

Implications for the Hydrogenation of Propyne and Propene with Parahydrogen due to the in†situ Transformation of Rh2C to Rh0/C.

NMR spectroscopy studies using parahydrogen-induced polarization have previously established the existence of the pairwise hydrogen addition route in the hydrogenation of unsaturated hydrocarbons over heterogeneous catalysts, including those based on rhodium (Rh0). This pathway requires the incorporation of both hydrogen atoms from one hydrogen molecule to the same product molecule. However, the underlying mechanism for such pairwise hydrogen addition must be better understood. The involvement of carbon, either in the form of carbonaceous deposits on the surface of a catalyst or as a metal carbide phase, is known to modify catalytic properties significantly and thus could also affect the pairwise hydrogen addition route. Here, we explored carbon's role by studying the hydrogenation of propene and propyne with parahydrogen on a Rh2C catalyst and comparing the results with those for a Rh0/C catalyst obtained from Rh2C via H2 pretreatment. While the catalysts Rh2C and Rh0/C differ notably in the rate of conversion of parahydrogen to normal hydrogen as well as in terms of hydrogenation activity, our findings suggest that the carbide phase does not play a significant role in the pairwise H2 addition route on rhodium catalysts.

Understanding the Synergy between Fe and Mo Sites in the Nitrate Reduction Reaction on a Bio-Inspired Bimetallic MXene Electrocatalyst.

Mo- and Fe-containing enzymes catalyze the reduction of nitrate and nitrite ions in nature. Inspired by this activity, we study here the nitrate reduction reaction (NO3 RR) catalyzed by an Fe-substituted two-dimensional molybdenum carbide of the MXene family, viz., Mo2 CTx : Fe (Tx are oxo, hydroxy and fluoro surface termination groups). Mo2 CTx : Fe contains isolated Fe sites in Mo positions of the host MXene (Mo2 CTx ) and features a Faradaic efficiency (FE) and an NH3 yield rate of 41 % and 3.2 µmol h-1 mg-1 , respectively, for the reduction of NO3 - to NH4 + in acidic media and 70 % and 12.9 µmol h-1 mg-1 in neutral media. Regardless of the media, Mo2 CTx : Fe outperforms monometallic Mo2 CTx owing to a more facile reductive defunctionalization of Tx groups, as evidenced by in situ X-ray absorption spectroscopy (Mo K-edge). After surface reduction, a Tx vacancy site binds a nitrate ion that subsequently fills the vacancy site with O* via oxygen transfer. Density function theory calculations provide further evidence that Fe sites promote the formation of surface O vacancies, which are identified as active sites and that function in NO3 RR in close analogy to the prevailing mechanism of the natural Mo-based nitrate reductase enzymes.

Advancing MXene Electrocatalysts for Energy Conversion Reactions: Surface, Stoichiometry, and Stability.

MXenes, due to their tailorable chemistry and favourable physical properties, have great promise in electrocatalytic energy conversion reactions. To exploit fully their enormous potential, further advances specific to electrocatalysis revolving around their performance, stability, compositional discovery and synthesis are required. The most recent advances in these aspects are discussed in detail: surface functional and stoichiometric modifications which can improve performance, Pourbaix stability related to their electrocatalytic operating conditions, density functional theory and advances in machine learning for their discovery, and prospects in large scale synthesis and solution processing techniques to produce membrane electrode assemblies and integrated electrodes. This Review provides a perspective that is complemented by new density functional theory calculations which show how these recent advances in MXene material design are paving the way for effective electrocatalysts required for the transition to integrated renewable energy systems.

Altering Oxygen Binding by Redox-Inactive Metal Substitution to Control Catalytic Activity: Oxygen Reduction on Manganese Oxide Nanoparticles as a Model System.

Establishing generic catalyst design principles by identifying structural features of materials that influence their performance will advance the rational engineering of new catalytic materials. In this study, by investigating metal-substituted manganese oxide (spinel) nanoparticles, Mn3 O4 :M (M=Sr, Ca, Mg, Zn, Cu), we rationalize the dependence of the activity of Mn3 O4 :M for the electrocatalytic oxygen reduction reaction (ORR) on the enthalpy of formation of the binary MO oxide, Δf H°(MO), and the Lewis acidity of the M2+ substituent. Incorporation of elements M with low Δf H°(MO) enhances the oxygen binding strength in Mn3 O4 :M, which affects its activity in ORR due to the established correlation between ORR activity and the binding energy of *O/*OH/*OOH species. Our work provides a perspective on the design of new compositions for oxygen electrocatalysis relying on the rational substitution/doping by redox-inactive elements.

Single-Atom-Substituted Mo2CTx:Fe-Layered Carbide for Selective Oxygen Reduction to Hydrogen Peroxide: Tracking the Evolution of the MXene Phase.

This work critically assesses the electrocatalytic activity, stability, and nature of the active phase of a two-dimensional molybdenum carbide (MXene) with single-atomic iron sites, Mo2CTx:Fe (Tx are surface terminating groups O, OH, and F), in the catalysis of the oxygen reduction reaction (ORR). X-ray absorption spectroscopy unequivocally confirmed that the iron single sites were incorporated into the Mo2CTx structure by substituting Mo atoms in the molybdenum carbide lattice with no other detectable Fe-containing phases. Mo2CTx:Fe, the first two-dimensional carbide with isolated iron sites, demonstrates a high catalytic activity and selectivity in the oxygen reduction to hydrogen peroxide. However, an analysis of the electrode material after the catalytic tests revealed that Mo2CTx:Fe transformed in situ into a graphitic carbon framework with dispersed iron oxyhydroxide (ferrihydrite, Fh) species (Fh/C), which are the actual active species. This experimental observation and the results obtained for the titanium and vanadium 2D carbides challenge previous studies that discuss the activity of the native MXene phases in oxygen electrocatalysis. Our work showcases the role of 2D metal carbides as precursors for active carbon-based (electro)catalysts and, more fundamentally, highlights the intrinsic evolution pathways of MXenes in electrocatalysis.

Tailoring Lattice Oxygen Binding in Ruthenium Pyrochlores to Enhance Oxygen Evolution Activity.

Ruthenium pyrochlores, that is, oxides of composition A2Ru2O7-δ, have emerged recently as state-of-the-art catalysts for the oxygen evolution reaction (OER) in acidic conditions. Here, we demonstrate that the A-site substituent in yttrium ruthenium pyrochlores Y1.8M0.2Ru2O7-δ (M = Cu, Co, Ni, Fe, Y) controls the concentration of surface oxygen vacancies (VO) in these materials whereby an increased concentration of VO sites correlates with a superior OER activity. DFT calculations rationalize these experimental trends demonstrating that the higher OER activity and VO surface density originate from a weakened strength of the M-O bond, scaling with the formation enthalpy of the respective MOx phases and the coupling between the M d states and O 2p states. Our work introduces a novel catalyst with improved OER performance, Y1.8Cu0.2Ru2O7-δ, and provides general guidelines for the design of active electrocatalysts.

Single Site Cobalt Substitution in 2D Molybdenum Carbide (MXene) Enhances Catalytic Activity in the Hydrogen Evolution Reaction.

Two-dimensional (2D) carbides, nitrides, and carbonitrides known as MXenes are emerging materials with a wealth of useful applications. However, the range of metals capable of forming stable MXenes is limited mostly to early transition metals of groups 3-6, making the exploration of properties inherent to mid or late transition metal MXenes very challenging. To circumvent the inaccessibility of MXene phases derived from mid-to-late transition metals, we have developed a synthetic strategy that allows the incorporation of such transition metal sites into a host MXene matrix. Here, we report the structural characterization of a Mo2CTx:Co phase (where Tx are O, OH, and F surface terminations) that is obtained from a cobalt-substituted bulk molybdenum carbide (ß-Mo2C:Co)  through a two-step synthesis: first an intercalation of gallium yielding Mo2Ga2C:Co followed by removal of Ga via HF treatment. Extended X-ray absorption fine structure (EXAFS) analysis confirms that Co atoms occupy Mo positions in the Mo2CTx lattice, providing isolated Co centers without any detectable formation of other cobalt-containing phases. The beneficial effect of cobalt substitution on the redox properties of Mo2CTx:Co is manifested in a substantially improved hydrogen evolution reaction (HER) activity, as compared to the unsubstituted Mo2CTx catalyst. Density functional theory (DFT) calculations attribute the enhanced HER kinetics of Mo2CTx:Co to the favorable binding of hydrogen on the oxygen terminated MXene surface that is strongly influenced by the substitution of Mo by Co in the Mo2CTx lattice. In addition to the remarkable HER activity, Mo2CTx:Co features excellent operational and structural stability, on par with the best performing non-noble metal-based HER catalysts. Overall, our work expands the compositional space of the MXene family by introducing a material with site-isolated cobalt centers embedded in the stable matrix of Mo2CTx. The synthetic approach presented here illustrates that tailoring the properties of MXenes for a specific application can be achieved via substitution of the host metal sites by mid or late transition metals.

Cobalt Oxide Materials for Oxygen Evolution Catalysis via Single-Source Precursor Chemistry.

The utilization of metal alkoxides as single-source precursors for (mixed-)oxide materials offers remarkable benefits, such as the possibility to precisely control the metal ratio in the resulting material, highly homogeneous distribution of the elements in the film, and the low temperatures required for film processing. Herein we report on the isolation and characterization of the bimetallic Co-Mo alkoxide [Co3 Mo4 O10 (OCH3 )10 (dmf)4 ] (Co3 Mo4 ; dmf=N,N-dimethylformamide), which was prepared by the anion metathesis reaction of the corresponding metal chlorides. The Co-Mo alkoxide was explored as a well-defined precursor of cobalt oxide catalysts for the oxygen evolution reaction (OER) in alkaline electrolyte MOH. The catalysts demonstrated excellent activity in the OER, manifested in low onset potentials and Tafel slopes and superb stability under the operating conditions both in alkaline and nearly neutral media. It was observed that the nature of the metal cation of the alkaline electrolyte MOH (M+ =Li+ , Na+ , K+ , Cs+ ) greatly affected the catalytic performance of the material. We propose that the positive effect of larger metal cations on the film activity in the OER could be explained by the higher hydration enthalpies of larger ions and enhanced mass transport within a larger interlayer space between the [CoO2 ]δ-∞ sheets of the in situ formed binary oxides. It may be deduced that this trend is universal and may be extended to other types of metal oxides forming layered structures during the OER.

Photo-Switchable Nanoripples in Ti3C2Tx MXene.

MXenes are two-dimensional materials with a rich set of chemical and electromagnetic properties, the latter including saturable absorption and intense surface plasmon resonances. To fully harness the functionality of MXenes for applications in optics, electronics, and sensing, it is important to understand the interaction of light with MXenes on atomic and femtosecond dimensions. Here, we use ultrafast electron diffraction and high-resolution electron microscopy to investigate the laser-induced structural dynamics of Ti3C2Tx nanosheets. We find an exceptionally fast lattice response with an electron-phonon coupling time of 230 fs. Repetitive femtosecond laser excitation transforms Ti3C2Tx through a structural transition into a metamaterial with deeply sub-wavelength nanoripples that are aligned with the laser polarization. By a further laser illumination, the material is reversibly photo-switchable between a flat and rippled morphology. The resulting nanostructured MXene metamaterial with directional nanoripples is expected to exhibit an anisotropic optical and electronic response as well as an enhanced chemical activity that can be switched on and off by light.

Correction: Ni-based heterogeneous catalyst from a designed molecular precursor for the efficient electrochemical water oxidation.

Correction for 'Ni-based heterogeneous catalyst from a designed molecular precursor for the efficient electrochemical water oxidation' by Denis A. Kuznetsov et al., Chem. Commun., 2016, 52, 9255-9258.

Tri-, tetra-, and hexanuclear mixed-valence molybdenum clusters: structural diversity and catalysis of acetylene hydrogenation.

A series of novel cluster compounds comprising molybdenum in a low valence state was synthesized by means of a disproportionation of the dimeric compound [Mo+42Cl4(OCH3)4(CH3OH)2] (1). The reaction of 1 with CH3OH leads to the disproportionation of Mo+4 yielding an unusual mixed-valence cluster [Mo+3.54Cl4O2(OCH3)6(CH3OH)4] (2). By exploring this synthetic approach further, tri-{[Mo3Cl3(OCH3)7(CH3OH)3] (3)}, tetra-{[Mo4Cl4(OCH3)10(CH3OH)2] (4), [Mo4Cl3O(OCH3)9(CH3OH)3] (5), [Mo4Cl2(OCH3)12(CH3OH)2] (6)}, and hexanuclear {[Mo6Cl4O6(OCH3)10(CH3OH)2] (7)} molybdenum alkoxides were synthesized by the reaction of 1 with methanol and stoichiometric amounts of magnesium methoxide, thus providing a general facile access to the polynuclear methoxide complexes of a low-valence molybdenum. Due to the feasibility to adopt multiple oxidation states in a reversible manner and the documented competence of molybdenum alkoxide compounds to catalyze the reduction of inert molecules, including N2, the synthesized compounds were expected to function as catalysts of small molecule substrates reduction/hydrogenation. Accordingly, the reduction of acetylene (C2H2) to an ethylene (C2H4) and ethane (C2H6) mixture, in methanol (with water additives) serving as a reaction medium and a proton donor, and using sodium or europium amalgams as reducing agents, was performed in the presence of 2. Preliminary kinetic studies evidently point to a catalytic function of molybdenum species derived from 2, thus establishing the observed reactivity as a rare example of non-precious metal-catalyzed acetylene hydrogenation, providing, in addition, a convenient model for further mechanistic studies.

Ni-based heterogeneous catalyst from a designed molecular precursor for the efficient electrochemical water oxidation.

Bimetallic Ni-Mo alkoxide was synthesized and exploited as the single-source precursor for the solution-processed deposition of the mixed-oxide layers on different conducting surfaces. Upon potential cycling in 1 M NaOH, these composites convert, in situ, into highly porous NiOx/NiOOH catalysts characterized by the high electrocatalytic activity for water oxidation under both basic (pH 13.6) and near neutral (pH 9.2) conditions.

Retrosynthetic approach to the design of molybdenum-magnesium oxoalkoxides.

The reaction of MoCl5 methanolysis in the presence of magnesium ions was shown to produce an extensive row of heterobimetallic Mg-Mo(V, VI) oxomethoxides of different nuclearity ranging from 4 for [Mg2(CH3OH)4Mo2O2(OCH3)10] (1) to 26 for [Mg(DMF)3(CH3OH)3]2[Mo22Mg4O48(OCH3)28(DMF)6] (2) with the latter possessing a ring morphology. Examination of [Mo6O12(OCH3)16Mg4(CH3OH)6] (3), [Mo6O12(OCH3)12Mg2(DMF)4] (4a), and [Mo6O16(OCH3)4Mg2(DMF)8] (5a) X-ray structures revealed the presence of the well known tetranuclear core {Mo4O8(OCH3)2}(2+) thus similar reactivity patterns leading to their formation were assumed. For convenient synthesis of such heterobimetallic oxoalkoxides, the retrosynthetic approach based on speculative deconstruction of a target molecule onto simpler fragments was suggested and successfully employed. Namely, the reaction of the stoichiometric amounts of appropriately chosen Mo(V), Mo(VI) and Mg(2+) synthons led to their assembling resulting in the formation of heterometallic clusters 3, 5a and [Mo6O12(OCH3)12Mg2(CH3OH)4]·2CH3OH (4b) characterized by means of elemental analysis, UV-Vis, IR spectroscopy, and X-ray crystallography.