Effect of Doping on Rutile TiO2 Surface Stability and Crystal Shapes

Transition-metal-(TM-)doped TiO₂ has been considered as promising electrode material for the oxygen evolution reaction (OER). OER activity is expected to depend on the coordination of the surface atoms. In this study, we theoretically investigate the stability of low-index surfaces of TM-doped rutile, (110), (100), (101) and (001), with 50 % of the Ti atoms substituted by Sc, Y, V, Nb or Ta. For Sc and Y, we also consider models with O vacancies providing the most stable oxidation state of Sc and Y. Surface energies are calculated with DFT(+U). Based on the Gibbs-Wulff theorem, the shape of the single crystals is predicted. It is observed that p-doping leads to spontaneous oxygen loss and O vacancies cause surface reconstruction. The Wulff shapes of n-doped TiO₂ have smaller contributions of the (110) facet and, for Nb and Ta, larger contributions of other facets. Given the higher coordinative unsaturation of the TM atoms in the latter, a higher catalytic activity is expected.     

Anticorrelation between Surface and Subsurface Point Defects and the Impact on the Redox Chemistry of TiO2(110)

By using a combination of scanning tunneling microscopy (STM), density functional theory (DFT), and secondary‐ion mass spectroscopy (SIMS), we explored the interplay and relative impact of surface versus subsurface defects on the surface chemistry of rutile TiO₂. STM results show that surface O vacancies (V_O) are virtually absent in the vicinity of positively charged subsurface point defects. This observation is consistent with DFT calculations of the impact of subsurface defect proximity on V_O formation energy. To monitor the influence of such lateral anticorrelation on surface redox chemistry, a test reaction of the dissociative adsorption of O₂ was employed and was observed to be suppressed around them. DFT results attribute this to a perceived absence of intrinsic (Ti), and likely extrinsic interstitials in the nearest subsurface layer beneath inhibited areas. We also postulate that the entire nearest subsurface region could be devoid of any charged point defects, whereas prevalent surface defects (V_O) are largely responsible for mediation of the redox chemistry at the reduced TiO₂(110).     

Rendering Photoreactivity to Ceria: The Role of Defects

The photoreactivity of ceria, a photochemically inert oxide with a large band gap, can be increased to competitive values by introducing defects. This previously unexplained phenomenon has been investigated by monitoring the UV‐induced decomposition of N₂O on well‐defined single crystals of ceria by using infrared reflection‐absorption spectroscopy (IRRAS). The IRRAS data, in conjunction with theory, provide direct evidence that reducing the ceria(110) surface yields high photoreactivity. No such effects are seen on the (111) surface. The low‐temperature photodecomposition of N₂O occurs at surface O vacancies on the (110) surface, where the electron‐rich cerium cations with a significantly lowered coordination number cause a local lowering of the huge band gap (ca. 6 eV). The quantum efficiency of strongly reduced ceria(110) surfaces in the photodecomposition of N₂O amounts to 0.03 %, and is thus comparable to that reported for the photooxidation of CO on rutile TiO₂(110).     

Structure of Mn2(CO)10 and Mn2(CO)9: Insights from ab initio calculations

Optimization of the Mn–Mn distance in Mn₂(CO)₁₀ with various basis sets of at least doublezeta quality results in Mn–Mn bond lengths in the range of 3.07–3.15 Å, 0.2–0.25 Å longer than the experimental value of 2.895 Å. Incrementing the basis set with diffuse p functions (exponent 0.0332) on the carbon atoms improves the calculated bond length to a value of 2.876 Å at the CI level, as a consequence of a charge transfer between each Mn atom and the equatorial carbonyls of the other Mn atom. For Mn₂(CO)₉ four structures have been studied at the SCF and CI levels with assumed geometries. The structure with a symmetric bridging carbonyl turns to be much higher in energy at the SCF level. The two structures which are purely metal–metal bonded [corresponding to the departure of an axial or equatorial carbonyl from Mn₂(CO)₁₀] are nearly degenerate in energy and more stable than the structure with a semibridging carbonyl by 5 kcal/mol at the SCF level and 10–11 kcal/mol at the CI level (seemingly at variance with the conclusions of matrix experiments that favor the semibridging structure).     

The Solid-State-Grinding Synthesis of Maganese-Modified Cobalt Oxides and Application in the Low-Temperature CO Preferential Oxidation in H<Subscript>2</Subscript>-Rich Gases

A series of MnOx modified cobalt oxides with different atomic molar ratios of Mn/(Mn?+?Co) were prepared by a soft reactive grinding route and investigated for CO preferential oxidation in H₂. It was found that as-prepared Mn-doped cobalt oxides exhibited superior activity compared to the single constituted oxides, other Mn–Co–O mixed oxides synthesized by solution-based route, and other grinding-derived mixed metal oxides M–Co–O (M?=?Zn, Ni, Cu, Fe). The grinding-derived MnCo10 catalyst with Mn/(Mn?+?Co) molar ration of 10% showed the best CO oxidation activity and higher selectivity at low temperature. The surface richness of Co³⁺ was not found as increasing the Mn molar ratio in the present work. However, the incoporation of MnOx with proper amount into Co₃O₄ could produce high surface area, high structure defects, and rich surface active oxygen species, while the ability to supply the active oxygen species was suggested to play the crucial role in promoting the catalytic performance of Mn–Co–O mixed oxides.     

Adsorption of carbon monoxide on Pd low‐index surfaces and (110) reconstructed surface

Adsorption and diffusion of carbon monoxide on Pd low‐index surfaces and missing‐row Pd (110) reconstructed surface have been investigated by the extended London–Eyring–Polyani–Sato (LEPS) method constructed by means of a five‐parameter Morse potential. All critical characteristics, such as adsorption site, adsorption geometry, binding energy, CO vibrational frequency have been obtained and compared with the experimental and theoretical data. On these surfaces, the stable adsorption sites of CO are changed with increasing CO coverage. On the missing‐row Pd (110) reconstructed surface, there are five stable adsorption sites: H₁, H₂ (H₁ and H₂ are threefold hollow sites on (111) subsurface), B (bridge site on the second layer), SB (short‐bridge site), and T (top site). Copyright © 2010 John Wiley & Sons, Ltd.     

Tricarbonyl(η5‐formylcyclopentadienyl)manganese(I) and tricarbonyl(η5‐formylcyclopentadienyl)rhenium(I) containing short π(CO)...π(CO) and π(CO)...π interactions

The structures of tricarbonyl(formylcyclopentadienyl)manganese(I), [Mn(C₆H₅O)(CO)₃], (I), and tricarbonyl(formylcyclopentadienyl)rhenium(I), [Re(C₆H₅O)(CO)₃], (II), were determined at 100 K. Compounds (I) and (II) both possess a carbonyl group in a trans position relative to the substituted C atom of the cyclopentadienyl ring, while the other two carbonyl groups are in almost eclipsed positions relative to their attached C atoms. Analysis of the intermolecular contacts reveals that the molecules in both compounds form stacks due to short attractive π(CO)...π(CO) and π(CO)...π interactions, along the crystallographic c axis for (I) and along the [201] direction for (II). Symmetry‐related stacks are bound to each other by weak intermolecular C—H...O hydrogen bonds, leading to the formation of the three‐dimensional network.     

Physical properties of mixed-valence Tl 3 I TlIIICl6 investigated by conductivity and positron annihilation lifetime measurements: Effect of crushing the crystals

For a better understanding of isotope exchange in solid Tl₄Cl₆ the effects of crushing the crystals were investigated by conductivity and by positron annihilation lifetime measurements. As in untreated Tl₄Cl₆, the conductivity variation with temperature shows a break at about 450 K. The activation energies, 0.53 eV and 0.70 eV, respectively, below and above 450 K, are very close to those in the untreated material but the absolute values of conductivity are lower after crushing which is attributed to the trapping of the mobile defects of dislocation. The positron lifetime variation with increasing temperature shows some contribution of extrinsic defects, annealing at about 413 K. On further heating or cooling cycles, the lifetime changes are controlled by the production of intrinsic cation vacancies, whose formation enthalpy, 0.39 eV, is close to that derived for untreated Tl₄Cl₆. The shape of the initial part of the curves would indicate that crushing does not directly create appreciable concentrations of cation vacancies but would rather produce annealable defects, possibly dislocations, favouring the formation and/or trapping of such vacancies.     

Decomposition of methane on polycrystalline thick films of Ga2O3 investigated by thermal desorption spectroscopy with a mass spectrometer

Methane in air can be detected by the conductivity increase of Ga₂O₃ films. Films (200 μm) of β-Ga₂O₃ were prepared by depositing a suspension of β-Ga₂O₃ powder (Johnson Matthey; 32102; 99,99%) on alumina substrates. The films were exposed to 20 kPa O₂ for 15 min at 934 K. In thermal desorption spectroscopy (TDS, β = 4,6 K/s, UHV conditions) only O₂ occured at temperatures above 934 K. On reduction in 100 Pa H₂ for 5 min at 800 K, only a suboxide, Ga₂O (above 880 K), indicating a destabilisation of the lattice [1], a broad hydrogen peak (440–930 K) and the formation of water (700–900 K) were observed. No Ga₂O₃ and O₂ were found in desorption. At temperatures between 260 K and 934 K the film was exposed to methane (100 Pa, 5 min). For exposure temperatures between 630 K and 934 K, CO, CO₂, H₂, and small amounts of CH₄ and the suboxide Ga₂O appeared in desorption. A reaction scheme for the decomposition of methane is proposed. It includes the adsorption of CH₄, the dissociation of CH₄, the desorption of H₂O and the formation of oxygen vacancies. These vacancies and the adsorbed hydrogen both acting as donors may explain the conductance increase on exposure to methane observed by other authors.     

Interaction of methanol with ruthenium

The interaction of methanol with a clean (110) ruthenium surface has been studied using temperatures programmed desorption methods. Methanol dissociates upon adsorption at 300 K and yields H₂(g) and chemisorbed CO as the dominant products. Randomization of evolved hydrogen was shown to occur during methanol adsorption and also upon subsequent thermal desorption using isotopically labeled methanol, CH₃OD. In addition to hydrogen and CO, small amounts of H₂CO, CH₃OH, CO₂, and H₂O, are also observed upon thermal desorption. In contrast with a previous study of formaldehyde on Ru(110), no detectable CH₄ product is found upon methanol desorption.     

Theoretical Insights into the Mechanism of Carbon Monoxide (CO) Release from CO‐Releasing Molecules

We used density functional theory to investigate the capacity for carbon monoxide (CO) release of five newly synthesized manganese‐containing CO‐releasing molecules (CO‐RMs), namely CORM‐368 ( 1 ), CORM‐401 ( 2 ), CORM‐371 ( 3 ), CORM‐409 ( 4 ), and CORM‐313 ( 5 ). The results correctly discriminated good CO releasers ( 1 and 2 ) from a compound unable to release CO ( 5 ). The predicted Mn? CO bond dissociation energies were well correlated (R²≈0.9) with myoglobin (Mb) assay experiments, which quantified the formation of MbCO, and thus the amount of CO released by the CO‐RMs. The nature of the Mn? CO bond was characterized by natural bond orbital (NBO) analysis. This allowed us to identify the key donor–acceptor interactions in the CO‐RMs, and to evaluate the Mn? CO bond stabilization energies. According to the NBO calculations, the charge transfer is the major source of Mn? CO bond stabilization for this series. On the basis of the nature of the experimental buffers, we then analyzed the nucleophilic attack of putative ligands (L′=HPO₄^2?, H₂PO₄^?, H₂O, and Cl^?) at the metal vacant site through the ligand‐exchange reaction energies. The analysis revealed that different L′‐exchange reactions were spontaneous in all the CO‐RMs. Finally, the calculated second dissociation energies could explain the stoichiometry obtained with the Mb assay experiments.     

Evolved gas analysis–ion attachment mass spectrometric analysis of decacarbonyl dimanganese pyrolysis

Evolved gas analysis?Cion attachment mass spectrometric analysis of the principal species produced by the pyrolysis of Mn₂(CO)₁₀ in an infrared image furnace indicated the presence of Mn(CO)₅ in the gas phase. This observation indicates that Mn₂(CO)₁₀ was in equilibrium with Mn(CO)₅. We also studied the temperature dependence of the mass spectrum to obtain information about the kinetics of the Mn₂(CO)₅ dissociation reaction. From the temperature dependence of the peak for Mn(CO)₅Li⁺ (m/z 202), we calculated the apparent activation energy of Mn(CO)₅ dissociation from solid Mn₂(CO)₁₀. The calculated activation energy (274.57?kJ/mol) is compared with previously reported experimental and calculated values of Mn?CMn bond dissociation energies.     

Construction of Long-Range Magnetic Sequences on Different Surfaces of BaTiO3

Using the first-principles spin-density-functional theory calculations, we studied the origin of ferromagnetism from non-magnetic ferroelectric barium titanate (BaTiO₃) and found out vacancies in different surface can successfully contribute to the origin of ferromagnetism. Accurately, our findings demonstrate that both O and Ti vacancies induce ferromagnetism on the (001) and (010) surfaces of BaTiO₃, and the optimal Ti−O bond length can control the vacancy-induced spin density that is delocalized or concentrated in the real space outside the vacancy, and it helps to enhance our understanding on the long-range magnetic order induced by the vacancy. In addition, intrinsic magnetism is shown on the defect-free (110) surface, and the structure is found to be a near-ideal two-dimensional Ising ferromagnet with large magnetocrystalline anisotropy, and it supplies the platform for studying basic spin behavior of BaTiO₃ and more according materials.     

Thermal stability and surface structure of Mo/CeO<Subscript>2</Subscript> and Ce-doped Mo/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts

In order to explore the influence of CeO₂ on the structure and surface characteristics of molybdena, an investigation was undertaken by using N₂ adsorption (BET method), thermal analysis and in-situ diffuse reflectance infrared (DRIFT) techniques. In this work, the Mo/CeO₂ and Ce-Mo/Al₂O₃ samples were prepared by impregnation and co-precipitation methods with high Mo loadings. Combining the results one may notice that the presence of ceria led to the increase of polymerized surface Mo species so as to forming Mo-O-Ce linkages besides the formation of coupled O=Mo=O bonds indicative of polymeric MoO₃. From thermal analysis, it can be inferred that Mo/Al₂O₃ is the thermally most stable material in the temperature range used in the experiment (up to 900°C), whereas Ce-Mo/Al₂O₃ and Mo/CeO₂ samples undergo morphological modifications above 700°C resulting in lattice defects, which motivate the mobility of Mo and Ce ions and thus enhance the possibility of interaction between them. Additionally, their activity towards CO adsorption needs reduced ceria and molybdena containing coordinatively unsaturated sites (CUS), oxygen vacancies and hydroxyl groups to form various carbonate species.     

Study of K/Mn-MgO Supported Fe Catalysts with Fe（CO）5 and Fe（NO3）3 as Precursors for CO Hydrogenation to Light Alkenes

Two K/Mn-MgO supported catalysts were prepared by Fe（CO）5 and Fe（NO3）3 as precursor respectively. The obtained Fe-K/Mn-MgO catalysts were tested for CO hydrogenation to light alkenes and characterized by X-ray powder diffraction （XRD）, X-ray photoelectron spectra （XPS）, H2 temperature-programmed reduction （H2-TPR）, H2 CO and CO2 temperature-programmed desorption （H2, CO/CO2-TPD） and transmission electron microscope （TEM） The results indicated that the catalyst with 10 wt% Fe loading prepared by Fe（CO）5 as precursor showed better performance in syngas to light alkenes than ones obtained from Fe（NO3）3 as precursor, where the CO conversion was 62.50% and the selectivity was 55.95% at 350 ℃, 1.5 MPa and 1000 h^-1, respectively.     

CO\begin{document}$ _{2} $\end{document} Reduction on Metal-Doped SnO\begin{document}$ _{2} $\end{document}(110) Surface Catalysts: Manipulating the Product by Changing the Ratio of Sn:O

The electrocatalytic carbon dioxide reduction reaction (CO₂RR) producing HCOOH and CO is one of the most promising approaches for storing renewable electricity as chemical energy in fuels. SnO₂ is a good catalyst for CO₂-to-HCOOH or CO₂-to-CO conversion, with different crystal planes participating the catalytic process. Among them, (110) surface SnO₂ is very stable and easy to synthesisze. By changing the ratio of Sn: O for SnO₂(110), we have two typical SnO₂ thin films: fully oxidized (stoichiometric) and partially reduced. In this work, we are concerned with different metals (Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au)-doped SnO₂(110) with different activity and selectivity for CO₂RR. All these changes are manipulated by adjusting the ratio of Sn: O in (110) surface. The results show that stochiometric and reduced Cu/Ag doped SnO₂(110) have different selectivity for CO₂RR. More specifically, stochiometric Cu/Ag-doped SnO₂(110) tends to generate CO(g). Meanwhile, the reduced surface tends to generate HCOOH(g). Moreover, we also considered the competitive hydrogen evolution reaction (HER). The catalysts SnO₂(110) doped by Ru, Rh, Pd, Os, Ir, and Pt have high activity for HER, and others are good catalysts for CO₂RR.     

Na10(Na,Mn)7Mn43(PO4)36: a new synthetic fillowite‐type phosphate

Na₁₀(Na,Mn)₇Mn₄₃(PO₄)₃₆ (sodium manganese phosphate) was synthesized hydrothermally at 873 K and 0.35 GPa. The complex crystal structure is almost identical to that of natural fillowite‐type phosphates and can be described as a hexagonal packing of three types of rods parallel to the c axis. The rods are constituted by an alternation of five‐ to seven‐coordinated Mn sites [average Mn—O = 2.243 (3) Å], of six‐ to nine‐coordinated Na sites [average Na—O = 2.590 (3) Å], of PO₄ tetrahedra [average P—O = 1.548 (3) Å] and of cation vacancies.     

Density functional computation of 55Mn NMR parameters

Gradient-corrected (GGA) and hybrid variants of density functional theory are used to compute geometries and ⁵⁵Mn chemical shifts of MnO₄ ⁻, Mn(CO)₆ ⁺, Mn₂(CO)₁₀, Mn(CO)₅ X [X=H, Cl, C(O)Me], Mn(CO)₅ ⁻, Mn(NO)₃(CO), and Mn(C₅H₅)L _x [L _x =(CO)₃, C₆H₆, C₇H₈]. For this set of compounds, substituent effects on δ(⁵⁵Mn) are significantly underestimated with the pure GGA functional BPW91 and are well described with hybrid functionals such as mPW1PW91 and, in particular, B3LYP. The computed data provide evidence for solvent and counterion effects on δ(⁵⁵Mn) of MnO₄ ⁻ and Mn(CO)₆ ⁺, respectively. The latter, in the presence of Cl⁻, may be described as highly fluxional Mn(CO)₅C(O)Cl. Electric field gradients computed with the B3LYP functional can be used for a qualitative rationalization of observed trends in ⁵⁵Mn NMR line widths. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s002140020338x Received: 17 January 2002 / Accepted: 13 March 2002 / Published online: 3 June 2002     

The Interaction of O2, NO,and N2O with Surface Defects of Dispersed Titanium Dioxide

A combined study of intrinsic structural defects in reduced TiO₂ was performed using mass spectrometry, optical diffuse-reflectance spectroscopy, and UV photoelectron spectroscopy (UPS). It was found that the reduction of TiO₂ resulted in the appearance of absorption in the region 0.50 h 3.50 eV (400 2500 nm), which is formed by absorption due to free electrons (a continuum at h 1.50 eV), local centers—Ti³⁺ ions (a band at 2.00 eV), and oxygen vacancies (bands at 1.17, 2.81, and 2.55 eV). The spectrum of induced occupied electronic states in the forbidden gap and the position of oxygen vacancy levels with respect to the Fermi level were determined by UPS. The absorption of reduced TiO₂ was stable on the sample to T = 800 K in a vacuum; however, it weakened in contact with O₂, NO, and N₂O molecules beginning at T = 300 K (surface sites) and T 400 K (subsurface sites) as a result of filling oxygen vacancies with atomic oxygen in the course of dissociative adsorption. The adsorption complexes formed by the interaction of O₂, NO, and N₂O with defects were analyzed by temperature-programmed desorption. The distribution of sites over the energies of oxygen binding was found with the use of a nonuniform surface model, and specific oxygen adsorption species were revealed. It was found that the irradiation of TiO₂ activates the formation and decay of sites and results in the formation of specific O₂ and N₂O adsorption species.