Analysis of silica-supported vanadia by X-ray absorption spectroscopy: Combined theoretical and experimental studies

V-O Species-Doped Carbon Frameworks Loaded with Ru Nanoparticles as Highly Efficient and CO-Tolerant Catalysts for Alkaline Hydrogen Oxidation

Efficient and CO-tolerant catalysts for alkaline hydrogen oxidation (HOR) are vital to the commercial application of anion exchange membrane fuel cells (AEMFCs). Herein, a robust Ru-based catalyst (Ru/VOC) with ultrasmall Ru nanoparticles supported on carbon frameworks with atomically dispersed V-O species is prepared elaborately. The catalyst exhibits a remarkable mass activity of 3.44 mA μgPGM, which is 31.3 times that of Ru/C and even 4.7 times higher than that of Pt/C. Moreover, the Ru/VOC anode can achieve a peak power density (PPD) of 1.194 W cm-2, much superior to that of Ru/C anode and even better than that of Pt/C anode. In addition, the catalyst also exhibits superior stability and exceptional CO tolerance. Experimental results and density functional theory (DFT) calculations demonstrate that V-O species are ideal OH- adsorption sites, which allow Ru to release more sites for hydrogen adsorption. Furthermore, the electron transfer from Ru nanoparticles to the carbon substrate regulates the electronic structure of Ru, reducing the hydrogen binding energy (HBE) and the CO adsorption energy on Ru, thus boosting the alkaline HOR performance and CO tolerance of the catalyst. This is the first report that oxophilic single atoms distributed on carbon frameworks serve as OH- adsorption sites for efficient hydrogen oxidation, opening up new guidance for the elaborate design of high-activity catalysts for the alkaline HOR.

N and O multi-coordinated vanadium single atom with enhanced oxygen reduction activity

Recently, atomically dispersed transition-metal single atom in nitrogen-doped carbon matrix as electrocatalysts has aroused general interest. However, there is no report about vanadium single atom for ORR in the literature. According to d-band center theory for transition-metals, the performance of catalysts is regulated by the electronic structure of the catalytic center which determines the intermediate adsorption kinetics. Indeed, the valence of vanadium is variable, its electron structure could be modulated by an appropriate coordination structure. Here, a novel method is developed to prepare the N and O co-coordinated vanadium single atom (V-N₁O₄) embedded in the carbon matrix. The catalyst displays a half-wave potential of 865 mV in base solution which surpasses 20% Pt/C, and also shows a high power density of 180 mW/cm² in Zn-air batteries. DFT calculations reveal that the N and O coordination configuration could regulate the electron structure and geometry of vanadium to boost the electrocatalytic activity.

Synthesis and Characterization of Mesoporous Silica SBA-15 and ZnO/SBA-15 Photocatalytic Materials from the Ash of Brickyards

SBA-15 has been successfully synthesized from ash of brickyard with the surface area of 700.100 m2/g, pore volume of 0.813 cm3·g−1, and pore size of 7.5 nm. SBA-15 material has been modified by a simple impregnation method, and the ZnO/SBA-15 obtained material retains the hexagonal structure of SBA-15, but the hexagonal capillary tube size is reduced, in which the surface area, volume of pores, and pore size is 212.851 m2·g−1, 0.244 cm3·g−1, and 3.7 nm, respectively. The ZnO/SBA-15 materials were investigated by the photodegradation of methylene blue under ultraviolet light and exhibited significant photocatalytic activity with the reached MB removal efficiency about 96.69%.

Grafting metal complexes onto amorphous supports: from elementary steps to catalyst site populationsviakernel regression

We use transition state theory, kernel regression, and population balance modeling techniques to model the grafting of metal complexes onto amorphous catalyst supports.

The Comparison between Single Atom Catalysis and Surface Organometallic Catalysis

Single atom catalysis (SAC) is a recent discipline of heterogeneous catalysis for which a single atom on a surface is able to carry out various catalytic reactions. A kind of revolution in heterogeneous catalysis by metals for which it was assumed that specific sites or defects of a nanoparticle were necessary to activate substrates in catalytic reactions. In another extreme of the spectrum, surface organometallic chemistry (SOMC), and, by extension, surface organometallic catalysis (SOMCat), have demonstrated that single atoms on a surface, but this time with specific ligands, could lead to a more predictive approach in heterogeneous catalysis. The predictive character of SOMCat was just the result of intuitive mechanisms derived from the elementary steps of molecular chemistry. This review article will compare the aspects of single atom catalysis and surface organometallic catalysis by considering several specific catalytic reactions, some of which exist for both fields, whereas others might see mutual overlap in the future. After a definition of both domains, a detailed approach of the methods, mostly modeling and spectroscopy, will be followed by a detailed analysis of catalytic reactions: hydrogenation, dehydrogenation, hydrogenolysis, oxidative dehydrogenation, alkane and cycloalkane metathesis, methane activation, metathetic oxidation, CO₂ activation to cyclic carbonates, imine metathesis, and selective catalytic reduction (SCR) reactions. A prospective resulting from present knowledge is showing the emergence of a new discipline from the overlap between the two areas.

Characterization and Catalytic Performance of Modified SBA-16 in Liquid Phase Reaction

A new mesoporous silica SBA-16 (called SP-16) was prepared by the direct hydrothermal method using Pluronic F127 (triblock copolymer; EO106PO70EO106) as surfactant and TEOS (tetraethyl orthosilicate) as silica source. The catalyst property of the SBA-16 was attained by loading the STA (Silicotungstic acid) active compound. The loading contents of STA were determined between 5 % and 40 % based on weight ratio of W and Si (W/Si). Catalytic activities and sustainability of SP-16 (10–20 %, W/Si) catalysts were determined by esterification (liquid phase reaction methanol and acetic acid) reactions at 343-353K, under autogenic pressure, 1/1–1/2 feed molar ratios (methanol/acetic acid) and in the presence of 0.4 g catalyst in the semi-batch reactor for 6 - 24h. Acetic acid conversion values of 10 and 20 % catalysts with 1/1 molar ratio at the end of 24h were obtained as 32–52.9 % and 47–60 %, respectively. On the other hand, when 1/2 molar ratio at 353K was used, 20 % catalyst showed 82.2 % conversion. Moreover, a second reaction experiment of 10 % catalyst was also carried out in identical conditions in the presence of catalyst recovered after the first methyl acetate reaction. The first and second reaction results of 10 % catalyst indicated that catalytic activity and sustainability were preserved for both 6 and 24h analyses. The physical properties of the materials obtained were investigated by Nitrogen sorption at 77K (BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy(FT-IR), Multiple Internal Reflection (DRIFT), Thermo-gravimetry/Differential thermal analysis(TG/DTA), Scanning electron microscope (SEM) and MAPPING analysis methods.

A unified view on heterogeneous and homogeneous catalysts through a combination of spectroscopy and quantum chemistry

Identifying catalytically active structures or intermediates in homogeneous and heterogeneous catalysis is a formidable challenge. However, obtaining experimentally verified insight into the active species in heterogeneous catalysis is a tremendously challenging problem. Many highly advanced spectroscopic and microscopic methods have been developed to probe surfaces. In this discussion we employ a combination of spectroscopic methods to study two closely related systems from the heterogeneous (the silica-supported vanadium oxide VOx/SBA-15) and homogeneous (the complex K[VO(O2)Hheida]) domains. Spectroscopic measurements were conducted strictly in parallel for both systems and consisted of oxygen K-edge and vanadium L-edge X-ray absorption measurements in conjunction with resonance Raman spectroscopy. It is shown that the full information content of the spectra can be developed through advanced quantum chemical calculations that directly address the sought after structure-spectra relationships. To this end we employ the recently developed restricted open shell configuration interaction theory together with the time-dependent theory of electronic spectroscopy to calculate XAS and rR spectra respectively. The results of the study demonstrate that: (a) a combination of several spectroscopic techniques is of paramount importance in identifying signature structural motifs and (b) quantum chemistry is an extremely powerful guide in cross connecting theory and experiment as well as the homogeneous and heterogeneous catalysis fields. It is emphasized that the calculation of spectroscopic observables provides an excellent way for the critical experimental validation of theoretical results.

SOMC grafting of vanadium oxytriisopropoxide (VO(OiPr)3) on dehydroxylated silica; analysis of surface complexes and thermal restructuring mechanism

VO(OⁱPr)₃ was grafted on highly dehydroxylated silica by a surface organometallic chemistry approach and its thermal evolution was analyzed with support of DFT calculations.

A combined experimental and theoretical spectroscopic protocol for determination of the structure of heterogeneous catalysts: developing the information content of the resonance Raman spectra of M1 MoVO x

Absorption and multiwavelength resonance Raman spectroscopy are widely used to investigate the electronic structure of transition metal centers in coordination compounds and extended solid systems. In combination with computational methodologies that have predictive accuracy, they define powerful protocols to study the spectroscopic response of catalytic materials. In this work, we study the absorption and resonance Raman spectra of the M1 MoVO _x catalyst. The spectra were calculated by time-dependent density functional theory (TD-DFT) in conjunction with the independent mode displaced harmonic oscillator model (IMDHO), which allows for detailed bandshape predictions. For this purpose cluster models with up to 9 Mo and V metallic centers are considered to represent the bulk structure of MoVO _x . Capping hydrogens were used to achieve valence saturation at the edges of the cluster models. The construction of model structures was based on a thorough bonding analysis which involved conventional DFT and local coupled cluster (DLPNO-CCSD(T)) methods. Furthermore the relationship of cluster topology to the computed spectral features is discussed in detail. It is shown that due to the local nature of the involved electronic transitions, band assignment protocols developed for molecular systems can be applied to describe the calculated spectral features of the cluster models as well. The present study serves as a reference for future applications of combined experimental and computational protocols in the field of solid-state heterogeneous catalysis.

L-edge X-ray absorption study of mononuclear vanadium complexes and spectral predictions using a restricted open shell configuration interaction ansatz

A series of mononuclear V((V)), V((IV)) and V((III)) complexes were investigated by V L-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The spectra show significant sensitivity to the vanadium oxidation state and the coordination environment surrounding the vanadium center. The L-edge spectra are interpreted with the aid of the recently developed Density Functional Theory/Restricted Open Shell Configuration Interaction Singles (DFT/ROCIS) method. This method is calibrated for the prediction of vanadium L-edges with different hybrid density functionals and basis sets. For the B3LYP/def2-TZVP(-f) and BHLYP/def2-TZVP(-f) functional/basis-set combinations, good to excellent agreement between calculated and experimental spectra is obtained. A treatment of the spin-orbit coupling interaction to all orders is achieved by quasi-degenerate perturbation theory (QDPT), in conjunction with DFT/ROCIS for the calculation of the molecular multiplets while accounting for dynamic correlation and anisotropic covalency. The physical origin of the observed spectral features is discussed qualitatively and quantitatively in terms of spin multiplicities, magnetic sublevels and individual 2p to 3d core level excitations. This investigation is an important prerequisite for future applications of the DFT/ROCIS method to vanadium L-edge absorption spectroscopy and vanadium-based heterogeneous catalysts.

Size-dependent catalytic activity of supported vanadium oxide species: oxidative dehydrogenation of propane

Possible reaction pathways for the oxidative dehydrogenation of propane by vanadium oxide catalysts supported on silica are examined by density functional theory. Monomeric and dimeric vanadium oxide species are both considered and modeled by vanadyl-substituted silsesquioxanes. The reaction proceeds in two subsequent steps. In a first step, hydrogen abstraction from propane by a vanadyl (O═V) group yields a propyl radical bound to a HOV(IV) surface site. Propene is formed by a second hydrogen abstraction, either at the same vanadia site or at a different one. V(V)/V(IV) redox cycles are preferred over V(V)/V(III) cycles. Under the assumption of fast reoxidation, microkinetic simulations show that the first step is rate-determining and yields Arrhenius barriers that are lower for dimers (114 kJ/mol at 750 K) than for monomers (124 kJ/mol). The rate constants predicted for a mixture of monomers and dimers are 14% larger (750 K) than for monomers only, although the increase remains within experimental uncertainty limits. Direct calculations of energy barriers also yield lower values for dimeric species than for monomeric ones. Reactivity descriptors indicate that this trend will continue also for larger oligomers. The size distribution of oligomeric species is predicted to be rather statistical. This, together with the small increase in the rate constants, explains that turnover frequencies observed for submonolayer coverages of vanadia on silica do not vary with the loading within the experimental uncertainty limits.

First principles calculations of the structure and V L-edge X-ray absorption spectra of V2O5 using local pair natural orbital coupled cluster theory and spin-orbit coupled configuration interaction approaches

A detailed study of the electronic and geometric structure of V2O5 and its X-ray spectroscopic properties is presented. Cluster models of increasing size were constructed in order to represent the surface and the bulk environment of V2O5. The models were terminated with hydrogen atoms at the edges or embedded in a Madelung field. The structure and interlayer binding energies were studied with dispersion-corrected local, hybrid and double hybrid density functional theory as well as the local pair natural orbital coupled cluster method (LPNO-CCSD). Convergence of the results with respect to cluster size was achieved by extending the model to up to 20 vanadium centers. The O K-edge and the V L2,3-edge NEXAFS spectra of V2O5 were calculated on the basis of the newly developed Restricted Open shell Configuration Interaction with Singles (DFT-ROCIS) method. In this study the applicability of the method is extended to the field of solid-state catalysis. For the first time excellent agreement between theoretically predicted and experimentally measured vanadium L-edge NEXAFS spectra of V2O5 was achieved. At the same time the agreement between experimental and theoretical oxygen K-edge spectra is also excellent. Importantly, the intensity distribution between the oxygen K-edge and vanadium L-edge spectra is correctly reproduced, thus indicating that the covalency of the metal-ligand bonds is correctly described by the calculations. The origin of the spectral features is discussed in terms of the electronic structure using both quasi-atomic jj coupling and molecular LS coupling schemes. The effects of the bulk environment driven by weak interlayer interactions were also studied, demonstrating that large clusters are important in order to correctly calculate core level absorption spectra in solids.

Degradation of Toluene Using Modified TiO2 as Photocatalysts

Volatile organic compounds (VOCs), especially toluene as the typical indoor air pollutants, are toxic and environmentally persistent whose removal is undoubtedly becoming increasingly urgent matter over these years. Titania is one of the most promising photocatalysts for the degradation of organic compounds, whereas the large band gap of titania and massive recombination of photogenerated charge carriers limit its overall photocatalytic effciency. These defects can be tackled by modifying the electronic band structure of titania including various strategies like metal deposition, non-metal atoms substitution, transition metal ions doping, and coupling with a narrow band gap semiconductor, etc. This review encompasses several advancements made in these aspects, and also the influence factors such as physical morphologies changing, humidity, as well as the presence of O2 etc, are involved. To be practically considering, TiO2 photocatalysts require being fixed on the bulky supports like silica, alumina, clays and activated carbons. Moreover, photocatalytic coatings deposited on external building materials, like roofing tiles and corrugated sheets, is becoming the attractive application potentials to remove toluene from air.

Structure characterization of metal oxide clusters by vibrational spectroscopy: possibilities and prospects

This article summarizes the methodological progress that has been made in the vibrational spectroscopy of isolated polynuclear metal oxide clusters, with particular emphasis on free electron laser-based infrared action spectroscopy of gas phase clusters, over the last decade. The possibilities, limitations and prospects of the various experimental approaches are discussed using representative examples from pivotal studies in the field.

Chemistry with methane: concepts rather than recipes

Four seemingly simple transformations related to the chemistry of methane will be addressed from mechanistic and conceptual points of view: 1) metal-mediated dehydrogenation to form metal carbene complexes, 2) the hydrogen-atom abstraction step in the oxidative dimerization of methane, 3) the mechanisms of the CH(4)→CH(3)OH conversion, and 4) the initial bond scission (C-H vs. O-H) as well as the rate-limiting step in the selective CH(3)OH→CH(2)O oxidation. State-of-the-art gas-phase experiments, in conjunction with electronic-structure calculations, permit identification of the elementary reactions at a molecular level and thus allow us to unravel detailed mechanistic aspects. Where appropriate, these results are compared with findings from related studies in solution or on surfaces.

Pore structure and surface area of silica SBA-15: influence of washing and scale-up

The removal of the surfactant (EO(20)PO(70)EO(20)) by washing before final calcination is a critical step in the synthesis of silica SBA-15. In contrast to washing with pure water or ethanol, washing with water and ethanol may, depending on the quantity of solvent used, alter the homogeneity and order of the pores, but also lead to an increase of the surface area of SBA-15. A reduction of solvent volume and a controlled washing protocol allow the synthesis of high surface area SBA-15 materials with a narrow monomodal pore size distribution. For larger batch sizes the influence of the quantity of solvent on the quality of the SBA-15 is reduced.

Structural characterization of vanadium oxide catalysts supported on nanostructured silica SBA-15 using X-ray absorption spectroscopy

The local structure of vanadium oxide supported on nanostructured SiO2 (VxOy/SBA-15) was investigated by in situ X-ray absorption spectroscopy (XAS). Because the number of potential parameters in XAS data analysis often exceeds the number of "independent" parameters, evaluating the reliability and significance of a particular fitting procedure is mandatory. The number of independent parameters (Nyquist) may not be sufficient. Hence, in addition to the number of independent parameters, a novel approach to evaluate the significance of structural fitting parameters in XAS data analysis is introduced. Three samples with different V loadings (i.e. 2.7 wt %, 5.4 wt %, and 10.8 wt %) were employed. Thermal treatment in air at 623 K resulted in characteristic structural changes of the V oxide species. Independent of the V loading, the local structure around V centers in dehydrated VxOy/SBA-15 corresponded to an ordered arrangement of adjacent V2O7 units. Moreover, the V2O7 units were found to persist under selective oxidation reaction conditions.