Vibrational spectroscopy of dispersed ReVIIOx sites supported on monoclinic zirconia.

In situ Raman and FTIR spectra complemented by in situ Raman/18O isotope labelling are exploited for deciphering the structural properties and configurations of the (ReOx)n phase dispersed on monoclinic ZrO2 at temperatures of 120-400 °C under oxidative dehydration conditions and coverages in the range of 0.71-3.7 Re nm-2. The dispersed (ReOx)n phase is heterogeneous, consisting of three distinct structural units: (a) Species-I with mono-oxo termination ORe(-O-Zr)m (ReO mode at 993-1005 cm-1); (b) Species-IIa with di-oxo termination (O)2Re(-O-Zr)m-1 (symmetric stretching mode at 987-998 cm-1); and (c) Species-IIb with di-oxo termination (O)2Re(-O-Zr)u (symmetric stretching mode at 982-991 cm-1); all terminal stretching modes undergo blue shifts with increasing coverage. With increasing temperature, a reversible temperature-dependent Species-IIa ↔ Species-I transformation is evidenced. At low coverages, below 1 Re nm-2, isolated species prevail; at 400 °C the mono-oxo ORe(-O-Zr)m Species-I is the majority species, the di-oxo Species-IIa occurs in significant proportion and di-oxo Species-IIb is in the minority. At coverage ≥1.3 Re nm-2, at 400 °C the di-oxo Species-IIa prevails clearly over mono-oxo Species-I. Below 80 °C and at a low coverage of 0.71 Re nm-2, the occurrence of a fourth structural unit, Species-III taking on a tri-oxo configuration (symmetric stretching mode at 974 cm-1) is evidenced. All temperature-dependent structural and configurational transformations are fully reversible and interpreted by mechanisms at the molecular level.

Heterogeneity of the vanadia phase dispersed on titania. Co-existence of distinct mono-oxo VOx sites.

The structural and configurational characteristics of the species comprising the (VOx)n phase dispersed on TiO2(P25) are studied under oxidative dehydration conditions by in situ molecular vibrational spectroscopy (Raman, FTIR) complemented by in situ Raman/18O isotope exchange and Raman spectroscopy under static equilibrium at temperatures of 175-430 °C and coverages in the 0.40-5.5 V nm-2 range. It is found that the dispersed (VOx)n phase consists of distinct species with different configurations. At low coverages of 0.40 and 0.74 V nm-2, isolated (monomeric) species prevail. Two distinct mono-oxo species are found: (i) a majority Species-I, presumably of distorted tetrahedral OV(-O-)3 configuration with VO mode at 1022-1024 cm-1 and (ii) a minority Species-II, presumably of distorted octahedral-like OV(-O-)4 configuration with VO mode at 1013-1014 cm-1. Cycling the catalysts in the 430 → 250 → 175 → 430 °C sequence results in temperature-dependent structural transformations. With decreasing temperature, a Species-II → Species-I transformation with concomitant surface hydroxylation takes place by means of a hydrolysis mechanism mediated by water molecules retained by the surface. A third species (Species-III, presumably of di-oxo configuration with νs/νas at ∼995/985 cm-1) occurs in minority and its presence is increased when further lowering the temperature according to a Species-I → Species-III hydrolysis step. Species-II (OV(-O-)4) shows the highest reactivity to water. For coverages above 1 V nm-2, an association of VOx units takes place leading to gradually larger polymeric domains when the coverage is increased in the 1.1-5.5 V nm-2 range. Polymeric (VOx)n domains comprise building units that maintain the structural characteristics (termination configuration and V coordination number) of Species-I, Species-II, and Species-III. The terminal VO stretching modes are blue-shifted with increasing (VOx)n domain size. A lower extent of hydroxylation is evidenced under static equilibrium forced dehydrated conditions, thereby limiting the temperature dependent structural transformations and excluding the possibility of incoming water vapors as the cause for the temperature dependent effects observed in the in situ Raman/FTIR spectra. The results address open issues and offer new insight in the structural studies of VOx/TiO2 catalysts.

Evidence of Self-Association and Conformational Change in Nisin Antimicrobial Polypeptide Solutions: A Combined Raman and Ultrasonic Relaxation Spectroscopic and Theoretical Study.

The polypeptide Nisin is characterized by antibacterial properties, making it a compound with many applications, mainly in the food industry. As a result, a deeper understanding of its behaviour, especially after its dissolution in water, is of the utmost importance. This could be possible through the study of aqueous solutions of Nisin by combining vibrational and acoustic spectroscopic techniques. The velocity and attenuation of ultrasonic waves propagating in aqueous solutions of the polypeptide Nisin were measured as a function of concentration and temperature. The computational investigation of the molecular docking between Nisin monomeric units revealed the formation of dimeric units. The main chemical changes occurring in Nisin structure in the aqueous environment were tracked using Raman spectroscopy, and special spectral markers were used to establish the underlying structural mechanism. Spectral changes evidenced the presence of the dimerization reaction between Nisin monomeric species. The UV/Vis absorption spectra were dominated by the presence of π → π* transitions in the peptide bonds attributed to secondary structural elements such as α-helix, ß-sheets and random coils. The analysis of the acoustic spectra revealed that the processes primarily responsible for the observed chemical relaxations are probably the conformational change between possible conformers of Nisin and its self-aggregation mechanism, namely, the dimerization reaction. The activation enthalpy and the enthalpy difference between the two isomeric forms were estimated to be equal to ΔH1* = 0.354 ± 0.028 kcal/mol and ΔH10 = 3.008 ± 0.367 kcal/mol, respectively. The corresponding thermodynamic parameters of the self-aggregation mechanism were found to be ΔH2* = 0.261 ± 0.004 kcal/mol and ΔH20 = 3.340 ± 0.364 kcal/mol. The effect of frequency on the excess sound absorption of Nisin solutions enabled us to estimate the rate constants of the self-aggregation mechanism and evaluate the isentropic and isothermal volume changes associated with the relaxation processes occurring in this system. The results are discussed in relation to theoretical and experimental findings.

Rethinking the molecular structures of WVIOx sites dispersed on titania: distinct mono-oxo configurations at 430 °C and temperature-dependent transformations.

The structural properties of the (WOx)n phase dispersed on TiO2 (P25, anatase) at surface densities of 0.5-4.5 W nm-2 (i.e. up to approximately a monolayer) were explored by using in situ Raman and FTIR spectroscopy, in situ Raman/18O exchange and Raman spectroscopy in static equilibrium at temperatures of 175-430 °C. Deciphering the temperature and coverage dependence of the spectral features under oxidative dehydration conditions showed that (i) the (WOx)n dispersed phase is heterogeneous at 430 °C consisting of two distinct mono-oxo species: Species-I with C3v-like OW(-O-)3 configuration (WO mode at 1009-1014 cm-1) and Species-II with C4v-like OW(-O-)4 configuration (WO mode at 1003-1009 cm-1); (ii) the OW(-O-)3 site is formed with first order of priority and its formation ceases after the complete consumption of the most basic hydroxyls that are titrated first, hence is abundant at low coverage (<1.5 W nm-2); (iii) the OW(-O-)4 site prevails over the OW(-O-)3 site at medium to high coverage (≥2 W nm-2) and partially occurs in associated (polymerized) coverages above 2 W nm-2; (iv) lowering the temperature in the 430 → 250 → 175 °C sequence does not affect the structural and vibrational properties of OW(-O-)3 but leads to the gradual transformation of the OW(-O-)4 site to a di-oxo (O)2W(-O-)3 site (with a symmetric stretching mode at ∼985 cm-1) and the partial association of adjacent OW(-O-)4 units. All temperature-dependent structural/configurational transformations are fully reversible in the 430-175 °C range and are interpreted at the molecular level by a mechanism involving water molecules retained at the surface that act in a reversible temperature-dependent mediative manner resulting in hydroxylation (upon cooling, e.g. to 250 °C) and dehydroxylation (upon heating, e.g. to 430 °C). The Raman spectra obtained for the hydroxyl region confirm the successive hydroxylation/dehydroxylation steps during temperature cycles (e.g. 430 → 250 → 430 °C). One can tune the speciation of the dispersed (WOx)n phase under dehydrated conditions by appropriate control of temperature and coverage.

Heterogeneity of deposited phases in supported transition metal oxide catalysts: reversible temperature-dependent evolution of molecular structures and configurations.

In situ high-temperature Raman spectroscopy under steady state oxidative dehydrated conditions was used for determining the temperature dependence of the molecular structures and configurations of (MOx)n (M = Re, Mo, W) sites supported at low submonolayer loadings on TiO2(P25). Prior to the Raman analysis, the studied catalyst samples underwent calcination at 450-480 °C for 4-5 h. Regularly repeated random sequences of heating and cooling under flowing 20%O2/He (in the absence of incoming water vapor) in the 35-430 °C temperature range were shown to cause drastic changes in the vibrational properties of the M-O stretching modes and in the molecular structures and configurations of the deposited ReOx, MoOx, and WOx sites in a reversible and reproducible manner. A heterogeneity of the deposited oxometallic phase was evidenced with three distinctly different species (i.e., MOx-I, MOx-II, and MOx-III) present in each system, each one prevailing in a particular temperature range. It was shown that the temperature could tune the molecular structure of the deposited oxometallic phase presumably on account of minima in the surface free energy. In the direction of temperature lowering, a mechanism leading to a hydrolysis-like of the anchoring bonds by activation of the surface hydroxyls and/or water molecules extant on the uncovered TiO2(P25) surface took place. In situ FTIR spectroscopy under identical conditions and similar temperature sequence protocols complemented the Raman results and corroborated the proposed prevailing configurations and pertinent band assignments.

Molybdena deposited on titania by equilibrium deposition filtration: structural evolution of oxo-molybdenum(vi) sites with temperature.

The equilibrium deposition filtration (EDF) method, an advanced catalyst synthesis route that is based on a molecular level approach, can be used for tailoring the oxometallic phase deposited on a porous oxide support. Here, the EDF method is used for synthesizing (MoOx)n/TiO2 catalysts. In situ Raman spectroscopy in the temperature range of 25-450 °C, low temperature (77 K) EPR spectroscopy and DR-UV spectroscopy are used for studying the evolution of the structural configuration of oxo-Mo(VI) species on TiO2 with increasing temperature as well as the influence of the supported (MoOx)n species on the photo-generation of electrons and holes of TiO2. This study concerns (MoOx)n/TiO2 samples in which the surface densities after calcination are 0.3, 2.6 and 3.9 Mo per nm(2), thereby covering a very wide range of submonolayer coverage. The gradual heat treatment of the catalysts results in a transformation of the initially (prior to drying) deposited species and the pertinent species evolution at the nano-level is discussed by means of a number of mechanisms including anchoring, association, cleavage and surface diffusion.

Molybdenum(VI) oxosulfato complexes in MoO3-K2S2O7-K2SO4 molten mixtures: stoichiometry, vibrational properties, and molecular structures.

The structural and vibrational properties of molybdenum(VI) oxosulfato complexes formed in MoO(3)­K(2)S(2)O(7) and MoO(3)­K(2)S(2)O(7)­K(2)SO(4) molten mixtures under an O(2) atmosphere and static equilibrium conditions were studied by Raman spectroscopy at temperatures of 400­640 °C. The corresponding composition effects were explored in the X(MoO)(3)(0) = 0­0.5 range. MoO(3) undergoes a dissolution reaction in molten K(2)S(2)O(7), and the Raman spectra point to the formation of molybdenum(VI) oxosulfato complexes. The Mo═O stretching region of the Raman spectrum provides sound evidence for the occurrence of a dioxo Mo(═O)(2) configuration as a core. The stoichiometry of the dissolution reaction MoO(3) + nS(2)O(7)(2­) → C(2n­) was inferred by exploiting the Raman band intensities, and it was found that n = 1. Therefore, depending on the MoO(3 content, monomeric MoO(2)(SO(4))(2)(2­) and/or associated [MoO(2)(SO(4))(2)](m)(2m­) complexes are formed in the binary MoO(3)­K(2)S(2)O(7) molten system, and pertinent structural models are proposed in full consistency with the Raman data. A 6-fold coordination around Mo is inferred. Adjacent MoO(2)(2+) cores are linked by bidentate bridging sulfates. With increasing temperature at concentrated melts (i.e., high X(MoO)(3)(0)), the observed spectral changes can be explained by partial dissociation of [MoO(2)(SO(4))(2)](m)(2m­) by detachment of S(2)O(7)(2­) and formation of a Mo­O­Mo bridge. Addition of K(2)SO(4) in MoO(3)­K(2)S(2)O(7) results in a "follow-up" reaction and formation of MoO(2)(SO(4))(3)(4­) and/or associated [MoO(2)(SO(4))(3)](m)(4m­) complexes in the ternary MoO(3)­K(2)S(2)O(7)­K(2)SO(4) molten system. The 6-fold Mo coordination comprises two oxide ligands and four O atoms linking to coordinated sulfate groups in various environments of reduced symmetry. The most characteristic Raman bands for the molybdenum(VI) oxosulfato complexes pertain to the Mo(═O)(2) stretching modes: (1) at 957 (polarized) and 918 (depolarized) cm(­1) for the ν(s) and ν(as) Mo(═O)(2) modes of MoO(2)(SO(4))(2)(2­) and [MoO(2)(SO(4))(2)](m)(2m­) and (2) at 935 (polarized) and 895 (depolarized) cm(­1) for the respective modes of MoO(2)(SO(4))(3)(4­) and [MoO(2)(SO(4))(3)](m)(4m­). The results were tested and found to be in accordance with ab initio quantum chemical calculations carried out on [MoO(2)(SO(4))(3)](4­) and [{MoO(2)}(2)(SO(4))(4)(µ-SO(4))(2)](8­) ions, in assumed isolated gaseous free states, at the DFT/B3LYP (HF) level and with the 3-21G basis set. The calculations included determination of vibrational infrared and Raman spectra, by use of force constants in the Gaussian 03W program.

An operando Raman study of molecular structure and reactivity of molybdenum(VI) oxide supported on anatase for the oxidative dehydrogenation of ethane.

Supported molybdenum oxide catalysts on TiO(2) (anatase) with surface densities in the range of 1.8-17.0 Mo per nm(2) were studied at temperatures of 410-480 °C for unraveling the configuration and molecular structure of the deposited (MoO(x))(n) species and examining their behavior for the ethane oxidative dehydrogenation (ODH). In situ Raman and in situ FTIR spectra under oxidizing conditions combined with (18)O/(16)O isotope exchange studies provide the first sound evidence for mono-oxo configuration for the deposited (MoO(x))(n) species on anatase. Isolated O=Mo(-O-)(3) tetra-coordinated species in C(3v)-like symmetry prevail at all surface coverages with a low presence of associated (polymeric) species (probably penta-coordinated) evidenced at high coverages, below the approximate monolayer of 6 Mo per nm(2). A mechanistic scenario for (18)O/(16)O isotope exchange and next-nearest-neighbor vibrational isotope effect is proposed at the molecular level to account for the pertinent spectral observations. Catalytic measurements for ethane ODH with simultaneous monitoring of operando Raman spectra were performed. The selectivity to ethylene increases with increasing surface density up to the monolayer coverage, where primary steps of ethane activation follow selective reaction pathways leading to ∼100% C(2)H(4) selectivity. The operando Raman spectra and a quantitative exploitation of the relative normalized Mo=O band intensities for surface densities of 1.8-5.9 Mo per nm(2) and various residence times show that the terminal Mo=O sites are involved in non-selective reaction turnovers. Reaction routes follow primarily non-selective pathways at low coverage and selective pathways at high coverage. Trends in the initial rates of ethane consumption (apparent reactivity per Mo) as a function of Mo surface density are discussed on the basis of several factors.

Raman spectroscopic study of tungsten(VI) oxosulfato complexes in WO3-K2S2O7-K2SO4 molten mixtures: stoichiometry, vibrational properties, and molecular structure.

The dissolution reaction of WO3 in pure molten K2S2O7 and in molten K2S2O7-K2SO4 mixtures is studied under static equilibrium conditions in the XWO3(0) = 0-0.33 mol fraction range at temperatures up to 860 °C. High temperature Raman spectroscopy shows that the dissolution leads to formation of W(VI) oxosulfato complexes, and the spectral features are adequate for inferring the structural and vibrational properties of the complexes formed. The band characteristics observed in the W=O stretching region (band wavenumbers, intensities, and polarization characteristics) are consistent with a dioxo W(=O)2 configuration as a core unit within the oxosulfato complexes formed. A quantitative exploitation of the relative Raman intensities in the binary WO3-K2S2O7 system allows the determination of the stoichiometric coefficient, n, of the complex formation reaction WO3 + nS2O7(2-) --> C(2n-). It is found that n = 1; therefore, the reaction WO3 + S2O7(2-) > WO2(SO4)2(2-) with six-fold W coordination is proposed as fully consistent with the observed Raman features. The effects of the incremental dissolution and presence of K2SO4 in WO3-K2S2O7 melts point to a WO3 · K2S2O7 · K2SO4 stoichiometry and a corresponding complex formation reaction in the ternary molten WO3-K2S2O7-K2SO4 system according to WO3 + S2O7(2-) + SO4(2-) --> WO2(SO4)3(4-). The coordination sphere of W in WO2(SO4)2(2-) (binary system) is completed with two oxide ligands and two chelating sulfate groups. A dimeric [{WO2(SO4)2}2(µ-SO4)2](8-) configuration is proposed for the W oxosulfato complex in the ternary system, generated from inversion symmetry of aWO2(SO4)3(4-) moiety resulting in two bridging sulfates. The most characteristic Raman bands for the W(VI) oxosulfato complexes pertain to W(=O)2 stretching modes (i) at 972 (polarized) and 937 (depolarized) cm(-1) for the ν(s) and ν(as) W(=O)2 modes of WO2(SO4)2(2-), and (ii) at 933 (polarized) and 909 (depolarized) cm(-1) for the respective modes of [{WO2(SO4)2}2(µ-SO4)2](8-).

Stoichiometry, vibrational modes, and structure of niobium(V) oxosulfato complexes in the molten Nb(2)O(5)-K(2)S(2)O(7)-K(2)SO(4) system studied by Raman spectroscopy.

The structural and vibrational properties of NbV oxosulfato complexes formed in Nb2O5-K2S2O7 and Nb2O5-K2S2O7-K2SO4 molten mixtures with 0 C2n-; a simple formalism exploiting the relative Raman band intensities is used for determining the stoichiometric coefficient, n, pointing to n = 3 and to the following reaction: Nb2O5 + 3S2O72- --> 2NbO(SO4)33-, which is consistent with the Raman spectra of the molten mixtures. Nb2O5 could be dissolved much easier when K2SO4 was present in an equimolar (1:1) SO42-/Nb ratio; the incremental presence of K2SO4 in Nb2O5-K2S2O7 melts induces composition effects in the Raman spectra that terminate when n(SO42-)/n(Nb) = 1. The composition effects and the temperature-dependent features of the Raman spectra obtained for Nb2O5-K2S2O7-K2SO4 molten mixtures together with the spectral changes occurring upon freezing are accounted for by a Nb2O5.3K2S2O7.2K2SO4 stoichiometry for the complete reaction taking place: Nb2O5 + 3S2O72- + 2SO42- --> NbO(SO4)4S2O77- + NbO2(SO4)23-. The spectral data are discussed in terms of the most plausible structural models, for which consistent band assignments are made. The most characteristic Raman bands for the NbV oxosulfato complexes pertain to Nb=O modes: (i) at 937 cm-1 for the mono-oxo Nb=O mode of NbO(SO4)33-; (ii) at 958 cm-1 for the mono-oxo Nb=O mode of NbO(SO4)4S2O77-; and (iii) at 926 cm-1 for the symmetric dioxo Nb(=O)2 mode of NbO2(SO4)23-.

Thermodynamic analysis of reaction equilibria in ionic and molecular liquid systems by high-temperature Raman spectroscopy.

A formalism for correlating relative Raman band intensities with the stoichiometric coefficients, the equilibrium constant, and the thermodynamics of reaction equilibria in solution is derived. The proposed method is used for studying: (1) the thermal dissociation of molten KHSO(4) in the temperature range 240-450 degrees C; (2) the dinuclear complex formation in molten TaCl(5)-AlCl(3) mixtures at temperatures between 125 and 235 degrees C. The experimental and calculational procedures for exploiting the temperature-dependent Raman band intensities in the molten phase as well as (if applicable) in the vapors thereof are described and used for determining the enthalpy of the equilibria: (1) 2HSO(4)(-)(l) <--> S(2)O(7)(2-)(l) + H(2)O(g), DeltaH(0)=64.9 +/- 2.9 kJ mol(-1); and (2) 1/2Ta(2)Cl(10)(l) + 1/2Al(2)Cl(6)(l) <--> TaAlCl(8)(l), DeltaH(0)=-12.1 +/- 1.5 kJ mol(-1).

Thermal dissociation of molten KHSO4: temperature dependence of Raman spectra and thermodynamics.

Raman spectroscopy is used to study the thermal dissociation of molten KHSO4 at temperatures of 240-450 degrees C under static equilibrium conditions. Raman spectra obtained at 10 different temperatures for the molten phase and for the vapors thereof exhibit vibrational wavenumbers and relative band intensities inferring the occurrence of the temperature-dependent dissociation equilibrium 2HSO4(-)(l) <--> S2O7(2-)(l) + H2O(g). The Raman data are adequate for determining the partial pressures of H2O in the gas phase above the molten mixtures. A formalism for correlating relative Raman band intensities with the stoichiometric coefficients, the equilibrium constant, and the thermodynamics of the reaction equilibrium is derived. The method is used along with the temperature-dependent features of the Raman spectra to show that the studied equilibrium 2HSO4(-)(l) <--> S 2O7(2-)(l) + H2O(g) is the only process taking place to a significant extent in the temperature range of the investigation and for determining its enthalpy to be DeltaH degrees=64.9+/-2.9 kJ mol(-1). The importance of these findings for the understanding of the performance of the industrially important sulfuric acid catalyst under "wet" conditions is briefly addressed.