R. C. R. Santos

CO2 mitigation by carbon nanotube formation during dry reforming of methane analyzed by factorial design combined with response surface methodology

Abstract A factorial experimental design was combined with response surface methodology (RSM) to optimize the catalyzed CO2 consumption by coke deposition and syngas production during the dry reforming of CH4. The CH4/CO2 feed ratio and the reaction temperature were chosen as the variables, and the selected responses were CH4 and CO2 conversion, the H2/CO ratio, and coke deposition. The optimal reaction conditions were found to be a CH4/CO2 feed ratio of approximately 3 at 700 °C, producing a large quantity of coke and realizing high CO2 conversion. Furthermore, Raman results showed that the CH4/CO2 ratio and reaction temperature affect the systems response, particularly the characteristics of the coke produced, which indicates the formation of carbon nanotubes and amorphous carbon.

Cu, Fe, or Ni doped molybdenum oxide supported on Al2O3 for the oxidative dehydrogenation of ethylbenzene

Abstract Molybdenum-based catalysts supported on Al 2 O 3 doped with Ni, Cu, or Fe oxide were synthesized and used in ethylbenzene dehydrogenation to produce styrene. The molybdenum oxide was supported using an unconventional route that combined the polymeric precursor method (Pechini) and wet impregnation on commercial alumina. The samples were characterized by X-ray diffraction (XRD), N 2 adsorption-desorption isotherms, temperature-programmed reduction of H 2 (H 2 -TPR), and thermogravimetric (TG) analysis. XRD results showed that the added metals were well dispersed on the alumina support. The addition of the metal oxide (Ni, Cu, or Fe) of 2 wt% by wet impregnation did not affect the texture of the support. TPR results indicated a synergistic effect between the dopant and molybdenum oxide. The catalytic tests showed ethylbenzene conversion of 28%–53% and styrene selectivity of 94%–97%, indicating that the addition of the dopant improved the catalytic performance, which was related to the redox mechanism. Molybdenum oxides play a fundamental role in the oxidative dehydrogenation of ethylbenzene to styrene by its redox and acid–base properties. The sample containing Cu showed an atypical result with increasing conversion during the reaction, which was due to metal reduction. The Ni-containing solid exhibited the highest amount of carbon deposited, shown by TG analysis after the catalytic test, which explained its lower catalytic stability and selectivity.

Role of Cu, Ni and Co metals in the acidic and redox properties of Mo catalysts supported on Al2O3 spheres for glycerol conversion

The combined acid–base and redox properties of active Cu, Ni and Co-promoted Mo catalysts supported on Al2O3 spheres have been investigated for the gas-phase dehydration of glycerol. Al2O3 spheres used as supports were impregnated with an aqueous resin containing metal precursors that was synthesized by the polymeric precursor method. The supports and catalysts were characterized by means of chemical analysis, XRD, H2-TPR, N2-physisorption isotherms, SEM-EDS, CO2-TPD, TG/DTA and pyridine adsorption isotherms. The results show differences in morphological, textural and acid–base properties, whereas the XRD and H2-TPR analyses point to a good dispersion of the metal precursors. The H2-TPR analysis shows that the addition of Co, Cu or Ni affects the reducibility of the Mo6+ species, which results in a shift of the H2 consumption peaks to a lower temperature. The addition of Mo resulted in a higher catalytic conversion, particularly for the sample containing Cu (CuMoAl). This improvement in the catalytic performance is possibly related to the presence of active Cu1+/Cu0 and Mo5+/Mo4+ redox species. Although all catalysts favour glycerol dehydration with higher acrolein selectivity, it was found that the redox properties of the Mo species play an important role in the product distribution, which leads to allyl alcohol production with decreased hydroxyacetone yield.

Optimization Study in Biodiesel Production via Response Surface Methodology Using Dolomite as a Heterogeneous Catalyst

A carbonate mineral, dolomite, was used as a heterogeneous catalyst to produce methyl-esters from soybean oil. The samples were analyzed by XRF, TGA, XRD, TPD-CO2, and SEM. The calcination of dolomite at 800°C/1 h resulted in a highly active mixed metal oxides. In addition, the influence of the reaction variables such as the temperature, catalyst amount, and methanol/soybean oil molar ratio in methyl-ester production was optimized by the application of a central composite design in conjunction with the response surface methodology (RSM). The XRF analysis is carried out after the reuses procedure which shows that the deactivation process is mainly due to the selective calcium leaching. Overall, the calcined dolomite exhibited high catalytic activity at moderate operating conditions for biodiesel production.

First-generation antipsychotic haloperidol: optical absorption measurement and structural, electronic, and optical properties of its anhydrous monoclinic crystal by first-principle approaches

The fifty years old first-generation antipsychotic drug haloperidol is very effective in the therapy of the positive symptoms of schizophrenia and mania, but its solid-state properties remain veiled. To fill this gap, the anhydrous monoclinic crystal of haloperidol, was investigated through calculations employing density functional theory (DFT) within the generalized gradient approximation (GGA) and including the dispersion effects (TS). The optical absorption measurements revealed that the anhydrous monoclinic haloperidol crystal is a wide band gap material with a 3.8 eV indirect main gap energy and an optical absorption onset due to a Frenkel exciton with a binding energy of 1.1 eV and charge-transfer excitons with a binding energy of 0.2–0.5 eV. The DFT-GGA+TS modeling produced structural parameters in very good agreement with the X-ray data, with the calculated band gap energy smaller than the experimental data by about 36%, with an indirect band gap corresponding to α1 → β1 transition. After applying the Δ-sol gap correction scheme, the main gap increased to 3.72 eV, almost matching the experimental gap estimation. Theoretical simulations within the time-dependent DFT formalism, on the other hand, predicted a Frenkel exciton with a binding energy of 1.6 eV, about 45% larger than the experimental estimation. It was found that electronic wavefunctions mainly originating from the 2p states of oxygen (O1) and nitrogen (N1) atoms contribute to the highest valence energy band, while the 2p states of C15, C16, C19, C21, and O2 atoms contribute most significantly to the lowest conduction energy bands. Analysis showed that the unit cell was less stiff along the b direction than the a and c directions. Finally, calculations pointed to a high degree of optical anisotropy for the absorption and complex dielectric function, with more structured curves for incident light polarized along the 100 and 001 directions.

Vibrational Properties of Bulk Boric Acid 2A and 3T Polymorphs and Their Two-Dimensional Layers: Measurements and Density Functional Theory Calculations

Boric acid (H3BO3) is being used effectively nowadays in traps/baits for the management of Aedes aegypti L. and Aedes albopictus Skuse species of mosquitoes, which are the main spreading vectors worldwide for diseases such as malaria, dengue, and zika. Previously, we published results on the structural, electronic, and optical properties of its molecular triclinic H3BO3-2A and trigonal H3BO3-3T polymorphs within the framework of density functional theory (DFT). Because of the renewed importance of these materials, the focus of this work is on the vibrational properties of the bulk boric acid 2A and 3T polymorphs. We measured the infrared and Raman spectra of the former, which was accompanied and interpreted through state-of-the-art DFT calculations, supplemented by computations regarding the H3BO3 molecule and two-dimensional layers based on the bulk structures. We identify/assign their normal modes and find vibrational signatures for each polymorph as well as in- and out-of-plane motions and molecular vibrations, unveiling a nice agreement between the DFT level of theory employed and our improved spectroscopic measurements in the wavenumber ranges of 400-2000 cm-1 (infrared) and 0-1500 cm-1 (Raman). We show that a dispersion-corrected DFT functional within the generalized gradient approximation (GGA) can be very accurate in describing the vibrational properties of the boric acid polymorphs. Besides, several issues left open/not clearly resolved in previously published works on the vibrational mode assignments of the bulk and 2D sheets of boric acid are explained satisfactorily. Finally, phonon dispersions and associated densities of states were also evaluated for each polymorph along with their temperature-dependent DFT-calculated entropy, enthalpy, free energy, heat capacity, and Debye temperature. In particular, our DFT calculations suggest a possible way to differentiate the 2A and 3T boric acid polymorphs through Raman spectroscopy and heat capacity measurements.

Vibrational Modes and Phonon and Thermodynamic Properties of the Metaboric Acid Polymorphs α-, β-, and γ-(BOH)3O3 within a Density Functional Theory Framework

A combined study of vibrational and thermodynamic properties of metaboric acid (BOH)3O3 crystal polymorphs α, β, and γ were obtained through density functional theory (DFT) calculations in an attempt to resolve the conflicting assignments that currently exist in the literature for them. A complete correlation between the normal-mode assignment and vibrational signatures to distinguish particular features of each metaboric acid polymorph, in particular, those related to motions of the planar layers in α-(BOH)3O3, with a level of detail surpassing essays based on previous published experimental works has been achieved. Besides, no DFT-based research work was published early on the (BOH)3O3 polymorph vibrational properties, and our DFT-simulated infrared and Raman spectra for all metaboric acid polymorphs agree very well with experiment. Comparison of the previously published experimental IR and Raman spectroscopic results with predictions from higher levels DFT calculations allows identification of the in-plane and out-of-plane B-O bending modes. For example, the strongest measured (DFT-calculated) Raman modes of α-(BOH)3O3 at 591 and 797 cm-1 (599 and 810 cm-1) are identified as vibrational signatures of breathing B3O3/Ag in-plane modes, while the shoulder in the lattice modes region at 135 (143) cm-1 is the vibrational signature of the bending B3O3/B1g out-of-plane mode. Phonon-dispersion bands and their respective phonon densities of states were also evaluated for each system, as well as temperature-dependent curves for entropy, enthalpy, free energy, heat capacity, and Debye temperature. Phonon dispersion curves are singular for each (BOH)3O3 species, and a consistent gap decrease between the lowest and highest frequency vibrational bands was observed. The DFT-based calculations also revealed that the noncovalent interactions prevalent in the α and β crystals lead to significant differences with respect to the thermodynamic properties in comparison with the γ phase.

Copper promoter effect on acid–base and redox sites of Fe/Al2O3 catalysts and their role in ethanol–acetone mixture conversion

Active species of copper and iron oxide (Cu–Fe) catalysts supported on alumina were prepared by combining Pechini and wet impregnation methods. The effect of combined acid–base and redox sites of Cu and Fe species on gas-phase ethanol–acetone mixture conversion was investigated. The catalysts were characterized by chemical analyses, XRD, H2-TPR, Mossbauer spectroscopy, N2 physisorption, CO2-TPD, SEM-EDS, TG/DTA and pyridine adsorption isotherms. N2 adsorption/desorption isotherms and SEM-EDS analysis showed that the addition of copper caused an increase of BET surface area and Cu and Fe oxide dispersion. H2-TPR characterization showed that interactions between Cu and Fe oxides shift the reducibility of Fe3+ species to lower temperature improving the redox properties of the catalyst. The partial reduction of the Cu and Fe oxide species was found to be efficient in inhibiting the side decomposition reactions, improving the catalytic efficiency towards dehydrogenation and hydrogen transfer processes. It was found that acid–base pairs play an important role in the formation of dehydrogenation, dehydration and condensation products from ethanol, while redox sites are decisive for hydrogen transfer reactions with reduction of acetone to isopropanol. H2-TPR and Mossbauer spectroscopy results for the spent catalysts revealed that the highest catalytic performance of the Cu–FeAl catalysts may be attributed to the good dispersion of the Cu oxide and the site generated by the partial reduction which produces Cu+/Cu0 and Fe2+ active species. A reaction pathway with the participation of the acid–base and redox sites in the formation of products by consecutive dehydrogenation–condensation or dehydrogenation–hydrogenation reactions has been proposed.

Changing the gap type of solid state boric acid by heating: a dispersion-corrected density functional study of α-, β-, and γ-metaboric acid polymorphs

The metaboric acid α-, β-, and γ-polymorphs were investigated using density functional theory (DFT) calculations by employing the generalized gradient approximation improved using the Tkatchenko–Scheffler scheme to take into account dispersive interactions. The structural, electronic, and optical properties of these polymorphs were thus obtained, with unit cell deviations Δa, Δb, and Δc (in comparison with X-ray data) as small as −0.08, −0.09, and −0.06 A for α-(BOH)3O3, 0.00, 0.06, and −0.09 A for β-(BOH)3O3, and 0.01, 0.01, and 0.01 A for γ-(BOH)3O3. The layered α-MA polymorph is predicted by our simulations to have a direct gap of 6.26 eV, which is close to the values found recently for the boric acid triclinic (H3BO3-2A, 6.25 eV) and trigonal (H3BO3-3T, 6.28 eV) indirect gap systems. As the α-MA structure can be obtained by heating up the H3BO3-2A crystal above 100 °C, we have an interesting result that a change in temperature can modify the nature of the band gap of boric acid in the solid state from indirect to direct. van der Waals interactions between the metaboric acid α-(BOH)3O3 planes (distance of about 3.1 ± 0.01 A) are a relevant aspect determining its energy gap, while its direct or indirect character depends on the crystal structure and molecular components. On the other hand, the polymeric monoclinic β-(BOH)3O3 has an indirect gap of 6.56 eV, while the covalent cubic γ-(BOH)3O3 has a direct gap of 7.28 eV. Finally, the complex dielectric function and optical absorption considering polarized light along the 001, 010, 100 crystal planes and polycrystalline samples (POLY) of the three metaboric acid polymorphs were obtained.