Hudson W. P. Carvalho

Karlsruhe Institute of Technology, Carlsruhe, Baden-Württemberg, Germany

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Publications (32)100.58 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: A series of ceria-supported Rh and Pt catalysts was synthesized by flame spray pyrolysis for high temperature water gas shift (WGS) reactions in a membrane reactor. The structural and catalytic properties were compared with a Pt/CeO2 catalyst prepared conventionally by incipient wetness impregnation. BET results show that compared to conventionally prepared catalysts the specific surface area of the flame made catalysts is larger and decreases with increasing metal content. This is in line with X-ray diffraction (XRD) results uncovering a high nanocrystallinity of the ceria, whose crystallite size slightly increases with higher noble metal loading. Both Pt and Rh particles are not detected by XRD due to the small crystallite size. In fact, transmission electron microscopy (TEM) images of the flame made catalysts with high loading showed Pt and Rh particles with diameters of ca. 3 nm which remained stable after catalytic testing. X-ray absorption near edge spectra (XANES) at the Rh K- and Pt L3-edge showed that both Pt and Rh were in an oxidized state after preparation. In situ XANES during temperature programmed reduction in 5% H2/He indicated a lower reduction temperature than for the conventional Pt/CeO2 catalyst. Hence, Rh and Pt species are located on the surface of the CeO2 particles. Extended X-ray absorption fine structure (EXAFS) spectra provide further evidence for oxidized Rh, Pt before reduction and full reduction after activation in hydrogen, and small particle sizes in agreement with the TEM and XRD results. The catalytic activity measurements were performed at atmospheric pressure in two different gas mixtures by measuring CO conversion as a function of temperature. In all cases the Pt/CeO2 catalysts prepared by flame spray pyrolysis showed high catalytic activity and much higher selectivity than the Rh/CeO2 catalyst, where methane formation is observed above 300 °C. Furthermore, the reaction rate in terms of turnover frequencies is much higher on the Pt-based catalysts, which makes them promising candidates for high temperature WGS catalysts in membrane and microreactors.
  • Applied Catalysis B Environmental 01/2015; 162:611. DOI:10.1016/j.apcatb.2014.07.030 · 6.01 Impact Factor
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    ABSTRACT: In this work, we present a detailed study of the formation of supported intermetallic Ni-Ga catalysts for CO2 hydrogenation to methanol. The bimetallic phase is formed during a temperature-programmed reduction of the metal nitrates. By utilizing a combination of characterization techniques, in particular in situ and ex situ X-ray diffraction, in situ X-ray absorption spectroscopy, transmission electron microscopy combined with electron energy loss spectroscopy and X-ray fluorescence, we have studied the formation of intermetallic Ni-Ga catalysts of two compositions: NiGa and Ni5Ga3. These methods demonstrate that the catalysts with the desired intermetallic phase and composition are formed upon reduction in hydrogen and enable us to propose a mechanism of the Ni-Ga nanoparticles formation. By studying the effect of calcination prior to catalyst reduction, we show that the reactivity depends on particle size, which suggests that the reaction is structure sensitive.
    Journal of Catalysis 12/2014; 320. DOI:10.1016/j.jcat.2014.09.025 · 6.07 Impact Factor
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    ABSTRACT: The effect of Ni catalysts promoted with Mg on catalytic activity and carbon deposition was investigated in acetic acid steam reforming. Acetic acid was chosen as representative compound for the steam reforming of bio-oil derived from biomass pyrolysis. In this study, nickel catalysts modified with Mg have been prepared by sequential impregnation method with 15 wt.% Ni and variable loadings of Mg (1-10 wt.%). The samples were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), high resolution transmission electron microscopy (HRTEM), specific surface area (BET) and temperature programmed reduction (TPR-H-2). Steam reforming of acetic acid was conducted in a fixed bed reactor at a temperature of 500 degrees C and 600 degrees C. Under reactive conditions, the 15Ni5Mg/Al catalyst proved to be superior to the other catalysts at 600 degrees C at which it presented an effluent gaseous mixture with the highest H-2 selectivity and reasonable low coke formation.
    Applied Catalysis B Environmental 11/2014; s 160–161:188–199. DOI:10.1016/j.apcatb.2014.05.024 · 6.01 Impact Factor
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    ABSTRACT: Hydrodeoxygenation of guaiacol over Pt-based catalysts at mild conditions (< 200 °C).•Pt supported on SiO2 more active than catalysts based on Al2O3, TiO2, ZrO2, CeO2.•Hydrogenation to methoxycyclohexanol and its further deoxygenation observed under mild conditions.•Improved efficiency towards cyclohexane for a bifunctional Pt/zeolite catalyst.•Under mild conditions preferential pathway is hydrogenation followed by acid-catalysed dehydration.
    Applied Catalysis A General 10/2014; DOI:10.1016/j.apcata.2014.10.043 · 3.67 Impact Factor
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    ABSTRACT: An in-depth understanding of the active site requires advanced operando techniques and the preparation of defined catalysts. We elucidate here the mechanism of the selective catalytic reduction of NO by NH3 (NH3-SCR) over a Fe-ZSM-5 zeolite catalyst. 1.3 wt% Fe-ZSM-5 with low nuclearity Fe sites was synthesized, tested in the SCR reaction and characterized by UV-Vis, X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopy. Next, this defined Fe-zeolite catalyst was studied by complementary high-energy-resolution fluorescence-detected XANES (HERFD-XANES) and valence-to-core X-ray emission spectroscopy (V2C XES) under different model in situ and realistic working (operando) conditions identical to the catalyst test bench including the presence of water vapor. In particular, this included the presence of water vapor which strongly influences the catalytic performance and the structure of the active centre. HERFD-XANES uncovered that the coordination (between 4 and 5), geometry (tetrahedral, partly 5-fold), and oxidation state of the Fe centres (reduced in NH3, partly in SCR mixture, slight reduction in NO) strongly changed. V2C XES supported by DFT calculations provided important insight into the chemical nature of the species adsorbed on Fe sites. The unique combination of techniques applied under realistic reaction conditions and the corresponding catalytic data unraveled the adsorption of ammonia via oxygen on the iron site. The derived reaction model supports a mechanism where adsorbed NOx reacts with ammonia coordinated to the Fe3+ site yielding Fe(2+) which reoxidation is slow.
    Journal of the American Chemical Society 08/2014; 136(37). DOI:10.1021/ja5062505 · 11.44 Impact Factor
  • Microscopy and Microanalysis 08/2014; 20(S3):458-459. DOI:10.1017/S1431927614004012 · 1.76 Impact Factor
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    ABSTRACT: Maghemite (γ-Fe2O3)-poly(methyl methacrylate) (PMMA) nanocomposites were prepared by grafting 3-(trimethoxy-silyl) propyl methacrylate on the surface of maghemite nanoparticles, this process being followed by methyl methacrylate radical polymerization. Three different hybrids with 0.1, 0.5, and 2.5 wt% of maghemite nanoparticles were studied. The results indicate that these nanocomposites consist of a homogeneous PMMA matrix in which maghemite nanoparticles with a bimodal size distribution are embedded. The existence of covalent bonding between silane monomers and atoms on the maghemite surface was evidenced. AFM images showed a clear increase in surface roughness for increasing maghemite content. The thermal stability of PMMA-maghemite nanocomposites is higher than that of pure PMMA and increases for increasing maghemite content. The results of our theoretical studies indicate that the electron density in the maghemite nanoparticle is not homogenous, the low electron density volumes being supposed to be radical trappers during PMMA decomposition, thus acting as a thermal stabilizer. POLYM. COMPOS., 2014. © 2014 Society of Plastics Engineers
    Polymer Composites 07/2014; DOI:10.1002/pc.23154 · 1.48 Impact Factor
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    ABSTRACT: The long term stability and resistance toward carbon deposition, sulfur, chlorine, and potassium of Ni/ZrO2 as a catalyst for the hydrodeoxygenation (HDO) of guaiacol in 1-octanol (as a model compound system for bio-oil) has been investigated at 250 [degree]C and 100 bar in a trickle bed reactor setup. Without impurities in the feed good stability of the Ni/ZrO2 catalyst could be achieved over more than 100 h of operation, particularly for a sample prepared with small Ni particles, which minimized carbon deposition. Exposing the catalyst to 0.05 wt% sulfur in the feed resulted in rapid deactivation with complete loss of activity due to the formation of nickel sulfide. Exposing Ni/ZrO2 to chlorine-containing compounds (at a concentration of 0.05 wt% Cl) on-stream led to a steady decrease in activity over 40 h of exposure. Removal of the chlorine species from the feed led to the regaining of activity. Analysis of the spent catalyst revealed that the adsorption of chlorine on the catalyst was completely r
    Catalysis Science & Technology 06/2014; DOI:10.1039/C4CY00522H · 4.76 Impact Factor
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    ABSTRACT: PMMA-polysilsesquioxane (PMMA-Sil) class II organic-inorganic hybrids were prepared by the sol-gel method from a PMMA-based polymer precursor containing trimethoxysilane groups. An analysis was made of the effect of siloxane content, adjusted by addition of tetraethyl orthosilicate (TEOS), on the structure and thermal stability of the dried gels. 13C nuclear magnetic resonance measurements confirmed PMMA as the organic phase, while 29Si measurements revealed the presence of both T and Q silicon species, the most abundant being T2 and Q3. X-ray diffraction results showed that the inorganic SiO2 phase was amorphous, while small angle X-ray scattering analyses indicated that the average gyration radius size of the silicate particles and the correlation distance between the particles increased with greater addition of TEOS. Thermal stability was improved by increasing the amount of the inorganic phase. This effect was more evident under an air atmosphere than under N2, indicating that the silicate phase acted to limit oxygen diffusion.
    Polymer Degradation and Stability 06/2014; 104. DOI:10.1016/j.polymdegradstab.2014.03.031 · 2.63 Impact Factor
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    ABSTRACT: Herein, we present a cross-linked ureasil-polyether-siloxane hybrid (labeled PEO500) that can function as a stimuli-sensitive material-it swells or shrinks in response to changes in the environmental conditions and it can also, effectively and selectively, remove dyes from water solution. We also developed a methodology to separate a mixture of cationic and anionic dyes present in water. Addition of PEO500 to an aqueous solution of the anionic orange II (OII) or the Ponceau S (PS) dye rendered the solution colorless, but an aqueous solution of cationic Methylene Blue (MB) remained unchanged after 2h of contact with the insoluble matrix. In situ small-angle X-ray scattering (SAXS) showed that the distance of siloxane nanodomains are strongly affected by the swelling or shriking. By in situ UV-vis adsorption experiments we found that the kinetics of OII and PS removal followed a pseudo-first-order rate equation. We accomplished B3LYP calculations, to establish which sites on the matrix interacted with the dyes and to investigate the nature of the matrix-dye chemical bonds. On the basis of the experimental and theoretical investigations, we proposed some mechanisms to explain how PEO500 adsorbs anionic dyes efficiently. This "smart" matrix is potentially applicable as an efficient, fast, selective, and convenient device in water treatment and stimuli-sensitive response materials.
    Langmuir 03/2014; 30(13):3857. DOI:10.1021/la404812e · 4.38 Impact Factor
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    ABSTRACT: Poly(methylmetacrilate)-maghemite (PMMA-gamma-Fe2O3) hybrid material was studied by the electron stimulated ion desorption (ESID) techniques coupled with time-of-flight mass spectrometry (TOF-MS) and theoretical investigation about its fragmentation. Moreover, atomic force microscopy was utilized to characterize the morphology before and after ionic desorption. ESID results indicated differences of pattern fragmentation for different compositions of hybrid material in comparison with neat PMMA. Theoretical studies suggest that kinetics effects can take place in the fragmentation process and electrostatic contributions were important in the stabilization of PMMA on maghemite after the grafting process.
    Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 08/2013; 117C:276-283. DOI:10.1016/j.saa.2013.08.029 · 2.13 Impact Factor
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    ABSTRACT: The XAS/WAXS time-resolved method was applied for unraveling the complex mechanisms arising from the evolution of several metastable intermediates during the degradation of chlorine layered double hydroxide (LDH) upon heating to 450 °C, i.e., Zn2Al(OH)6·nH2O, ZnCuAl(OH)6·nH2O, Zn2Al0.75Fe0.25(OH)6·nH2O, and ZnCuAl0.5Fe0.5(OH)6·nH2O. After a contraction of the interlamellar distance, attributed to the loss of intracrystalline water molecules, this distance experiences an expansion (T > 175–225 °C) before the breakdown of the lamellar framework around 275–295 °C. Amorphous prenucleus clusters with crystallo-chemical local order of zinc-based oxide and zinc-based spinel phases, and if any of copper-based oxide, are formed at T > 175–225 °C well before the loss of stacking of LDH layers. This distance expansion has been ascribed to the migration of ZnII from octahedral layers to tetrahedral sites in the interlayer space, nucleating the nano-ZnO or nano-ZnM2O4 (M = Al or Fe) amorphous prenuclei. The transformation of these nano-ZnO clusters toward ZnO crystallites proceeds through an agglomeration process occurring before the complete loss of layer stacking for Zn2Al(OH)6·nH2O and Zn2Al0.75Fe0.25(OH)6·nH2O. For ZnCuAl(OH)6·nH2O and ZnCuAl0.5Fe0.5(OH)6·nH2O, a cooperative effect between the formation of nano-CuO and nano-ZnAl2O4 amorphous clusters facilitates the topochemical transformation of LDH to spinel due to the contribution of octahedral CuII vacancy to ZnII diffusion.
    Chemistry of Materials 07/2013; 25(14):2855–2867. DOI:10.1021/cm401352t · 8.54 Impact Factor
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    ABSTRACT: In this study, the short- and long-range chemical environments of Cu dopant in TiO2 photocatalyst have been investigated. The Cu-doped and undoped TiO2 specimens were prepared by the sol-gel approach employing CuSO4 center dot 5H(2)O and Ti(O-iPr)(4) precursors and subjecting the dried gels to thermal treatment at 400 and 500 A degrees C. The photocatalytic activity, investigated by methylene blue degradation under sunlight irradiation, showed a significantly higher efficiency of Cu-doped samples than that of pure TiO2. The X-ray diffraction results showed the presence of anatase phase for samples prepared at 400 and 500 A degrees C. No crystalline CuSO4 phase was detected below 500 A degrees C. It was also found that doping decreases the crystallite size in the (004) and (101) directions. Infrared spectroscopy results indicated that the chemical environment of sulfate changes as a function of thermal treatment, and UV-vis spectra showed that the band gap decreases with thermal treatment and Cu doping, showing the lowest value for the 400 A degrees C sample. X-ray absorption fine structure measurements and analysis refinements revealed that even after thermal treatment and photocatalytic assays, the Cu2+ local order is similar to that of CuSO4, containing, however, oxygen vacancies. X-ray photoelectron spectroscopy data, limited to the near surface region of the catalyst, evidenced, besides CuSO4, the presence of Cu1+ and CuO phases, indicating the active role of Cu in the TiO2 lattice.
    Journal of Materials Science 06/2013; 48(11):3904. DOI:10.1007/s10853-013-7192-1 · 2.31 Impact Factor
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    ABSTRACT: The structure and the thermodegradation behavior of both poly(methyl methacrylate)-co-poly(3-tri(methoxysilyil)propyl methacrylate) polymer modified with silyl groups and of intercalated poly(methyl methacrylate)-co-poly(3-tri(methoxysilyil)propyl methacrylate)/Cloisite 15A™ nanocomposite have been in situ probed. The structural feature were comparatively studied by Fourier transform infrared spectroscopy (FTIR), 13C and 29Si nuclear magnetic resonance (NMR), and small angle X-ray scattering (SAXS) measurements. The intercalation of polymer in the interlayer galleries was evidenced by the increment of the basal distance from 31 to 45 Å. The variation of this interlayer distance as function of temperature was followed by in situ SAXS. Pristine polymer decomposition pathway depends on the atmosphere, presenting two steps under air and three under N2. The nanocomposites are more stable than polymer, and this thermal improvement is proportional to the clay loading. The experimental results indicate that clay nanoparticles play several different roles in polymer stabilization, among them, diffusion barrier, charring, and suppression of degradation steps by chemical reactions between polymer and clay. Charring is atmosphere dependent, occurring more pronounced under air. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers
    Polymer Engineering and Science 06/2013; 53(6). DOI:10.1002/pen.23364 · 1.24 Impact Factor
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    ABSTRACT: In this study, the short-and long-range chem-ical environments of Cu dopant in TiO 2 photocatalyst have been investigated. The Cu-doped and undoped TiO 2 spec-imens were prepared by the sol–gel approach employing CuSO 4 Á5H 2 O and Ti(O-iPr) 4 precursors and subjecting the dried gels to thermal treatment at 400 and 500 °C. The photocatalytic activity, investigated by methylene blue degradation under sunlight irradiation, showed a signifi-cantly higher efficiency of Cu-doped samples than that of pure TiO 2 . The X-ray diffraction results showed the pres-ence of anatase phase for samples prepared at 400 and 500 °C. No crystalline CuSO 4 phase was detected below 500 °C. It was also found that doping decreases the crys-tallite size in the (004) and (101) directions. Infrared spectroscopy results indicated that the chemical environ-ment of sulfate changes as a function of thermal treatment, and UV–vis spectra showed that the band gap decreases with thermal treatment and Cu doping, showing the lowest value for the 400 °C sample. X-ray absorption fine struc-ture measurements and analysis refinements revealed that even after thermal treatment and photocatalytic assays, the Cu 2? local order is similar to that of CuSO 4 , containing, however, oxygen vacancies. X-ray photoelectron spec-troscopy data, limited to the near surface region of the catalyst, evidenced, besides CuSO 4 , the presence of Cu 1? and CuO phases, indicating the active role of Cu in the TiO 2 lattice.
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    ABSTRACT: We report on a novel transparent, flexible, rubbery, and insoluble tri-ureasil organic–inorganic hybrid material with multifunctional characteristics and potential application in drug delivery, water purification, and photochromic materials. We obtained the tri-ureasil gels by the one-pot sol–gel route using 3-isocyanatopropyltriethoxysilane and glyceryl poly(oxypropylene)triamine of molecular weight 5000 g mol−1 (Jeffamine® T5000). This approach generated a silica backbone covalently connected with poly(oxyalkylene) chains PPO(OCH–CH3CH2)n through urea bridges. We characterized the obtained materials by DSC, swelling tests, XRD, 29Si NMR, and small-angle X-ray scattering (SAXS). The results attested that the tri-ureasil hybrids are potentially applicable as photochromic devices, drug delivery systems, and adsorbents of pollutants from contaminated waters.
    02/2013; 4(5):1575-1582. DOI:10.1039/C2PY21049E
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    ABSTRACT: Mn-incorporated Fe3O4 photocatalysts were prepared by a simple co-precipitation method. Photocatalytic discoloration of Methylene Blue (MB) was used to evaluate the performance of these catalysts. The DSC results have shown that the insertion of Mn into Fe3O4 lattice has increased converting Fe3O4 to γ-Fe2O3. This is accompanied by a decrease of surface area and of crystallinity, as detected by XRD. The analysis of the chemical environment by XPS has shown that Mn2+ replaces Fe2+ preferentially in the octahedral sites while Mn3+ replaces Fe3+ of inverse spinel sites. The Mn-incorporated samples were significantly more efficient in MB discoloration assisted by UVA irradiation and H2O2. It was also found that ascorbic acid prevents H2O2 decomposition, by scavenging preferentially OOH radicals produced at Mn sites. Finally, the results reported here can contribute for a better comprehension of the activity of composite catalysts and the design of efficient systems for discoloration of organic pollutants.
    Materials Science and Engineering B 01/2013; 181. DOI:10.1016/j.mseb.2013.11.008 · 2.12 Impact Factor
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    ABSTRACT: The radical trapping flame retardant mechanism of polymer-clay nanocomposites is frequently claimed to be responsible for improving the thermal stability of polymers. However it had never been demonstrated. Herein using in situ time-resolved X-ray Absorption Spectroscopy (XAS) we present experimental evidence that Fe3+ embedded in the clay can act as electron acceptors during polymer thermal decomposition. Therefore it contributes to improve the polymer thermal stability.
    RSC Advances 01/2013; 3(45):22830. DOI:10.1039/c3ra44388d · 3.71 Impact Factor
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    ABSTRACT: In this work the effect of doping concentration and depth profile of Cu atoms on the photocatalytic and surface properties of TiO(2) films were studied. TiO(2) films of about 200 nm thickness were deposited on glass substrates on which a thin Cu layer (5 nm) was deposited. The films were annealed during 1s to 100°C and 400°C, followed by chemical etching of the Cu film. The grazing incidence X-ray fluorescence measurements showed a thermal induced migration of Cu atoms to depths between 7 and 31 nm. The X-ray photoelectron spectroscopy analysis detected the presence of TiO(2), Cu(2)O and Cu(0) phases and an increasing Cu content with the annealing temperature. The change of the surface properties was monitored by the increasing red-shift and absorption of the ultraviolet-visible spectra. Contact angle measurements revealed the formation of a highly hydrophilic surface for the film having a medium Cu concentration. For this sample photocatalytic assays, performed by methylene blue discoloration, show the highest activity. The proposed mechanism of the catalytic effect, taking place on Ti/Cu sites, is supported by results obtained by theoretical calculations.
    Journal of hazardous materials 12/2010; 184(1-3):273-80. DOI:10.1016/j.jhazmat.2010.08.033 · 4.33 Impact Factor

Publication Stats

164 Citations
100.58 Total Impact Points


  • 2013–2015
    • Karlsruhe Institute of Technology
      • Institute of Technical and Polymer Chemistry
      Carlsruhe, Baden-Württemberg, Germany
    • Federal University of Minas Gerais
      Cidade de Minas, Minas Gerais, Brazil
    • SOLEIL synchrotron
      Gif, Île-de-France, France
  • 2009–2013
    • São Paulo State University
      • Institute of Chemistry
      San Paulo, São Paulo, Brazil
  • 2007–2010
    • Universidade Federal de Lavras (UFLA)
      • Departamento de Química
      Lavras, Minas Gerais, Brazil