Carlos A.C. Perez

Stability and selectivity of bimetallic Cu-Co/SiO2 catalysts for cyclohexanol dehydrogenation

Abstract Bimetallic Cu–Co/SiO 2 catalysts with different metal loading (5:5, 15:15, 35:35 – Cu:Co) were prepared by deposition–precipitation method and evaluated by using the cyclohexanol dehydrogenation. XRD results show that only the 15%Cu–15%Co/SiO 2 oxide precursor exhibits mainly a phase containing Cu and Co with spinel-like structure, while other bimetallic precursors present CuO and Co 3 O 4 as distinct phases. The Rietveld method was used to quantify the different phases on the precursors and to determinate how much Cu there was in the spinel form and as CuO. After reduction and passivation, the 15%Cu–15%Co/SiO 2 catalyst showed a Cu–Co alloy formation and a different H 2 chemisorption capacity. The H 2 uptake was 2.5 greater than 35%Cu–35%Co/SiO 2 catalyst and the same order of magnitude for the monometallic 35%Co/SiO 2 . Catalytic results showed that the 15%Cu–15%Co/SiO 2 catalyst presented the highest stability and selectivity to cyclohexanone, with the lowest phenol production.

Quantitative XPS analysis of silica-supported Cu–Co oxides

Abstract Copper–cobalt oxides with Cu/Co=5:5, 15:15 and 35:35 bulk ratio have been prepared by deposition–precipitation method at constant pH from copper and cobalt nitrate solutions. Different oxides were obtained by decomposition of the precursors at 673 K for 7 h in air and analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). XRD data showed the formation of different oxide phases; for the bulk atomic ratio of 15Cu:15Co, a phase containing Cu and Co with spinel-like structure was observed, while the other bimetallic oxides presented CuO and Co3O4 as distinct phases. The XPS qualitative analysis has shown that all samples exhibited Cu2+ and Co3+ species at the surface. The Cu–Co spinel presented a displacement in Cu 2p binding energy value. A mathematical model was proposed from relative intensity ratios, which allowed the determination of the oxide particle thickness and the fraction of coverage at the support. This model described accurately the system and showed that cobalt improved the copper dispersion.

The promoting effect of noble metal addition on niobia-supported cobalt catalysts

Abstract The promoting effects of a noble metal (Pd, Pt, Rh) added to Co/Nb 2 O 5 catalysts were studied by varying the Me/Co atomic ratios. Acid niobium was calcined to niobium pentoxide. The surface and bulk structures of the calcined materials were characterized by XPS and TPR techniques. The catalytic performance was obtained with CO hydrogenation. The addition of a noble metal promoted the reduction of Co 3+ and Co 2+ phases at the surface. XPS results revealed that Co 2+ species are well dispersed as a thin layer around the niobium support together with Co 3 O 4 crystallites islands. The CO 3 O 4 /Co 2+ ratio depends on the surface area of the support. XPS measurements also revealed that PdO, Rh 2 O 3 and PtO 2 are the main phases in the mono and bimetallic catalysts. The activity of the bimetallic catalysts increased and the stability was already attained. The selectivities towards C 5 + and oxygenates increased with the addition of Rh up to an atomic ratio of 0.5 and decreased beyond that. This behavior is similar for both temperatures of reduction at 573 and 773 K.

Synthesis and characterization of niobium oxide layers on silica and the interaction with nickel

Abstract A surface phase oxide containing niobiuma oxide, deposited on non-porous silica was prepared by two different methods. The photoelectron spectroscopy (XPS) and infrared results show that the method using five-stage attaching reactions between Nb(OC2O5)5 and silanol led to a very close one-atomic-layer niobium oxide structure on SiO2. This sample presented two oxygen species comparable to model systems and it was possible to distinguish them in the XPS spectral even when using a silica powder. Cyclohexane dehydrogenation was used to characterize the reduced metallic surface. The nickel compounds interact preferentially with niobia rather than with silica. These nickel species favored lower dehydrogenation activity, suggesting the choice of niobia as a nickel trap.

Structure of vanadate in calcium phosphate and vanadate apatite solid solutions

Polycrystalline solid solutions of phosphate and vanadate calcium apatites were synthesized and studied by XRD, XPS, 31P and 51V NMR, FTIR and UV spectroscopies. Homogeneous distributions of vanadium within the solid and the surface were obtained. While 51V NMR spectra suggest an axial symmetry of the vanadate group, FTIR spectra indicate distortion from essentially C3v symmetry in solid solutions with low vanadium content to Cs for pure vanadate apatite, in agreement with the site symmetry group of phosphate in hydroxyapatite. Refinements of XRD pattern of vanadate apatite by Rietveld method confirm the Cs point group attribution. Three vanadium oxygen bond lengths were found around 168 pm and one close to 174 pm, suggesting that the Cs point group could be generated by a small distortion from C3v symmetry. Analysis of UV spectra confirms that distortion from Td symmetry increases with vanadium content and suggests some contraction of the vanadate ion in the apatite lattice.

Determination of cobalt species in niobia supported catalysts

Niobia-supported cobalt catalysts were prepared by incipient wetness impregnation and were characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), temperature-programmed reductioon (TPR) and magnetic measurements. At least two different types of cobalt species were present on the calcined catalysts: Co3O4 particles and Co2+ surface species. At high cobalt content, Co3O4 particles are the main species whereas the percentage of Co2+ species linked to the support increases as the cobalt loading is decreased. XPS results revealed that the Co2+ species could be better represented by a mixture of Co2Nb5O14 and CoNb2O6. TPR analyses allowed one to quantify the percentage of Co3O4 particles on the niobia-supported Co catalyst. However, due to partial reduction of niobia, TPR alone did not permit the quantification of the reduction degree of cobalt in these catalysts. Magnetic measurements linked to a TPR technique is a possible way to measure the reduction degree of cobalt in Co/Nb2O5 catalysts. After reduction at high temperature, the mixture of cobalt niobates was reduced and NbO2 produced led to the strong metal-support interaction (SMSI) effect.

Surface characterization of zirconia-coated alumina as support for Pt particles

ZrO 2 /Al 2 O 3 samples were prepared by impregnation of the alumina powder with a solution of zirconium hydroxide in nitric acid, containing ZrO 2 in the range of 1-20 wt%. Platinum was impregnated by incipient wetness technique, using an aqueous solution of H 2 PtCl 6 . The surface coverage of zirconia on alumina, as probed by XPS and ISS, increased up to 10 wt% of ZrO 2 and above this concentration crystallites of zirconia nucleated, which decreases the BET surface area. XRD measurements indicated that the average size of these crystallites was about 1 nm. ISS data of Pt/ZrO 2 and Pt/10%ZrO 2 /Al 2 O 3 catalysts, after reduction at 773 K indicated an increase in the surface population of Zr atoms which are able to catch CO. This interaction was also probed by FTIR spectra of CO adsorbed on Pt/ZrO 2 catalyst, where an intense band at 2130 cm -1 , associated to CO adsorption on Pt-ZrO x interface, was observed.

Influence of processing parameters on structural characteristics of porous calcium phosphate samples: a study using an experimental design method

Synthetic porous ceramics can be used as three-dimensional scaffolds for bone repair. The aim of this work is to correlate process parameters with scaffolds structural characteristics. The factorial 2 level experimental design was chosen to study the effect of the Ca/P ratio (1.58 or 1.67), calcination temperature (1000 or 1150 °C) and porogen content (20 or 40%) on the calcium phosphate samples characteristics. The influence of such parameters was determined by X-ray diffraction, infrared spectroscopy and image analysis. It was observed that phase composition was basically a function of the Ca/P ratio of the raw material. The use of the porogen did not alter the hydroxyapatite (HA)/tricalcium phosphate (TCP) content, but induced changes in the relative content of TCP phase (a or b). It is possible to design a porous sample with defined characteristics, and the model herein used can be considered as having a good predictive power.

Quantitative XPS Analysis of Bimetallic Cu–Co Catalysts

The surfaces of bimetallic Cu-Co/SiO 2 catalyst with 1 Cu:2 Co atomic ratio has been studied focusing on the determination of bimetallic phase thickness and its distribution using a mathematical formalism from X-ray photoelectron spectroscopy (XPS) intensities. X-ray diffraction analysis indicated a Cu-Co alloy formation and XPS qualitative analysis showed that Cu 0 and Co 0 species were present on the surface. Atomic ratios of Cu/Si, Co/Si and Co/Cu from XPS and atomic absorption (A.A.) analyses suggested homogeneous particle formation with a slight Cu surface enrichment in the catalyst. A mathematical model was proposed from relative intensity ratios, which was able to determine the metallic particle thickness and its coverage fraction at the support. Considering that these values were comparable to those for the particle size obtained from transmission electron microscopy (TEM) measurements, it may be said that the model describes the evaluated system accurately.

Thermal Behavior of Poly(ethylene terephthalate) Crystalline and Amorphous Phases by Wide Angle X‐ray Scattering

Structural changes in the crystalline and amorphous phases of semicrystalline, compression molded, isotropic poly(ethylene terephthalate (PET) samples obtained from bottle grade resin, when it was heated from room temperature to temperatures near the crystalline melting temperature, were investigated by wide angle X‐ray scattering (WAXS). Semicrystalline PET samples were obtained by thermally treating initially amorphous samples at 120°C and 200°C for 24 h, and WAXS curves were taken in intervals of approximately 20°C up to a temperature near Tm. For each temperature, crystallographic data were obtained by using Winplotr and Celref programs to perform fittings of the WAXS curves and to calculate related parameters like unit cell dimensions, density, and volume, as well as crystallite sizes in directions, degree of crystallinity (X c), and distance between chains in the amorphous phase. The crystallite sizes in the and directions slightly increased, indicating a growth in the crystal lateral dimensions, while in the direction practically no variation was observed. The unit cell volume had a discrete increase due to thermal expansion, and its density decreased. The X c had a significant decrease, changing from 34% to 14% in the sample treated at 120°C, and from 34% to 21% in the sample treated at 200°C. The X c decrease with increasing temperature was accomplished by the increase in the intensity and symmetry of reflections and crystal rearrangement due to melting/recrystallization processes of crystals formed at lower annealing temperatures. The increase in temperature leads to variation in the two amorphous halos between 0 and 35°. While the distance between chains remains constant and the area increases for the first halo (located at approximately 17.5°), the distance between chains and the area of the second halo (which is located at about 24.5°) decreases. This second halo was associated with paracrystallinity.