F.B. Noronha

Characterization of palladium-copper bimetallic catalysts supported on silica and niobia

Abstract Silica and niobia Pd-Cu supported bimetallic catalysts were characterized by temperature-programmed reduction (TPR), hydrogen chemisorption and infrared spectroscopy of adsorbed carbon monoxide. On catalysts reduced at 573 K, the metal-metal interaction was confirmed from the analysis of TPR profiles and the decreased capacity of hydrogen chemisorption with increasing copper content. After reduction at 773 K, the palladium/niobia catalyst displays a SMSI effect; however, the increase in copper content in the bimetallic catalyst led to a suppression of the SMSI.

Characterization of PdCeO2 interaction on alumina support and hydrogenation of 1,3-butadiene

Abstract The influence of the preparation method and loading of CeO 2 on the interaction of Pd/CeO 2 /Al 2 O 3 catalysts was studied. The materials were characterized by temperature-programmed reduction, hydrogen chemisorption and X-ray photoelectron spectroscopy measurements. The 1,3-butadiene hydrogenation was used as a model reaction. The Pd Ce interaction affected the reduction behavior of the catalysts. Temperature-programmed reduction (TPR) results showed that the presence of CeO 2 shifts the reduction temperature of PdO to lower values, while palladium similarly facilitates the reduction of the cerium surface species. The hydrogen uptake decreased when increasing the ceria content. XPS results showed that the Pd/Al ratio decreased with CeO 2 addition being affected by the preparation method and content. The formation of catalytic sites at the interface of Pd Ce was postulated from turnover frequency (TOF) results of the 1,3-butadiene hydrogenation. The number of these new sites increases with the addition of ceria. The nature of these sites is discussed in terms of the influence on the selectivity in butadiene hydrogenation.

NO reduction with ethanol on Pd-Mo/Al2O3 catalysts

Abstract The NO reduction with ethanol was studied on Pd/Al 2 O 3 and Pd–Mo/Al 2 O 3 catalysts. The Pd/Al 2 O 3 catalyst was more active for NO conversion, although the selectivity for N 2 formation was the same on both catalysts. IR and TPD analysis of adsorbed ethanol showed formation of stable acetate species upon dehydrogenation of ethanol to acetaldehyde at reaction temperature. TPSR measurements suggested that NO preferentially adsorbs on Pd while ethanol selectively adsorbs on alumina and/or partially reduced molybdenum oxide (MoO x ). The catalytic results further suggested that the reaction mechanism was the same on both catalysts and that the reaction occurred between adsorbed NO and acetate species at the metal/oxide interface. A reaction mechanism was proposed where the limiting step was the dehydrogenation of ethanol to acetaldehyde.

A study of the promoting effect of noble metal addition on niobia and niobia alumina catalysts

The catalytic activity of Nb 2 O 5 and Nb 2 O 5 /Al 2 O 3 -supported metal catalysts was evaluated in the n-heptane conversion, CO hydrogenation and butadiene hydrogenation. After high temperature of reduction (HTR), the metal adsorption capacity decreases on all the samples, due to the reduction of Nb 2 O 5 with subsequent blocking of metal atoms and bimetallic effect. It was also observed that the activity decay caused by metal-support interaction was remarkably inhibited on the bimetallics with respect to the monometallics by comparing reaction rates after HTR. Thus, the addition of Rh to Co, Cu to Pd and Sn to Pt on niobia catalysts significantly altered the product distribution in Fischer-Tropsch synthesis (FTS) and in the hydrogenation and dehydrogenation of hydrocarbons, respectively. In addition, an unusual bifunctional effect was obtained in Pt/Nb 2 O 5 /Al 2 O 3 catalyst.

Characterization of Pt-Sn bimetallic catalysts supported on alumina and niobia

Abstract Alumina and niobia Pt-Sn supported bimetallic catalysts were characterized by TPR, hydrogen chemisorption and cyclohexane dehydrogenation. The TPR profiles of niobia supported Pt-Sn catalysts showed the presence of different precursors from the ones obtained on alumina. Furthermore, the hydrogen uptakes indicated that the amount of metallic tin formed in the niobia supported catalysts was higher than in the alumina supported catalysts. After reduction at 773 K, the platinum/niobia catalyst displayed a strong metal-support interaction (SMSI) effect, with the creation of new interfacial active sites. The addition of a small tin content led to a suppression of the SMSI. Increasing the amount of tin, however, induced a marked poisoning of the platinum.

SMSI effect in the butadiene hydrogenation on Pd-Cu bimetallic catalysts

Abstract Niobla Pd-Cu supported bimetallic catalysts have been studied In the 1,3-butadlene hydrogenation. On catalysts reduced at 573K, the addition of copper to palladium decreased the hydrogen adsorption capacity and the turnover frequency but Increased the trans/cis 2-butene ratio. These results are ascribed to a bimetallic formation. After reduction at 773K. the hydrogen chemlsorptlon and turnover frequency are drastically reduced due to SMSI effect. The 1,3-butadlene hydrogenation seems to be a structure sensitive reaction.

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.

The nature of metal oxide on adsorptive and catalytic properties of Pd/MeOx/Al2O3 catalysts

Abstract The role of ceria, niobium and molybdenum oxides on the promotion of the NO reduction by CO was studied. A bifunctional mechanism was discussed as a function of both the nature of interaction between metal oxide and palladium and the redox properties of each metal oxide. The NO dissociation was better on the Pd/MoO 3 /Al 2 O 3 catalyst than on the Pd/CeO 2 /Al 2 O 3 and Pd/Nb 2 O 5 /Al 2 O 3 catalysts. The explanation for the very high N 2 production on Pd–Mo catalyst during the TPD analysis may be attributed to the NO+Me δ + stoichiometric reaction. The promoting effect of a reducible oxide for the NO+CO reaction at low temperature can be ascribed mainly to its easiness for a redox interchange and its interaction with the noble metal particles. This would increase the surface redox ability and favor the dynamic equilibrium needed for high N 2 selectivity.

Oxidation and reduction effects of propane-oxygen on Pd-chlorine/alumina catalysts

Pd–chloride precursor salt was used to prepare Pd/Al2O3 catalysts. TPSR measurements showed three distinct reactions for the oxidation of propane on palladium surface under excess of hydrocarbon: complete oxidation, steam reforming and propane hydrogenolysis. Propane oxidation on palladium catalysts was related to the Pd2+ sites observed on Pd/Al2O3 through infrared of adsorbed carbon monoxide. In fresh catalysts reduced by H2, the IR spectra showed the linear and bridge adsorbed CO species on the Pd0 surface. After propane reaction, a new band at 2130 cm-1 related to CO adsorption on Pd2+ species was noted. Carbon monoxide species adsorbed on Pd0 were also observed in all samples after reaction. Our results suggest surface ratios of Pd0/PdO during the propane oxidation. On the other hand, time on stream conversions of the complete oxidation of propane were affected by either the water generated during the reaction or added as a reactant at 10 vol%. The water generated by the reaction helped to eliminate chlorine residues in the form of oxychloride species leading to an increasing of the activity. However, the presence of water into the reaction mixture caused a strong decreasing of the activity. The inhibition mechanism of propane oxidation in the presence of water consisted in the dissociative adsorption of water on palladium sites with the possible formation of palladium hydroxide (Pd–OH) at the surface, diminishing the number of active surface sites. Dynamic fluctuations into the reaction conditions supported the idea that a pseudo‐equilibrium adsorption–desorption of water was reached. After water removal or increasing in the reaction temperature the equilibrium was shifted to the direction of OH–Pd decomposition. This behavior suggests that the inhibitory effect of water is a reversible phenomenon, being a function of the amount of water and the reaction temperature.

NO reduction with ethanol on MoO3/Al2O3 and CeO2-ZrO2-supported Pd catalysts

Abstract The reduction of NO with ethanol on MoO 3 /Al 2 O 3 and CeO 2 -ZrO 2 -supported Pd catalysts was studied. The Pd/CeO 2 -ZrO 2 sample showed higher activity for the conversion of NO and higher selectivity for N 2 formation when compared to the Pd-MoO 3 /Al 2 O 3 sample. The CO chemisorption results showed that the CeZr mixed oxide chemisorbed a higher amount of CO, and this is related to a higher reducibility capacity of this compound when compared to MoO 3 . Furthermore, TPD analysis of adsorbed NO and ethanol showed that the Pd/CeO 2 -ZrO 2 catalyst has a higher ability of dissociating NO to N 2 and of decomposition of ethanol with the formation of CO 2 . Also, TPSR experiments showed that on Pd-8Mo/Al 2 O 3 , ethanol competes with NO for adsorption sites on partially reduced molybdenum oxide. This was not the case for the Pd/Ce 0.75 Zr 0.25 O 2 catalyst, where both NO and ethanol adsorb and decompose on the partially reduced mixed oxide surface. These facts are probably related to the better performance of this catalyst in relation to the Pd-MoO 3 /Al 2 O 3 catalyst for the NO + ethanol reaction.