Promoting Effect of Mo on Rh/Al2O3 Catalysts for CO Oxidation
Victor Teixeira da Silva
Departamento de Engenharia Química, IME, Rio de Janeiro, Brazil
and
M. A. S. Baldanza, R. L. Martins and M. Schmal
NUCAT / PEQ / COPPE/ UFRJ, Rio de Janeiro, Brazil
ABSTRACT
The effect of the variation of molybdenum content in 1% Rh x% Mo / Al2O3 catalysts (x = 2, 8, 14, 20 wt. %) was studied using the CO oxidation as probe reaction. The catalysts were characterized by Diffuse Reflectance Spectroscopy (DRS), Temperature-Programmed Reduction (TPR), Temperature-Programmed Desorption (TPD), elemental analysis, and Infrared (IR) of adsorbed CO. For the different reaction conditions studied, the obtained results show that the 1%Rh2%Mo/Al2O3 catalyst presented a superior performance than 1%Rh/Al2O3 and the others containing molybdenum. This enhancement of the noble metal activity can be attributed to a molybdenum promoter effect which in low levels avoids the noble metal sintering during catalyst activation. On the other hand, IR results show that when the CO temperature desorption is increased there is hydrocarbon formation over the catalyst surface, a phenomena that is not observed in Pd, Pt, Pd-Mo, and Pt-Mo alumina supported catalysts.
INTRODUCTION
The traditional three-way catalysts are constituted by Pd, Pt and Rh finely dispersed over ceramic monoliths. While Pd and Pt are used for hydrocarbons and CO oxidation, Rh is active not only for those reactions but also for the NOx selective reduction to N2 avoiding NH3 formation (1,2).
Several studies in the literature (3) have shown that when molybdenum is incorporated to a PdRh/Al2O3 not only its performance in exhaust reactions is enhanced, but also its selectivity to N2 formation. However, there are few fundamental studies dealing with the RhMo/Al2O3 system.
The main objective of this work was to evaluate the influence of the variation of the molybdenum content in a 1%Rh/Al2O3 catalyst on the CO oxidation reaction.
EXPERIMENTAL
Details of catalyst preparation, characterization and catalytic evaluation can be found elsewhere (4). Briefly, the catalytic evaluation experiments were carried out in a differential reactor, being the reactants and products quantified by online gas chromatography. Reducing (CO/O2 molar ratio = 4), stoichiometric (CO/O2 =2) and oxidizing (CO/O2=1) reaction conditions were used.
RESULTS AND DISCUSSION
DRS spectra of a 2Mo/Al2O3 sample show two adsorption maxima situated at 248 and 285 nm which can be related to Mo+6 in tetrahedral and octahedral coordination (polymolybdates or three-dimensional MoO3), respectively. Increasing of the molybdenum content from 2 to 8 wt.% causes the band at 248 nm to disappear and a displacement of the absorption band related to Mo+6 in octahedral coordination from 285 to 288 nm. Further increases in Mo content to 14 and 20 wt.% cause absorption maximum to shift towards higher wavenumbers, 289 and 291 nm, respectively.
Catalytic results show that catalysts containing 8, 14, and 20 wt.% do not enhance noble metal performance. On the other hand, the Rh2Mo/Al2O3 catalyst presented superior performance than that of Rh/Al2O3 catalyst. This superiority in performance can be attributted either to i) formation of a highly active Rh-Mo species or ii) a molybdenum promoting effect.
In view of DRS results which show no other features than those related to Rh or Mo species, hypothesis i) can be discarted. As a matter of fact, results from the literature (5) using XPS and TEM show that after reduction of RhMo catalyst there is no formation of any Rh-Mo species. In this way, the promoting effect of molybdenum can be attributed to a noble metal dispersion increase. In fact, when in low content and below the monolayer, molybdenum in tetrahedral coordination has a strong interaction with the support and forms barriers which isolate rhodium particles avoiding its sintering during reduction (5).
IR spectra of adsorbed CO show the characteristic features of gem-dicarbonyl for all catalysts and the higher the molybdenum content, the lower the gem-dicarbonyl intensity. In RhMo/Al2O3 catalysts there is, depending on the reducing temperature a absorption band associated to Mo+5. When samples are evacuated in increasing temperatures, there is an intensity decrease of the gem-dicarbonyl absorption bands. Interestingly, simultaneously to this decrease there is the appearing of adsorption bands related to Csp3 bands in C-H bonds. Since no H2 was introduced in the system during evacuation, the C-H bond formation can only be explained if during reduction there is formation of a compound of the type HxMoyOz which loses H2 during heat treatment in vacuum forming adsorbed hydrocarbons on the catalyst surface. The hypothesis of bronze formation is conformed by the association of TPR and TPD data.
CONCLUSIONS
During reduction of RhMo/Al2O3 catalysts there is, due to H2 spillover, formation of a HxMoyOz species.
The promoting effect of Mo on Rh/Al2O3 catalysts is observed only for low Mo loadings and can be explained by the blocking of the Rh particles which avoids noble metal sintering during reduction.
REFERENCES