D.E. Damiani y C.E. Gigola^*

Planta Piloto de Ing. Química - CC 717- 8000 Bahía Blanca - Argentina

According to present knowledge, palladium catalysts can successfully compete with those of Pt-Rh for the removal of contaminants present in exhaust gases . In addition they exhibit a high catalytic activity and stability for the combustion of light hydrocarbons. Due to these important characteristics considerable attention is given to oxidation (HCs) and reduction (NO_x) reactions on palladium and palladium promoted catalysts in order to obtain fundamental information that can be used to develop more efficient catalysts for emission control and the production of energy. Among other alternatives to modify the catalytic properties of alumina-supported palladium catalysts we have considered the addition of W, V and Mo. The research effort is focused on the development of preparation methods that lead to interaction between active species using inorganic and organometalic precursors. The laboratory preparations are mainly characterized by chemisorption of H₂ and CO, oxygen uptake measurements, TPR and FTIR spectra of adsorbed CO and NO. Monometallic and bimetallic samples are tested with several probe reactions like NO decomposition, NO reduction with CO (and HC) and CH₄ oxidation.

In order to explore the potential of Pd as a substitute for Pt-Rh preliminary studies were performed on Pd /a -Al₂O₃ catalysts [1]. At 460 ^oC, we have demonstrated that the metal is capable of removing CO, C₂H₆ and NO in the presence of oxygen if the reaction mixture is stoichiometric. At oxidizing conditions the presence of NO has a detrimental effect on hydrocarbon oxidation. On the other hand a strong reducing environment affected the elimination of CO, and the steam reforming reaction seems to be present leading to H₂ and possible NH₃ formation. When the Pd only catalyst was compared with a commercial Pt-Rh-Pd monolithic, under oxidizing and stoichiometric conditions the results were similar.

To obtain the Pd-W catalysts two procedures were used. Pd/g -Al₂O₃ samples derived from aqueous precursors were impregnated with (O₄₁W₁₂ (NH₄)₁₀).5H₂O, calcined and reduced. Another Pd-W series was obtained by the photochemical reaction of W(CO)₆ in the presence of Pd/g -Al₂O₃, with and without PPh₃ as carbonyl ligands. The calcination step was avoided to favor the presence of cations in a low oxidation state. Chemisorption and FTIR measurements confirmed the interaction between Pd and W. The catalytic activity and selectivity of these samples for the NO decomposition reaction was examined as function of time. At low temperature, 473 K, they exhibit a high initial activity followed by a deactivation period where N₂O formation was observed [1]. Moreover evolution of oxygen was not detected, which suggests that this product leads to catalyst deactivation. Increasing the reaction temperature to 673 K the catalytic activity was maintained for a longer time. However the detection of CO₂ among the reaction products indicated the presence of carbon residuals, mainly on catalysts prepared with organometallic precursors. In order to avoid these problems the addition of W via W(CO)₆ was carried out in the absence of PPh₃, and the reaction temperature was increased up to a level where steady state activity was observed. After reduction at 573K and heating in He up to 1023 K the Pd/g -Al₂O₃ and Pd-W/g -Al₂O₃ catalysts exhibit a steady state activity for NO decomposition, while WO_x/g -Al₂O₃ is inactive. Moreover the activity is only determined by the fraction of exposed Pd atoms. Upon reduction at 973 K all samples show a very high initial activity at 1023 K but deactivate to a conversion level similar to that observed on samples reduced at 573 K. In addition oxygen retention is observed. On Pd/g -Al₂O₃ this oxygen uptake was close to the fraction of exposed Pd atoms measured after reduction at 573 K. Therefore this behaviour was tentatively ascribed to a "sintering" (H₂) - "redispersion " (NO) process. On Pd-W/g - Al₂O₃ and W/ g -Al₂O₃ the oxygen uptake after reduction at 973 K was close to the amount of W, indicating that the high initial activity for NO decomposition is solely due to a change on the W oxidation state (WO_x+1 Û WO_x). To understand this complicated picture characterization of the samples prior and after reaction is in progress. Preliminary results indicate that the metal and the support surface are modified during high temperature pretreatment and both play a role in the decomposition reaction.

In a parallel study we have paid attention to the activity and selectivity of Pd/g -Al₂O₃ , VO_x/g -Al₂O₃ and Pd-VO_x/ g -Al₂O₃ catalysts for the decomposition of NO and the reduction of NO with CO. In this case samples of vanadia supported on g -Al₂O₃ were first prepared using aqueous solutions of NH₄VO₃ and subsequently Pd was introduced as Pd(AcAc)₂ ( using toluene solution). Considering the low vanadium loading, and that vanadia highly interacts with alumina samples, it is molecularly dispersed. Samples with a VO_x monolayer and half a monolayer were obtained. However a low concentration of crystalline V₂O₅ may be present.

Characterization of the binary samples by hydrogen chemisorption shows that the fraction of exposed Pd atoms decreases but the particle size is not altered. It is assumed that partially reduced vanadia decorate noble metal particles, TPR measurements indicate that the reduction of VO_x is enhanced by the presence of palladium . The TPR profiles show that the reduction of VO_x/g -Al₂O₃ begins at approximately 300 ^oC. On the other hand, the TPR profile for Pd-VO_x/g -Al₂O₃ presents a low temperature peak (293 K) assigned to the reduction of PdO and a hydrogen consumption peak at about 180 ^oC, which is assigned to a partial reduction of VO_x species. The presence of palladium lowers the vanadia reduction temperature by 300 ° C approximately. This palladium-vanadium interaction alters the catalytic properties of palladium. The initial activity of Pd/ and VO_x for NO decomposition, measured at 573 K, is high but a deactivation process similar to that observed on Pd-W is observed. On VO_x/g -Al₂O₃ N₂ is the only product. On the other hand the binary sample is active for a longer period of time. Regarding the reduction of NO with CO , VO_x/g -Al₂O₃ seems inactive towards CO while Pd/g -Al₂O₃ and Pd-VO_x/g -Al₂O₃ remain active after 2 hours of reaction time. In addition the selectivity to N₂ was always higher on the binary catalysts [3].

Regarding Pd/g -Al₂O₃ catalysts modified with Mo the main research effort was on catalyst preparation, introducing a variety of procedures and precursors. The first attempt was based on the use of Pd(NH₃)₄Cl₂ or Pd(NO₃)₂ and Mo₇O₂₄(NH₄)₆.4H₂O. The former Pd compound was coimpregnated with the molybdenum precursor to obtain one sample. In another preparation a sequential impregnation procedure was used adding Mo₇O₂₄(NH₄)₆.4H₂O first. Finally a Pd-Mo sample was prepared by impregnation of a Mo/g -Al₂O₃ with Pd(NO₃)₂. Characterization of these samples by XRD, TPR and H₂ chemisorption demonstrated the presence of Pd-Mo interaction despite the use of low metal loading.

A second series of Pd-Mo catalysts with metal contents of about 1% were prepared using organometallic precursors ; Pd(AcAc)₂ and MoO₂(AcAc)₂ . The order of addition was also a preparation variable. The samples were calcined at 773K . More recently a another Seri of catalysts containing Pd and/or Mo was obtained by the sol-gel procedure. Pure alumina, MoO₃/Al₂O₃ and Pd-MoO₃/Al₂O₃ samples were synthesized via sol-gel. The alumina was then impregnated with Pd(AcAc)₂ or Mo₇O₂₄(NH₄)₆.4H₂O to derive monometallic samples. In addition the sol-gel prepared MoO₃/Al₂O₃ was impregnated with Pd(AcAc)₂ to obtain another bimetallic catalyst. These samples are currently being characterized by XRD, H₂ chemisorption and TPR.

Catalysts from the first Seri already characterized have been tested for CH₄ combustion [4] using a stoichiometric mixture. The promotional effect of Mo on coimpregnated Pd-Mo samples prepared with Pd(NH₃)₄Cl₂ was clear. Catalytic activity was observed at 470 K , much lower than that required by the Pd sample (570 K). When the preparation was done in sequence, the molybdenum added first, chlorine has a poisoning effect as observed in other studies.

As mentioned above Pd catalysts are also attractive from the point of view of energy producing processes that burn light hydrocarbons with a flameless operation. Our interest is in process fluid heaters. If the fuel/air mixture is available at room temperature, it should be preheated up to the temperature where the catalytic reaction begins. For Pd/ g-Al₂O₃ catalysts the light-off temperature for CH₄ is about 573 K depending on the design and operating parameters . To avoid the use of a preburner an autothermal operation where the feed stream is preheated by a fraction of the total amount of heat generated in the catalytic combustor is being considered. However the heat feedback, inherent to all autothermal processes, is a source of reactor instability. Consequently we are also interested in the development of promoted Pd catalysts with lower light-off temperatures .

References:

[1] A. Pisanu, Ph. D. Thesis, UNS, 1997.

[2] A. M. Sica, J.H.Z. Dos Santos , I. M. Baibich and C. E. Gigola, J. Mol. Catal. 137 (1999) 287.

[3] C. Neyertz, M. A. Volpe and C. Gigola ; to be published in Catal. Today.

[4] L. W. Konopny, A. Juan and D. E. Damiani, Appl. Catal. B, 15 (1998) 115.

Collaboration:

The research work described above is in part the result of a cooperative research program between researchers of PLAPIQUI, Bahía Blanca, Argentina and the Chemistry Institute of UFRGS, Porto Alegre, Brasil, which is still in progress with mutual benefits. Financial support for a visitors exchange program is being provided by FRAPERGs (Brasil ) and ANPCyT (Argentina). The researchers from Brasil have ample experience and adequate facilities for catalysts preparation using organometallic and air sensitive compounds, as well as instrumentation for DRIFT studies. In PLAPIQUI our experience is mainly in the preparation of mono and bimetallic catalysts from inorganic precursors, activity and selectivity test and characterization work by conventional techniques.