Preparation and characterization of alumina-supported Co and Ag/Co catalysts

SHORT COMMUNICATION

Preparation and characterization of alumina-supported Co and Ag/Co catalysts

Marina Simionato; Elisabete Moreira Assaf^*

Instituto de Química de São Carlos - Universidade de São Paulo Av. Trabalhador Sãocarlense, 400, 13560-970 São Carlos - SP, Brazil

ABSTRACT

The effect of silver addition as a promoter in alumina supported cobalt oxide catalysts is investigated in this study. The Co/Al₂O₃ and Ag/Co/Al₂O₃ catalysts were prepared by incipient wetness impregnation. The catalysts were characterized by inductively-coupled plasma spectrometry, X-ray diffraction (XRD), temperature programmed reduction with hydrogen (TPR-H₂), Raman spectroscopy and nitrogen adsorption at 77 K. XDR and Raman Spectroscopy results indicated the presence of Co₃O₄ species. The peaks observed in the TPR-H₂ patterns were attributed to the reduction of Ag₂O, Co₃O₄ crystallites and Co⁺³, Co⁺² species interacting with alumina. It is suggested that silver used as a promoter favors the reduction of cobalt, facilitating the formation of active Co⁰ sites.

Keywords: cobalt catalysts, Ag-Co/Al₂O₃

1. Introduction

Alumina supported Cobalt catalysts has been studied for several reactions, such as mitigation of nitrogen oxides (NOx)¹³, oxidation of volatile organic compounds (VOCs)⁴, Fischer-Tropsch synthesis⁵ and hydrodesulfurization reaction⁶.

The catalytic removal of nitrogen oxides from the exhaust stream of various combustion sources has become increasingly important, because of the serious environmental damages, which they cause. The active catalysts known for this reaction are mainly the zeolite-based catalysts. However, these catalysts present some disadvantages as the deactivation in presence of water steam and sulfur compounds⁷. Therefore, there is an interest in the study of other catalytic systems, which can be active, selective and resistant to the deactivation process.

Alumina-supported catalysts, such as Co/Al₂O₃^1-3 and Ag/Al₂O₃⁸ have shown promising perspectives to the NO_x reduction reactions. It has been reported that metal-supported interactions may appreciably affect the surface properties and hence, catalytic activity. These interactions depend upon factors such as the concentration of metal species, nature of the support and calcinations temperature².

Hamada et al.⁹ verified that for NO decomposition reaction, the catalytic activity of Co₃O₄ with the addition of small amounts of silver is superior to that the obtained with simple metallic oxide. The introduction of silver as a promoter generates a Ag-Co₃O₄ system that reduce the reaction inhibition caused by the irreversible deposition of oxygen. However, the cobalt dispersion on a high surface support, such as alumina, can increase the number of active Co⁰ sites, improving the catalytic activity of these catalysts.

In order to study the potential of the catalysts Co/Al₂O₃ and Ag/Co/Al₂O₄ it is necessary to elucidate the structural and textural characteristic caused by different preparation. In this paper we study the effect of different Co e Ag loading on the final properties of the catalysts, as well as the effect caused by the addition of silver in Co/Al₂O₃ catalysts. Co/Al₂O₃ and Ag/Co/Al₂O₃ catalysts were characterized using X-ray diffraction (XRD), Raman spectroscopy and temperature programmed reduction (TPR) to identify the species formed and to obtained evidences on the interaction between the metallic compounds.

2. Experimental

2.1. Catalyst Preparation

The Co/Al₂O₃ catalysts were prepared using the impregnation method. The support (Al₂O₃, Degussa) was first calcined at 650 °C in order stabilize the g-Al₂O₃ form with a surface area of 198 m²/g. The support was impregnated with cobalt nitrate (Co(NO₃)₂ × 6 H₂O (Riedel-deHaën) solution with concentration adequate to produce catalysts with 5 and 9% cobalt. The excess of water was removed in a rotary evaporator, the solids were dried for 24 h at 60 °C and calcined at 600 °C, for 3 h. The addition of silver on Co/Al₂O₃ catalysts was accomplished by the same method, using AgNO₃ (Cenna Bras) solution. After the promoter addition, the solids were dried at 60 °C for 24 h and calcined at 600 °C, in a synthetic air atmosphere for 3 h.

2.2. Catalyst Characterization

Chemical analysis

The metal contend in the catalysts were determined by Inductively Coupled Plasma Optical Emisson Spectrometry, using a Perkin Elmer equipment.

Adsorption of N₂ at 77 K

The textural characterization of catalysts was determined by Adsorption of N₂ at 77 K using a Quantachrome Nova 2.0 instrument. Surface area was obtained by application of the BET equation and pore size distributions were obtained using BJH method.

X-Ray diffraction

The X-ray powder diffraction patterns of the samples were obtained using an automatic Rigaku diffractometer model D Max 2050 PC with monochromatized Cu Ka radiation (l = 1.5406 Å). The spectra were scanned at a rate of 2°/min in the range 2q = 3° 80°.

Raman Spectroscopy

The Raman spectroscopy measures were carried out in a Jobin-Yvon T64000 spectrometer with a laser power of 73 µW. The Raman spectra of the catalysts were collected between 150 and 800 cm^-1, projecting a continuous wave laser of argon ion (Ar⁺) green (5.145 Å) through the samples exposed to air at room temperature.

Temperature programmed reduction with hydrogen

TPR profiles of catalysts were recorded using a Micromeritics Pulse Chemisorb model 2705.

The sample was heated from 25 to 1000 °C using a heating ramp of 10 °C/min, the gas reducer employed was a mixture of 5%H₂/N₂ with a flow of 30 ml. The consumption of H₂ was measured in a thermal conductivity detector.

3. Results and Discussion

Table 1 shows the results of chemical analysis and textural characterization of the catalysts.

Results of surface area measurements by adsorption of nitrogen at 77 K shown that the BET surface of the alumina (198 m²/g) decreases 17% for the 5% Co catalyst and 27% for the 9% Co catalysts. It is interesting to observe that the cobalt content practically does not alter the average pore radius, but increasing cobalt from 5 to 9% causes a small decrease in the pore volume of the catalyst. This occurs because the impregnation of the support with metal clogs the pores with smaller diameter of the alumina. As a consequence, it occurs a decrease in the surface area and in the pore volume and an increase in the average pore radius. The silver addition to the Co/Al₂O₃ catalysts practically did not alter the values of surface area, the pore volume and average pores radius, probably due to the low contends impregnated.

TPR-H₂ patterns obtained for the Co/Al₂O₃ and Ag/Co/Al₂O₃ catalysts are presented in Fig. 1.

TPR-H₂ pattern of unsupported cobalt oxide (Fig. 3g) exhibits a single peak at 460 °C, where the reduction of Co⁺³ ® Co⁺²® Co⁰ occurs. This result is in agreement with those obtained by Arnoldy and Moulijn¹⁰, which also presents a pattern with a single peak for the Co₃O₄ reduction. According to these authors, the reduction peaks observed in the TPR-H₂ pattern (Figs. 3a-3f)) can be attributed as follows:

• Phase I (120 °C): small peak in the samples which contain silver, attributed to the Ag₂O reduction;

• Phase II (380 °C): weak interaction of Co₃O₄ with the support; it is observed in the Co (5) catalyst and is very hard to be distinguished in the Co (9) catalyst TPR-H₂ curve;

• Phase III (430 - 460 °C): interaction of Co₃O₄ crystallites with alumina;

• Phase IV (600 - 630 ^oC): Co⁺³ ions in crystallites, probably with stoichiometry Co₃AlO₆ (Co₃O₄-AlO₂);

• Phase V (700 °C): Co⁺² ions in well-dispersed surface species, just observed in the samples that do not contain silver;

• Phase VI (770 - 800 °C): Co⁺² ions interacting with the support, forming a CoO-Al₂O₃ phase.

Arnoldy and Moulijn⁶ suggested that these last two existing phases IV and V are formed by diffusion of cobalt ions with the aluminum ions of the support, resulting in non-stoichiometric spinel structures, which is not the CoAl₂O₄, since it is reduced at temperatures higher than 1000 °C. These authors also mention that the number of Al⁺³ ions that surround the Co ions determines the reduction temperature, while the valence and the cobalt coordination are less important.

It can be observed in Fig. 1 that the silver addition to the catalysts induces a decrease of the phases III and VI in favor of the phase IV. An evidence for this hypothesis it is the fact that calcination of the catalyst impregnated with silver nitrate causes the formation of Ag₂O and Ag⁰. Therefore, the reduction of the silver oxide during calcination causes the oxidation of cobalt Co⁺² to Co⁺³, which is confirmed by the increase of the area of under the peak IV and the decrease of the areas under the peaks III and disappear of peak V.

On the other hand, the reduction peaks of the cobalt species in Ag/Co/Al₂O₃ catalysts present a lower reduction temperature than the Co/Al₂O₃ catalysts, indicating that silver promotes the reduction of the cobalt species in the presence of molecular hydrogen. According to Luo et al.⁴, silver oxide is reduced at lower temperature, then H₂ is dissociated over the reduced silver and hydrogen spillover reduce the surface of cobalt oxides.

As the silver favors the formation of the Co₃O₄-AlO₂ phase, which are easier to be reduced than the CoO-Al₂O₃ phase, this promoter addition facilitates the formation of active Co⁰ sites.

XRD patterns of the Co/Al₂O₃ and Ag/Co/Al₂O₃ catalysts are shown in Fig. 2 and the main lines of Co₃O₄ and g-Al₂O₃, from JCPDS¹¹ files are also presented for comparison. In this figure it can be observed the peaks of high intensity at 2q = 36.8°, 65.5°, 31.6°, 59.5° and 19.2°. The diffraction peaks at 2q = 67.1° and 45.8° of the g-Al₂O₃ for the catalysts containing 5% of cobalt are quite intense, however they decrease with the increase of the metal loading, evidencing the peaks of cobalt specie at 65.5° and 45.2°.

It is important to stress that both Co₃O₄ and CoAl₂O₄ have a cubic spinel structure with almost identical diffraction peaks. Therefore it is not possible to distinguish which species was formed using the only diffraction measurements. However, stoichiometric aluminates were not observed in the TPR-H₂ curves. This suggests that the observed peaks in XRD patterns can be attributed to Co₃O₄.

In the XRD patterns the presence of peaks of Ag₂O and Ag⁰ were not observed (2q = 38.6°, 50.4°, 66.7°)¹¹. According to Luo et al.⁴ only in Ag/Al₂O₃ catalysts with Ag loading larger than 15%, the crystalline Ag₂O and Ag⁰ diffraction peaks become apparent.

In Fig. 2, the AgCoO₂ diffraction peaks (2q = 37.6°, 65.3°, 41.4° and 29.2°)¹¹ were not observed due to the low Ag loading. This species could be formed by the Ag-Co interaction¹².

Raman spectra of the Co/Al₂O₃ and Ag/Co/Al₂O₃ catalysts are shown in Fig. 3. These spectra show five bands at 670, 600, 505, 460 and 182 cm^-1. These bands, according to Ohtsuka et al.¹³, are characteristic of Co₃O₄ spinel structure. In the Raman spectra of the catalysts were not observed bands at 753 e 412 cm^-1 that characterize the presence of the CoAl₂O₄¹⁴.

Thus, associating the DRX results with Raman Spectroscopy analyses, we can confirm the presence of Co₃O₄.

4. Conclusions

The association of XRD and Raman Spectroscopy techniques revealed formation of Co₃O₄ species in the g-alumina supported Co and Ag/Co prepared by impregnation. In the of TPR-H₂ analyses both the existence of Co₃O₄ phases and Co⁺² and Co⁺³ ions interacting with Al₂O₃ were evidenced, indicating the possible formation of non-stoichiometric aluminates.

The presence of silver in the catalyst was observed only in the chemical analyses and in the TPR-H₂ measurements. Both DRX and Raman Spectroscopy are not sensitive to the small amount of silver added to the catalyst. From the TPR-H₂ results it is suggested that the use of silver as promoter favors the reduction of cobalt, facilitating the formation of active Co⁰ sites.

Acknowledgments

The authors tanks CNPq, DEQ/UFSCAR and IF/UNICAMP.

11. JCPDS - Joint Committee on Powder Diffraction Standars, International Center of Diffraction Data. Pensilvânia USA, (CD-ROM), 1994.

Received: January 30, 2003

Revised: July 27, 2003

Trabalho apresentado no XV CBECIMAT, Natal - RN, Novembro de 2002

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Publication Dates

• Publication in this collection
  19 Jan 2004
• Date of issue
  Dec 2003

History

• Accepted
  27 July 2003
• Received
  30 Jan 2003