Preparation and Characterization of Alumina-Supported Co and Ag / Co Catalysts

The effect of silver addition as a promoter in alumina supported cobalt oxide catalysts is investigated in this study. The Co/Al 2 O 3 and Ag/Co/Al 2 O 3 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-H 2 ), Raman spectroscopy and nitrogen adsorption at 77 K. XDR and Raman Spectroscopy results indicated the presence of Co 3 O 4 species. The peaks observed in the TPR-H 2 patterns were attributed to the reduction of Ag 2 O, Co 3 O 4 crystallites and Co, Co species interacting with alumina. It is suggested that silver used as a promoter favors the reduction of cobalt, facilitating the formation of active Co sites.

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 compounds 7 .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/Al 2 O 3 [1][2][3] and Ag/Al 2 O 3 8 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 temperature 2 .
Hamada et al. 9 verified that for NO decomposition reaction, the catalytic activity of Co 3 O 4 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-Co 3 O 4 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 Co 0 sites, improving the catalytic activity of these catalysts.
In order to study the potential of the catalysts Co/Al 2 O 3 and Ag/Co/Al 2 O 4 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/Al 2 O 3 catalysts.Co/Al 2 O 3 and Ag/Co/Al 2 O 3 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.

Catalyst Preparation
The Co/Al 2 O 3 catalysts were prepared using the impregnation method.The support (Al 2 O 3 , Degussa) was first calcined at 650 °C in order stabilize the γ-Al 2 O 3 form with a surface area of 198 m 2 /g.The support was impregnated with Short communication cobalt nitrate (Co(NO 3 ) 2 × 6 H 2 O (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/Al 2 O 3 catalysts was accomplished by the same method, using AgNO 3 (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.

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 N 2 at 77 K
The textural characterization of catalysts was determined by Adsorption of N 2 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 Kα radiation (λ = 1.5406Å).The spectra were scanned at a rate of 2°/min in the range 2θ = 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%H 2 /N 2 with a flow of 30 ml.The consumption of H 2 was measured in a thermal conductivity detector.

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 m 2 /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/Al 2 O 3 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-H 2 patterns obtained for the Co/Al 2 O 3 and Ag/Co/Al 2 O 3 catalysts are presented in Fig. 1.
TPR-H 2 pattern of unsupported cobalt oxide (Fig. 3g) exhibits a single peak at 460 °C, where the reduction of Co +3 → Co +2 → Co 0 occurs.This result is in agreement with those obtained by Arnoldy and Moulijn 10 , which also presents a pattern with a single peak for the Co 3 O 4 reduction.According to these authors, the reduction peaks observed in the TPR-H 2 pattern (Figs.3a-3f)) can be attributed as follows: • Phase I (120 °C): small peak in the samples which contain silver, attributed to the Ag 2 O reduction;   6 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 CoAl 2 O 4 , 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 Ag 2 O and Ag 0 .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/Al 2 O 3 catalysts present a lower reduction temperature than the Co/Al 2 O 3 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 H 2 is dissociated over the reduced silver and hydrogen spillover reduce the surface of cobalt oxides.
As the silver favors the formation of the Co 3 O 4 -AlO 2 phase, which are easier to be reduced than the CoO-Al It is important to stress that both Co 3 O 4 and CoAl 2 O 4 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-H 2 curves.This suggests that the observed peaks in XRD patterns can be attributed to Co 3 O 4 .
In the XRD patterns the presence of peaks of Ag 2 O and Ag 0 were not observed (2θ = 38.6°,50.4°, 66.7°) 11 .According to Luo et al. 4 only in Ag/Al 2 O 3 catalysts with Ag loading larger than 15%, the crystalline Ag 2 O and Ag 0 diffraction peaks become apparent.In Fig. 2, the AgCoO 2 diffraction peaks (2θ = 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 interaction 12 .
Raman spectra of the Co/Al 2 O 3 and Ag/Co/Al 2 O 3 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 Co 3 O 4 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 CoAl 2 O 4 14 .Thus, associating the DRX results with Raman Spectroscopy analyses, we can confirm the presence of Co 3 O 4 .

Conclusions
The association of XRD and Raman Spectroscopy techniques revealed formation of Co 3 O 4 species in the γ-alumina supported Co and Ag/Co prepared by impreg-  nation.In the of TPR-H 2 analyses both the existence of Co 3 O 4 phases and Co +2 and Co +3 ions interacting with Al 2 O 3 were evidenced, indicating the possible formation of nonstoichiometric aluminates.
The presence of silver in the catalyst was observed only in the chemical analyses and in the TPR-H 2 measurements.Both DRX and Raman Spectroscopy are not sensitive to the small amount of silver added to the catalyst.From the TPR-H 2 results it is suggested that the use of silver as promoter favors the reduction of cobalt, facilitating the formation of active Co 0 sites.

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Phase III (430 -460 °C): interaction of Co 3 O 4 crystallites with alumina; • Phase IV (600 -630 o C): Co +3 ions in crystallites, probably with stoichiometry Co 3 AlO 6 (Co 3 O 4 -AlO 2 ); • 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-Al 2 O 3 phase.Arnoldy and Moulijn 2 O 3 phase, this promoter addition facilitates the formation of active Co 0 sites.XRD patterns of the Co/Al 2 O 3 and Ag/Co/Al 2 O 3 catalysts are shown in Fig. 2 and the main lines of Co 3 O 4 and γ-Al 2 O 3 , from JCPDS 11 files are also presented for comparison.In this figure it can be observed the peaks of high intensity at 2θ = 36.8°,65.5°, 31.6°,59.5° and 19.2°.The diffraction peaks at 2θ = 67.1°and 45.8° of the γ-Al 2 O 3 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°.

Table 1 .
Composition and surface area, pore volume and pore radius of the catalysts.