(508a) Characterizing the Processes of Calcination and Reduction in the Preparation of Supported Cobalt Catalysts

Abstract:

A comprehensive study utilising a range of characterisation techniques was carried out on a number of supported cobalt catalysts to investigate potential support effects. A range of complementary techniques including in-situ hot stage XRD and TGA-DSC coupled to online mass spec were employed. Thermal treatments in varying atmospheres showed that the silica support had a mild catalysing effect on catalyst decomposition whereas the alumina was found to stabilise portions of the precursor. In all of the supported catalysts the reducibility was increased in comparison with the bulk cobalt salt.

Keywords: Cobalt, characterization, support.

1. Introduction

Although it was initially thought that the support played no role in Fischer-Tropsch chemistry, it has been shown [1] that in fact it has a major role in influencing the overall hydrocarbon production rate.  Indeed it is believed that variation of the support can have a much more significant impact on cobalt dispersion, and hence catalytic activity, than the overall cobalt loading [2].  In this study we have used a range of techniques to study the conversion of cobalt nitrate supported on silica and alumina into metal.  We have examined both calcination and reduction stages.

2. Experimental

Catalysts containing 20 wt% cobalt were prepared by impregnation of silica (Degussa Aerosil 200) and alumina (Engelhard Al-3992) with aqueous solutions of Co(NO₃)₂.6H₂O.  Prior to impregnation, the supports were dried at 373 K overnight.  After impregnation, water was slowly removed at 353 K by a rotary evaporator before the catalyst was dried further in an oven at 373 K overnight.  The catalysts were characterised using in situ hot stage XRD with a controlled gas environment using a Siemens D5000 X-ray diffractometer (40kV, 40mA) using monochromatic CuK_a x-ray source (1.5418Å), TGA/DSC with evolved gas analysis was performed using a SDT Q600 thermal analyser coupled to a ESS mass spectrometer.  Each catalyst was subjected to heating to 1200 K in four gas environments, argon, oxygen, hydrogen and hydrogen after treatment in oxygen to 773 K.

3. Results and discussion

The decomposition of cobalt nitrate in an inert gas flow is reported to occur as detailed below [5], with the release of nitrogen dioxide, water and oxygen:  3[Co(NO₃)₂.6H₂O] → Co₃O₄ + NO₂ + O₂ + 18H₂O

In contrast however we observed the evolution of nitrogen monoxide as well as water, oxygen and carbon dioxide during the decomposition of cobalt nitrate in argon. Suggesting that the following equation may be a more accurate representation: 3[Co(NO₃)₂.6H₂O] → Co₃O₄ + 6NO + 18H₂O + 4O₂.  This decomposition was complete by 573 K.  Decomposition of the spinel oxide to the thermodynamically more stable divalent cobalt oxide with the release of oxygen was also observed at 1107 K.

Decomposition studies in argon of cobalt nitrate impregnated onto silica and alumina were undertaken. The silica support reduced the temperature of decomposition by 50 deg.K suggesting that the silica has a mild catalysing effect. In contrast, the alumina support stabilised a portion of the cobalt nitrate, causing one of the decomposition events to occur 75 deg.K higher in temperature. Similar effects were noticed when decomposition was followed under oxidative conditions, revealing that to generate a similar species on both silica and alumina the calcination temperature needs to change by 125 deg.K.

Insight into the reduction process was provided by characterisation studies of the supported cobalt catalysts in a hydrogen atmosphere. In the case of cobalt nitrate supported on silica this was a complex process with multiple endothermic and exothermic events below 573 K. Similar to the effect observed with the decomposition in argon and oxygen, the silica support caused reduction to occur at a lower temperature. For the alumina supported system the reduction was a far simpler process with only a single exothermic event below 573 K (Figure 1). In contrast to the treatments in oxygen and argon, the temperature of reduction was also lowered.

Using a combination of TGA-DSC coupled to MS and hot-stage XRD in different gas environments, it is possible to characterise the thermal, electronic and structural changes occurring during calcination and reduction of supported cobalt catalysts.  Using this methodology it is possible to optimise the preparation and subsequent processing to deliver a catalyst with optimal dispersion and reduction properties. 

References

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S.K. Park and S.I. Woo, Appl. Catal. A, 306 (2006) 1.

2.     B.C. Gates, ?Catalytic Chemistry?, John Wiley & Sons, Inc.,
New York, 1992. p.77.

3.     J.J. Lee, K. Chan, Y.J. Yoon, T. Hyeon, S.H. Moon, Proceedings of the 12th International Congress on Catalysis, Granada, Spain, 2000, p.2777.

4.     J.S. Lee, K.Y. Yu, H.D. Won, J.S. Jang, S.M. Ji, B.S. Won, Korea Patent 567,704 (2006).

5.     T. Cersi,
S. Békássy, G. Kenessey, G. Liptay, F. Figures, Thermochim. Acta, 288 137 (1996)