National Aeronautics and Space Administration

Glenn Research Center

Robyn Bradford

“BET (Brunauer, Emmett, and Teller) Analysis to Determine Surface Area of Cobalt Catalysts for Use in Fischer-Tropsch Gas-to-Liquid (GTL) Processes”

ABSTRACT

The Brunauer, Emmett and Teller (BET) method to determine the surface area (SA) of a solid is based on detecting the amount of nitrogen physisorbed on the surface. This study uses the BET method to find SA from the amount of nitrogen (N2) gas that physically adsorbs onto a Fischer-Tropsch (F-T) cobalt-alumina (Co/Al2O3) catalyst from a continuously flowing gas mixture of 30% N2 and 70% helium (He) at a cryogenic temperature. The BET surface areas of various species of Co/Al2O3 catalysts were measured using a Micromeritics FlowSorb II 2300 surface analyzer. Since adsorption and desorption values are theoretically the same, specific surface area (SSA) was determined by dividing the desorption value by the dry sample weight of the Co/Al2O3 catalyst. Target cobalt metal loadings and promoters, which increase catalyst activity, were both varied to examine the effect on SSA. Elemental analysis to determine the weight percent of cobalt metal present in the catalysts was done in-house by energy dispersive spectroscopy (EDS) and off-site by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Previous F-T catalysis research indicates that as the metal content on an alumina substrate increases, SA decreases. This study is a direct investigation into this phenomenon and results coincide with earlier studies. It was observed here that upon increased loadings of cobalt metal, the SSAs of the F-T catalysts decreased. In addition, SSAs decreased further upon addition of promoters such as Ag, Pt, Mn, Ru, and Ni. The objective here is to qualitatively construct a calibration curve to equate SSA to the amount of cobalt metal present in Co/Al2O3 catalysts for the F-T process.

Introduction

 

The Fischer-Tropsch (F-T) process is a series of chemical reactions that convert hydrogen and carbon monoxide into liquid hydrocarbons. The process is an essential component in gas-to-liquid technology and is a significant and viable alternative fuel production mechanism because the products of the chemical reactions (paraffins and olefins) can be upgraded and used to produce synthetic aviation fuel. The most common catalysts used are cobalt/alumina (Co/Al2O3) catalysts: cobalt metal supported on the porous structure of aluminum oxide (alumina). Although other metals can be used, cobalt has proven itself to be the most active.

The Brunauer, Emmett and Teller (BET) method to determine surface area (SA) is based on physisorption: the amount of gas that physically adsorbs onto a surface at a given pressure. Physisorption is determined by temperature, gas pressure, the interaction between surface and gas (vapor pressure) and surface area. The method is used extensively in surface area studies because it is inexpensive and reliable. However, it does not apply to all isotherm types. Surface area is important in catalysis research because it is used to monitor the activity and stability of catalysts.1

BET theory is an expansion of an earlier surface area technique known as the Langmuir isotherm. The Langmuir theory described how gas molecules accumulate in a single layer (monolayer) over exposed solid surfaces and pores due to attractive forces. The drawback of the Langmuir method is that it doesn’t take into consideration that gas molecules can form more than one layer (multilayer). In contrast, BET theory does account for multilayer formation; and the theory’s characteristic equation, known as the BET equation, mathematically explains how this occurs.1

(P/P0)/V[1-(P/P0)] = 1/(VmC)+[(C-1)/(VmC)]P/P0

The BET multilayer adsorption model is based on several assumptions. First, it is presumed that gas molecules physically adsorb onto a solid surface in infinite layers. Second, there is no interaction between the layers; and third, the monolayer adsorption model applies to each layer.1

In general, to obtain the BET surface area, a dried, powdered sample is placed in a sample tube and then exposed to a flowing gas mixture of helium and nitrogen at a certain pressure. Other gas mixtures can be used, but He and N2 are the most common. Thermal conductivity is measured at the start of the experiment; and at room temperature, there is no change in thermal conductivity. To induce adsorption, the sample tube is immersed in a Dewar flask of liquid nitrogen (LN2). This immediately changes the pressure and nitrogen molecules begin to adsorb onto the surface of the F-T catalyst from the continuously flowing gas mixture; thermal conductivity changes also. After adsorption, it returns to its initial value. To obtain desorption measurements, the LN2 Dewar flask is removed. Desorption instigates another change in thermal conductivity. Theoretically, the adsorption and desorption values are the same since the amount desorbed can be no more or no less than the amount adsorbed.2 

References

1 Brunauer, S., Emmett, P.H. and Teller, E., J. Am. Chem. Soc., 1938, 60, 309.

2 Fagerlund, G. Determination of specific surface area by the BET method. (1973) Materiaux Et Constructions, Vol. 6, No. 33, 239-245.

3Jones, S., Hepp, A., Hoops, M. and Cowen, J., “Synthesis and characterization of cobalt/alumina catalysts for Fischer- Tropsch processing,” NASA, 2009.

4 Jacobs, G., Das, T.K., Zhang, Y., Li, J., Racoillet, G., Davis, B.H. Applied Catalysis A: General. 233 (2002) 263-281.