(761e) Passivation Agents to Preserve Mo2C and W2C Catalyst Activity | AIChE

(761e) Passivation Agents to Preserve Mo2C and W2C Catalyst Activity

Authors 

Mehdad, A. - Presenter, University of Oklahoma
Jentoft, R. E., University of Oklahoma
Jentoft, F. C., University of Oklahoma


                                               Passivation Agents to Preserve Mo2C and W2C Catalyst Activity

Ali Mehdad, Rolf E. Jentoft, and Friederike C. Jentoft

Chemical, Biological & Materials Engineering, University of Oklahoma

Norman, OK 73019-1004 USA

mehdad@ou.edu, rejentoft@ou.edu, fcjentoft@ou.edu

 

Transition metal carbides are promising catalysts for demanding applications in heavy oil processing because they can withstand high temperatures and can exhibit catalytic properties similar in nature to those of platinum, while potentially being less costly [1]. The structure of early transition-metal carbides has been described as the insertion of carbon atoms into the lattice of a metal. By this carbon insertion, the electronic structure is changed and the reactivity is modified [2]. Several high surface area monometallic carbides, particularly those of molybdenum and tungsten, have been described in the literature [3, 4] and have been successfully employed for hydrogenation and hydrotreating reactions. We have used temperature programmed reaction (TPR) to produce phase pure (XRD) transition metal carbides. While the high surface area can be beneficial for catalysis, these carbides can be pyrophoric, making characterization of relevant samples problematic. Here, we investigate the passivation of carbide surfaces with several passivation agents to determine the effectiveness of the passivation, to investigate re-activation of the surfaces, and to test the affects of passivation and reactivation on catalytic activity.

In the majority of published catalytic studies concerning transition metal carbides the samples are passivated with a low concentration of oxygen (0.5-1%) at room temperature [4-6].   Wu et al. [7] have investigated the use of oxygen, water, and carbon dioxide for the passivation of molybdenum carbide supported on aluminum oxide, and reported that carbide surface passivated in either CO2 or water could be more fully regenerated than that passivated in oxygen. Their conclusions were based on IR spectroscopy studies using carbon monoxide as a probe molecule to access changes to the Mo oxidation state and the number of surface sites that could be regenerated after passivation. Here, we investigate the passivation of  Mo2C and W2C samples with either air or CO2, determine the conditions required for re-activation, and the catalytic activity of the reactivated samples compared to that of carbide samples which have not been exposed to air.

Samples were prepared by TPR carburization of oxide precursors, MoO3 (99.95%, Alfa Aesar) and WO3 (99.995%, Aldrich ) in a mixture of C2H6 (99.95%, Matheson)/H2 (UHP, Airgas)/Ar (UHP, Airgas). The formation of carbides was monitored by thermogravimetry with evolved gas analysis by online mass spectrometry (Netzsch STA 449 F1 with QMS 403 C). The carbide structures were verified by powder X-ray diffraction, and their surface areas were measured by N2 physisorption with Brunauer–Emmett–Teller (BET) analysis. The prepared carbides were passivated with gas mixtures including air or CO2 at 40 °C and the weight gain was measured to determine the degree of surface reaction and the time required for passivation. Subsequent re-activation was also monitored with thermogravimetry with evolved gas analysis to help determine optimum conditions for surface reactivation and to elucidate the effect of the passivation atmosphere on reduction (or activation) temperature and the catalytic activity of the resulting material. The catalytic activity of the carbides for hydrogenation of toluene was tested at a pressure of 30 bar and reaction temperatures of 250 and 300 °C, the H2to toluene molar ratio was 13, and W/F was 0.059 hr. The main product of the hydrogenation in all cases was methylcyclohexane.

Results show that samples passivated by CO2 are re-activated at a lower temperature and show a higher catalytic activity than those passivated in air. For example, W2C that was passivated in 1% oxygen at 40 °C required a temperature of about 400 °C to be re-reduced, while passivation in 30% CO2 at 40 °C required a temperature of only 300 °C to re-activate. After exposure to air for several hours and regeneration, the rate of methylcyclohexane formation at 300 °C for the oxygen passivated W2C sample was 0.0064 (mol/g cat. min), while the rate for the CO2 passivated W2C sample was 0.022 (mol/g cat. min), about 3.5 times greater.

These results will allow us to discuss the surface chemistry that is occurring during passivation and reactivation of transition metal carbide catalysts. In addition, we will compare the behavior of Mo2C and W2C, and contrast the differences in activity to changes in the surface stoichiometry during passivation and re-activation. Understanding changes in the surfaces during passivation and reactivation can help us assure that we are characterizing catalytically significant surfaces when testing new carbide catalysts. 

 

References:

[1]. Levy, R.B. and M. Boudart, Platinum-like behavior of tungsten carbide in surface catalysis. Science, 1973, 181(4099): 547-549

[2]. Kitchin, J.R., Nørskov, J.K., Barteau, M.A., Chen, J.G. Trends in the chemical properties of early transition metal carbide surfaces: A density functional study. Catalysis Today. 2005, 105(1): 66-73

[3].  Oyama, S.T., Preparation and catalytic properties of transition metal carbides and nitrides. Catalysis Today, 1992, 15(2): 179-200

[4]. Lee, J.S., Oyama, S.T., Boudart, M., Molybdenum carbide catalysts: I. Synthesis of unsupported powders. Journal of Catalysis. 1987, 106(1): 125-133

[5]. Hanif, A., Xiao, T., York, A.P.E., Sloan, J., Green, M.L.H., Study on the Structure and Formation Mechanism of Molybdenum Carbides. Chemistry of Materials. 2002, 14(3): 1009-1015

[6]. Neylon, M.K., Choi, S., Kwon, H., Curry, K.E., Thompson, L.T., Catalytic properties of early transition metal nitrides and carbides: n-butane hydrogenolysis, dehydrogenation and isomerization. Applied Catalysis A: General. 1999, 183(2): 253-263

[7]. Wu, W., Wu. Z., Liang, C., Ying, P., Fenga, Z., and Li, C., An IR study on the surface passivation of Mo2C/Al2O3 catalyst with O2, H2O and CO2. Physical Chemistry Chemical Physics, 2004, 6(24): 5603-5608

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