471040 Carbon Decoration in Supported Palladium Catalysts: Discrepancy in STEM-Chemisorption Particle Sizes

Thursday, November 17, 2016: 3:45 PM
Franciscan C (Hilton San Francisco Union Square)
Ritubarna Banerjee and John R. Regalbuto, Department of Chemical Engineering, University of South Carolina, Columbia, SC

Introduction

        Metal-support interactions have been known to stabilize metal nanoparticles in harsh reaction environments or cause hindrances in the chemisorption surface area during chemical reactions [1, 2]. Although it is necessary for the active sites to remain exposed without any poisoning to allow the maximum adsorption of reactants in heterogeneous reactions, it is also vital to maintain the structural stability in chemical reactions at elevated temperatures. Carbon-supported palladium catalysts find a wide variety of applications in the fine chemicals industry, as electrodes in fuel cells and in a large number of hydrogen-mediated aqueous phase catalytic reactions.   While it has been frequently seen in the literature that active sites in the metal are blocked by carbonaceous deposits [3], there is also ample evidence that carbon coatings are frequently used to coat oxide supports and metal nanoparticles for enhancing their hydrothermal stability in aqueous phase hydrogenation reactions at elevated temperatures [4] due to their inherent hydrophobicity and inertness.

During the preparation of palladium catalysts on carbon black in our laboratory, it has been found that well dispersed catalysts are formed as confirmed with Scanning Transmission Electron Microscopy (STEM) and X-Ray Diffraction (XRD) but the chemisorption of hydrogen is substantially suppressed on the catalysts leading to a disagreement between particle sizes obtained by STEM and chemisorption [5]. This work focusses on understanding the STEM-chemisorption discrepancy based on the hypothesis that during the preparation of carbon supported palladium nanoparticles, the metal surface becomes coated from the surface/sub-surface/interstitial carbon that migrates from the support to the metal nanoparticles or with a partial surface blockage by residual chloride from the chloride precursor used. This suppresses the hydrogen chemisorption and leads to larger particle sizes as compared to transmission electron microscopy TEM/X-ray diffraction (XRD). The discrepancy is also believed to be dependant on the type of carbon used, the functional groups on the surface, the precursor chosen and pre- treatment conditions of the support.

Materials and Methods

        Strong Electrostatic Adsorption (SEA) was used to deposit Pd-nanoparticles onto two sets of different carbons: VXC72 (carbon black) and Darco G60 (activated carbon) that were oxidized and heat-treated to different temperatures. The pre-treated catalysts were then dried at 120°C and reduced in hydrogen at 180°. The catalysts were characterized for particle sizes using STEM, XRD and chemisorption. Systematic Temperature Programmed Oxidation (TPO) studies were conducted to burn off the surface carbon and pre and post TPO chemisorption sizes were compared. Chloride and non-chloride precursors were also compared to determine the effect of chloride ions on the discrepancy.

       To understand the discrepancy better, the ratio of chemisorption and STEM surface average sizes were plotted against the pretreatment temperatures. The discrepancy for both DarcoG60 and VXC72 is the greatest when they are oxidized and decreases with the increase in pretreatment temperature which confirms that there is a relation between the surface oxygen groups and carbon decoration. It is also seen that the unoxidized Darco series did not show any discrepancy. Incidentally, this was the only series of samples that had very large Pd particles (>10 nm) with wide standard deviations (Fig 1). After the TPO burn off experiment, the discrepancy is reduced by 50-60% for the highest discrepancy (oxidized carbon samples) (Fig 2). For the unoxidized Darco sample, there was no change in the ratio since the discrepancy did not exist in the first place. However, it is seen that burning off the surface carbon does not recover the chemisorption surface completely. This led to the comparison of results using chloride and non-chloride precursors which showed that chemisorption gives a 30-50% reduction in the discrepancy for the nitrate precursor as compared to chloride precursor. It may, thus, be concluded that carbon decoration coupled with chloride poisoning is responsible for the discrepancy in the STEM-chemisorption particle sizes.

References

[1] D. Nerhing, H. Dreyer. Chem.Tech. 1960, 12, 343.

[2] G.M. Schwab. Adv. Catalysis, 1978, 27, 1-22.

[3] P. Forzatti , L. Lietti, Catalysis Today, 1999, 52,165-181.

[4] H. N. Pham, A. E. Anderson, R. L. Johnson, T. J. Schwartz,B. J. O’Neill, P. Duan, K. Schmidt-Rohr, J. A. Dumesic and A. K. Datye, ACS Catalysis, DOI: 10.1021/acscatal.5b00329 ACS Catal. 2015, 5, 4546−4555.

[5] J.M.M.Tengco, Y.K.Lugo-Jose, J.R.Monnier, J.R.Regalbuto, Catalysis Today, 2015, 246, 9.

 


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