466737 Characterization of Tungsten and Zirconium Decorated Multi-Walled Carbon Nanotube Catalysts By XAS and NOx-Temperature Programmed Desorption

Monday, November 14, 2016: 8:50 AM
Imperial B (Hilton San Francisco Union Square)
Gary L. Haller1, Patrick Kelleher1 and Cyril Thomas2, (1)Chemical and Environmental Engineering, Yale University, New Haven, CT, (2)Laboratoire de Réactivité de Surface, Sorbonne Universités, UPMC Univ Paris 6, Paris, France

The goal of this research is to develop strong solid acid catalysts supported on multi-walled carbon nanotubes (MWCNT) suitable for aqueous biomass processing. We have previously shown that zirconia supported on MWCNT (ZrO2/MWCNT) is hydrothermally stable and can be made a solid acid catalyst by sulfating the zirconia.1 Although this makes a relatively good acid catalyst, we have found that the sulfur species is not particularly stable in hot liquid water. On the other hand we have demonstrated that the tungsten-modified analogue (WOx-ZrO2/MWCNT) is hydrothermally stable and it is expected to have similar, but somewhat lower, acid catalytic activity.2 We have studied the effects that catalyst annealing temperature and oxide loading on the oxide particle size. This was accomplished with XRD and TEM and there was good agreement in the ZrO2 particle size of ~2 nm and the WOx particle size of ~6 nm at high loadings, 30 wt%, (but immeasurable at low loading, < 10wt%) using these two techniques. X-Ray absorption near edge spectroscopy (XANES), has shown changes in the W L3 edge and Zr K edge white line intensities of WOx-ZrO2/MWCNT upon annealing at different temperatures, indicating a change in the interaction and/or oxidation state of the tungsten and/or zirconium. To further interpret the XANES, it is essential to estimate accessible zirconia surface as a function of WOx loading and annealing temperature at constant ZrO2 loading and particle size of the ZrO2/MWCNT. This was done using NOx-TPD.3,4

The XAS results show that as the annealing temperature is increased (250-850 °C) for a given WOx loading, the white line intensity of the Zr K edge decreases. Furthermore, we have found that as the WOx loading is increased, the white line intensity of the Zr K edge again decreases. Data collected at the W L3 edge shows similar trends to the data collected at the Zr K edge. The decrease in the white line intensity results from increased electron density at the Fermi level. This indicates an apparent reduction and/or rehybridization of Zr/W at higher temperatures and with higher WOx loadings. Mutual reduction of the W and Zr is chemically unlikely, and moreover, it is probable that the coverage of ZrO2 by WOx varies with both temperature of annealing and WOx loading. Thus, we use NOx-TPD to estimate the accessible zirconia surface as a function of WOx loading and annealing temperature in order to assess the WOx-ZrO2 interaction at constant ZrO2 loading and constant WOx coverage. From the NOx-TPD data it can be deduced that: 1) As synthesized, the ZrO2/MWCNT annealed in He at 450 °C retains amorphous C on the ZrO2 that is removed by repetitive NOx-TPD (presumably oxidized by NO2), as indicated by the observed increase in the accessible ZrO2 surface as the number of successive NOx-TPD experiments increases, 2) the C free ZrO2 particle size, deduced from NOx-TPD, is 2.2 nm and consistent with that measured by XRD and TEM, and 3) this amorphous C contamination, that interferes with WOx/ZrO2 interaction, appears to be partially removed by interaction with WOx (enhanced interaction by higher WOxloading).

1. C. Liu, S. Lee, D. Su, B. Lee, S. Lee, R. E. Winans, C.Yin, S. Vajda, L. Pfefferle, G. L. Haller, Langmuir 28 (2012) 17159-17167.

2. J. Macht, R. T. Carr, E. Iglesia, J. Catal. 264 (2009) 54-66.

3. H. Y. Law, J. Blanchard, X. Carrier, C. Thomas, J. Phys. Chem. C 114 (2010) 9731-9738.

4. C. Thomas, J. Phys. Chem. C 115 (2011) 2253-2256.

5. C. Liu, S. Lee, D. Su, Z. Zhang, L. Pfefferle, G. L. Haller, J. Phys. Chem C 116 (2012) 21742-21752.

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