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

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 (WO_x-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 WO_x 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 WO_x-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 WO_x loading and annealing temperature at constant ZrO₂ loading and particle size of the ZrO₂/MWCNT. This was done using NO_x-TPD.^3,4

The XAS results show that as the annealing temperature is increased (250-850 °C) for a given WO_x loading, the white line intensity of the Zr K edge decreases. Furthermore, we have found that as the WO_x 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 WO_x loadings. Mutual reduction of the W and Zr is chemically unlikely, and moreover, it is probable that the coverage of ZrO₂ by WO_x varies with both temperature of annealing and WO_x loading. Thus, we use NO_x-TPD to estimate the accessible zirconia surface as a function of WO_x loading and annealing temperature in order to assess the WO_x-ZrO₂ interaction at constant ZrO₂ loading and constant WO_x coverage. From the NO_x-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 NO_x-TPD (presumably oxidized by NO₂), as indicated by the observed increase in the accessible ZrO₂ surface as the number of successive NO_x-TPD experiments increases, 2) the C free ZrO₂ particle size, deduced from NO_x-TPD, is 2.2 nm and consistent with that measured by XRD and TEM, and 3) this amorphous C contamination, that interferes with WO_x/ZrO₂ interaction, appears to be partially removed by interaction with WO_x (enhanced interaction by higher WO_xloading).

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.