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Langmuir 1999, 15, 8336-8336
Comments Comment on “Characterization of Zirconia-Supported Tungsten Oxide Catalyst”
In a recent paper by J. R. Sohn and M. Y. Park entitled “Characterization of Zirconia-Supported Tungsten Oxide Catalyst” (ref 1), the authors reported X-ray photoelectron spectroscopy (XPS) results for WO3/ZrO2 catalysts calcined in air at 973 K (ref 1, Figure 4). In addition to the W 4f doublet characteristic of W6+, their XPS spectra show a line at 32.0 eV which was attributed by the authors to metallic W. While it is true that the line position is close to that of the W 4f7/2 level for W metal (ca. 31.3 eV; ref 2), the authors assignment of the observed line at 32 eV to W metal is in error. The probability that metallic W can be present on the surface of a catalyst which has been calcined in air at high temperature is very low. The authors, apparently, did not consider the XPS spectrum for the ZrO2 support in their interpretation. It would have shown that the peak at 32.0 eV is, in fact, due to the Zr 4p level. Figure 1 shows the XPS spectra of ZrO2 and representative WO3/ZrO2 catalysts from recent work in our laboratory.3 Note that the ZrO2 support exhibits a peak at ca. 32 eV due to the Zr 4p level.2 The same peak appears in the XPS spectra of WO3/ZrO2 catalysts. The XPS Zr 4p/Zr 3d intensity ratio measured for the ZrO2 support is similar to that obtained for the WO3/ZrO2 catalysts. The Zr 4p3/2 and Zr 4p1/2 peaks are not resolved. This is consistent with the authors observation that the peak at 32.0 eV does not appear as a typical W 4f doublet. The assignment of the XPS line at 32 eV to the Zr 4p level explains why the peak is more prominent (compared to the W 4f envelope) at low W loadings (ref 1, Figure 4). Our assignment also explains why the so-called W metal peak is absent in the case of WO3/Al2O3 catalysts. For WO3/ ZrO2 catalysts containing the same W loading and calcined at different temperatures (ref 1, Figure 5), the apparent increase in the area of the peak at 32 eV compared to the W 4f envelope can, in part, be attributed to a change in the morphology of the catalysts at high temperatures. * Corresponding author. (1) Sohn, J. R.; Park, M. Y. Langmuir 1998, 14, 6140. (2) Practical Surface Analysis, 2nd ed.; Briggs, D., Seah, M. P., Eds.; John Wiley and Sons: West Sussex, England, 1990. (3) Vaidyanathan, N.; Houalla, M.; Hercules, D. M. In preparation.
Figure 1. XPS spectra of ZrO2 and representative WO3/ZrO2 catalysts from ref 3. The numbers refer to the W loading expressed as wt % WO3.
Only after taking into consideration the contribution of the Zr 4p peak from the support can these results be appropriately examined. It must be added, however, that the above comments only impact the XPS part of the paper. They do not significantly affect the main conclusions of the authors which were essentially derived from other characterization techniques. Nithya Vaidyanathan, Marwan Houalla,* and David M. Hercules
Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235 Received January 18, 1999
10.1021/la990049m CCC: $18.00 © 1999 American Chemical Society Published on Web 08/31/1999
LA990049M
