J. Phys. Chem. B 2001, 105, 12427-12428
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COMMENTS Comment on “Sintering and Phase Transformation of V-Loaded Anatase Materials Containing Bulk and Surface V Species” Chinnakonda S. Gopinath* and Thirumalaiswamy Raja Catalysis DiVision, National Chemical Laboratory, Pune 411 008, India ReceiVed: June 18, 2001 Recently Balikdjian et al.1 presented a combined macroscopic and spectroscopic results on V-loaded anatase (V/TiO2) materials calcined at different temperatures up to 900 °C with three different concentrations of vanadium (V/Ti molar ratio 0.02, 0.03, and 0.06) and an associated phase transformation to rutile (V/TiO2). It is suggested from X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) measurements that vanadium ions migrate to surface below 600 °C and above this temperature there is a phase transformation to rutile occurs. It is also suggested that rutile formation is initiated near the surface of anatase particles and the phase transformation from anatase to rutile is due to the vanadium species that are migrated to the surface. The above conclusions are just in contrary to what Balikdjian et al.1 have observed from their XPS results and given in detail in the following. Poor presentation of XPS results and its analysis were the main reasons for this comment article. First of all, the XPS results shown in ref 1 (Figure 4 in ref 1) displays the same binding energy (BE) values (between 520 and 540 eV) on the x axis for different elements (O 1s, V 2p, and Ti 2p core levels). The top panel in the XPS figure shows the O 1s core level at a BE less than 526 eV for a sample calcined at 900 °C under flowing oxygen with a V/Ti molar ratio of 0.06 (indicated as C900-0.06 in ref 1). However, the above O 1s core level does not show any indication of V 2p1/2 core level and the same normally appears in the BE range 522-525 eV.2 In the bottom panel of the same figure shows both V 2p and O 1s features in the BE range 526-536 and 535-540 eV, respectively. In the middle panel, again with same BE range on the x axis, Ti 2p levels are shown. However, Table 2 in ref 1 shows the correct BE values for V and Ti 2p3/2 core levels. The BE value for O 1s level is not mentioned anywhere in the text, and it is clear from the above discussion that the same peak appears at about 526 eV in top panel and at 537 eV in the bottom panel. This inconsistency in Figure 4 in ref 1 might be, at least in part, either due to the charge correction not applied to the spectrum or charge correction applied in a wrong way. From the XPS results, the V/Ti ratio (mentioned in ref 1 as relative integrated surface areas SV/STi) is calculated for all of the samples (V/Ti molar ratio 0.02, 0.03, and 0.06) after heat treatment at 450 and 900 °C.1 Generally, the V/Ti ratio is high for the samples that are heat treated at high temperatures. For instance, the V/Ti ratio increases from 0.07 at 450 to 0.53 at 900 °C heat treated samples with a V/Ti molar ratio of 0.06. It * To whom correspondence should be addressed. Fax: 91-20-589 3761. E-mail:
[email protected].
is concluded from the above points that the phase transformation from anatase to rutile is due to the migration of vanadium to the surface.1 However, it is very clear from the XPS results1 that the V/Ti ratio cannot be of such a high value (0.53) for C900-0.06. This is mainly due to the very different intensities associated with V and Ti. A very poor and good signal-to-noise ratio associated with V and Ti 2p3/2 core levels, respectively, clearly indicates that at least there is an order of magnitude difference in intensity among the two elements. It is strongly suggested here to refer to the XPS results of Ti and V 2p core levels from 5% V2O5 loaded on TiO2 given in ref 2 to get a complete picture of the intensity difference of about an order of magnitude between Ti and V in this material. A simple calculation3 was performed with the following equation:
V/Ti ) (IV2p3/2/ITi2p3/2)(σTi2p/σV2p) where IV2p3/2/ITi2p3/2 is photoelectron count or area ratio of V/Ti assumed to be 0.1 from the above discussion and σ is the photoionization cross section of a given orbital of an element. σ for the Ti and V 2p levels are 0.1746 and 0.2117 Mb, respectively.4 This calculation gives a value of 0.082 for V/Ti atomic ratio on surface. When the ratio of sensitivity factors for V/Ti 1.356 determined from V2O5 and TiO2 is employed,5 it also gives a value of 0.074 for the V/Ti atomic ratio. These values are in close agreement with the bulk value of 0.06 and suggests that the V/Ti atomic ratio on the surface as well as in the bulk is nearly the same and that there is hardly any significant difference in the atomic ratio of V/Ti. Furthermore, even if the IV2p3/2/ITi2p3/2 is assumed to be a “generous” 0.33, it gives a V/Ti atomic ratio to be 0.24 and not 0.53 as claimed in ref 1. Even if σ or the sensitivity factor was not employed in the V/Ti calculation in ref 1, by including the same, it gives a V/Ti atomic ratio of g0.4 for C900-0.06. This simple calculation questions the validity of the conclusion derived that there was a migration of vanadium to the surface under the experimental conditions employed in ref 1. The above discussion clearly suggests that there is hardly any migration of vanadium to the surface, and hence, this cannot be the reason for anatase to rutile phase transformation observed. Anatase is one of the well studied systems in the recent past, and there are a number of reports on its thermal behavior and with different amounts of vanadia loading. In fact, this material is taken as the subject of a joint research program studying vanadia on anatase involving 25 European laboratories under the name “EUROCAT Oxide Group”.6 There is a gradual change in (V/Ti)XPS atomic ratio of anatase materials loaded with 8 wt % of V2O5 (used by the EUROCAT group laboratories and known as EL10V8)7 with heat treatment from its initial (V/Ti)XPS high value (0.44). However, the extent of change in (V/Ti)XPS depends strongly on the calcination temperature and duration of calcination. When the calcination temperature and its duration increases, V/Ti also increases, but it saturates at some point without any more change. However, the (V/Ti)XPS value increases only after prolonged heating and phase transformation also occurs. For instance, calcination of EL10V8 in an oxygen atmosphere at 600 °C yields 3% of the rutile phase
10.1021/jp012289s CCC: $20.00 © 2001 American Chemical Society Published on Web 11/14/2001
12428 J. Phys. Chem. B, Vol. 105, No. 49, 2001 after 48 h, 26% of the rutile after 72 h, and complete transformation to the rutile phase occurs after 288 h.7 Similar results were obtained by Depero et al.8 It is also to be noted here that the low V/Ti ratio for C450-0.06 (0.07)1 than the high value for similar EL10V8 (0.44) and both materials were calcined at 450 °C.7 This directly indicates that the nature and surface composition of V/TiO2 prepared in ref 1 is different from the same material prepared by other groups.2,6-8 The above anatase-rutile phase transformation observed by other groups7-9 also clearly contradicts the incomplete phase transformation even at 900 °C observed with V/Ti molar ratio of 0.06 after an in situ heat treatment in X-ray diffractometer to 900 °C with a heating rate of 10 °C/min in ref 1. It is to be noted here that the crystallite size is dependent on the temperature, duration of heat treatment, and V content and this directly influence the X-ray diffraction. The formation of Magneli phases with defective rutile structures and those that cannot be distinguished experimentally by XRD in the present system also cannot be ruled out.8 At present, it is not known to what extent it influences the XRD and surface properties. From specific surface area and transmission electron microscope measurements on V-loaded anatase materials,1 it is clear that the surface area decreases with heat treatment temperature, and particle size has been found to be elongated between 2500 Å and 2 µm, irrespective of the amount of V loading. This directly indicates that the V influences the structure of anatase to assist to increase in domain size. Additionally, incorporation of V within the anatase bulk framework might exhibit the low surface area of V-loaded anatase due to agglomeration. The above points also suggest that the rutile phase formation is not hindered by more V species at high V loadings, as reported in ref 1. It is explained in this paper that in ref 1 XPS results are presented in a poor form and the surface V/Ti atomic ratio
Comments calculated from XPS measurements are inconsistent with the values reported by Balikdjian et al. Hardly there is any migration of V species to surface on anatase, and the V/Ti surface atomic ratio calculated is in good agreement with the bulk values. The above point clearly hints that the anatase to rutile phase transformation is not assisted by vanadium migration to the surface. It has been observed in the earlier results that vanadium content decreases the threshold temperature of anatase-rutile phase transformation, and this fact is attributed to V incorporation into the bulk of anatase lattice.8 Results observed by Balikdjian et al.1 also indicates the same from XPS and XRD measurements; however, phase transformation temperature increases with vanadium content. These observations can be attributed to the totally different nature and surface composition of V/TiO2 in ref 1 compared to the earlier studies. Acknowledgment. T.R. thanks CSIR, New Delhi for a Research Associate fellowship. References and Notes (1) Balikdjian, J. P.; Davidson, A.; Launay, S.; Eckert, H.; Che, M. J. Phys. Chem. B 2000, 104, 8931. (2) Bond, G. C.; Zurita, J. P.; Flamerz, S. Appl. Catal. 1986, 27, 353. (3) Balasubramanian, S.; Gopinath, C. S.; Subramanian, S.; Balasubramanian, N. Semicond. Sci. Technol. 1994, 9, 1604. (4) Yeh, J. J.; Lindau, I. At. Data Nucl. Data Tables 1985, 32, 1. (5) Nogier, J. P.; Jammul, N.; Delawar, M. J. Electron Spectrosc. Relat. Phenom. 1991, 56, 279. (6) EUROCAT Oxide Group, Catal. Today 1994, 20. (7) Nogier, J. Ph.; De Kersabiec, A. M.; Fraissard, J. Appl. Catal. A: General 1999, 185, 109. (8) Depero, L. E.; Bonzi, P.; Musci, M.; Casale, C. J. Solid State Chem. 1994, 111, 247. (9) Olivieri, G.; Ramis, G.; Busca, G.; Esesibano, V. S. J. Mater. Chem. 1993, 3, 1239.
