Calcined catalysts cobalt oxide species

The present research showed a dependence of various ratios of rutile anatase in titania as a catalyst support for Co/Ti02 on characteristics, especially the reduction behaviors of this catalyst. The study revealed that the presence of 19% rutile phase in titania for CoATi02 (C0/RI9) exhibited the highest number of reduced Co metal surface atoms which is related the number of active sites present. It appeared that the increase in the number of active sites was due to two reasons i) the presence of ratile phase in titania can fadlitrate the reduction process of cobalt oxide species into reduced cobalt metal, and ii) the presence of rutile phase resulted in a larger number of reduced cobalt metal surface atoms. No phase transformation of the supports further occurred during calcination of catalyst samples. However, if the ratios of rutile anatase were over 19%, the number of active sites dramatically decreased. 

The presence of two peaks in the TPR profile for the Co/Ti02 system suggested that two cobalt oxide species were present on the calcined catalyst. Using XPS, these species were identified as C03O4 and Co+. The Co V /2 xpS spectrum of the calcined Co/Ti02 catalyst had a satellite peak due to a Co+ ... 

X-ray diffraction patterns of all catalysts confirm presence of C03O4 [23]. Apart from the peaks indicative of C03O4 and the various supports, the supported catalysts did not show any other peaks. Accordingly, C03O4 is the dominant crystalline cobalt species after calcination. The average cobalt oxide particle sizes are included in Table 3. 

In a previous XPS study of NiHZSM-5 catalysts, Badrinarayanan et al. (3) concluded that nickel was present always as Ni, even after exposure to harsh reducing conditions. As previously revealed, cobalt exchanged within the zeolite pores was in the +2 oxidation state. The stable oxidation state of nickel is +2. Thus, it seems logical that if nickel occupied zeolite framework sites, it would be present predominantly in the +2 oxidation state. Indeed, the Ni/(A1-Na) atomic ratio measured by ICP-AES after the initial calcination (500°C in air) was 1.1 (Table I), suggesting that nickel probably occupied interstitial sites. However, nickel was reduced completely in hydrogen, implying that the nickel probed by XPS was a surface species. This is substantiated by data in Table II after in situ oxidation, the surface Ni/Si ratio was 6 times higher than that measured for the bulk, i.e. 13 x lO " vs. 2.2 x 10 ... 

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