Cobalt oxide deactivation

The catalytic deep oxidation of methane is initiated at 250 °C and completed at 500-530 °C. It is worth noting that no methane was converted to CO2 over non-coated monoliths up a temperature of 650 °C. The performed isothermal catalytic deep oxidation at 500 °C shows a slow deactivation during the first 50 hours of time-on-stream as demonstrated in Fig. 4b. Beyond this time, the catalyst retains 80 % of its initial activity for an additional 50 hours on stream. The investigation of the deactivation process above 600 °C, not shown, indicates a strong deactivation, which was attributed to the decomposition of the spinel. The surface analysis [17] and the observed pronounced reforming activity after deactivation [21] provide evidence that deep reduction is the major cause of cobalt oxide deactivation above 600 °C. 

The oxidation of cobalt metal to inactive cobalt oxide by product water has long been postulated to be a major cause of deactivation of supported cobalt FTS catalysts.6 10 Recent work has shown that the oxidation of cobalt metal to the inactive cobalt oxide phase can be prevented by the correct tailoring of the ratio Ph2cJPh2 and the cobalt crystallite size.11 Using a combination of model systems, industrial catalyst, and thermodynamic calculations, it was concluded that Co crystallites > 6 nm will not undergo any oxidation during realistic FTS, i.e., Pi[,()/I)i,2 = 1-1.5.11-14 Deactivation may also result from the formation of inactive cobalt support compounds (e.g., aluminate). Cobalt aluminate formation, which likely proceeds via the reaction of CoO with the support, is thermodynamically favorable but kinetically restricted under typical FTS conditions.6... 

Li et al.22 investigated the effect of water for a platinum-promoted Co/y-Al203 catalyst during Fischer-Tropsch synthesis in a CSTR-type reactor. The catalyst lost activity in the presence of water, and it was found that small quantities of water (3-25 vol%) led to mild and reversible deactivation, whereas large amounts of water (>28 vol%) deactivated the catalyst more severely and permanently. The deactivation was attributed to the formation of cobalt oxide or cobalt aluminate. 

However cobalt oxide does have some drawbacks. Lower ammonia conversion efficiencies have been reported - as low as 88% to 92% in a high pressure plant compared with a typical value between 94% and 95% for Pt-Rh gauzes. The optimum operating temperature is 70 to 80°C lower than for Pt-Rh gauzes, and this could result in difficulties with the steam balance in a revamped plant. Cobalt oxide catalysts also suffer from reversible deactivation due to the reduction ofCo304 to CoO in the upper parts of the catalyst bed222. 

A gold-based material has been formulated for use as a three-way catalyst in gasoline and diesel applications.28 This catalyst, developed at Anglo American Research Laboratories in South Africa, consisted of 1% Au supported on zirconia-stabilized-Ce02, ZrC>2 and TiC>2, and contained 1% CoOx, 0.1% Rh, 2% ZnO, and 2% BaO as promoters. The catalytically active gold-cobalt oxide clusters were 40-140 nm in size. This catalyst was tested under conditions that simulated the exhaust gases of gasoline and diesel automobiles and survived 773 K for 157 h, with some deactivation (see Section 11.2.7). 

One example is a catalyst consisting of gold on cobalt oxide particles supported on a mechanical mixture of zirconia-stabilised ceria, zirconia and titania, that survived 773 K for 157 h, with some deactivation [145]. Grisel and Nieuwenhuys [38,127] have shown that the addition of transition metal oxides to form Au/MO j/ADOs catalysts, massively suppresses Au particle sintering in methane oxidation tests up to 973 K. Also, Seker and Gulari [30] found that Au/ADOs catalysts survive rigorous pre-treatments of 873 K in air for 24 h, followed by several cycles of 423-773 K and they were then kept... 

Products of incomplete combustion have been shown to increase as the catalyst deactivates. Agarwal et al. report that the oxidation of a mixed stream of trichloroethylene and C5-C9 hydrocarbons over a chromia alumina catalyst produced CO equal to 32% of the total CO + CO2 with fresh catalyst. With a deactivated catalyst, CO had risen to 54% of the total carbon oxides produced. Pope et al. report products of incomplete combustion for the oxidation of 1,1,1-trichloroethane over a cobalt oxide catalyst. The cause of the catalyst deactivation has not been established, but both Agarwal et al. and Michalowiczl reference evidence of carbonaceous deposits on the catalyst after oxidation of halogenated hydrocarbons. ESCA studies by Hucknall et al. O have always shown a carbon residue on palladium alumina catalysts in addition to adsorbed halogen. 

Co catalysts in a comparable loading range were prepared with variable cobalt oxide-support interactions. Up to 20 molar % co-fed H2O, a positive effect of co-fed water was found for cobalt supported on a weakly interacting support, silica, which included an increase in CO conversion, a decrease in CH4 selectivity, and a corresponding improvement in C5+ production. At the same time, the CO2 selectivity remained low. At 25% H2O addition, the deactivation rate of the catalyst accelerated by a factor of four. 

According to van der Vaail et al. (1990), gases containing chlorinated compounds have been successfully oxidized over metal oxide catalysts such as chromia/alumina, cobalt oxide, and copper oxide/manganese oxide while sulfur-containing VOCs can be oxidized by the use of platinum-based catalysis, which are rapidly deactivated by chlorides. 

Some studies of potential commercial significance have been made. For instance, deposition of catalyst some distance away from the pore mouth extends the catalyst s hfe when pore mouth deactivation occui s. Oxidation of CO in automobile exhausts is sensitive to the catalyst profile. For oxidation of propane the activity is eggshell > uniform > egg white. Nonuniform distributions have been found superior for hydrodemetaUation of petroleum and hydrodesulfuriza-tion with molybdenum and cobalt sulfides. Whether any commercial processes with programmed pore distribution of catalysts are actually in use is not mentioned in the recent extensive review of GavriUidis et al. (in Becker and Pereira, eds., Computer-Aided Design of Catalysts, Dekker, 1993, pp. 137-198), with the exception of monohthic automobile exhaust cleanup where the catalyst may be deposited some distance from the mouth of the pore and where perhaps a 25-percent longer life thereby may be attained. 

In the absence of bromide ion the p-xylene undergoes rapid autoxidation to p-toluic acid but oxidation of the second methyl group is difficult, due to deactivation by the electron-withdrawing carboxyl group, and proceeds only in low yield at elevated temperatures. Although bromide-free processes were subsequently developed (ref. 5) they require the use of much higher amounts of cobalt catalyst and have not achieved the same importance as the Amoco-MC process. Indeed, the... 

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