Coke oxidation nickel effect

In order to improve the resistance of Ni/Al203-based catalysts to sintering and coke formation, some workers have proposed the use of cerium compounds [36]. Ceria, a stable fluorite-type oxide, has been studied for various reactions due to its redox properties [37]. Zhu and Flytzani-Stephanopoulos [38] studied Ni/ceria catalysts for the POX of methane, finding that the presence of ceria, coupled with a high nickel dispersion, allows more stability and resistance to coke deposition. The synergistic effect of the highly dispersed nickel/ceria system is attributed to the facile transfer of oxygen from ceria to the nickel interface with oxidation of any carbon species produced from methane dissociation on nickel. 

Quantitative Estimates of Density Variations. Quantitative estimates of the relative contribution of Factors 1-5 to changes in catalyst density are given in Table IX. In carrying out these calculations, the average skeletal density of coke on catalyst was taken to be 1.2 g/cc (Appendix A). Nickel, vanadium, and rare earth were assumed to be present as the oxides NiO, V20A, and RE203 with densities of 6.7, 4.3, and 6.9, respectively. Similar assumptions were made for iron and titanium (Appendix A), but the effect of Fe and Ti was included only for the heavier Fractions E, F, and H, which exhibited increased levels of these two metals (Table III). For the float Fractions A-F, this approach, based on bulk oxide densities, is expected to overestimate the density increase due to metals that are present as well-dispersed species. 

The primary routes of potential human exposure to coke oven emissions are inhalation and dermal contact. Occupational exposure to coke oven emissions may occur for those workers in the aluminum, steel, graphite, electrical, and construction industries. Coke oven emissions can have a deleterious effect on human health. Coke oven emissions contain literally several thousand compounds, several of which are known carcinogens and/or cocarcinogens including polycyclic organic matter from coal tar pitch volatiles, jS-naphthylamine, benzene, arsenic, beryllium, cadmium, chromate, lead, nickel subsulfide, nitric oxide, and sulfur dioxide. Most regulatory attention has been paid to coal tar pitch volatiles. 

Studies of catalysts deactivation by coke are abundant in the literature most of them are usually conducted at high temperatures (around 500°C) using metal catalysts supported on oxides with low surface area such as silica, aluminas or silica-alumina [2 and references therein]. The deactivation by coke of zeolite catalysts has also been studied and such studies have mostly been done for high temperature reactions such as the conversion of n-hexane or the isomerization of xylenes [2,4]. However, low temperature coke formation (20-25°C) combining the effect of high acidity and size specificity for a high coking component such as nickel, has not yet been considered from the point of view of the presence of compounded effects of crystalline structure and location of metal particles. 

In the partial oxidation of CH4 to synthesis gas, coke formation over the catalyst frequently takes place, resulting in catalyst deactivation. Claridge et al. [4] observed that the relative rate of coke formation follows the order Ni>Pd Rh, Ru, Pt, Ir. Nickel catalysts are highly effective for partial oxidation of CH4 to synthesis gas, but they are unsatisfactory with respect to coke formation [5]. From the industrial view point, Ni is preferable as the active species compared to expensive precious metal such as Rh, Pd or Ru. High dispersion of metal species over catalyst may reduce coke formation [6]. Nickel-supported catalysts are conventionally... 

The ESC deposit contained appreciable levels of potential catalytic elements. These appeared to have promoted gasification, as the oxidation of the ESC coke was nearly five orders of magnitude greater than that of effectively pure graphite (SP-1), with an ash content of 1 ppm (Table 3). Such a wide discrepancy would be less likely to be attributable to structural or textural differences. Also, metallic nickel dispersed in the SP-1 graphite at concentrations of 303 ppm and 1.4 wt.% increased its gasification rate by factors of 10 and 4 x 10 respectively. 

In marked contrast to the effects observed with iron, vanadium and copper oxides, prior impregnation of silica-alumina with nickel oxide led to a considerable increase in the initial rate of subsequent coke gasification, up to a coke conversion level of a>30% (Figure 2). 

Kiylov et aZ. studied the catalytic reaction of methane with carbon dioxide over MnO -containing catalysts for syngas production. The effect of the modification of nickel by different additives, such as copper, chromium, iron and manganese oxides, was evaluated and the best effect was obtained with manganese oxide an increase of manganese content originated an increase of the carbon dioxide conversion and a decrease of coke formation. 

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