Carbon monoxide on nickel oxide

Mechanisms of the Room Temperature Oxidation of Carbon Monoxide on Nickel Oxide... 

The oxidation of carbon monoxide on nickel oxide has often been investigated (4, 6, 8, 9, II, 16, 17, 21, 22, 26, 27, 29, 32, 33, 36) with attempts to correlate the changes in the apparent activation energy with the modification of the electronic structure of the catalyst. Published results are not in agreement (6,11,21,22,26,27,32,33). Some discrepancies would be caused by the different temperature ranges used (27). However, the preparation and the pretreatments of nickel oxide were, in many cases, different, and consequently the surface structure of the catalysts—i.e., their composition and the nature and concentration of surface defects— were probably different. Therefore, an explanation of the disagreement may be that the surface structure of the semiconducting catalyst (and not only its surface or bulk electronic properties) influences its activity. 

The influence of the surface structure upon the catalytic activity is likely to be particularly important in the case of finely divided nickel oxides, prepared at a moderate temperature, which present catalytic activity for this reaction at room temperature. In a previous work, we studied the room-temperature oxidation of carbon monoxide on nickel oxide prepared by dehydration of the hydroxide under vacuum (p = 10"6 torr) at 200°C., by means of a microcalorimetric technique (8, 20). The object of this work is to re-investigate, by the same method, the mechanism of the same reaction on a nickel oxide prepared at 250°C. [NiO(250)] instead of 200°C. [NiO(200)]. 

Figure 3. Adsorption of carbon monoxide on nickel oxide containing preadsorbed oxygen...

Deren et al. (271) have reported compensation behavior in the oxidation of carbon monoxide on nickel oxide containing various amounts of chromium oxide the effect is attributed to the modification of the Fermi level at... 

The catalytic oxidation of carbon monoxide on nickel oxides prepared at 200 and 250° has been studied at room temperature. First, chemisorption of the reactants (CO, O2) and of the product of the reaction... 

FIG. 11 Infrared spectra of decreasing amounts of carbon monoxide on nickel oxide in the C—O stretching vibrations region. (From Ref. 31 courtesy of Elsevier Science B.V.)... 

The action of carbon tetrachloride or a mixture of chlorine with a hydrocarbon or carbon monoxide on the oxide.—H. N. Warren 9 obtained aluminium chloride by heating the oxide to redness with a mixture of petroleum vapour and hydrogen chloride or chlorine, naphthalene chloride or carbon tetrachloride was also used. The bromide was prepared in a similar manner. E. Demarpay used the vapour of carbon tetrachloride, the chlorides of chromium, titanium, niobium, tantalum, zirconium, cobalt, nickel, tungsten, and molybdenum H. Quantin, a mixture of carbon monoxide and chlorine and W. Heap and E. Newbery, carbonyl chloride. 

Nickel oxide, prepared by dehydration of nickel hydroxide under vacuum at 250°C. [NiO(250)]y presents a greater activity in the room-temperature oxidation of carbon monoxide than nickel oxide prepared according to the same procedure at 200° C. [NiO(200)]> although the electrical properties of both oxides are identical. The reaction mechanism was investigated by a microcalorimetric technique. On NiO(200) the slowest step of the mechanism is CO. i(ads) + CO(ads) + Ni3+ 2 C02(g) + Ni2+, whereas on NiO(250) the rate-determining step is O (0ds) + CO(ads) + Ni3+ - C02(g) + Ni2+. These reaction mechanisms on NiO(200) and NiO(250), which explain the differences in catalytic activity, are correlated with local surface defects whose nature and concentration vary with the nature of the catalyst. 

NiO(250°) contains more metallic nickel than NiO(200°). Magnetic susceptibility measurements have shown that carbon monoxide is adsorbed in part on the metal (33) and infrared absorption spectra have confirmed this result since the intensity of the bands at 2060 cm-i and 1960-1970 cm-1 is greater when carbon monoxide is adsorbed at room temperature on samples of nickel oxide prepared at temperatures higher than 200° and containing therefore more metallic nickel (60). Differences in the adsorption of carbon monoxide on both oxides are not explained entirely, however, by a different metal content in NiO(200°) and NiO(250°). Differences in the surface structures of the oxides are most probably responsible also for the modification of their reactivity toward carbon monoxide. In the surface of NiO(250°), anionic vacancies are formed by the removal of oxygen at 250° and cationic vacancies are created by the migration of nickel atoms to form metal crystallites. Carbon monoxide may be adsorbed in principle on both types of surface vacancies. Adsorption experiments on doped nickel oxides, which are reported in Section VI, B, have shown, however, that anionic vacancies present a very small affinity for carbon monoxide whereas cationic vacancies are very active sites. It appears, therefore, that a modification of the surface defect structure of nickel oxide influences the affinity of the surface for the adsorption of carbon monoxide. The same conclusion has already been proposed in the case of the adsorption of oxygen. 

Uhdeova and Rezl [172] determined oxygen in organic substances by reaction-frontal GC. The method involves pyrolysis of the sample in a stream of helium, conversion of the oxygen-containing products to carbon monoxide on nickel-coated carbon packing at 1050°C, and oxidation of the produced hydrogen and carbon monoxide to water and carbon dioxide on copper (II) oxide at 650°C. The homogenized mixture... 

From the results of other authors should be mentioned the observation of a similar effect, e.g. in the oxidation of olefins on nickel oxide (118), where the retardation of the reaction of 1-butene by cis-2-butene was greater than the effect of 1-butene on the reaction of m-2-butene the ratio of the adsorption coefficients Kcia h/Kwas 1.45. In a study on hydrogenation over C03O4 it was reported (109) that the reactivities of ethylene and propylene were nearly the same (1.17 in favor of propylene), when measured separately, whereas the ratio of adsorption coefficients was 8.4 in favor of ethylene. This led in the competitive arrangement to preferential hydrogenation of ethylene. A similar phenomenon occurs in the catalytic reduction of nitric oxide and sulfur dioxide by carbon monoxide (120a). 

The use of equation (3.2) to study the behaviour of catalysts is known as solid electrolyte potentiometry (SEP). Wagner38 was the first to put forward the idea of using SEP to study catalysts under working conditions. Vayenas and Saltsburg were the first to apply the technique to the fundamental study of a catalytic reaction for the case of the oxidation of sulfur dioxide.39 Since then the technique has been widely used, with particular success in the study of periodic and oscillatory phenomena for such reactions as the oxidation of carbon monoxide on platinum, hydrogen on nickel, ethylene on platinum and propylene oxide on silver. 

Just as in the case of the H2-D2 exchange on ZnO, two mechanisms are also discernible for the carbon monoxide oxidation [stage (b)] on nickel oxide below 300°C. There is a low-temperature mechanism operative between 100° and 180°C. characterized by a low activation energy of 2 kcal./mole and a high-temperature mechanism, above 180°C., with a higher activation energy of 13 kcal./mole. The kinetics are different and are respectively ... 

The method outlined above in the case of zinc oxide will now be applied to the carbon monoxide oxidation on nickel oxide catalysts modified in both ways. If it is assumed, as before, that semiconductivity trends in the bulk and in the surface layer are qualitatively the same, a correlation between semiconduetivity and catalysis will be established if cationic impurities of valences lower and higher than 2 are found to affect the catalytic rate in opposite directions. 

Activation Energies for Carbon Monoxide Oxidation on Nickel Oxide Catalysis... 

Thus, again the limiting step of both Interactions 2 and 4 cannot be the adsorption of carbon monoxide (Equation 3) since the rates of adsorption of CO, without interaction with preadsorbed species, on nickel oxides containing nearly the same quantity of 0"(ads) ions or C03"(ads) ions should be the same for the same coverage of the surface and the rate of production of heat would be the same. 

Catalysts help customers comply cost-effectively with clean-air regulations. Hydrocarbons, carbon monoxide, and nitrogen oxides can be removed using supported precious metal catalysts. Organic sulfur compounds are converted to H2S using nickel/molybdenum or cobalt/molyb-denum on alumina catalysts. Sulfur can be recovered in a Claus process unit. The Claus catalytic converter is the heart of a sulfur recovery plant. 

Oxide catalysts are known to be effective for oxidation reactions. In this study, we wanted to produce carbon monoxide through partial oxidation of the biomass, as this could be expected to lead to a conversion of carbon monoxide into hydrogen via the water-gas shift reaction. An oxidization of the tarry product is also expected. By these two effects, improvement of the efficiency of the gasification is expected. Oxide catalyst is expected to enhance the oxidation reaction needed for this scenario. Since oxide catalyst is considerably cheaper than nickel catalyst, its use would make the whole gasification process more economical. Hence, we decided to examine the effect of oxide catalysts on gasification with partial oxidation using cellulose as a model compound. 

The surface reaction of carbon monoxide and oxygen on nickel oxide 16) and on cobalt oxide 17) was also investigated by the same method of... 

Fig. 9. Apparent activation energies for the oxidation of carbon monoxide on doped nickel oxide. O G. M. Schwab and J. Block [Z. physik. Chem. Frankfurt) [N.S.] 1, 42 (1954)] M. E. Dry and F. S. Stone [Discussions Faraday Soc. 28, 192 (1959)].

At room temperature, carbon monoxide is adsorbed on nickel oxide [NiO(200°)J to a larger extent than oxygen. At a maximum pressure of 30 torr, 14.08 cm of carbon monoxide is adsorbed per gram of oxide (0 = 0.24) compared to 3.67 cm of oxygen (0 = 0.12) (23). During the adsorption, there is no modification of the electrical conductivity of the... 

The spectrum of carbon monoxide adsorbed on nickel oxide prepared at 200° may be divided into two regions (Table I, la) (60). The first includes two bands at 2060 and 1960-1970 cm i, the second three bands at 1620, 1575, and 1420-1440 cm-i. Bands at 2060 and 1960-1970 cm- are typical of carbonyl structures and are found in the spectrum of carbon monoxide on metallic nickel (61). It has been suggested by some authors (62) that, in our experiments, these bands were also produced by the adsorption on the metal, the oxide being supposed oxygen-deficient. Chemical analyses (30) have shown, however, that, NiO(200°) contains an excess of oxygen and magnetic susceptibility measurements (33) have demonstrated that the quantity of metal is very small. Since the intensity of these bands is strong, we believe that they are not produced exclusively by the chemisorption of carbon monoxide on the metal but mainly by the adsorption on exposed nickel ions. 

The reactivity of carbon dioxide toward oxygen was also studied (25, 66). First, carbon dioxide was adsorbed on nickel oxide containing preadsorbed oxygen. The black color and the high electrical conductivity of the sample remain unaltered (25). However, a reaction does occur since a subsequent adsorption of carbon monoxide produces a desorption of carbon dioxide, while adsorption of carbon monoxide is impossible on a sample precovered by carbon dioxide (25). It is believed that... 

Fio. 17. Differential heats for the successive adsorptions at 30° of oxygen and carbon monoxide on hydrated nickel oxide. 

---