Nickel oxide chemisorption

It is well established that sulfur compounds even in low parts per million concentrations in fuel gas are detrimental to MCFCs. The principal sulfur compound that has an adverse effect on cell performance is H2S. A nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Chemisorption on Ni surfaces occurs, which can block active electrochemical sites. The tolerance of MCFCs to sulfur compounds is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, and gas cleanup). Nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Moreover, oxidation of H2S in a combustion reaction, when recycling system is used, causes subsequent reaction with carbonate ions in the electrolyte [1]. Some researchers have tried to overcome this problem with additional device such as sulfur removal reactor. If the anode itself has a high tolerance to sulfur, the additional device is not required, hence, cutting the capital cost for MCFC plant. To enhance the anode performance on sulfur tolerance, ceria coating on anode is proposed. The main reason is that ceria can react with H2S [2,3] to protect Ni anode. 

Butanol, reaction over reduced nickel oxide catalysts, 35 357-359 effect of ammonia, 35 343 effect of hydrogen, 35 345 effect of pyridine, 35 344 effect of sodium, 35 342, 351 effect of temperature, 35 339 over nickel-Kieselguhr, 35 348 over supported nickel catalysts, 35 350 Butanone, hydrogenation of, 25 103 Butene, 33 22, 104-128, 131, 135 adsorption on zinc oxide, 22 42-45 by butyl alcohol dehydration, 41 348 chemisorption, 27 285 dehydrogenation, 27 191 isomerization, 27 124, 31 122-123, 32 305-308, 311-313, 41 187, 188 isomerization of, 22 45, 46 isomers... 

Fig. 7a. Rate of chemisorption of oxygen on nickel oxide at 25°C and at various oxygen partial pressures, according to Engell and Hauffe. The chemisorbed volume in cubic centimeters is plotted against log (< + Zo), where (is the time of chemisorption in minutes, and to a constant.

In studying the chemisorption of hydrogen on carefully reduced nickel the author has actually observed that a minute quantity of the vapor of stop-cock grease or of mercury vapor from a pressure gage appreciably affect the rate of chemisorption in so far as these contaminants reduce considerably the rate of adsorption and produce the effects typical for the so-called activated adsorption. Incomplete reduction of nickel oxide to the metal leads to a similar result. This can be avoided by repeated reduction and subsequent evacuations of the metal sample at 400°C. for a week. A typical result obtained with an exhaustively reduced nickel specimen is shown in Fig. 1. In view of these findings, the activated adsorption of hydrogen on other reduced metal catalysts frequently reported in the earlier literature might have been caused by contamination effects. 

The studies of Garner and his co-workers in the years 1928-1939, which had established the existence of two types of carbon monoxide and hydrogen chemisorption on oxides and which identified irreversible chemisorption with incipient reduction, were followed in the immediate postwar period by an intensive study of the properties of copper oxide (12-15). The work was later extended to nickel oxide (16) and cobalt oxide (17,18). With each of these oxides it was established that carbon monoxide was capable of reacting not only with lattice oxygen, but also with adsorbed oxygen. The concept of irreversible chemisorption involving a carbonate ion and ulti-... 

In general, chemisorption will produce new spectral bands which are not characteristic of the adsorbate or the adsorbent. However, absence of such bands cannot be taken as evidence of an absence of chemisorption. A difficulty present in any attempt to make kinetic measurements is that extinction coefficients are often significantly altered as a result of adsorption. These changes, which cannot as yet be interpreted theoretically, make it difficult to correlate the observed absorbance with the coverage of adsorbed molecules. The change in extinction coefficient is dependent on both the adsorbate and the adsorbent. For example, an increase of e was observed with increasing coverage for ethylene adsorbed on copper oxide, whereas the reverse occurred with nickel oxide . ... 

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... 

Different chemisorbed ionic species may exist on the surface of nickel oxide 0 (ads), 0 (ads), 02-(ads). From a consideration of the enthalpy changes of gas-phase reactions involving oxygen species. Winter (46) concluded that the most likely species first formed on the chemisorption of oxygen is 02 (ads), followed by O (ads). The direct formation of 02-(ads) was shown to be most unlikely and the following results are in agreement with this conclusion. 

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. 

Removal of lattice oxygen from the surface of nickel oxide in vcumo at 250° or incorporation of gallium ions at the same temperature [Eq. (14)] causes the reduction of surface nickel ions into metal atoms. Nucleation of nickel crystallites leaves cationic vacancies in the surface layer of the oxide lattice. The existence of these metal crystallites was demonstrated by magnetic susceptibility measurements (33). Cationic vacancies should thus exist on the surface of all samples prepared in vacuo at 250°. However, since incorporation of lithium ions at 250° creates anionic vacancies, the probability of formation of vacancy pairs (anion and cation) increases and consequently, the number of free cationic vacancies should be low on the surface of lithiated nickel oxides. Carbon monoxide is liable to be adsorbed at room temperature on cationic vacancies and the differences in the chemisorption of this gas are related to the different number of isolated cationic vacancies on the surface of the different samples. 

---