Nickel hydroxide, dehydration

Formed [Ni(OH)2] becomes entrapped in the deposit to a certain extent During the procedure of powder drying at 120°C, reaction (5.33) can take place. The transition temperature at which nickel hydroxide dehydrates to form the oxide is ca. 75°C [Eq. (5.33)] [157]. Thus, the most possible way for the NiO formation is dehydration of entrapped [Ni(OH)2] during the procedure of powder drying. In such a case smaller amount of the NiO phase than the amounts of other phases (M0O3 and M0NL4) should be expected in the deposit, as it was the case in the TEM analysis [123]. 

The mixed chromium and nickel (5% and 10% Ni atomic) catalysts were prepared by dehydration of mixed chromium and nickel hydroxides prepared by adding an ammonia solution to a solution of chromium and nickel nitrate in order to maintain a pH = 7 1. The final pH was equal to 7.S. The hydroxyde treatment was the same as the one already described for chromium oxide. 

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. 

In this work, nickel oxides are prepared by dehydration under vacuum (10 torr) at moderate temperatures (200-300 ) of a pure nickel hydroxide. The hydroxide itself is prepared by the steam distillation of a solution of reagent grade Ni(NOs)2 in an excess of aqueous ammonia (22). As has been shown (22), this method yields Ni(OH)2 containing less than 0.08% of NHs and N2O6. When dried at about 60°, the product has the composition NiO, 1.05 H2O and its BET surface area amounts to 34 m /gm. The external aspect is that of a fine crystalline powder and not of a gel. There are no small-diameter pores in the hydroxide particles since the adsorption-desorption isotherm of nitrogen at —195° does not present an hysteresis loop. The X-ray diagram is that of a well-crystalized nickel hydroxide. 

When the pressure over nickel hydroxide is reduced to 10- torr, dehydration begins at 210° but a black nickel oxide is formed which therefore contains a stoichiometric excess of oxygen (23). If the residual pressure is decreased to 10 torr, dehydration proceeds at a measurable rate at 200°. The color of the oxide is then yellowish green. When the decomposition of the hydroxide is carried out to the point of a constant weight, the composition of the product is NiO, 0.16 H2O (24). 

Since dehydration produces a change of color from light green for the hydroxide to yellowish green for the oxide, it has been possible to observe that decomposition does not take place simultaneously at the surface of all the particles in a thick layer ( 10 mm) of nickel hydroxide. On the contrary, a well-defined reaction interface moves downward in the powdered sample at a constant rate. It has been ascertained that the reaction rate is then proportional to the interface area and that the apparent reaction order for the dehydration process changes if the shape of the pan containing the reactants is modified (23). However, the... 

In these experiments, nickel hydroxide was mixed in a proportion of 12.4% with finely divided silica (Cabosil), pressed in a die and dehydrated at 200°, under vacuum, in the infrared analysis cell. Composition of the sample was therefore different from the composition of the samples used in the gravimetric or calorimetric work [NiO(200°)] and possible effects of the support cannot be, a priori, completely excluded. Calorimetric experiments with the supported samples have shown, however, that their reactivity toward CO is very similar to the reactivity ofNiO(200°). 

It is of interest to determine the reasons for the different behavior of NiO(200°) and NiO(250°). The water content of the solids differs NiO(200°), which is prepared at a lower temperature, retains more water (0.16 H20/mole) than NiO(250°) (0.11 H20/mole). Although it has been shown that the decomposition of nickel hydroxide is a topo-chemical reaction (23) and although the residual hydroxide should be located in the interior of the particles, water molecules may remain adsorbed on the surface of the newly formed oxide phase. Moreover, since dehydration produces a large increase of surface area (from 34 to 130 m /gm), fragmentation of oxide particles is likely and, thence, hydroxyl groups may also remain on the exposed surface. For these reasons, participation of adsorbed water or surface hydroxyl groups in adsorptions and interactions is not, a priori, precluded. 

Mixtures of lithium and nickel hydroxides were prepared by impregnating nickel hydroxide with solutions of lithia in distilled water. Mixed gallium and nickel hydroxides were coprecipitated, by steam distillation, from solutions of Ni(OH)a and Ga203, 1.75 H2O in aqueous ammonia. These mixtures of hydroxides were dehydrated at 250° under vacuum (p = 10 torr). Preliminary experiments (40) have shown that incorporation of foreign ions does not occur at temperatures lower than 250° and that, in order to obtain a constant value of the electrical properties of doped samples, it is necessary to heat them at least 24 hours at 250° in vacuo. Incorporation is therefore a slow process at 250°. Dehydration is not complete at 250° NiO-f 10 at. % Li... 

Indoxyl Fusion.2—A mixture of 15 g. of sodium hydroxide and 20 g. of potassium hydroxide is fused and carefully dehydrated by heating to about 500° in a nickel crucible. When the mass has barely solidified it is just remelted by gentle heating and poured into a Jena glass conical flask (capacity 100 c.c.) which is at a temperature of 220° in an oil bath. If this procedure is adopted there need be no fear that the glass will crack. 

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)]. 

A conventional wastewater treatment system with an average flow rate of 160,000 gpd produces effluent suitable for NPDES discharge. Metal hydroxide sludges are dewatered in a 15 cu. ft filter press producing more than one half ton of filter cake per day. The filter cake is further dewatered in a 7 cu. ft, batch-type sludge dryer. Based upon recommendations by their consultant, the firm also uses the sludge dryer to dehydrate nickel strip solutions. Two reverse osmosis systems are used for partial nickel recovery. Trivalent chromium is recovered by drag-out control and evaporation. 

Nickel oxides prepared by dehydration of the hydroxide, under vacuum (p 10 torr), at temperatures higher than 200°, contain less residual water and present a larger surface area than NiO(200°). The surface area of an oxide prepared at 250° [NiO(250°)], for instance, is 156m /gm and its composition is NiO, 0.11 H2O compared to 142 m /gm and NiO, 0.16 H20 for NiO(200°) prepared from the same batch of hydroxide. For NiO(300°), the corresponding results are... 

The charge and discharge cycles of nickel batteries involve two different pairs of solid phases. Oxidation of p-Ni(OH)2 produces p-NiOOH, oxidation of a-Ni(OH)2 produces y-NiOOH. The end-products of these cycles are interconnected by dehydration and overcharge. For the crystallographic properties of Ni(II) hydroxides and Ni(Ill, IV) hydroxides see Section V.3.2.2.1 and Sections V.3.2.3.1, V.3.2.3.2, respectively. 

A study based on in situ/ex situ FTIR and Al MAS NMR and pyridine (Py) as a probe material in nickel-loaded herrlandite crystals revealed that the Lewis acid sites can be attributed prirrrarily to Ni ions, whereas the Brorrsted ones can probably be associated with surface-supported hydroxide phases [OlGl]. The distorted A1 tetrahedra were formed drrring the dehydration process arrd Py cherrrisorption/complexation, whereas the crystal stmcture was preserved in the rehydrated sartqrle. 

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