Nickelous oxide, desorption

Truong CM, Wu M-C, Goodman DW (1993) Adsorption of formaldehyde on nickel oxide studied by thermal programmed desorption and high-resolution electron energy loss spectroscopy. J Am Chem Soc 115 3647... 

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. 

Evacuation of the oxygen atmosphere down to 10- torr does not produce a desorption (24) and no thermal effect is registered (68). COa fads) is therefore a stable species at room temperature, in the presence of oxygen or under vacuum. Its spontaneous decomposition to yield carbon dioxide is not possible at room temperature but is observed at 200° (24). A subsequent adsorption of carbon monoxide is possible on the samples of NiO(200°) and NiO(250°) (Table V) and, in both cases, carbon dioxide is then found in the cold trap. This adsorption of carbon monoxide decreases the electrical conductivity of the samples which, however, remains higher than the conductivity of the initial nickel oxide [7 X 10-i< ohm-i cm-i for NiO(200°) 1.6 x lO- o ohm-i cm-i for NiO(250°)] (25, 41). It was concluded from these experiments that a fraction of C03 (ads) ions is decomposed at room temperature by carbon monoxide and that the interaction product is carbon dioxide, which is, at least partially, desorbed to the gas phase (0.5 cm /gm) (25). 

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... 

Reduction of the oxide begins with some difficulty, in the absence of metal nuclei, and this accounts for the slow exothermic phenomenon whose intensity is maximum at the beginning of reduction and which results probably in the formation of metal nuclei on the oxide surface. Since the intensity of the fast exothermic phenomenon increases when the extent of reduction is larger, it must be related to a reduction process now occurring at the metal-oxide interface, carbon monoxide being adsorbed on metal crystallites. All carbon monoxide in dose G is adsorbed on the metal and reacts with nickel oxide at the metal-oxide interface since the slow exothermic phenomenon does not appear on curve G (Fig. 34). Calorimetric curves similar to curve G are obtained during the reaction of subsequent doses of carbon monoxide. Finally, it appears from curves B to G (Fig. 34) that desorption of carbon dioxide is a slower process than the adsorption of carbon monoxide and its interaction with nickel oxide. 

Fig. 2. Temperature-programmed desorption (TPD) spectra from 4.0 L of 2-C3H7I adsorbed on Ni(lOO) surfaces predosed with various amounts of oxygen. Three regimes are observed for this system (1) that for the clean nickel, where only the hytbogenation-dehydrogenation steps typical of transition metals are seen (left) (2) that for nickel oxide, where there is little reactivity, and where only complete oxidation is observed (right) and (3) that for an intermediate oxygen surface coverage, where some partial oxidation is manifested by the appearance of a TPD peak for acetone around 350 K (center).

Direct study of the desorption of stearic acid from platinum and from NiO was carried out by Timmons and Zisman [10]. Their findings are shown in Table 10-5. The methylene iodide contact angle and the surface potential measurements indicate that a stearic acid monolayer adsorbed on platinum can be removed completely by heating to 130 C or by extraction with diethyl ether. But if the adsorbent is nickel oxide, heating to 150 C or extraction with diethyl ether fails to restore the original contact angle behavior or the surface potential of the adsorbent surface. 

Therefore, if in the N2O decomposition, the rate-determining step is assumed to be the desorption of oxygen, we would expect that, of the three oxides, the best catalyst would be ferric oxide, followed by chromic oxide, and then nickel oxide. However, just the opposite is found (2). Of the three... 

Therefore, it seems that the total amount of oxygen desorbed (24gmol/g) during the TPD (20% considering the formation of 0203), is more representative of a desorption than a bulk decomposition of the support. This total amount of O2 desorbed can be compared with the chemisorption data on TWC without nickel oxide [4],... 

Regarding samples 2 to 5, the spectrum in Figure 6.29 shows simple desorption of sulfur dioxide, the mass ratio keeping a constant value of 0.4. The interaction between nickel oxide and sulfur dioxide therefore comes down to a simple adsorption process. The results we obtained for sample 1 (see Figure 6.30) are much more interesting if we consider the appearanee of new species on the spectrum beyond 600°C. 

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

Budde et have recently observed the ultraviolet laser-induced desorption of NO from oxidized Ni(lOO). The 193 nm excitation wavelength used was resonant with gas phase NO transitions to a predissociative upper state. Desorption yields of NO from clean Ni(lOO) were essentially zero. Comparison of TPD results from clean and oxidized nickel surfaces indicated that an oxidized nickel surface could support a weakly bound NO state not found on clean Ni(100). 

Each of the various processes of adsorption may have desorptions of the reverse forms, for example, dissociative adsorption may have as its reverse, associative desorption. However, the process of chemisorption may not be reversible [ 1.2.2(c)]. Desorption may lead to species other than that adsorbed, for example, ethane dissociatively adsorbed on clean nickel gives little or no ethane upon desorption, 1-butene dissociatively adsorbed to methylallyl and H on zinc oxide gives mainly 2-butenes upon desorption, and some W03 may evaporate from tungsten covered with adsorbed oxygen. 

Foley and Ayuso (2008) suggest that typical processes that could explain the release of arsenic from minerals in bedrock include oxidation of arsenian pyrite or arsenopyrite, or carbonation of As-sulfides, and these in general rely on discrete minerals or on a fairly limited series of minerals. In contrast, in the Penobscot Formation and other metasedimentary rocks of coastal Maine, oxidation of arsenic-bearing iron—cobalt— nickel-sulfide minerals, dissolution (by reduction) of arsenic-bearing secondary arsenic and iron hydroxide and sulfate minerals, carbonation and/or oxidation of As-sulfide minerals, and desorption of arsenic from Fe-hydroxide mineral surfaces are all thought to be implicated. All of these processes contribute to the occurrence of arsenic in groundwaters in coastal Maine, as a result of the variability in composition and overlap in stability of the arsenic source minerals. Also, Lipfert et al. (2007) concluded that as sea level rose, environmental conditions favored reduction of bedrock minerals, and that under the current anaerobic conditions in the bedrock, bacteria reduction of the Fe-and Mn-oxyhydroxides are implicated with arsenic releases. 

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