Reduction of NiO

To investigate the electrode activation procedure. X-ray photoelectron spectroscopy (XPS) is typically used. A controlled transfer from the solution to the ultrahigh vacuum chamber has to be realized with an appropriate setup to prevent the surface 

The nickel catalyst surface is reduced after 4.5 h at a current density of 5 mAm. The electrodes are not expected to be reduced after a short activation time because the catalysts are still significantly covered by the PTFE film, observed with XPS and specific surface area measurements. Perforations in the PTFE film are sufficient to allow contact between the electrolyte and the nickel catalyst and therefore to reduce the nickel oxide. During the activation process, the perforations in the PTFE film are increased or a new contact between the electrolyte and the metal catalyst is formed. The contact area may be created by the hydrogen that is formed on the nickel surface and that separates the PTFE film from the catalyst surface. Therefore, the initial nickel signal of the activated electrode (observed by XPS) is increased. The specific surface area increases without the complete removal of the PTFE film. 

From the specific surface area measurements, it can be assumed that a large area of the catalyst is stiU covered by the PTFE film after short activation times. This film obstmcts the contact area for the gas and also the access of the electrolyte to the catalyst In conclusion, the activation process commences at the surface of the electrode, and the activation zone increases in depth with increasing activation time. Two effects cause this behavior. The electrolyte penetration into the electrode is hindered by the hydrophobic character of PTFE. With increasing PTFE damage and time, the electrolyte and the activation zone penetrate further into the electrode. The perforation in the PTFE film can enhance the electrolyte penetration into the electrode. 

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Another possibility is that one of the reactants is particularly mobile, this is apparent in certain solid—gas reactions, such as the reduction of NiO with hydrogen, which is a well-characterized nucleation and growth process [30,1166]. Attempts have been made to use the kinetic equations developed for interface reactions to elucidate the mechanisms of reactions between the crystalline components of rocks under conditions of natural metamorphism [1167,1168]. 

Effect of volume fraction of Ni particles on the fracture strength of Ni-Al203 nanocomposites, produced by the reduction of NiO particles ( ), and the reduction of Ni-nitrate ( ). Reproduced from Sekino ef a/.,12 with permission from the Journal of the American Ceramic Society. 

The detailed theoretical treatment of these rate equations, as well as others for the second case discussed below, was reviewed extensively by Harrison [3]. Examples of recent studies showing this type of behaviour are the reduction of NiO with H2 [36] and the reduction of Mo03, Mo03— A1203 and Co3 04 —Mo03— Al2 03, which are commonly used as hydrosulfurization catalysts [37]. In both these cases, reduction kinetics were fitted to eqn. (3). 

For the reduction of NiO by CO in the temperature range 566—796°C, the kinetics were determined by the initial rate method and the rate expression was found to be of the first order with respect to CO [14]. 

Figure 5. Results of TGA tests. Weight of reduced Ni is normalized to be 10 mg. Atomic ratio, Nc/NNi> shows the value at 100 min after the reduction of NiO.

IC reactions can be of two types. The nucleation of the new phase (e.g. Ni produced by the reduction of NiO) may be rate determining. In that case, separate nuclei of the new phase can be detected (Fig. 1(a)). These are called nucleation-controlled interface or NCI reactions. In other cases, all the surface of the initial solid reacts, and a continuous interface entirely covers the solid reactant. With respect to kinetics, the interfacial process entirely controls the reaction in this last case. This is an ICI reaction (Fig. 1(b)). 

The conclusion that nucleation limited the rate of activation by reduction of NiO/Si02 precursor was simultaneously proposed following the above deductions [79] and by Cocnen, using an impressive series of converging arguments [80],... 

Figure 13. Influence of calcination temperature on the reduction of NiO/Si02 at 598K by pure H2 [81] SiOz 334 m2g-, 18 9% wt Ni, calcined at 773 K, 5 h, to 2s is the time necessary to reduce 25% of the NiO, the degree of reduction obtained after 40 min is indicated on the nght-hand axis...

Figure 20. Additional reduction of NiO due to copper in NiO/ Si02 catalysts. Samples of different NiO loadings (wt% of total sample, upper scale or wt in gram in a 4gNi0/Si02 sample, lower scale) prepared on the same silica and with the same procedure as the samples used for Figs 13 and 19. Copper was introduced as explained in the legend of Fig. 19, in the proportion Cu Ni = 5.3 wt%. Reduction by pure H2 (100 kPa) at 598 K. The weight of NiO reduced corresponds to amax after 40 min. The double arrow indicates the magnitude of the additional reduction due to copper [81].

One form of nickel is an important catalyst known as Raney nickel that is prepared by the reduction of NiO with hydrogen. Nickel also is used in several alloys that have wide application. For example, Monel is a type of alloy that contains nickel and copper in a ratio of about 2 1. It is frequently used in making bathroom fixtures. 

For XQ > 10 5, 0 decreases steeply and tends towards zero as reactivity increases. This reactivity consists first of dissolution reactions (e.g. Cu/NiO, Cu/Fe-tO, then of reactions to form new phases at the interface (e.g., the Ti/MgO and Zr/MgO systems, which form Ti and Zr oxides). In the Sn/NiO and Sn/CoO systems studied in high vacuum, the reduction of NiO and CoO by liquid Sn is possible and as dissolved oxygen is eliminated continuously by pumping the amount of dissolved Ni or Co produced by this reaction can become greater than that... 

Regarding the reduction profiles i) the TPR of Ni-Ti shows a narrow sharp peak at around 623 K, caused by the reduction of NiO, and a small shoulder at 773 K that is caused by the reduction of NiTiOs intermetallic species, ii) the TPR of Ni-Al shows two peaks, the first... 

The temperature programmed reduction (TPR) of NiST proceeds in two steps with maxima at around 700 and 815 K (Fig. 2). The observed TPR peaks are indicative for the presence of Ni with different reactivity for hydrogen toward the zero-valent state. Since the non-crystalline silica-alumina phase gave a much poorly resolved TPR profile (Fig. 2), the peaks should be related to the crystal structure of NiST. The nickel cations placed inside the octahedral sheets being coordinated with sbc framework oxygen atoms should be more stable than Ni exposed at the edges of the clay platelets. Consequently, the first reduction step can most likely be attributed to the reduction of the N cations located at those positions. Since the reduction of NiO takes place at approximately 570 K, these data show... 

The TPR profiles of calcined LDHs precursors show two peaks of H2 consumption (Fig.l). The first peak around 570 K corresponds to the release of NO3 anions as NO2, and their subsequent reduction to NO and N2O as identified by mass spectrometry [9]. The second peak with maxima at 705,920 and 1000 K for HA, HC and HG samples respectively, corresponds to the reduction of NiO particles. These experiments show that the reducibility of the nickel oxide particles decreases when the Mg content increases. This could be compared with the decrease of the Ni crystal size measured by XRD in HC and the lack of detection of these particles in HG samples. This behaviour has been attributed to the formation of excess Ni aluminate and Ni spinel type phases decreasing the size of the mixed oxide particles and hindering their reducibility [6]. 

Research on supported Ni catalysts, used for steam reforming and other applications " , has dealt with factors affecting their activity and stability. Catalyst formulation and the extent to which interaction occurs between NiO and the support are important factors influencing the reduction of NiO to Ni in the catalyst and the catalysts subsequent behavior. The influence of the support on the metal is illustrated by NiO on AI2O3 or MgO. It is well known that NiO deposited on oxide supports is less readily reduced than bulk NiO. Furthermore, growth of crystallites of the metal oxide can be retarded by a suitable support. For instance, the presence of MgO retards the growth of NiO. When NiO is calcined at 500°C for 4 h, NiO crystallites increase to about 30 nm, whereas in a NiO/40% MgO solid solution, the crystallites grow to only 8 nm (Fig. 1). ... 

The modified samples have a higher catalytic acid activity in the isomerization of m-xylene and the disproportionation of toluene as compared to pure SAPO-5. The selectivity is close to that of SAPO-5 containing no Nl. Reduction of NiO-contalnlng samples formation of metallic nickel possessing activity in a hydrogen stream. 

In addition, equilibrium constants for the reduction of NiO with H2 and CO are reported in [26PEA/COO], [73RAU/GUE] and [33WAT], [38BOG], [42FRI/WEI], [64ALC/BEL], [67ANTAVAR], respectively. The equilibrium constants for the reactions ... 

Colombier measured the potential of NiSO4(0.5 M) Ni against a calomel reference electrode at 20°C. When the electrolyte was appropriately degassed and kept air-free the same equilibrium potentials were found for massive or powdered nickel, the latter having been electrolytically deposited or prepared by hydrogen reduction of NiO. As the author presented no details about the experimental data, the reference potential selected, how the liquid junction potential and the activity coefficients were accounted for, the final result, °(Ni Ni, 293.15 K) = (0.227 + 0.002) V, cannot be recalculated and was not being considered any further. 

If the oxygen partial pressure in the surrounding of an Ni,NiO mixture is greater than that calculated from Equations (25-54) and (25-11) then nickel is oxidized. In the reverse case, (the oxygen partial pressure is smaller) it is reduced. If a gas flows over a Ni,NiO mixture, then by comparing the electrode potential measured in the gas with that calculated from Equation (25-54) and taking temperature into account, we can decide whether oxidation of Ni or reduction of NiO occurs. 

In general, nickel catalysts prepared by reduction of NiO are very sensitive to preparation variations, even to the mode of manufacture of commercial NiO ee of obtained MHB can change from 68 to 86%). Modifications of HNi with tartaric acid + NaBr solution were presented in Figures 4.2.-4.4. This catalyst was characterized by high ee s but exhibited the disadvantage of slow reaction rates therefore, the reaction had to be performed at temperatures above 100°C, in which case by-products were formed. 

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