Nickel oxide surface structure

It must be acknowledged, however, that the determination of the number of the different surface species which are formed during an adsorption process is often more difficult by means of calorimetry than by spectroscopic techniques. This may be phrased differently by saying that the resolution of spectra is usually better than the resolution of thermograms. Progress in data correction and analysis should probably improve the calorimetric results in that respect. The complex interactions with surface cations, anions, and defects which occur when carbon monoxide contacts nickel oxide at room temperature are thus revealed by the modifications of the infrared spectrum of the sample (75) but not by the differential heats of the CO-adsorption (76). Any modification of the nickel-oxide surface which alters its defect structure produces, however, a change of its energy spectrum with respect to carbon monoxide that is more clearly shown by heat-flow calorimetry (77) than by IR spectroscopy. 

Nature of Active Sites. There is no apparent correlation between the increase of catalytic activity and a modification of the electronic structure of nickel oxide, since the electrical properties of both catalysts are identical. It is probable that local modifications of the nickel oxide surface are responsible for the change of its activity and of the reaction mechanism. It should be possible to associate these structural modification with local modifications of the height of the Fermi level, but it would be difficult to explain the results by the electronic theory of catalysis which considers only collective electrons or holes. A discussion based only on the influence of surface defects seems, therefore, to be more straightforward. 

Surface structure modifications do not infiuence the adsorption of carbon dioxide as much as the adsorptions of carbon monoxide or oxygen. Differential heats of adsorption at 30° on NiO(200°) (Fig. 13) and on NiO(250°) (Fig. 14) are indeed very similar. It must be noted, however, that the high initial heat (46 kcal/mole) that was obtained in the case of the adsorption on NiO(200°) (59) is not recorded when carbon dioxide is adsorbed on NiO(250°) (initial heat, 29 kcal/mole) (40). This difference, which was already observed in the case of the adsorption of carbon monoxide, is explained, as in the former case, by the interaction, on XiO(200°), of carbon dioxide with labile surface oxygen ions which do not exist on the surface of NiO(250°). This result is considered as additional evidence of the removal of labile oxygen ions from the nickel oxide surface, under vacuum, at 250°. 

It has been shown previously from the results of oxygen exchange experiments that the mobility of surface ions in nickel oxide increases markedly between 200 and 250°. At 200°, the structure of the nickel oxide surface is modified in the course of the catalytic oxidation of carbon monoxide. If, as is probable, this structural modification is related to the surface ionic mobility, changes of surface structure will become even more important during the catalytic reaction at higher temperatures. 

An effect which is frequently encountered in oxide catalysts is that of promoters on the activity. An example of this is the small addition of lidrium oxide, Li20 which promotes, or increases, the catalytic activity of dre alkaline earth oxide BaO. Although little is known about the exact role of lithium on the surface structure of BaO, it would seem plausible that this effect is due to the introduction of more oxygen vacancies on the surface. This effect is well known in the chemistry of solid oxides. For example, the addition of lithium oxide to nickel oxide, in which a solid solution is formed, causes an increase in the concentration of dre major point defect which is the Ni + ion. Since the valency of dre cation in dre alkaline earth oxides can only take the value two the incorporation of lithium oxide in solid solution can only lead to oxygen vacaircy formation. Schematic equations for the two processes are... 

Before any attempt to establish a correlation between the surface structure of the oxidized alloys and their CO conversion activity one must stress that the surface composition of the samples under reaction conditions may not necessarily be Identical to that determined from ESCA data. Moreover, surface nickel content estimates from ESCA relative Intensity measurements are at best seml-quantlta-tlve. This can be readily rationalized If one takes Into consideration ESCA finite escape depth, the dependence of ESCA Intensity ratio... 

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)]. 

Anodic oxidation of a nickel electrode will give a surface structure that can be represented as Ni—surface—Ox Hy (A). Reaction with, say, a dichiorosilane will then follow (equation 29) ... 

The best example of a study of this type of intermediate is found in the oxidation of CO over a nickel-nickel oxide catalyst 24). The latter term is used because there is doubt as to the specific nature of the catalyst surface. The spectrum in Fig. 14 was obtained during the oxidation of CO over nickel-nickel oxide at 35° C. The band at 4.56 u is tentatively attributed to an intermediate complex having the structure Ni- 0 C rr.O. The bands at 6.5 and 7.2 u are due to C02 chemisorbed on the catalyst surface. This C02 is considered to be adsorbed product rather than as a reaction intermediate because these bands remain after the reaction is completed. The 4.56- u band in Fig. 14 is attributed to the asymmetrical 0—C—0 vibration rather than to the C—O vibration of chemisorbed CO. This interpretation implies that there should be a second band due to the symmetrical vibration. The symmetrical 0—C—O vibration of C02 produces a Raman band at 7.2 ju. The symmetrical 0—C—0 vibration of Ni - -O—C=0 would be expected to produce an infrared band near 6 or 7 u- Thus far this band has not been observed. This failure is not considered a serious obstacle to the structure assignment,... 

The samples prepared have a good surface area after calcination at 500°C, as can be seen in table 1. Alumina-titania mixed oxide supported samples have surface areas larger than those of the alumina and titania single oxides. As expected x-ray diffraction results show that the mixed oxide catalysts are amorphous, but alumina shows a y phase structure, and Ti02 is a well crystallized anatase phase. No nickel metal or nickel oxide was detected in any of the samples, including Ti02 sample, suggesting the metal was well dispersed, and present as small crystallites (< 50A). 

Nickel oxide, like MgO, usually adopts the relatively simple rocksalt lattice. The natural cleavage plane of NiO is (100), and studies have shown that the resulting surfaces are of high quality, relaxing only slightly away from the ideal bulk terminated (100) surface (see Fig. 1). Structural determinations of adsorbates have been performed on both this surface and the polar (111) surface. To circumvent surface charging problems almost all of these studies have been performed on highly oriented NiO thin films. 

Nickel-loaded KTiNbOs was studied by Takahashi et al. [76] for the photodecomposition of water. It was found that the activity of compounds prepared by a polymerized complex (PC) method was 10 times higher than the activity of the oxide prepared by conventional methods. The increased activity for the PC preparation was attributed partly to the larger surface area (ca. 23 vs. ca. 3 m- g ) and partly to probable improvement in surface structure. 

A method for determining the particle size distribution from a single X-ray diffraction profile when strain is present was applied to co-precipitated nickel oxide on alumina and silica. Appreciable strain occurred in the NiO, possibly due to the pressure developed in the small particles to balance the surface tension forces and the distortion produced by the deformation of the f.c.c. structure into a rhombohedral form. Apart from errors in the size distribution created by neglected lattice strain, the measurement of strain itself is important because its correlation with catalytic activity has been suggested. 

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