Nickel surface area

To give an idea of the wide rai e of catalytic systems that have been investigated where chemisorption data were essential to interpret the results, some of the author s papers will be discussed. Measurements were reported on the surface areas of a very wide range of metals that catalyze the hydrogenation of ethane. In the earliest paper, on nickel, the specific catalytic activity of a supported metal was accurately measured for the first time it was shown also that the reaction rate was direcdy proportional to the nickel surface area. Studies on the same reaction... 

Figure 8 shows the spectra of CO adsorbed on our samples they were taken at a CO pressure of about 1 torr. To facilitate comparison, the extinction per square meter of nickel surface, as calculated from the spectra and the analytical data of the samples, has been plotted versus the wave-number. It should be borne in mind that in the calculation of the extinction values, use has been made of the experimentally determined nickel surface areas. Hence, all inaccuracies in the surface area measurements will be reflected in the values of extinction per square meter (E/m2). 

The specific metal surface area of our nickel samples was established by means of deuterium chemisorption, the amount of deuterium adsorbed being determined by exchange with a known quantity of hydrogen followed by mass speetrometric analysis. It was assumed in the calculation that 1 cm3 (NTP) of deuterium corresponds to 3.64 m2 of nickel surface area. 

The nickel surface area was measured by the thiophene method (ref. 5) although 3 methylthiophene was used for greater accuracy. 

We have also controlled the effect of hydrogen pressure of pretreatmenc on tne activity. In isopropanol tne hydrogen pressure of pretreatment nas no effect on the initial rate (V0A = 11 + 0.5 mmol rmn g oetween U.2 and 4.6 MPa. On the contrary, in cyclonexane (Taole 3) we observed a slight decrease in activity as tne hydrogen pressure increases. Tnis activity loss is accompanied oy a decrase in specific surface area (S y), out tne nickel surface area determined oy methyltniopnene reaction remains constant. We assume that tne activity less could result from a partial surface poisoning oy... 

Comparison of Nickel Surface Areas as Determined from H2-Chemisorption (Fh) and Estimated from Magnetic Measurements (Vm)... 

Bartholomew and Sorensen [23] also measured loss of nickel surface area, BET surface area, and pore radius/volume after sintering of 15% Ni/alumina and 13.5% Ni/silica in H2 at 923, 973, and 1023 K. Their results for Ni/alumina were generally consistent with those of Bartholomew et al. [27] that is, percentage losses in nickel surface area of 5-13% at 923 and 973 K were comparable with observed losses in BET surface area and pore volume (e.g., 14% at 973 1C), while a 25% observed decrease in nickel area at 1023 K was twice as large as the... 

The thermal treatment is one of the factors which controls the properties of the final catalyst [56]. The total surface area (in the range between 100 and 300m2g l) decreases with increasing reduction temperature however, the nickel surface area (typically 20-50 m2g l) increases which is probably due to a higher degree of reduction. The best precursor with respect to a high surface area is the hydroxycarbonate. The surface areas of catalysts prepared from hydroxy-chlorides and nitrates are smaller by about a factor of two. Nickel particle sizes are in the order of 5nm for such catalysts. 

The dramatic increase in irreversible CO adsorption on presulfided supported nickel catalysts at moderate pressures (162) has significant, practical implications in regard to the use of CO chemisorption to measure nickel dispersion. For example, it is often desirable to determine nickel surface areas for catalysts used in a process where sulfur impurities are present in the reactants. Substantial differences in the measurements of nickel surface area by H2 or CO adsorption are possible depending upon the catalyst history and choice of adsorption conditions. In view of the ease with which catalysts may be poisoned by sulfur contaminants at extremely low concentrations in almost any catalytic process, and since large CO uptakes may be observed on supported Ni not necessarily representative of the unpoisoned nickel surface area, the use of CO adsorption to measure nickel surface areas is highly questionable under almost any circumstance. 

An ESCA analysis of the molybdenum promoted Raney nickel showed that when a low molybdenum content alloy was used, the activated catalyst had greater amounts of nickel on the surface than the unpromoted active catalyst. At a 2% molybdenum content the nickel surface area in the activated catalyst reached a maximum and then decreased with the presence of more molybdenum. These findings correlate with the observed maximum in catalytic activity observed with the 2% molybdenum Raney nickel catalyst. The initial increase in... 

Reduction of the nickel oxide present in the mixed nickel-aluminum oxides is usually done in a flow of hydrogen at elevated temperatures (Eqn. 13.1). The extent of reduction depends on the conditions used. Fig. 13.1 shows that the most complete reduction and maximum nickel surface area were obtained when the reduction was run at 700°C. ... 

Fig. 13.1. Dependence of a) nickel surface area and b) percent reduction on the temperature used for the reduction of a coprecipitated Ni/Al203 catalyst. (Redrawn using data from Ref. 4.)...

In summary, although a good correlation between nickel surface area and activity for hydrogenation of benzene has often been found on clean nickel, this seems not to be the case on coked nickel catalysts. Indeed, the growth of well-defined forms of carbon is expected to change the nature of the active sites, potentially giving rise to a new catalyst. 

To determine nickel surface areas of fresh catalysts, hydrogen chemisorption measurements were performed with the atmospheric TPH apparatus described above. Nickel surface areas of the catalysts were also measured by static volumetric hydrogen chemisorption (Coulter Omnisorp 100 cx). Before these measurements, samples were reduced in situ in a pure hydrogen atmosphere by raising the temperature up to 900°C (20°C/min). The samples were then cooled in a helium flow and the measurements were performed at 30°C. The catalyst materials and the analytical methods are described in [4,6,7]. 

TPH tests with pure alumina (alpha) indicated that sulfur was not adsorbed on this material during fixed-bed poisoning tests, although sulfur adsorbed on nickel catalysts supported on alumina, thus indicating it adsorbs on the surface of nickel only. Tests with a pure alumina (alpha) bed also indicated that hydrogen was not adsorb on it at the conditions for nickel surface area measurements by hydrogen. 

Chemisorption uptakes of Hj at 298-303 K for alumina supported nickel catalysts were measured. The corresponding nickel surface areas were calculated and subsequently sulfur contents at saturation were determined. The average values were of the same magnitude as the amounts of sulfur that were not desorbed from the catalysts during the TPH treatments. Hence, it may be concluded that the saturation layer of sulfor remains on the catalyst even after regeneration in hydrogen atmosphere. 

Figure 4 Catalyst reduction effect of reduction conditions on nickel surface area. (25 % Ni in catalyst pressure 1 atm H2O/H2 8.0)...

With this objective the present work was undertaken to investigate the efrects of some parameters of the impregnation process on the selectivity of promoted Ni/MgO catalysts for the reaction of propane oxidation by air to CO and H2. The properties of catalysts relevant for their selectivity such as the Ni-loading in produced catalyst samples, the nickel surface area and mean Ni-crystallite size as well the pore size distribution of catalyst samples are presented. 

The steam reforming catalyst is normally based on nickel. The properties are dictated by the severe operating conditions. The activity depends on the nickel surface area and particle size. The shape should be optimized to achieve maximum activity with minimum increase in pressure drop. 

A number of recent studies of CO2 reforming of methane (6,12,13) indicate that promoters may also inhibit (12,13) the dehydrogenation of the adsorbed CH -species. The results in Table 4 illustrate the difference in rates of methane activation for two catalysts (6). The Ni/MgO catalyst shows an exchange rate significantly lower than that of the Ni/MgAl204 catalyst, although the catalysts have similar nickel surface areas and show similar rates for steam reforming. The MgO based catalyst may therefore inhibit the carbon formation by this mechanism as well as by enhanced steam adsorption. 

Case (1) Bodrov et al. (1964) carried out this set of experiments at 1073 K using a nickel surface area of 0.0775 m with the following reacting mixture composition ... 

We prepared Ni-M (M = Al, Cr, Cu, Co and Mo) catalysts supported on graphite, at low temperature, by coreduction of metal salt mixtures (NiXa, MX2) deposited on this support with sodium naphthalene as reducting agent. Quantitative microanalyses performed by STEM/EDX showed that the two metals were evenly distributed over graphite leaflets. The activity and the selectivity of these catalysts in the hydrogenation of citral to citronellal and citronellol have been compared with that of unsupported bimetallic catalysts, with the same atomic composition and prepared by the same procedure. It appeared that the nickel surface area of the supported catalysts was notably higher than that of the unsupported ones, but the support had almost no effect on the catalytic properties. 

But with metals of higher oxidisability, in particular with aluminium, the atomic ratio percentage is much lower that the initial salt ratio. One could assume that aluminium is leached out by the alcoholate ion which results from the large excess of sodium naphthalene i.e the alcoholate ion would act in a way similar to that of the hydroxide ion in the preparation of Raney nickel. The very high value measured for the nickel surface area might support this assumption. 

Also Included in Table 2, are the initial hydrogenation rates < in mmol.S per square metre of nickel surface area ) together with the results obtained with unsupported bimetallic Ni-M catalysts which have been prepared with the same procedure [7]. It appears that whereas the nickel surface areas of the supported catalysts are two times (Ni), three times (NiAIg.s) and seven times (NiCria) higher than that of the unsupported cataiysts, the hydrogenation rates are the same for both series of catalysts. 

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