Chemisorption metal surface area

In this article, we will discuss the use of physical adsorption to determine the total surface areas of finely divided powders or solids, e.g., clay, carbon black, silica, inorganic pigments, polymers, alumina, and so forth. The use of chemisorption is confined to the measurements of metal surface areas of finely divided metals, such as powders, evaporated metal films, and those found in supported metal catalysts. 

In principle any standard catalytic metal surface area measuring technique, such as H2 or CO chemisorption can be used to measure the metal/gas interface area Aq or Nq. This is because solid electrolytes such as YSZ chemisorb practically no H2 or CO at any temperature. 

Whereas determination of chemisorption isotherms, e.g., of hydrogen on metals, is a means for calculating the size of the metallic surface area, our results clearly demonstrate that IR studies on the adsorption of nitrogen and carbon monoxide can give valuable information about the structure of the metal surface. The adsorption of nitrogen enables us to determine the number of B5 sites per unit of metal surface area, not only on nickel, but also on palladium, platinum, and iridium. Once the number of B5 sites is known, it is possible to look for other phenomena that require the presence of these sites. One has already been found, viz, the dissociative chemisorption of carbon dioxide on nickel. 

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. 

Throughout these studies, no product other than propane was observed. However, subsequent studies by Sinfelt et al. [249—251] using silica-supported Group VIII metals (Co, Ni, Cu, Ru, Os, Rh, Ir, Pd and Pt) have shown that, in addition to hydrogenation, hydrocracking to ethane and methane occurs with cobalt, nickel, ruthenium and osmium, but not with the other metals studied. From the metal surface areas determined by hydrogen and carbon monoxide chemisorption, the specific activities of... 

Examination of automotive catalysts by various chemisorption techniques has shown that a loss in noble metal surface area caused by higher temperatures correlates monotonically with various activity indices (62, 63). Moreover, Dalla Betta and co-workers (64) were able to separate the additional effect of poisons on the surface of the precious metal by painstaking attention to detail. They developed techniques for accurately measuring the crystallite-size distribution in used automotive catalysts by... 

On metallic catalysts, sulfur is strongly adsorbed, and even if only minute amounts are found in the feedstock, accumulation can occur on a significant part of the metallic surface area. In the adsorbed state, the poison molecule will deactivate the surface on which it is adsorbed then the toxicity will depend on the number of geometrically blocked metal atoms. On the other hand, the chemisorption bond between the poison and the metal can modify the properties of the neighboring metallic atoms responsible for the adsorption of reactants. If the interaction between the poison and the metal is weak, the structure of the metal will remain unchanged, but it can induce a perturbation all around the adsorption site, which will be able to modify the catalytic properties of this surface. Yet if the interaction between the metal and the adsorbate is strong, it can go as far as to modify the metal-metal bond. The mobility of the surface atoms can be increased and a new superficial structure can appear. 

Selective chemisorption methods have been used with success for the determination of metal surface area and particle size in supported catalysts, and for titration of acid sites on silica-alumina and zeolite catalysts. The chemisorption methods are sometimes neglected in the quest for a more physical description of the catalyst surface, possibly with the penalty of missing an important and quantitative piece of information about the catalyst surface. 

The metal surface area for one of the catalysts, catalyst A, was determined by hydrogen chemisorption. Prior to the adsorption experiment, the sample was reduced in hydrogen at 370°C for 16 hours and then evacuated for one and a half hours. The adsorption experiment was conducted at room temperature using one gram of catalyst. 

As shown in Table I, complete deactivation for these three catalysts occurs around 0.6 to 0.8 wt% sulfur, based on the active site content. These values are typical for complete deactivation in a commercial reactor. The metal surface area measured by hydrogen chemisorption is almost three times the active site concentration determined from the fit of the model to the accelerated aging data. Some of this difference may be due to a poor separation of the product kg C" into the individual constants. How-... 

The metal surface areas of Pt and Pt-Sn catalysts after carbon deposition can be detemiiRed by in sitn H2 chemisorptioR in our combined apparatus. According to tbe H2 chemisorption isobars shown in Hg.S, two kinds of adsorption sites corresponding to adsorbed H2 at different temperatures are tecogiuzed. Hydrogen uptakes determined at low temperature (0 O and hi temperature ( 200 C and 330°C) repiesem low temperature... 

In the first group belong the techniques which are also used in heterogeneous catalysis for determining the surface area of catalysts. Two such techniques are widely used The Brunauer-Emett-Teller (BET) method, based on the physical adsorption of N2 or Ar at very low temperatures [8, 44] and the H2 or CO chemisorption method [8, 44], The first method leads to the total catalyst surface area, whereas the second leads to the specific (active metal) surface area. In the case of supported electrocatalysts (e.g., Pt/C electrocatalysts used as anodes in PEM fuel cells) the two techniques are complementary, as the former can lead to the total electrocatalyst surface... 

Pt by hydrogen chemisorption. Since Ni reduction is slow compared with the reduction of Pd or Pt, we related the amount of H2 chemisorbed after 1 h reduction at 350 to the H/Pt or H/Pd molar ratio. Pd-beidellite as reference shows that Pd reduction is then complete. The measured H/Pt and H/Pd ratios are rather low and range from 0.13 to 0.5, depending on the history of the catalyst (Table II). After prolonged reduction the H2 chemisorption capacity of the catalysts increases due to Ni reduction. After 16 h reduction at 350 C, the standard reduction procedure, a substantial part of the metal surface area consists of reduced Ni. The relative contribution of Ni and Pt or Pd to the metal activity of the catalysts further depends on the intrinsic activity of the various metals under the given reaction conditions. 

Within a single secondary washcoat particle, the distribution of the precious metals can be assumed to be relatively homogeneous. The precious metals are typically present in a highly dispersed state. Dispersions measured by CO chemisorption methods are typically in the range 10-50% or even higher, for fresh catalysts. This means that the precious metals are present as single atoms or as small clusters of about ten atoms. For a catalyst with about 1.8 g precious metal per liter of catalyst volume, this corresponds to a precious metal surface area in the range of about 3-30 m 1 catalyst volume. 

The cobalt metal area of the reduced perovskites was determined by hydrogen chemisorption experiments. The results are shown in Table 1. The chemisorption measurements revealed that the cobalt metallic surface area was similar for all the perovskites. This is supported by the Co/Ln surface ratio (Table I) obtained by XPS which also suggests similar metallic dispersion. The XPS analyses of the reduced perovskites showed the presence of Co" (778.6 eV) but also a doublet at approximately 780.5 and 796.2 eV which correspond to Co 2p, and Co 2p , peaks respectively, for the Co ion. Shake-up satellite lines with 4.7 eV over the Co lines were also detected indicating the presence of Co [12]. These oxidised species of cobalt are probably formed by air oxidation during the transference of the reduced sample from the reactor to the XPS spectrometer. Also, Marcos el al. [15] have shown that the reduction of the perovskite LaCoO, produced a La,0, oxide covered by hydroxyl groups which upon heating and evacuation in the XPS pretreatment chamber partly reoxidises the cobalt crystallites. 

The BET surface area as well as the palladium metal surface area of the precursor increases by more than two orders of magnitude during the in situ activation. The solid-state reactions occurring in the metallic glass during in situ activation result in a large increase of the BET surface area from 0.02 to 45.5 m2/g. The palladium metal surface area of the as-prepared catalyst determined by CO chemisorption is 6.9m2/g, which corresponds to a palladium dispersion of about 6%. 

The metallic surface area and dispersion are measured by CO pulse chemisorption, performed on a Micromeritics Pulse ChemiSorb 2700. 

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