Nickel catalysts hydrogen chemisorbed

In this series of experiments the reactivity of hydrogen chemisorbed on granular skeletal nickel catalyst towards different tin alkyl compounds, i.e. SnEt, SnBu, SnEt Cl and SnBz2Cl2 [Et =... 

Acetylene hydrogenation. Selective hydrogenation of acetylene to ethylene is performed at 200°C over sulfided nickel catalysts or carbon-monoxide-poisoned palladium on alumina catalyst. Without the correct amount of poisoning, ethane would be the product. Continuous feed of sulfur or carbon monoxide must occur or too much hydrogen is chemisorbed on the catalyst surface. Complex control systems analyze the amount of acetylene in an ethylene cracker effluent and automatically adjust the poisoning level to prepare the catalyst surface for removing various quantities of acetylene with maximum selectivity. 

Aluminium dissolves with H2 evolution, and this hydrogen remains chemisorbed on nickel, presumably in a dissociated form. Raney nickel catalysts are often doped with other metals in order to improve the catalytic activity the selectivity decreases in the order. Mo > Cr > Fe > Cu > Co. These metals are fused with the Ni-Al alloy and remain on the final catalyst, probably as oxides. It is believed that the role of the doping metals is to strengthen the selective adsorption of nitrogenous substrates. 

Carbon Deposited on Nickel Catalysts by Exposure to Carbon Monoxide. In a previous TPSR study (9) carbon was deposited on at 25-wt% Ni/A Oj catalyst (G-65) by exposure to CO at temperatures between 550 and 610 K. TPSR (t ) of carbon produced by the dissociation/Qisproportionation of chemisorbed CO showed the presence of large a and p states (Figure 7). Although some chemisorbed CO may have contributed to the a states, it was, conclusively shown that most of the a carbon state was more reactive than a monolayer of chemisorbed CO. In this study, we extended the deposition of carbon by CO exposure to higher temperatures to deter-mine if the 6 and 6 filament carbon states in addition to the a and p states can be populated by a hydrogen-free source. The TPSR (H2) results (Figure 8) clearly show the presence of the filament carbon (6T carbon) state at 773 K. Comparison of the peak temperatures for carbon deposited by C2H and CO exposure (Table IV) shows little difference in the reactivity of the carbon states. 

In addition to actual synthesis tests, fresh and used catalysts were investigated extensively in order to determine the effect of steam on catalyst activity and catalyst stability. This was done by measurement of surface areas. Whereas the Brunauer-Emmett-Teller (BET) area (4) is a measure of the total surface area, the volume of chemisorbed hydrogen is a measure only of the exposed metallic nickel area and therefore should be a truer measure of the catalytically active area. The H2 chemisorption measurement data are summarized in Table III. For fresh reduced catalyst, activity was equivalent to 11.2 ml/g. When this reduced catalyst was treated with a mixture of hydrogen and steam, it lost 27% of its activity. This activity loss is definitely caused by steam since a... 

Evidence for a marked difference between the surface and bulk compositions of dilute copper-nickel alloys has been reported recently by a number of investigators (82, 87-90). Much of the experimental evidence comes from hydrogen adsorption data (74, 82, 87, 90). The conclusions of van der Plank and Sachtler were based on the premise that nickel chemisorbs hydrogen while copper does not (82, 87). The total adsorption of hydrogen at room temperature was taken as a measure of the amount of nickel in the surface. However, in hydrogen adsorption studies on the catalysts used to obtain the catalytic results in Fig. 6, the amount of adsorption on the copper catalyst, while small compared to the adsorption on nickel, is not negligible (74) However, the amount of strongly adsorbed... 

Similarly, we can ask what would be the best catalyst for hydrogenating an olefin such as ethylene. Since olefins (alkenes) are more strongly chemisorbed than hydrogen, we choose a metal that just barely chemisorbs H2—this means Co, Rh, Ir, Ni, Pd, or Pt. In practice, nickel is the least expensive choice. Again, it should be finely divided (maximum surface area) for greatest catalytic efficiency and be dispersed on the internal surfaces of a porous support such as alumina with surface area on the order of 200 m2 g 1.8... 

THE RELATIONSHIP BETWEEN infrared spectra of chemisorbed carbon monoxide and the catalytic activity of metais for the methanization reaction is discussed in conjunction with experiments dealing with the effect of dissolved hydrogen on the catalytic activity of nickel. The purpose of this discussion is to illustrate the type of reasoning involved in seeking a relationship between spectra of chemisorbed molecules and catalytic activity. The underlying concepts of this relationship are extended to include recent advances made in studies of the effect of the semiconductor properties of the carrier on the activity of sup -ported metal catalysts. 

The pairing of copper with platinum and nickel with palladium was reminiscent of the work of Taylor and McKinney (3) who pointed out that copper and platinum were relatively poor catalysts for the hydrogenation of carbon monoxide to methane, while nickel and palladium were active catalysts for this reaction. Although this pattern could be coincidental, it is more reasonable and productive to assume a relationship between the spectroscopic results and the catalytic activities and to conclude that metals which chemisorb carbon monoxide in... 

The idea is not new that dissolved hydrogen can modify the activity of metal catalysts. Hall and Emmett (9) list a large number of workers who have supplied supporting evidence. Emmett, Kokes, and Hall showed that the behavior of dissolved hydrogen can be quantitatively related to the behavior Of copper-nickel alloys. This important contribution bears directly on the present discussion because the effect of adding copper to nickel is also amenable to study by means of the spectra of chemisorbed carbon monoxide. 

Physisorption involves only a weak attraction between the substrate and the adsorbent but in chemisorption a chemical reaction takes place between the adsorbent and atoms on the catalyst surface. As a result, chemisorbed species are attached to the surface with chemical bonds and are more difficult to remove. If the adsorption of hydrogen on nickel is considered as an example, the reaction involves the breaking of an H-H bond and the formation of two Ni-H bonds on the surface. As shown in Fig. 2.3, this adsorption occurs by way of an initially adsorbed dihydrogen molecule. It proceeds via a electron donation and back bonding to the a orbitals of the hydrogen molecule with the final formation of the two surface M-H species. 

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