Rhodium nitrogen adsorption

Two samples were measured before activation (226A, containing 17% Pt and 249A containing 0.5% rhodium). The nitrogen adsorption isotherms were classed as type 2 and hence BET total surface areas were calculated for these samples (5 and 33 m g respectively). They showed no micropore volume on inspection of Ae t-plots and the whole surface area for each sample was accounted for by the combined area of the meso and macropores. These t-plot values are in good agreement with the total surface areas (table 1) and show that the subsequent activation process develops a microporous structure not present in the chars. 

Numerous quantum mechanic calculations have been carried out to better understand the bonding of nitrogen oxide on transition metal surfaces. For instance, the group of Sautet et al have reported a comparative density-functional theory (DFT) study of the chemisorption and dissociation of NO molecules on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium to estimate both energetics and kinetics of the reaction pathways [75], The structure sensitivity of the adsorption was found to correlate well with catalytic activity, as estimated from the calculated dissociation rate constants at 300 K. The latter were found to agree with numerous experimental observations, with (111) facets rather inactive towards NO dissociation and stepped surfaces far more active, and to follow the sequence Rh(100) > terraces in Rh(511) > steps in Rh(511) > steps in Pd(511) > Rh(lll) > Pd(100) > terraces in Pd (511) > Pd (111). The effect of the steps on activity was found to be clearly favorable on the Pd(511) surface but unfavorable on the Rh(511) surface, perhaps explaining the difference in activity between the two metals. The influence of... 

Table 1 summarizes the most important surface complexes formed when NO and CO are adsorbed on noble metal catalysts. According to the literature NO and CO are adsorbed as nitrites, nitrates and carbonates on alumina [2]. The most important surface complexes for CO and NO adsorption on rhodium are a gem-dicarbonyl (Rh(CO)2) and a linear Rh-NO complex [1]. However, tricarbonyl and bridged Rhx-CO complexes have been proposed to be formed and different kinds of linear Rh nitrosyl complexes are possible [1-4]. The adsorption of CO on R and Pd catalysts depends much on the oxidation stage of R and Pd [5]. CO adsorption on R forms mostly linear and bridged carbonyls [6-11]. NO is adsorbed linearly on R [12]. In the case of Pd the most common surface complexes are linear carbonyls [13], strong multilaterally-bonded carbonyls, bridged carbonyls [5,14,15] and triply-bonded CO [5]. Isocyanate, nitrous oxide or nitrogen dioxide are proposed to be coimected to the reaction mechanism of the NO-CO reactions [2,16-19]. 

Bunsen worked out a method for the separation of platinum metals (preparation of pure rhodium). He determined the composition, NIg,NH3, of nitrogen iodide, developed a volumetric method of iodimetry, using a solution of sulphurous acid, and worked on water analysis. Bunsen and L. Schischkoff investigated the chemical reactions in the explosion of gunpowder. Bunsen discovered arsenic pentasulphide, and measured the adsorption of carbon dioxide on glass. ... 

H2 on noble metal catalysts, platinum, and rhodium. Since one of the antibonding orbitals of NO bond is occupied by an electron, the N-O bond is weaker than the bond of CO. The rate of dissociative NO adsorption is generally faster than that of CO. Rhodium is more reactive than platinum. Therefore the rate of dissociative adsorption of NO is higher on Rh than on Pt, as is the steady-state coverage of adsorbed nitrogen atoms. 

D. Alfe, S. de Gironcoli, S. Baroni, The reconstruction of nickel and rhodium (001) surfaces upon carbon, nitrogen or oxygen adsorptions. Surf. Sci. 437(1-2), 18-28 (1999)... 

Suggest how nickel catalyses this reaction by referring to the processes of adsorption, reaction on the metal surface and desorption, in catalytic converters, rhodium catalyses the reduction of nitrogen(ll) oxide, NO, to nitrogen. Draw diagrams to suggest ... 

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