Surface carbon, reactivity

It was previously reported that magnesium oxide with a moderate basicity formed reactive surface carbonate species, which reacted with carbon deposited on foe support by foe methane decjomposition [6]. Upon addition of Mg to foe Ni/HY catalyst, reactive carbonate was formed on magnesium oxide and carbon dioxide could be activated more easily on the Mg-promoted Ni/HY catal t. Reactive carbonate species played an important role in inhibiting foe carbon deposition on the catalyst surface. 

Steady state and non steady state kinetic measurements suggest that methane carbon dioxide reforming proceeds in sequential steps combining dissociation and surface reaction of methane and CO2 During admission of pulses of methane on the supported Pt catalysts and on the oxide supports, methane decomposes into hydrogen and surface carbon The amount of CH, converted per pulse decreases drastically after the third pulse (this corresponds to about 2-3 molecules of CH< converted per Pt atom) indicating that the reaction stops when Pt is covered with (reactive) carbon CO2 is also concluded to dissociate under reaction conditions generating CO and adsorbed... 

Carbonaceous species on metal surfaces can be formed as a result of interaction of metals with carbon monoxide or hydrocarbons. In the FTS, where CO and H2 are converted to various hydrocarbons, it is generally accepted that an elementary step in the reaction is the dissociation of CO to form surface carbidic carbon and oxygen.1 The latter is removed from the surface through the formation of gaseous H20 and C02 (mostly in the case of Fe catalysts). The surface carbon, if it remains in its carbidic form, is an intermediate in the FTS and can be hydrogenated to form hydrocarbons. However, the surface carbidic carbon may also be converted to other less reactive forms of carbon, which may build up over time and influence the activity of the catalyst.15... 

The decomposition of acetic acid was studied by TRPS in order to determine the similarities of its surface reactivity to formic acid. Reactions on Fe(100) (95), Ni(llO) 118), and Cu/Ni(110) (100) alloys were studied. On Ni(l 10) acetic acid adsorbed at 300 K with a sticking probability near unity to form the anhydride intermediate and release HjO. The decomposition of this intermediate proceeded by the two-dimensional autocatalytic process observed for formic acid to yield CO2,, CO, and surface carbon. The rate... 

S. Park, D. Srivastava, K. Cho, Generalized chemical reactivity of curved surfaces Carbon nanotubes. Nano Lett., 3(9) (2003) 1273-1277. 

Much work has been done on the effect of the addition of impurities (salts and metals, chiefly) on the reactivity of carbon. Quantitatively, the effects are difficult to understand, since they are functions of the location of the impurity in the carbon matrix and the extent of interaction of the impurity with the matrix. Long and Sykes (94) suggest that impurities affect carbon reactivity by interaction with the 7r-electrons of the carbon basal plane. This interaction is thought to change the bond order of surface carbon atoms, which affects the ease with which they can leave the surface with a chemisorbed species. Since the 7r-electrons in carbon are known to have high mobility in the basal plane, it is not necessary that the impurity be adjacent to the reacting carbon atom. Indeed, it is thought that the presence of the impurity at any location on the basal plane is sufficient for it to affect the reaction. 

Usually it is difficult to separate the effect of ciystallite size on carbon reactivity from the effects of crystallite orientation and impurity content. However, Armington (62) attempted to do so by reacting a series of graphi-tized carbon blacks with oxygen and carbon dioxide, as discus.sed earlier in this article. Assuming that upon graphitization all the carbon blacks are converted to polyhedral particles with the surface composed almost completely of basal plane structure, it is possible to eliminate crystallite orientation as a variable. Spectroscopically, the total impurity content of all the graphitized carbon blacks is quite low and to a first approximation, the analyses of the individual constituents are similar. 

Carbon may exist in different forms, of which carbidic surface carbon (Cs) is the most reactive species. It is readily transformed to carbenic species [Eq. (3.25], which participates in chain growth. This is supported by reevaluation of earlier labeling experiments. New studies showed - that certain surface carbon species may be very reactive and do incorporate into products mainly into methane. Hydrogenation of adsorbed CO without previous dissociation may also lead to surface carbenic species244 [Eq. (3.26)] ... 

There are four surface species in this reaction mechanism (1) CH(s), a surface carbon atom bonded to a hydrogen (2) C (s), a reactive surface carbon radical, namely species 1 with the hydrogen stripped away (3) CM(s), a surface CH3 group atop a surface carbon atom and (4) CM (s), a surface CH2 radical atop a surface carbon atom, namely species 3 with the hydrogen stripped away. 

Most likely, one has to consider the following information [22]. A working FTS or methanation catalyst has (at least) three kinds of carbon on its surface a reactive type, an almost unreactive (Cs) (perhaps carbidic) and coke (Cg h very unreactive). The working (in the FTS) surface is proportional to (1-0S - Qgraph) and the rate of C + Hads is then r = ktot (1 - 0S - 0grapiO 0c6h-... 

The existence of surface metal-oxygen bonds can also be indirectly deduced by the reachvity of these bonds with molecules from the gas phase, such as the surface hydration producing new hydroxyls, the surface carbonation producing carbonates, and also some more complex reactivity, such as the reactivity with alkoxysilanes producing surface alkoxides that may later be converted to surface hydroxides by ehminahon [81]. 

FT synthesis kinetics are similar on Co and Ru catalysts and reflect similar CO activation and chain growth pathways on these two metals. These kinetic expressions are consistent with the stepwise hydrogenation of surface carbon formed in fast CO dissociation steps (26). Chemisorbed CO and CH t species are the most abundant reactive intermediates, as expected from the high binding energy of CO and carbonaceous deposits on Co and Ru surfaces (7, 76-80). As a result, reaction orders in CO remain negative at the usual inlet pressures but can become positive as CO reactants are depleted by transport limitations within pellets or by high levels of conversion within the catalyst bed. 

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