Adsorbates carbon dioxide

Transition into the Kolbe region at platinum is associated with the formation of an oxide layer. Acetate ions are believed to be more strongly adsorbed on this layer than is water. Conversion of water to oxygen is then suppressed in favour of the oxidation of acetate ions [59, 60. Electron transfer from acetate is synchronous with cleavage of the alkyl-carboxylate bond leaving adsorbed carbon dioxide and... 

The same sequence of adsorptions has been studied on NiO(200) (8, 20). The interaction between oxygen preadsorbed on NiO(200) and carbon monoxide yields only adsorbed carbon dioxide. Therefore, on NiO(200), gaseous carbon dioxide is produced during the catalytic reaction through Mechanism I (8, 20), whereas on NiO(250) two reaction paths are probable (Mechanisms I and II). These results show clearly... 

Co. surface area = 300 m2/g ) with aqueous solutions of Cu, Cr, Mg, Ca, Sr, and Ba in Nitrate. All the catalysts have Cu to Si02 weight ratio of 14/86. For promoted catalyst, the Cr to Cu molar ratio was varied from 1/4 0 to 1/4, and the alkaline earth metal to Cu molar ratio was kept at 1/10. The impregnated catalysts were dried at 100 °C overnight, calcined at 450 for 3 h and then reduced in a stream of 10% H2 in Ar at 300 °C for 2 h. The copper surface areas of catalysts were determined by the N20 decomposition method described elsewhere [4-5J. The basic properties of the catalysts were determined by temperature-programmed desorption ( TPD ) of adsorbed carbon dioxide. Ethanol was used as reactant for dehydrogenation reaction which was performed in a microreactor at 300°C and 1 atm. 

The volumes of adsorbed carbon dioxide are not very much influenced by the degree of activation of the chars. Figure 5b... 

Nickel oxide prepared at 250° [NiO(250°)] presents a greater adsorption affinity toward carbon monoxide at 30° than NiO(200°) [at 2 torr, 4.5 cm /gm on NiO(200°), 5.5 cm /gm on NiO(250°)] and the differential heats of adsorption on NiO(250°) (Fig. 12) decrease more progressively than on NiO(200°) (Fig. 11). The initial heat of adsorption is lower on NiO(250°) (29 kcal/mole) than on NiO(200°) (42 kcal/mole). However, on the latter catalyst, surface oxygen ions react with carbon monoxide to give a small quantity (0.3 cm /gm) of adsorbed carbon dioxide, which accounts for the high initial heat of adsorption of carbon monoxide (42 kcal/mole) on NiO(200°). Because of the higher temperature of its... 

Nickel oxide adsorbs carbon dioxide at room temperature to a much higher extent than it does oxygen. At a maximum pressure of 30 torr, 14.20 cm of carbon dioxide is adsorbed per gram of oxide (6 = 0.24) (23). Part of the adsorbate is removed during the evacuation at room temperature, the quantity of irreversibly held carbon dioxide then being 8.81 cm /gm (0 = 0.15). The adsorption is pressure-dependent since an increase in pressure to 450 torr increases the surface coverage to 23.14 cm3/gm (irreversible fraction, 9.87 cm /gm) (42). Kinetics of adsorption were not determined because most of the adsorption (85%) is a very rapid process. Color and electrical conductivity of nickel... 

Calculated values for the heat of the homogeneous process (Table III) are close to 68 kcal/mole for cycle 1 (formation of adsorbed carbon dioxide) but not for cycle 2 (formation of gaseous carbon dioxide). It is, therefore, concluded from the calorimetric data, that the interaction between carbon monoxide and oxygen ions adsorbed on NiO(200°) yields adsorbed carbon dioxide exclusively. 

The interaction between oxygen adsorbed on NiO(200°) and carbon monoxide yields only adsorbed carbon dioxide. Mechanism I is therefore not probable on NiO(200°). Thus, the calorimetric results show clearly the influence that a modification in the temperature of the catalyst preparation may have upon the catal3rtic reaction itself. 

Thermochemical cycles were used to test the validity of Eqs. (5a) and (7) by means of the calorimetric data (Table VI) 68). In cycle 4, the formation of gaseous carbon monoxide at the end of the sequence of adsorptions CO-O2-CO is assumed whereas, in cycle 5, the formation of adsorbed carbon dioxide is supposed. Calculations presented in Table VI show that, in the case of NiO(250°), cycle 5 is balanced for a low surface coverage whereas cycle 4 is balanced for high surface coverages. The calorimetric results confirm therefore the formation of the C03"(ads) ions and moreover they show that, when the heat of formation of this complex is high (120 kcal/mole), its interaction with carbon monoxide produces adsorbed carbon dioxide. When the heat of formation is lower (110 kcal/mole), gaseous carbon dioxide is evolved to the gas phase. 

The reverse interaction, between preadsorbed carbon dioxide and oxygen from the gas phase, was also studied (66). A small quantity of oxygen can be adsorbed (1.08 cra /gm) and the electrical conductivity of the sample increases. The initial heat of adsorption (83 kcal/mole) shows that oxygen reacts with adsorbed carbon dioxide. A subsequent adsorption of carbon monoxide is again possible (1.05 cm /gm) which produces a decrease of the electrical conductivity of the sample. These results were explained, from the calorimetric data, by the formation of a small number of C03"(ads) ions by the interaction of oxygen with preadsorbed carbon dioxide. These complex ions are then converted by carbon monoxide into carbon dioxide which remains adsorbed on the oxide surface. From these experiments it was concluded (66) that, although a small quantity of preadsorbed carbon dioxide may react with oxygen, this interaction is not a possible step of the reaction mechanism and that, consequently, adsorbed carbon dioxide is an inhibitor of the catalytic process. 

Table IX presents the results for stoichiometric mixtures (Po = 3 torr) when the temperature of the cold trap is higher than —195° and therefore for increased partial pressures of carbon dioxide. Apparent orders in Table IX were determined by the differential method (plot of log dPjdt as function of log P). They show that as the partial pressure of carbon dioxide increases, the autoinhibition increases. The rate of the reaction is also greatly decreased and the order increased if a constant activity catalyst has adsorbed carbon dioxide previous to the...

The surface coverage by adsorbed oxygen is smaller than the coverage by adsorbed carbon monoxide [which is partially transformed by oxygen into C08"(ads) ions]. Moreover, interaction between adsorbed oxygen ions and carbon monoxide yields adsorbed carbon dioxide. 

Since gaseous carbon dioxide may be produced by interaction (7c) on surface sites that adsorb carbon dioxide irreversibly, we believe that production of gaseous carbon dioxide is the consequence of the cooperative interaction of CO(ads) with COs"(ads) ions (46). A fraction of the reaction product remains, however, on the surface and the reaction is self-inhibited. 

Interaction (1) which produces gaseous carbon dioxide in the case of NiO(250°) causes the inhibition of the surface of Ni0(200°) because it produces adsorbed carbon dioxide. Since it has been shown that the different behavior of NiO(200°) and NiO(250°) with respect to this interaction is related to their different surface structure (Section IV, C), it is concluded that the catalytic activity of a divided nickel oxide in the room-temperature oxidation of carbon monoxide is determined primarily by the number and the nature of the lattice defects which are formed on the catalyst surface during its preparation at a low temperature. 

It has been shown in Section III, A that a fraction of oxygen ions irreversibly adsorbed on nickel oxide at elevated temperatures (250°) reacts at room temperature with carbon monoxide to form adsorbed carbon dioxide. This interaction evidently also occurs on the surface of oxygenated or regenerated samples during the catalytic reaction (76). It has been observed, for instance, that adsorption of carbon monoxide, at room temperature, on the regenerated sample, although it decreases its electrical conductivity from 10- to IO-12 ohm- cm-, does not... 

Adsorption of carbon monoxide at 30° decreases the electrical conductivity of lithium-doped nickel oxide [NiO(10 Li)(250°)] which has been precovered by oxygen, at the same temperature from 1.8 x 10- to 6.2 X 10-12 ohm- cm-1. Formation of neutral species during this interaction is thus observed on all samples. Thermochemical cycles 1, 2 (Table XIII) and 3 (Table XIV) yield, however, ambiguous results in the case of NiO(10 Li)(250°). It appears from cycle 3 (Table XIV) that the intermediate formation of COs-(ads) ions is possible but the direct formation of adsorbed carbon dioxide is also probable from cycle 1 (Table XIII). Cycle 2, on the other hand, testing the formation of gaseous carbon dioxide, is balanced neither for low nor for intermediate surface coverages, although carbon dioxide is found in the cold trap after the adsorption of carbon monoxide (1.5 cm /gm). 

It may be concluded from the calorimetric data presented in Section IV and in the present section that, on one hand, interaction of oxygen with adsorbed carbon monoxide proceeds in a similar manner on all samples. Complex ions, C03 (ads), are formed and their conversion by carbon monoxide produces gaseous and adsorbed carbon dioxide. These interactions are the different steps of a reaction mechanism (II) which is probable on all catalysts. On all catalysts also, carbon dioxide should remain partially adsorbed and inhibit the catalytic reaction. 

Physical sorbents for carbon dioxide separation and removal were extensively studied by industrial gas companies. Zeolite 13X, activated alumina, and their improved versions are typically used for removing carbon dioxide and moisture from air in either a TSA or a PSA process. The sorption temperatures for these applications are usually close to ambient temperature. There are a few studies on adsorption of carbon dioxide at high temperatures. The carbon dioxide adsorption isotherms on two commercial sorbents hydrotalcite-like compounds, EXM911 and activated alumina made by LaRoche Industries, are displayed in Fig. 8.F23,i24] shown in Fig. 8, LaRoche activated alumina has a higher carbon dioxide capacity than the EXM911 at 300° C. However, the adsorption capacities on both sorbents are too low for any practical applications in carbon dioxide sorption at high temperature. Conventional physical sorbents are basically not effective for carbon dioxide capture at flue gas temperature (> 400°C). There is a need to develop effective sorbents that can adsorb carbon dioxide at flue gas temperature to significantly reduce the gas volume to be treated for carbon sequestration. 

Carbon dioxide is a hard acid, and as such, it is expected to initially adsorb on hard base sites, presumably oxygen anions and surface hydroxyl groups. However, the adsorbed carbon dioxide can be as carboxylate or carbonate species, and the carbonate may be monodentate or bidentate ... 

---