Equilibrium of COS

At low temperatures the reaction is negatively affected by the lack of oxygen on the surface, while at higher temperatures the adsorption/desorption equilibrium of CO shifts towards the gas phase side, resulting in low coverages of CO. As discussed in Chapter 2, this type of non-Arrhenius-like behavior with temperature is generally the case for catalytic reactions. 

Figure 53 shows relative rates of C02 formation under steady-state conditions that were recorded with various single-crystal surfaces of Pd as well as with a polycrystalline Pd wire (173). It must be noted that with these experiments no determination of the effective surface areas was performed so that no absolute turnover numbers per cm2 are obtained. Instead, the reaction rates were normalized to their respective maximum values. As can be seen from Fig. 53, all data points are close to a common line which indicates that, in fact, with this reaction the activity is influenced very little by the surface structure. As has been outlined in Section II, the adsorption of CO exhibits essentially quite similar behavior on single-crystal planes with varying orientation. Since the adsorption-desorption equilibrium of CO forms an important step in the overall kinetics of steady-state C02 formation, this effect forms at least a qualitative basis on which the structural insensitivity may be made plausible. 

BasUe et aL [116] studied the WGS reaction using a MR consisting of a composite palladium-based membrane realized with an ultrathin palladium film ( 0.1 pm) coated on the inner surface of a porous ceramic support (y-Al203) by the co-condensation technique. The authors pointed out the benefit of applying a palladium MR, taking into account that, at 320°C and 1.1 bar, the thermodynamic equilibrium of CO conversion is around 70%, while the authors obtained with the MR CO conversion of around 100%. Moreover, the same authors illustrated that a complete CO conversion could be reached by using a composite membrane with a thinner palladium layer (10 pm Pd film coated on a ceramic support) [117]. 

CO shift reaction. CO shift reaction (1.16) is reversible and exothermic. The reaction heat and the equilibrium constant of the reaction decrease with temperature. The following factors have effects on chemical equilibrium of CO shift reaction. 

It will be seen in Section 5.2.1.4 that, since these rotational transitions of CO are associated with the zero-point (v = 0) level, the value of B obtained here is, in fact. Bo and not the equilibrium value B. Equation (5.25) shows how these values are related. 

H. Lennartz and co-workers. Vaporisation Equilibrium of the Water—Sulfuric Acid System, Rep. Fur. 6783, Commission of European Communities, Hydrogen Energy Vector, Europe, 1980, pp. 60—70. 

T. Vermeulen and co-workers. Vapor—Eiquid Equilibrium of the Sulfuric Acidj Water System, AIChE meeting, Anaheim, Calif., (June 10, 1982). 

A closer analysis of die equilibrium products of the 1 1 mixture of methane and steam shows the presence of hydrocarbons as minor constituents. Experimental results for die coupling reaction show that the yield of hydrocarbons is dependent on the redox properties of the oxide catalyst, and the oxygen potential of the gas phase, as well as die temperamre and total pressure. In any substantial oxygen mole fraction in the gas, the predominant reaction is the formation of CO and the coupling reaction is a minor one. 

The equilibrium constant Kg is determined at any temperature from standard state information on reactants and product. Considering the synthesis of CH3OH, the equilibrium conversion Xg is determined for a stoichiometric feed of CO and Hj at the total pressure. These conversions are determined by the number of moles of each species against conversion X by taking as a basis, 1 mole of CO. 

Substituting the expressions for the mole fraetions of CO, Hj, CH3OH, respeetively, for the equilibrium eonstant yields... 

Bergmann has suggested that oxidation is ruled out at positions (where hydration occurs readily) which are not accessible to the enzyme after the pteridine is adsorbed on it. Alternatively, the destruction of co-planarity by hydration may prevent adsorption of the pteridine on the enzyme. The case of xanthopterin (2-amino-4,6-dihydroxypteridine) may be relevant. The neutral species of this substance exists as an equilibrium mixture of approximately equal parts of the anhydrous and 7,8-hydrated forms (in neutral aqueous solution at 20°). Xanthine oxidase cataljrzes the oxidation of the anhydrous form in the 7-position but leaves the hydrated form unaffected and about two hours is required to re-establish the former equilibrium. 

An increase in carbonate-ion concentration moves the equilibrium in favour of calcium carbonate deposition. Thus one secondary effect of cathodic protection in seawater is the production of OH , which favours the production of CO, , which in turn promotes the deposition of CaCOj. Cathodically protected surfaces in seawater will often develop an aragonite (CaCOj) film. This film is commonly referred to as a calcareous deposit. 

The general theoretical treatment of ion-selective membranes assumes a homogeneous membrane phase and thermodynamic equilibrium at the phase boundaries. Obvious deviations from a Nemstian behavior are explained by an additional diffusion potential inside the membrane. However, allowing stationary state conditions in which the thermodynamic equilibrium is not established some hitherto difficult to explain facts (e.g., super-Nemstian slope, dependence of the selectivity of ion-transport upon the availability of co-ions, etc.) can be understood more easily. 

Because of the opposite effects of temperature on the stability of CO and CH4, the odd result is that, as the temperature increases, graphite deposition is less likely for starting mixtures which are near stoichiometric, but it is more difficult to produce pure methane by removing water and allowing the mixture to react further. Because of equilibrium considerations, the final approach to pure methane must be done at a relatively low temperature. 

Figure 8.11 Molar volumes of CO (s) and COi(g) at p = 0.100 MPa. At this pressure, the solid is in equilibrium with gas at T = 194.52 K. (Note the change in the volume scale.)...

The reaction of Si02 with SiC [1229] approximately obeyed the zero-order rate equation with E = 548—405 kJ mole 1 between 1543 and 1703 K. The proposed mechanism involved volatilized SiO and CO and the rate-limiting step was identified as product desorption from the SiC surface. The interaction of U02 + SiC above 1650 K [1230] obeyed the contracting area rate equation [eqn. (7), n = 2] with E = 525 and 350 kJ mole 1 for the evolution of CO and SiO, respectively. Kinetic control is identified as gas phase diffusion from the reaction site but E values were largely determined by equilibrium thermodynamics rather than by diffusion coefficients. 

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