CO reactions

Chapter 12, to symmetry-caused energy barriers on the H2CO ==> H2 + CO reaction potential energy surface. 

The manufacture of hexamethylenediamine [124-09-4] a key comonomer in nylon-6,6 production proceeds by a two-step HCN addition reaction to produce adiponittile [111-69-3] NCCH2CH2CH2CH2CN. The adiponittile is then hydrogenated to produce the desired diamine. The other half of nylon-6,6, adipic acid (qv), can also be produced from butadiene by means of either of two similar routes involving the addition of CO. Reaction between the diamine and adipic acid [124-04-5] produces nylon-6,6. 

The catalytic reaction of NO and CO on single crystal substrates, under ultra-high vacuum conditions, has been extensively studied. Neglecting N2O formation and CO desorption, the Langmuir-Hinshelwood mechanism of the NO + CO reaction can be described by the following sequence of steps [16,17] ... 

B. J. Brosilow, R. M. Ziff. Comment on NO-CO reaction on square and hexagonal surfaces A Monte Carlo simulation. J Catal 136 275-278, 1992. 

The initiating radical is derived from the monomer by addition of the H2O molecule with a reduction of Co " to Co ". (reaction Scheme [29])... 

In the case of catalysts activated with the help of reduction by CO, reactions (8) and (9) are excluded from the kinetic scheme. Such catalysts contain the centers Nr before their contact with the polymerization medium. 

The effect of alkali presence on the adsorption of oxygen on metal surfaces has been extensively studied in the literature, as alkali promoters are used in catalytic reactions of technological interest where oxygen participates either directly as a reactant (e.g. ethylene epoxidation on silver) or as an intermediate (e.g. NO+CO reaction in automotive exhaust catalytic converters). A large number of model studies has addressed the oxygen interaction with alkali modified single crystal surfaces of Ag, Cu, Pt, Pd, Ni, Ru, Fe, Mo, W and Au.6... 

Table 10.4. Kinetic parameters of the elementary steps involved in the NO + CO reaction.

The NO + CO reaction is only partially described by the reactions (2)-(7), as there should also be steps to account for the formation of N2O, particularly at lower reaction temperatures. Figure 10.9 shows the rates of CO2, N2O and N2 formation on the (111) surface of rhodium in the form of Arrhenius plots. Comparison with similar measurements on the more open Rh(llO) surface confirms again that the reaction is strongly structure sensitive. As N2O is undesirable, it is important to know under what conditions its formation is minimized. First, the selectivity to N2O, expressed as the ratio given in Eq. (7), decreases drastically at the higher temperatures where the catalyst operates. Secondly, real three-way catalysts contain rhodium particles in the presence of CeO promoters, and these appear to suppress N2O formation [S.H. Oh, J. Catal. 124 (1990) 477]. Finally, N2O undergoes further reaction with CO to give N2 and CO2, which is also catalyzed by rhodium. 

Figure 10.9. Arrhenius plots of the NO + CO reaction over rhodium (111) and (110). [Adapted from C.H.F. Peden, D.N. Belton and S.J. Schmieg,j. Catal. 155 (1995) 204.1...

The mechanism of the NO -1- CO reaction at realistic pressures is thus very complicated. In addition to the reaction steps considered above, one also has to take into account that intermediates on the surface may organize into islands or periodically ordered structures. Monte Carlo techniques are needed to account for these effects. Consequently, we are still far from a complete kinetic description of the CO -1- NO reaction. For an interesting review of the mechanism and kinetics of this reaction we refer to Zhdanov and Kasemo [V.P. Zhdanov and B. Kasemo, Suif. Sci. Rep. 29 (1997) 31],... 

Figure 3. (a) Plots of rjjjj versus T for NHj decomposition on clean Rh. (b) versus T for the NO+CO reaction on clean Pt. Solid curves are fit to data using LH rate models Indicated In the text. 

Figure 2. Showing the beginning of CO reaction with the surface along the long Rh-0 islands, which are between the missing rows, as bright streaks.

Figure 5 summarises results for the CO2, N2 and N2O formation rates for the dependence of apparent activation energies on catalyst potential. Although there is a notable increase in activation energy with increased Na coverage in each case, the variation is not as abrupt at that characteristic of EP CO oxidation [24] and NO+CO reactions. 

Another approach to the determination of surface kinetics in these systems has been to combine molecular beams in the 10 2-10 1 mbar pressure range with the use of the infrared chemiluminescence of the C02 formed during steady-state NO + CO reactions. This methodology has been used to follow the kinetics of the reaction over Pd(110) and Pd(l 11) surfaces [49], The activity of the NO + CO reaction on Pd(l 10) was determined to be much higher than on Pd(lll), as expected based on the UHV work discussed in previous sections but in contrast with result from experiments under higher pressures. On the basis of the experimental data on the dependence of the reaction rate on CO and NO pressures, the coverages of NO, CO, N, and O were calculated under various flux conditions. Note that this approach relied on the detection of the evolution of gas-phase... 

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