CO+ O reaction

Fig. 9. Theoretical data for the variation of the oxygen ((-)<,) and CO (Mcf>) coverages, as well as of the fraction of the surface present in the nonreconstructed 1 x I structure (a) as a function of time, as evaluated from the solution of Eqs. (4), (5a), and (6), modeling the oscillations for the CO/O, reaction on Pl( 110). Parameters adopted from experimental data for Pq2 — 5 x 10 5 torr, pco = 2.3 x I O torr, and T = 540 K. (From Ref. 69.)...

Fig. 11. Conditions pm and p(), at fixed T = 480 K for the occurrence of oscillations in the CO/O, reaction on Pt(IOO) and Pt( 110) surfaces. The dashed line marks the lower limit for the CO pressure to cause the hex —> I x I structural transformations on Pt(IOO), which also coincides with the lower boundary of the oscillatory range for this plane. (From Ref.

Fig. 13. Kinetic oscillations during the CO/O reaction on Pt(110) at I = 540 K, />0, = 7.5 x 10-5 torr, and for varying pm. (From Ref. 71.) (a) pco = 3.90 x 10 lorr constant behavior (fixed point), (b) pt0 = 3.K4 x I0"5 torr onset of harmonic oscillations with small amplitudes (Hopf bifurcation), (c)pco = 3.66 x 10 5 torr harmonic oscillation with increased amplitude, (d) pc0 = 3.61 x I0-5 torr first period doubling, (e) pc0 = 3.52 x 10 torr second period doubling, (f) pco = 3.42 x 10 5 torr aperiodic (chaotic) behavior.

Fig. 14. Bifurcation diagram for the oscillatory CO/O, reaction on Pt(IIO) as derived from the experimental data of Fig. 13. (From Ref. 71.)...

Fig. 17. Experimental dynamic phase diagram for forced oscillations in the CO/O, reaction on Pt(l 10). Periodic modulation of the 02 pressure with amplitude A (as a percentage of the constant base pressure) and period length, Tex, with respect to that of the autonomous oscillations, T . (From Ref. 91.) Fixed parameters, for the subharmonic (superharmonic) range p(h = 3.0 (4.15) x 10 5 torr, p , = 1.6 (2.1) x 10 5 torr, T = 525 (530) K.

Fig. 25. CO/O, reaction on Pt(llO) pco = 2.3 x 10 5 torr, T = 470 K. At the point marked by an arrow, p,h was changed from 1.5 to 2.0 x 10 4 torr. The stationary reaclion rate starts to increase slowly due to faceting until oscillatory conditions are reached. (From Ref. 24.)...

Fig. 34. Development of kinetic oscillations with the CO/O, reaction on Pt(2IO). After establishing the indicated conditions at point C, the system slowly evolved oscillations with continuously changing periods and amplitude. (From Ref.. . )...

Fig. 35. The CO/O, reaction on Pdf 110). Slight variation of the conditions caused a transition from harmonic to period-doubling behavior. (From Ref. 147.)...

The occurrence of both reaction pathways has been demonstrated in recent time-resolved STM investigations (Kim and Wintterlin, 2004), which also showed that the reaction takes place at random positions, because the O and CO diffusion barriers are comparable to the activation energy for the CO + O reaction. The most recent theoretical calculations support these findings fully they even suggest that the dominating reaction is between the adsorbed CO and O (Reuter et al., 2004). 

Figure 2. Temperature map and profile in the ignited state for the CO-O, reaction.

Generally, almost all catalytic surface reactions proceed via the LH mechanism. Experimental verification was, for example, obtained in studies in which after adsorption of the reactants A + B, the gas phase is pumped off and the sample temperature continuously increased until the product molecules come off. This procedure is denoted as temperature-programmed reaction spectroscopy (TPRS). In particular, operation of the LH mechanism may be concluded if the relaxation time for product formation is long enough (>10ps) to reach thermal equilibrium of the adsorbates with the surface. For the CO + O reaction on a Pd (111) surface, this delay time between the adsorption and product formation was determined in molecular beam experiments down to about 10 s, leading to derivation of the action energy for the LH step [15]. 

Because the synthesis reactions are exothermic with a net decrease in molar volume, equiUbrium conversions of the carbon oxides to methanol by reactions 1 and 2 are favored by high pressure and low temperature, as shown for the indicated reformed natural gas composition in Figure 1. The mechanism of methanol synthesis on the copper—zinc—alumina catalyst was elucidated as recentiy as 1990 (7). For a pure H2—CO mixture, carbon monoxide is adsorbed on the copper surface where it is hydrogenated to methanol. When CO2 is added to the reacting mixture, the copper surface becomes partially covered by adsorbed oxygen by the reaction C02 CO + O (ads). This results in a change in mechanism where CO reacts with the adsorbed oxygen to form CO2, which becomes the primary source of carbon for methanol. 

Figure 14.5 (a) Reaction of Al,Al -ethylenebis(3-Bu -salicylideniminato)cobalt(II) with dioxygen and pyridine to form the superoxo complex [Co(3-Bu Salen)2(02)py] the py ligand is almost coplanar with the Co-O-O plane, the angle between the two being 18°.< (b) Reversible formation of the peroxo complex [Ir(C0)Cl(02)(PPh3)2]. The more densely shaded part of the complex is accurately coplanar. ... 

Co-condensation reaction of the vapors of l,3-di-rcrt-butylimidazol-2-ylidene and nickel, palladium, or platinum gives the coordinatively unsaturated 14-electron sandwiches [L M] (M=Ni, Pd, Pt) of the carbene type (990M3228). Palladium(O) carbene complexes can also be prepared by the direct interaction of l,3-R2-imidazol-2-ylidenes (R=/-Pr, r-Bu, Cy, Mes) (L) with the palladium(O) compound [Pd(P(o-Tol)3)2] (OOJOM(595)186), and the product at the first stage is [(L)PdP(o-Tol)3l, and then in excess free carbene [PdL ]. 

This reaction is diffusion controlled in solid CO. The binuclear carbonyl is unstable and even at these low T decomposes to Ag2 or higher clusters. Similarly, Au forms Au(CO)[ o, 2> which does not dimerize. However, Cu cocondensed with CO and Ar forms compounds Cu (CO) (n = 1-4), which after warming to 35 K decompose to larger Cu carbonyl clusters of indeterminate composition, the IR spectra of which resemble CO chemisorbed onto bulk Cu. 

The crystal structure of an interesting complex, HFe3(CO),oCN(CHj)2, has been reported (59). This species, which arises from the reaction of Fe3(CO),2 and CgHjCOCl in dimethylformamide, has structure (XXII). This compound can be viewed as a derivative of the anion Fe3(CO),o-(/i-CNCH3)H, analogous to Fe3(CO),iH . No doubt such an anion (as yet unknown) would be extremely basic, and readily alkylated. 

The several polymeric metal carbonyls studied have led to some surprisingly high yields [e.g., Fe3(CO),2 and Ruj(CO)j2 in Table IV] but to no substantiated mechanisms. The 17% yield of Fe3(CO),2 in neutron-irradiated Fe(CO)j was interpreted as a reaction of Fe(CO)4 with the Fe(CO)5, but no further evidence is available. The study of Mn2(CO),o has been fruitful (44, 46). The insensitivity of the parent yield MnMn(CO),o to heat indicates that the molecule is formed by a reaction quite early in the sequence, perhaps epithermal. The discovery (46) of a species which reacts rapidly with I2 and exchanges with IMn(CO)5 led to the conclusion that the Mn(CO)5 radical is produced prominently (4.5%) by nuclear reactions in the solid decacarbonyl. The availability of this labeled Mn(CO)5 has made possible several interesting observations about the exchange properties of this radical in the solid (45) and in solution (42). 

CO oxidation, an important step in automotive exhaust catalysis, is relatively simple and has been the subject of numerous fundamental studies. The reaction is catalyzed by noble metals such as platinum, palladium, rhodium, iridium, and even by gold, provided the gold particles are very small. We will assume that the oxidation on such catalysts proceeds through a mechanism in which adsorbed CO, O and CO2 are equilibrated with the gas phase, i.e. that we can use the quasi-equilibrium approximation. 

CoCl(PPh3)3], Reaction of [Co(TIMEN )]Cl 9 with oxygen in the presence of NaBPh leads to the formation of the peroxo-complex [Co(r -02)(TIMEN 5 )] BPh 10, which is a rare example of a side-on r -peroxo cobalt complex (the majority of Co-O -adducts are rj -O -complexes, i.e. end-on). The authors also showed that 10 is capable of converting molecular oxygen to benzoylchloride. 

For the same reason, Ru(OOOl) modihcation by Pt monolayer islands results in a pronounced promotion of the CO oxidation reaction at potentials above 0.55 V, which on unmodified Ru(OOOl) electrodes proceeds only with very low reaction rates. The onset potential for the CO oxidation reaction, however, is not measurably affected by the presence of the Pt islands, indicating that they do not modify the inherent reactivity of the O/OH adlayer on the Ru sites adjacent to the Pt islands. At potentials between the onset potential and a bending point in the j-E curves, COad oxidation proceeds mainly by dissociative H2O formation/ OHad formation at the interface between the Ru(OOOl) substrate and Pt islands, and subsequent reaction between OHad and COad- The Pt islands promote homo-lytic H2O dissociation, and thus accelerate the reaction. At potentials anodic of the bending point, where the current increases steeply, H2O adsorption/OHad formation and COad oxidation are proposed to proceed on the Pt monolayer islands. The lower onset potential for CO oxidation in the presence of second-layer Pt islands compared with monolayer island-modified Ru(OOOl) is assigned to the stronger bonding of a double-layer Pt film (more facile OHad formation). 

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