Desorption recombinative

Studying the kinetics of the interaction of N2 with the Ru catalysts revealed that the Cs promoter enhances both the rate of dissociative chemisorption and the rate of recombinative desorption. Ru catalysts were found to be rather inactive for NH3 synthesis without alkali... 

Experimental probes of Born-Oppenheimer breakdown under conditions where large amplitude vibrational motion can occur are now becoming available. One approach to this problem is to compare theoretical predictions and experimental observations for reactive properties that are sensitive to the Born-Oppenheimer potential energy surface. Particularly useful for this endeavor are recombinative desorption and Eley-Rideal reactions. In both cases, gas-phase reaction products may be probed by modern state-specific detection methods, providing detailed characterization of the product reaction dynamics. Theoretical predictions based on Born-Oppenheimer potential energy surfaces should be capable of reproducing experiment. Observed deviations between experiment and theory may be attributed to Born-Oppenheimer breakdown. 

Calculations carried out by Gottesfeld et al. [52], who borrowed from studies of thermal desorption of H2 from Cu [56, 57], indicate that H2 rather than H20 should be a product of formaldehyde oxidation at Cu at potentials up to ca. +0.4 V vs. RHE. This is provided conditions are such that the activation energy for hydrogen recombination and desorption does not exceed 10 kcal/mole. Obviously a relatively high activation energy (which appears never to be observed at Cu) favors eventual oxidation of adsorbed H atoms, before recombinative desorption can occur. Gottesfeld et al. s calculation is interesting, but perhaps not a definitive calculation since it is... 

The extra oxygen adsorbed on the Mo(l 12)-(1 X2)-0 surface drastically changes the selectivity of the reaction. TPR spectra of methanol from the (lX2)-0 surface with 0.20 ML of preadsorbed extra oxygen after exposure to 4 L of methanol at 200 K are different from the spectra for the surface without the extra oxygen on the following points (1) considerable reduction of the peaks of CH4 and H2 at 560 K, the second is (2) disappearance of the peak of recombinative desorption of CO at 800 K, and (3) appearance of the peak of H20 at 580 K. The amounts of desorption products are summarized in Table 8.2. Selectivity to CH20 increased to 88%. Particularly, reduction of re-combinative desorption of CO at 800 K indicates that complete decomposition of methoxy to C(a) and O(a) is considerably suppressed by the presence of extra oxygen. Detection of H20 and... 

CI2 evolution reaction, 38 56 electrochemical desorption, 38 53-54 electrode kinetics, 38 55-56 factors that determine, 38 55 ketone reduction, 38 56-57 Langmuir adsorption isotherm, 38 52 recombination desorption, 38 53 surface reaction-order factor, 38 52 Temkin and Frumkin isotherm, 38 53 real-area factor, 38 57-58 regular heterogeneous catalysis, 38 10-16 anodic oxidation of ammonia, 38 13 binding energy quantification, 38 15-16 Haber-Bosch atrunonia synthesis, 38 12-13... 

Boland. J. J. (1991b). Evidence of pairing and its role in the recombinative desorption of hydrogen from the Si(100)-2 X 1 structure. Phys. Rev. Lett. 67, 1539-1542. 

Lewis, L. B., Segall, J. and Janda, . C. Recombinative desorption of hydrogen from the Ge(100)-(2 x 1) surface a laser-induced desorption study. Journal of Chemical Physics 102, 7222-8 (1995). 

Two results appear immediately from the study of CO on Mo2C. First, the TPD data show that CO only partially dissociates on Mo2C. Dissociation on Mo2C leads to high temperature recombinative desorption of CO. However, the ratio of the area of the latter peak to the saturation area of the 325 K feature is 0.2. Second, the RAIRS spectra display a single v(CO) stretching mode with a frequency characteristic of on-top bonded... 

The right-hand panel of Figure 2.9 corresponds to the recombinative desorption of N-atoms on Rh(100). As two adsorbed N-atoms are involved, the rate of desorption depends on the coverage squared this leads to TPD peaks which shift to lower temperatures with increasing coverage [19]. The presence of lateral interactions is difficult to recognize without detailed analysis of the kinetics (this point is discussed later). 

Product state analysis offers a flexible way to obtain detailed state resolved information on simple surface reactions and to explore how their dynamics differ from the behaviour observed for H2 desorption [7]. In this chapter, we will discuss some simple surface reactions for which detailed product state distributions are available. We will concentrate on N2 formation in systems where the product desorbs back into the gas phase promptly carrying information about the dynamics of reaction. Different experimental techniques are discussed, emphasising those which give fully quantum state resolved translational energy distributions. The use of detailed balance to relate recombinative desorption measurements to the reverse, dissociation process is outlined and the influence of the surface temperature on the product state distributions discussed. Simple low dimensional models which provide a reference point for discussing the product energy disposal are described and then results for some surface reactions which form N2 are discussed in detail, emphasising differences with the behaviour of H2. 

Once (Ei — E0) becomes greater than the energy spacing to10 a vibrational population inversion may be obtained, the exact point depending on the temperature. Excess vibrational excitation has been observed in most recombinative desorption systems, a result of the role of the vibrational coordinate during creation of the new bond. 

The extensive surface reconstruction in the presence of N has implications for our discussion of the recombination process, since we must consider whether N2 forms from recombination on the unreconstructed Cu(l 1 1) surface or is formed by decomposition of copper nitride islands. In the latter case N recombination may either leave the local Cu atoms in a metastable (100) arrangement or else recombination might be associated with substantial motion of the Cu atoms as they relax from the nitride adsorption geometry. If N recombination occurs at nitride islands then the dynamics of recombinative desorption will sample a phase space which is completely different to that for dissociation on clean flat Cu terraces, making it impossible to relate these two processes by detailed balance. This is the behaviour of H recombination on Si where the large change in the Si equilibrium geometry induced by H adsorption ensures that the adsorption and desorption processes sample very different channels [13]. 

The two phase model describes all the principle features of the desorption kinetics, suggesting that recombinative desorption under conditions where the coverage is less than saturation occurs by the recombination of N atoms from a dilute phase on the Cu(l 11) surface. This behaviour is the same as that observed for H recombinative desorption on many surfaces [63]. Desorption from the dilute phase is preferred over direct decomposition of the nitride islands because this leaves the copper surface in its equilibrium (111) orientation, rather than as an unstable Cu(l 00) overlayer [99]. As a result we expect that detailed balance can be used to relate measurements of recombination from the N covered Cu(l 1 1) surface with nitrogen dissociation on bare Cu(l 1 1) terraces. In contrast, if desorption occurred via decomposition of reconstructed copper nitride islands then detailed balance arguments would not reveal anything about the energetics or dynamics of N2 dissociation on a Cu(l 1 1) surface. 

Before A and B can react, they must both adsorb on the catalyst surface. The next event is an elementary step that proceeds through a reaction of adsorbed intermediates and is often referred to as a Langmuir-Hinshelwood step. The rate expression for the bimolecular reaction depends on the number density of adsorbed A molecules that are adjacent to adsorbed B molecules on the catalyst surface. This case is similar to the one developed previously for the recombinative desorption of diatomic gases [reverse reaction step in Equation (5.2.20)] except that two different atomic species are present on the surface. A simplified rate expression for the bimolecular reaction is ... 

Methanol desorbs from the (110) surface in low and high temperature states, as recently verified in separate studies by Henderson et al. [71,72] and Vohs et al. [73] The low temperature desorption state has been assigned to desorption of molecularly adsorbed methanol. The higher temperature states have been assigned as recombinative desorption of methanol with surface hydroxyl groups based on SSIMS and HREELS identifications of surface species [52,71-73]. 

When appreciably strong lateral interactions between electrosorbed intermediates arise, for instance, when they are partially charged species, the R values and the corresponding Tafel slopes can be worked out for various mechanisms they are usually closely related, functionally. We show the results of calculations for the heterogeneous recombination and the electrochemical desorption steps, for example, for the case of H in the HER or Cl in CI2 evolution. The equations for recombination desorption are... 

---