CO desorption

Negleeting CO desorption, as in the standard ZGB model, the CO-poi-soned state is irreversible sinee there is no possibility of removing CO from the surfaee. So, CO desorption has to be eonsidered in order to avoid the fully CO-poisoned state. The adsorption and desorption of X then drives the system from a state with high eoneentration of adsorbed CO to the reaetive state and baek. This proeess ean be understood with the aid of Fig. 8. At low X eoverage only the reaetive state is stable. Inereasing X eoverage eauses site bloeking and eonsequently the adsorption of both CO and O2 is redueed. 

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] ... 

FIG. 16 Variation of the steady-state rate of production, Pcoj, with Pco in the NO + CO lattice gas model with NO desorption (rate d o = 0.5), and CO desorption at various rates (shown). The inset shows the reaction rate measured experimentally at 410 K. (From Ref. 81.)... 

The effect of electronegative modifiers on the activation energy of CO desorption, Ed, and on the corresponding pre-exponential factor, vd, can be quantified by analysis of the TPD spectra at very low CO coverages. The... 

J.L. Falconer, and R J. Madix, Flash desorption activation energies DCOOH decomposition and CO desorption fromNi(l 10), Surf. Sci. 48, 393-405 (1975). 

If the desorption process follows straightforward first-order kinetics, one may divide the rate at any temperature by the actual coverage, and plot this logarithmically against the reciprocal temperature, to construct an Arrhenius plot. This procedure usually works well in cases where the initial coverage is sufficiently low that lateral interactions play no role. For example, it would work well for CO desorption with an initial coverage below 0.3 ML (Fig. 7.7). 

The carbon monoxide reaction is well studied and the observed kinetics are well understood. Of particular interest is the so-called CO-inhibiting regime , characterized by carbon dioxide covering and blocking the surface, so that the reaction rate is governed by CO desorption rate (see original citations in [78]). 

Figure 2 displays a qualitative correlation between the increase or decrease in CO desorption temperature and relative shifts in surface core-level binding energies (Pd(3d5/2), Ni(2p3/2), or Cu(2p3/2) all measured before adsorbing CO) [66]. In general, a reduction in BE of a core level is accompanied by an enhancement in the strength of the bond between CO and the supported metal monolayer. Likewise, an opposite relationship is observed for an increase in core-level BE. The correlation observed in Figure 2 can be explained in terms of a model based on initial-state effects . The chemisorption bond on metal is dominated by the electron density of the occupied metal orbital to the lowest unoccupied 27t -orbital of CO. A shift towards lower BE decreases the separation of E2 t-Evb thus the back donation increases and vice versa. 

Davies JC, Bonde J, Logadottir A, Nprskov JK, Chorkendorff I. 2005. The ligand effect CO desorption from PtRu catalysts. Euel Cells 4 429. 

In agreement with the DEMS results the ECTDMS work of Wilhelm et at. (1987b) also showed that the nature of the predominant adsorbate is a function of both coverage and methanol concentration. The authors performed ECDTMS measurements at two different methanol concentrations. The areas under the H2 and CO desorption peaks were calibrated and allowed the number of desorbed H and CO particles, nH and nc0, to be calculated. nH represents the number of COH species present on the surface and nco the total number of CO and COH adsorbates. From these values the mole fraction of C=Oada and COHads can be calculated, as shown in Figure 3.40. 

Combined TPD and theoretical investigation of CO desorption from Cu-K-FER zeolite... 

CO desorption from photoexcited free (carbonyl)gold clusters has been studied by photoelectron spectroscopy. For the particular example [Au2(CO)]-, the unimolecular desorption threshold has been approximated by statistical calculations using the experimentally determined rate constants.294... 

For the present analysis, it is assumed that the activation energy for CO desorption decreases linearly with CO coverage. 

Implicit in the use of Equation (10) is the assumption that desorption occurs from the surface populated by one type of adsorbed species. This single-state model will be shown to be adequate to describe the dynamic behavior of CO desorption at the high temperature (723 K) considered here. The analysis of the more general case of multi-state desorption can be found in Donnelly et al. (32) and Winterbottom (3). 

Figure 6. Integral-averaged CO coverage as a function of time during CO desorption. Key -----------------------, measured and------, calculated.

Here again, good agreement between theory and experiment was obtained. Notice that the CO desorption occurred much more slowly than the CO adsorption that is, a ten-fold difference in the 50% relaxation time (2 s vs 0.2 s) was observed. 

Figure 8 shows how the intrapellet concentration profiles vary with time during the course of CO desorption. Both the gas-phase (solid lines) and surface (dotted lines) CO concentration profiles exhibit relatively mild gradients inside the pellet, in contrast to the steep profiles established during the adsorption process. This can be attributed to the fact that the intrinsic rate of desorption is slower than that of adsorption. 

Figure 10. Effects of internal and external transport resistances on the computed step-response of CO desorption. Curve A corresponds to our experimental conditions. Key A, km — 60 cm/s, Deff = 0.0246 cms/s B, km —r oo, Def, = 0.0246 cm2/s and C,km- oo, Delt oo.

The rate of CO removal from the Pt surface is also affected by the presence of O2 in the gas phase, as demonstrated in Figure 12. In this experiment the catalyst, initially in equilibrium with 1 vol % CO (in N ), was suddenly exposed to a feedstream of 0.7 vol % O2 (in 1 7 It can be seen from Figure 12 that the Pt-CO band decays much faster in O2 (Curve B) than in (Curve A for reference). This indicates that the surface reaction between CO and oxygen is faster than the rate of CO desorption. 

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