Poisons surface reconstruction

Poisoning is caused by chemisorption of compounds in the process stream these compounds block or modify active sites on the catalyst. The poison may cause changes in the surface morphology of the catalyst, either by surface reconstruction or surface relaxation, or may modify the bond between the metal catalyst and the support. The toxicity of a poison (P) depends upon the enthalpy of adsorption for the poison, and the free energy for the adsorption process, which controls the equilibrium constant for chemisorption of the poison (KP). The fraction of sites blocked by a reversibly adsorbed poison (0P) can be calculated using a Langmuir isotherm (equation 8.4-23a) ... 

The hydrogenolysis of EtaSiH over silica-supported Pd and Pt catalysts resulted in significant poisoning, specifically, the loss of activity in the hydrogenation of cyclohexene.375 Oxidation, however, fully restored the activity of catalysts with small metal particles (>50% dispersion) as a result of surface reconstruction. 

Gallagher et al. (2003) Electroanalysis Pt3Sn Surface reconstruction, structural stability from UHV to fluid, electroadsorption of CO + + + CO poisoning... 

C0(ads)+0(ads)— C02(ads)+, 4.C02(ads) — C02(gas)+. The probabilities of steps 1 and 2 are between 0 and 1, while probabilities of other steps are P(3) = 1, P 4) = 1, P(-l)= 0 P(-2)= 0, P(-4)=0. The ZGB-model shows the effect of heterogeneity in the adlayer because of the infinitely fast formation of C02, there is a segregation of the reactants in CO and oxygen islands. The original model has later been extended and modified by numerous people to include desorption of the reactants, diffusion, an Eley Rideal mechanism for the oxidation step, physisorption of the reactants, lateral interactions, an oxidation step with a finite rate constant, surface reconstruction and additional poisoning adsorbates. 

The half-cell enclosed by the dashed line in Figure 15-1 is called a silver-silver chloride electrode. Figure 15-3 shows how the electrode is reconstructed as a thin tube that can be dipped into an analyte solution. Figure 15-4 shows a double-junction electrode that minimizes contact between analyte solution and KCI from the electrode. The silver-silver chloride and calomel reference electrodes (described soon) are used because they are convenient. A standard hydrogen electrode (S.H.E.) is difficult to use because it requires H2 gas and a freshly prepared catalytic Pt surface that is easily poisoned in many solutions. 

One of the challenges for the future is to refine existing analytical techniques and to develop new ones for characterizing catalysts and species adsorbed on catalyst surfaces. Of particular need are methods that allow the observation of a catalyst under actual working conditions, because the structure and composition of a catalyst surface in the working environment are often different from those existing prior to reaction. Examples of such effects include the reconstruction of metal surfaces, the appearance of defects in oxides, and the deposition of poisons. 

Atomic C and O are yielded by the dissociation of CO in FTS, the C adsorption on Co surfaces is crucial for this reaction. As listed in Table 2, the adsorption of C on most Co models is much stronger than the adsorption of H and O. Weststrate et al. made a systematic investigation on the C adsorption on Co surfaces. The experimental results indicated that atomic carbon weakened the adsorption of CO and H2, whereas a saturated (reconstructed) atomic carbon-covered surface can still adsorb 60% of the CO and H compared to the clean surface. Thus, a high coverage of atomic carbon may not be a strong poison to FT cobalt catalysts. In other words, FTS on Co catalysts is feasible in spite of the strong adsorption of C atoms on Co surfaces. 

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