Surface poisoning

Anchored amine materials can be prepared through a number of synthetic methodologies. Because of the potential importance of these materials to organic synthesis, a ninhydrin assay was developed as a rapid laboratory determination of available surface amines. The assay agreed well with expected values for aminopropyltriethoxysilane grafted onto commercial silica. The assay also distinguished between reactive amines and protonated or poisoned surface amines on co-condensed SBA-15 materials. 

Following the procedure used in Section 12.3.1.1 for the first-order case, it is readily shown that the effectiveness factor for the poisoned surface is given by... 

This equation indicates that a small amount of poisoned surface can lead to a sharp decline in apparent activity. For example, if only 10% of the catalyst surface has been deactivated in the case where the Thiele modulus for the unpoisoned reaction is 40, 3F = 0.200 so that the... 

As in the case of Ni it appears that the effects of sulfur on the adsorption of organic molecules on other metals are more complex than for either H2 or CO. For example, formaldehyde (HCHO), which dissociates at 80 K on a clean Ru(110) surface (190), adsorbs molecularly on the sulfur-poisoned surface (78). Similarly, Cosyns et al. (191) suggest that sulfiding of the platinum enhances the adsorption of the alkylaromatics compounds. 

AES studies of in situ sulfur-poisoned Ni and Ru showed the complete absence of carbon on the surface or in the near-surface region for Co, on the other hand, a fraction of a monolayer of carbon was present but there was no bulk carburization. For Co which was carbon deactivated prior to sulfur poisoning, surface carbon and bulk carburization were both observed (205). 

For completeness, we mention that SFG was able to detect the final product of CH3OFI decomposition (CO), whereas the frequency range of intermediate products such as formaldehyde was not accessible. Similarly, HP-XPS carried out with a laboratory X-ray gun cannot accurately differentiate CO from CH , 0. In contrast, the wider frequency range of PM-IRAS allowed the identification of, for example, CH2O (in addition to CO) at elevated methanol partial pressures on a strongly CH c-poisoned surface (177). As these PM-IRAS investigations are relevant to methanol oxidation, they are discussed below. 

Surface hydroxyls help to bind molecules (substrates and physisorbed reagents) onto surfaces and hence encourage reaction and inhibit leaching. Unfortunately, they can also bind product molecules creating difficulties in catalytic turnover and in site poisoning. Surface hydroxyls might be... 

A besetting problem with the study of alkene hydrogenation is deactivation of the catalyst by strongly-adsorbed carbonaceous deposits - or acetylenic residues that form even well below room temperature the base metals of Groups 8 and 10 are especially prone to this, but even the noble metals are not immune, and even the most fundamental studies are necessarily made on equilibrium surfaces, much of which is permanently inactivated. This raises the vexed question of whether such poisoned surfaces truly reflect the character of the metal, and indeed whether it is not on the carbonaceous overlayer that the catalysis occurs. Another idea that has been seriously suggested is that reaction actually occurs, in... 

In their treatment, Herington and Rideal reduce the surface to a geometric array of uniformly active sites, and consider the accessibility of different sizes of reactant molecule to surfaces poisoned to varying degrees by localized poison molecules of varying dimensions. The essential result of their investigation is that the extent to which a reactant can be adsorbed on a poisoned surface depends on the geometry of reactant, poison, and surface. 

FIGURE 3.9 Change in surface coverage versus time for the irreversible adsorption (poisoning) of Pt by CO gas. Starting from an initial CO surface coverage (Oq) at time f = 0, approaches a value of 1 (completely poisoned surface) at long times. 

Surface preparations enhance the quality of a bonded joint by performing one or more of the following functions (1) remove contaminants, (2) control adsorbed water, (3) control oxide formation, (4) poison surface atoms that catalyze adhesive breakdown, (5) protect the adhesive from tbe adherend and vice versa, (6) match the adherend crystal structure to the adhesive molecular structure, and (7) control surface roughness. Thus, surface preparations can affect the permanence of the joint as well as its initial strength. 

Surface poisoning. Surface poisoning, which is caused by the adsorption of contaminants such as C, CO, CO2, or hydrocarbons on the membrane surface, results in a decrease in the adsorption rate of H2. If the poisoning is too severe, the dissociative reactions at the surface may dominate the H2 permeation. 

Deactivation and self-regeneration thus proceed simultaneously, being essentially slower than the reaction steps. The quasi-steady-state conditions can be appHed for the main reaction, but not for deactivation, similar to the considerations above. The equation for the poison surface coverage is... 

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