Effect of Electronegative Additives

The influence of electronegative additives on the CO hydrogenation reaction corresponds mainly to a reduction in the overall catalyst activity.131 This is shown for example in Fig. 2.42 which compares the steady-state methanation activities of Ni, Co, Fe and Ru catalysts relative to their fresh, unpoisoned activities as a function of gas phase H2S concentration. The distribution of the reaction products is also affected, leading to an increase in the relative amount of higher unsaturated hydrocarbons at the expense of methane formation.6 Model kinetic studies of the effect of sulfur on the methanation reaction on Ni(lOO)132,135 and Ru(OOl)133,134 at near atmospheric pressure attribute this behavior to the inhibition effect of sulfur to the dissociative adsorption rate of hydrogen but also to the drastic decrease in the 

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The effect of electronegative additives on the adsorption of ethylene on transition metal surfaces is similar to the effect of S or C adatoms on the adsorption of other unsaturated hydrocarbons.6 For example the addition of C or S atoms on Mo(100) inhibits the complete decomposition (dehydrogenation) of butadiene and butene, which are almost completely decomposed on the clean surface.108 Steric hindrance plays the main role in certain cases, i.e the addition of the electronegative adatoms results in blocking of the sites available for hydrocarbon adsorption. The same effect has been observed for saturated hydrocarbons.108,109 Overall, however, and at least for low coverages where geometric hindrance plays a limited role, electronegative promoters stabilize the adsorption of ethylene and other unsaturated and saturated hydrocarbons on metal surfaces. 

Model studies on single crystal surfaces are also helpful in developing an understanding of the effects of surface additives on catalyst performance. Electronegative, electroneutral (i.e. metals) and electropositive additives can all be studied. The influence of additives on the bond strengths and structure of... 

Brook and coworkers have studied the effects of electronegative groups on silicon on the reactivities of vinylsilanes115. Unlike the substitution reaction of 1-trimethylsilylprop-1-ene with acetyl chloride (equation 51a) the major product of the reaction of fi-trichlorosilylstyrene with phenylacetyl chloride arises from addition, as shown in equation 51b. 

N-C=C-N fragment which serves as an electron donor and the kinetic stabilization provided by the two bulky adamantanyl substituents. Still, these groups are not mandatory as illustrated by the perdeu-terio derivative 16 which was shown to be a true carbene with negligible ylidic character by X-ray and neutron diffraction studies. [13] A quantitative study of the influence of steri-cally demanding substituents demonstrated that dimesitylcarbene 17 is ca. 160 times more stable than diphenylcarbene at room temperature [14] and for didurylcarbene a still further increase of life-time has been observed because of the buttressing effect of the additional methyl substituents. 

To illustrate the primary effects of adatom addition, single-crystal electrodes are discussed here. Feliu and Herrero have extensively studied formic acid electrooxidation on Pt single-crystal substrates modified with an array of various adatoms. They have established a connection between the electronegativity of the adatoms in relation to Pt and the type of active enhancement mechanism incurred as a function of adatom coverage [42]. Their results support inhibition of the indirect pathway on Pt(lll) terraces and they have demonstrated that COads formation occurs at step and defect sites. For Pt(l 11) substrates decorated with electropositive adatoms, such as Bi, Pb, Sb, and Te, the electronic enhancement is extended to the second or third Pt atom shell from the adatom. While for electronegative adatoms, in respect to Pt, the third-body effect dominates with increased coverages, such as S and Se. 

The effect of electronegative substituents is even more marked when they are attached to sp -hybridized carbons. Thus Vch in methyl cyanide is 136 Hz but in vinyl cyanide it is 177 Hz. Some ch values are presented in Table 4.26. The additivity of substituent effects on aliphatic carbons is given by the equation... 

E. As in carbonyls, however, attaching an electronegative atom to P raises the P=0 stretch. The effect of each additional electronegative oxygen is roughly additive. 

The effect induced by different electronegative additives is more pronounced in the case where the additive adatoms occupy the most coordinated sites forming ordered structures (e.g Cl addition onNi(lOO)). In this case (Fig. 2.28) one modifier adatom affects 3-4 CO adsorption sites and complete disappearance of the CO p2-peak is observed above modifier coverages of -0.25 or less. The lack of ordering and the tendency of the modifier to form amorphous islands (e.g. P on Ni(100)) diminishes the effect. Thus in the case of P on Ni(100) the disappearance of the CO p2-peak is observed at P coverages exceeding 0.6. 

In the above discussion the effect of difference in electronegativity of unlike atoms on bond length (usually a decrease) has been ignored. There is the possibility also of a small change in bond length between unlike atoms, such as of a metal and a metalloid, that reflects the difference in the nature of the overlapping orbitals, in addition to the effects of partial ionic character and of electron transfer. I believe that a thorough... 

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