Macs most abundant catalyst-containing

If significant fractions of the catalyst material may be bound in the form of reaction intermediates, the rules for reaction orders in noncatalytic simple pathways no longer apply. However, if one of the cycle members—the free catalyst or an intermediate—is a macs (most abundant catalyst-containing species, containing practically all of the catalyst material), the rules for noncatalytic pathways can be adapted The rate equation and reaction orders for the cycle are the same as for an imaginary equivalent linear pathway that starts and ends with macs. A cycle member that contains only an insignificant fraction of catalyst material is a lacs (low-abundance catalyst containing species), and the denominator terms it contributes can be dropped. 

If the catalyst is present almost completely in the form of one member of the cycle, be it as the free catalyst or an intermediate Xj, that species is called the "most abundant catalyst-containing species," or macs for short. If a macs exists, one row of the matrix 8.31 dominates all others the first row if the macs is the free catalyst, the (j + l) th row if the macs is Xj. The Christiansen rate equation 8.30 is thereby reduced to the lower degree of complexity of those of bulk-catalytic and noncatalytic simple pathways, with only k instead of k2 terms in the denominator (see eqns 8.2 and 6.4 to 6.6). 

The concept of a most abundant catalyst-containing species (macs in this book) was originally introduced by Boudart for heterogeneous catalysis under the name of masi, for "most abundant surface intermediate" [42]. 

A concept of a most abundant catalyst-containing species macs) or most abundant surface intermediate (masi) is often used, which could reduce the complex mutistep reaction to two-step sequence. 

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