In Eq. 2, n C2H4,out and n C2H4,in are molar flow rates of ethylene leaving and entering the reactor, respectively. Note that if X C2H2 = 0, the denominator of Eq. 2 is equal to zero and S C2H4 = 0 by convention. The values of S C2H4 are in the range [-1,1]. Ethylene is both a product of the acetylene selective hydrogenation reaction and part of the reaction feed. S C2H4 < 0 indicates that more ethylene (from the feed) was consumed than formed from acetylene, with S C2H4 = -1 corresponding to the full consumption of the ethylene of the feed. S C2H4 > 0 indicate selective materials and conditions. The ideal, desired behavior corresponds to S C2H4 = 1. In addition to the formation of the desired ethylene, the undesirable formation of the total-hydrogenation product ethane and the formation of oligomeric species (e.g., C 4 ) were also quantified (see ESI for details). The reactivity was measured at each 13.5 minutes (min) during t OS . Additional details on the catalytic tests are provided in ESI and elsewhere. B. Experimental and Calculated Candidate Descriptive Parameters