At a minimum, therefore, multiple supports must also be screened. Deposition methodology also is a factor. Since these conditions are not exactly known a priori, one has to screen all candidates at a series of conditions. It is also an endemic problem of catalyst research that making direct observations of catalysts at operating conditions challenges research creativity. Reproducibility of results more art than science - Beside reaction conditions, catalyst performance also depends on the history of the catalyst.
Some catalysts decline rapidly with time, but can be regenerated in situ.
All of these effects mean that screening at a variety of conditions is necessary. In the limit, the library becomes secondary to the screening of operating variables. Because of the existence of catalyst- support interactions described above, screening multiple samples on the same support, analogous to a 96 well plate, is of limited use, unless a variety of chemically different supports is available. Libraries must be generated by algorithms and are always therefore inherently rational. One is less likely to encounter surprising results searching a rationally assembled library than a truly random one.