  Catalyzing reactions effectively by vacuum fluctuations of electromagnetic
fields is a significant challenge within the realm of chemistry. Different from
most studies based on vibrational strong coupling, we introduce an innovative
catalytic mechanism driven by weakly coupled polaritonic fields. Through the
amalgamation of macroscopic quantum electrodynamics (QED) principles with
Marcus electron transfer (ET) theory, our results reveal that ET reaction rates
can be precisely modulated across a wide dynamic range by controlling the size
and structure of nanocavities. Comparing to QED-driven radiative ET rates in
free space, plasmonic cavities induce substantial rate enhancements spanning
from orders of magnitude ranging from 10^3-fold to 10^1-fold. By contrast,
Fabry-Perot cavities engender rate suppression spanning from 10^{-2}-fold to
10^{-1}-fold. This work overcomes the necessity of using strong light-matter
interactions in QED chemistry, opening up a new era of manipulating QED-based
chemical reactions in a wide dynamic range.
