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The green biogenic PdAu nanoparticles (bio-PdAu NPs) exhibits remarkable catalytic performance in hydrogenation, which is highly desired. However, the catalytic principles and effectiveness of bio-Pd x Au y NPs in response to various catalytic systems (electrocatalysis and suspension-catalysis) are unclear. Herein, a facile synthetic strategy for bio-Pd x Au y NPs synthesis with controlled size and the catalytic principles for hydrogen evolution reaction (HER) and azo dye degradation is reported. In the biosynthetic process, the size and composition of the bio-Pd x Au y NPs could be precisely controlled by predesigning the precursor mass ratio of Pd/Au, and the Au proportion showed a linear relationship with the size of NPs (R 2  = 0.92). The obtained bio-Pd x Au y NPs exhibit variable activity in electrocatalysis (HER) and suspension-catalysis (azo dye degradation). For electrocatalysis, the formation of conductive networks that facilitates the extracellular electron transfer is crucial. It was revealed that the bio-Pd 2 Au 8 exhibited superior electrocatalytic performance in HER/toward hydrogen evolution, with a maximum current density of 1.65 mA cm -2 , which was 1.54 times higher than that commercial Pd/C (1.07 mA cm -2 ). The high electrocatalytic activity was attributed to its appropriate size (81.38 ± 6.14 nm) and uniform distribution on the cell surface, which promoted the extracellular electron transfer by constructing a conductive network between catalyst and electrode. However, for suspension-catalysis, the size effect and synergistic effect of bimetallic NPs have a more prominent effect on the degradation of azo dyes. As the increase of Au proportion the particle size decreases, and the catalytic activity of bio-Pd x Au y improved significantly. The response principles of bio-Pd x Au y proposed in this study provide a reliable reference for the rational design of bio-based bimetallic catalysts with enhanced catalytic performance., (Copyright © 2021 Elsevier Inc. All rights reserved.)