| Summary: | Improving the residual char of polypropylene (PP) is difficult due to the preferential complete combustion. Here, we designed a combination catalyst that not only provides physical barrier effects, but also dramatically promotes catalytic charring activity. We successfully synthesized WS<sub>2</sub> monolayer sheets decorated with isolated Ni atoms that bond covalently to sulfur vacancies on the basal planes via thiourea. Subsequently, PP blends composed of 8 wt.% Ni-decorated WS<sub>2</sub>, NiO, and activated carbon (AC) were obtained (<sup>E</sup>Ni-<sup>S</sup>WS<sub>2</sub>-AC-PP). Combining the physical barrier effects of WS<sub>2</sub> monolayer sheets with the excellent catalytic carbonization ability of the <sup>E</sup>Ni-<sup>S</sup>WS<sub>2</sub>-AC combination catalyst, the PP blends showed a remarkable improvement in flame retardancy, with the yield of residual char reaching as high as 41.6 wt.%. According to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations, it was revealed that the microstructure of residual char contained a large number of carbon nanotubes. The production of a large amount of residual char not only reduced the release of pyrolytic products, but also formed a thermal shield preventing oxygen and heat transport. Compared to pure PP, the peak heat release rate (pHRR) and total heat release rate (THR) of <sup>E</sup>Ni-<sup>S</sup>WS<sub>2</sub>-AC-PP were reduced by 46.32% and 26.03%, respectively. Furthermore, benefiting from the highly dispersed WS<sub>2</sub>, the tensile strength and Young’s modulus of <sup>E</sup>Ni-<sup>S</sup>WS<sub>2</sub>-AC-PP showed similar values to pure PP, without sacrificing the toughness.
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