Quasi-Monolayer Rh Nanoclusters Stabilized on Spinel MgAl2O4 Nanosheets for Catalytic CO2 Reforming of Methane
A unique nanocomposite of Rh quasi-monolayer clusters supported on spinel MgAl2O4 nanosheets with a low Rh loading of 0.15 wt % (Rh/MgAl2O4-MC) was synthesized by a facile method. By the catalyst design, nearly all of Rh atoms are utilized like well-known surface single-atom catalysts. More importantly and surprisingly, Rh/MgAl2O4-MC shows extremely high specific reaction rates of CH4 (rCH4) and CO2 (rCO2) per mole Rh and turnover frequency (TOF) for catalytic CO2 reforming of CH4 (DRM). Its rCH4 and rCO2 are as high as 51.14 and 62.65 mol molRh–1 s–1, which are 4.7 and 4.8 times higher than those of conventional Rh nanoparticles supported on spinel MgAl2O4 nanosheets with a higher Rh loading of 0.96 wt % (Rh/MgAl2O4-NP), respectively. Its TOF values of CH4 and CO2 are as much as 53.9 and 66.1 s–1, which are 3.5 and 3.5 times higher than those of Rh/MgAl2O4-NP, respectively. Meanwhile, Rh/MgAl2O4-MC retains the excellent catalytic durability of supported nanoparticles with negligible side reaction of carbon deposition, which is in striking contrast to the prone deactivation of the single-atom catalysts of precious metals (e.g., Rh and Ru) reported. Experimental evidence and DFT calculations reveal the presence of a strong interaction between Rh quasi-monolayer clusters and MgAl2O4 for Rh/MgAl2O4-MC. The interaction not only makes energetically unstable clusters stable but also alters the reaction pathway of DRM. Compared to DRM on Rh nanoparticles, the oxidations of C* and CH* species as the rate-determining step of DRM are significantly accelerated on Rh quasi-monolayer clusters due to their activation energies being considerably reduced, thus improving intrinsic catalytic activity and inhibiting the side reaction of carbon deposition.
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