Fractals, being "exactly the same at every scale or nearly the same at different scales" as defined by Benoit B. Mandelbrot, are complicated yet fascinating patterns that are important in aesthetics, mathematics, science and engineering. Extended molecular fractals formed by the self-assembly of small-molecule components have long been pursued but, to the best of our knowledge, not achieved. To tackle this challenge we designed and made two aromatic bromo compounds (4,4″-dibromo-1,1':3',1″-terphenyl and 4,4‴-dibromo-1,1':3',1″:4″,1‴-quaterphenyl) to serve as building blocks. The formation of synergistic halogen and hydrogen bonds between these molecules is the driving force to assemble successfully a whole series of defect-free molecular fractals, specifically Sierpiński triangles, on a Ag(111) surface below 80 K. Several critical points that govern the preparation of the molecular Sierpiński triangles were scrutinized experimentally and revealed explicitly. This new strategy may be applied to prepare and explore various planar molecular fractals at surfaces.

Utilization of CO2 and its conversion to value-added chemicals are highly desirable to alleviate the environmental concerns caused by the massive anthropogenic CO2 emission. Although In2O3/Pd have been employed as efficient catalysts for hydrogenation of CO2 to methanol, the electronic effects by strong metal–support interaction (SMSI) between Pd and In2O3 are poorly understood, which is greatly affected by the morphology of In2O3. Herein, we use MIL-68­(In) nanorod as a morphological template for the synthesis of hollow In2O3 nanotubes (h-In2O3) and the preparation of supported Pd catalysts for CO2 hydrogenation to methanol. Interestingly, loading Pd on h-In2O3 showed a much higher performance than In2O3 with other morphologies, which exhibited almost unchanged CO2 conversion of 10.5%, methanol selectively of 72.4%, and methanol space-time yield of 0.53 gMeOH h–1 gcat –1 over 100 h on stream at 3 MPa and 295 °C. After in-depth characterizations, we found that the different electronic properties of Pd species on In2O3 can be finely tuned by diverse synthetic conditions, which were responsible for high activity and stability. The molar fraction of Pd2+ species in the h-In2O3/Pd catalyst reached 67.6%, 3.2 times that of the In2O3@Pd catalyst (21.3%), due to the different surface chemistry of In2O3. Density function theory results indicated that the Pd donated more electrons to the curved In2O3 (222) surface than the pristine surface, and Pd2+ was critical to facilitate H2 adsorption and formation of the surface oxygen vacancy. This work demonstrates that controlling the morphology of In2O3 can modify both the Pd electronic property and SMSI between Pd and In2O3, which are the origins of the high catalytic performance.

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