(558u) Catalysis inside Hollow Nanostructured Materials: The Balanced Adsorption-Diffusion Effect

Hydrogenation reactions are extensively applied in industrial chemical process. However, most hydrogenations need a high partial pressure or excess hydrogen to achieve a high activity. Meanwhile, the selectivity of consecutive hydrogenations is difficult to manipulate, leading to an unsatisfied distribution of product. These two drawbacks result in massive energy consumption and high-cost industrial devices.

We here propose a nanostructured catalyst with morphology of nanotube-assembled hollow sphere (NAHS) to solve these problems. In dimethyl oxalate (DMO) hydrogenation, this NAHS achieves favorable activity (95% of ethylene glycol yield) and stability (> 300 h) when the H₂/DMO molar ratio is as low as 20, while the typical value of H₂/DMO in industry ranges from 80 to 120. The high-pressure hydrogen adsorption and Monte Carlo simulation were further conducted, demonstrated that hydrogen molecules enrich inside nanotube and hollow sphere, leading to an enhanced activity in such a low H₂ proportion.

Furthermore, the structure effect of NAHS on catalytic performance is further investigated. By a modified hydrothermal-dissolution treatment, the nanotube length and the hollow-sphere size of NAHSs are adjusted separately. We find the selectivity of the intermediate product methyl glycolate is easily controlled by decreasing the nanotube-length of NAHS. Meanwhile, combining adsorption experiment and DFT calculation, we conclude that the concave surface of hollow sphere tends to absorb hydrogen molecules more strongly than the planar surface or the convex surface, leading to a higher local concentration of hydrogen inside hollow sphere. With the increased surface curvature, the interaction between curved surface and hydrogen molecules becomes stronger, accelerates the reaction rate. However, by calculating the diffusion flux, we find that with the increased hollow sphere size, the concentration difference between the inner and outer space of hollow sphere become larger, resulting in an enhanced diffusion. The adsorption and diffusion influence the reaction rate, resulting in a volcano trend between the hollow-sphere size and activity. This balancing effect between adsorption and diffusion provides a new approach to design more efficient and selective hollow nanostructured catalysts.