Xiaojun Gu

Porous nitrogen-doped carbon-immobilized bimetallic nanoparticles as highly efficient catalysts for hydrogen generation from hydrolysis of ammonia borane

Two N-doped carbon materials, N-doped Vulcan XC-72 carbon labelled as NXC and graphite carbon nitride labelled as C3N4, were selected as supports to prepare a series of bimetallic AuM (M = Co, Ni) nanoparticles (NPs) using three different reduction ways towards mixed metal ions AuCl4− and M2+, and then the as-synthesized supported bimetallic AuM NPs were used as catalysts for hydrolytic dehydrogenation of ammonia borane (NH3BH3). All the catalysts exhibited high dispersion and small size of bimetallic NPs; however, they exhibited remarkably different catalytic activities featuring total turnover frequency (TOF) values from 6.4 to 42.1 molH2 molcat−1 min−1. Among all the catalysts, the NXC-immobilized AuCo NPs through in situ synthesis using NaBH4 and NH3BH3 as reductants exhibited the highest activity with a total TOF value of 42.1 molH2 molcat−1 min−1, which was among the highest values for Co-based catalysts ever reported for hydrolytic dehydrogenation of NH3BH3. This remarkably enhanced activity may be attributed to the synergistic effect of the N-doped NXC support and AuCo NPs and the resulting highly efficient activation of N–B bond in NH3BH3. In addition, the AuCo catalysts showed good recyclability, demonstrating that they had high stability/durability.

Exceptional size-dependent catalytic activity enhancement in the room-temperature hydrogen generation from formic acid over bimetallic nanoparticles supported by porous carbon

In this work, we report a prominent size-dependent activity in the catalytic dehydrogenation of formic acid (HCOOH, FA) over carbon-supported AuPd alloy nanoparticles (NPs) directed by amino acids (lysine, serine and glutamic acid) with different isoelectric points. Through decreasing the average size of AuPd NPs from 12.8 ± 0.5 to 3.8 ± 0.5 nm using different amino acids as structure-directing agents, drastic activity enhancement in the generation of hydrogen without CO impurity from FA was observed, and the initial turnover frequency (TOF) value was enhanced from 14 to 718 h−1 at 298 K, which was among the highest values for the reported heterogeneous catalysts for FA dehydrogenation under the same conditions. Through optimizing the molar ratio of Au/Pd and the metal loading in the catalysts, the initial TOF value for FA dehydrogenation was enhanced to 1153 h−1 at 298 K. In addition, the catalyst with the smallest size of AuPd NPs gave an initial TOF value of 2972 h−1 at 323 K with 100% H2 selectivity, comparable to the values acquired from the most active homogeneous catalysts. The investigation of UV-vis spectroscopy showed that the reaction between mixed metal ions (Pd2+ and AuCl4−) and the three amino acids before the reduction by NaBH4 led to the formation of three different coordination complex intermediates, which induced the formation of different-sized AuPd alloy NPs featuring remarkably different catalytic performances for FA dehydrogenation.

Highly Efficient Catalytic Hydrogen Evolution from Ammonia Borane Using the Synergistic Effect of Crystallinity and Size of Noble-Metal-Free Nanoparticles Supported by Porous Metal–Organic Frameworks

A series of nonprecious metal nanoparticles (NPs) supported by metal-organic framework MIL-101 were synthesized using four methods and their catalytic performance on hydrogen evolution from ammonia borane (NH3BH3) was studied. The results showed that the crystalline Co NPs with size of 4.5-8.5 and 14.5-24.5 nm had low activities featuring the total turnover frequency (TOF) values of 9.9 and 4.5 molH2 molcat-1 min-1, respectively. In contrast, the amorphous Co NPs with size of 1.6-2.6 and 13.5-24.5 nm exhibited high activities featuring the total TOF values of 51.4 and 22.3 molH2 molcat-1 min-1, respectively. The remarkably different activities could be ascribed to the different crystallinity and size of Co NPs in the catalysts. Moreover, the ultrasound-assisted in situ method was also successfully applied to bimetallic systems, and MIL-101-supported amorphous CuCo, FeCo and NiCo NPs had the catalytic activities with total TOF values of 51.7, 50.8, and 44.3 molH2 molcat-1 min-1, respectively, which were the highest in the values of the reported non-noble metal Co-based catalysts. The present approach, namely, using the synergistic effect of crystallinity and size of metal NPs, may offer a new prospect for high-performance and low-cost nanocatalysts.

Highly efficient visible-light-driven catalytic hydrogen evolution from ammonia borane using non-precious metal nanoparticles supported by graphitic carbon nitride

A series of non-precious monometallic (Co, Ni and Fe) and bimetallic (CuCo, FeCo, NiCo, CuNi and FeNi) nanoparticles (NPs) supported by semiconductor graphitic carbon nitride (g-C3N4) was synthesized, which was used to catalyze hydrogen evolution from ammonia borane (NH3BH3) under visible light irradiation at 298 K. The systematic investigation showed that in comparison with the activities of all the catalysts in the dark, their activities were remarkably enhanced under visible light irradiation. Particularly, the in situ-synthesized Co, FeCo and CuCo catalysts exhibited visible-light-driven activities featuring total turnover frequency (TOF) values of 55.6, 68.2 and 75.1 min−1, respectively, which were the highest amongst the values of the reported noble-metal-free catalysts at 298 K. The enhancement of activities could be ascribed to the enrichment of the electron density of the active metal NPs under visible light irradiation, which was caused by the Mott–Schottky effect at the g-C3N4–metal interface. Furthermore, the catalytic activities of the catalysts strongly depended on the wavelength and intensity of the incident light, indicating that the visible light irradiation indeed played a key role in the enhancement of catalytic activities.

Controlling catalytic dehydrogenation of formic acid over low-cost transition metal-substituted AuPd nanoparticles immobilized by functionalized metal–organic frameworks at room temperature

A series of nonprecious transition metal (Fe, Co and Ni)-substituted AuPd nanoparticles (NPs) immobilized by porous metal–organic frameworks (MOFs) containing different functional groups were synthesized, and their catalytic performance for the dehydrogenation of formic acid (HCOOH, FA) was studied. In comparison with AuPdCo NPs immobilized by bare MIL-101 and MIL-101 functionalized by electron-accepting groups NO2 and SO3H, which had very low H2 selectivity and activity, AuPdCo NPs immobilized by MIL-101 functionalized by electron-donating group NH2 exhibited 100% H2 selectivity and drastically high activity featuring a turnover frequency (TOF) value of 347 h−1 at 298 K, comparable to the reported values of highly active heterogeneous noble-metal catalysts and homogeneous catalysts. The remarkably different and controllable activity and H2 selectivity may be attributed to the different bonding modes between AuPdCo NPs and functionalized MOFs, which might lead to the different interaction pathways between the intermediates from the splitting of FA molecules and the metallic atoms on the catalyst surface.

Non-Noble Metal Nanoparticles Supported by Postmodified Porous Organic Semiconductors: Highly Efficient Catalysts for Visible-Light-Driven On-Demand H2 Evolution from Ammonia Borane

From the viewpoint of controlling the visible-light-driven activities of catalysts containing metal nanoparticles (NPs) by tuning the microstructures of semiconducting supports, we employed a postsynthetic thermal modification approach to prepare carbon nitride (C3N4) species featuring different microstructures and then we synthesized Co and Ni NPs supported by these C3N4 species, which were used to catalyze the room-temperature H2 evolution from ammonia borane (NH3BH3). The systematic investigation showed that the catalysts had different activities under light irradiation. Compared with the pristine C3N4-based catalyst, all the modified C3N4-based catalysts had enhanced activities. The highest active Co catalyst with a total turnover frequency of 93.8 min-1 was successfully obtained, which exceeded the values of all the reported heterogeneous noble metal-free catalysts. The structure characterizations indicated that the postmodified porous C3N4 species had the different band structures, photoluminescence lifetime, and photocurrent density under visible light irradiation, leading to the different separation efficiency of photogenerated charge carriers. These characteristics helped us regulate the electronic characteristics of Co and Ni NPs in the supported catalysts and then led to the significantly different and enhanced activity in the visible-light-driven H2 evolution.