Asymmetric Sites on the ZnZrOx Catalyst for Promoting Formate Formation and Transformation in CO2 Hydrogenation.

The role of formate species for CO2 hydrogenation is still under debate. Although formate has been frequently observed and commonly proposed as the possible intermediate, there is no definite evidence for the reaction of formate species for methanol production. Here, formate formation and conversion over the ZnZrOx solid solution catalyst are investigated by in situ/operando diffuse reflectance infrared Fourier transform spectroscopy-mass spectroscopy (DRIFTS-MS) coupled with density functional theory (DFT) calculations. Spectroscopic results show that bidentate carbonate formed from CO2 adsorption is hydrogenated to formate on Zn-O-Zr sites (asymmetric sites), where the Zn site is responsible for H2 activation and the Zr site is beneficial for the stabilization of reaction intermediates. The asymmetric Zn-O-Zr sites with adjacent and inequivalent features on the ZnZrOx catalyst promote not only formate formation but also its transformation. Both theoretical and experimental results demonstrate that the origin of the excellent performance of the ZnZrOx catalyst for methanol formation is associated with the H2 heterolytic cleavage promoted by the asymmetric Zn and Zr sites.

Carbonization of self-assembled nanoporous hemin with a significantly enhanced activity for the oxygen reduction reaction.

The scarcity and high cost of Pt-based electrocatalysts for the oxygen reduction reaction (ORR) hinder the practical application of proton exchange membrane fuel cells (PEMFCs). It is critical to replace platinum with non-noble metal electrocatalysts (NNMEs). Carbonized metalloporphyrins represent an important class of NNMEs, but most metalloporphyrins are costly and the corresponding NNMEs do not possess a high ORR activity. Herein, we report that the self-assembly of inexpensive hemin leads to porous nanomaterials in water under ambient conditions and subsequent heat-treatment of the unprecedented nanoporous hemin results in a magnetic NNME with a much enhanced ORR activity compared with directly carbonized hemin without self-assembly. The improvement of the ORR activity likely originates from the exposure of more ORR active sites, caused by the surface area increase of the nanoporous hemin after carbonization over that of micro-scale pristine hemin crystals. Moreover, the ORR activity of heat-treated nanoporous hemin is actually comparable to that of commercial Pt/C in alkaline solution. Additionally, the carbonized nanoporous hemin is much better than commercial Pt/C in terms of durability and tolerance to methanol. This study opens up a new avenue to the production of inexpensive metalloporphyrin-based NNMEs with a high ORR performance by using a self-assembly method in combination with traditional pyrolysis.

Interwoven Molecular Chains Obtained by Ionic Self-Assembly of Two Iron(III) Porphyrins with Opposite and Mismatched Charges.

We report ionic self-assembly of positively charged FeIII meso-tetra(N-methyl-4-pyridyl) porphyrin (FeIIINMePyP) with negatively charged FeIII meso-tetra(4-sulfonatophenyl) porphyrin (FeIIITPPS4), leading to the formation of flower-like nanostructures composed of unprecedented three-dimensional (3D) entangled chains of porphyrin dimers. Molecular dynamics (MD) simulations show that the 3D entanglement of porphyrin chains closely correlates to mismatched charges present in porphyrin dimers like [FeIII(H2O)2NMePyP]5+/[FeIII(H2O)2TPPS4]3- that requires extra interactions or entanglement with neighboring ones to achieve electric neutrality. Interestingly, the interwoven chains bring in excellent thermal stability as evidenced by well maintenance of the flower-like morphology after pyrolysis at 775 °C in argon, which is in good agreement of high-temperature MD simulations. Meanwhile, heat treatment of the flower-like porphyrin nanostructure leads to the formation of a non-noble metal electrocatalyst (NNME) with largely inherited morphology. This exemplifies a new approach by combining ionic self-assembly with subsequent pyrolysis for the synthesis of NNMEs with desired control over the morphology of template-free NNMEs that has rarely been achieved prior to this study. Furthermore, our electrocatalyst exhibits excellent activity and durability toward oxygen reduction reaction as well as much better methanol tolerance compared with commercial Pt/C in alkaline solutions.

A highly selective and stable ZnO-ZrO2 solid solution catalyst for CO2 hydrogenation to methanol.

Although methanol synthesis via CO hydrogenation has been industrialized, CO2 hydrogenation to methanol still confronts great obstacles of low methanol selectivity and poor stability, particularly for supported metal catalysts under industrial conditions. We report a binary metal oxide, ZnO-ZrO2 solid solution catalyst, which can achieve methanol selectivity of up to 86 to 91% with CO2 single-pass conversion of more than 10% under reaction conditions of 5.0 MPa, 24,000 ml/(g hour), H2/CO2 = 3:1 to 4:1, 320° to 315°C. Experimental and theoretical results indicate that the synergetic effect between Zn and Zr sites results in the excellent performance. The ZnO-ZrO2 solid solution catalyst shows high stability for at least 500 hours on stream and is also resistant to sintering at higher temperatures. Moreover, no deactivation is observed in the presence of 50 ppm SO2 or H2S in the reaction stream.