We have studied the pH-dependent speciation of aqueous HAuCl4 and its influences on the synthesis, structure, and property of Au colloids. Aqueous HAuCl4 consists of [AuCl x (OH)4−x ]− (x ≥ 2) at low pH but [AuCl x (OH)4−x ]− (x < 2) at high pH. By employment of ascorbic acid as the reducing agent and sodium benzenesulfonate (SDBS) as the protecting agent, reduction of aqueous HAuCl4 at low pH leads to the synthesis of well-dispersed and uniform fine Au colloids, whereas that at high pH forms large Au colloids and ensembles of fine Au colloids. These large Au colloids and ensembles of fine Au colloids exhibit strong surface plasmon resonance in the near-infrared region. The SDBS molecules bind to the surface of Au colloids through the S element, and the charge transfer from Au atoms to S elements occurs. The charge is localized around Au atoms directly interacting with SDBS for fine Au colloids but delocalized to the entire Au colloid for large Au colloids and ensembles of fine Au colloids.

Recently, reactive iron species (RFeS) have shown great potential for the selective degradation of emerging organic contaminants (EOCs). However, the rapid generation of RFeS for the selective and efficient degradation of EOCs over a wide pH range is still challenging. Herein, we constructed FeN4 structures on a carbon nanotube (CNT) to obtain single-atom catalysts (FeSA-N-CNT) to generate RFeS in the presence of peroxymonosulfate (PMS). The obtained FeSA-N-CNT/PMS system exhibited outstanding and selective reactivity for oxidizing EOCs over a wide pH range (3.0–9.0). Several lines of evidences suggested that RFeS existing as an FeN4O intermediate was the predominant oxidant, while SO4 ·– and HO· were the secondary oxidants. Density functional theory calculation results revealed that a CNT played a key role in optimizing the distribution of bonding and antibonding states in the Fe 3d orbital, resulting in the outstanding ability of FeSA-N-CNT for PMS chemical adsorption and activation. Moreover, CNT could significantly enhance the reactivity of the FeN4O intermediate by increasing the overlap of electrons of the Fe 3d orbital, O 2p orbital, and bisphenol A near the Fermi level. The results of this study can advance the understanding of RFeS generation in a heterogeneous system over a wide pH range and the application of RFeS in real practice.

Solid catalysts usually consist of multicomponents, within which interfacial interactions have been recognized as a key factor affecting structures and catalytic performance. Metal−support interactions (MSI) have been extensively studied in oxidesupported metal catalysts (metal/oxide catalysts), in which the important concepts of strong metal−support interactions (SMSI) and electronic metal−support interactions (EMSI) have been well established and their effects on the metal catalysis have been extensively demonstrated. Recently, metal-supported oxide inverse catalysts (oxide/metal inverse catalysts) have emerged as a new type of efficient catalysts, in which the oxide−metal interactions (OMI) strongly influence the oxide catalysis. Herein we comprehensively review the progresses on the MSI of metal/oxide catalysts and OMI of oxide/metal inverse catalysts with aims to emphasize structure sensitivity of MSI and OMI and to introduce the concepts of electronic oxide−metal interactions (EOMI) and electronic oxide−metal strong interactions (EOMSI) in oxide/metal inverse catalysts, in analogy to the concepts of EMSI and SMSI in metal/oxide catalysts. First, we briefly introduce the background of the topic and the interfacial interactions between metals and oxides with emphasis on the nature of metal−support interfacial interactions depending on the electronic structures. Second, the MSI, with an emphasis on the EMSI and SMSI, in metal/oxide catalysts is reviewed with an emphasis on the recently exported size and facet effects on the electronic structures and MSI. Third, the OMI in oxide/metal inverse catalysts is reviewed with an emphasis on introducing the EOMI and EOMSI. Finally, a summary and outlook is given with emphasis on the local nature and structure sensitivity of MSI and OMI.