Nanoporous gold functionalized with praseodymia-titania mixed oxides as a stable catalyst for the water-gas shift reaction

Praseodymia–titania mixed oxide supported gold as efficient water gas shift catalyst: modulated by the mixing ratio of oxides

Modulating the active sites for controllable tuning of the catalytic activity has been the goal of much research, however, this remains challenging. The O vacancy is well known as an active site in reducible oxides. To modify the activity of O vacancies in praseodymia, we synthesized a series of praseodymia–titania mixed oxides. Varying the Pr : Ti mole ratio (2 : 1, 1 : 2, 1 : 1, 1 : 4) allows us to control the electronic interactions between Au, Pr and Ti cations and the local chemical environment of the O vacancies. These effects have been studied study by X-ray photoelectron spectroscopy (XPS), CO diffuse reflectance Fourier transform infrared spectroscopy (CO-DRIFTS) and temperature-programmed reduction (CO-TPR, H₂-TPR). The water gas shift reaction (WGSR) was used as a benchmark reaction to test the catalytic performance of different praseodymia–titania supported Au. Among them, Au/Pr₁Ti₂O_x was identified to exhibit the highest activity, with a CO conversion of 75% at 300 °C, which is about 3.7 times that of Au/TiO₂ and Au/PrO_x. The Au/Pr₁Ti₂O_x also exhibited excellent stability, with the conversion after 40 h time-on-stream at 300 °C still being 67%. An optimal ratio of Pr content (Pr : Ti 1 : 2) is necessary for improving the surface oxygen mobility and oxygen exchange capability, a higher Pr content leads to more O vacancies, however with lower activity. This study presents a new route for modulating the active defect sites in mixed oxides which could also be extended to other heterogeneous catalysis systems.

Schematic illustration of H₂O activation on the Pr-TiO_x support and the following reaction with CO in the Au–oxide interface.

Nanoporous Gold for Energy Applications

Research activities using nanoporous gold (NPG) were reviewed in the field of energy applications in three categories: fuel cells, supercapacitors, and batteries. First, applications to fuel cells are reviewed with the subsections of proof-of-concept studies, studies on fuel oxidations at anode, and studies on oxygen reduction reactions at cathode. Second, applications to supercapacitors are reviewed from research activities on active materials/NPG composites to demonstrations of all-solid-state flexible supercapacitors using NPG electrodes. Third, research activities using NPG for battery applications are reviewed, mainly about fundamental studies on Li-air and Na-air batteries and some model studies on improving Li ion battery anodes. Although NPG based studies are the main subject of this review, some of meaningful studies using nanoporous metals are also discussed where relevant. Finally, summary and future outlook are given based on the survey on the research activities.

Water Splitting: From Electrode to Green Energy System

Hydrogen (H2) production is a latent feasibility of renewable clean energy. The industrial H2 production is obtained from reforming of natural gas, which consumes a large amount of nonrenewable energy and simultaneously produces greenhouse gas carbon dioxide. Electrochemical water splitting is a promising approach for the H2 production, which is sustainable and pollution-free. Therefore, developing efficient and economic technologies for electrochemical water splitting has been an important goal for researchers around the world. The utilization of green energy systems to reduce overall energy consumption is more important for H2 production. Harvesting and converting energy from the environment by different green energy systems for water splitting can efficiently decrease the external power consumption. A variety of green energy systems for efficient producing H2, such as two-electrode electrolysis of water, water splitting driven by photoelectrode devices, solar cells, thermoelectric devices, triboelectric nanogenerator, pyroelectric device or electrochemical water-gas shift device, have been developed recently. In this review, some notable progress made in the different green energy cells for water splitting is discussed in detail. We hoped this review can guide people to pay more attention to the development of green energy system to generate pollution-free H2 energy, which will realize the whole process of H2 production with low cost, pollution-free and energy sustainability conversion.

Praseodymium hydroxide/gold-supported precursor: a new strategy for preparing stable and active catalyst for the water-gas shift reaction

Rod-shaped praseodymium hydroxide (Pr(OH)_x) as a hydroxyl- and O vacancy-rich support can promote the dispersion and stabilization of Au species show high activity and stability for water gas shift reaction, and holds great promise in the field of heterogeneous catalysis.