Recent trends in vanadium-based SCR catalysts for NOx reduction in industrial applications: stationary sources

Vanadium-based catalysts have been used for several decades in ammonia-based selective catalytic reduction (NH₃-SCR) processes for reducing NO_x emissions from various stationary sources (power plants, chemical plants, incinerators, steel mills, etc.) and mobile sources (large ships, automobiles, etc.). Vanadium-based catalysts containing various vanadium species have a high NO_x reduction efficiency at temperatures of 350-400 °C, even if the vanadium species are added in small amounts. However, the strengthening of NO_x emission regulations has necessitated the development of catalysts with higher NO_x reduction efficiencies. Furthermore, there are several different requirements for the catalysts depending on the target industry and application. In general, the composition of SCR catalyst is determined by the components of the fuel and flue gas for a particular application. It is necessary to optimize the catalyst with regard to the reaction temperature, thermal and chemical durability, shape, and other relevant factors. This review comprehensively analyzes the properties that are required for SCR catalysts in different industries and the development strategies of high-performance and low-temperature vanadium-based catalysts. To analyze the recent research trends, the catalysts employed in power plants, incinerators, as well as cement and steel industries, that emit the highest amount of nitrogen oxides, are presented in detail along with their limitations. The recent developments in catalyst composition, structure, dispersion, and side reaction suppression technology to develop a high-efficiency catalyst are also summarized. As the composition of the vanadium-based catalyst depends mostly on the usage in stationary sources, various promoters and supports that improve the catalyst activity and suppress side reactions, along with the studies on the oxidation state of vanadium, are presented. Furthermore, the research trends related to the nano-dispersion of catalytically active materials using various supports, and controlling the side reactions using the structure of shaped catalysts are summarized. The review concludes with a discussion of the development direction and future prospects for high-efficiency SCR catalysts in different industrial fields.

Electrocatalytic hydrodechlorination system with antiscaling and anti-chlorine poisoning features for salt-laden wastewater treatment

The high salinity and coexistence of scaling ions (Ca²⁺, Mg²⁺, HCO₃^-) in wastewater challenge the efficacy and durability of palladium (Pd)-mediated electrocatalytic hydrodechlorination (EHDC) reaction for chlorinated organic pollutant detoxification, due to the accompanying Cl^- poisoning at Pd sites and scaling on electrode. In a concentrated NaCl solution (5.8 g L ^- 1) with Ca²⁺ (80.0 mg L ^- 1), Mg²⁺ (30.0 mg L ^- 1) and HCO₃^- (180.0 mg L ^- 1), the EHDC efficiency of Pd towards 2,4-dichlorophenol decreases significantly from 67.8% to 33.1% in 72.0 h of reaction, and the electrode is covered with layers of fluffy aragonite precipitate. Herein we demonstrate the inclusion of a commercial antiscalant 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) can prevent both scale formation and Cl^- poisoning, leading to an efficient and steady EHDC process. A mechanistic study reveals that the unique dual function of PBTC primarily originates from the bearing phosphonate and carboxyl groups. With the large affinity of these groups (especially the phosphonate group) for scaling cations and Pd, the PBTC can chelate and stabilize the scaling cations in water and replace Cl^- at Pd surface. It can also release protons, and trigger the formation of more electron-deficient Pd^δ+ species via PBTC-Pd binding, leading to an enhanced EHDC. This work provides effective solutions to the scaling/poisoning issues that commonly encountered in real wastewater and paves a solid road for EHDC application in pollution abatement.

Carbon Monoxide Poisoning Resistance and Structural Stability of Single Atom Alloys

Platinum group metals (PGMs) serve as highly active catalysts in a variety of heterogeneous chemical processes. Unfortunately, their high activity is accompanied by a high affinity for CO and thus, PGMs are susceptible to poisoning. Alloying PGMs with metals exhibiting lower affinity to CO could be an effective strategy toward preventing such poisoning. In this work, we use density functional theory to demonstrate this strategy, focusing on highly dilute alloys of PGMs (Pd, Pt, Rh, Ir and Ni) with poison resistant coinage metal hosts (Cu, Ag, Au), such that individual PGM atoms are dispersed at the atomic limit forming single atom alloys (SAAs). We show that compared to the pure metals, CO exhibits lower binding strength on the majority of SAAs studied, and we use kinetic Monte Carlo simulation to obtain relevant temperature programed desorption spectra, which are found to be in good agreement with experiments. Additionally, we consider the effects of CO adsorption on the structure of SAAs. We calculate segregation energies which are indicative of the stability of dopant atoms in the bulk compared to the surface layer, as well as aggregation energies to determine the stability of isolated surface dopant atoms compared to dimer and trimer configurations. Our calculations reveal that CO adsorption induces dopant atom segregation into the surface layer for all SAAs considered here, whereas aggregation and island formation may be promoted or inhibited depending on alloy constitution and CO coverage. This observation suggests the possibility of controlling ensemble effects in novel catalyst architectures through CO-induced aggregation and kinetic trapping.

Ultrasonic radiation to enable improvement of direct methanol fuel cell

To improve DMFC (direct methanol fuel cell) performance, a new method using ultrasonic radiation is proposed and a novel DMFC structure is designed and fabricated in the present paper. Three ultrasonic transducers (piezoelectric transducer, PZT) are integrated in the flow field plate to form the ultrasonic field in the liquid fuel. Ultrasonic frequency, acoustic power, and methanol concentration have been considered as variables in the experiments. With the help of ultrasonic radiation, the maximum output power and limiting current of cell can be independently increased by 30.73% and 40.54%, respectively. The best performance of DMFC is obtained at the condition of ultrasonic radiation (30 kHz and 4 W) fed with 2M methanol solution, because both its limiting current and output power reach their maximum value simultaneously (222 mA and 33.6 mW, respectively) under this condition. These results conclude that ultrasonic can be an alternative choice for improving the cell performance, and can facilitate a guideline for the optimization of DMFC.