Highly Efficient Photocatalyst Based on a CdS Quantum Dots/ZnO Nanosheets 0D/2D Heterojunction for Hydrogen Evolution from Water Splitting

A novel CdS/ZnO heterojunction constructed of zero-dimensional (0D) CdS quantum dots (QDs) and two-dimensional (2D) ZnO nanosheets (NSs) was rationally designed for the first time. The 2D ZnO NSs were assembled into ZnO microflowers (MFs) via an ultrasonic-assisted hydrothermal procedure (100 °C, 12 h) in the presence of a NaOH solution (0.06 M), and CdS QDs were deposited on both sides of every ZnO NS in situ by using the successive ionic-layer absorption and reaction method. It was found that the ultrasonic treatment played an important role in the generation of ZnO NSs, while NaOH was responsible to the assembly of a flower-like structure. The obtained CdS/ZnO 0D/2D heterostructures exhibited remarkably enhanced photocatalytic activity for hydrogen evolution from water splitting in comparison with other CdS/ZnO heterostructures with different dimensional combinations such as 2D/2D, 0D/three-dimensional (3D), and 3D/0D. Among them, CdS/ZnO-12 (12 deposition cycles of CdS QDs) exhibited the highest hydrogen evolution rate of 22.12 mmol/g/h, which was 13 and 138 times higher than those of single CdS (1.68 mmol/g/h) and ZnO (0.16 mmol/g/h), respectively. The enhanced photocatalytic activity can be attributed to several positive factors, such as the formation of a Z-scheme photocatalytic system, the tiny size effect of 0D CdS QDs and 2D ZnO NSs, and the intimate contact between CdS QDs and ZnO NSs. The formation of a Z-scheme photocatalytic system remarkably promoted the separation and migration of photogenerated electron–hole pairs. The tiny size effect effectively decreased the recombination probability of electrons and holes. The intimate contact between the two semiconductors efficiently reduced the migration resistance of photogenerated carriers. Furthermore, CdS/ZnO-12 also presented excellent stability for photocatalytic hydrogen evolution without any decay within five cycles in 25 h

Rationally Designed Porous MnO_<i>x</i>–FeO_<i>x</i> Nanoneedles for Low-Temperature Selective Catalytic Reduction of NO_<i>x</i> by NH₃

In this work, a novel porous nanoneedlelike MnO_<i>x</i>–FeO_<i>x</i> catalyst (MnO_<i>x</i>–FeO_<i>x</i> nanoneedles) was developed for the first time by rationally heat-treating metal–organic frameworks including MnFe precursor synthesized by hydrothermal method. A counterpart catalyst (MnO_<i>x</i>–FeO_<i>x</i> nanoparticles) without porous nanoneedle structure was also prepared by a similar procedure for comparison. The two catalysts were systematically characterized by scanning and transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, hydrogen temperature-programmed reduction, ammonia temperature-programmed desorption, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFT), and their catalytic activities were evaluated by selective catalytic reduction (SCR) of NO_<i>x</i> by NH₃. The results showed that the rationally designed MnO_<i>x</i>–FeO_<i>x</i> nanoneedles presented outstanding low-temperature NH₃-SCR activity (100% NO_<i>x</i> conversion in a wide temperature window from 120 to 240 °C), high selectivity for N₂ (nearly 100% N₂ selectivity from 60 to 240 °C), and excellent water resistance and stability in comparison with the counterpart MnO_<i>x</i>–FeO_<i>x</i> nanoparticles. The reasons can be attributed not only to the unique porous nanoneedle structure but also to the uniform distribution of MnO_<i>x</i> and FeO_<i>x</i>. More importantly, the desired Mn⁴⁺/Mn^<i>n</i>+ and O_α/(O_α + O_β) ratios, as well as rich redox sites and abundant strong acid sites on the surface of the porous MnO_<i>x</i>–FeO_<i>x</i> nanoneedles, also contribute to these excellent performances. In situ DRIFT suggested that the NH₃-SCR of NO over MnO_<i>x</i>–FeO_<i>x</i> nanoneedles follows both Eley–Rideal and Langmuir–Hinshelwood mechanisms

Understanding the Promotional Effect of Mn₂O₃ on Micro-/Mesoporous Hybrid Silica Nanocubic-Supported Pt Catalysts for the Low-Temperature Destruction of Methyl Ethyl Ketone: An Experimental and Theoretical Study

Pt_0.3Mn_<i>x</i>/SiO₂ nanocubic (nc) micro-/mesoporous composite catalysts with varied Mn contents were synthesized and tested for the oxidation of methyl ethyl ketone (MEK). Results show that MEK can be efficiently decomposed over synthesized Pt_0.3Mn_<i>x</i>/SiO₂-nc materials with a reaction rate and turnover frequency respectively higher than 12.7 mmol g_Pt^–1 s^–1 and 4.7 s^–1 at 100 °C. Among these materials, the Pt_0.3Mn₅/SiO₂-nc catalyst can completely oxidize MEK at just 163 °C under a high space velocity of 42600 mL g^–1 h^–1. The remarkable performance of these catalysts is attributed to a synergistic effect between the Pt nanoparticles and Mn₂O₃. NH₃-TPD and NH₃-FT-IR experiments revealed that exposed Mn₂O₃ (222) facets enhance the quantity of Brønsted acid sites in the catalyst, which are considered to be responsible for promoting the desorption of surface-adsorbed O₂ and CO₂. It is suggested that the desorption of these species liberates active sites for MEK molecules to adsorb and react. ¹⁸O₂ isotopic labeling experiments revealed that the presence of a Pt–O–Mn moiety weakens the Mn–O bonding interactions, which ultimately promotes the mobility of lattice oxygen in the Mn₂O₃ system. It was determined that the Mn⁴⁺/Mn³⁺ redox cycle in Mn₂O₃ allows for the donation of electrons to the Pt nanoparticles, enhancing the proportion of Pt⁰/Pt²⁺ and in turn increasing the activity and stability of catalyst. In situ DRIFTS, online FT-IR, and DFT studies revealed that acetone and acetaldehyde are the main intermediate species formed during the activation of MEK over the Pt_0.3Mn₅/SiO₂-nc catalyst. Both intermediates were found to partake in sequential reactions resulting in the formation of H₂O and CO₂ via formaldehyde