Shibo Xi

XAFCA: a new XAFS beamline for catalysis research

A new X-ray absorption fine-structure (XAFS) spectroscopy beamline for fundamental and applied catalysis research, called XAFCA, has been built by the Institute of Chemical and Engineering Sciences, and the Singapore Synchrotron Light Source. XAFCA covers the photon energy range from 1.2 to 12.8 keV, making use of two sets of monochromator crystals, an Si (111) crystal for the range from 2.1 to 12.8 keV and a KTiOPO4 crystal [KTP (011)] for the range between 1.2 and 2.8 keV. Experiments can be carried out in the temperature range from 4.2 to 1000 K and pressures up to 30 bar for catalysis research. A safety system has been incorporated, allowing the use of flammable and toxic gases such as H2 and CO.

Encapsulating porous SnO2 into a hybrid nanocarbon matrix for long lifetime Li storage

To overcome the low conductivity and large volume variation of metal oxide anodes, the electrode microstructure design for these metal oxides appeared to be the most promising strategy for achieving the desired Li storage performance. In this article, we report on a rational design of the carbon/SnO2 microstructure, in which porous SnO2 nanoparticles are encapsulated into the graphene matrix and additional carbon coating layer. As an anode material for LIBs, the as-prepared G@p-SnO2@C composite exhibited an ultra-long cycling life up to 1800 cycles. It can sustain high specific capacities of 602 and 418 mA h g−1 at 1.5 A g−1 after 1000 and 1800 cycles, respectively. The excellent battery performance could be attributed to the unique architecture of this composite, which enhances electrical conductivity, provides sufficient interior void space to accommodate the volume variation of SnO2, mitigates the aggregation, and preserves the integrity of electrodes during cycling.

Identifying the Origin and Contribution of Surface Storage in TiO2(B) Nanotube Electrode by In Situ Dynamic Valence State Monitoring

Fundamental insight into the surface charging mechanism of TiO2 (B) nanomaterials is limited due to the complicated nature of lithiation behavior, as well as the limitations of available characterization tools that can directly probe surface charging process. Here, an in situ approach is reported to monitor the dynamic valence state of TiO2 (B) nanotube electrodes, which utilizes in situ X-ray absorption spectroscopy (XAS) to identify the origin and contribution of surface storage. A real-time correlation is elucidated between the rate-dependent electrode performance and dynamic Ti valence-state change. A continuous Ti valence state change is directly observed through the whole charging/discharging process regardless of charging rates, which proves that along with the well-known non-faradaic reaction, the surface charging process also originates from a faradaic reaction. The quantification of these two surface storage contributions at different charging rates is further realized through in situ dynamic valence state monitoring combined with traditional cyclic voltammetry measurement. The methodology reported here can also be applied to other electrode materials for the real-time probing of valence state change during electrochemical reactions, the quantification of the faradaic and non-faradaic reactions, and the eventual elucidation of electrochemical surface charging mechanisms.

Enlarged CoO Covalency in Octahedral Sites Leading to Highly Efficient Spinel Oxides for Oxygen Evolution Reaction

Cobalt-containing spinel oxides are promising electrocatalysts for the oxygen evolution reaction (OER) owing to their remarkable activity and durability. However, the activity still needs further improvement and related fundamentals remain untouched. The fact that spinel oxides tend to form cation deficiencies can differentiate their electrocatalysis from other oxide materials, for example, the most studied oxygen-deficient perovskites. Here, a systematic study of spinel ZnFex Co2-x O4 oxides (x = 0-2.0) toward the OER is presented and a highly active catalyst superior to benchmark IrO2 is developed. The distinctive OER activity is found to be dominated by the metal-oxygen covalency and an enlarged CoO covalency by 10-30 at% Fe substitution is responsible for the activity enhancement. While the pH-dependent OER activity of ZnFe0.4 Co1.6 O4 (the optimal one) indicates decoupled proton-electron transfers during the OER, the involvement of lattice oxygen is not considered as a favorable route because of the downshifted O p-band center relative to Fermi level governed by the spinels cation deficient nature.

Selective conversion of lactic acid to acrylic acid over alkali and alkaline-earth metal co-modified NaY zeolites

Alkali and alkaline-earth metal cation co-modified NaY zeolites were systematically synthesized and comprehensively investigated as catalysts for gas-phase dehydration of lactic acid (LA) to acrylic acid (AA). The long-term (time-on-stream >55 h) catalytic performance in four repeated reaction–regeneration cycles was studied. The best performing catalyst shows a consistently high AA selectivity of ∼84% at different weight hourly space velocity (WHSV) values ranging from 0.48 to 4.8 h−1. Most importantly, the catalyst can still deliver a high AA selectivity of ∼82% after four long-term reaction–regeneration cycles. The investigation shows that mild etching increases the defect density of the zeolite and thus leads to poor hydrothermal stability in the long-term reaction–regeneration cycles. The strong acidic adsorbate/catalyst surface interaction (base property) and the acidity of the catalyst are responsible for the catalyst deactivation. The role of the alkali and alkaline-earth metal cations and the transformation of these cations during the reaction and regeneration process are presented.

Single Cobalt Atoms Anchored on Porous N-Doped Graphene with Dual Reaction Sites for Efficient Fenton-like Catalysis

The Fenton-like process presents one of the most promising strategies to generate reactive oxygen-containing radicals to deal with the ever-growing environmental pollution. However, developing improved catalysts with adequate activity and stability is still a long-term goal for practical application. Herein, we demonstrate single cobalt atoms anchored on porous N-doped graphene with dual reaction sites as highly reactive and stable Fenton-like catalysts for efficient catalytic oxidation of recalcitrant organics via activation of peroxymonosulfate (PMS). Our experiments and density functional theory (DFT) calculations show that the CoN4 site with a single Co atom serves as the active site with optimal binding energy for PMS activation, while the adjacent pyrrolic N site adsorbs organic molecules. The dual reaction sites greatly reduce the migration distance of the active singlet oxygen produced from PMS activation and thus improve the Fenton-like catalytic performance.

Spinel Manganese Ferrites for Oxygen Electrocatalysis: Effect of Mn Valency and Occupation Site

AbstractSpinel catalysts have been widely explored for the electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). To consolidate the understanding on electrocatalysis by spinel family, intermediate spinels should be deliberately examined because most spinel oxides are of intermediate structure. Here, we report an investigation on the ORR and OER performance of intermediate spinel MnFe2O4. The modulation of cation oxidation state and inversion degree of spinel MnFe2O4 were achieved by a simple annealing process. X-ray absorption spectroscopy analysis reveals that the Mn occupancy in octahedral sites varied from 0.25 ~ 0.41 and Mn cations were oxidized from 2+ to 3+ with increasing temperature treatment. Convinced by the leading role of octahedral-geometric, we further reveal the role of Mn oxidation state through normalizing the activity to active Mn[Oh] site number. Our findings clearly indicate that Mn3+ was more catalytically active than Mn2+ in catalyzing ORR and OER. Graphical AbstractBoth Mn occupancy in octahedral site and its oxidation state play dominant roles in determining the catalytic activities of spinel manganese ferrites toward oxygen electrocatalysis.

Heteroatomic Zn-MWW Zeolite Developed for Catalytic Dehydrogenation Reactions: A Combined Experimental and DFT Study

To incorporate divalent transition metals into molecular sieve frameworks remains a great challenge in zeolite chemistry. In this study, Zn was successfully incorporated inside the MWW zeolite framework, namely Zn‐MWW (Zn−Si), through hydrothermal synthesis. A combined DFT and X‐ray absorption near edge structure (XANES) simulation was applied to unravel the location of Zn inside the structure, indicating that the most plausible Zn position is T5 with two sodium cations close to the neighboring oxygen to neutralize the charge of the zeolite. The as‐synthesized material exhibits superior catalytic performance for 1‐butene oxidative dehydrogenation with CO2 compared with MCM‐22 and ITQ‐1 supported ZnO. The 1,3‐butadiene (BD) selectivity is doubled at similar 1‐butene conversions. On the other hand, more than 51 % acetaldehyde is produced during ethanol conversion on the Zn‐MWW zeolite, whereas MCM‐22 mainly gives ethylene. The remarkable performance is associated with the efficient combination of acidity and redox properties owing to the successful insertion of atomic Zn inside the MWW framework, as demonstrated by XRD, FTIR, 29Si MAS NMR, XANES analysis, and DFT simulations.