62 research outputs found
Surface and Interface Engineering in Copper-Based Bimetallic Materials for Selective CO2 Electroreduction
No full textThe electrochemical CO₂ reduction reaction (CO₂RR) can couple carbon-capture storage with renewable energy to convert CO₂ into chemical feedstocks. For this process, copper is the only metal known to catalyze the CO₂RR to hydrocarbons with adequate efficiency, but it suffers from poor selectivity. Copper bimetallic materials have recently shown an improvement in CO₂RR selectivity compared with that of copper, such that the secondary metal is likely to play an important role in altering inherent adsorption energetics. This review explores the fundamental role of the secondary metal with a focus on how oxygen (O) and hydrogen (H) affinity affect selectivity in bimetallic electrocatalysts. Here, we identify four metal groups categorized by O and H affinities to determine their CO₂RR selectivity trends. By considering experimental and computational studies, we link the effects of extrinsic chemical composition and physical structure to intrinsic intermediate adsorption and reaction pathway selection. After this, we summarize some general trends and propose design strategies for future electrocatalysts.Anthony Vasileff, Chaochen Xu, Yan Jiao, Yao Zheng and Shi-Zhang Qia
A boron imidazolate framework with mechanochromic and electrocatalytic properties
No full textA new alkaline-stable anionic boron imidazolate framework, CdB(im)4(btec)0.5·H2O (BIF-89, im = imidazolate, btec = benzene-1,2,4,5-tetracarboxylate), was designed and synthesized. The resulting BIF-89 with a three-dimensional molecular structure exhibits a unique mechanochromic behavior with a maximum emission blue shift of about 23 nm in the blue emitting range. Additionally, the uncoordinated carboxylic groups in the BIF-89 framework can stabilize incorporation of Fe(III) at the atomic level to form Fe-immobilized BIF-89 (Fe@BIF-89), which can serve as an effective electrocatalyst for the oxygen evolution reaction in strongly alkaline media. The new BIF-89 expands the field of MOFs to luminescence mechanochromism and electrocatalytic applications without involving precious metals.Tian Wen, Yao Zheng, Chaochen Xu, Jian Zhang, Mietek Jaroniec and Shi-Zhang Qia
Transition Metal-Based Electrocatalysts for Highly Selective C02 Reduction
No full textThe electrochemical CO2 reduction reaction (CRR) can combine carbon cycling with renewable energy to convert CO2 into high-value carbonaceous feedstocks. However, this process su ers from kinetically sluggish because of the complicated electron transfer and high energy barriers involved. Well-designed transition metal materials as promising electrocatalysts show remarkable catalytic activities for the CRR. Therefore, this Thesis is to study the catalytic activity and selectivity on these transition metal catalysts, and a fundamental understanding of the catalytic mechanism is given through a series of experimental and computational results using advanced synthesis methods, electrochemical measurements, material characterization including microscopy and spectroscopy, synchrotron-based X- ray spectroscopy, in situ spectroscopy, and density functional theory (DFT) calculations. The scope of this Thesis is narrowed to nanoscale and sub-nanoscale engineered 3d-block transition metal (mainly, Fe, Co, Ni, Cu) catalysts for the CRR process. In this Thesis, the rst section introduces research progress including catalytic performance and mechanisms on sub-nanoscale 3d-block transition metal catalysts for the CRR. The second section consists of published and submitted works: (1) The rst project starts with the investigation of the CRR on Ni catalysts. We engineered and alloyed Ni with Cu to obtain ultrasmall graphene-encapsulated Ni-Cu bimetallic catalysts. The Cu-lean catalyst exhibited signi cant activity and selectivity, and the highest Faradaic e ciency (FE) toward CO was 90% at -1.0 V vs. RHE. By coupling synchrotron-based X-ray absorption and in situ Raman spectroscopy studies, we found that there is a negative correlation with the Cu content in Ni-Cu catalyst and CO selectivity due to redistribution of the 3d electrons from Ni and Cu. (2) Because of the high catalytic activity was received on ultrasmall Ni-Cu particles, the second project aims to fabricate sub-nanoscale transition metal catalysts for the CRR. We synthesized atomically dispersed Fe immobilized within N-doped carbon nanosheets. The optimal Fe catalyst achieved FE of 90% toward CO at -0.58 V vs. RHE. A series of controlled tests revealed that there is a synergistic e ect between the Fe sites and the pyrrolic-N-framework which promotes the catalytic activity of CO evolution. (3) The third work is based on the previous Fe catalyst and investigates the unique single-atom Cu catalyst (Cu-N4-NG). The chemical structure and coordination environment of Cu-N4-NG were identi ed using synchrotron-based characterization. Compared to a traditional bulk Cu catalyst, Cu-N4-NG performed a FE of 80.6% towards CO at -1.0 V vs. RHE. The experimental results revealed that the presence of Cu-N4 moieties largely promotes CO2 activation and water dissociation, showing CO2 reduction is kinetically preferred on Cu-N4-NG. Also, the computational investigation suggested a thermodynamic explanation that CO2 reduction is less hindered on Cu-N4-NG compared to hydrogen evolution. (4) Although high FEs were obtained on single-atom transition metal catalyst shown in the previous two works, the two catalysts were not strictly single-atom catalysts with a uniform structure of M-N4, some coordination defects existed. Thus, graphene- supported metal phthalocyanine catalysts with M-N4 structure were reported in the fourth work, which achieved almost 100% CO2 conversion to CO on graphene- supported cobalt phthalocyanine. Further experimental studies showed that the phthalocyanines with graphene were signi cantly activated than the pure ones. A series of control tests uncovered that the graphene substrate facilitates electron transfer between the catalyst and CO2 molecules, which increased CO selectivity.Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering and Advanced Materials, 202
Selectivity control for electrochemical CO2 reduction by charge redistribution on the surface of copper alloys
No full textCopper is a significant platform for CO2 electroreduction catalysts because it is the only known metal to produce multi-carbon products but suffers from poor selectivity. In the early stages of the reaction pathway, a selectivity-determining step dictates if the pathway leads to formate (a dead-end) or to CO (and on to multi-carbon products). Therefore, controlling the adsorption of key intermediates, in order to steer the reaction pathway as desired, is critical for selective CO2 electroreduction. Alloying copper is a strategy in which the composition and electronic properties of the alloy surface can be finely tuned to alter the reaction intermediate adsorption behavior. Herein, through in situ Raman spectroscopy and density functional theory (DFT) calculations, we investigate a composition-dependent selectivity toward CO and formate during CO2 electroreduction on a range of Cu–Sn alloy catalysts. We find that the selectivity shifts from CO to formate generation as the Sn content in the alloy catalyst increases because of a shift in adsorption preference from the C-bound *COOH intermediate to the O-bound *OCHO intermediate. Theoretical DFT calculation results indicate that this selectivity shift is due to a gradual weakening of *COOH adsorption and strengthening of *OCHO that occurs with increasing Sn content. A combination of theoretical Bader charge analysis and experimental X-ray photoelectron spectroscopy revealed the origin of such transformation: upon alloying, charge is redistributed from Sn to Cu, which creates regions of localized positive charge on the Sn sites. Therefore, with increasing tin content, these localized positive sites hinder the nucleophilic attack of the CO2 carbon, making *COOH adsorption (and the CO pathway) less favorable.Anthony Vasileff, Xing Zhi, Chaochen Xu, Lei Ge, Yan Jiao, Yao Zheng, and Shi-Zhang Qia
Anomalous hydrogen evolution behavior in high-pH environment induced by locally generated hydronium ions
No full textMost fundamental studies of electrocatalysis are based on the experimental and simulation results obtained for bulk model materials. Some of these mechanistic understandings are inapplicable for more active nanostructured electrocatalysts. Herein, considering the simplest and most typical electrocatalytic process, the hydrogen evolution reaction, an alternative reaction mechanism is proposed for nanomaterials based on the identification of a new intermediate, which differs from those commonly known for the bulk counterparts. In-situ Raman spectroscopy and electrochemical thermal/kinetic measurements were conducted on a series of nanomaterials under different conditions. In high-pH electrolytes with negligible hydronium (H₃O⁺) concentration in bulk phase, massive H₃O⁺ intermediates are found generating on the catalytic surface during water dissociation and hydrogen adsorption processes. These H₃O⁺ intermediates create a unique acid-like local reaction environment on nanostructured catalytic surfaces and cut the energy barrier of the overall reaction. Such phenomena on nanostructured electrocatalysts explain their widely observed anomalously high activity under high-pH conditions.Xuesi Wang, Chaochen Xu, Mietek Jaroniec, Yao Zheng, Shi-Zhang Qia
Reduction
No full textRecent studies have shown that single-atom catalysts (SACs) have significantly better catalytic activity and selectivity for the electrochemical CO₂ reduction reaction (CRR) compared to their bulk metal and nanostructured counterparts. However, there are few relevant articles reviewing SACs for the CRR, despite their importance in the field. Herein, the scope of this review is the recent development of single-atom 3d-block transition metal catalysts (metal = Mn, Fe, Co, Ni, Cu, Zn) and their application as electrocatalysts for the CRR. The recent representative works by metal are summarized. Results show that 1) Ni and Fe SACs exhibit superior catalytic performance for CO evolution; 2) Co, Mn, Zn SACs are less reported due to their relative inertness for the CRR; 3) Cu SACs have ordinary catalytic activity for the CRR, however, C2 products are observed in a few reports. Point (3) is attractive to the prospective study of CO₂ to highly reduced products. Finally, some suggestions for the future development of SACs for the CRR are briefly proposed.Chaochen Xu, Anthony Vasileff, Yao Zheng, and Shi-Zhang Qia
Unlocking the Value of Phosphogypsum Waste: Steam Calcination for Efficient Decomposition and Resource Recovery
No full textPhosphogypsum is a waste by-product of phosphate fertilizer production. It contains calcium sulfate (CaSO4.xH2O), which can be decomposed to produce calcium oxide (CaO) and sulfur dioxide (SO2). Steam calcination is a process that uses steam to decompose CaSO4. This study represents the first comprehensive investigation of steam calcination applied to phosphogypsum, comparing its performance to pure CaSO4 at various temperatures (1050–1200 °C) and steam molar ratios (0, 4%, and 30%). The results showed that steam calcination can significantly lower the calcination temperature of both CaSO4 and phosphogypsum. The presence of steam can significantly accelerate the decomposition reaction of pure CaSO4 and phosphogypsum when compared to decomposition in nitrogen or air. It was found that the decomposition reaction in steam follows the contracting cylinder reaction model in the case of CaSO4 and the first-order reaction model for the phosphogypsum. Additionally, the activation energy for the steam calcination of CaSO4 and phosphogypsum were found to be 421.2 kJ/mol and 418.6 kJ/mol, respectively, which are lower than the activation energy for the calcination in oxyfuel combustion products (O2, CO2, SO2, and H2O gases) 531.4 kJ/mol demonstrating its potential for enhanced energy efficiency. As a result, steam calcination emerges as a promising sustainable solution for valorizing phosphogypsum while reducing energy consumption and greenhouse gas emissions.Authors acknowledge the support from Clean Energy Research Platform at King Abdullah University of Science and Technolog
Optimization and Construction of Forestland Ecological Security Pattern: A Case Study of the Huai River Source–Dabie Mountains in China
No full textIn this research, we chose six indicators—soil conservation, water conservation, carbon sequestration, windbreak and sand fixation, biodiversity conservation, and forest recreation—to compute the forestland ecosystem service index for forestland within the study region, utilizing time series data. The outcomes reveal that the aggregate index of forestland ecosystem services exhibits a spatial distribution characterized by higher values in the northeastern part and lower values in the southwestern part, with an upward trend over time. Among these functions, windbreak and sand fixation, water conservation, carbon sequestration, and forest recreation all maintained relatively high growth rates. We selected 10 factors that are closely related to the natural environment and human activities and employed spatial principal component analysis to develop a comprehensive resistance surface. Based on the assessment results of forestland ecosystem functions, in conjunction with morphological spatial pattern analysis (MSPA) as well as landscape connectivity analysis, we optimized the method for identifying ecological source sites and extracted 38 ecological source sites. Subsequently, leveraging circuit theory, we extracted 91 ecological corridors and pinpointed 25 ecological nodes, ultimately constructing a forestland ecosystem security pattern (ESP) in the study area and proposing restoration strategies
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