锡量子点制备及其电催化还原二氧化碳产甲酸性能

锡基材料在自然界含量丰富、价格低廉, 在电催化还原CO₂制液体燃料反应中具有巨大潜力。但是较低的产物选择性和较差的稳定性限制了其应用。本工作制备的锡量子点电催化剂(Sn-QDs), 具有高效、高稳定性和高选择性的电催化还原CO₂产HCOOH活性。Sn-QDs的平均颗粒尺寸仅为2~3 nm, 结晶性良好。小的颗粒尺寸增大了电化学活性面积(ECSA), Sn-QDs的ECSA约为锡颗粒的4.4倍。ECSA增大以及CO₂还原反应动力学加速, 促进了CO₂电化学转化。在-1.0 V (vs RHE)下, Sn-QDs/CN催化剂的HCOOH法拉第效率(FE_HCOOH)达到95%, 并且在宽约0.5 V的电势范围内能够保持在83%以上。此外, Sn-QDs/CN可以在24 h内保持良好的电化学稳定性。     

Synthesis of highly active and stable Pd/C catalysts toward HCOOH dehydrogenation by a simple NH3 adsorption method

Pd catalysts supported on activated carbon (Pd/C–NH₃) toward HCOOH dehydrogenation were prepared by a simple adsorption method using ammonia (NH₃) and Ar as the working gas. The results show that the TOF_initial of Pd/C–NH₃ was 459.8 h⁻¹ at 50 °C. When the reaction was carried out for 4 h, the HCOOH dehydrogenation ratio over Pd/C–NH₃ was about 81.2%, which was 1.15 and 1.13 times, respectively, as that of the as-prepared Pd/C catalyst without any treatment (Pd/C–As) and the Pd/C catalyst purchased from Sigma-Aldrich (Pd/C-CM). The total amount of H₂ and CO₂ produced by using Pd/C–NH₃ to decompose HCOOH in the third cycle was 99.4% of the gas produced by the first reaction cycle, and 1.80 and 12.60 times, respectively, as that of Pd/C–As and Pd/C-CM. The characterization results indicated that the Pd active species in Pd/C–NH₃ migrated to the outer surface of the carbon support during the reaction, and the pore volume of the carbon support became larger, which were beneficial to the reaction. These factors made Pd/C–NH₃ exhibit excellent HCOOH dehydrogenation activity and stability. NH₃ adsorption is a simple and effective method for preparing high-performance Pd/C HCOOH dehydrogenation catalysts, and has important guiding significance for the preparation of other carbon supported noble metal catalysts.     

Synthesis of Au/UiO-66-NH2/Graphene composites as efficient visible-light photocatalysts to convert CO2

The production of new solar fuel through CO₂ photocatalytic reduction has aroused tremendous attention in recent years because of the increased demand of global energy sources and global warming caused by the mass concentration of CO₂ in the earth's atmosphere. In this work, UiO-66-NH₂ was co-modified by the Au nanoparticles (Au-NPs) and Graphene (GR). The resultant nanocomposite exhibits a strong absorption edge in visible light owing to the surface plasmon resonance (SPR) of Au-NPs. More attractively, Au/UiO-66-NH₂/GR displays much higher photocatalytic activity (49.9 μmol) and selectivity (80.9%) than that of UiO-66-NH₂/GR (selectivity: 71.6%) and pure UiO-66-NH₂ (selectivity: 38.3%) for the CO₂ reduction under visible light. The enhanced photocatalytic performance is primarily dued to the surface plasmon resonance (SPR) of Au-NPs, which could enhance the visible light absorption. The GR sheets could play as an electron acceptor with superior conductivity and thus suppress the recombination of electrons and holes. Besides, the GR could also improve the dispersibility of UiO-66-NH₂ so as to expose more active sites and strengthen the capture of CO₂. The contact effect and synergy effect among different samples are strengthened in the ternary composites and the photocatalytic performance is therefore improved. This study demonstrates a MOF based hybrid composite for efficient photocatalytic CO₂ reduction, the findings not only prove great potential for the design and application of MOFs-based materials but also bring light to novel chances in the development of new high performance photocatalysts.     

Effect of temperature, oxidant and catalyst loading on the performance of direct formic acid fuel cell

We investigated the effect of temperature, oxidant and catalyst loading on the performance of direct formic acid fuel cell (DFAFC). When oxidant was changed from air to oxygen, the power density was increased to 17.3 mW/ cm² at 25 ‡C. The power density of DFAFC operated with oxygen showed a maximum value of 40.04 mW/cm² with the temperature rise from room temperature to 70 °C. The highest power density of DFAFC using air was observed for Pt-Ru black catalyst with loading of 8 mgPt/cm² at room temperature. At 70 ‡C; however, the performance of catalyst with the loading of 4 mgPt/cm² was higher than that of 8 mgPt/cm². The DFAFC, operated with oxygen and catalyst of 4 mgPt/cm² loading, showed the best performance at all temperature range. The enhancement of cell performance with an increase of catalyst loading is believed to come from an increase of catalyst active sites. However, operated at higher temperature or with oxygen, the cell with higher catalyst loading showed lower performance than expected. It is speculated that the thick catalyst layer inhibits the proton transport.     

碳载体的碱处理对Pd/C催化剂电催化活性的影响

采用处理与未处理的活性炭分别制备Pd/C催化剂,运用循环伏安法和计时电流法来检测两种Pd/C催化剂对甲酸的电催化氧化活性和稳定性。结果表明,用NH3.H2O处理过的活性炭所制备的Pd/C催化剂在对甲酸的电催化氧化的活性上和稳定性上都有不同程度的提高。     

Surface modification and electrocatalytic properties of Pt(100), Pt(110), Pt(320) and Pt(331) electrodes with Sb towards HCOOH oxidation

Behaviors of irreversibly adsorbed Sb adatoms on Pt(100), Pt(110), Pt(320) and Pt(331) single crystal surfaces and electrocatalytic properties of the modified electrodes towards formic acid oxidation were investigated. It was determined that Sb adatoms are stable at potentials below 0.45 V (SCE) on Pt(100) and Pt(110), below 0.40 V on Pt(320), and below 0.35 V on Pt(331). Different coverage of Sb_ad was obtained conveniently by partially stripping Sb_ad from saturation coverage of Sb_ad. It has demonstrated that the redox behaviors of Sb adatoms and the coadsorption properties of Sb_ad with H_ad depend strongly on the orientations of the Pt single crystal electrode. Significant catalytic effects towards HCOOH oxidation were observed on Pt single crystal electrodes modified with Sb adatoms, which consist of (1) the inhibition of dissociative adsorption of HCOOH, (2) the enhancement of oxidation current, and (3) the negative shift of oxidation potential that was measured about 220 mV on Pt(110)/Sb, 110 mV on (110) sites of Pt(320)/Sb, and 100 mV on Pt(331)/Sb electrode. Neither enhancement of oxidation current nor negative shift of oxidation potential can be observed on Pt(100)/Sb electrode. The results suggested that electronic effect is the main effect presented on Pt(110), Pt(320) and Pt(331) surface upon Sb modification, while geometric effect is considered to the major effect on Pt(100) electrode.     

Fabrication of heterostructured UIO-66-NH2 /CNTs with enhanced activity and selectivity over photocatalytic CO2 reduction

Developing photocatalysts with superior efficiency and selectivity is an important issue for photocatalytic converting CO₂. Hierarchically heterostructured one-dimensional nanomaterials represent a kind of promising catalysts for photocatalytic CO₂ reduction on account of the high surface area and synthetic effect between different components. Herein, we synthesized UIO-66-NH₂/carbon nanotubes (CNTs) heterostructures via a hydrothermal method, and investigated their photocatalytic performance. The element mapping, X-ray diffraction, and X-ray photoelectron spectroscopy collectively confirmed that the UIO-66-NH₂ was successfully loaded on the surface of the CNTs. The specific surface area of the UIO-66-NH₂/CNTs is 1.5 times higher than that of UIO-66-NH₂. The photocurrent and electrochemical impedance spectroscopy measurements showed that the CNTs could enhance the electron mobility and reduce the recombination of photogenerated electron-hole pairs, which was also confirmed by the Photoluminescence spectroscopy (PL). The CNTs can improve the conductivity of composites and the dispersion of UIO-66-NH₂, exposing more active sites, therefore the UIO-66-NH₂ can increase the absorption of carbon dioxide and thus enhance the selectivity. The composites remarkably promoted the separation and transition of electrons and thus improved the photocatalytic efficiency of CO₂ reduction. More importantly, it was found that the as-prepared composites suppress the hydrogen generation reaction during the CO₂ reduction process.     

Rh single atoms embedded in CeO2 nanostructure boost CO2 hydrogenation to HCOOH

CO₂ hydrogenation to value-added chemicals is a promising pathway to solve CO₂-relevant environmental problems but still remains a great challenge. Herein, we report a CeO₂ nanostructure supported Rh single atoms (Rh-SAs/CeO₂) catalyst and was used for the efficient CO₂ hydrogenation to HCOOH. The Rh-SAs/CeO₂ exhibited high catalytic activity with turnover numbers (TON) up to 221 at 200 ℃, which was 4-fold to that of CeO₂ supported Rh nanoparticles (Rh-NPs/CeO₂). Moreover, HCOOH selectivity for Rh-SAs/CeO₂ reached 85%, much higher than that of Rh-NPs/CeO₂ (46%). Mechanism studies revealed that Rh single atoms in the Rh-SAs/CeO₂ with high metal atoms utilization efficiency not only provided abundant active sites to promote the catalytic activity, but also suppressed the decomposition of HCOOH to CO and benefited the formation of HCOOH.     

NH3 plasma synthesis of N-doped activated carbon supported Pd catalysts with high catalytic activity and stability for HCOOH dehydrogenation

Plasma is a simple and effective method to prepare N-doped carbon materials and supported metal catalysts. In this work, Pd/C–C(NH₃) and Pd/C–P(NH₃) catalysts are prepared by heat treatment and cold plasma methods using Ar and NH₃ as the working gas. The activity and stability of obtained catalysts are tested by formic acid dehydrogenation reaction. The results show that TOF_initial of Pd/C–P(NH₃) is 527.1 h⁻¹ at 50 °C, and the HCOOH decomposition rate is about 89.2% at 4 h. The hydrogen production of Pd/C–P(NH₃) when used in first and third cycle are 1.14 and 1.14 times than that of Pd/C–C(NH₃), and 1.24 and 13.24 times than that of commercial Pd/C. Various characterization techniques are used to characterize the structure of the prepared Pd/C catalysts. The results indicate that NH₃ plasma is milder than NH₃ thermal treatment. The high activity and stability of Pd/C–P(NH₃) are mainly due to the NH₃ cold plasma effectively achieving N-doping of the carbon support, and Pd nanoparticles with a small size and high dispersion. Atmospheric pressure NH₃ cold plasma provides an effective method to prepare high-performance Pd/C catalysts for HCOOH dehydrogenation and plays a guiding role in the preparation of high-performance carbon-supported noble metal catalysts.     

Formic Acid: A Promising Bio‐Renewable Feedstock for Fine Chemicals

In light of the growing scarcity of petroleum‐based raw materials, carbon dioxide (CO₂) is becoming increasing attractive as organic carbon source. In this perspective, formic acid (HCOOH) might be an interesting bio‐renewable solution to store, transport, and activate carbon dioxide for the synthesis of value‐added chemicals. Herein, HCOOH has been successfully used as C₁ building block for the synthesis of a library of alcohols via a catalysed oxo‐synthesis, under green experimental conditions.