Polyaniline coated Pt/CNT as highly stable and active catalyst for catalytic hydrogenation reduction of Cr(VI)

Liquid phase catalytic hydrogenation reduction is a feasible method to eliminate Cr(VI) in water, while supported noble metal catalysts are liable to deactivation. In this study, carbon nanotube supported Pt catalyst (Pt/CNT) coated by polyaniline (Pt/CNT@PANI) was prepared and applied in the liquid phase catalytic hydrogenation of Cr(VI). Characterization results disclose that after coating Pt/CNT is completely wrapped by PANI layers and active Pt particles are no longer accessible. Despite complete embedment of Pt particles by PANI layers, Pt/CNT@PANI remains highly active for Cr(VI) reduction in liquid phase catalytic hydrogenation. The catalytic Cr(VI) reduction on Pt/CNT@PANI can be described by a PANI oxidation-reduction mechanism, by which PANI is first oxidized by Cr(VI) to form Cr(III), and oxidized PANI is reduced by catalytic hydrogenation. The Cr(VI) reduction on Pt/CNT@PANI complies with the Langmuir-Hinshelwood model, reflecting the pivotal role of Cr(VI) adsorption. Furthermore, the catalytic activity of Pt/CNT@PANI differs with PANI layer thickness and Cr(VI) reduction is positively correlated with reaction temperature. Catalyst recycling results show that after 4 cycles Pt/CNT loses 92.4% of catalytic activity, while the initial activity of Pt/CNT@PANI slightly decreases by 11.6%, demonstrating its high catalyst stability.

Oxygen Vacancy-Governed Opposite Catalytic Performance for C3H6 and C3H8 Combustion: The Effect of the Pt Electronic Structure and Chemisorbed Oxygen Species

Revealing the role of engineered surface oxygen vacancies in the catalytic degradation of volatile organic compounds (VOCs) is of importance for the development of highly efficient catalysts. However, because of various structures of VOC molecules, the role of surface oxygen vacancies in different catalytic reactions remains ambiguous. Herein, a defective Pt/TiO_2-x catalyst is proposed to uncover the different catalytic mechanisms of C₃H₆ and C₃H₈ combustion via experiments and theoretical calculations. The electron transfer, originated from the oxygen vacancy, facilitates the formation of reduced Pt⁰ species and simultaneously interfacial chemisorbed O₂, thus promoting the C₃H₆ combustion via efficient C═C cleavage. The reduced Pt nanoparticles facilitate the robust chemisorption of bridging dimer O₂^2- (Pt-O-O-Ti) species. This chemisorbed oxygen inhibits the C₃H₈ combustion by depressing C₃H₈ adsorption. This work offers insights for the rational design of highly efficient catalysts for activating the C═C bond in alkene or C-H bond in alkane.

One-step elimination of Cr(VI) by catalytic hydrogenation of Cr(VI) and simultaneous Cr(OH)3 recovery on Pt catalysts encapsulated in N-doped mesoporous carbon

Hexavalent chromium Cr(VI) is a highly toxic heavy metal, which is commonly eliminated by stepwise reduction at acidic pH and precipitation of Cr(OH)₃ at alkaline pH. A unique Pt catalyst with Pt particles embedded in the framework of N-doped mesoporous carbon CMK-3 (denoted as Pt@NCMK-3) was designed and fabricated to one-step eliminate Cr(VI) pollution at near neutral pH via simultaneous Cr(VI) reduction by catalytic hydrogenation and Cr(OH)₃ recovery. Structural characterization showed that Pt particles of Pt@NCMK-3 were effectively embedded in the carbon rods of NCMK-3. Batch experiments revealed that Pt@NCMK-3 exhibited a higher catalytic activity and stability than other test catalysts. Fixed-bed column reaction results indicated that under the experimental conditions Pt@NCMK-3 had better breakthrough performances than other catalysts. Additionally, after 4 treatment-recovery cycles Pt@NCMK-3 maintained nearly identical breakthrough performance, whereas other catalysts displayed markedly decreased breakthrough bed volumes, reflecting a substantially higher stability of Pt@NCMK-3.

Advanced Nanocarbons for Enhanced Performance and Durability of Platinum Catalysts in Proton Exchange Membrane Fuel Cells

Insufficient stability of current carbon supported Pt and Pt alloy catalysts is a significant barrier for proton-exchange membrane fuel cells (PEMFCs). As a primary degradation cause to trigger Pt nanoparticle migration, dissolution, and aggregation, carbon corrosion remains a significant challenge. Compared with enhancing Pt and PtM alloy particle stability, improving support stability is rather challenging due to carbon's thermodynamic instability under fuel cell operation. In recent years, significant efforts have been made to develop highly durable carbon-based supports concerning innovative nanostructure design and synthesis along with mechanistic understanding. This review critically discusses recent progress in developing carbon-based materials for Pt catalysts and provides synthesis-structure-performance correlations to elucidate underlying stability enhancement mechanisms. The mechanisms and impacts of carbon support degradation on Pt catalyst performance are first discussed. The general strategies are summarized to tailor the carbon structures and strengthen the metal-support interactions, followed by discussions on how these designs lead to enhanced support stability. Based on current experimental and theoretical studies, the critical features of carbon supports are analyzed concerning their impacts on the performance and durability of Pt catalysts in fuel cells. Finally, the perspectives are shared on future directions to develop advanced carbon materials with favorable morphologies and nanostructures to increase Pt utilization, strengthen metal-support interactions, facilitate mass/charge transfer, and enhance corrosion resistance.

T porous PtIr bimetallic nanotubes with core shell structure for enhanced electrocatalysis on methanol oxidation

Sluggish methanol oxidation brings challenges to the commercialization of the direct methanol fuel cells (DMFCs). Herein, porous PtIr bimetallic nanotubes were prepared via galvanic replacement using Ag nanowires as template. These PtIr catalysts show a core-shell nanostructure with a tunable Pt-rich surface. The mass activity of methanol oxidation reaction (MOR) at these porous PtIr nanotubes can reach up to 1.42 A mg^-1based on Pt loading, which is better than the commercial Pt/C catalysts and can be comparable with most of one-dimensional Pt-based MOR catalysts reported recently. In addition, these PtIr catalysts can maintain structural integrity after long-term durability test. The superior catalytic performance of the novel porous PtIr nanotubes will make it possible used in the commercial DMFCs as advanced MOR catalysts at industrial scale.

Efficient NH3 Detection Based on MOS Sensors Coupled with Catalytic Conversion

A solution of NH₃ detection based on catalytic conversion of NH₃ into NO_x was proposed by using MOS gas detectors and Pt-supported catalysts. The catalysts convert NH₃ into NO_x, which is a very sensitive analyte for MOS detectors. Catalysts based on Pt-loaded HZSM-5 and Al₂O₃ were prepared by wet impregnation. MOS detectors were fabricated from nanosized In₂O₃ and WO₃ using screen-printing techniques. As expected, MOS sensors based on In₂O₃ and WO₃ have an extremely high sensitivity to NO₂; nevertheless, they have a relatively low response to NH₃ and a large cross-sensitivity to typical interfering gases such as CO and ethanol. By the present solution, MOS sensors could very sensitively respond to NH₃, even down to 0.25 ppm. In addition, it was also found that the catalysis also combusts the reducing gases into CO₂ and water and consequently significantly improves the selectivity of NH₃. Lastly, we would to like to stress that the proposed concept of the catalytic conversion method suggests the potential utility for broader measurements by using different catalysts and gas detectors and that only a part of the usage for NH₃ was presented here.

Impact of Membrane Types and Catalyst Layers Composition on Performance of Polymer Electrolyte Membrane Fuel Cells

Performance of a low temperature polymer electrolyte membrane fuel cell (PEMFC) is highly dependent on the kind of catalysts, catalyst supports, ionomer amount on the catalyst layers (CL), membrane types and operating conditions. In this work, we investigated the influence of membrane types and CL compositions on MEA performance. MEA performance increases under all practically relevant load conditions with reduction of the membrane thickness from 50 to 15 μm, however further decrease in membrane thickness from 15 to 10 μm leads to reduction in cell voltage at high current loads. A thick anode CL is found to be beneficial under wet operating conditions assuming more pore space is provided to accommodate liquid water, whereas under dry operating conditions, an intermediate thickness of the anode CL is beneficial. When studying the impact of catalyst layer thickness, too thin a catalyst layer again shows reduced performance due to increased ohmic resistance ruled out the performance of the MEAs which have identical Pt crystallite sizes on the cathode CLs i. e. the thinnest the cathode CL, the highest the voltage were achieved at a defined current load. Adaptation of the operating conditions is highly anticipated to achieve the highest MEA performance.

Encapsulating Pt Nanoparticles inside a Derived Two-Dimensional Metal-Organic Frameworks for the Enhancement of Catalytic Activity

The development of highly active and stable electrocatalysts toward oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is a key for commercial application of fuel cells and water splitting. Here, we report a highly active and stable Pt nanoparticles (NPs) encapsulated in ultrathin two-dimensional (2D) carbon layers derived from the ultrathin 2D metal-organic framework precursor (ZIF-67). Electrochemical tests reveal that our approach not only stabilized Pt NPs successfully but also boosted Pt activities toward ORR and HER. We found that our Pt catalysts encapsulated in ultrathin 2D carbon layers exhibited an ORR activity of 5.9 and 12 times greater than those of the commercial Pt/C and Pt/RGO without 2D carbon layer protection. Our encapsulated Pt catalysts also show more than nine times higher stability than those of Pt/C catalysts. In addition to ORR, our novel encapsulated Pt catalysts display an extraordinary stability and activity toward HER, with a lower overpotential (14.3 mV in acidic media and 37.2 mV in alkaline media) at a current density of 10 mA cm^-2 than Pt/C catalysts (23.1 mV in acidic media and 92.1 mV in alkaline media). The enhanced electrochemical activities and stability of our encapsulated Pt catalysts are attributed to the synergistic effect of Pt-based NPs and ultrathin 2D carbon layers derived from ZIF-67 with enriched active sites Co-N_x. First-principles simulations reveal that the synergistic catalysis of Pt-based NPs and Co-N_x derived from ZIF-67 improves the activity for ORR and HER.

Atomic Carbon Layers Supported Pt Nanoparticles for Minimized CO Poisoning and Maximized Methanol Oxidation

Maximizing activity of Pt catalysts toward methanol oxidation reaction (MOR) together with minimized poisoning of adsorbed CO during MOR still remains a big challenge. In the present work, uniform and well-distributed Pt nanoparticles (NPs) grown on an atomic carbon layer, that is in situ formed by means of dry-etching of silicon carbide nanoparticles (SiC NPs) with CCl₄ gas, are explored as potential catalysts for MOR. Significantly, as-synthesized catalysts exhibit remarkably higher MOR catalytic activity (e.g., 647.63 mA mg^-1 at a peak potential of 0.85 V vs RHE) and much improved anti-CO poisoning ability than the commercial Pt/C catalysts, Pt/carbon nanotubes, and Pt/graphene catalysts. Moreover, the amount of expensive Pt is a few times lower than that of the commercial and reported catalyst systems. As confirmed from density functional theory (DFT) calculations and X-ray absorption fine structure (XAFS) measurements, such high performance is due to reduced adsorption energy of CO on the Pt NPs and an increased amount of adsorbed energy OH species that remove adsorbed CO fast and efficiently. Therefore, these catalysts can be utilized for the development of large-scale and industry-orientated direct methanol fuel cells.

Enhanced CO Oxidation and Cyclic Activities in Three-Dimensional Platinum/Indium Tin Oxide/Carbon Black Electrocatalysts Processed by Cathodic Arc Deposition

There have been extensive efforts to develop competitive electrocatalysts using carbon black (CB) supports for high-performance proton-exchange membrane fuel cells with less usage of Pt. Herein, we propose a very promising electrocatalyst architecture based on the three-dimensional Pt/indium tin oxide (ITO)/CB support structure which was enabled by a nonconventional deposition process ensuring very uniform impregnation of Pt and ITO nanoparticles into the CB network. The unusual scales of the Pt (∼1.9 nm) and ITO (∼5.6 nm) nanoparticles were directly related to unexpectedly better performance of the electrocatalytic activities. As a highlight, the electrochemical surface area of the electrocatalyst was maintained very well after the 3000 cycle-accelerated durability evaluation by demonstrating an excellent retention of ∼74.9%. Particularly, the CO tolerance exhibited a low value of ∼0.68 V as the absorption current peak, compared to ∼0.79 V for a commercial Pt/CB catalyst containing twice more Pt.

Influence of Boron, Tungsten and Molybdenum Modifiers on Zirconia Based Pt Catalyst for Glycerol Valorization

The influence of boron, tungsten and molybdenum modifiers on zirconia-based Pt catalyst was studied for glycerol valorization. Zirconia modified supports were prepared by impregnation of ZrO₂ with either boric, silicontungstic or phosphomolybdic acids to obtain supports with enhanced Brönsted acidic properties. The modified supports were subsequently impregnated with chloroplatinic acid to obtain Pt-based catalysts. Pt incorporation resulted in the increase in Lewis acidity of the solids, being more significant for the Pt//W/ZrO₂ catalyst. Reduced Pt catalysts were tested for the liquid-phase glycerol hydrogenolysis, observing a synergistic effect between catalyst acid sites and metal function that proved to be crucial in glycerol hydrogenolysis. The Pt//W/ZrO₂ catalyst was the most active catalyst in this reaction, being the only leading to 1,3-PDO (45% sel., 160 °C) while Pt//Mo/ZrO₂ is the best option for 1,2-PDO (49% sel., 180 °C). Reusability studies carried out for Pt//W/ZrO₂ showed that catalytic activity dropped after the first use, remaining constant for the second and subsequent ones. Selectivity to reaction products also changes during reuses. Therefore, the selectivity to 1,2 PDO increases in the first reuse in detriment to the selectivity to n-propanol whereas the selectivity to 1,3-PDO remains constant along the uses. This behavior could be associated to the lixiviation of W species and/or catalyst fouling during reaction runs.