Electron penetration triggering interface activity of Pt-graphene for CO oxidation at room temperature.
ABSTRACT: Achieving CO oxidation at room temperature is significant for gas purification but still challenging nowadays. Pt promoted by 3d transition metals (TMs) is a promising candidate for this reaction, but TMs are prone to be deeply oxidized in an oxygen-rich atmosphere, leading to low activity. Herein we report a unique structure design of graphene-isolated Pt from CoNi nanoparticles (PtǀCoNi) for efficiently catalytic CO oxidation in an oxygen-rich atmosphere. CoNi alloy is protected by ultrathin graphene shell from oxidation and therefore modulates the electronic property of Pt-graphene interface via electron penetration effect. This catalyst can achieve near 100% CO conversion at room temperature, while there are limited conversions over Pt/C and Pt/CoNiOx catalysts. Experiments and theoretical calculations indicate that CO will saturate Pt sites, but O2 can adsorb at the Pt-graphene interface without competing with CO, which facilitate the O2 activation and the subsequent surface reaction. This graphene-isolated system is distinct from the classical metal-metal oxide interface for catalysis, and it provides a new thought for the design of heterogeneous catalysts.
Project description:We investigated the H and H<sub>2</sub> adsorption effects on the stability of a Pt atom on various heteroatom-doped graphene supports using first-principles calculations based on density functional theory. We show that H and H<sub>2</sub> adsorptions on the Pt atom weaken the interaction between the Pt atom and graphene support and decrease the adsorption energy of Pt atoms. H<sub>2</sub> adsorption on Pt atoms decreases the adsorption energy of Pt atoms on all graphene supports by more than 30%, whereas H adsorption only affects pristine, O-, and S-doped graphene. These results indicate that the hydrogen atmosphere enhances the detachment of Pt catalysts. However, the B-, O-, Si-, P-doped, and monovacant graphene still maintained large adsorption energies of PtH and PtH<sub>2</sub> of more than 1.5 eV. In addition, the diffusion barriers of PtH and PtH<sub>2</sub> on pristine graphene were calculated to be less than 0.07 eV, which further demonstrated that H and H<sub>2</sub> enhance the degradation of Pt catalysts. Even after H and H<sub>2</sub> adsorptions on a Pt atom, O-, Si-, P-doped, and monovacant graphene still maintained large diffusion barriers of more than 1 eV. Therefore, we concluded that O-, Si-, and P-doped graphene are suitable supports for Pt catalysts in a hydrogen atmosphere.
Project description:In this paper, a simple "one pot" methodology to synthesize snowflake-like dendritic CoNi alloy-reduced graphene oxide (RGO) nanocomposites has been reported. First-principles quantum mechanical calculations based on density functional theory (DFT) have been conducted to understand the electronic structures and properties of the interface between Co, Ni, and graphene. Detailed investigations have been conducted to evaluate the performance of CoNi alloy and CoNi-RGO nanocomposites for two different types of applications: (i) as the catalyst for the reduction reaction of 4-nitrophenol and Knoevenagel condensation reaction and (ii) as the active electrode material in the supercapacitor applications. Here, the influence of microstructures of CoNi alloy particles (spherical vs snowflake-like dendritic) and the effect of immobilization of CoNi alloy on the surface of RGO on the performance of CoNi-RGO nanocomposites have been demonstrated. CoNi alloy having a snowflake-like dendritic microstructure exhibited better performance than that of spherical CoNi alloy, and CoNi-RGO nanocomposites showed improved properties compared to CoNi alloy. The <i>k</i> <sub>app</sub> value of the (CoNi<sub>D</sub>)<sub>60</sub>RGO<sub>40</sub>-catalyzed reduction reaction of 4-nitrophenol is 20.55 × 10<sup>-3</sup> s<sup>-1</sup>, which is comparable and, in some cases, superior to many RGO-based catalysts. The (CoNi<sub>D</sub>)<sub>60</sub>RGO<sub>40</sub>-catalyzed Knoevenagel condensation reaction showed the % yield of the products in the range of 80-93%. (CoNi<sub>D</sub>)<sub>60</sub>RGO<sub>40</sub> showed a specific capacitance of 501 F g<sup>-1</sup> (at 6 A g<sup>-1</sup>), 21.08 Wh kg<sup>-1</sup> energy density at a power density of 1650 W kg<sup>-1</sup>, and a retention of ∼85% of capacitance after 4000 cycles. These results indicate that (CoNi<sub>D</sub>)<sub>60</sub>RGO<sub>40</sub> could be considered as a promising electrode material for high-performance supercapacitors. The synergistic effect, derived from the hierarchical structure of CoNi<sub>D</sub>-RGO nanocomposites, is the origin for its superior performance. The easy synthetic methodology, high catalytic efficiency, and excellent supercapacitance performance make (CoNi<sub>D</sub>)<sub>60</sub>RGO<sub>40</sub> an appealing multifunctional material.
Project description:Improving the low-temperature activity (below 100?°C) and noble-metal efficiency of automotive exhaust catalysts has been a continuous effort to eliminate cold-start emissions, yet great challenges remain. Here we report a strategy to activate the low-temperature performance of Pt catalysts on Cu-modified CeO<sub>2</sub> supports based on redox-coupled atomic layer deposition. The interfacial reducibility and structure of composite catalysts have been precisely tuned by oxide doping and accurate control of Pt size. Cu-modified CeO<sub>2</sub>-supported Pt sub-nanoclusters demonstrate a remarkable performance with an onset of CO oxidation reactivity below room temperature, which is one order of magnitude more active than atomically-dispersed Pt catalysts. The Cu-O-Ce site with activated lattice oxygen anchors deposited Pt sub-nanoclusters, leading to a moderate CO adsorption strength at the interface that facilitates the low-temperature CO oxidation performance.
Project description:Metal sulfides are commonly used in energy storage and electrocatalysts due to their redox centers and active sites. Most literature reports show that their performance decreases significantly caused by oxidation in alkaline electrolyte during electrochemical testing. Herein, S and N co-doped graphene-based nickel cobalt sulfide aerogels are synthesized for use as rechargeable alkaline battery electrodes and oxygen reduction reaction (ORR) catalysts. Notably, this system shows improved cyclability due to the stabilization effect of the S and N co-doped graphene aerogel (SNGA). This reduces the rate of oxidation and the decay of electronic conductivity of the metal sulfides materials in alkaline electrolyte, i.e., the capacity decrease of CoNi<sub>2</sub>S<sub>4</sub>/SNGA is 4.2% for 10?000 cycles in a three-electrode test; the current retention of 88.6% for Co-S/SNGA after 12?000 s current-time chronoamperometric response in the ORR test is higher than corresponding Co-S nanoparticles and Co-S/non-doped graphene aerogels. Importantly, the results here confirm that the Ni-Co-S ternary materials behave as an electrode for rechargeable alkaline batteries rather than supercapacitors electrodes in three-electrode test as commonly described and accepted in the literature. Furthermore, formulas to evaluate the performance of hybrid battery devices are specified.
Project description:CO oxidation is a widely used model system for understanding fundamental aspects of heterogeneous catalysis. While platinum (Pt) continues to be a reference material for CO oxidation catalysis, poisoning of Pt catalysts presents a critical issue that blocks reaction sites and impedes subsequent reaction steps. Fabrication of CO poison-free Pt catalysts remains a great challenge due to its CO-philic nature. Herein, we report a Pt based catalyst to effectively tackle CO poisoning by tightly anchoring Pt sub-nanoclusters onto Mn-mullite oxide (SmMn2O5) via atomic layer deposition. Superior CO oxidation activity has been observed with a significantly lowered light-off temperature and apparent activation energy. In situ diffuse reflectance infrared Fourier transform spectroscopy analysis, oxygen isotope experiments and density functional theory calculations confirm that the low-temperature activity originates from active oxygen atom sources at the bifunctional interface structure.
Project description:Since trace amounts of CO in H<sub>2</sub> gas produced by steam reforming of methane causes severe poisoning of Pt-based catalysts in polymer electrolyte membrane fuel cells (PEMFCs), research has been mainly devoted to exploring CO-tolerant catalysts. To test the electrochemical property of CO-tolerant catalysts, chronoamperometry is widely used under a CO/H<sub>2</sub> mixture gas atmosphere as an essential method. However, in most cases of catalysts with high CO tolerance, the conventional chronoamperometry has difficulty in showing the apparent performance difference. In this study, we propose a facile and precise test protocol to evaluate the CO tolerance via a combination of short-term chronoamperometry and a hydrogen oxidation reaction (HOR) test. The degree of CO poisoning is systematically controlled by changing the CO adsorption time. The HOR polarization curve is then measured and compared with that measured without CO adsorption. When the electrochemical properties of PtRu alloy catalysts with different atomic ratios of Pt to Ru are investigated, contrary to conventional chronoamperometry, these catalysts exhibit significant differences in their CO tolerance at certain CO adsorption times. The present work will facilitate the development of catalysts with extremely high CO tolerance and provide insights into the improvement of electrochemical methods.
Project description:A setup for fluorescence-detected X-ray absorption spectroscopy (XAS) with sub-second time resolution has been developed. This technique allows chemical speciation of low-concentrated materials embedded in highly absorbing matrices, which cannot be studied using transmission XAS. Using this setup, the reactivity of 1.5?wt% Pt/CeO<sub>2</sub> catalyst was studied with 100?ms resolution during periodic cycling in CO- and oxygen-containing atmospheres in a plug-flow reactor. Measurements were performed at the Pt L<sub>3</sub>- and Ce L<sub>3</sub>-edges. The reactivity of platinum and cerium demonstrated a strong correlation. The oxidation of the catalyst starts on the ceria support helping the oxidation of platinum nanoparticles. The new time-resolved XAS setup can be applied to various systems, capable of reproducible cycling between different states triggered by gas atmosphere, light, temperature, etc. It opens up new perspectives for mechanistic studies on automotive catalysts, selective oxidation catalysts and photocatalysts.
Project description:As the technology development, the future advanced combustion engines must be designed to perform at a low temperature. Thus, it is a great challenge to synthesize high active and stable catalysts to resolve exhaust below 100 °C. Here, we report that bismuth as a dopant is added to form platinum-bismuth cluster on silica for CO oxidation. The highly reducible oxygen species provided by surface metal-oxide (M-O) interface could be activated by CO at low temperature (~50 °C) with a high CO<sub>2</sub> production rate of 487 μmol<sub>CO2</sub>·g<sub>Pt</sub><sup>-1</sup>·s<sup>-1</sup> at 110 °C. Experiment data combined with density functional calculation (DFT) results demonstrate that Pt cluster with surface Pt-O-Bi structure is the active site for CO oxidation via providing moderate CO adsorption and activating CO molecules with electron transformation between platinum atom and carbon monoxide. These findings provide a unique and general approach towards design of potential excellent performance catalysts for redox reaction.
Project description:Despite its importance in oxidation catalysis, the active phase of Pt remains uncertain, even for the Pt(111) single-crystal surface. Here, using a ReactorSTM, the catalytically relevant structures are identified as two surface oxides, different from bulk ?-PtO<sub>2</sub>, previously observed. They are constructed from expanded oxide rows with a lattice constant close to that of ?-PtO<sub>2</sub>, either assembling into spoked wheels, 1-5?bar O<sub>2</sub>, or closely packed in parallel lines, above 2.2?bar. Both are only ordered at elevated temperatures (400-500?K). The triangular oxide can also form on the square lattice of Pt(100). Under NO and CO oxidation conditions, similar features are observed. Furthermore, both oxides are unstable outside the O<sub>2</sub> atmosphere, indicating the presence of active O atoms, crucial for oxidation catalysts.Improving platinum as an oxidation catalyst requires understanding its structure under catalytic conditions. Here, the authors discover that catalytically important surface oxides form only when Pt is exposed to high pressure and temperature, highlighting the need to study catalysts in realistic environments.
Project description:Active and stable catalysts are essential for effective hydrogenation of gaseous CO<sub>2</sub> into valuable chemicals. This work focuses on the structural and catalytic features of single metals, i.e., Co and Ni, as well as bimetallic CoNi alloy catalysts synthesized via combustion of reactive sol-gels. Different characterization methods were used for studying the relationships between the structure, composition, and catalytic activity of the fabricated materials. All catalysts exhibited highly porous sponge-like microstructure. The outermost surfaces of the CoNi alloys were more saturated with Co, while a stoichiometric Co/Ni ratio was observed for the particle's bulk. Catalytic properties of the as-synthesized powders were studied in the CO<sub>2</sub> hydrogenation reaction at 300 °C for over 80 h of time on stream. All the catalysts demonstrated exceptional selectivity with respect to CH<sub>4</sub> formation. However, the combination of elemental Co and Ni in a single phase resulted in a synergistic effect in bulk alloy catalysts, with activity twofold to threefold that of single-metal catalysts. The activity and stability of the CoNi<sub>3</sub> catalyst were higher than those previously reported for Ni-based catalysts. The reasons for this behavior are discussed.