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Enhanced activity and stability of MgO-promoted Ni/Al 2 O 3 catalyst for dry reforming of methane: Role of MgO
Highly dispersed nickel nanoparticles (NPs) were deposited on Al 2 O 3 NPs by atomic layer deposition (ALD). Various amounts of MgO were loaded on Ni/Al 2 O 3 catalysts by the incipient wetness (IW) method for dry reforming of methane. Fresh and used catalysts were systematically characterized to reveal the effects of MgO on the catalytic performance. MgO was found to increase the basic amount and basic strength of catalyst surfaces, which provided additional surface oxygen species and assisted the adsorption and activation of CO 2 . Although the formation of NiO-MgO solid solution, during the calcination of incipient wetness, did not improve the overall reducibility, the Ni NPs from NiO-MgO solid solution after reduction formed an intimate interaction with MgO, which could inhibit Ni NPs from sintering and provide sufficient metal-support interface for CO 2 activation. The MgO-promoted Ni/Al 2 O 3 reached a methane reforming rate of 1780 L CH4 g Ni -1 h -1 at 850 °C, which is 26% more than that of the pristine Ni/Al 2 O 3 . Finally, the higher CO 2 activity enhanced the oxidation rate of the surface carbon generated from side-reactions, thereby resulting in a higher reforming rate and inhibiting coke formation, especially the detrimental graphitic encapsulating carbon on the active nickel surface.
Activated carbon supported Ni, Fe, and bimetallic NiFe catalysts for CO x -free H 2 production by microwave methane pyrolysis
The goal of this study was to test the effect of metal-impregnated carbon-based catalysts on the conversion of methane to hydrogen gas and solid carbon using microwave reactor technology. Monometallic and bimetallic catalysts on activated carbon supports (Ni/AC, Fe/AC, Ni-Fe/AC) are compared during methane pyrolysis testing. Catalytic methane pyrolysis was carried out in a microwave reactor at reaction temperatures of 600 °C and 800 °C. For comparison, one of the catalysts (Ni-Fe/AC) was tested in a conventionally heated reactor at 800 °C. The prepared catalysts were characterized by X-ray diffraction (XRD), while post-reaction catalysts were characterized by XRD and SEM. During reaction testing, the monometallic Ni/AC catalyst exhibited the best catalytic activity (CH 4 conversion: 46.0 and H 2 yield: 46.9%) when reacted in the microwave reactor, however, it suffered from rapid deactivation from carbon deposition (carbon yield: 0.39 gC/g catalyst). The bimetallic Ni-Fe/AC catalyst was slightly less active (CH 4 conversion: 36.9 and H 2 yield: 40.5%) but it was more resistant to carbon formation (carbon yield: 0.27 gC/g catalyst) suggesting it may have greater long-term stability. The Ni-Fe/AC catalyst was also the most energy efficient as it required the least microwave power to maintain the 800 °C reaction temperature compared to the other catalysts tested. Methane conversion of the bimetallic Ni-Fe/AC at 800 °C under microwave irradiation was three times the conversion under conventional heating at the same reaction temperature. Finally, this work demonstrates the use of microwave-specific catalysts for catalytic methane pyrolysis in a microwave reactor, and can be used as a foundation for further methane pyrolysis process and catalyst optimization for CO x -free H 2 production.
La-Based Perovskite Oxide Catalysts for Alkaline Oxygen Reduction: The Importance of Electrochemical Stability
Alkaline fuel cells represent a promising energy conversion technology since they enable the use of precious metal-free electrocatalysts for the oxygen reduction reaction. Among these, La-based perovskite oxides, with great compositional and structural tunability, are especially attractive. However, we believe that the current literature lacks a thorough understanding of their inherent (in)stability issues. Here, in this study, we report on nine La-based perovskite oxide electrocatalysts for the oxygen reduction reaction, including LaMO 3 (M = Mn, Co, Ni) and LaCo x M 1–x O 3 (M = Mn, Ni; x = 0.9, 0.5, 0.1). While some exhibited initial promising activity, they all degraded significantly, even after brief electrochemical testing. Through comprehensive structural characterization of LaCo 0.9 Mn 0.1 O 3 , particularly scanning transmission electron microscopy, we found evidence to support a degradation mechanism in which the B-site cation species irreversibly leach from the perovskite under testing conditions. A sample cycled 10 000 times from +0.4 to +1.2 V vs reversible hydrogen electrode (RHE) showed an amorphous La oxide shell 1–2 nm thick. Bulk analyses showed that B-site leaching also occurs for samples soaked in 1 M KOH or pure EtOH. The mechanistic insights provided here should help inform the future design of La-based perovskite oxide electrocatalysts for alkaline fuel cells and stress the importance of stability metrics validating claims of promising electrocatalytic activity.
Comment on “Non-PGM electrocatalysts for PEM fuel cells: effect of fluorination on the activity and stability of a highly active NC_Ar + NH 3 catalyst” by Gaixia Zhang, Xiaohua Yang, Marc Dubois, Michael Herraiz, Régis Chenitz, Michel Lefèvre, Mohamed Cherif, François Vidal, Vassili P. Glibin, Shuhui Sun and Jean-Pol Dodelet, Energy Environ. Sci. , 2019, 12 , 3015–3037, 10.1039/C9EE00867E
Proper kinetic and thermodynamic models are important in addressing durability as a major issue facing PGM-free ORR electrocatalysts.
Alternative Fuel Research in Fischer-Tropsch Synthesis
NASA Glenn Research Center has recently constructed an Alternative Fuels Laboratory which is solely being used to perform Fischer-Tropsch (F-T) reactor studies, novel catalyst development and thermal stability experiments. Facility systems have demonstrated reliability and consistency for continuous and safe operations in Fischer-Tropsch synthesis. The purpose of this test facility is to conduct bench scale Fischer-Tropsch (F-T) catalyst screening experiments while focusing on reducing energy inputs, reducing CO2 emissions and increasing product yields within the F-T process. Fischer-Tropsch synthesis is considered a gas to liquid process which reacts syn-gas (a gaseous mixture of hydrogen and carbon monoxide), over the surface of a catalyst material which is then converted into liquids of various hydrocarbon chain length and product distributions1. These hydrocarbons can then be further processed into higher quality liquid fuels such as gasoline and diesel. The experiments performed in this laboratory will enable the investigation of F-T reaction kinetics to focus on newly formulated catalysts, improved process conditions and enhanced catalyst activation methods. Currently the facility has the capability of performing three simultaneous reactor screening tests, along with a fourth fixed-bed reactor used solely for cobalt catalyst activation.
Thermo-Catalytic Decomposition Comparisons: Carbon Catalyst Structure, Hydrocarbon Feed and Regeneration
Thermo-catalytic decomposition (TCD) activity and stability depend upon the initial carbon catalyst structure. However, further transitions in the carbon structure depend on the carbon material (structure and composition) originating from the TCD process. In this article, reaction data are presented that illustrates the time-dependent TCD activity as TCD-formed carbon contributes and then dominates conversion. A variety of initial carbon catalysts are compared, including sugar char, a conductive carbon black (AkzoNobel Ketjenblack), a rubber-grade carbon black (Cabot R250), and its graphitized analogue as formed and partially oxidized. Regeneration of carbon catalysts by partial oxidation is evaluated using nascent carbon black as a model, coupled with subsequent comparative TCD performance relative to the nascent, non-oxidized carbon black. Activation energies for TCD with nascent and oxidized carbons are evaluated by a leading-edge analysis method applied to TCD. Given the correlation between nanostructure and active sites, two additional carbons, engine soots, are evaluated for regeneration and dependence upon nanostructure. Active sites are quantified by oxygen chemisorption, followed by X-ray photoelectron spectroscopy (XPS). The structure of carbon catalysts is assessed pre- and post-TCD by high-resolution transmission electron microscopy (HRTEM). Last, energy dispersive X-ray analysis mapping (EDS) is carried out for its potential to visualize oxygen chemisorption.
Stabilizing *CO 2 Intermediates at the Acidic Interface using Molecularly Dispersed Cobalt Phthalocyanine as Catalysts for CO 2 Reduction
Abstract CO 2 electroreduction (CO 2 R) operating in acidic media circumvents the problems of carbonate formation and CO 2 crossover in neutral/alkaline electrolyzers. Alkali cations have been universally recognized as indispensable components for acidic CO 2 R, while they cause the inevitable issue of salt precipitation. It is therefore desirable to realize alkali‐cation‐free CO 2 R in pure acid. However, without alkali cations, stabilizing *CO 2 intermediates by catalyst itself at the acidic interface poses as a challenge. Herein, we first demonstrate that a carbon nanotube‐supported molecularly dispersed cobalt phthalocyanine (CoPc@CNT) catalyst provides the Co single‐atom active site with energetically localizeddstates to strengthen the adsorbate‐surface interactions, which stabilizes *CO 2 intermediates at the acidic interface (pH=1). As a result, we realize CO 2 conversion to CO in pure acid with a faradaic efficiency of 60 % at pH=2 in flow cell. Furthermore, CO 2 is successfully converted in cation exchanged membrane‐based electrode assembly with a faradaic efficiency of 73 %. For CoPc@CNT, acidic conditions also promote the intrinsic activity of CO 2 R compared to alkaline conditions, since the potential‐limiting step, *CO 2 to *COOH, is pH‐dependent. This work provides a new understanding for the stabilization of reaction intermediates and facilitates the designs of catalysts and devices for acidic CO 2 R.
Catalytic reforming of methane with H 2 S via dynamically stabilized sulfur on transition metal oxides and sulfides
Reforming of methane with H 2 S is a promising path to directly utilize sour natural gas reserves, although some aspects of the mechanism and structure–function relations remain elusive. In this report we show that metal oxides of group 4–6 elements, which are inert for steam and dry methane reforming reactions, are active and stable (pre)catalysts for the H 2 S reforming of methane. The key active sites are sulfur species (S*) that are dynamically bound to metal cations during catalysis. Similar H/D isotope exchange patterns and universal rate inhibition by H 2 on representative catalysts indicate that H 2 S decomposition and recombination of surface hydrogen atoms are quasi-equilibrated, whereas CH 4 dissociation steps are reversible, yet not quasi-equilibrated. An in-depth analysis of the kinetic data and isotopic substitution effects identifies S*-mediated C–H bond cleavage as the most plausible rate-limiting step common for all catalysts, with subtle yet essential differences between 3d and 4d/5d catalysts in the thermodynamic stability of S*.
Role of Mg:Al ratio and Pt promotion on mitigating the deactivation of Ni-based methane steam reforming catalysts
Ni-based catalysts for CH 4 steam reforming require fine-tuning to withstand deactivation under dynamic operation conditions relevant to emerging H 2 -driven technologies. This study investigates the impact of the Mg:Al ratio and Pt presence in catalyst composition on the activity and stability of industrially relevant Ni-based mono- and bimetallic catalysts during simulated daily start-up and shut-down cycles in various gas atmospheres. The evolution of the catalyst structure during extensive testing procedures was investigated in detail by complementary electron microscopy, in situ/operando XAS and XRD. The results obtained revealed that catalyst deactivation is promoted at high Mg:Al ratios and especially affects the monometallic catalysts. By converting CH 4 at lower temperatures, Pt regulates the extent of Ni oxidation and its incorporation into MgO lattice. Further prevention of catalyst deactivation was achieved by optimizing the reactor shut-down procedure to minimize the simultaneous exposure to high temperatures and H 2 O vapors. By flushing the reactor with N 2 only around the reaction extinction temperature, a fraction of Ni species is maintained in metallic state, which is beneficial for the long-term activity and reaction operation economy.
HydroGEN: Low Temperature Electrolysis
In low temperature electrolysis (LTE), it is imperative to both enhance and explore durability and demonstrate the opportunities for anion exchange membrane-based water electrolysis (AEMWE). The advantage of alkaline-based systems is primarily reduced capital cost: high pH enabling platinum group metal (PGM)-free catalysts and coatings, and the enhanced stability of those components compared to proton exchange membrane (PEM) -based systems. Compared to the water-only fed AEMWE in previous LTE 2.0 work, supporting electrolytes can allow for a significant improvement in performance through higher site-access and stability by reducing utilization and overpotential stresses that lead to catalyst layer delamination.
HydroGEN: Low Temperature Electrolysis
In low temperature electrolysis (LTE), it is imperative to both enhance and explore durability and demonstrate the opportunities for anion exchange membrane-based water electrolysis (AEMWE). The advantage of alkaline-based systems is primarily reduced capital cost: high pH enabling platinum group metal (PGM)-free catalysts and coatings, and the enhanced stability of those components compared to proton exchange membrane (PEM) -based systems. Compared to the water-only fed AEMWE in previous LTE 2.0 work, supporting electrolytes can allow for a significant improvement in performance through higher site-access and stability by reducing utilization and overpotential stresses that lead to catalyst layer delamination.
Highly efficient Pt catalyst on newly designed CeO 2 -ZrO 2 -Al 2 O 3 support for catalytic removal of pollutants from vehicle exhaust
Pt-CeO 2 catalysts have been widely studied for the vehicle emission control. Designing novel CeO 2 based supports with improved physical-chemical properties has become a research hotspot to further promote the catalytic performance and stability of Pt-CeO 2 catalysts. In this work, through utilizing a unique, two-step incipient wetness impregnation (T-IWI) method for ceria-zirconia-alumina (CZA-T) support preparation, a Pt single site catalyst (Pt/CZA-T) with excellent thermal stability was synthesized. Higher oxidation activity and Oxygen storage capacity (OSC) were achieved on activated Pt/CZA-T, comparing to Pt catalysts on regular CeO 2 /Al 2 O 3 (Pt/CA) and one-step prepared CeZrO x /Al 2 O 3 (Pt/CZA). Via the modification of hydrophilic/hydrophobic properties of γ-Al 2 O 3 by this unique T-IWI method, finer Ce 0.9 Zr 0.1 O 2 particles with higher density of surface defects were formed on CZA-T, on which a higher Pt dispersion and stronger Pt-O-Ce interaction were achieved. Upon activation, smaller, well-dispersed Pt clusters on CZA-T were generated. It was concluded that the CO oxidation performance and OSC were highly related to the size of Pt clusters on different supports that we have developed. Finally, more Pt sites located at Pt cluster-CeZrO x interfaces, which were the real active sites, were responsible for the highest OSC function and CO oxidation activity of activated Pt/CZA-T catalyst.
Non-iridium-based electrocatalyst for durable acidic oxygen evolution reaction in proton exchange membrane water electrolysis
Iridium-based electrocatalysts remain the only practical anode catalysts for proton exchange membrane (PEM) water electrolysis, due to their excellent stability under acidic oxygen evolution reaction (OER), but are greatly limited by their high cost and low reserves. Here, in this study, we report a nickel-stabilized, ruthenium dioxide (Ni-RuO 2 ) catalyst, a promising alternative to iridium, with high activity and durability in acidic OER for PEM water electrolysis. While pristine RuO 2 showed poor acidic OER stability and degraded within a short period of continuous operation, the incorporation of Ni greatly stabilized the RuO 2 lattice and extended its durability by more than one order of magnitude. When applied to the anode of a PEM water electrolyser, our Ni-RuO 2 catalyst demonstrated >1,000 h stability under a water-splitting current of 200 mA cm -2 , suggesting potential for practical applications. Density functional theory studies, coupled with operando differential electrochemical mass spectroscopy analysis, confirmed the adsorbate-evolving mechanism on Ni-RuO 2 , as well as the critical role of Ni dopants in stabilization of surface Ru and subsurface oxygen for improved OER durability.
Epitaxial heterointerfacial electron bridge synchronizes oxygen evolution activity and stability on a layered double hydroxide surface
Scalable green hydrogen production via electrocatalytic water splitting is largely restricted by the insufficient activity and stability of oxygen evolution reaction (OER) catalysts at the anode. As a class of the most active OER catalysts in alkaline electrolyzers, the application of layered double hydroxides (LDHs) remains a main challenge owing to the unstable lattice oxygen dissolution due to the dominant lattice oxygen-involving OER mechanism during long-term operation. Herein, we found that using an epitaxial hetero-interfacing nickel hydroxide (namely Ni(OH) 2 ) as an electron bridge between an active FeCo LDH and Ni foam support to form an LDH*/NFO catalyst, the electronic storage capacity around the Fermi level (-0.5 to +0.5 eV, e-D FE ) sharply increases from 0.93 per cell to 1.51 per cell. Subsequently, we demonstrate that this high e-D FE enables ceaseless and fast power injection into the kinetic process of intermediate species conversion and inhibits lattice oxygen dissolution in the active FeCo LDH. Consequently, it demonstrated a low OER overpotential of 246 mV at a current density of 100 mA cm -2 and ultrahigh stability for up to 3500 hours with an ultraslow overpotential increase rate of 9.4 × 10 -3 mV h -1 . Therefore, we developed an epitaxial hetero-interfacial electron bridging strategy to synchronize the activity and stability of available catalysts for scalable green hydrogen production via electrocatalytic water splitting.
Improving productivity and stability for CO 2 hydrogenation by using pincer-ligated Mn complexes with hemilabile ligands
Pincer ligands are widely used to support transition metal catalysts, but first-row systems often deactivate too quickly for widespread use. Here, pincer ligands bearing an additional hemilabile donor were designed to stabilize Mn catalysts for CO 2 hydrogenation to formate. Ligands of the type (RCH 2 CH 2 )N(CH 2 CH 2 P i Pr 2 ) 2 ( iPr PN R P; R = OMe, CH 2 OMe, NMe 2 , P i Pr 2 ) were synthesized and coordinated to Mn to form complexes of the type ( iPr PN R P)Mn(CO) 2 H. Their performance was compared to {MeN(CH 2 CH 2 P i Pr 2 ) 2 }Mn(CO) 2 H, which lacks a pendant donor. The pendant donor identity and arm length strongly influenced catalytic activity, but all pendant arm containing systems extended catalyst lifetime. Notably, ( iPr PN CH2OMe P)Mn(CO) 2 H achieved turnover frequencies of 158,000 h -1 and turnover numbers of 838,000, exceeding most state-of-the-art catalysts. Mechanistic studies revealed that hemilabile coordination enhances stability by avoiding deactivation, although excessive binding can lead to inactive states. Here, this work demonstrates hemilabile donors dramatically improve pincer-based Mn catalysts and provides design principles for broader applications.
Surface Lattice-Embedded Pt Single-Atom Catalyst on Ceria-Zirconia with Superior Catalytic Performance for Propane Oxidation
Tuning the metal–support interaction and coordination environment of single-atom catalysts can help achieve satisfactory catalytic performance for targeted reactions. Herein, via the facile control of calcination temperatures for Pt catalysts on pre-stabilized Ce 0.9 Zr 0.1 O 2 (CZO) support, Pt single atoms (Pt 1 ) with different strengths of Pt–CeO 2 interaction and coordination environment were successfully constructed. With the increase in calcination temperature from 350 to 750 °C, a stronger Pt–CeO 2 interaction and higher Pt-O-Ce coordination number were achieved due to the reaction between PtO x and surface Ce 3+ species as well as the migration of Pt 1 into the surface lattice of CZO. The Pt/CZO catalyst calcined at 750 °C (Pt/CZO-750) exhibited a surprisingly higher C 3 H 8 oxidation activity than that calcined at 550 °C (Pt/CZO-550). Here, through systematic characterizations and reaction mechanism study, it was revealed that the higher concentration of surface Ce 3+ species/oxygen vacancies and the stronger Pt–CeO 2 interaction on Pt/CZO-750 could better facilitate the activation of oxygen to oxidize C 3 H 8 into reactive carbonate/carboxyl species and further promote the transformation of these intermediates into gaseous CO 2 . The Pt/CZO-750 catalyst can be a potential candidate for the catalytic removal of hydrocarbons from vehicle exhaust.