Replacing scarce and expensive platinum (Pt) with metal–nitrogen–carbon (M–N–C) catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells has largely been impeded by the low ...oxygen reduction reaction activity of M–N–C due to low active site density and site utilization. Herein, we overcome these limits by implementing chemical vapour deposition to synthesize Fe–N–C by flowing iron chloride vapour over a Zn–N–C substrate at 750 °C, leading to high-temperature trans-metalation of Zn–N4 sites into Fe–N4 sites. Characterization by multiple techniques shows that all Fe–N4 sites formed via this approach are gas-phase and electrochemically accessible. As a result, the Fe–N–C catalyst has an active site density of 1.92 × 1020 sites per gram with 100% site utilization. This catalyst delivers an unprecedented oxygen reduction reaction activity of 33 mA cm−2 at 0.90 V (iR-corrected; i, current; R, resistance) in a H2–O2 proton exchange membrane fuel cell at 1.0 bar and 80 °C.Replacing platinum with metal–nitrogen–carbon catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells has been impeded by low activity. These limitations have now been overcome by the trans-metalation of Zn–N4 sites into Fe–N4 sites.
Through coupled experimental analysis and computational techniques, we uncover the origin of anodic stability for a range of nonaqueous zinc electrolytes. By examination of electrochemical, ...structural, and transport properties of nonaqueous zinc electrolytes with varying concentrations, it is demonstrated that the acetonitrile–Zn(TFSI)2, acetonitrile–Zn(CF3SO3)2, and propylene carbonate–Zn(TFSI)2 electrolytes can not only support highly reversible Zn deposition behavior on a Zn metal anode (≥99% of Coulombic efficiency) but also provide high anodic stability (up to ∼3.8 V vs Zn/Zn2+). The predicted anodic stability from DFT calculations is well in accordance with experimental results, and elucidates that the solvents play an important role in anodic stability of most electrolytes. Molecular dynamics (MD) simulations were used to understand the solvation structure (e.g., ion solvation and ionic association) and its effect on dynamics and transport properties (e.g., diffusion coefficient and ionic conductivity) of the electrolytes. The combination of these techniques provides unprecedented insight into the origin of the electrochemical, structural, and transport properties in nonaqueous zinc electrolytes.
A catalytic architecture, comprising a mesoporous silica shell surrounding platinum nanoparticles (NPs) supported on a solid silica sphere (mSiO2/Pt-X/SiO2; X is the mean NP diameter), catalyzes ...hydrogenolysis of melt-phase polyethylene (PE) into a narrow C23-centered distribution of hydrocarbons in high yield using very low Pt loadings (∼10–5 g Pt/g PE). During catalysis, a polymer chain enters a pore and contacts a Pt NP where the C–C bond cleavage occurs and then the smaller fragment exits the pore. mSiO2/Pt/SiO2 resists sintering or leaching of Pt and provides high yields of liquids; however, many structural and chemical effects on catalysis are not yet resolved. Here, we report the effects of Pt NP size on activity and selectivity in PE hydrogenolysis. Time-dependent conversion and yields and a lumped kinetics model based on the competitive adsorption of long vs short chains reveal that the activity of catalytic material is highest with the smallest NPs, consistent with a structure-sensitive reaction. Remarkably, the three mSiO2/Pt-X/SiO2 catalysts give equivalent selectivity. We propose that mesoscale pores in the catalytic architecture template the C23-centered distribution, whereas the active Pt sites influence the carbon–carbon bond cleavage rate. This conclusion provides a framework for catalyst design by separating the C–C bond cleavage activity at catalytic sites from selectivity for chain lengths of the products influenced by the structure of the catalytic architecture. The increased activity, selectivity, efficiency, and lifetime obtained using this architecture highlight the benefits of localized and confined environments for isolated catalytic particles under condensed-phase reaction conditions.
Promoters are ubiquitous in industrial heterogeneous catalysts. The wider roles of promoters in accelerating catalysis and/or controlling selectivity are, however, not well understood. A model system ...has been developed where a heterobimetallic active site comprising an active metal (Rh) and a promoter ion (Ga) is preassembled and delivered onto a metal–organic framework (MOF) support, NU-1000. The Rh–Ga sites in NU-1000 selectively catalyze the hydrogenation of acyclic alkynes to E-alkenes. The overall stereoselectivity is complementary to the well-known Lindlar’s catalyst, which generates Z-alkenes. The role of the Ga in promoting this unusual selectivity is evidenced by the lack of semihydrogenation selectivity when Ga is absent and only Rh is present in the active site.
This paper summarizes a XANES, XPS, XRD, and Mössbauer study of an oxygen reduction reaction (ORR) catalyst obtained via a heat treatment of polyaniline, iron, and carbon black. The catalyst was ...characterized at several critical synthesis stages and following heat treatment at various temperatures. The effect of sulfur during the synthesis was also investigated. XANES linear combination fitting (XANES-LCF) was used to determine the speciation of iron using 16 iron standards. The highest ORR activity was measured with a catalyst heat-treated at 900 °C, with the largest Fe–N x content, as determined by the XANES-LCF, also characterized by the highest microporosity. An absence or a reduction in the amount of a sulfur-based oxidant in the aniline polymerization was found to lead to an increase in the amount of iron carbide formed during the heat treatment and a decrease in the number of Fe–N4 centers, thus attesting to an indirect beneficial role of sulfur in the catalyst synthesis. Using principal component analysis (PCA), a good correlation was found between the ORR activity and the presence of Fe–N x structures.
High-throughput synthesis of a series of monometallic and bimetallic catalysts (45 bimetallic and 50 monometallic samples) consisting of nickel and one of nine different metal promoters (B, Co, Cu, ...Fe, Mg, Mn, Sn, V and Zn) supported on one of five different metal oxides (alumina, ceria, magnesia, silica and titania) is carried out via organometallic grafting using a robotic platform. The catalysts are evaluated for their activity and selectivity for the dry reforming of methane at a feed ratio of CH4:CO2 of 1 at 650–800 °C in a parallel flow reactor system. The type of oxide support prevails over the type of additive for both catalyst activity and stability. On Al2O3 and MgO, Fe was found to be the best promoter; on SiO2, Cu is the best promoter at 700 °C and higher, while on TiO2, Mn is found to enhance the conversion at 800 °C. On CeO2, all additives except Fe have beneficial effects. Twenty-five catalysts show > 90% methane conversion with ten catalysts showing > 95% conversion at 800 °C with the H2:CO ratios ranging from 0.8 to 1.2. Amongst the ten highest performers, NiFe/Al2O3 and NiFe/MgO are more active than Ni/Al2O3 and Ni/MgO, respectively and were stable over a period of 25 h at 800 °C. Characterization on the as-prepared samples reveals highly dispersed phase, while after reduction in H2, highly dispersed and reduced nickel particles up to 10 nm are formed. The particles do not increase in size under dry reforming reaction conditions at 800 °C. An increased hydrogen consumption observed during H2-TPR of the nickel particles is positively correlated with methane conversion for Al2O3-based catalysts. The resistance to deactivation by coking and variation in coke structure are investigated by spectroscopic and microscopic methods to identify the relationship between metal promoters, alloy formation, and type of surface carbon deposits. Carbon whiskers were observed on the ten selected spent samples and are preferentially deposited on Ni rather than on the promoters. Carbon nanotube formation and metal particle removal from support were not observed to cause deactivation while amorphous carbon formation was clearly linked to catalyst deactivation, as amorphous carbon could encapsulate Ni, either on the support or at the end of the carbon nanotube. The organometallic grafting technique is an efficient and suitable technique for synthesizing highly dispersed and homogeneous phases which lead to high conversion and high durability for dry reforming of methane.
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•Monometallic and bimetallic Ni DRM catalysts on Al2O3, CeO2, SiO2 and TiO2 were synthesized using organometallic grafting.•10 catalysts, especially NiFe/Al2O3 and NiFe/MgO, showed stable and high methane conversion > 95 % at 800 °C.•Highly dispersed phase led to highly dispersed nickel particles after in-situ reduction that retain size after DRM.•Amorphous carbon formation is linked to catalyst deactivation by encapsulation of the nickel nanoparticles.
Abstract
The electrochemical reduction of carbon dioxide to formic acid is a promising pathway to improve CO
2
utilization and has potential applications as a hydrogen storage medium. In this work, a ...zero-gap membrane electrode assembly architecture is developed for the direct electrochemical synthesis of formic acid from carbon dioxide. The key technological advancement is a perforated cation exchange membrane, which, when utilized in a forward bias bipolar membrane configuration, allows formic acid generated at the membrane interface to exit through the anode flow field at concentrations up to 0.25 M. Having no additional interlayer components between the anode and cathode this concept is positioned to leverage currently available materials and stack designs ubiquitous in fuel cell and H
2
electrolysis, enabling a more rapid transition to scale and commercialization. The perforated cation exchange membrane configuration can achieve >75% Faradaic efficiency to formic acid at <2 V and 300 mA/cm
2
in a 25 cm
2
cell. More critically, a 55-hour stability test at 200 mA/cm
2
shows stable Faradaic efficiency and cell voltage. Technoeconomic analysis is utilized to illustrate a path towards achieving cost parity with current formic acid production methods.
Our civilization relies on synthetic polymers for all aspects of modern life; yet, inefficient recycling and extremely slow environmental degradation of plastics are causing increasing concern about ...their widespread use. After a single use, many of these materials are currently treated as waste, underutilizing their inherent chemical and energy value. In this study, energy-rich polyethylene (PE) macromolecules are catalytically transformed into value-added products by hydrogenolysis using well-dispersed Pt nanoparticles (NPs) supported on SrTiO3 perovskite nanocuboids by atomic layer deposition. Pt/SrTiO3 completely converts PE (M n = 8000–158,000 Da) or a single-use plastic bag (M n = 31,000 Da) into high-quality liquid products, such as lubricants and waxes, characterized by a narrow distribution of oligomeric chains, at 170 psi H2 and 300 °C under solvent-free conditions for reaction durations up to 96 h. The binding of PE onto the catalyst surface contributes to the number averaged molecular weight (M n) and the narrow polydispersity (Đ) of the final liquid product. Solid-state nuclear magnetic resonance of 13C-enriched PE adsorption studies and density functional theory computations suggest that PE adsorption is more favorable on Pt sites than that on the SrTiO3 support. Smaller Pt NPs with higher concentrations of undercoordinated Pt sites over-hydrogenolyzed PE to undesired light hydrocarbons.
A study was conducted on the steam reforming of biogas mixtures over a 4 wt.% Rh/La–Al2O3 catalyst, where the effects of temperature (590–685 °C), steam (S/C molar ratio = 1.28–3.86), CO2/CH4 molar ...ratio (0.55–1.51), and the gas hourly space velocity (9810–27,000 hr−1) on the conversions and product yields were evaluated. Within these ranges, temperature and steam had the most pronounced effect on methane and carbon dioxide conversions. The highest methane conversion observed was 99%. Low temperatures and high S/C resulted in a net CO2 production. The water gas shift reaction appeared to have a stronger effect on the CO2 conversion than the CO2 reforming reaction. Experimental methane conversions were lower than the equilibrium predicted values. Lower temperature operations yielded a lower carbon balance suggesting the tendency to form carbonaceous species other than CO, CO2, and CH4. The presence of CO2 in the biogas contributed to the CO yield (beyond that from CH4 steam reforming) only above certain CO2/CH4 ratios.
•A study of the steam reforming of biogas was conducted with a rhodium catalyst.•The effects of temperature, feed composition (S/C, CO2/CH4), and GHSV were studied.•99% of the methane was converted at 650 °C, 19,600 hr−1, and H2O/(CO2+CH4) = 3.87.•Net conversion of CO2 is favored at high CO2/CH4 ratios, high temperatures, and low H2O/C ratios.
This paper describes the stability of Fe species in a heat-treated polyaniline–iron–carbon (PANI–Fe–C) oxygen reduction reaction (ORR) catalyst in an aqueous acidic electrolyte and in a ...membrane–electrode assembly (MEA) at various potentials. Linear combination fitting of ex situ and in situ X-ray absorption near-edge structure (XANES) spectra to the spectra for a suite of Fe standards was used to determine the catalyst iron speciation at various potentials, after potential cycling in an aqueous electrolyte, and after 200h potentiostatic holds in MEAs. XANES edge-step analysis and inductively-coupled mass spectrometry were used to determine the amount of Fe lost from the catalyst into the aqueous electrolyte and from the MEA cathodes. Results showed that the Fe was lost from the catalyst in the electrochemical environment and the rate and extent of this loss were dependent on potential and on the type of electrolyte. The Fe specie primarily responsible for this loss was iron sulfide. Despite the large overall loss of Fe species from the catalyst that had been subjected to potentiostatic holds in an MEA at either 0.4V or 0.6V for 200h, H2–air polarization curves showed only moderate loss of cathode kinetic performance while the performance in the mass transport region improved. Correlating the performance loss to the XANES speciation, the kinetic losses may be attributed to the oxidation of active site(s) and/or loss of pyrrolic-like and pyridinic-like coordination, as well as the mass transport improvement due to removal of inactive Fe species, predominantly sulfides. Species with porphyrazin-like coordination were stable in both the aqueous and MEA environments. It is speculated that the stability of the porphyrazin is responsible for the durability of this catalyst.
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