Catalyst Species Research Articles

Strategies for Improving Anion Exchange Membrane Fuel Cell Performance by Optimizing Electrode Conditions

We systematically study anion exchange membrane fuel cells (AEMFCs) based on poly(aryl-co-aryl piperidinium) (c-PAP) copolymers and provide a scalable scenario for high-performance AEMFCs, covering the optimization of the relative humidity (RH), catalyst species, catalyst interfaces, and hydrophobic treatment. Specifically, high-water-permeable c-PAP ionomers in the presence of moderate relative humidity (RH) (75%/100%) can be used to address anode flooding and cathode dry-out issues. The composition of the catalyst layer and the anode hydrophobic treatment significantly impact the power density of AEMFCs. c-PAP-based AEMFCs with optimum catalyst composition achieve a peak power density (PPD) of 2.70 W cm−2 at 80 °C in H2-O2 after hydrophobic treatment. Pt1Co1/C cathode-based AEMFCs reach a PPD of 1.80 W cm−2 along with an outstanding specific power of 13.87 W mg−1. Moreover, these AEMFCs can be operated under a 0.2 A cm−2 current density at 60 °C for over 300 h with a voltage decay rate of ∼300 μv h−1.

Evolution of the active species of homogeneous Ru hydrodeoxygenation catalysts in ionic liquids.

This work establishes structure–property relationships in Ru-based catalytic systems for selective hydrodeoxygenation of ketones to alkenes by combining extensive catalytic testing, in situ X-ray absorption spectroscopy (XAS) under high pressures and temperatures and ex situ XAS structural characterization supported by density functional theory (DFT) calculations. Catalytic tests revealed the difference in hydrogenation selectivity for ketones (exemplified by acetone) or alkenes (exemplified by propene) upon changing the reaction conditions, more specifically in the presence of CO during a pretreatment step. XAS data demonstrated the evolution of the local ruthenium structure with different amounts of Cl/Br and CO ligands. In addition, in the absence of CO, the catalyst was reduced to Ru0, and this was associated with a significant decrease of the selectivity for ketone hydrogenation. For the Ru–bromide carbonyl complex, selectivity towards acetone hydrogenation over propene hydrogenation was explained on the basis of different relative energies of the first intermediate states of each reaction. These results give a complete understanding of the evolution of the Ru species, used for the catalytic valorization of biobased polyols to olefins in ionic liquids, identifying the undesired deactivation routes as well as possibilities for reactivation.

A nanoclay-confined single atom catalyst: tuning uncoordinated N species for efficient water treatment

Uncoordinated nitrogen species in a single atom catalyst accelerate the release of intermediates and the regeneration of active sites via downshifting its d-band center to a lower energy level.

Mechanistic Insight into Surface Oxygen Species of the Polyoxometalate-Supported Pd Single-Atom Catalysts for Highly Efficient Co Oxidation

Mechanistic Insight into Surface Oxygen Species of the Polyoxometalate-Supported Pd Single-Atom Catalysts for Highly Efficient Co Oxidation

The role of Mo species in Ni-Mo catalysts for dry reforming of methane.

The Ni-Mo catalyst has attracted significant attention due to its excellent coke-resistance in dry reforming of methane (DRM) reaction, but its detailed mechanism is still vague. Herein, Mo-doped Ni (Ni-Mox) and MoOx adsorbed Ni surfaces (MoOx@Ni) are employed to explore the DRM reaction mechanism and the effect of coke-resistance. Due to the electron donor effect of Mo, the antibonding states below the Fermi level between Ni and C increase and the adsorption of C decrease, thereby inhibiting the carbonization of Ni. On account of the strong Mo and O interaction, more O atoms gather around Mo, which inhibits the oxidation of Ni and may promote the formation of MoOx species on the Ni-Mo catalyst. The presence of Mo-O species promotes the carbon oxidation, forming a unique redox cycle (MoOx ↔ MoOx-1) similar to the Mars-van Krevelen (MvK) mechanism, explaining the excellent anti-carbon deposition effect on the Ni-Mo catalyst.

New Insights into the Relationship between Cu-Ce Interaction and the Properties of Reactive Cu Species in Cuox-Ceo2 Catalysts for Co-Prox Derived from Tpd and in Situ Drifts Techniques

New Insights into the Relationship between Cu-Ce Interaction and the Properties of Reactive Cu Species in Cuox-Ceo2 Catalysts for Co-Prox Derived from Tpd and in Situ Drifts Techniques

Remarkable COx tolerance of Ni3+ active species in a Ni2O3 catalyst for sustained electrochemical urea oxidation

Ni2O3 exhibits a high current density for urea electro-oxidation due to efficient interaction of NiO(OH) active catalyst species with reactants compared to the as-synthesized NiO and also shows sustained UOR facilitated by better COx tolerance.

DNP NMR spectroscopy enabled direct characterization of polystyrene-supported catalyst species for synthesis of glycidyl esters by transesterification.

Polymer-supported catalysts have been of great interest in organic syntheses, but have suffered from the difficulty in obtaining direct structural information regarding the catalyst species embedded in the polymer due to the limitations of most analytical methods. Here, we show that dynamic nuclear polarization (DNP)-enhanced solid-state NMR is ideally positioned to characterize the ubiquitous cross-linked polystyrene (PS)-supported catalysts, thus enabling molecular-level understanding and rational development. Ammonium-based catalysts, which show excellent catalytic activity and reusability for the transesterification of methyl esters with glycidol, giving glycidyl esters in high yields, were successfully characterized by DNP 15N NMR spectroscopy at 15N natural abundance. DNP 15N NMR shows in particular that the decomposition of quaternary alkylammonium moieties to tertiary amines was completely suppressed during the catalytic reaction. Furthermore, the dilute ring-opened product derived from glycidol and NO3− was directly characterized by DNP 15N CPMAS and 1H–15N and 1H–13C HETCOR NMR using a 15N enriched (NO3) sample, supporting the view that the transesterification mechanism involves an alkoxide anion derived from an epoxide and NO3−. In addition, the detailed analysis of a used catalyst indicated that the adsorption of products on the cationic center is the major deactivation step in this catalysis.

High-performance nickel/iron catalysts for oxygen evolution in pH-near-neutral borate electrolyte synthesized by mechanochemical approach

In this work, a novel method of mechanochemistry is used to directly prepare high-performance Ni-Fe based catalysts for oxygen evolution reaction (OER) in an environmental-friendly borate electrolyte with near-neutral pH. Such a procedure is simple, effective, and time-saving. The effect of iron-based raw materials and mechanochemical reaction on OER of catalysts are investigated. It shows that iron nitrate nonahydrate is the best iron resource compared to iron hydroxide and iron oxide due to amorphous iron species in catalysts for strengthening the “Fe inductive effect” and generating active Ni species with high valence. Besides, the grinding time can control the mechanochemical reaction from mixed reaction to solid reaction in the catalyst synthesis process. When the time is 30 min, the catalyst is composed with amorphous Ni(OH)2 and the precursors of NiFe-LDH. This catalyst only needs 413 mV overpotential to reach 1 mA cm−2 in 0.1 M K-Bi (pH=9.2), which is better than precious RuO2. It is proven that the mechanochemical approach is an alternative method for synthesizing superior OER catalysts.

In Situ Raman Spectroscopic Study of Species in Co-MnCeOx Catalysts under COPrOx Reaction Conditions

This work is focused on the study of Co-MnCeOx catalysts by in situ Raman spectroscopy, under COPrOx conditions, and aims to establish a correlation with the catalytic results of different catalytic formulations. Through XRD, the catalysts structures were explored, and the oxidation states and relative concentrations of surface species were analyzed by XPS. Catalysts were prepared with Co (10 wt %) and different Mn concentrations. The more active catalyst contains 2.5 wt % of Mn and Co/Ce ≈ 1 (CO conversion 99.5%, 150–180 °C). The Co-Mn2.5Ce catalyst is composed of Co3O4 spinel, Mn inserted in the CeO2 lattice, and probably MnOx dispersed, which remains stable up to 250 °C under reaction conditions. XRD results confirm the crystalline structure of Co3O4 spinel; however, there was no evidence of crystalline phases of MnOx oxides. A lower lattice parameter suggests the insertion of Mnn+ ions in the ceria framework. The catalytic surface is composed of Co3+-Co2+ belonging to Co3O4 and Ce4+ of ceria and Mn4+-Mn3+ species.

An investigation on the promoting effect of Pr modification on SO2 resistance over MnOx catalysts for selective reduction of NO with NH3.

Pr-modified MnOx catalyst was synthesized through a facile co-precipitation process, and the results showed that MnPrOx catalyst exhibited much better selective catalytic reduction (SCR) activity and SO2 resistance performance than pristine MnOx catalyst. The addition of Pr in MnOx catalyst led to a complete NO conversion efficiency in 120-220°C. Moreover, Pr-modified MnOx catalyst exhibited a superior resistance to H2O and SO2 compared with MnOx catalyst. After exposing to SO2 and H2O for 4h, the NO conversion efficiency of MnPrOx catalyst could remain to 87.6%. The characterization techniques of XRD, BET, hydrogen-temperature programmed reduction (H2-TPR), ammonia-temperatureprogrammed desorption (NH3-TPD), XPS, TG and in situ diffuse reflectance infrared spectroscopy (DRIFTS) were adopted to further explore the promoting effect of Pr doping in MnOx catalyst on SO2 resistance performance. The results showed that MnPrOx catalyst had larger specific surface area, stronger reducibility, and more L acid sites compared with MnOx catalyst. The relative percentage of Mn4+/Mnn+ on the MnPrOx-S catalyst surface was also much higher than those of MnOx catalyst. Importantly, when SO2 exists in feed gas, PrOx species in MnPrOx catalyst would preferentially react with SO2, thus protecting the Mn active sites. In addition, the introduction of Pr might promote the reaction between SO2 and NH3 rather than between SO2 and Mn active sites, which was also conductive to protect the Mn active sites to a great extent. Since the presence of SO2 in feed gas had little effect on NH3 adsorption on the MnPrOx catalyst surface, and the inhibiting effect of SO2 on NO adsorption was alleviated, SCR reactions could still proceed in a near-normal way through the Eley-Rideal (E-R) mechanism on Pr-modified MnOx catalyst, while SCR reactions through the Langmuir-Hinshelwood (L-H) mechanism were suppressed slightly.

Iron and copper nanoparticles inside and outside carbon nanotubes: Nanoconfinement, migration, interaction and catalytic performance in Fischer-Tropsch synthesis

Carbon materials have attracted increasing attention as supports for metal catalysts. Iron-containing carbon nanotubes often promoted with copper have found application in Fischer-Tropsch synthesis, which provides an alternative way for conversion of renewable feedstocks to chemicals and fuels. In carbon nanotubes, the active phase can be nanoconfined inside the channels or localized on the outer surface. In most of previous work, the distribution of metal nanoparticles inside or outside carbon nanotubes is considered to be immobile during the catalyst activation or catalytic reaction.In this paper, we uncovered remarkable mobility of both iron and copper species in the bimetallic catalysts between inner carbon nanotube channels and outer surface, which occurs in carbon monoxide and syngas, while almost no migration of iron species proceeds in the monometallic catalysts. This mobility is enhanced by noticeable fragility and defects in carbon nanotubes, which appear on their impregnation with the acid solutions of metal precursors and precursor decomposition. Remarkable mobility of iron and copper species in bimetallic catalysts affects the genesis of iron active sites, and enhances interaction of iron with the promoter. In the bimetallic iron-copper catalysts, the major increase in the activity was attributed to a higher reaction turnover frequency over iron surface sites located in a close proximity with copper.

Migration mechanism on Cu species in CHA-type catalyst for the selective catalytic reduction of NOx during hydrothermal treatment

CHA-type aluminosilicate zeolites, have aroused widespread concern recently due to their remarkable physicochemical properties. Meanwhile, Cu-based CHA-type catalysts have been commercialized for desirable low-temperature activity in selective catalytic reduction of NOx. In this work, thermal aging (TA) and hydrothermal aging (HTA) samples were prepared to investigate the respective effects of heat and water on Cu-based CHA-type catalysts during hydrothermal aging treatment. Impressively, TA sample preformed greater SCR activity in low and high temperature range, but HTA sample exhibited a slight decrease compared to the TA sample. The characterization data (obtained by XPS, H2-TPR and in situ DRIFTS techniques) revealed that high temperature promoted the transformation from ZCu(OH) to Z2Cu while high temperature and water facilitated the migration from CuO to Cu2+ during hydrothermal aging. The increase of isolated Cu2+could improve the adsorption capacity of Lewis acid sites at low temperature. However, the gradual drop for the adsorption capacity of Lewis acid sites with the rise of aging temperature indicated the reduction in acid strength after thermal and hydrothermal aging. Additionally, addition of water aggravated the dealumination and reduced the conversion of NOx in the temperature range from 300 °C to 450 °C. While the dealumination occurring at Lewis sites was greatly weakened due to the protection of interaction between Cu and support.

Relay catalysis of copper-magnesium catalyst on efficient valorization of glycerol to glycolic acid

The exploration of potential application of glycerol is of particular importance for solving the oversupply of glycerol as an unavoidable byproduct of biodiesel and affording profitable possibilities for sustainable biodiesel. Herein, we developed a simple, cheap but robust Cu1Mg4 catalyst, which exhibited outstanding catalytic activity for the selective conversion of glycerol towards valuable glycolic acid, eliminating the addition of extra inorganic base. As high as 71.8% yield of glycolic acid was achieved, the highest value to date as far as we know. In combination of experiment and DFT calculations, it was revealed that Cu, Mg species in catalyst enabled the relay catalysis for the cascade reactions in glycerol transformation. Cu species in catalyst dominated the first dehydrogenation reaction of glycerol to glyceraldehyde, which was promoted by Mg species. Mg species followed to catalyze the next cleavage of C2-C3 bond in glyceraldehyde to generate glycolaldehyde. The final oxidation of glycolaldehyde to glycolic acid was achieved by the catalysis of Cu species. This relay catalysis of Cu1Mg4 catalyst significantly inhibited the formation of glyceric acid and lactic acid by-products, thereby enabling the selective generation of glycolic acid.

Mobility and Reactivity of Cu+ Species in Cu-CHA Catalysts under NH3-SCR-NOx Reaction Conditions: Insights from AIMD Simulations.

The mobility of the copper cations acting as active sites for the selective catalytic reduction of nitrogen oxides with ammonia in Cu-CHA catalysts varies with temperature and feed composition. Herein, the migration of [Cu(NH3)2]+ complexes between two adjacent cavities of the chabazite structure, including other reactant molecules (NO, O2, H2O, and NH3), in the initial and final cavities is investigated using ab initio molecular dynamics (AIMD) simulations combined with enhanced sampling techniques to describe hopping events from one cage to the other. We find that such diffusion is only significantly hindered by the presence of excess NH3 or NO in the initial cavity, since both reactants form with [Cu(NH3)2]+ stable intermediates which are too bulky to cross the 8-ring windows connecting the cavities. The presence of O2 modifies strongly the interaction of NO with Cu+. At low temperatures, we observe NO detachment from Cu+ and increased mobility of the [Cu(NH3)2]+ complex, while at high temperatures, NO reacts spontaneously with O2 to form NO2. The present simulations give evidence for recent experimental observations, namely, an NH3 inhibition effect on the SCR reaction at low temperatures, and transport limitations of NO and NH3 at high temperatures. Our first principle simulations mimicking operating conditions support the existence of two different reaction mechanisms operating at low and high temperatures, the former involving dimeric Cu(NH3)2-O2-Cu(NH3)2 species and the latter occurring by direct NO oxidation to NO2 in one single cavity.

DFT and kinetic evidences of the preferential CO oxidation pattern of manganese dioxide catalysts in hydrogen stream (PROX)

The oxidation functionality of Mn(IV) sites has been assessed by density functional theory (DFT) analysis of adsorption and activation energies of CO, H2 and O2 on a model Mn4O8 cluster. DFT calculations indicate that Mn(IV) atoms prompt an easy CO conversion to CO2 via a reaction path involving both catalyst and gas-phase oxygen species, while much greater energy barriers hinder H2 oxidation. Accordingly, a MnCeOx catalyst (Mnat/Ceat, 5) with large exposure of Mn(IV) sites shows a remarkable CO oxidation performance at T ≥ 293 K and no H2 oxidation activity below 393 K. Empiric kinetics disclose that the catalyst-oxygen abstraction step determines both CO and H2 oxidation rate, although different activation energies favor the preferential oxidation (PROX) pattern of the studied catalyst (353–423 K). Conversion-selectivity of 100%, high stability during 72 h reaction time and moderate inhibiting effects of water and CO2 feeding reveal the potential of MnO2 materials as efficient, low-cost and robust PROX catalysts.

Identify the Activity Origin of Pt Single-Atom Catalyst via Atom-by-Atom Counting.

Atom dispersion in metal supported catalysts is vital as it structurally accounts for their catalytic performances. Since practical catalysts normally present structural diversity, such as the coexistence of single atoms, clusters, and particles, traditional spectroscopy methods including chemisorption, titration, and X-ray absorption, however, provide only an averaged description about the atom dispersion but are not able to distinguish localized structural divergence. In this work, through developing a methodology of electron-microscopy-based atom recognition statistics (EMARS), catalyst dispersion has been redefined at atomic precision in real space via the statistically counting 18 000+ Pt atoms for a Pt/Al2O3 industrial reforming catalyst. The EMARS results combined with in situ microscopy evidence disclose that the activity for aromatics production quantitatively correlates with the density of Pt single-atoms, while Pt clusters contribute no direct activity but could kinetically transform into single-atoms when being heated under an oxidative atmosphere. Compared to EMARS, the traditional hydrogen-oxygen titration method is found to induce serious bias in the Pt dispersion in reference to actual activity. This distinctive capability of EMARS for metal dispersion quantification offers a possibility of directly identifying the catalysis roles of different metal species in a practical catalyst via atom-resolved statistics.

Reaction Behaviors and Crystal Transformation of Industrial Vanadium–Phosphorus–Oxygen (VPO) Catalysts for n-Butane Oxidation

A long-time evaluation of n-butane oxidation over an industrial vanadium–phosphorus–oxygen (VPO) catalyst was implemented. The catalytic performances for n-butane oxidation during this period were obtained. It was shown that the conversion of n-butane increased with the evaluation time, but the selectivity of the maleic anhydride (MA) product decreased gradually. To investigate the crystal transformation of the VPO catalyst, the properties of fresh and evaluated VPO catalysts were measured by a series of characterization methods, including X-ray diffraction (XRD), N2 adsorption and desorption, NH3-temperature programmed desorption (TPD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The results showed that the acidities and valence of vanadium increased after evaluation due to the appearance of a β-VOPO4 phase. The crystal transformation would increase the activity for n-butane oxidation. Meanwhile, during the evaluation period, the decrease in selectivity of the MA product should be related to the decreasing percentage of Lat-O species in the VPO catalyst.

Promotional effect of ZrO2 and WO3 on bimetallic Pt-Pd diesel oxidation catalyst.

Diesel oxidation catalysts Pt-Pd-(y)ZrO2-(z)WO3/CeZrOx-Al2O3 with total Pt & Pd loading of only 0.68 wt.% were prepared and investigated for oxidation activity and stability of CO, C3H6, and NO. Introduction of ZrO2 greatly improved low-temperature activities and retained stability especially for CO and C3H6 oxidation after treated at 800 °C. With the optimal loading amount of 6 wt% ZrO2, 2 wt% WO3 was introduced to the system and showed higher activity. Reaction temperature for 50% CO and C3H6 conversiondeclined to 160 and 181 °C, and the maximal NO conversion increased to 50%. By using XRD, TEM, CO chemisorption, XPS, and H2-TPR analysis, it was found that ZrO2 could inhibit aggregation of Pt and Pd, improve metal dispersion, and increase surface-chemisorbed oxygen after high-temperature treatment, accounting for promoted performance. Also, there were more reducible oxide species in ZrO2-doped catalysts. ZrO2 could induce reduction of noble metal oxides and surface ceria by weakening Pt-O-Ce interaction, which increased the ability to dissociate H2 and spillover effect of dissociated hydrogen to ceria. Doping WO3 increased metal dispersion of fresh samples and brought more Pt0 species that were active sites for oxidation reactions. Thus, ZrO2 and WO3 could be effective additives for oxidation catalysts to synergistically improve their activities and thermal stability.

Ethanol Dry Reforming over Mn-Doped Co/CeO2 Catalysts with Enhanced Activity and Stability

This work mainly focused on the promotional effect of the Mn dopant on the catalytic behavior of the Co/CeO2 catalyst by selecting ethanol dry reforming as the model reaction. Herein, the as-prepared catalysts (CoCe and CoCeMn) were characterized by Brunauer–Emmett–Teller, X-ray diffraction, hydrogen temperature programmed reduction, X-ray photoelectron spectroscopy, Raman spectroscopy, and transmission electron microscopy to depict the influence of the Mn promoter on the physicochemical features. The results indicated that the presence of Mn species in the CoCeMn catalyst could improve the structural properties such as increasing the specific surface area, strengthening the active metal–support interaction, enhancing catalyst reducibility, and generating more oxygen vacancies. Generally, it was accepted that these positive factors could improve the catalytic performance. Therefore, the catalytic activity/stability of the CoCeMn catalyst was much better than that of the undoped CoCe sample. In addition, the possible mechanism of the catalyst antisintering/anticoking was also proposed to further elucidate the important role of the dopant Mn.