Thermal Activation Treatments Research Articles

Elucidating Activity-Stability Trade-Off to Design Highly Durable Fe-N-C Catalysts

In this work, we developed a Fe2O3@ZIF-8 composite precursors and combined with a novel thermal activation treatment under an Ar/H2 mixture atmosphere (i.e., forming gas), which successfully prepares a highly active Fe-N-C catalyst. 57Fe Mossbauer spectroscopy analysis experimentally verified that the catalyst only contains S1 sites. The highly active Fe-N-C catalyst achieved exceptional ORR initial activity in acids, exceeding that of a Pt/C baseline catalyst (60 µgPt cm−2) in rotating disk electrode (RDE) tests. The MEA tests further verified that the Fe-N-C catalyst's activity is competitive with Pt/C cathode (0.1 mgPt cm−2) in the kinetic range. Notably, the catalyst demonstrated remarkable activity of 50.8 mA cm−2 (@0.9 ViR-free) under H2-O2 conditions, exceed the U.S. DOE target. As expected, the catalyst degraded significantly during the stability ASTs.To increase S1 sites and address the stability issues, we further developed in-situ CVD methods to treat Fe2O3@ZIF-8 precursors under forming gas flow but with additional N/C precursors that was put at the upstream sides of the tube furnace. Therefore, additional nitrogen and carbon sources are added to the Fe2O3@ZIF-8 precursors during the thermal activation of catalysts. As a results, we can populate the intrinsically stable S2 sites to design highly durable Fe-N-C catalyst. The resulting catalyst presented outstanding stability in both RDE and fuel cell tests. The E 1/2 gained 21 mV after 100,000 cycles of accelerated degradation test (ADT), achieving 0.869 VRHE at the end of the test, which is close to Pt/C (60 µgPt cm−2) in acid electrolyte. The current density of MEA at 0.8 V retained unchanged, and the peak power density increased from 454.0 to 512.0 mW cm−2 in H2-fuel cell after 30,000 cycles of AST.

Incorporation of Magnesium into GaN Regulated by Intentionally Large Amounts of Hydrogen during Growth by MOCVD

Herein, metal–organic chemical vapor deposition (MOCVD) of GaN layers doped with Mg atoms to the recognized optimum level of [Mg] ≈2 × 1019 cm−3 is performed. In a sequence of MOCVD runs, operational conditions, including temperature and flow rate of precursors, are maintained except for intentionally larger flows of hydrogen carrier gas fed into the reactor. By employing the largest hydrogen flow of 25 slm in this study, the performance of the as‐grown Mg‐doped GaN layers is certified by a room‐temperature hole concentration of p ≈2 × 1017 cm−3 in the absence of any thermal activation treatment. Experimental evidence is delivered that the large amounts of hydrogen during the MOCVD growth can regulate the incorporation of the Mg atoms into GaN in a significant way so that MgH complex can coexist with a dominant and evidently electrically active isolated MgGa acceptor.

Preparation and Characterization of Porous Carbon Composites from Oil-Containing Sludge by a Pyrolysis-Activation Process

Large amounts of oil-containing sludge (OS) are produced in the energy, industry and services sectors. It is mainly composed of residual oil and particulate matter, thus posing an environmental threat and leading to resource depletion if it is improperly handled. In this work, the OS feedstock was thermally treated to produce porously magnetic carbon composites (CC) using a pyrolysis-activation process. Using the data on the thermogravimetric analysis (TGA) of the OS feedstock, the thermal activation experiments were performed at 850 °C as a function of residence time (30, 60 and 90 min). The results of pore analysis for the resulting CC products showed that the Brunauer–Emmett–Teller (BET) surface area greatly decreased from 21.59 to 0.56 m2/g with increasing residence time from 30 to 90 min. This decline could be associated with continuous gasification by CO2, thus causing the removal of limited carbon on the surface of CC for a longer reaction time. Furthermore, the physical properties of the resulting CC products can be enhanced by post acid-washing due to the development of new pores from the leaching-out of inorganic minerals. The BET surface area increased from 21.59 to 40.53 m2/g at the residence time of 30 min. Obviously, the resulting CC products were porous materials with mesopores and macropores that were concurrently formed from the thermal activation treatment. These porous features were also observed by scanning electron microscopy (SEM).

Utilizing thermal activation treatment to improve the properties of waste cementitious powder and its newmade cementitious materials

The low activity of waste cementitious powder from construction and demolition waste (CDW) is the main reason that limits its application in cementitious materials, and thus, this work investigates the utilization of thermal activation treatment in improving the properties of waste cementitious powder and its newmade mortar. The un-treated waste powder mainly consists of inert hydrated products, quartz or calcite, and new active components are formed in thermal-activated waste powder, and the utilization of thermal-activated waste powder promotes the hydration reaction of new cementitious materials. The waste powder has an irregular micro-structure, but it becomes smooth after 1200 °C activation. When the thermal activation temperature on waste powder is 600–800 °C, the properties of mortar including thermal-activated waste powder are better than those of mortar with un-treated waste powder, and specifically, the activity index of waste paste powder ranges from 71.7% to 90.1% when the activation temperature ranges from 20 °C to 600 °C. But the properties of mortar including 1200 °C thermal-activated waste powder are generally inferior to those of mortar with other thermal-activated waste powders. The 600–800 °C thermal-activated temperature is suggested to improve the properties of waste cementitious powder by taking energy consumption and thermal-activated efficiency into consideration. And although higher than untreated waste powder preparation, the costs and energy consumption for preparing thermal-activated waste powder still has advantages in contrast with cement preparation. Replacing partial cement with thermal-activated waste powder in new cementitious materials is feasible and beneficial.

Activation Process and Mechanism of ZrCoCe Getter Films

In order to study the activation process and mechanism of ZrCoCe, highly porous ZrCoCe getter films were grown by the DC magnetron sputtering method. The effect of activation temperature on the surface composition of the porous ZrCoCe getter films were studied by X-ray photoelectron spectroscopy (XPS). The results shows that the surface of air-exposed porous ZrCoCe film is covered with H2O, CO2 and hydrocarbons, both Zr and Ce exist in the oxidized state, and zirconium oxide starts to reduce at 300 °C. The activation process also results in a sizable Co segregation at surface. In addition, zirconium carbide can be found in the subsurface region of the film after thermal activation treatment.

Passivation of phosphorus diffused silicon surfaces with Al2O3: Influence of surface doping concentration and thermal activation treatments

Thin layers of Al2O3 are well known for the excellent passivation of p-type c-Si surfaces including highly doped p+ emitters, due to a high density of fixed negative charges. Recent results indicate that Al2O3 can also provide a good passivation of certain phosphorus-diffused n+ c-Si surfaces. In this work, we studied the recombination at Al2O3 passivated n+ surfaces theoretically with device simulations and experimentally for Al2O3 deposited with atomic layer deposition. The simulation results indicate that there is a certain surface doping concentration, where the recombination is maximal due to depletion or weak inversion of the charge carriers at the c-Si/Al2O3 interface. This pronounced maximum was also observed experimentally for n+ surfaces passivated either with Al2O3 single layers or stacks of Al2O3 capped by SiNx, when activated with a low temperature anneal (425 °C). In contrast, for Al2O3/SiNx stacks activated with a short high-temperature firing process (800 °C) a significant lower surface recombination was observed for most n+ diffusion profiles without such a pronounced maximum. Based on experimentally determined interface properties and simulation results, we attribute this superior passivation quality after firing to a better chemical surface passivation, quantified by a lower interface defect density, in combination with a lower density of negative fixed charges. These experimental results reveal that Al2O3/SiNx stacks can provide not only excellent passivation on p+ surfaces but also on n+ surfaces for a wide range of surface doping concentrations when activated with short high-temperature treatments.

Reaction kinetics during the thermal activation of the silicon surface passivation with atomic layer deposited Al2O3

The excellent surface passivation of crystalline silicon provided by Al2O3 requires always an activation by a thermal post-deposition treatment. In this work, we present an indirect study of the reaction kinetics during such thermal activation treatments for Al2O3 synthesized by atomic layer deposition. The study was performed for Al2O3 deposited at varying temperatures, which results in different micro-structures of the films and, in particular, different hydrogen concentrations. The effective carrier lifetime was measured sequentially as a function of the annealing time and temperature. From these data, the reaction rate Ract and the activation energy EA were extracted. The results revealed a rather constant EA in the range of 1.4 to 1.5 eV, independent of the deposition temperature. The reaction rate, however, was found to increase with decreasing deposition temperature, which correlates with an increasing amount of hydrogen being incorporated in the Al2O3 films. This is a strong indication for an interface hydrogenation that takes place during the thermal activation, which is limited by the amount of hydrogen provided by the Al2O3 layer.

Supported Ir–Pd nanoalloys: Size–composition correlation and consequences on tetralin hydroconversion properties

A series of Ir–Pd/SiO2–Al2O3 catalysts has been prepared by incipient wetness co-impregnation of acetylacetonate precursors and treated by reductive thermal decomposition. The decomposition has been followed by combined thermogravimetry, differential thermal analysis, and mass spectrometry, showing a marked difference between the bimetallic and monometallic cases. The catalysts characterization by transmission electron microscopy and single-nanoparticle energy dispersive X-ray spectroscopy reveals that the particles are bimetallic and exhibit a size–composition correlation. More generally, the enrichment of larger particles with the less cohesive element (Pd in the present case) via Ostwald ripening is here claimed to be inherent to the thermal activation treatments used in the preparation of bimetallic catalysts. The catalysts have been tested in the hydroconversion of tetralin, which is considered as a model molecule for gas oil upgrading by selective ring opening. At 350°C, 4MPa H2, and in the presence of ppm amounts of H2S, hydrogenation, dehydrogenation, and ring-opening/contraction products (ROCPs) are formed. While the activity increases with the Pd content, the selectivity to ROCPs reaches a maximum for the Ir55–Pd45 composition and increases with the sulfur concentration. The structural and catalytic results are combined to propose a qualitative model relying on metal–acid bifunctionality and size–composition correlation. While the particle size drives the number of surface sites, the corresponding surface composition governs the hydrogenation efficiency of these sites. Therefore, by controlling the mean metal composition of the catalyst, it is possible to tune the activity–selectivity balance.

Stability and Photoelectronic Properties of Layered Titanate Nanostructures

Layered titanate nanostructures offer promising photoelectronic properties that are subject to surface chemistry-induced morphology changes. For a systematic evaluation of the bulk and surface contributions to the photoactivity of these structures, we investigated their photoelectronic properties and in particular their dependence on the condition of the gas-solid interface. We comprehensively explored the stability of Na(2)Ti(3)O(7) nanowires and scrolled up H(2)Ti(3)O(7) nanotubes by means of transmission electron microscopy, Raman, and FT-IR spectroscopy and subjected both titanate sheet-based structures to controlled thermal activation treatment under high vacuum conditions. We found that throughout thermal annealing up to T = 870 K the structure and morphology of Na(2)Ti(3)O(7) nanowires are retained. Consistent with the significant photoluminescence emission that is attributed to radiative exciton annihilation in the bulk, UV-induced charge separation is strongly suppressed in these structures. H(2)Ti(3)O(7) nanotubes, however, undergo transformation into elongated anatase nanocrystals during annealing at temperatures T >OR= 670 K. Photoexcitation experiments in O(2) atmosphere reveal that these structures efficiently sustain the separation of photogenerated charges. Trends in the abundance of trapped holes and scavenged electrons were characterized quantitatively by tracking the concentration of paramagnetic O(-) and O(2)(-) species with electron paramagnetic resonance spectroscopy EPR, respectively. An incisive analysis of these results in comparison to those obtained on airborne anatase nanocrystals underlines the critical role of surface composition and structure on charge separation and, in consequence, on the chemical utilization of photogenerated charge carriers.

Characterization of Carbon-Supported AuPt Nanoparticles for Electrocatalytic Methanol Oxidation Reaction

In view of the recent finding that the bimetallic AuPt nanoparticles prepared by molecular-capping-based colloidal synthesis and subsequent assembly on carbon black support and thermal activation treatment exhibit alloy properties, which is in sharp contrast to the bimetallic miscibility gap known for the bulk counterparts in a wide composition range, there is a clear need to assess the electrocatalytic properties of the catalysts prepared with different bimetallic composition and different thermal treatment temperatures. This paper reports recent results of such an investigation of the electrocatalytic methanol oxidation reaction (MOR) activities of the carbon-supported AuPt nanoparticle catalysts with different bimetallic composition and thermal treatment temperatures. Au(m)Pt(100)(-)(m) nanoparticles of 2-3 nm core sizes with different atomic compositions ranging from 10% to 90% Au (m = 10 approximately 90) have been synthesized by controlling the feeding of the metal precursors used in the synthesis. The electrocatalytic MOR activities of the carbon-supported AuPt bimetallic catalysts were characterized in alkaline electrolytes. The catalysts with 65% to 85% Au and treated at 500 degrees C were found to exhibit maximum electrocatalytic activities in the alkaline electrolytes. The findings, together with a comparison with some well-documented catalysts as well as recent experimental and theoretical modeling results, have revealed important insights into the participation of CO(ad) and OH(ad) on Au sites in the catalytic reaction of Pt in the AuPt alloys with approximately 75% Au. The insights are useful for understanding the correlation of the bifunctional electrocatalytic activity of the bimetallic nanoparticle catalysts with the bimetallic composition and the thermal treatment temperatures.

Physical techniques and catalyst preparation: Determining the interactions of transition-metal complexes with oxide surfaces

Abstract This contribution presents the molecular approach used to investigate the different preparation steps of important catalytic systems, based on Mo, Ni, and Co. The first preparation step consists of impregnating oxide supports by aqueous solutions of transition-metal complexes (TMCs), called precursor complexes, and involves the liquid-solid (wet) interface. The following steps consist of thermal activation treatments, involving the gas-solid (dry) interface. The molecular approach is based on (i) the use of the TMC to monitor and probe the nature of its own interactions with the oxide support, (ii) the selection of the most appropriate techniques to monitor such interactions, and (iii) the design of experiments to identify and isolate the key parameters controlling the "TMC-oxide support" interactions. Physical techniques play a key role in the characterization of catalyst preparation both at the macroscopic [temperature-programmed reduction (TPR), X-ray diffraction (XRD)] and molecular [Raman, X-ray absorption near-edge spectroscopy-extended X-ray absorption fine structure (XANES)-EXAFS, diffuse reflectance UV-vis] levels, particularly when they are used simultaneously. They give some insight into the structure of the species existing on the surface, with the oxide support behaving as a solid ligand, when exposed for a short time to the precursor complex solution or after thermal treatment, or, for longer contact times, as a reactant.

Operando resonance Raman spectroscopic characterisation of the oxidation state of palladium in Pd/γ-Al2O3catalysts during the combustion of methane

Resonance Raman spectroscopy is used in order to investigate the behaviour of Pd/γ-alumina (2 wt.%) in the catalytic combustion of methane. Simultaneous spectroscopic and catalytic measurements allow an operando spectroscopic investigation of the catalyst under operational conditions. Temperature cycles were performed by alternately heating and cooling the system under the reaction conditions after various in situ gaseous pre-treatments of the catalyst. Our results show that neither the Raman features nor the conversion of methane are influenced by initial thermal activation treatment under reducing or oxidizing atmospheres. Pure metallic palladium was found to be inactive for the catalytic combustion of methane. Under the reaction conditions, the Pd/γ-Al2O3 catalyst is always in an oxide PdO form with its surface in an intermediate state between surface lacunary PdO and crystalline PdO species.

Electron spin resonance evidence for isomorphous substitution of titanium into titanosilicate TiMCM-48 mesoporous materials

Hydrothermal synthesis of TiMCM-48 with varying Si/Ti ratios has been achieved. Comparative electron spin resonance (ESR) studies between synthesized TiMCM-48 and ion-exchanged Ti–AlMCM-48 show significant differences with respect to location and adsorbate interactions. Gamma-irradiation at 77 K of TiMCM-48 after a thermal activation treatment produces a strong ESR signal due to V centers and an axial signal with g∥ = 1.976 and g⊥ = 1.903 for isolated trivalent titanium in framework tetrahedral sites. An axial ESR signal with reversed g values of g∥ = 1.900 < g⊥ = 1.977 typical of Ti(III) with distorted octahedral symmetry is observed after γ-irradiation at 77 K of ion-exchanged Ti–AlMCM-48. Both TiMCM-48 and Ti–AlMCM-48 also differ in their behavior towards various adsorbates. The Ti(III) species observed in TiMCM-48 and Ti–AlMCM-48 after adsorption of D2O, CO, NH3 and CH3OD change their ESR characteristics indicating significant interaction with adsorbates. The ESR characteristics of the Ti(III) species in TiMCM-48 after adsorption of these adsorbates are different from those of Ti(III) in Ti–AlMCM-48. The contrasting characteristics of Ti(III) in TiMCM-48 and Ti–AlMCM-48 suggest that the titanium ion is in a framework site in TiMCM-48.

Electron Spin Resonance Evidence for Isomorphous Substitution of Titanium into Titanosilicate TiMCM-41 Mesoporous Molecular Sieve

New evidence for the framework substitution of titanium into TiMCM-41 mesoporous molecular sieve is found from electron spin resonance (ESR) studies. TiMCM-41 samples with varying Si/Ti ratios have been synthesized as have titanosilicate TS-1, siliceous MCM-41, AlMCM-41, and Ti−AlMCM-41 in the last of which titanium ions are incorporated by ion exchange. These materials were examined by ESR spectroscopy after a thermal activation treatment followed by γ-irradiation at 77 K. γ-Irradiation at 77 K of TiMCM-41 having a high Si/Ti ratio produces a strong orthorhombic ESR signal due to V centers and an axial signal with g∥ = 1.971 and g⊥ = 1.901 best explained as arising from trivalent titanium situated at a framework tetrahedral site. An upper limit for the substitution of titanium into a framework tetrahedral site is observed. As the titanium concentration in the synthesis gel increases, the resulting material contains both tetrahedral and octahedral titanium. γ-Irradiation of ion-exchanged Ti−AlMCM-41 gives an axial ESR signal characterized by reverse g values g∥ = 1.898 and g⊥ = 1.967 typical of Ti(III) in distorted octahedral symmetry. Both TiMCM-41 and Ti−AlMCM-41 differ in their behavior toward interaction with various adsorbate molecules. The Ti(III) species observed in TiMCM-41 after adsorption of D2O, CO, NH3, and CD3OH changes its ESR characteristics from that of the original Ti(III) species observed in the absence of these adsorbates. The ESR characteristics of the Ti(III) species in Ti−AlMCM-41 after adsorption of the above adsorbates are different from those of TiMCM-41, indicating that the Ti in these two materials are in different coordination environments. This independently supports the different g tensors for Ti(III) in TiMCM-41 versus Ti−AlMCM-41.

Effect of Thermal Treatments on the Properties of Nickel and Cobalt Activated-Charcoal-Supported Catalysts

The effect of thermal pretreatment in N 2 up to 723 K and the activation treatments in H 2 and an inert atmosphere on the properties of Ni and Co activated-charcoal-supported catalysts were studied. Catalysts were characterized by means of N 2 adsorption at 77 K, H 2 chemisorption at room temperature, thermogravimetric analysis (TGA), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The catalysts′ activity and selectivity for acetone hydrogenation to 2-propanol under unusual and severe conditions (473 K and high overall acetone conversion) were also measured. TGA and XRD evidence was found for the charcoal-support-promoted NiO and CoO reduction to the metallic states when the catalysts were subjected to an inert atmosphere at 723 K. The thermal activation treatments under H 2 or inert atmosphere above 723 K caused a loss of acetone hydrogenation activity (calculated on a metal load basis) for both the Ni and Co activated-charcoal-supported catalysts, with respect to that of the low-temperature (573 K) activation treatments. In a series of activated-charcoal-supported Ni catalysts, a large decrease in the H 2 chemisorption uptake was also found for a sample pretreated in N 2 at 723 K prior to H 2 reduction. These results were not due to nickel or cobalt sintering, as shown by XRD line broadening measurements. The catalytic activity loss was accompanied by a decrease (in the case of Ni) and an increase (in the case of Co) in the 2-propanol selectivity.

XRD characterization of highly dispersed metal catalysts on carbon support

Carbon-supported Pt catalysts were prepared from H2PtCl6 or K2PtCl6 aqueous solutions. Particle size and structure after several thermal activation treatments were studied by X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Cyclic Voltammetry (CV), and the results of the three techniques were compared. As the catalysts were highly dispersed on an amorphous support, a conventional XRD profile analysis for crystallite size determination could not be performed properly, because of the strong overlapping between the broad Pt peaks superposed to the halos of the amorphous phase. Thus, a new procedure of whole XRD pattern fitting, based on the Rietveld method, was used to have reliable data of Pt particle size (surface area) and lattice parameter. All structural and microstructural parameters were refined within the same procedure, also considering the transparency of the carbon supported catalysts and minimizing the effect of the amorphous background. The method can also take into account the presence of bimodal particle size distributions, which is difficult to study by CV or TEM.

Characterization of nickel/silica catalysts during impregnation and further thermal activation treatment leading to metal particles

Nickel nitrate impregnated onto silica, identified as such after drying at 25[degrees]C, is gradually transformed into basic nickel nitrate, Ni(NO[sub 3])[sub 2][center dot]2Ni(OH)[sub 2], and then into nickel phyllosilicate (1:1 nickel phyllosilicate) upon drying at 90[degrees]C. At this temperature, the amount of phyllosilicates which depends on the drying time, can reach 20% of the overall Ni. The temperature programmed reduction (TPR) profiles of impregnated nickel exhibit three peaks of hydrogen consumption at 300, 380, and about 500[degrees]C, attributed to the decomposition of nickel nitrate or basic nitrate into NiO, the reduction of NiO, and the reduction of 1:1 nickel phyllosilicates, respectively. The last peak corresponds to the reduction of 20% of the overall Ni content regardless of the Ni loading and the conditions of drying at 25 or 90[degrees]C. This fixed percentage proves that phyllosilicate may be still formed at the beginning of the TPR. After TPR up to 700[degrees]C, the nickel is totally reduced and the metal nickel particles have a constant average size of about 65 A while the concentration of particles per surface unit increases with the Ni loading. Before complete reduction, the nickel phyllosilicates are probably located at the interface between silica and the remaining nickel,more » for which they act as anchoring sites. During TPR, after nitrate decomposition, the NiO particles located on the phyllosilicates are reduced into Ni[sup O] at 400[degrees]C without any migration. Between 400 and 700[degrees]C, the increase in metal particle size is due not only to the reduction of the phyllosilicates but also to Ni[sup 0] migration induced by thermal effect and probably also by the weakening of the anchoring strength because of the reduction of phyllosilicates. 47 refs., 8 figs., 12 tabs.« less