Formation Of Active Phase Research Articles

Effect of Li promoter on FeMn/CNTs for light olefins from syngas

FeMnLi/CNTs catalysts with different Li content were prepared by the incipient wetness impregnation method for light olefins from syngas and investigated by N2 adsorption-desorption, XRD, H2-TPR, CO2-TPD and Mӧssbauer techniques. The addition of Li increased the surface basicity of catalysts and promoted the transformation of Fe3+ specious and formation of active phase, χ-Fe5C2, which was favorable to CO conversion and light olefins selectivity. The best catalytic performance was achieved over 8Fe8Mn1.5Li catalyst with CO conversion of 14.18% and light olefins selectivity of 48.42%.

Controlled Synthesis of Copper-Doped Molybdenum Carbide Catalyst with Enhanced Activity and Stability for Hydrogen Evolution Reaction

As a promising alternative to precious metal catalysts, Molybdenum carbide (MoC) still cannot compete with commercial Pt catalysts in many aspects. In the present work, we develop a simple carburization method to synthesize synergistic MoC catalysts with highly dispersed copper. Addition of copper could effectively induce formation of α-phase of MoC and markedly improve electrochemical activity for HER reaction. When the doping ratio of Cu–Mo reaches 10:90, the Cu doped MoC displays the best catalytic performance, even compared with the commercial Pt/C catalyst. The enhanced HER activity for Cu–MoxCy can be attributed to the formation of active phase and also synergistic surface.

γ-Fe2O3 as the precursor of iron based catalyst prepared by solid-state reaction at room temperature for Fischer-Tropsch to olefins

Iron catalyst was fabricated via simple solid-state reaction of metal nitrates mixture (Fe, K, Al, Ca, Mg, Sm) with oxalic acid dihydrate at room temperature. Iron catalysts prepared by conventional methods, such as co-precipitation and fusing, are also included as comparison. The performances of catalysts were evaluated by the Fischer-Tropsch Synthesis (FTS) in a fixed bed reactor. The results indicate that uniform nanoparticles were obtained with surface area as high as 38.7 m2/g prepared by solid-state reaction. Following calcination at 500 °C, γ-Fe2O3 were obtained as the catalyst precursor. By contrast, α-Fe2O3 was the main precursor derived from co-precipitation. In addition, γ-Fe2O3 prepared by solid-state reaction facilitates the formation of active phase, Fe5C2 with the exposure of (021) facet. High Miller index surface (021) facilitates the adsorption and the dissociation of CO, which contributes the high activity and high selectivity to light olefins. Consequently, compared with conventional catalysts, solid-state reaction derived catalysts exhibit much higher FTO reaction activity and selectivity to lower olefins. The FTY value of 5.62 × 10 ̶ 3 molCO gFe̶ 1 s ̶ 1 was obtained which is almost two times higher than that of catalyst prepared by co-precipitation (3.19 × 10 ̶ 3 molCO gFe̶ 1 s ̶ 1) or 3 times higher than that of catalyst prepared by fusing method (1.98 × 10 ̶ 3 molCO gFe̶ 1 s ̶ 1). The selectivity to light olefins of around 40% is obtained which is significantly higher than that of the reference catalysts (25% and 33% respectively). The present study provides a promising pathway to prepare iron based catalysts at room temperature.

Structural Evolution of Highly Active Multicomponent Catalysts for Selective Propylene Oxidation

Multicomponent Bi-Mo-Fe-Co oxide catalysts prepared via flame spray pyrolysis were tested for selective propylene oxidation, showing high conversion (>70%) and selectivity (>85%) for acrolein and acrylic acid at temperatures of 330 °C. During extended time-on-stream tests (5–7 days), the catalysts retained high activity while undergoing diverse structural changes. This was evident on: (a) the atomic scale, using powder X-ray diffraction, Raman spectroscopy, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy; and (b) the microscopic scale, using synchrotron X-ray nanotomography, including full-field holotomography, scanning X-ray fluorescence, and absorption contrast imaging. On the atomic scale, sintering, coke formation, growth, and transformation of active and spectator components were observed. On the microscopic scale, the catalyst life cycle was studied at various stages through noninvasive imaging of a ~50-µm grain with 100-nm resolution. Variation of catalyst synthesis parameters led to the formation of notably different structural compositions after reaction. Mobile bismuth species formed agglomerates of several hundred nanometres and segregated within the catalyst interior. This appeared to facilitate the formation of different active phases and induce selectivity for acrolein and acrylic acid. The combined multiscale approach here is generally applicable for deconvolution of complex catalyst systems. This is an important step to bridge model two-component catalysts with more relevant but complex multicomponent catalysts.

Effect of Pretreatment Method on the Nanostructure and Performance of Supported Co Catalysts in Fischer–Tropsch Synthesis

Understanding precursor transformation to active catalysts is crucial to heterogeneous Fischer–Tropsch (FT) catalysis directed toward production of hydrocarbons for transportation fuels. Despite considerable literature on FT catalysis, the effect of pretreatment of supported cobalt catalysts on cobalt dispersion, dynamic atomic structure, and the activity of the catalysts is not well understood. Here we present systematic studies into the formation of active catalyst phases in supported Co catalyst precursors in FT catalysis using in situ environmental (scanning) transmission electron microscopy (E(S)TEM) with single-atom resolution under controlled reaction environments for in situ visualization, imaging, and analysis of reacting atomic species in real time, EXAFS, XAS, DRIFTS analyses, and catalytic activity measurements. We have synthesized and analyzed dried reduced (D) and dried calcined reduced (DC) Co real-world (practical) catalysts on reducible and nonreducible supports, such as SiO2, Al2O3, TiO2, and ZrO2. Comparisons of dynamic in situ atomic structural observations of reacting single atoms, atomic clusters, and nanoparticles of Co and DRIFTS, XAS, EXAFS, and catalytic activity data of the D and DC samples reveal in most cases better dispersion in the D samples, leading to a larger number of low-coordination Co0 sites and a higher number of active sites for CO adsorption. The experimental findings on the degree of reduction of D and DC catalysts on reducible and nonreducible supports and correlations between hexagonal (hcp) Co sites and the activity of the catalysts generate structural insights into the catalyst dynamics, important to the development of efficient FT catalysts.

Sub-3 nm pores in two-dimensional nanomesh promoting the generation of electroactive phase for robust water oxidation

Herein, abundant and uniform nanopores with sub-3-nm sizes are introduced to 1.5 nm-thick nickel-iron layered double hydroxide (NiFe LDH) ultrathin nanosheets via an etching-aging process, realizing remarkable enhancement in oxygen evolution reaction (OER) performance. Detailed analyses revealed that the NiFe LDH phase around the nanopores can be preferentially oxidized into electroactive Fe:NiOOH phase. In addition, the buffering space provided by the nanopores can effectively avoid structural deformation during repeated redox cycling, leading to better electrochemical stability, which synergistically make the catalyst a promising candidate for commercial water splitting. This work highlights the positive role of porous structure in accelerating the formation of active phases beyond just increment of surface area, which can provide insight in designing advanced catalysts.

Pd Doped La0.1Sr0.9TiO3 as High-Temperature Water-Gas Shift Catalysts: In-Situ Formation of Active Pd Phase

A series of La0.1Sr0.9Ti1−xPdxO3 with x up to 0.1 were prepared through a modified sol–gel method. Combining X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS) and Temperature programmed reduction (TPR), we proved that solid solutions formed with a Pd substitution (in the B-sites) of no more than 5 at.%. Further addition of Pd resulted in segregation of PdOx species. An in-depth analysis of in-situ CO adsorption confirmed that the metallic Pd, formed in-situ during the water-gas shift reactions (WGSR), was the major contributor for this reaction.

Controlled formation of iron carbides and their performance in Fischer-Tropsch synthesis

Iron carbides are unmistakably associated with the active phase for Fischer-Tropsch synthesis (FTS). The formation of these carbides is highly dependent on the catalyst formulation, the activation method and the operational conditions. Because of this highly dynamic behavior, studies on active phase performance often lack the direct correlation between catalyst performance and iron carbide phase. For the above reasons, an extensive in situ Mössbauer spectroscopy study on highly dispersed Fe on carbon catalysts (Fe@C) produced through pyrolysis of a Metal Organic Framework was coupled to their FTS performance testing. The preparation of Fe@C catalysts via this MOF mediated synthesis allows control over the active phase formation and therefore provides an ideal model system to study the performance of different iron carbides. Reduction of fresh Fe@C followed by low-temperature Fischer-Tropsch (LTFT) conditions resulted in the formation of the ε′-Fe2.2C, whereas carburization of the fresh catalysts under high-temperature Fischer-Tropsch (HTFT) resulted in the formation of χ-Fe5C2. Furthermore, the different activation methods did not alter other important catalyst properties, as pre- and post-reaction transmission electron microscopy (TEM) characterization confirmed that the iron nanoparticle dispersion was preserved. The weight normalized activities (FTY) of χ-Fe5C2 and ε′-Fe2.2C are virtually identical, whilst it is found that ε′-Fe2.2C is a better hydrogenation catalyst than χ-Fe5C2. The absence of differences under subsequent HTFT experiments, where χ-Fe5C2 is the dominating phase, is a strong indication that the iron carbide phase is responsible for the differences in selectivity.

Impact of the Cu2O microcrystal planes on active phase formation in the Rochow reaction and an experimental and theoretical understanding of the reaction mechanism

In heterogeneous catalysis, on one hand, people always want to know about reaction details, such as what the most active phase is, how it is formed, and what is the reaction mechanism. On the other hand, it is still very challenging to probe these reaction details, particularly at high reaction temperatures and pressures. Here, we report the microcrystal plane-controlled catalytic performance on well-defined Cu2O cube, octahedron, and rhombic-dodecahedron catalysts in the Rochow reaction. It was found that the Cu2O cube exposing {1 0 0} crystal planes gave the highest dimethyldichlorosilane selectivity and yield, while the Cu2O rhombic-dodecahedron exposing {1 1 0} crystal planes showed the lowest selectivity and yield. Our experimental observation, as well as density functional theory calculations, demonstrated that the enhanced selectivity and yield stemmed from the stronger dissociative adsorption of methyl chloride on the Cu2O{1 0 0} plane, which greatly promoted the transformation of Cu2O into Cu3Si active phases under the reaction conditions. This work reveals a new strategy for controlling the surface structure of catalysts in order to enhance their catalytic performance.

Enhanced catalytic performance and promotional effect of molybdenum sulfide cluster-derived catalysts for higher alcohols synthesis from syngas

A series of catalysts were prepared using molybdenum sulfide clusters as precursors and their catalytic performance for higher alcohols synthesis was evaluated. It was found that the cluster precursors could provide well-dispersed MoS2 particles and a number of coordinated unsaturated Mo sites with all the catalysts in varying degrees, which were favorable for the formation of intermediate active phases thereby enhancing catalytic performance. In particular, K-NiMo3Sx catalyst obtained by using heterobimetallic cluster NiMo3S13 as precursor exhibited the highest activity with the space-time-yield (STY) of total alcohols (497 mg/g/h) and the selectivity of higher alcohols (59.71%), which could be attributed to the interaction effect of the Ni promoter and Mo species and the resultant formation of active phase NiMo3Sx. Thus, the most key factor for obtaining highly efficient molybdenum sulfide catalysts was to produce a number of coordinated unsaturated sites, on which abundant NiMoS active phase can be formed and the formation of bulk inactive phase should be avoided.

Cu-ZnO catalysts for CO2 hydrogenation to methanol: Morphology change induced by ZnO lixiviation and its impact on the active phase formation

Synthesis of Cu-ZnO catalysts for the direct hydrogenation of CO2 to methanol requires catalyst precursors able to display a strong Cu-Zn interaction. Copper and zinc mixed hydroxynitrates are good candidates to fulfill this specification. The synthesis of such a phase was achieved by a wet impregnation of copper nitrate on ZnO. Solvent evaporation and catalyst drying at moderate temperatures allowed retaining the metals hydroxynitrate structure, leading to a memory effect for the reduced catalysts. Depending on the Cu content in the reaction mixture, a ZnO lixiviation gradually occurs, what induces an important change in the morphology of the reduced catalyst. Catalyst activity is proportional to the amount of Zn migrated in the active phase, the latter consisting most probably of an oxygen deficient mixed oxide comprising both Cu and Zn. The most active catalyst is obtained when ZnO is fully lixiviated during the catalyst precursor synthesis, giving a core-shell structure after reduction.

Active Fischer-Tropsch synthesis Fe-Cu-K/SiO2 catalysts prepared by autocombustion method without a reduction step

ABSTRACT The purpose of this study was to prepare iron-based catalysts supported on silica by autocombustion method for directly using for Fischer–Tropsch synthesis (FTS) without a reduction step. The effect of different citric acid (CA):iron nitrate (N) molar ratios and acid types on the FTS performance of catalysts were investigated. The CA:N molar ratios had an important influence on the formation of iron active phases and FTS activity. The iron carbide (FexC), which is known to be one of the iron active phases, was demonstrated by the X-ray diffraction and X-ray photoelectron spectroscopy. Increasing the CA:N molar ratios up to 0.1 increased CO conversion of catalyst to 86.5%, which was then decreased markedly at higher CA:N molar ratios. An excess of CA resulted in carbon residues covering the catalyst surface and declined FTS activity. The optimal catalyst (CA:N molar ratio = 0.1) achieved the highest CO conversion when compared with other autocombustion catalysts as well as reference catalyst prepared by impregnation method, followed by a reduction step. The autocombustion method had the advantage to synthesize more efficient catalysts without a reduction step. More interestingly, iron-based FTS catalysts need induction duration at the initial stage of FTS reaction even after reduction, because metallic iron species need time to be transformed to FexC. But here, even if without reduction, FexC was formed directly by autocombustion and induction period was eliminated during FTS reaction.

MoW synergetic effect supported by HAADF for alumina based catalysts prepared from mixed SiMonW12-n heteropolyacids

MoW catalysts supported on Al2O3 with equal surface content of metals (Mo+W=3.9 at/nm2) were synthesized by using mixed Keggin type heteropolyacids (HPAs) H4[SiMo1W11O40] and H4[SiMo3W9O40] and corresponding mixture of monometallic H4[SiMo12O40] and H4[SiW12O40] HPAs. After liquid phase sulfidation, catalysts were characterized by high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). For the first time, High Angle Annular Dark Field microscopy (HAADF) was used to evidence the morphology and composition of the active sulfide phase. The catalysts were tested in hydrotreating of model feed that contained dibenzothiophene (DBT) and naphthalene. Using mixed SiMonW12-nHPAs as starting precursors had a beneficial effect on the catalytic activity. Incorporation of molybdenum in the structure of SiW Keggin-type HPA led to a decrease of the size of active phase crystallites, to an increase of the metal sulfidation degree and of the number of active sites. Substitution of a quarter of tungsten atoms by molybdenum allowed to achieve DBT conversion comparable to value obtained on pure SiMo12HPA based catalyst and higher conversion of naphthalene. Both SiMonW12-nHPAs based catalysts had higher rate constants in studied reactions compared to their corresponding references prepared from two separate monometallic HPAs. Moreover, MonW12-n/Al2O3 catalysts had the highest selectivity in respect of hydrogenation (HYD) pathway of DBT hydrodesulfurization (HDS). The presence of mixed MoxW1-xS2 slabs was evidenced by HAADF analysis. On Mo3W9/Al2O3, small randomly distributed islands of Mo were present in the WS2 slabs, while on Mo3+W9/Al2O3, Mo islands appeared larger and in the core of the WS2 slabs. Moreover, on this last sample, some monometallic slabs were also present. It was concluded that using mixed HPA precursors resulted in the formation of mixed MoxW1-xS2 active phase possessing higher synergetic effect between the two metals and consequently higher catalytic activity.

Synthesis and Mössbauer spectroscopic investigation of copper-manganese ferrite catalysts for water-gas shift reaction and methanol decomposition

Copper-manganese ferrites with nominal compositions Cu1-хMnхFe2O4, where x=0; 0.2; 0.4; 0.6; 0.8 and 1 are synthesized by sol-gel auto-combustion method. Formation of ferrites with cubic spinel structure and crystallite size of 11–26nm was observed. A decrease of iron located in tetrahedral positions with the increase of Mn content in ferrite structure was found. The prepared ferrite materials are tested as catalysts in water-gas shift reaction (WGSR) and methanol decomposition (MD). The samples are arranged according the WGS activity in following order: Cu0.8Mn0.2Fe2O4>Cu0.5Mn0.5Fe2O4>CuFe2O4>Cu0.2Mn0.8Fe2O4>MnFe2O4. All mixed copper-manganese ferrites exhibit higher catalytic activity in methanol decomposition as compared to single CuFe2O4 and MnFe2O4 ones. A significant phase transformation with ferrites under the reduction reaction medium which influences the formation of real catalytic active phase is registered. Formation of magnetite or Mn-substituted magnetite is registered during the WGSR, while iron carbides and Mn-rich oxides are the dominant phases during the MD reaction.

Insight into the promotion mechanism of K and Ni in sulfide molybdenum-based catalysts for higher alcohols synthesis from syngas

Three catalysts, K-Mo3Sx, K-Mo3Sx-NiAc (Ac=acetate) and K-Mo3Sx-NiL (L=Cl−), were synthesized using in situ decomposition method, which were applied to the conversion of syngas (H2 and CO) to higher alcohols. The catalytic results revealed that K-Mo3Sx-NiAc exhibited a high C2+ alcohol selectivity of 41.9% and a space-time-yield of 440mg/g/h, while K-Mo3Sx-NiL had a dramatically low performance. X-ray photoelectron spectra (XPS) results suggested that the different performances of the catalysts were caused by the formation of new active phase K-Mo-S, and Ni only played a role in the further enhancement of catalytic performance.

Preparation of high temperature water gas shift catalyst with coprecipitation method in microemulsion system

In this paper nanocrystalline mesoporous iron-based high temperature water gas shift catalysts were prepared by coprecipitation method in water-in-oil (W/O) microemulsion. The prepared samples were characterized by using X-ray diffraction (XRD), N2 adsorption (BET), temperature-programmed reduction (TPR), Fourier transform infrared spectroscopy (FTIR), transmission and scanning electron microscopies (TEM, SEM) techniques. The results revealed that the Fe2O3 possessed low specific surface area and catalytic activity in high temperature water gas shift reaction. The addition of copper to Fe2O3 favored the active phase formation and significantly decreased the reduction temperature of hematite to magnetite and consequently improved the catalytic activity of Fe2O3. The incorporation of chromium in hematite increased the surface area and catalytic activity of Fe2O3. The results also showed that the prepared Fe⿿Cr⿿Cu catalyst showed higher activity than the commercial catalyst. In addition, the effects of GHSV, steam/gas ratio, and calcination temperature were investigated on the catalytic performance of Fe⿿Cr⿿Cu catalyst.

Application of microemulsion method for development of methanol steam reforming Pd/ZnO catalysts

Thermal decomposition of palladium acetylacetonate adsorbed on zinc oxide (ZnO), and the formation of palladium oxide (PdO) and palladium zinc alloy (PdZn) phases were studied. Two types of ZnO supports were prepared by the microemulsion method using different surfactants, i.e. hexadecyltrimethylammonium bromide (CTAB) and (Bis(2-ethylhexyl) sulfosuccinate sodium salt, AOT). The nanoparticles of ZnO synthesized in the presence of CTAB surfactant showed higher specific surface area, smaller crystallite size and more irregular shape. Palladium was loaded on the surface of obtained supports by the impregnation method from the acetone solution of palladium acetylacetonate. Thermogravimetric studies indicated that palladium precursor loaded on CTAB-modified ZnO support was less stable and simultaneously decomposed in broader range of temperatures. Slight differences between the forms of precursors adsorbed on the supports were demonstrated by the diffuse reflectance infrared Fourier transform spectroscopy studies. Thermal decomposition of palladium acetylacetonate precursors in the air led to the formation of PdO species. The influence of ZnO morphology on the metal–oxygen bonds strength in PdO and formation of active phases were observed. Strongly dispersed PdZn crystallites on ZnO supports were formed upon reduction at 350 °C. Smaller crystallites of the size equal to 6.5 nm were detected in the Pd/ZnO-AOT catalysts.

Impact of carbon on the surface and activity of silica–carbon supported copper catalysts for reduction of nitrogen oxides

Composite catalysts, prepared by one or more active components supported on a support are of interest because of the possible interaction between the catalytic components and the support materials. The supports of combined hydrophilic-hydrophobic type may influence how these materials maintain an active phase and as a result a possible cooperation between active components and the support material could occur and affects the catalytic behavior. Silica–carbon nanocomposites were prepared by sol–gel, using different in specific surface areas and porous texture carbon materials. Catalysts were obtained after copper deposition on these composites. The nanocomposites and the catalysts were characterized by nitrogen adsorption, TG, XRD, TEM- HRTEM, H2-TPR, and XPS. The nature of the carbon predetermines the composite's texture. The IEPs of carbon materials and silica is a force of composites formation and determines the respective distribution of the silica and carbon components on the surface of the composites. Copper deposition over the investigated silica–carbon composites leads to formation of active phases in which copper is in different oxidation states. The reduction of NO with CO proceeds by different paths on different catalysts due to the textural differences of the composites, maintaining different surface composition and oxidation states of copper.

Relation between composition and morphology of K(Co)MoS active phase species and their performances in hydrotreating of model FCC gasoline

K-Mo/Al2O3 and K-CoMo/Al2O3 catalysts with various amount of potassium (up to 15wt. %) were synthesized using H3PMo12O40, CoCO3 and KOH. The catalysts were characterised by the following techniques: low-temperature N2 adsorption, Raman spectroscopy, temperature-programmed reduction, temperature-programmed desorption of NH3, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy. Synthesized catalysts were tested in hydrotreating of model fluid catalytic cracking gasoline containing 1000ppm of sulphur from thiophene and 36wt. % of n-hexene-1. It was found that potassium incorporation led to the strong changes in both physical-chemical characteristics and catalytic behaviour of sulphide catalysts. Using potassium favoured the growth of average slab length and sulphidation degree of metals with selective formation of CoMoS active phase. Addition of potassium also led to the partial poisoning of the active sites what resulted in decrease of catalyst activities compared with the reference Mo/Al2O3 and CoMo/Al2O3 samples. Olefin hydrogenation was more sensitive to potassium than thiophene hydrodesulphurization what resulted in the growth of the dependence of selectivity factor on potassium content in the catalysts. Obtained results allowed us to suppose that potassium partially inserts in sulphide slabs with changing the nature of active sites and probably with formation of new type of active “KCoMoS” sites.

Influence of alumina precursor on the physico-chemical properties of V–Sb–P–W/Al2O3 catalyst studied for the ammoxidation of propane

Influence of alumina precursor on active phase formation in V1.0–Sb3.5–P0.5–W1.0/50% Al2O3 catalyst was studied for the ammoxidation of propane in the temperature range of 490–530°C at W/F=2.8gcatsml−1. Three different alumina precursors such as, aluminum chloride (AlC), aluminum hydroxide (AlH) and aluminum nitrate (AlN) were employed in the study. The physico-chemical characterisitics of the synthesized catalysts were investigated by BET surface area-pore size method, XRD, FTIR, HAADF/EDS, NH3 TPD-mass and XPS techniques. XRD result demonstrates AlPO4 phase for all the three catalysts which was likely due to P interaction with alumina precursor. Formation of propylene and acrylonitrile (ACN) in this study was influenced by the nature of the active surface on Al2O3 synthesized from different precursors. Among the catalysts, AlC exhibited greater propane conversion (76%) and ACN selectivity (28%) at 530°C. The higher activity of AlC was associated with (i) active surface enriched in V–Sb–W phase as established by XPS and HAADF/EDS results (ii) higher adsorption ability for NH3 in the presence of residual chloride (coming from AlCl3) followed by efficient nitrogen insertion in the intermediate molecule of propane to yield ACN. The surface with Sb–W–O related phase in AlN and AlH possibly contributes to the degradation of adsorbed propane/propylene and NH3 to yield methane, ethane, ethylene, COx and NOx. To our knowledge, this is the first report for the influence of alumina precursor on the active phase formation in V–Sb–P–W/Al2O3 catalyst studied for the propane ammoxidation.