Potassium Promoter Research Articles

Effects of ordered mesoporous bimodal structures of Fe/KIT-6 for CO hydrogenation activity to hydrocarbons

Highly ordered mesoporous bimodal structures of KIT-6 with average pore diameters of 3.6 and 5.4 nm originated from its different wall thickness were newly synthesized and applied for CO hydrogenation to hydrocarbons through Fischer-Tropsch Synthesis (FTS) reaction using the supported iron nanoparticles. The ordered bimodal mesopore structures of the KIT-6 with smaller than 5.4 nm in size showed a significantly enhanced FTS activity by selectively forming heterogeneous bimodal crystallite size distributions of iron nanoparticles. The positive effects of the mesoporous bimodal structures of the KIT-6 were mainly attributed to the facile formation of the spatially confined smaller iron nanoparticles, which can be strongly interacted with the structurally stable ordered mesoporous KIT-6 with the co-presence of the larger iron nanoparticles on the outer surfaces of the mesoporous bimodal KIT-6. After adding small amount of potassium promoter with 2 wt%K on the mesoporous bimodal Fe/KIT-6, a slight increase of CO conversion as well as C5+ selectivity with less extent of CO2 formation was observed due to the suppressed activity of water-gas shift reaction. However, the structurally unstable monomodal KIT-6 having a relatively larger mesopore above 7.3 nm showed a lower FTS activity by preferentially forming an inactive amorphous carbon species. A superior FTS activity on the ordered mesoporous bimodal Fe/KIT-6 was attributed to the easy formations of smaller iron carbides, where the active smaller iron carbides were strongly interacted and spatially confined inside of the highly ordered mesoporous bimodal KIT-6 surfaces.

Determination of the Deactivation Model of Iron-potassium/γ-Al2O3 Catalyst in a Fixed Bed Reactor

Catalyst activity and performance are the most important factors for selecting a catalyst in different processes. The Fischer-Tropsch synthesis is a very important synthesis that extensive action has been taken place to increase the activity of its catalysts in the recent century. Deactivation of the catalysts of the process is influenced by many factors, among which coking and sintering have come to be considered as key causes of the catalysts deactivation over time. Determination of the deactivation model for catalysts based on empirical data causes the catalyst activity and efficiency to be predictable, for this reason, the cost of construction and processing of various processes reduce significantly at industrial scale up. An Fe-K/Al2O3 catalyst was prepared by using an impregnation method in this research. Experimental data were measured in a fixed-bed tube reactor under low operating pressure (0.2 MPa) and high temperature (310 0C). By comparing the experimental data (in this study), with different models of deactivation, was obtained the best model for the deactivation of Fe-K/Al2O3 catalyst that amounts of kd, aeq and m are 0.425, 0.459 and 1, respectively. Then, the experimental data obtained from this study were evaluated with data of other references to evaluate the effectiveness of potassium promoter and alumina support on the parameters of the deactivation rate equation. It was found that to reduce the deactivation rate of iron catalyst in the presence of K promoter, it is better than the support of alumina is used at high operating pressures and low temperature.

Effect of barium and potassium promoter on Co/CeO2 catalysts in ammonia synthesis

The Co/CeO2 catalysts promoted with Ba or K were prepared to study the effect of promoter on the catalytic performance of ammonia synthesis. The results show that the presence of Ba or K promoter changes the properties of CeO2-supported Co catalysts including the surface area, the crystallite size and the morphology of CeO2, the reduction degree of cobalt species and the adsorption performance of hydrogen and nitrogen. As a consequence, the samples promoted with an appropriate amount of Ba show higher ammonia synthesis rates, while the catalysts with high Ba loading or K promoter all exhibit low catalytic activities.

Designing, optimization and performance evaluation of the K-Zn0.4Co2.6O4|α-Al2O3|cordierite catalyst for low-temperature N2O decomposition

A series of K-Zn0.4Co2.6O4|α-Al2O3|cordierite monolith was developed and optimized with respect to the active phase loading and concentration of potassium promoter. The catalysts were thoroughly characterized by XRD, μRS, XRF, SEM/TEM and their reactivity was evaluated in low-temperature N2O decomposition in 5% N2O and in a model mixture of gases containing typical contaminates for nitric acid plants (1.5% N2O, 1% NOx, 1% H2O, 2.0% O2) in TPSR and isothermal stepwise modes. The optimized catalysts exhibit high performance (X > 90% at T < 400 °C) due to favorable control of the active phase promotion (bulk and surface), morphology (~30 nm grains) and dispersion over the α-Al2O3 micrograins that constitute a compact washcoat.

Effect of Potassium Promotion on the Formation of a Fe/Cact Catalyst in the Course of Reduction in CO and CO/H2

The process of formation of a Fe/C catalyst was considered as a sequence of topochemical reactions. It was demonstrated that potassium promotion accelerated the formation of magnetite at the stage of catalyst calcination in a flow of argon. The particle size of iron oxides decreased with the concentration of potassium. Potassium promotion also led to an increase in the concentration of Hagg carbide in the course of catalyst reduction by synthesis gas.

Understanding of potassium promoter effects on oxychlorination of ethylene by operando spatial-time resolved UV–vis–NIR spectrometry

Obtaining the comprehensive kinetics of elementary reaction steps in catalytic redox cycles remains a challenge. We report here an operando study by combined mass and UV–vis–NIR spectrometry to obtain kinetic data for reduction and oxidation steps in the catalytic cycle of ethylene oxychlorination on potassium-promoted CuCl2/Al2O3 catalysts and understand the catalytic consequences of potassium promotion for ethylene oxychlorination through a comprehensive kinetic analysis of the catalytic cycle. The results demonstrate that the Cu2+ and Cu1+ concentration, the number of active sites, and the activity of the active sites are highly dynamic, depending on the catalyst composition and the operating conditions. A pseudo-steady-state approach was applied to deconvolute the effect of promoters on the oxychlorination reaction into the effects on the individual reduction and oxidation steps, as well as their consequences in steady state in terms of the active site number and the site activity. It has been found that the rate of oxidation of Cu1+ by oxygen is much lower than the rate of reduction of Cu2+ by ethylene at relatively high Cu2+ content. This resulted in CuCl dominating at the surface and thus a low rate in the steady state on the neat CuCl2 catalyst. Potassium promotion increased the reducible Cu2+ content from 0.61mol/molCu of the neat CuCl2 catalyst to 0.8mol/molCu by reducing the interaction between Cu and the alumina support. Further increasing the potassium loading from 0.92 to 1.54wt.% decreases the reducible Cu2+ from about 0.8 to 0.7mol/molCu. Potassium promotion significantly increased the site activity of the Cu1+ oxidation, but decreased the site activity of the Cu2+ reduction by ethylene. The positive effect of potassium promotion is due to the increase in the number of Cu2+ active sites, which was more significant than the negative impact on turnover frequency (TOF); thus, potassium promotion increased the steady-state reaction rate. In addition, owing to the kinetic balance between the higher Cu1+ oxidation rate and the lower Cu2+ reduction rate, potassium promotion yielded an increased Cu oxidation state with Cu2+ dominating the steady-state catalyst surface. This led to remarkably improved stability of CuCl2 catalysts.

Fischer–Tropsch Synthesis: XANES Spectra of Potassium in Promoted Precipitated Iron Catalysts as a Function of Time On-stream

XANES K-edge spectra of potassium promoter in precipitated Fe catalysts were acquired following activation by carburization in CO and as a function of time on-stream during the course of a Fischer–Tropsch synthesis run for a 100Fe:2K catalyst by withdrawing catalysts, sealed in wax product, for analysis. CO-activated and end-of-run spectra of the catalyst were also obtained for a 100Fe:5K catalyst. Peaks representing electronic transitions and multiple scattering were observed and resembled reference spectra for potassium carbonate or potassium formate. The shift in the multiple scattering peak to higher energy was consistent with sintering of potassium promoter during the course of the reaction test. The catalyst, however, retained its carbidic state, as demonstrated by XANES and EXAFS spectra at the iron K-edge, suggesting that sintering of potassium did not adversely affect the carburization rate, which is important for preventing iron carbides from oxidizing. The method serves a starting point for developing better understanding of the chemical state and changes in structure occurring with alkali promoter.

Promotion effects of potassium on the activity and selectivity of Pt/zeolite catalysts for reverse water gas shift reaction

Catalytic hydrogenation of carbon dioxide to CO by reverse water gas shift reaction plays a vital role in the recycling of the abundant carbon source. Here, the reaction over a series of zeolite L-supported platinum catalysts was systematically investigated to study the promoting role of potassium. The results of activity tests showed that controlled addition of potassium promoter to Pt/L catalyst was beneficial for the catalytic activity and selectivity. It was observed that cage encapsulation by zeolite pores could stabilize Pt nano-particles even at severe reaction conditions. Furthermore, X-ray absorption near edge structure and X-ray photoelectron spectra results demonstrated that the chemical state of platinum was altered by the additional potassium promoter, providing evidence for the formation of Pt-O(OH)-K interfacial sites. Thereafter, the Pt-O(OH)-K interfaces, acting as main active sites to adsorb CO2 and produce bridge-bonded formate intermediates, were identified by a set of in-situ diffuse reflectance Fourier transform infrared spectroscopy and microcalorimetry measurements. This finding provides new insights into the use of a variety of zeolite as carriers for highly dispersed noble metal in catalytic CO2 reduction.

Effect of potassium promotion on the structure and performance of alumina supported carburized molybdenum catalysts for Fischer-Tropsch synthesis

Fischer-Tropsch synthesis produces light olefins and long chain hydrocarbons from syngas generated from fossil and renewable feedstocks. Molybdenum carbide exhibits catalytic properties approaching those of metal catalysts in a number of hydrogenation reactions. The present paper focuses on the effect of promotion with potassium on the structure and performance of alumina supported carburized molybdenum catalysts in Fischer-Tropsch synthesis. Molybdenum carbide catalysts were synthesized by temperature programmed carburization in methane and hydrogen. Molybdenum was highly dispersed in both calcined and carburized alumina supported catalysts. Mixed oxides of potassium, molybdenum and aluminium were uncovered in the calcined promoted catalysts. These oxides are converted in molybdenum carbide species on their carburization. Potassium promotion strengthens interaction between molybdenum and aluminium. The potassium promoted catalysts exhibited higher selectivity to light olefins and long chain hydrocarbons but lower Fischer-Tropsch reaction rate compared to the unpromoted counterpart. The observed lower Fischer-Tropsch reaction rate on the promoted catalysts was attributed to more difficult carburization of the molybdenum species and electronic effects due to the presence of potassium. The light olefin and C5+ selectivities decrease at higher carbon monoxide conversion probably because of secondary olefin hydrogenation and hydrocarbon hydrogenolysis.

Deactivation study of carbon-supported ruthenium catalyst with potassium promoter

Carbon-supported Ru catalyst with alkali promoter is arguably the most significant advancement in ammonia synthesis. However, the insufficient stability of this type of catalysts greatly limits their applications, and carbon methanation is often considered as the reason for deactivation. Here we report that there is no clear correlation between carbon methanation and the loss of activity. Rather, carbon oxidation at high temperature, especially via the formation of carbon monoxide, was found to have a strong influence on the activity and stability of Ru catalysts. As a result, there is a significant increase in carbon loss, leading to the sintering of Ru particles and thereby altered hydrogen adsorption. Consequently, catalytic activity and stability of Ru/C catalysts are reduced. This finding contributes to the development of strategies to design carbon-supported metal catalysts with high activity and stability.

Optimization of cesium and potassium promoter loading in alkali-doped Zn0.4Co2.6O4|Al2O3 catalysts for N2O abatement

A series of potassium or cesium doped Zn0.4Co2.6O4|Al2O3 catalysts with different alkali loadings were prepared, characterized with respect to chemical composition (XRF), structure (XRD, RS) morphology (TEM), and the alkali promoter thermal stability. A strong beneficial effect on the deN2O activity of the Zn0.4Co2.6O4|Al2O3 catalyst (decrease in the T50% by about 80 °C) was observed for both promoters at different surface coverages. It was found that in comparison to a rather narrow range of optimal cesium loading (0.5–2 atoms/nm2) a comparable promotional effect of potassium doping was observed for a slightly wider surface concentrations (0.5–3 atoms/nm2). Such difference was attributed to surface dispersion of potassium over the alumina support and the spinel active phase, while cesium was found to be located mainly on the spinel phase. For practical applications, the superiority of potassium over cesium consist in fact that a similar beneficial effect is associated with much higher thermal stability in the temperature range of the catalyst deN2O operation and lower price of the promoter precursor.

In Situ XRD Study on Promotional Effect of Potassium on Carburization of Spray‐dried Precipitated Fe2O3 Catalysts

AbstractA study of the promotional effect of potassium on the carburization behavior of a series of spray‐dried precipitated Fe2O3 catalysts (100 Fe, 100 Fe/0.52 K, 100 Fe/1.54 K, and 100 Fe/2.4 K) has been performed by using in situ XRD. The average crystallite size evolution for species such as α‐Fe2O3, Fe3O4, χ‐Fe5C2, and θ‐Fe3C were followed. The potassium promoter clearly inhibits the reduction of α‐Fe2O3 to Fe3O4, where the decreasing binding energy in the Fe 2p3/2, O 1s and K 2p3/2 spectra from X‐ray photoelectron spectroscopy (XPS) suggests an electron density increase in Fe and O upon potassium promotion, leading to the enhanced covalency in the Fe−O bond. In terms of crystallite size during carburization, an optimum potassium loading exists in catalyst 100 Fe/0.52 K, which shows the fastest reduction to Fe3O4 with minimum crystallite sizes of around 7 to 15 nm. Potassium has no clear effect in determining the final crystallite size of χ‐Fe5C2. An arch‐shaped curve in the evolution of crystallite size of the Fe3O4 intermediate was observed, which can be explained by the activation energy difference between the bidirectional steps of the outward oxygen diffusion and the inward carbon diffusion.

Potassium promotion on CO hydrogenation on the χ-Fe5C2(111) surface with carbon vacancy

The mechanisms of CO hydrogenation with the formation of C1 surface species on the clean and K2O-doped χ-Fe5C2(111) surfaces with carbon vacancies have been comparatively elucidated. On both surfaces, CO direct dissociation (CO→C+O) is not favored. On the clean surface, the preferred H-assisted pathway is CO→HCO→CH2O→CH3O→CH3OH. On the K2O-doped surface, the preferred paths are HCO→CHOH→CH+OH and HCO→CH2O→CH2OH→CH2+OH due to the barrier decrease of the CO bond cleavage of COH, CHOH and CH2OH by K2O promotion. These results might explain the K2O promoted carburization process and the formation of long-chain hydrocarbons.

Thermal stability and repartition of potassium promoter between the support and active phase in the K-Co2.6Zn0.4O4|α-Al2O3 catalyst for N2O decomposition: Crucial role of activation temperature on catalytic performance

The effect of calcination temperature of potassium promoted cobalt spinel catalyst supported on α-alumina extrudates was investigated for understanding its diverge role in N2O decomposition. The catalyst were thoroughly characterized by XRD, μRS, IR, XPS, X-ray μ-tomography, SEM, STEM-EDX techniques. Potassium stability and surface location were evaluated by species resolved thermal alkali desorption (SR-TAD). It was found that the potassium promoter is unevenly distributed between the spinel active phase and the support, due to different energetic stability of 2.4-2.6eV and 1.7eV, respectively. Strong beneficial influence of the K-dopant on the catalyst activity, observed for the catalysts calcined at 500°C, substantially attenuated upon increasing the calcination temperature above 600°C. It was accounted for by the migration of potassium from the support to the spinel nanocrystals (below 500°C) and subsequent formation of potassium cobaltate overlayer (above 600°C). Thus while the support is acting as a potassium reservoir, the spinel active phase acts as a potassium sink. The presence of the cobaltate shell around the spinel nanocrystallites was revealed by FTIR and SR-TAD techniques, and corroborated by STEM-EDX elemental mapping. The results point out that in the case of the supported cobalt spinel catalysts promoted by alkali careful selection of the proper activation conditions is crucial for their optimal performance.

Influence of copper and potassium on the structure and carbidisation of supported iron catalysts for Fischer–Tropsch synthesis

In silica supported iron Fischer–Tropsch catalysts, promotion with copper strongly enhances both hematite reduction and magnetite carbidisation, while potassium promotion hinders reduction of hematite to magnetite but enhances magnetite carbidisation in carbon monoxide.

How to Efficiently Promote Transition Metal Oxides by Alkali Towards Catalytic Soot Oxidation

The effect of surface and bulk potassium promotion on the transition metal oxides (Mn, Fe, Co) catalytic activity in catalytic soot oxidation was investigated. The surface promotion was obtained via incipient wetness impregnation, whereas the bulk promotion—nanostructuration—was obtained via wet chemical synthesis or solid state reaction leading to mixed oxide materials. The introduction of potassium into the transition metal oxide matrix results in the formation of tunneled or layered structures, which enable high potassium mobility. The structure of the obtained materials was verified by powder X-ray diffraction and Raman spectroscopy and the catalytic activity in soot oxidation was determined by TPO-QMS and TGA. It was found that the surface promotion does not alter the metal oxide structure and leads to either enhancement (Mn, Fe) or deterioration (Co) of the catalytic activity. Simultaneously, the catalytic activity in soot combustion of the potassium structured oxides is strongly enhanced. The most active, cobalt oxide based catalyst significantly lowers the temperature of 50 % of soot conversion by ~350 °C comparing to the non-catalyzed process. The study allows for the establishment of rational guidelines for designing robust materials for DPF applications based on ternary metal oxides.

Fabrication of K-promoted iron/carbon nanotubes composite catalysts for the Fischer–Tropsch synthesis of lower olefins

K-promoted iron/carbon nanotubes composite (i.e., FeK-OX) was prepared by a redox reaction between carbon nanotubes and K2FeO4 followed by thermal treatments on a purpose as the Fischer–Tropsch catalyst for the direct conversion of syngas to lower olefins. Its catalytic behaviors were compared with those of the other two Fe-IM and FeK-IM catalysts prepared by impregnation method followed by thermal treatments. The novel FeK-OX composite catalyst is found to exhibit higher hydrocarbon selectivity, lower olefins selectivity and chain growth probability as well as better stability. The catalyst structure-performance relationship has been established using multiple techniques including XRD, Raman, TEM and EDS elemental mapping. In addition, effects of additional potassium into the FeK-OX composite catalyst on the FTO performance were also investigated and discussed. Additional potassium promoters further endow the catalysts with higher yield of lower olefins. These results demonstrated that the introduction method of promoters and iron species plays a crucial role in the design and fabrication of highly active, selective and stable iron-based composite catalysts for the FTO reaction.

The role of potassium promoter in isobutanol synthesis over Zn–Cr based catalysts

The potassium promoter would tailor the microstructure of the Zn–Cr spinel and stabilize the surface hydroxyl species which greatly facilitate the formate of isobutanol.

Fischer–Tropsch synthesis on potassium-modified Fe3O4 nanoparticles

Synthesized magnetite (Fe3O4) nanoparticles, with an average crystallite size of 8 and 22 nm, were investigated for Fischer–Tropsch synthesis reaction to clarify the effects of iron crystallite size and a potassium promoter. The larger crystallite size of Fe3O4 without the potassium promoter showed an increased CO conversion with a lower selectivity of C5+ and olefinic hydrocarbons. This was mainly attributed to the enhanced reduction degree and an abundance of active edge sites of cube-like morphology. However, a modification of Fe3O4 nanoparticles with the potassium promoter significantly changed the product distribution by increasing C5+ and olefinic hydrocarbons with much higher CO conversions. The enhanced activity on the large crystallite size of K-modified Fe3O4 was mainly attributed to an enhanced secondary reaction of olefins formed on the active iron carbide sites. The positive effect of the potassium promoter was much significant on the larger Fe3O4 nanoparticles due to an abundant presence of potassium promoter on the outer surface of the large Fe3O4 nanoparticles with a bigger inter-particular pore diameter for an easy mass transport.

Carbon Formation in Steam Reforming and Effect of Potassium Promotion

Carbon Formation in Steam Reforming and Effect of Potassium Promotion