Effect Of Potassium Promotion Research Articles

New insight to migration and influence of potassium element on combustion of coal/biomass char-slag interface

Co-combustion of biomass and coal can both support heat requirement of a boiler and reduce carbon emission while the high contents of alkali metals (e.g. Na, K) in ash are key factors in the stable operation. In this study, the migration behavior and influence of K on the combustion of coal/biomass char particles on the slag surface were investigated with visualization experiments and surface analytical techniques. The results showed that the addition of K in the slag can accelerate the combustion process of coal char particles on the slag surface, which proved that K+ was not deactivated as ionic state in the slag. The promotion effect of potassium was mainly reflected in accelerating the combustion rate at the beginning stage, where the carbon conversion was below 0.2, compared to the resting stage. K was proved to migrate from the slag side to the interface of char particles and slag and aggregated with Al during the combustion process, while other elements were enriched on the slag surface outside the combustion zone. Results also showed that the combustion of biomass char particles (rice straw char) on the slag surface with less K was inhibited. However, K was found to aggregate on the interface of biomass char and slag throughout the combustion process with no diffusion. A migration mechanism was proposed that the combustion of char particles on the slag surface caused an increasing temperature and further depolymerization of aluminosilicates, with alumina replacing silicon to form aluminum tetrahedra while adsorbing K+ to the interface and promote the combustion.

Effect of Potassium Promoter on the Performance of Nickel-Based Catalysts Supported on MnOx in Steam Reforming of Ethanol

The effect of a potassium promoter on the stability of and resistance to a carbon deposit formation on the Ni/MnOx catalyst under SRE conditions was studied at 420 °C for different H2O/EtOH molar ratios in the range from 4/1 to 12/1. The catalysts were prepared by the impregnation method and characterized using several techniques to study their textural, structural, and redox properties before being tested in a SRE reaction. The catalytic tests indicated that the addition of a low amount of potassium (1.6 wt.%) allows a catalyst with high stability to be obtained, which was ascribed to high resistance to carbon formation. The restriction of the amount of carbon deposits originates from the potassium presence on the Ni surface, which leads to (i) a decrease in the number of active sites available for methane decomposition and (ii) an increase in the rate of the steam gasification of carbon formed during SRE reactions.

Effect of potassium on carbon adsorption on the Co(0001) surface.

The effect of potassium on carbon adsorption and deposition on the Co(0001) surface was studied on the basis of theoretical calculations. Thermodynamically, the surface Cn species is expected, and C2 dimer may be a critical elementary unit. With the increase of carbon coverage, a fraction of the carbon atoms may diffuse into the subsurface. But kinetically, the formation of Cn species is more favorable, and there is no driving force for carbon to migrate into the subsurface. As the surface carbon concentration increases, the adsorbed carbon atoms turn into carbon chains and then into graphene sheets parallel to the surface. Potassium promoter has little effect on the most stable adsorption configurations of carbon atoms but increases the adsorption energy of carbon species, which can be explained by the decreasing of the surface work function resulting from the electron effect of potassium promoter. The potassium promotes carbon deposition and carbonization of the cobalt surface to a certain extent. These results could support some useful information for the carbon deposition and cobalt carbide formation.

A DFT Insight into the Tuning Effect of Potassium Promoter on the Formation of Carbon Atoms via Carburization Gases Dissociation on Iron-Based Catalysts

The research of the formation mechanism of iron carbides is significant to design the high-performance catalysts for the Fischer–Tropsch synthesis (FTS) process. In this paper, the effect of potassium promoter on the formation of atomic carbon via carburization gases dissociation on the iron-based catalyst, the C2H4, C2H2 and CO/H2 adsorption energies and dissociation paths as well as the rate constants of the corresponding elementary steps are investigated by DFT on the Fe(110), Fe(110)-K2O, Fe(211) and Fe(211)-K2O surfaces. The calculation results demonstrated that the K2O promoter can modify the capabilities of surface C formation via the thermodynamic method as well as the kinetical method. The K2O promoter can increase the CO adsorption energy while decreasing the C2H4 adsorption energy both on Fe(110) and Fe(211) surfaces. Kinetically, via tuning the catalyst surfaces from Fe(110) to Fe(211), the K2O promoter can inhibit the ability of C2H4/C2H2 dissociation to atomic carbon, while enhancing the ability of CO/H2 decomposition to atomic carbon. The C2H4/C2H2 dissociation rate constants on Fe(211) and Fe(211)-K2O are about 107 times slower than that on Fe(110) and Fe(110)-K2O, whereas the dissociation rate constants of CO/H2 on Fe(211) are about 106 times faster than that on Fe(110), and about 107 times faster on Fe(211)-K2O than on Fe(110)-K2O.

Effect of potassium promoter on phase transformation during H2 pretreatment of a Fe2O3 Fischer Tropsch synthesis catalyst precursor

• H 2 reduction behaviors of Fe 2 O 3 based catalysts are investigated by in situ XRD. • Potassium promoter notably inhibits the reduction from α-Fe 2 O 3 to Fe 3 O 4 . • The formation of α-Fe crystallite is accelerated or unchanged during reduction. • Potassium effect on FTS performance of the Fe 2 O 3 based catalysts is studied. Effect of potassium promoter on phase transformation during the isothermal and temperature programmed reduction of the α-Fe 2 O 3 catalyst were systematically investigated by in situ XRD. The reduction pathway of Fe 2 O 3 based catalysts follows the same sequence of α-Fe 2 O 3 → Fe 3 O 4 → α-Fe during the isothermal reduction, in contrast to that of α-Fe 2 O 3 → Fe 3 O 4 → FeO/ α-Fe→ α-Fe during the temperature programmed reduction. The potassium promoter notably inhibits the reduction from α-Fe 2 O 3 to Fe 3 O 4 due to the enhanced strength of Fe-O bonds, whereas it either accelerates or has no effect on the formation rate of α-Fe crystallites during reduction. The potassium promoter increases the CO conversion during FTS, while enhances C 5 +, olefins, CO 2 selectivity, and decreases CH 4 selectivity.

Porous Graphene-Confined Fe-K as Highly Efficient Catalyst for CO2 Direct Hydrogenation to Light Olefins.

We devised iron-based catalysts with honeycomb-structured graphene (HSG) as the support and potassium as the promoter for CO2 direct hydrogenation to light olefins (CO2-FTO). Over the optimal FeK1.5/HSG catalyst, the iron time yield of light olefins amounted to 73 μmolCO2 gFe-1 s-1 with high selectivity of 59%. No obvious deactivation occurred within 120 h on stream. The excellent catalytic performance is attributed to the confinement effect of the porous HSG on the sintering of the active sites and the promotion effect of potassium on the activation of inert CO2 and the formation of iron carbide active for CO2-FTO.

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.

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.

The promoter effect of potassium in CuO/CeO2 systems supported on carbon nanotubes and graphene for the CO-PROX reaction

Improved performance of catalysts promoted with K was obtained in the CO-PROX reaction.

Carbon Formation in Steam Reforming and Effect of Potassium Promotion

Carbon Formation in Steam Reforming and Effect of Potassium Promotion

Potassium promotion effects in carbon nanotube supported molybdenum sulfide catalysts for carbon monoxide hydrogenation

The paper focuses on the effect of potassium promotion on the structure and catalytic performance of carbon nanotube supported molybdenum sulfide catalysts for carbon monoxide hydrogenation. A combination of characterization techniques showed the presence of MoO2 and mixed K-Mo oxides in the calcined catalysts. The sulfidation of oxide phases leads to MoS2 and K-Mo sulfides. MoO2 showed somewhat lower extent of sulfidation compared to other molybdenum oxide species. MoS2 was principally responsible for CH4 production, while lighter olefins, paraffins, alcohols and higher hydrocarbons were produced on the mixed K-Mo sulfides. The catalyst basicity seems to be one of the important factors controlling the reaction selectivity; moderate basicity is essential for higher rates of olefin and alcohol synthesis.

Effect of potassium addition on a long term performance of Co–ZnO–Al2O3 catalysts in the low-temperature steam reforming of ethanol: Co-precipitation vs citrate method of catalysts synthesis

The promotion effect of potassium on Co–ZnO–Al2O3 catalysts in the steam reforming of ethanol has been studied with samples prepared by co-precipitation and citrate methods. Experimental results of catalytic activity, selectivity and coke deposition phenomena are reported. Potassium addition (1 and 2wt.%) had a significant influence on catalytic efficiency at various ethanol+water vapours streams (H2O:EtOH=21:1, 15:1, 12:1 molar ratios). It acts as a promoter improving catalyst stability, activity as well as marginalizing coke deposition on the catalyst surface. Catalysts prepared by the co-precipitation method, with smaller cobalt active phase crystallites and with potassium deposited on the catalyst surface are more stable in the steam reforming of ethanol than those with large crystallites prepared by citrate method, into which potassium was introduced together with all others components. The efficiency of the potassium promotion is higher when it is deposited on the catalyst surface with smaller cobalt active phase crystallites. The water excess equivalent to the H2O:EtOH molar ratio higher than 15:1 has been necessary for the stable SRE. Even potassium-promoted K–Co–ZnO–Al2O3 catalyst needs the ethanol–water feed with a large excess of water.

Effects of potassium promoter on the performance of PdCl 2 –CuCl 2 /AC catalysts for the synthesis of dimethyl carbonate from CO and methyl nitrite

The effect of potassium (K) promoter on the catalytic performance of activated carbon (AC) supported Wacker-type catalysts (PdCl2–CuCl2/AC) for the synthesis of dimethyl carbonate (DMC) from CO and methyl nitrite (MN) was investigated by means of N2 adsorption, H2-temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS). The experimental results showed that the space time yield of DMC on Wacker-type catalysts with different K promoters ranked in the following order: KCl > KOH > CH3COOK > K2CO3. Especially, the addition of KCl significantly improved the catalytic activities of PdCl2–CuCl2/AC catalyst for DMC synthesis from CO and MN. N2 adsorption data indicated that the addition of K promoters did not change the textural properties of Wacker-type catalysts greatly. H2-TPR and XPS results demonstrated that the existence of KCl promoted the reducibility of Cu2+ species and increased the proportion of Cu2+ species on catalyst surface, which is favorable for oxidizing Pd0 to active Pd2+. Further, the addition of KCl benefited the reactivity of PdCl2–CuCl2/AC catalyst for DMC synthesis from CO and MN.

Guidelines for optimization of catalytic activity of 3d transition metal oxide catalysts in N 2O decomposition by potassium promotion

The effect of potassium promotion on deN 2O activity of various 3d electron spinels (Mn 3O 4, Fe 3O 4, Co 3O 4) was investigated by TPSR in conjunction with parallel work function measurements. The results were interpreted in terms of a surface dipole model (K δ+ –O surf δ− ) supported by DFT molecular modelling. The substantial enhancement of deN 2O reactivity of Co 3O 4 and Mn 3O 4 upon K addition (decrease of Δ T 50% by 150 °C) was observed, whereas for Fe 3O 4 the promotional effect was unexpectedly small. The maximum deN 2O activity was found for potassium surface coverage of 2, 6, 8 K atoms/nm 2 for cobalt, iron and manganese spinels, respectively. In each case the optimal level of doping was found for the minimum of the measured work function indicating that this parameter can be use for optimization of the catalyst deN 2O reactivity.

Study on the carburizing character of iron catalysts by Temperature Programmed Surface reaction of carburization

The promotion effect of potassium has been studied with the method of Temperature Programmed Surface Carburization (TPSC) technique and H2-TPR. The process of carburization on the catalyst surface was enhanced by addition of potassium was added, so that the probability for the generated alkenes species to be further hydrogenated was increased. As a result, the percentage of the alkenes from C2 to C4 in the products was increased.

Effect of potassium promoter on precipitated iron-manganese catalyst for Fischer?Tropsch synthesis

A systematic study has been carried out to investigate the impact of potassium promoter on the performance of a precipitated iron-manganese catalyst for Fischer–Tropsch synthesis (FTS). Characterization technologies of N2 physisorption, X-ray diffraction (XRD), Mössbauer effect spectroscopy (MES) and H2 thermal gravimetric analysis (H2-TGA) were used to study the effect of potassium on the textural properties, bulk phase composition and reduction behavior. FTS reaction test was performed in a fixed bed reactor. The results of characterization showed that the addition of potassium leads to the relatively large crystallite size of α-Fe2O3 and inhibits the reduction of catalyst. The carbonization of the catalyst is enhanced with the increase in both the potassium content and the reaction temperature. A maximum in FTS and water-gas shift (WGS) activity is noted upon increasing K content (0.7 wt.% K), followed by a sharp decline in activity at the potassium level in excess of the maximum. It is found that potassium is an effective promoter to suppress the hydrogenation function of the catalyst. The selectivity to olefins is promoted and the formation of methane and light hydrocarbons is restrained with the increasing potassium level. The selectivity to oxygenates shows a rapid and monotonic decrease with the increase of potassium loading and passes through a minimum at potassium loading of 0.7 wt.%. After the point, it increases slowly with further increasing in potassium content. At the same time, increasing reaction temperature results in a monotonic decrease in the weight percent of oxygenates over the un-promoted and potassium-promoted catalysts.

Effect of potassium promoter on precipitated iron-manganese catalyst for Fischer–Tropsch synthesis

A systematic study has been carried out to investigate the impact of potassium promoter on the performance of a precipitated iron-manganese catalyst for Fischer–Tropsch synthesis (FTS). Characterization technologies of N 2 physisorption, X-ray diffraction (XRD), Mössbauer effect spectroscopy (MES) and H 2 thermal gravimetric analysis (H 2-TGA) were used to study the effect of potassium on the textural properties, bulk phase composition and reduction behavior. FTS reaction test was performed in a fixed bed reactor. The results of characterization showed that the addition of potassium leads to the relatively large crystallite size of α-Fe 2O 3 and inhibits the reduction of catalyst. The carbonization of the catalyst is enhanced with the increase in both the potassium content and the reaction temperature. A maximum in FTS and water-gas shift (WGS) activity is noted upon increasing K content (0.7 wt.% K), followed by a sharp decline in activity at the potassium level in excess of the maximum. It is found that potassium is an effective promoter to suppress the hydrogenation function of the catalyst. The selectivity to olefins is promoted and the formation of methane and light hydrocarbons is restrained with the increasing potassium level. The selectivity to oxygenates shows a rapid and monotonic decrease with the increase of potassium loading and passes through a minimum at potassium loading of 0.7 wt.%. After the point, it increases slowly with further increasing in potassium content. At the same time, increasing reaction temperature results in a monotonic decrease in the weight percent of oxygenates over the un-promoted and potassium-promoted catalysts.

Promoter Effect of Potassium on an Iron Catalyst in the Carbon Dioxide Hydrogenation Reaction

The effect of the K content on a Fe catalyst for the CO2 hydrogenation was studied. Very pronounced changes in the selectivities to C2+ hydrocarbons, as well as in the olefin ratios were observed. The addition of K favored the reoxidation of Fe during the reaction

Effect of Potassium Promotion on Iron-Based Catalysts for Fischer–Tropsch Synthesis

The effect of potassium on Fischer–Tropsch catalyst activity, kinetic parameters, and selectivity has been investigated for a precipitated iron catalyst that was employed with low H2/CO ratio synthesis gas. A wide range of synthesis gas conversions have been obtained by varying space velocities over catalysts with various potassium loadings. Differing trends in catalyst activity with potassium loading were observed depending on the space velocity of synthesis gas conversion. As potassium loading increased, the catalyst activity either decreased (low conversion), passed through a maximum (intermediate conversion), or increased (high conversion). This is shown to be a result of the increasing dependency of the Fischer–Tropsch synthesis on the hydrogen formed by the water-gas shift reaction with increasing synthesis gas conversions. Both the rate constant and the adsorption parameter in a common two-parameter Fischer–Tropsch rate expression decreased with potassium loading; therefore, observed maxima in Fischer–Tropsch rate with potassium loading can be due to the opposing influences of these parameters. The effect of potassium on alkene selectivity was dependent on the number of carbon atoms of the hydrocarbons as well as the carbon monoxide conversion level. The extent of isomerization of 1-alkene product decreased with potassium loading, while the selectivity to methane decreased only slightly with increasing potassium content at CO conversions about 50% and higher.