Effect Of Alkali Promotion Research Articles

Effect of alkali promoters (Li, Na, K) on the performance of Ru/Al2O3 catalysts for CO2 capture and hydrogenation to methane

The impact of alkali promotion on the CO2 hydrogenation activity and selectivity over 1 %Ru/Al2O3 catalyst was investigated varying the type of metal (Li, Na, K) and precursor salt (carbonates vs. nitrates) while keeping constant the equivalent weight load. Catalysts prepared by sequential impregnations of Ru and alkali precursors on highly resistant γ-Al2O3 spheres (1 mm) were characterized by XRD, BET, H2 chemisorption, PSD, FT-IR, CO2-TPD, H2-TPRx and CO2 catalytic hydrogenation tests at atmospheric pressure under continuous flow conditions in the temperature range 200–430 °C. It was found that both CO2 capture capacity and methanation activity of Ru/Al2O3 catalyst were simultaneously increased only via doping with Li (nitrate), which largely reacted with alumina to form a mixed spinel phase. Therefore, Li-Ru/Al2O3 catalyst was successfully tested as a dual functional material operating the cyclic chemical looping process of CO2-capture followed by its methanation under alternating flow conditions at a fixed temperature as low as 230 °C.

Effects of alkali promoters on tri-metallic Co-Ni-Cu-based perovskite catalyst for higher alcohol synthesis from syngas

• The addition of alkali promotes the formation of Co 2 C during reaction. • High Co 2 C/Co ratio enhances the C 2+ OH distribution but reduces CO conversion. • High alkali basicity stabilizes the catalyst by eliminating the carbon deposition. • K-promoted Co-Ni-Cu based perovskite catalyst resulted in excellent C 2+ OH formation. Direct conversion of natural gas-derived syngas into higher alcohols via the Fischer-Tropsch process offers a more sustainable pathway to address fuels and chemicals demands for various industries in the near future. Large scale application of this process nonetheless relies on the availability of a highly active and selective catalyst. In this work, tri-metallic Co-Ni-Cu catalyst derived from La 0.9 Sr 0.1 Co 0.8 Ni 0.1 Cu 0.1 O 3 perovskite precursor with two different alkali promoters were prepared and tested for higher alcohol synthesis from syngas. Active components and catalyst performances were found to vary with same weight loading of Na and K. K-promoted catalyst demonstrated the best higher alcohol formation with the higher alcohol distribution of 82.2% and alcohol chain growth probability factor of 0.53 at 325 °C, followed by the Na-promoted and non-promoted counterparts. The rationale behind the alkali addition—performance relationship is two-fold. On one hand, the addition of alkali promotes the formation of stable Co 2 C; resulting in high Co 2 C/Co ratio that enhances the higher alcohol distribution but decreases the CO conversion. On the other hand, the increased catalyst basicity obtained from alkali promotion facilitates the carbon chain propagation of alcohols, especially for the conversion of methanol to ethanol. In addition, the alkali promoters, especially K, prevent the deactivation of catalyst Co-Ni-Cu by eliminating the carbon deposition during the reaction; resulting in a stable catalyst even at 340 °C.

Effect of the promoter presence in catalysts on the compositions of Fischer–Tropsch synthesis products

The effects of alkali promoters namely potassium, copper, and manganese, on the performance of the bi-functional iron–low acidity ZSM-5 catalysts in the Fischer–Tropsch (FT) synthesis were studied. ZSM-5 with very high silica alumina ratio (SAR) of 280 was used to take advantage of its shape selectivity and suppressing the effect of alkali migration on zeolite cation exchange sites. One bi-functional iron base catalyst (FeZ), three single promoted (KFeZ, CuFeZ, MnFeZ) and four multiple promoted (CuKFeZ, CuMnFeZ, KMnFeZ, CuKMnFeZ) catalysts were synthesized by the incipient to wetness impregnation method. They were characterized by XRD, BET, TPR, TEM and TPD analyses methods and tested for their FT synthesis activities.Promoters had considerable impacts on the compositions and the hydrocarbon distributions of the FT synthesis products. Addition of promoters resulted in an increase of up to 8.5–56% and 20–743% in the synthesis of gasoline and diesel range hydrocarbons, respectively. Consequently, the production of the total liquid fuel (gasoline+diesel) also increased from 48% (base catalyst) to 64–79% (promoted catalysts). The highest gasoline range hydrocarbon production was obtained from MnFeZ and CuKMnFeZ (≈70%) while CuKMnFeZ produced a FT product having the highest total liquid fuel (≈79%).

Effects of alkali promoters on the textural and catalytic properties of mesoporous Fe–Al–Cu catalysts for water gas shift reaction

ABSTRACTMesoporous Fe2O3–Al2O3–CuO catalysts promoted with alkali oxides were synthesized and used in water gas shift reaction (WGSR) at high temperatures for hydrogen purification. These chromium-free catalysts were characterized using nitrogen adsorption/desorption, hydrogen temperature programmed reduction, X-ray diffraction (XRD), and transmission electron microscopy techniques. The synthesized catalysts with narrow single-modal pore size distribution in mesopore region possessed high specific surface area. The catalytic results revealed that except Cs, the addition of other alkali promoters declined the catalytic activity. However, all catalysts showed higher catalytic performance than the conventional commercial catalyst. The results showed an optimum content of Cs promoter (3 wt.%) for the promoted Fe–Al–Cu catalyst (3 wt.% Cs-FAC), which exhibited the highest activity in WGSR at high temperature.

The effect of alkali metals on the synthesis of methanethiol from CO/H2/H2S mixtures on the SBA-15 supported Mo-based catalysts

The effects of alkali promoters on catalytic behaviors for synthesizing CH3SH over Mo-based catalysts were investigated. The selectivity of CH3SH for Na, K, Cs and non-alkali promoted catalysts are following in the order of Cs-Mo/SBA–15> K-Mo/SBA–15> Na-Mo/SBA–15> Mo/SBA-15. The addition of alkali metals dramatically improved the CH3SH selectivity and CO conversion. The highest selectivity of CH3SH was 66% obtained by incorporating Cs into Mo/SBA-15 which was superior to those reported in the current literature. Based on N2 adsorption-desorption, XRD, Raman, XPS and NO-TPD characterizations, active MoS2 species on oxidized Mo O-S species, obtained from the sulfidation temperature as low as 553K, was considered as active sites for synthesizing CH3SH. And it deemed that the different alkali metals (Na, K, Cs) could give rise to the difference in the sulfidation degree of Mo oxidized species and modify the chemical and electronic environment of the active sites, which resulted in the generation of distinguished concentration of Mo containing coordinatively unsaturated sites (Mo-CUS). Those concentrations of Mo-CUS were detected by NO-TPD to be 161, 210 and 222μmol/g, respectively, which had a positive relationship with the selectivity of CH3SH.

Promoter effect of alkalis on CuO/CeO2/carbon nanotubes systems for the PROx reaction

The effect of alkali promotion (Li, Na, K and Cs) on the CO preferential oxidation (CO PROX) reaction has been studied over Cu-CeO2 catalyst supported on carbon nanotubes (CNT). The catalysts were prepared with 2.5wt.% Cu and 20wt.% CeO2 loadings, and alkali/Cu atomic ratios of 0.68. The catalytic performance and the characterization by powder X-ray diffraction (XRD), TEM-STEM, H2-temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) has been presented. TEM and XRD analyses showed that the addition of alkali reduces the size of ceria crystallites. It was found that in general the incorporation of alkali favored the conversion of CO at low temperatures, being more pronounced for the K-doped catalyst. The characterization showed that the addition of K favored the Cu-CeO2 interaction, the activity of the catalyst was enhanced and the opposite was observed for Cs-doped sample. The characterization results reveal that the observed increase in the Cu+ species proportion and in lattice oxygen are related to the better catalytic performance.

Effect of Alkali Promoters (K) on Nitrous Oxide Abatement Over Ir/Al2O3 Catalysts

The promoting impact of potassium (0–1 wt% K) on nitrous oxide (N2O) catalytic decomposition over Ir/Al2O3 is investigated under both oxygen deficient and oxygen excess conditions. All samples were characterized by means of X-ray powder diffraction (XRD), temperature-programmed reduction (H2-TPR), ammonia desorption (NH3-TPD) and Fourier Transform Infrared Spectroscopy of pyridine adsorption (FTIR-Pyridine). The results reveal that the K-free Ir/Al2O3 catalyst consists mainly of the IrO2 phase, exhibiting also significant Lewis acidity, which is gradually eliminated by the addition of K. Catalytic performance results showed that the deN2O performance in the absence of O2 in the feed mixture is negatively affected upon increasing potassium loading. However, under oxygen excess conditions, a pronounced effect of K is observed. Although the catalytic performance of the un-doped catalyst is drastically hindered by the presence of O2, the K-promotion notably prohibits the oxygen poisoning. The optimum deN2O performance under oxygen excess conditions is obtained with potassium loading of 0.5 wt% K, which offers complete conversion of N2O at 580 °C, instead of the corresponding 50 % N2O conversion achieved with the un-modified sample. On the basis of characterization results, it was concluded that alkali-doping in combination with oxygen excess conditions are required towards the formation of active Ir entities.

Study on the effect of alkali promoters on the formation of cobalt carbide (Co2C) and on the performance of Co2C via CO hydrogenation reaction

The present investigation deals with an alkali metal assisted synthesis of cobalt carbide (Co2C), starting from carburizing reduced complex precursors (obtained from Co3O4 with the addition of Li2O, Na2O and K2O) using CO as a carburization source. It is found that the Co2C formation could be significantly accelerated by the Li component. A comparative study reveals that the promotion effect of Li component may be related to the H2 adsorption on the reduced precursor, which enhanced the ability of the precursor to react with CO. Furthermore, the Co2C prepared from the precursor containing Li component shows a considerable increase in activity for CO hydrogenation and an improved fraction of higher alcohols in the total alcohol products, compared to the Co2C prepared without promoters. An attempt is made to elucidate the catalytic behavior of the as-prepared Co2C samples.

Strong electronic promotion of Co3O4 towards N2O decomposition by surface alkali dopants

The effect of alkali promoters on N2O decomposition in dry and wet (1% H2O) conditions over Co3O4 was investigated. The beneficial influence of dopants on the catalyst activity increases in the order: Li ≪ Na < K < Cs. The N2O reactivity was complemented by the work function measurements by Kelvin method. The strong promotional effect for sodium, potassium and cesium was found to be of electronic nature and was discussed in terms of lowering of the spinel work function caused by the surface alkali adspecies.

Methanation on K+-modified Pt/SiO2: the impact of reaction conditions on the effective role of the promotor

This paper reports on the first study that the authors know of of the effect of alkali promotion of Pt on methanation. Methanation was investigated on a 4.5 wt% Pt/SiO2 catalyst promoted with different amounts of K+ (K+/Pt=0, 0.1, and 0.2) for two different temperature ranges (503-552 K and 573-665 K). The methanation rate was 10-70% lower on the promoted catalysts for reaction temperatures of 573 to 665 K. In this temperature range, the relative decrease in rate upon promotion was a function of K+ loading and did not vary with temperature, \(p_{H_2 } ,p_{{\text{CO}}} \), or time-on-stream. In addition, there was no significant effect of K+-promotion on activation energy (ca. 29 kcal/mol) or methanation reaction orders with respect to CO and H2 (-0.1-0.0 and 0.4-0.6, respectively). However, there was a decrease in the number of methane-destined surface intermediates upon promotion as determined by steady-state isotopic transient kinetic analysis (SSITKA). All these observations lead to the conclusion that, in this higher reaction temperature range, K+ acts mainly as a site-blocking agent for methanation on Pt and does not change the reaction rate of the limiting step, probably hydrogenation. Between 503 and 552 K, the activation energy and reaction orders with respect to H2 and CO were also not affected by K+. However, the catalyst with a K+/Pt ratio of 0.1 showed the highest methanation activity. In this lower temperature range and for all the catalysts, the apparent activation energies were also found to be lower, 18 vs. 29 kcal/mol, compared to those at higher temperatures. The reaction order with respect to CO was higher (0.2-0.3) in comparison with what was observed in the higher temperature range (ca. -0.1-0.0). These results suggest, that, in the low temperature range and for low loadings of K+, K+ affects the rate-determining step resulting in a rate increase greater than the decrease due to the blockage effect. Thus, K+ serves as a rate promoter at low reaction temperatures while its only effective function is site blockage at higher temperatures.

The Effect of Alkali Promoters on Cu-Na-ZSM-5 Catalysts in the Oxidation of Benzyl Alcohol

The oxidation activities of alkali-added Cu-Na-ZSM-5 zeolites in the gas-phase catalytic oxidation of benzyl alcohol were studied. The main products were benzaldehyde and carbon oxides (carbon dioxide + carbon monoxide). The partial oxidation activity was found to markedly increase when an alkali-metal-added Cu-Na-ZSM-5 was used, in comparison with that obtained when the Cu-Na-ZSM-5 zeolite itself was used. On the other hand, the increase in yield of deep oxidation products (CO2 + CO) catalyzed by the alkali-added Cu-Na-ZSM-5 was rather low. The alkali added to the Cu-Na-ZSM-5 zeolite was thus found to selectively promote catalytic activity for the partial oxidation of benzyl alcohol. Certain types of alkali salts were used as the additive with the results that Na+, K+, and Rb+, but not Li+, had similar behavior for benzyl alcohol oxidation. An alkali oxide such as Na2O, rather than the alkali salt itself, was suggested to substantially function as an active promoter for the oxidation of benzyl alcohol as judged from the pretreatment condition of the alkali-added Cu-Na-ZSM-5 zeolite and its similar catalytic activity in the oxidation of alkali salts with the same alkali cation but different counteranions. The added alkali was found to act more effectively as a promoter for oxidation when alkali and Cu ions were accessible to each other; therefore the alkali was suggested to interact directly with the Cu ion to promote the oxidation of benzyl alcohol, rather than this promotion being a separate function of the alkali itself. One of the roles of the alkali added to the Cu-Na-ZSM-5 zeolite was promoting the dissociation of the oxygen molecule, as evidenced by the results of the exchange reaction of the oxygen molecule. The alkali was also suggested to neutralize the acid sites in the Cu-Na-ZSM-5 zeolite and prevent its deactivation by retarding the formation of coke-like materials on the zeolite.

Isotopic tracing study of K promotion of NH3 synthesis on Ru

There has been much discussion based on indirect evidence about the site heterogeneity of catalysts for ammonia as well as the effect of alkali promotion. This paper presents the results from direct measurements using steady-state isotopic transient kinetic analysis (SSITKA) of surface site heterogeneity on Ru/SiO2 induced by K promotion. This heterogeneity is caused by the creation of a set of super active sites. It is the high activity of these sites, more than just the increase in number of surface intermediates, which produces the high activity of K promoted Ru for ammonia synthesis.

Role of alkali promoters in K/MoS 2 catalysts for CO-H 2 reactions

The effect of alkali promoters on selectivity of CO-H 2 reactions was studied for potassium-promoted MoS 2 employing different potassium salts and pretreatment conditions (oxidized vs. fresh samples). Promoters assisted either chain growth of hydrocarbon products or alcohol formation. A good correlation was observed between p K a of the conjugate acid of each promoter and its spacetime yield of alcohol formation. Alcohol selective promoters such as K 2CO 3, KOH and K 2S readily removed their counter anions under the reaction conditions to form a new potassium complex and spread themselves uniformly over MoS 2. This complex appears to serve as an active site which adsorbs carbon monoxide molecularly and, at the same time, cover the majority of the MoS 2 surface which is responsible for dissociative carbon monoxide adsorption and hydrogenation. Promoters for chain growth such as K 2SO 4 and KCl maintained their initial chemical states throughout the reactions and showed highly nonuniform lateral distributions. Thus, the promoters have a limited coverage over MoS 2, yet modify the electronic state of MoS 2 which interacts directly with carbon monoxide. Exposure of K 2CO 3- or KOH-promoted MoS 2 to atmosphere for an extended period oxidized the catalyst and caused segregation of potassium into the bulk of MoS 2 Thus, the most of MoS 2 surface is now exposed, yet modified by potassium located in the subsurface region of MoS 2. These modified catalysts promoted hydrocarbon chain growth without forming alcohols. The results demonstrate that the distribution of promoter is one of the primary factors determining its role in catalytic CO-H 2 reactions.

13C NMR studies of CO adsorbed on supported rhodium catalysts: Effects of alkali promotion

Changes in the chemical properties of silica-supported Rh catalysts on potassium promotion are observed in the 13C NMR spectra of adsorbed CO. Catalysts prepared by incipient wetness with 2.5% Rh loading and Rh: K ratios of 1:0, 3 : 1, and 1 : 3 show relatively constant CO adsorption site character but have large changes in relative populations of linear, bridging, isolated dicarbonyl, and metal-bound CO. Back-bonded metal electrons on CO produce a Gaussian peak near 275 ppm and indicate a transition to metallic behavior of the underlying Rh at high promotion levels. Purely electrostatic effects are shown to be too small to cause the shifts reported for IR and NMR peak positions. Alkali promotion inhibits the breakup of Rh particles and formation of isolated dicarbonyl sites during CO adsorption, probably by the conversion of surface -OH groups to -OK groups.

The effect of alkali promoters in oxidative coupling of methane with Mn-oxide catalysts

The catalytic oxidalive coupling of metnane to ethylene and ethane with manganese oxide catalysts promoted with alkali metal and alkali metallic-chloride has been studied at atmospheric pressure in a fixed bed flow reactor.

The effects of Cs promotion on [formula omitted] catalysts

The adsorptive and reactive properties of a series of Rh Al 2O 3 catalysts promoted with various levels of Cs were investigated by IR spectroscopy of CO adsorption, temperature-programmed desorption of H 2, and CO hydrogenation reactions. The results indicate that a significant fraction of the added Cs interacts directly with the support rather than with the Rh crystallites. The degree of interaction between the alkali species and both metal and support sites on the catalyst depends strongly on the method of preparation of the catalyst, which may affect the chemical state of the alkali species. Trends for CO hydrogenation reaction activity and product selectivity were consistent with those reported for promoted Rh on other supports except that the level of promotion required to achieve similar results is much higher for Rh supported on Al 2O 3. These investigations demonstrate how a combination of experimental techniques may provide significantly more information about the effects of alkali promotion than would any of the methods used alone.

The effect of alkali promotion on CO hydrogenation over [formula omitted

To develop a better understanding of the link between alkali promotion of hydrocarbon synthesis and that of oxygenate synthesis, CO hydrogenation over both unpromoted and promoted Rh TiO 2 was studied in a differential reactor at 250–300 °C, 1–10 atm, and CO H 2 = 2 . The ability of the alkali species to promote the selectivity for oxygenated compounds increased in the order unpromoted < Li < K = Cs, while the overall activity decreased in the order: unpromoted > Li > K > Cs. The promoters also showed a strong hydrogenation-suppression ability which may have caused the decrease in hydrocarbon selectivity and activity. The activities and selectivities of Rh TiO 2 and alkali-promoted Rh TiO 2 appear to be correlated with their catalytic abilities for hydrogenation, CO dissociation, and CO insertion. Results for unpromoted Rh SiO 2 are also presented for Comparison purposes.