Species-resolved Thermal Alkali Desorption Research Articles

Optimization of the potassium promotion of the Co|α-Al2O3 catalyst for the effective hydrogen production via ethanol steam reforming

A series of K-Co|α-Al2O3 catalysts doped with potassium in the range of 0–4 wt% were synthesized and studied in the ethanol steam reforming process. The enhancement of activity and selectivity of the Co|α-Al2O3 catalyst upon the K doping was explained by potassium impact on the reducibility of the cobalt oxide precursor and acidity of the catalyst: K promotor inhibited the cobalt reduction and substantially increased the number of strong Lewis sites. The investigations of the surface state of potassium using a species resolved thermal alkali desorption method allowed for defining optimal potassium dispersion accurately, corresponding to the concentration of 0.3 wt%. Samples with higher potassium contents due to the overloading did not exhibit any improvement of the catalyst efficiency. Presented studies revealed the key importance of the precise optimization of the K loading and dispersion on the Co|α-Al2O3 catalyst for efficient hydrogen production via the ethanol steam reforming process.

Magnesium Effect in K/Co-Mg-Mn-Al Mixed Oxide Catalyst for Direct NO Decomposition

Emission of nitric oxide represents a serious environmental problem since it contributes to the formation of acid rain and photochemical smog. Potassium-modified Co-Mn-Al mixed oxide is an effective catalyst for NO decomposition. However, there are problems related to the thermal instability of potassium species and a high content of toxic and expensive cobalt. The reported research aimed to determine whether these shortcomings can be overcome by replacing cobalt with magnesium. Therefore, a series of Co-Mg-Mn-Al mixed oxides with different Co/Mg molar ratio and promoted by various content of potassium was investigated. The catalysts were thoroughly characterized by atomic absorption spectroscopy (AAS), temperature-programmed reduction by hydrogen (TPR-H2), temperature-programmed desorption of CO2 (TPD-CO2), X-ray powder diffraction (XRD), N2 physisorption, species-resolved thermal alkali desorption (SR-TAD), and tested in direct NO decomposition with and without the addition of oxygen and water vapor. Partial substitution of magnesium for cobalt did not cause an activity decrease when the optimal molar ratio of K/Co on the normalized surface area was maintained; it means that the portion of expensive and toxic cobalt can be successfully replaced by magnesium without any decrease in catalytic activity.

Cobalt catalyst for steam reforming of ethanol–Insights into the promotional role of potassium

The promotional effect of potassium doping on the ESR performance of the CoZn|α-Al2O3 catalyst was thoroughly investigated in the context of its impact on the selectivity and stability of the catalyst, and discussed in terms of cobalt dispersion, oxidation state, cobalt-alumina interactions, catalyst acidity, and the deactivation resistance of the catalyst. The potassium slightly inhibited the cobalt reducibility (XPS, TPR, STEM/EELS), significantly increased the surface concentration and the strength of Lewis sites (FT-IR), enhancing the selectivity to hydrogen. The surface state of potassium at various stages of the catalyst evolution was examined with a species resolved thermal alkali desorption (SR-TAD) method. These studies revealed a pronounced migration of K from the cobalt nanograins towards the alumina support upon the activation and during the ESR processes. Potassium stabilizes the high dispersion of cobalt crystallites and prevents their detachment from the surface and subsequent encapsulation by the growing carbon deposit.

On the stability of alkali metal promoters in Co mixed oxides during direct NO catalytic decomposition

Co-Mn-Al mixed oxide and Co3O4 catalysts with alkali metal promoters (K, Cs) were tested for direct NO decomposition with the aim to determine their activity and stability. The catalysts were prepared by coprecipitation with subsequent impregnation by alkali metal salts and characterized by AAS, MP-AES, XRD, N2 physisorption and species-resolved thermal alkali desorption (SR-TAD). Long-term experiments of NO catalytic decomposition performed above the temperature of alkali metal desorption showed slow deactivation which has not yet been reported by other authors, since only short laboratory tests of NO catalytic decomposition have been presented. The reason for the observed deactivation was identified as volatilization of alkali metal promoters from the catalyst surface, which was revealed by SR-TAD and chemical analysis of alkali metals. Measurements conducted below the temperature of alkali metal desorption yielded an increase in NO conversion during the initial period of measurement, which can be explained by rearrangement of surface species.

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.

Studies of potassium-promoted nickel catalysts for methane steam reforming: Effect of surface potassium location

The effect of potassium addition to the Ni/Al2O3 steam reforming catalyst has been investigated on several model systems, including K/Al2O3 with various amounts of alkali promoters (1–4wt% of K2O), a model catalyst 90%NiO-10%Al2O3 promoted with potassium and a commercial catalyst. The potassium surface state and stability were investigated by means of the Species Resolved Thermal Alkali Desorption method (SR-TAD). The activity of the catalysts in the steam reforming of methane and their coking-resistance were also evaluated. The results reveal that the beneficial effect of potassium addition is strongly related to its location in the catalysts. The catalyst surface should be promoted with potassium in order to obtain high coking-resistant catalysts. Moreover, the catalyst preparation procedure should ensure a direct interaction of potassium with the Al2O3 support surface. Due to the low stability of potassium on θ-Al2O3 this phase is undesirable during the preparation of a stable steam reforming catalyst.

DIFFUSION, SEGREGATION AND DESORPTION OF POTASSIUM FROM K2Fe22O34 FERRITE

The samples of K 2 Fe 22 O 34 phase were synthesized and characterized by XRD, BET, SEM-EDX and XPS. K+ diffusion was investigated by impedance spectroscopy, whereas the surface segregation and desorption were monitored by Kelvin Probe (changes in work function) and Species Resolved Thermal Alkali Desorption, at the temperature range of 600–1000 K. The analysis of changes in work function and desorption signals ( K and K+ ) at various temperatures allowed to resolve the kinetic parameters (flux, rate constants, activation energies) of potassium diffusion, surface accumulation and desorption. Based on the obtained results, the simple, functional model of bulk and surface changes of K -β-ferrite was proposed.

Irreversible deactivation of styrene catalyst due to potassium loss—Development of antidote via mechanism pinning

Potassium loss processes involved in irreversible deactivation of the β-ferrite component of the styrene catalyst were investigated by means of Species Resolved Thermal Alkali Desorption and work function measurements in the process temperature range. Two main mechanisms of the stabilization were developed depending on the dopant location. It was found that incorporation of Cr promoter into the structure of β-ferrite slows down potassium diffusion from the bulk towards the surface, whereas the additive segregated at the basal planes (such as CeO 2), favoring the cationic state of potassium, inhibits the probability of potassium atoms to leave the K 2Fe 22O 34 surface via work function increase. From the obtained results it may be concluded that K loss can be effectively extinguished via appropriate doping of the less stable β-ferrite active phase of the iron-oxide catalyst.

Potassium surface stability and electronic promotion in K-NbN 0.9O 0.1 catalysts

Potassium promoted niobium oxynitride catalysts were investigated by species resolved thermal alkali desorption (SR-TAD) method. Energetic barriers for potassium ions and atoms desorbing from the catalyst surface were determined. They are in the range of 1.0–2.3 and 2.3–3.0 eV for K and K +, respectively, depending on the way of the promoter introduction (via nascent- or post-doping). The work function (3.0–3.8 eV) was evaluated from the Schottky energy cycle. The shifts of the Fermi edge upon K doping were confirmed by parallel XPS studies. The surface stability of the promoter and its electronic action were found to be inimical. The lowering of the work function was greater for the post-doped sample, but the stability of potassium was distinctly lower than that observed in the case of the nascent one.