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Abstract

The effect of potassium additive on the catalytic activity of nickel–molybdenum alumina-supported systems has been studied by varying the molybdenum content within 5–18 mass% MoO3, reaction temperature from 180 to 400 (500)°C, and steam to gas ratio of 0.3, 0.7, and 1. It has been established that potassium reduces the activity of one-component Mo-containing samples, while, independent of molybdenum loading, nickel promotes activity within the whole temperature range studied and extends the temperature range of catalytic activity by about 70°C to lower reaction temperatures. A symbatic or additive, or antibatic catalytic behavior was observed with NiMo-containing samples depending on the atomic Ni/Mo ratio and temperature range. Potassium, being a third component in tri-component KNiMo-containing samples, enhances the water–gas shift (WGS) activity depending on the atomic K/(Ni + Mo) ratio. The activity approaches the equilibrium conversion degree in the interval of 320–500 °C. A decrease in the specific surface area of calcined and tested samples relative to the bare support shows close values indicating that the overall dispersion of the species is not changed during the catalytic test. Close examination indicated that the sample containing K2O, NiO, and MoO3 of 4.9, 2.5, and 12.7 mass%, respectively, was found to be the most suitable catalyst for water–gas shift reaction with sulfur containing feed since it attains equilibrium conversion even at 300 °C, and at a low steam to gas ratio of 0.3 atm. This catalyst demonstrates a stable and reproducible catalytic activity as inlet gas loading is increased.

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Reaction Kinetics, Mechanisms and Catalysis
Authors: Viorel Chihaia, Karl Sohlberg, Margarita Gabrovska, Rumeana Edreva-Kardjieva, Dorel Crişan, Peter Tzvetkov, Maya Shopska, and Iskra Shtereva

Abstract

The effect of nickel content on the structure and activity of co-precipitated Ni–Al layered double hydroxides (LDHs) as catalyst precursors for CO2 removal by methanation was studied by variation of the Ni2+/Al3+ molar ratio (Ni2+/Al3+ = 3.0, 1.5 and 0.5), and of the reduction and reaction temperatures as well as of the space velocities. Powder X-ray diffraction (PXRD), H2 chemisorption, and temperature programmed reduction (TPR) techniques were applied for physicochemical characterization of the samples. It was specified that the nano-scaled dimensions of the as-synthesized samples also generate nano-metrical metallic nickel particles (PXRD). The existence of readily and hardly reducible Ni2+–O species in the studied samples (TPR), affects catalytic performance. The studied catalysts hydrogenate CO2 effectively to residual concentrations of the latter in the range of 0–10 ppm at reaction temperatures from 400 to 220 °C and space velocities between 22,000 and 3000 h−1. The variation of the CO2 methanation activity with the changes of space velocities depends on the nickel content, and reduction and reaction temperatures. After reduction at 400 and 450 °C, a sample of Ni2+/Al3+ = 3.0 has demonstrated the highest conversion degree at all the reaction temperatures and space velocities, while a catalyst of Ni2+/Al3+ = 0.5 dominated in the methanation activity after reduction within 530–600 °C. The Ni2+/Al3+ = 1.5 catalyst data take intermediate position between Ni2+/Al3+ = 3.0 and Ni2+/Al3+ = 0.5 often closer to Ni2+/Al3+ = 3.0 ones. The studied Ni–Al LDH systems are found to be promising catalyst precursors for fine CO2 removal from hydrogen-rich gas streams through the methanation reaction, depending on the technological regime of catalyst activation.

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