Phenomenological Kinetics of the Thermal Decomposition of Sodium Hydrogencarbonate

Abstract

Aiming to find rigorous understanding and novel features for their potential applications, the physico-geometrical kinetics of the thermal decomposition of sodium hydrogencarbonate (SHC) was investigated by focusing on the phenomenological events taking place on a single crystalline particle during the course of the reaction. The overall kinetics evaluated by systematic measurements of the kinetic rate data by thermogravimetry under carefully controlled conditions were interpreted in association with the morphological studies on the precursory reaction, mechanism of surface reaction, structure of the surface product layer, diffusion path of evolved gases, crystal growth of the solid product, and so on. The precursory reaction was identified as the decomposition of impurity, taking place at the boundary between the surface of the SHC crystal and the adhesive small SHC particles deposited on the surface. In flowing dry N(2), the thermal decomposition of SHC proceeds by two-dimensional shrinkage of the reaction interface controlled by chemical reaction with the apparent activation energy of about 100 kJ mol(-1), after rapid completion of the surface reaction and formation of porous surface product layer. Atmospheric CO(2) and water vapor influence differently on the overall kinetics of the thermal decomposition of SHC. Added gas phase of CO(2) slightly inhibits the overall rate because of the increasing contribution of the surface reaction. Under higher water vapor pressure, the physico-geometrical mechanism of the surface reaction changes drastically, indicating the preliminary reformation of reactant surface and the formation of needle crystals of solid product on the surface. The mechanistic change and extended contribution of the surface reaction result in the deceleration of the surface reaction and acceleration of the established reaction.

Effects of Particle Size on the Kinetics of Physico-geometrical Consecutive Reactions in Solid–Gas Systems: Thermal Decomposition of Potassium Hydrogen Carbonate

“…In this reaction stage, the particles were sticky and difficult to separate from one another, indicating the formation of gelatinated surface layers during IP. Similar gelation of reacting particles and contribution of the liquefied phase to the thermal decomposition process have been confirmed in thermal dehydration of several crystalline hydrates , and were expected in the thermal decomposition of NaHCO 3 . The scale-like structure on the particle surfaces resulted from the solidification of the once liquefied surface layer, while cooling to room temperature.…”

“…This effect of the reaction equilibrium was observed only during the initial part of the reaction for the atmospheric p (H 2 O), indicating that the effect of p (H 2 O) is restrictive to the SR. In either case, the effect of atmospheric conditions is limited compared to the thermal decomposition of other solids, and thus, the effects of the self-generated CO 2 and water vapor are negligible as in the thermal decomposition of sodium hydrogen carbonate. , …”

“…The thermal decomposition of alkali-metal hydrogen carbonates has emerged as a candidate based on the previously reported kinetic behaviors. The thermal decomposition of sodium and potassium hydrogen carbonates (NaHCO 3 and KHCO 3 , respectively) has been studied for preparing MHCO 3 /M 2 CO 3 sorbents for CO 2 and others. − Detailed kinetic analyses of the thermal decomposition of NaHCO 3 have been reported by different groups. − Several studies have reported a sigmoidal shape for the mass-loss curve of the thermal decomposition of NaHCO 3 under isothermal conditions and the empirical fitting of the nucleation and growth-type model known as the Johnson–Mehl–Avrami model (A­( m )) − to the experimental kinetic curves under linear nonisothermal conditions. , In contrast, kinetic fitting of the PBR model (R­( n )) to the kinetic data recorded under linear nonisothermal and constant-transformation-rate thermal analysis (CRTA) , conditions has also been reported. − However, the contribution of SR in the early stage of the reaction is evidenced by the microscopic observations in different studies. − Therefore, the apparent fitting of the A­( m ) model to experimental kinetic curves can be interpreted based on the physico-geometrical consecutive process comprising the initial acceleration period owing to SR and subsequent deceleration period owing to PBR . The fitting of the R­( n ) model showed detectable deviations in the initial part of the kinetic curves, which could be explained by the contribution of SR.…”

Impact of atmospheric water vapor on the thermal decomposition of calcium hydroxide: a universal kinetic approach to a physico-geometrical consecutive reaction in solid–gas systems under different partial pressures of product gas

“…Acceleration of the overall reaction rate at a constant temperature and significant shifts in the mass-loss curves to the lower temperatures under nonisothermal conditions at a heating rate b with increasing p(H 2 O) value in the reaction atmosphere are often observed during the thermal decomposition of several solids, in which water vapor evolves as the product. [21][22][23][24][25][26][27][28][29] Examples of these types of reactions include the thermal decomposition of Cu 2 CO 3 -(OH) 2 , [21][22][23][24] NaHCO 3 , 25,26 Zn 5 (CO 3 ) 2 (OH) 6 , 27,28 and CH 3 COOAg. 29 Conversely, the atmospheric CO 2 , which is the other gaseous product during the thermal decompositions of Cu 2 CO 3 (OH) 2 , NaHCO 3 , and Zn 5 (CO 3 ) 2 (OH) 6 , decreases the reaction rate at a temperature.…”

Universal Kinetic Description for Thermal Decomposition of Copper(II) Hydroxide over Different Water Vapor Pressures

“…5 Previous studies have reported a variety of kinetic phenomena induced by p(C(g)) effects, mainly for thermal dehydration and decarbonation reactions. The effects include deceleration interpreted as a normal effect with respect to chemical equilibrium for the reversible reaction, [6][7][8][9][10][11][12][13][14][15][16] acceleration via the catalytic effects, [17][18][19][20][21][22][23][24][25] opposite effects from two product gases on the overall rate, 21,23,26 and the fluctuation of the overall reaction rate with p(C(g)) that appears as a combined effect of deceleration and acceleration, which is typically known as the Smith − Topley effect [27][28][29] that has been observed for the thermal dehydration of crystalline hydrates. 30 The restraining effect that partial pressure of the product gas p(C(g)) has on the overall reaction rate of thermal decomposition, which is the most widely observed phenomenon, [6][7][8][9][10][11][12][13][14][15][16] has been subjected to rigorous kinetic description.…”