Negative active-mass thickness

In addition, the reduction of NOj is a very fast reaction and is controlled by external and internal diffusion [27, 30]. In contrast, the oxidation of SO2 is very slow and is controlled by the chemical kinetics [31]. Accordingly, the SCR activity is increased by increasing the catalyst external surface area (i.e. the cell density) to favor gas-solid mass transfer while the activity in the oxidation of SO2 is reduced by decreasing the volume of the catalyst (i.e. the wall thickness) this does not affect negatively the activity in NO removal because significant ammonia concentrations are confined near the external geometric surface of the catalyst. 

A decrease in current efficiency with increasing current density is observed in many cases. This can enhance the uniformity of thickness of the deposit due to a negative feedback effect. Thus, in areas on the surface where the total current density is higher, the fraction of the current consumed for metal deposition is lower. The dependence of the current efficiency on current density is governed by the kinetic parameters of the two reactions involved. Thus, in the commonly encountered situation where hydrogen evolution is activation controlled while metal deposition is partially controlled by mass transport (0.05 < j/ji) < 0.7), the current efficiency will decrease with increasing current density. 

---