Electrocatalytic activity deactivation

The activation observed in titania-supported Au electrocatalysts is unlikely to arise from electronic effects in monolayer or bilayer Au [Valden et al., 1998 Chen and Goodman, 2004], since the electrocatalytic activity was correlated with the size of three-dimensional titania-supported Au particles [Guerin et al., 2006b Hayden et al., 2007a, c]. The possibility that titania-induced electronic modification of three-dimensional particles below 6.5 nm is responsible for the induced activity, however, could not be excluded. It was pointed out, though, that such electronic effects should dominate for the smaller particle regime (<3 nm), where deactivation of the Au is observed on all supports. 

If the Ru loss in the deactivated anode is a result of uniform dissolution across the entire coating layer, resulting in a Ru loading of less than 2 g m-2, the anode has to be recoated to regain its electrocatalytic activity for the chlorine evolution reaction. Under these conditions, the existing anode coating must be stripped prior to recoating. However, if surface depletion of Ru is the cause for increased anode potential, then replenishment of these surface sites should result in the rejuvenation of the deactivated anodes. 

The study of inactive adatoms on noble (precious) metals has little impact on the practical problems of cathode activation for two reasons (i) deactivation is the more common occurrence (ii) adatoms are not stable in the absence of ions in solution where a finite level of precursors must be maintained, which in fact corresponds to the approach of in situ activation. The presence of ionic impurities in solution may pose serious technical problems. Studies of adatoms activation of Raney Ni, a material of current use in technology, can have a greater practical impact. It is interesting that the adsorption of Cd or Pb normally results in a sizable enhancement of the catalytic activity of Raney Ni [307-312]. The Tafel slope of the Raney Ni used by these authors is reported to decrease to ca. 30 mV as the catalyst is first soaked in a solution of the nitrates of the above metals [307, 308] (Fig. 15). The electrocatalytic activity is observed to increase slowly with time of adsorption as well as of polarization. 

The surface area is a function of the temperature of preparation of the oxides [169, 486]. Thus, the apparent electrocatalytic activity decreases with increasing temperature of calcination (which is usually in the range 350°-500 °C) [227, 475]. However, if the calcination is carried out at too high temperature, the electrode surface is deactivated probably as a consequence of some dehydration, and the observed Tafel slope can be very high [487], The important relationship between the acid-base properties of an oxide surface in solution and its electrocatalytic properties has been pointed out by the present author [488]. 

Nogami et al. and Augustynski et al. showed that the activity of copper electrode, deactivated during the CO2 reduction, is recovered by anodic polarization of the electrode. Periodic anodic pulses are effective to maintain the electrocatalytic activity in prolonged electrolysis of CO2 reduction. 

Another possibility for the deactivation may be derived from the water from which electrolytic solutions are prepared. Very tiny amount of organic substances, such as surface active reagent, is recently contained in water These organic substances are sometimes very hard to remove by distillation. Adsorption of surface active reagents may deteriorate electrocatalytic activity of any electrode. 

The electrocatalytic reduction of dimethylacetylenyl carbinol at an electrode activated with Raney Ni has been studied and it was found that more activity and selectivity than by the ordinary catalytic process could be achieved. In the catalytic process 150 mg KCNS per g of catalyst is sufficient to deactivate the catalyst completely however, in the electro-catalytic process a ten-fold increase in the poison led to selective poisoning and a 99% yield of dimethylvinyl carbinol was obtained. The authors suggested that an electrochemical process is superimposed on the catalytic process. 

---