No abstract

No abstract

nonstoichiometry will improve the electrocatalytic behavior of RuO2 films. Unlike normal semiconductor films, those of RuO2 are generally highly conducting; the main resistance component is probably due to intergranular contact resistance in these microcrystalline layers. S. Ardizzone, 15 A. Carugati, ~5 G. Lodi, ~5 and S. Trasatti: ~5The main point of Dr. Burke's remarks, as we understand it, is that there are no reasons to expect that the electrocatalytic activity of RuO2 electrodes should be influenced by nonstoichiometry since the active surface sites at high anodic potentials are invariably Ru(VI) species. While we do not feel like agreeing on this concept in principle, we contend that Dr. Burke's criticism is not properly addressed. Figure 1 on p. 1690 and the related comment in our paper clearly point out that "cracked" and "compact" electrodes differ in the surface morphology rather than in the nonstoichiometry. Figure 3 shows that no appreciable difference is observed within each group of electrodes although the nonstoichiometry varies largely, yet a difference possibly exists between the two groups, which may, thus, be related to the surface morphology. Therefore, the message from this paper is that morphology rather than nonstoichiometry is the crucial factor in electrocataIysis at RuO2 anodes.That the surface morphology can affect the electrocatalytic properties of RuO2 has been shown in our previous work ~6 on O3 evolution on some sets of electrodes. The effect in that case is admittedly more striking, and our conclusions have been confirmed in different laboratories. ~7' ,8 It has been neatly found TM that the degree of crystallinity has a definite effect on the O3 evolution mechanism. The point of zero charge of RuO2 samples has been found ,8, 38 to depend on the temperature of preparation and to be related, as expected from theories, with the crystal parameters of the oxide. 2' All of these observations emphasize the extreme sensitivity of the nature of the active sites to the morphology of the surface.Dr. Burke contends that the degree of hydration is not expected to be important in imparting the electrocatalytic properties. It is well established, however, that a number of properties of RuO2 are closely interrelated as a function of the temperature of preparation. ~6~ 22 Thus, the residual hydration decreases as T increases and at the same time the crystallinity increases. Hydration is presumably located in grain boundaries or at "inner" surfaces (pores, etc.). As the crystallites of RuO2 grow, defect-rich regions will shrink. That is what Fig. 9 in our paper is devised to point out.In Dr. Burke's opinion, we have not paid much attention to the above aspect apparently because we have not appreciated some points he touches upon in his comments. (i) Reversibility of the C12 reaction: This is easily proved by the fact that we were able to measure the exchange current from equilibrium i-E curves [cf. footnote 23]. The effect of mass transfer on the Tafel slope has been discussed by one of us in a pr...

Periodontal regeneration is still a challenge for periodontists and tissue engineers, as it requires the simultaneous restoration of different tissues-namely, cementum, gingiva, bone, and periodontal ligament (PDL). Here, we synthetized a chitosan (CH)-based trilayer porous scaffold to achieve periodontal regeneration driven by multitissue simultaneous healing. We produced 2 porous compartments for bone and gingiva regeneration by cross-linking with genipin either medium molecular weight (MMW) or low molecular weight (LMW) CH and freeze-drying the resulting scaffolds. We synthetized a third compartment for PDL regeneration by CH electrochemical deposition; this allowed us to produce highly oriented microchannels of about 450-µm diameter intended to drive PDL fiber growth toward the dental root. In vitro characterization showed rapid equilibrium water content for MMW-CH and LMW-CH compartments (equilibrium water content after 5 min >85%). The MMW-CH compartment degraded more slowly and provided significantly more resistance to compression (28% ± 1% of weight loss at 4 wk; compression modulus H = 18 ± 6 kPa) than the LMW-CH compartment (34% ± 1%; 7.7 ± 0.8 kPa) as required to match the physiologic healing rates of bone and gingiva and their mechanical properties. More than 90% of all human primary periodontal cell populations tested on the corresponding compartment survived during cytocompatibility tests, showing active cell metabolism in the alkaline phosphatase and collagen deposition assays. In vivo tests showed high biocompatibility in wild-type mice, tissue ingrowth, and vascularization within the scaffold. Using the periodontal ectopic model in nude mice, we preseeded scaffold compartments with human gingival fibroblasts, osteoblasts, and PDL fibroblasts and found a dense mineralized matrix within the MMW-CH region, with weakly mineralized deposits at the dentin interface. Together, these results support this resorbable trilayer scaffold as a promising candidate for periodontal regeneration.

No abstract

In the frame of study on the electrochemical incineration of organic pollutants, the reactivity of glucose toward mineralization was studied under different electrolysis conditions. The process was followed at Pt, SnO2‐normalPt composite, and PbO2 electrodes, at different current densities and temperatures. In all cases, the supporting electrolyte was 2NH2SO4 . Chemical oxygen demand and total organic carbon content of the solutions and amount of oxygen evolved were measured as functions of the electrolysis time. From these data, an initial electrochemical oxidation index (EOI) was evaluated following standard methods. Larger values for this parameter were found at the PbO2 electrodes under all conditions of current density and temperature. At Pt and SnO2‐normalPt , in the room‐temperature range, the efficiency of the electrochemical mineralization of glucose was low, particularly at longer electrolysis times. The situation improved by increasing the temperature to 56°C. The extent of mineralization was quite low at SnO2‐normalPt electrodes under all conditions explored. At Pt and SnO2‐normalPt electrodes, the main oxidation intermediate was glucaric acid, apparently quite stable toward further attack at these electrodes. In the case of PbO2 electrodes, smaller concentrations of intermediates were detected. Gluconic and 2‐ketogluconic acids were also present in amounts comparable with that of glucaric acid. Study of the initial electrochemical oxidation index for gluconic acid and glucaric acid was also carried out, confirming the stability of the latter at Pt and SnO2‐normalPt electrodes. At PbO2 , on the contrary, it was found to be even more reactive than glucose. An explanation for the reactivity of carboxylic acids toward mineralization at the PbO2 electrodes is proposed. © 1999 The Electrochemical Society. All rights reserved.

No abstract

No abstract