| Summary: | 碩士 === 東海大學 === 化學工程學系 === 92 === Ammonia sensor plays important roles in the environmental protection and medical applications. The concentration of ammonia in tap water must be controlled under 1.0 ppm, and under 0.3 ppm in environmental conservation. The detecting limit of amperometric ammonia sensor based on metal electrode can not match these requirements. The activity of amperometric ammonia sensor based on conductive polymers decays rapidly in the detecting processes. In this thesis, the properties of Pt electrodeposited on Pt(sputtering)/porous ceramic plate and Au(sputtering)/porous ceramic plate, namely SPt and SAu, were studied. The reaction activity and mechanisms of the anodic oxidation of NH3 on the preparing Pt electrodes were discussed. Using the preparing Pt/SAu and Pt/SPt sensing electrodes, the sensing properties of the ameperometric NH3 sensor in the aqueous phase were systematically investigated.
The stability of Pt/SAu or Pt/SPt prepared at various potentials were found quite different except for 0.25 V.
Using Pt(0.5C, 0.25V)/SAu or SPt as working electrode, the electrochemical activity of the anodic oxidation of NH3 on Pt(0.5C, 0.25V)/SAu was higher than that of Pt(0.5C, 0.25V)/SPt proved by the cyclic voltammetry (CV). The experimental results indicated that the peak current of anodic oxidation of 374 ppm NH3 on Pt(0.5C, 0.25V)/SAu was found to be 5.94 mA cm-2 greater than 3.03 mA cm-2 obtained on Pt(0.5C, 0.25V)/SPt. These results were deduced to the total electrochemical area and the electrochemical area of Pt(100) of Pt(0.5C, 0.25V)/SAu greater than that of Pt(0.5C, 0.25V)/SPt.
On the reaction mechanisms of the anodic oxidation of NH3 on the preparing Pt electrode, NH3 was firstly adsorbed on the surface to form Pt－NH2. Then the formation of Pt＝NH in the presence of OH- was believed to be a rate determining step (rds) of the anodic oxidation of NH3 on Pt electrode. The intermediates adsorbed on Pt electrode could be easily desorbed from the Pt surface by combining with Pt－NHx. When the potential was swept to a relative higher potential, the decrease in the peak current of cathodic reduction of PtO revealed that Pt＝NH was oxidized to Pt≡N and combined with PtO to form Pt．NOx which was a strong adsorbed species. The strong adsorbed NOx could be reduced to be NHx, which was a good leaving species, for potential lower than 0.2V. The peak current of the cathodic reduction of NO2- at 0.18 V on Pt(10C, 0.25V)/SAu was greater than that on Pt(10C, 0.25V)/SPt found from the adsorption-desorption experiments. Compared with Pt(10C, 0.25V)/SPt the fast desoprtion of NO2- species from Pt(10C, 0.25V)/SAu was due to the higher ratio of SAD(100).
The occupation of electroactive sites by the strong adsorbed species and the reconstruction of the electrode surface were believed to cause the decay of electrode activity. The surface morphologies became denser for Pt electrode after the anodic oxidation of ammonia. The reconstruction of Pt caused the increase of the grain size of Pt by 2∼3 nm, and decrease in the total electrochemical area and the area of Pt(100). When the reaction was taken in relative higher ammonia concentration, the reconstruction rate and the decay of electrochemical activity of Pt increased.
Using Pt(10C, 0.25V)/SAu stabilized by CV method as sensing electrode, the sensitivity, the response time and the detecting limit of the amperometric ammonia sensor for monitoring NH3 in 1.0M KOH aqueous solution in the concentration range of 0∼46.5 ppm were obtained to be 32.7 μA ppm-1cm-2, less than 30 s and 0.1 ppm, respectively. Furthermore, when Pt(0.5C, 0.25V)/SAu stabilized by the potentiostate method was used as sensing electrode, the sensitivity and the response time of the amperometric ammonia sensor for monitoring 0∼1.0 ppm NH3 were obtained to be 194.0 μA ppm-1cm-2 and 15 s, respectively. Using a new electrode as sensing electrode in the same concentration range, the sensitivity was obtained to be 215.0 μA ppm-1cm-2. The sensitivity of amperometric NH3 sensor based on Pt(0.5C, 0.25V)/SAu decayed 13% when the sensor was continuously used to sensing NH3 in the range of 0∼1.0 ppm for ten times.
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