本研究主要探討鉑奈米顆粒的製備與其對電催化氧氣還原反應特性的影響,以鹽類還原法來製備鉑奈米顆粒,選擇氯鉑(Ⅳ)酸做為前驅物,尋找一適當之還原劑,在液相反應中,且期望在保持反應系統成分簡單,將前驅物氯鉑酸中之鉑(Ⅳ)還原至零價的金屬態,製備出具有高分散度、粒徑分布範圍集中且穩定不發生聚集的鉑奈米級顆粒,並直接將鉑奈米顆粒用在電催化氧氣還原反應,測試其電催化活性效能。 當以檸檬酸鈉還原之鉑奈米顆粒,在353K下反應3小時,無論在2.25 mM或是較高之11.25、67.5 mM前驅物濃度製備之樣品,檸檬酸鈉在這反應溫度下,還原力並不足以將氯鉑酸中的鉑(Ⅳ)還原至零價金屬態,且即使提高反應溫度至363K,仍無法達到還原的效果。 而當使用甲醇為還原劑時,在353K下反應15分鐘後,開始有鉑顆粒的生成,反應進行20分鐘之後,反應轉化率可達到90%以上;而當另行添加檸檬酸鈉之後,還可能可以吸附於鉑奈米顆粒表面,但若提高反應系統中之前驅物濃度或著是提高甲醇濃度,會減緩還原反應的發生。 在電催化陰極氧氣還原反應特性探討中,嘗試比較各製備條件下得到的鉑奈米顆粒電催化活性,製備反應時間較短所得之較小粒徑(2.4 nm)鉑微粒,相較於製備反應時間較長所得的鉑微粒擁有較佳之電催化活性,可使氧氣還原反應更驅向於直接還原反應之反應機制。
This study investigates the preparation of Pt nanoparticle and its electrocatalytic oxygen reduction reaction. The dihydrogen hexachloro platinate(Ⅳ) is used as the precursor. We aim to find an efficient reducing agent in the absence of complex protection agent. Though sodium citrate is frequently used as a reduction agent. Nevertheless, it can not reduce Pt precursor into Pt metallic state at 343-363K even after 3h of reaction. However, methanol can efficiently reduce Pt precursor and a conversion of 90% was achieved at 353K. The presence of citrate when using methanol as the reducing agent resulted in a somewhat slowlier reduction and a more stable Pt sol. The higher the sodium citrate concentration the stabler the Pt sols is. We proposed that the sodium citrate adsorbs on the surface of Pt sol to protects them from agglomeration. Higher concentrations of methanol and Pt precursor was found to slow down the reduction of Pt(Ⅳ) and thus resulted in lower reduction efficiency. Pt nanoparticles prepared at different reaction conditions were compared in electrocatalytic oxygen reduction reaction characteratics. The smaller size Pt particle (2.4 nm) was more active than bigger particles (2.7 nm). The smaller size Pt particles also drive the reduction reaction more toward the direct reduction mechanism showing electron transfer number of 4.