ELECTROCATALYTIC PROPERTIES OF METALS

在水介质中，二氧化碳的电化学还原产生各种各样的产物，这些产物的分布在很大程度上取决于金属电极。根据CO_2还原产物的选择性，将金属电极分为四类。铜电极只产生碳氢化合物，如甲烷、乙烯和醇。(2)金、银、锌和钯从CO_2中放出CO。(3)在锡、铅、汞和镉电极处产生甲酸。(4)镍、铁和铂几乎不能还原二氧化碳，只有氢是水还原的结果。目前的研究人员先前发现，在铜电极上碳氢化合物的形成是通过吸附一氧化碳的中间形成进行的。因此，对CO_2还原有活性的金属电极可分为CO金属和FA金属。本文的工作是为了揭示CO_2还原过程中产物选择性的原因。采用原位电沉积的方法，研究了CO_2在其他金属修饰电极上的电还原。结果表明，金属吸附原子在CO生产中具有一定的电催化活性。排列顺序为Au> Ag> Cu> Zn>> Sn> Cd> Pb。附着物的电催化活性也受到底金属性质的很大影响。铜有一种独特的性质，这种金属只能产生碳氢化合物和醇。目前的研究表明，吸附铜不具有电催化成烃的性质，需要多层铜沉积才能有效地将CO_2还原成烃。

Electrochemical reduction of carbon dioxide in aqueous media yields various kinds of products, the distribution of which depends greatly upon metal electrodes. Metal electrodes are thus divided into four groups in accordance with the product selectivity in CO_2 reduction. (1)Copper electrode exclusively yields hydrocarbons, such as methane and ethylene, and alcohols. (HC metal) (2) Gold, silver, zinc and palladium give CO from CO_2. (CO metals) (3)Formic acid is produced at tin, lead, mercury and cadmium electrodes. (FA metals) (4)Nickel, iron and platinum can hardly reduce carbon dioxide and only hydrogen is evolved as a result of water reduction. The present investigators previously revealed that hydrocarbon formation at copper electrode proceeds with intermediate formation of adsorbed carbon monoxide. Thus metal electrodes which are active in CO_2 reduction may be classified into CO metals and FA metals.The present work has been carried out in order to reveal the origin of the product selectivity in CO_2 reduction. We studied electroreduction of CO_2 at electrodes modified with other metal adatoms by in-situ electrodeposition. We obtained conclusions that metal adatoms have an order of electrocatalytic activity in CO production. The order may be Au> Ag> Cu> Zn>> Sn> Cd> Pb. Electrocatalytic activity of adatoms is also greatly affected by the properties of substrate metals.Copper has a unique property that this metal exclusively yields hydrocarbons and alcohols. The present research work has revealed that adatom copper does not show electrocatalytic property of hydrocarbon formation and multilayer copper deposition is required for effective reduction of CO_2 to hydrocarbons.