吸附——定义、机制和类型
吸附是原子、离子或分子从气体、液体或溶解的固体粘附到表面上。该过程在吸附剂表面形成吸附膜。
这与吸收不同,吸收是在流体溶解或渗透液体或固体时发生的。吸附是一种表面现象,而吸附发生在材料的整个体积中,尽管吸附通常先于吸附。术语吸附指的是这两个过程,而解吸指的是相反的过程。
吸附机理
发生吸附是因为吸附剂的表面颗粒与本体中的颗粒不在同一环境中。在吸附剂内部,所有作用在颗粒之间的力都是相互平衡的,但在表面上,颗粒并没有被同类原子或分子所包围,因此它们具有不平衡或残留的吸引力。这些吸附力负责将吸附质颗粒吸引到它们的表面。
在给定的温度和压力下,吸附程度随着每单位质量吸附剂的表面积增加而增加。吸附热是吸附的另一个重要方面。在吸附过程中,表面残余力总是降低,即表面能降低,表现为热量。
吸附总是一个放热反应。换句话说,吸附的 ΔH 总是负的。当气体被吸附时,分子的运动自由度受到限制。这对应于吸附后气体熵的降低,即 ΔS 为负。因此,吸附伴随着系统焓和熵的降低。
The thermodynamic requirement for a process to be spontaneous is that ∆G must be negative at constant temperature and pressure, implying a decrease in Gibbs energy. Based on the equation,
∆G = ∆H – T∆S
If H has a sufficiently high negative value and – ∆TS is positive, ∆G can be negative. Thus, in a spontaneous adsorption process, the combination of these two factors causes ∆G to be negative. As adsorption progresses, H becomes increasingly negative. At some point, ∆H equals ∆TS, and ∆G equals zero. At this point, equilibrium has been achieved.
吸附类型
固体上的气体吸附主要有两种类型。吸附定义为当气体由于弱范德华力而积聚在固体表面时的物理吸附或物理吸附。
当气体分子或原子通过化学键保持在固体表面时,就会发生化学吸附或化学吸附。
化学键本质上可以是共价键或离子键。化学吸附具有高活化能,因此通常被称为活化吸附。有时这两个过程同时发生,因此很难确定吸附的类型。随着温度的升高,低温下的物理吸附可以发展为化学吸附。例如,氢气首先通过范德华力吸附在镍上。然后氢分子解离形成氢原子,这些氢原子通过化学吸附保持在表面上。
物理吸附特性
- 缺乏特异性:由于范德华力是普遍的,给定的吸附剂表面对特定气体没有偏好。
- 吸附物的性质:固体吸收的气体量由气体的性质决定。通常,易于液化(即具有较高临界温度)的气体很容易被吸附,因为范德华力在临界温度附近更强。结果,1g活性炭吸附二氧化硫(临界温度630K)比甲烷(临界温度190K)多,甲烷(临界温度33K)仍多4.5mL。
- 可逆性:固体对气体的吸附通常是可逆的。当压力增加时,随着气体体积的减小,更多的气体被吸附(Le-Chateliers 原理),通过降低压力可以去除气体。由于吸附过程的放热性质,物理吸附在低温下很容易发生,并随着温度的升高而降低(Le-Chateliers 原理)。
- 吸附剂的表面积:吸附的程度随着吸附剂表面积的增加而增加。因此,具有大表面积的细碎金属和多孔物质是有效的吸附剂。
- 吸附焓:毫无疑问,物理吸附是一个放热过程,但它的吸附焓非常低(20-40 kJ mol-1)。这是因为气体分子和固体表面之间的吸引力完全是由弱范德华力引起的。
化学吸附特性
- 高特异性:化学吸附具有高度特异性,只有在吸附剂和被吸附物之间可能发生化学键合时才会发生。例如,氧由于氧化物的形成而被吸附在金属上,而氢由于氢化物的形成而被吸附在过渡金属上。
- 不可逆性:化学吸附在本质上通常是不可逆的,因为它涉及化合物的形成。化学吸附也是一个放热过程,但由于高活化能,它在低温下非常缓慢。与大多数化学变化一样,吸附通常随着温度的升高而增加。在高温下,在低温下吸附的气体的物理吸附可以转化为化学吸附。高压通常有利于化学吸附。
- 表面积:化学吸附与物理吸附一样,随着吸附剂表面积的增加而增加。
- 吸附焓:化学吸附具有高焓(80-240 kJ mol-1),因为它涉及化学键的形成。
示例问题
问题1:为什么会发生吸附?
回答:
Adsorption occurs because the adsorbent’s surface particles are not in the same environment as the particles within the bulk. Inside the adsorbent, all of the forces acting between the particles are mutually balanced, but on the surface, the particles are not surrounded on all sides by atoms or molecules of their kind, and thus they have unbalanced or residual attractive forces. These adsorbent forces are responsible for attracting adsorbate particles to its surface.
问题2:吸附是放热过程吗?如何?
回答:
Adsorption is always an exothermic reaction. To put it another way, the ∆H of adsorption is always negative. When a gas is adsorbed, the molecules’ freedom of movement is restricted. This corresponds to a decrease in the entropy of the gas following adsorption, i.e., ∆S is negative. Adsorption is thus accompanied by a decrease in the system’s enthalpy as well as entropy.
问题3:吉布斯能量与自发过程之间的关系是什么?
回答
The thermodynamic requirement for a process to be spontaneous is that ∆G must be negative at constant temperature and pressure, implying a decrease in Gibbs energy. Based on the equation,
∆G = ∆H – T∆S
If H has a sufficiently high negative value and – T∆S is positive, ∆G can be negative. Thus, in a spontaneous adsorption process, the combination of these two factors causes G to be negative.
问题四:勒夏特列原理是什么?
回答:
It states that when a system that has been in equilibrium for a long time is subjected to a change in concentration, temperature, volume, or pressure, the system shifts to a new equilibrium, which partially offsets the applied change.
问题 5:什么是范德华力?
回答:
A distance-dependent interaction between atoms or molecules is the Van der Waals force. These attractions, unlike ionic or covalent bonds, are not the result of a chemical electronic bond; they are therefore comparatively weak and more susceptible to disturbance. At greater distances between interacting molecules, the van der Waals force quickly vanishes.
问题6:什么是临界温度?临界温度与吸附气体有什么关系?
回答:
The critical temperature is the temperature above which a gas cannot be liquefied even with high pressure. The higher the critical temperature, the more gas adsorbed on the surface.