Collaborative Research: Adsorption Equilibration of Binary Mixtures on Planar and Porous Sorbents

【非技术内容】将两种气体的混合物引入放置了吸收材料的容器后，由于特定分子与吸收表面结合，容器内混合物的压力和两种气体的比例随时间而变化。经过足够的时间后，压力和气体的组成达到恒定值，并且在表面形成与气相平衡的凝聚相。本项目研究吸附动力学，即从引入气体混合物到达到平衡的这一过程的演变，以及它如何依赖于诸如起始气体混合物的组成和温度，每个气体分子附着在吸附剂上的强度，以及不同分子达到最终状态的速度等参数。虽然这是一个非常复杂的过程，许多方面还没有得到很好的理解，但大多数吸附的实际应用（如气体分离或气体储存）要么依赖于这些动力学，要么受到这些动力学的限制。对混合物吸附动力学的更全面的了解将有助于更好地控制过程和更有效的实际应用。通过对模型系统进行计算机模拟来研究吸附动力学，并将结果与通过确定压力和成分作为时间函数的实验得到的结果进行比较。计算机模拟提供了对这一过程的微观理解，实验是对模拟有效性的检验。参与该项目的研究生和本科生可以获得更广阔的视角来看待这种合作关系的实验-理论特征所提供的研究现象。通过面向当地高中生的暑期项目和为学生提供关键支持的一对一辅导项目，政府在招募和留住代表性不足群体的学生方面做出了特别努力。技术摘要：该项目解决了气体混合物在与各种固体表面相互作用和/或被限制在小孔隙中时如何接近吸附平衡的问题。这是通过结合一系列涉及理论、实验和模拟的研究结果来实现的。关于单一组分平衡的信息（对于许多系统来说，这本身往往很难找到）不足以理解混合物的吸附动力学，其中物种之间的相互作用具有明显的决定性作用。理解和充分利用吸附剂的吸附特性，当两种不同的物种竞争被吸附在其表面时，需要进一步明确地研究混合物中存在的动力学过程，最终导致吸附相的最终平衡状态。研究了这些现象的三个主要方面：a)二元气体混合物（及其各组分）的吸附速率如何取决于分压和温度？（实验和模拟）；b)平衡过程中竞争所涉及的基本过程是什么？它们如何影响吸附速率？（理论与模拟）；c)地表各分量的空间分布如何随时间变化？（理论和模拟）。这些问题针对两种吸附剂（平面和多孔）和两种吸附剂（球形和线性），重点关注分子相互作用的作用（特别是不同物种之间的相互作用），吸附剂的取向自由度的影响以及多孔吸附剂施加的限制。实验是在一个专门建造的装置中进行的，该装置允许进行体积吸附测量，以及在系统接近平衡时通过质谱法确定气体混合物的组成，并允许在较长时间内控制温度。建模和仿真研究利用动力学蒙特卡罗方法模拟吸附动力学，探索了实验中研究的相同系统和行为。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-Technical AbstractAfter a mixture of two gases is introduced into a container where an absorbing material has been placed, the pressure and the ratio of the two gases of the mixture inside the container change with time, as particular molecules bind to the absorbing surface. After enough time elapses, the pressure and the composition of the gas reach constant values, and a condensed phase forms on the surface, in equilibrium with the gas phase. This project studies the kinetics of adsorption, that is, the evolution of this process from the introduction of the gas mixture until equilibrium is reached, and how it depends on parameters such as the composition and temperature of the starting gas mixture, how strongly each of the gas molecules attaches to the sorbent, and how fast the different molecules reach their final state. While this is a very complex process, with numerous aspects that are not well understood, most practical applications of adsorption (such as gas separation or gas storage) either depend on, or are limited by, these dynamics. A more complete understanding of the mixture adsorption dynamics will allow for better control of the process and more efficient practical applications. The kinetics of adsorption is studied by performing computer simulations for model systems, and comparing the results with those obtained through experiments in which pressure and composition are determined, as a function of time. The computer simulations provide a microscopic understanding of the process, and the experiments are a test for the validity of the simulations. Graduate and undergraduate students participating in the project can gain a broader perspective of the investigated phenomena provided by the experimental-theoretical character of this partnership. Special efforts are made towards recruitment and retention of students from underrepresented groups, through summer programs for local high school students and one-on-one mentoring programs that provide critical student support.Technical AbstractThis project addresses the question of how a mixture of gases approaches adsorption equilibrium as it interacts with various kinds of solid surfaces and/or is confined within small pores. This is achieved by combining results from a series of studies that involve theory, experiments and simulations. Information on the equilibration of a single component (which is itself often difficult to find for many systems) is not sufficient to understand the sorption kinetics of mixtures, where the interplay between the species has a distinct and defining role. Understanding and taking full advantage of the adsorption characteristics of a sorbent when two distinct species compete to become adsorbed onto its surfaces requires further and explicit investigation of the kinetic processes present in the mixture that eventually lead to the final equilibrium state of the adsorbed phase. Three main aspects of these phenomena are investigated:a) How does the adsorption rate of a binary mixture of gases (and of its individual components) depend on the partial pressures and temperature? (experiments and simulations); b) What are the elementary processes involved in the competition during equilibration, and how do they affect the adsorption rates? (theory and simulations); c) How does the spatial distribution of each component on the surface change with time? (theory and simulations). These questions are addressed for two kinds of sorbents (planar and porous) and two kinds of adsorbates (spherical and linear), with a focus on the role of molecular interactions (especially between the distinct species), the effects of the orientational degrees of freedom of the adsorbates and, those of the confinement imposed by the porous sorbents. The experiments are conducted in a specially built setup that allows for volumetric adsorption measurements as well as for the determination of the composition of the gas mixture through mass spectrometry as the system approaches equilibrium, and which allows control of the temperature over extended periods of time. The modeling and simulation studies explore the same systems and behavior studied in the experiments by using a Kinetic Monte Carlo approach to simulate adsorption dynamics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.