Adsorption

The sorption of acid dyes from aqueous effluents onto activated carbon has been studied. The effects of initial dye concentration and activated carbon mass on the rate of Acid Blue 80, Acid Red 114 and Acid Yellow 117 removal have been investigated. A three-resistance mass transport model based on film, pore and surface diffusion control has been applied to model the concentration decay curves. The model incorporates an effective diffusion coefficient D eff, which is dependant on the equilibrium solid phase concentration or fractional surface coverage. The results of the film-pore-surface diffusion model are compared with the data obtained from the basic film-pore diffusion model. It has been found that the film-pore-surface diffusion model provides a major improvement over the data correlated by the film-pore diffusion model. Also, the relationship between surface diffusion and fractional surface coverage has been investigated for the adsorption of acid dyes on activated carbon.

A series of Tb–CeO2 mixed oxides were investigated as novel oxygen sorbents. The relationship between %Tb content-oxygen uptake, alongside the selectivity and reversibility of these materials, was determined via chemisorption (400, 500 and 600 °C) and supporting thermogravimetric studies at 500 and 600 °C. Oxygen chemisorption experiments conducted at 600 °C showed higher uptakes were achieved by incorporating more Tb into the CeO2 crystal lattice. The uptake of 40 mol% Tb–CeO2 was 121 μmol g−1 and for 10 mol% Tb–CeO2 the uptake was 34 μmol g−1. Increasing the analysis temperature for each material resulted in an increase in uptake as more oxygen was able to be removed. All materials exhibited good reversibility and cyclic stability during alternating N2 and air atmospheres at 600 °C. High O2/N2 selectivity was also demonstrated as no detectible uptake was observed at 600 °C using N2 as the adsorbate. The data suggests that these materials may have applications in air trace gas removal or as membranes for air separation applications.

Sorption processes are incorporated in a wide range of applications. A heat and mass transfer model which calculates the temperature distribution and pressure in a sorption cell is desired for developing any sorption system. In this paper, we present a one-dimensional dynamic numerical model for closed sorption cell systems, which is based on adsorption isotherm measurements of the working pair. The model is governed by energy and mass balances, where a uniform pressure inside the cell is assumed, the thermal contact resistances between different materials are incorporated, and the material properties are temperature dependent. The model is implemented by an implicit finite differences method, aiming to predict the system performance and to allow the optimization of the mechanical design and operating conditions. A design-dependent correction factor is suggested to compensate for the one-dimensional assumption, especially for predicting the pressure in the cell. The numerical model is successfully validated against experimental results of nitrogen—activated carbon systems, at several operating conditions. The numerical model allows extensive investigations on sorption system designs, for example, the dependency of the sorption cell performances on the heater and cooler designs are discussed. The results show that different designs are required for providing either high thermal efficiencies or high inner temperature and pressure. The model is an essential tool in our laboratory for researching and developing different sorption based systems.

The well-known simple adsorption methods used to evaluate the micropore size distribution from low pressure adsorption isotherms were examined by employing model isotherms for slit-like graphite micropores obtained from nonlocal density functional theory. It was shown that in the range of pore sizes from about 0.4 to 0.9 nm, the Horvath Kawazoe (HK) method satisfactorily reproduces the shape of the micropore size distribution, but the pore sizes are underestimated. In the case of micropores wider than 0.9 nm, the method fails as the formation of the monolayer on the pore walls produces a peak corresponding to 0.6 nm micropores on the HK pore size distribution. Therefore, the HK method indicates the presence of microporosity even for nonporous samples. The Dubinin-Astakhov adsorption isotherms were also examined and it was shown that their application to represent local adsorption isotherms for homogeneous pores is questionable. However, the adsorption potential distributions seem to be promising for micropore analysis.

Khả năng hấp phụ của CH4 và H2 siêu tới hạn trên các bức tường ống bên ngoài và bên trong của ống nano carbon đơn tường (SWCNT) đã được xác định. Bức tường ống bên trong có độ cong âm cho thấy khả năng hấp phụ cao hơn cho CH4 và H2 siêu tới hạn so với bức tường ống bên ngoài có độ cong dương, điều này là do sự khác biệt trong tiềm năng tương tác của chúng. Các bó SWCNT tinh tế đã được chuẩn bị bằng phương pháp sấy hỗ trợ lực mao dẫn sử dụng toluene hoặc methanol nhằm tạo ra các khoảng không gian lỗ rỗng giữa các cấu trúc với tiềm năng tương tác mạnh nhất cho CH4 hoặc H2; các bó SWCNT này thể hiện khả năng hấp phụ cao nhất cho CH4 và H2 siêu tới hạn. Điều này cho thấy rõ rằng các cấu trúc nano này của SWCNT là rất quan trọng cho khả năng hấp phụ khí siêu tới hạn.

The simultaneous adsorption of two herbicides—2,4-dichlorophenoxyacetic acid (2,4-D) and 4-chloro-2-methylphenoxyacetic acid (MCPA)—from their aqueous binary mixtures onto granular activated carbon was studied. The quantities adsorbed were determined by HPLC with UV detection. The experimental data were analysed using the Freundlich adsorption isotherm. The high correlation coefficients indicated that the adsorption equilibrium fitted the Freundlich isotherm well. A multilayer perceptron (MLP) (an artificial neural network model—ANN) was applied to describe the adsorption equilibrium in multicomponent systems. This enabled sorption isotherms to be predicted for all possible combinations of the two herbicides. The experimental results and the calculated data obtained from MLP for the solutions of the individual components and their mixtures suggest that MCPA is better adsorbed onto activated carbon than 2,4-D.

Two series of Pd–Pt/Al2O3 catalysts, characterized by high metal dispersions were prepared and investigated in the reaction of n-hexane conversion at the temperature < 300 °C. Incipient wetness co-impregnation of γ-alumina with the solutions of Pd(acac)2 and Pt(acac)2 led to relatively good Pd–Pt alloying. However, a similar catalyst preparation using solutions of PdCl2 and H2PtCl6 resulted in unsatisfactory homogenization; furthermore, significant amounts of chlorine were retained in the catalysts. The Pd–Pt alloy homogeneity has a significant effect on the relations of catalytic activities and product selectivities with Pd–Pt alloy composition. The relationship between the catalytic activity of chlorine-free Pd–Pt/Al2O3 catalysts and bulk Pd–Pt composition matches the accepted relation between the surface composition and bulk composition of Pd–Pt, reflecting a high surface enrichment in palladium. In contrast, for the chloride series of Pd–Pt/Al2O3, variations of the turnover frequency are directly correlated with the bulk composition, indicating that unalloyed, more active, Pt particles decide about the catalytic behavior. Pd/Al2O3 shows better than Pt/Al2O3 selectivity for isomerization, especially after reduction at higher temperatures, ≥ 500 °C. On alloying with Pt, for the Cl-free series the isomerization selectivity was very high for nearly all bimetallic catalysts and showed a synergistic effect for 20 at.% Pt. In contrast, for the Cl-containing series, the isomerization selectivity was proportional to the palladium content. The effect of reduction temperature on the isomerization propensity of Cl-containing Pd–Pt/Al2O3 catalysts was rather small, suggesting that possible variations in support acidity associated with chloride presence do not have large catalytic consequences, confirming the metal-only catalytic action in alkane isomerization carried out at the temperatures below 300 °C.

In modeling of activated carbons, the pores are often assumed to be slit-shaped formed of a constant number of graphene layers. X-ray diffraction studies show that micropores are formed between stacks of different numbers of graphene layers. In this study, we investigate, through the grand canonical Monte Carlo method, the influence on the adsorbed alkanes densities of pore walls with different graphene layers thickness and the related interpore adsorbate interaction when the pore wall has only one graphene layer. All studies of thickness and interpore interaction to date were performed using the Steele 10-4-3 potential model. Instead of Steele model, we propose explicit models made up of graphene layers of discrete carbon atoms. We also investigated the sensitivity of the system to the cut-off and solid-fluid parameter. With our explicit model we found that the influence of the number of carbon layers is not significant for n>2 as previously observed by Steele model—DFT studies. The system was also insensitive to cut-off and well deep parameter variations. A new pore model with an extra dummy graphene wall was proposed to investigate the interpore interaction. The interpore interaction study with the alkanes series C1 to C4 shows that the retention capacity of heavier alkanes is the same whether for activated carbons with few layers (stronger interpore interaction) as for carbons with two or more layers (stronger solid-fluid interaction) assuming negligible surface mediation. The explicit models proposed can be successfully used in the elaboration of virtual porous carbon models to reproduce wall thickness and interpore adsorbate interactions phenomena.

Việc ứng dụng mô hình MPTA đã được mở rộng sang hấp phụ lỏng liên kết. Mô hình MPTA mô tả các tương tác giữa chất lỏng–chất lỏng bằng cách sử dụng một phương trình trạng thái (EoS), và các tương tác giữa chất lỏng–chất rắn bằng một phương trình tiềm năng. Để mở rộng ứng dụng cho hấp phụ lỏng liên kết, một thuật ngữ liên kết đã được xem xét cho các tương tác chất lỏng–chất lỏng. Mười sáu hỗn hợp nhị phân chứa các thành phần liên kết và không liên kết trong trạng thái cân bằng với nhiều vật liệu hấp phụ khác nhau đã được nghiên cứu; các tương tác chất lỏng–chất lỏng đã được mô hình hóa bằng phương trình trạng thái Peng–Robinson, Soave–Redlich–Kwong, SRK chuyển đổi thể tích, và CPA, trong khi các hiệu ứng tương tác chất lỏng–chất rắn đã được đưa vào xem xét bằng cách sử dụng các hàm tiềm năng Dubinin–Radushkevich–Astakhov (DRA) và Steele. Các tham số của mô hình đã được xác định bằng cách phù hợp mô hình với dữ liệu thực nghiệm về sự dư thừa bề mặt. Đối với các hệ thống đã nghiên cứu, độ chính xác của các đẳng nhiệt đã được xác định rằng phụ thuộc nhiều hơn vào phương trình tiềm năng chất lỏng–chất rắn hơn là EoS được áp dụng. Các phép tính cho thấy rằng phương trình SRK là lựa chọn phù hợp cho các hệ không liên kết, trong khi phương trình CPA được tìm thấy là thích hợp hơn cho các hệ liên kết, như mong đợi. Các kết quả cũng cho thấy rằng hàm tiềm năng Steele phù hợp hơn với dữ liệu thực nghiệm so với hàm tiềm năng DRA.