Reference surface excess isotherms for carbon dioxide adsorption on ammonium ZSM-5 at various temperatures
Huong Giang T. Nguyen, Blaza Toman, Julie Colon Martinez, Daniel W. Siderius, Roger D. van Zee
Abstract
Abstract This work is part of the effort at the Facility for Adsorbent Characterization and Testing (FACT lab) at the National Institute of Standards and Technology (NIST) to develop reference materials, reference data, and measurement best practices for adsorption metrology. High-pressure surface excess isotherms for CO 2 adsorption on NIST Reference Material 8852 (ammonium ZSM-5) at 15 °C, 25 °C, and 35 °C are reported, expanding on a FACT Lab-organized international interlaboratory study on this sorbent/sorbate pair at 20 °C. The range of temperatures of the present study is of interest for many CO 2 adsorption measurements and applications. Measurements were made using five different adsorption instruments, both manometric and gravimetric. Excellent agreement in the measured isotherms among the instruments was found. An empirical reference equation of the form, $${n}_{ex,ref}({P}_{eq})=\frac{c}{\{{1+\mathrm{exp}[\left(-\mathrm{ln}\left(P\right)+a\right)/b ]\}}^{b }} ,$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi>e</mml:mi> <mml:mi>x</mml:mi> <mml:mo>,</mml:mo> <mml:mi>r</mml:mi> <mml:mi>e</mml:mi> <mml:mi>f</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo>(</mml:mo> <mml:msub> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>eq</mml:mi> </mml:mrow> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> <mml:mo>=</mml:mo> <mml:mfrac> <mml:mi>c</mml:mi> <mml:mrow> <mml:mo>{</mml:mo> <mml:msup> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>+</mml:mo> <mml:mi>exp</mml:mi> <mml:mo>[</mml:mo> <mml:mfenced> <mml:mo>-</mml:mo> <mml:mi>ln</mml:mi> <mml:mfenced> <mml:mi>P</mml:mi> </mml:mfenced> <mml:mo>+</mml:mo> <mml:mi>a</mml:mi> </mml:mfenced> <mml:mo>/</mml:mo> <mml:mi>b</mml:mi> <mml:mo>]</mml:mo> <mml:mo>}</mml:mo> </mml:mrow> <mml:mi>b</mml:mi> </mml:msup> </mml:mrow> </mml:mfrac> <mml:mo>,</mml:mo> </mml:mrow> </mml:math> [ n ex,ref is surface excess uptake (mmol/g); P eq is equilibrium pressure (MPa); P is ( P eq )/(1 MPa)]; a , b , c are constants] and the 95% uncertainty interval were determined for the isotherms at each of the temperatures. Lastly, the isosteric heat of adsorption is estimated from absolute isotherms derived from the surface excess reference data.