Andrew D’Amico, Senior Research Engineer at Micromeritics, talks to AZoM about the importance chemical adsorption measurements and his recent research in the field of chemisorption.
Could you explain to our readers what chemisorption is and provide them with some information about how it works?
As it applies to characterization, chemical adsorption is the strong, selective interaction of a gas or vapor with a chemically active site, such as an acid site, basic site, vacancy site or redox site.
During chemical adsorption characterization methods, active (probe) gas is selectively exposed to the sample such that adsorption, desorption, or reaction can be observed and quantified.
Selective exposure is accomplished by either isothermal pulses or temperature programming (increase temperature until adsorption, desorption, or reaction occurs).
Why are chemical adsorption measurements useful?
Chemical adsorption (or chemisorption) measurement techniques, which also include reactions, are useful for evaluating physical and chemical properties of materials that are critical for process / reaction performance.
Such properties can include the (reduction) temperature at which metals become catalytically active, amount of surface metal or active species available for reaction, strength of specific types of active sites, or ability of materials to perform after reduction/oxidation cycles.
The characterizations can provide information on how the material should be used within a process or why a material has a given behaviour within a process.
In which industries are chemical adsorption analysis techniques used?
Industries that deal with catalysts rely heavily on chemical adsorption techniques, generally automotive or petrochemical industries. Industries what use metals that can be oxidized or reduced may all benefit from chemical adsorption techniques and the field of battery research is another area that has utilized chemical adsorption techniques.
How does chemisorption differ to physisorption?
The terms 'chemisorption' and 'physisorption' are not strictly defined but generally refer to 'irreversible' and 'reversible' adsorption, respectively. In addition, the strength of adsorption associated with chemisorption is generally above 100 kJ/mole while the heats for physisorption are generally below 30 kJ/mole, although reversible adsorption can occur in the range of 80 kJ/mole.
The consequence of this is that a constant partial pressure of adsorptive gas is in equilibrium with the fluid in the adsorbed phase for physisorption. In chemisorption, gas can adsorb on a surface without a corresponding concentration of active gas in the bulk gas phase.
Additionally, different results are obtained from physisorption analyses, such as pore size, pore volume, BET surface area, and heat of adsorption.
Why is Micromeritics considered a market leader within the field of chemical adsorption analysis?
Micromeritics is considered a leader in the field of chemical adsorption analyses with over 30 years of experience in developing cutting edge chemical adsorption instrumentation to expand the analytical characterization of catalytic materials. These instruments are specifically designed for the demanding and sensitive measurements required for proper chemisorption analyses.
Micromeritics instruments are backed by an expert team of scientists with decades of experience in chemical adsorption techniques, most notably Dr. Jeff Kenvin and Dr. Simon Yunes, who have presented numerous conference presentations and have advanced the state of the art of chemical adsorption by their peer-reviewed journal publications.
Could you provide our readers with a brief overview of the chemisorption instruments which Micromeritics has designed and developed?
I would first like to clarify the definitions of static and dynamic chemisorption experiments. In static adsorption, generally two adsorption 'isotherms' (quantity of gas adsorbed as a function of pressure at constant temperature) are collected at sub-atmospheric pressures: the first representing total adsorption, the second representing reversible adsorption, and the difference between the two representing the irreversible adsorption (i.e. 'chemisorption').
This technique provides information on the quantity of adsorption sites, from which subsequent calculations may be done. In dynamic chemisorption, the identity, quantity, and strength of active sites can be quantified by adsorption, desorption, or reaction under isothermal or temperature-programmed conditions.
The chemisorption instruments offered by Micromeritics can be group into either dynamic adsorption instruments (AutoChem II 2920, AutoChem HP 2950, or Chemisorb 2720/2750) or static adsorption instruments with some level of dynamic capability (ASAP 2020 Chemi, 3FLEX Chemi, Chemisorb HTP).
For the dynamic instruments, the Autochem II 2920 is capable of the TPR, TPO, TPD, and pulse chemisorption techniques (including vapor use); the AutoChem HP 2950 extends the pressure range of the AutoChem II 2920 up to 1000 psia on the carrier gas flow path; the Chemisorb 2720/2750 is a lower-cost, manual-controlled dynamic chemisorption instrument.
For the static chemisorption instruments with some dynamic capabilities, adsorption isotherms are collected on all units; the 3FLEX offers hard-seal valves, VCR manifold seals, and temperature-programmed experiments when used along with a mass spectrometer. The Chemisorb HTP is a 6-port, high-throughput static analyser.
How does the choice of which instrument to use vary with the application area?
The choice of instrument often depends on funding and instrument technology is continually changing, so I will focus on the application differences between the static and dynamic techniques.
The biggest differences between the two techniques (static vs dynamic chemisorption) are the partial pressure of active gas over the sample and the residence time of the active gas, or time that the active gas is in contact with the sample.
For the static system, an increasingly higher partial pressure of the active gas is continually in contact with the sample. Selective exposure of the active gas to the sample is controlled by volumetric doses of gas under vacuum conditions.
For the dynamic system, depending on the experiment, a lower concentration of active gas may be in contact with the sample for only a few seconds. Depending on the sample, equilibration and access of the active gas to the adsorption sites of interest may be faster in the static system.
What have been your recent contributions in the field of chemical adsorption?
I have recently been part of two research collaborations that resulted in peer-reviewed journal publications (ChemSusChem 2015, 8, 2073 – 2083 and Journal of Catalysis 329 (2015) 335–347).
I have also been part of developing an ASTM standard on acid site characterization as a member of the ASTM D32 committee on catalysis.
Where can our readers find out more information about chemical adsorption analysis and Micromeritics’ instruments?
The Micromeritics website is the best place to browse the most up-to-date content on Micromeritics’ chemisorption instruments.
The website offers a variety of resources such as application notes, technical articles, numerous educational videos, blog articles, and links to our scientists’ published works.
I would encourage anyone that is interested in learning more about chemisorption and its unique applications to check out this valuable resource.
About Andrew D’Amico
Andrew D’Amico received his B.S. degree in Chemical and Biomolecular Engineering from the Georgia Institute of Technology in 2009 and M.S. degree in Chemical Engineering from the University of Oklahoma in 2012.
Over the course of four semesters, between 2006 and 2008, Andrew worked for Micromeritics as part of the cooperative education (co-op) program at Georgia Tech.
During this time, he helped develop PID control for a physisorption cryostat, performed pressure swing adsorption measurements on zeolites using the ASAP 2050, developed partial-least-squares models for the DVVA 4000, investigated the characterization of zeolite acidity using propylamines as probe molecules using the AutoChem II 2920, and assisted in the development of methods for zeolite micropore analysis using the ASAP 2420.
After graduation, Andrew pursued graduate studies before returning to Micromeritics in 2012 as a Senior Research Engineer. Andrew’s research interests include catalyst characterization (especially metals supported on oxides and zeolite catalysts), trickle-bed reactors, and fungible hydrocarbon production from conversion of plant oils.
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