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• Chemisorption is often found to occur at temperatures far above the critical temperature of the adsorbate.
• As is true for most chemical reactions, chemisorption is usually associated with an activation energy, which means that adsorbate molecules attracted to a surface must go through an energy barrier before they become strongly bonded to the surface.
• Because chemisorption involves a chemical bond between adsorbate and adsorbent, unlike physisorption, only a single layer of chemisorbed species can be realized on localized active sites such as those found in heterogeneous catalysts.
• However, further physical adsorption on top of the chemisorbed layer and diffusion of the chemisorbed species into the bulk solid can obscure the fact that chemisorbed material can be only one layer in depth
Graduated as a metallurgical engineer from the School of Mines at Columbia University in 1903
1903-1906 M.A. and Ph.D. in 1906 from Göttingen.
1906-1909 Instructor in Chemistry at Stevens Institute of Technology, Hoboken, New Jersey.
1909 –1950 General Electric Company at Schenectady where he eventually became Associate Director
1913 -Invented the gas filled, coiled tungsten filament incandescent lamp.
1919 to 1921, his interest turned to an examination of atomic theory, and he published his "concentric theory of atomic structure" . In it he proposed that all atoms try to complete an outer electron shell of eight electrons
1927 Coined the use of the term "plasma" for an ionized gas.
1932 The Nobel Prize in Chemistry "for his discoveries and investigations in surface chemistry"
1935-1937 With Katherine Blodgett studied thin films.
1948-1953 With Vincent Schaefer discovered that the introduction of dry ice and iodide into a sufficiently moist cloud of low temperature could induce precipitation.
• Because the formation of a chemical bond takes place between an adsorbate molecule and a localized, or specific, site on the surface of the adsorbent, the number of active sites on catalysts can be determined simply by measuring the quantity of chemisorbed gas
The gas-sorption stoichiometry is defined as the number of metal atoms with which each gas molecule reacts.
Since, in the gas adsorption experiment to determine the quantity of active sites in a catalyst sample, it is the quantity of adsorbed gas which is actually measured, the knowledge of (or at least a reasonably sound assumption of) the stoichiometry involved is essential in meaningful active site determinations (area, size, dispersion).
For the same cube of unit mass, the area is then the area per unit mass A and l is rewritten d (crystallite size), the length required to give a cube whose mass is unity. Equating both terms for volume:
dA/6=1/ or
d=6/A
For a supported metal, the loading, L, must be taken into consideration.
d=L6/A
Other geometries can be treated in a similar fashion. For example, a rectangular particle whose length is three times its width has a shape factor of 14/3.
Exposed metal atomsSince these islands vary in size due to both the intrinsic
nature of the metal and the support beneath, plus the method of manufacture more or less of the metal atoms in the whole sample are actually exposed at the surface. It is evident therefore that the method of gas adsorption is perfectly suited to the determination of exposed active sites.
• Dispersion is defined as the percentage of all metal atoms in the sample that are exposed.
• The total amount of metal in the sample is termed the loading, χ , as a percentage of the total sample mass, and is known from chemical analysis of the sample.
3.3.5 Accessible vs. Non-accessible Sites1. Adventitious moisture2. Reducing gas accessibility3. Diffusion4. Purge5. Physisorption blocks6. Bulk hydride7. Spillover8. Stoichiometry9. Characterization gas vs. Process gas
• Whenever a gas molecule adsorbs on a surface, heat is (generally) released, i.e. the process of adsorption is exothermic.
• This heat comes mostly from the loss of molecular motion associated with the change from a 3-dimensional gas phase to a 2-dimensional adsorbed phase.
• Heats of adsorption provide information about the chemical affinity and the heterogeneity of a surface, with larger amounts of heat denoting stronger adsorbate-adsorbent bonds.
• There are at least two ways to quantify the amount of heat released upon adsorption: in terms of (i) differential heats, q, and (ii) integral heat, Q.
Differential Heats of Adsorption• q, is defined as the heat released upon adding
a small increment of adsorbate to the surface. • Its value depends on (i) the strength of the
bonds formed and (ii) the degree to which surface is already covered.
• i.e a plot of q vs. θ provides a curve illustrating the energetic heterogeneity of the surface.
• Use it to fingerprint surface energetics and to test of the validity of any Vm evaluation method used (see earlier) since each method assumes a different relationship between q and θ.
Integral Heat of Adsorption• This is simply defined as the total
amount of heat released, Q, when one gram of adsorbent takes up X grams of adsorbate. It is equivalent to the sum, or integral, of q over the adsorption range considered, that is:
where Vm is expressed in mL at STP, and θ ideally ranges from
θmin = 0 to θmax = maximum coverage attained experimentally.
Pulse Titration • Metal area, dispersion and crystallite size are
calculated from the amount of analysis (reactive) gas adsorbed.
• Variable volumes of analysis gas are injected into the inert carrier gas stream, which continuously flows over the sample.
• Detector measures the volume of gas that remains unadsorbed by the sample. Subtraction from the total amount injected gives the total amount adsorbed to within 1uL accuracy.
• A low concentration of pre-mixed hydrogen (e.g.5%) in nitrogen or argon (or other reducing gas for custom research applications) flows over the sample as it is heated during a linear increase (ramp) in temperature.
• Peak reduction temperature is also a function of heating rate and may be used to calculate activation energy for the reduction process.
Zhang and Verykios reported that three types of carbonaceous species designated as C, C, and C were found over Ni/Al2O3 and Ni/CaO±Al2O3 catalysts in the TPO experiments.
Zhang ZL and Verykios XE,. Catal. Today 21 589-595 (1994).
Goula et al identified two kinds of carbon species on Ni/CaO Al2O3 catalysts from TPO experiments. The high-temperature peak was assigned to amorphous and/or graphite forms of carbon. The lower temperature peak suggested a filamentous form.
Goula MA, Lemonidou AA and Efstathiou AM, J Catal 161 626-640 (1996).