Page 404 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 6 EVALUATION METHODS FOR PROPERTIES OF NANOSTRUCTURED BODY
layer on the surface of pores in the calculation for cap- saturation after several CO-pulses . The amount of
illary condensation. The analysis methods based on adsorbed CO is calculated from the difference of the
the capillary condensation theory can be applied amount of injected and exhausted CO. Total surface
mainly for the mesoscopic pore size range (2–100nm). area is obtained from adsorbed amount of CO and
The one-side opened tubular pore morphology model occupation area of an adsorbed CO molecule.
is also often used in the gas adsorption method. The Dispersion ratio and average particle size of a noble
different kinds of analysis method must be used for metal can be estimated from the total surface area.
under the size range of mesoscopic level because the Dispersion ratio means exposed ratio of atoms of a
capillary condensation phenomenon does not happen noble in the total amount of its loading. 100% of dis-
there. Many analysis methods have been proposed for persion corresponds to that where all of the atoms
under mesoscopic pore size range, for example, MP are exposed at the surface. The shape of a metallic par-
method, HK (Horvath–Kawazoe) method, SF ticle is assumed to be a cube of the same size, and
(Saito–Foley) method, DA (Dubinia–Astakhov) average noble metal particle size is estimated.
method, etc. The analysis methods affect the result of There are three kinds of measurement methods for
the pore size especially smaller than mesoscopic level. surface acidity and basicity, they are titration method,
In the gas adsorption method specific surface area can adsorption heat method and thermal desorption
be calculated from the relationship between the method [6]. A thermal desorption method of ammo-
amount of adsorbed molecules and its partial pressure nia for measurement of density and strength of acid-
by using BET (Brunauer, Emmet and Teller’s) equa- ity is exemplified here. A sample cell of a catalyst is
tion and occupation area of a nitrogen molecule [3]. evacuated at 500 C for 1 h by a rotary pump to elim-
The bubble point method is used for measurement of inate adsorbed water from a catalyst surface at the
maximum pore size of continuous pores in porous beginning. Then 100 Torr (13.3 kPa) of ammonia gas
films. In the bubble point method, a liquid is impreg- is injected in the sample cell at 100 C. After 30 min
nated into the sample and to measure the pressure of exposure to ammonia, residue of ammonia vapor in
when a bubble is observed at the beginning [4]. Mean the sample cell is eliminated by helium gas flow.
pore size of continuous pores measured by a half-dry Finally, the sample cell is heated in the heater whose
method [4], and pore size distribution of continuous temperature is increased by 10 K per min, and des-
pores can be estimated from the gas flow rate and dif- orbed ammonia is measured at each temperature. The
ferential pressure. As described in the Section 4.4.1, amount of ammonia gas which is desorbed at rela-
pores in catalyst or catalyst support consist of clear- tively lower temperature corresponds to the amount of
ance between crystallites or secondary particles; there- acid site of relatively weak acidity, and high tempera-
fore pore size distribution strongly depends on their ture desorption of that corresponds to relatively
aggregation and agglomeration structure. Pore size strong acidity [7]. As described for Au loaded catalyst
distribution affects heat resistance. As a suitable exam- in Section 4.4.1, surface acidity increases catalytic
ple of reaction selectivity by controlling pore size dis- activity of loaded catalyst in some cases and
tribution of a catalyst, catalytic removal reaction of decreases it in another case.
sulfur, vanadium and asphaltene are explained. The In addition, IR (infrared spectroscopy) is used to
most appropriate pore sizes for them are 10, 18 and 25 know the state of the adsorbed molecule. IR is used
nm respectively [5]. in a detector for qualitative and quantitative analysis
The active site is an adsorption point with the sur- in catalyst activity test described in the next para-
face of the catalyst where the chemical adsorption is graph as well. Moreover, there are many analysis
first caused. The surface of the metal particles and methods used, for example, ESR, XPS, NMR, STM,
their neighborhood on their catalyst support corre- AFM, EELS, LEED, AES and XAFS [8]. As an
spond to active site in metal-supported catalyst. Acidic example of a particular observation of adsorbed mol-
site has the role of active site in solid acid catalyst for ecules, there is an in situ observation of dehydro-
example zeolite or composite oxide catalyst, and basic genation of individual trans-2-butene molecules
site has that role in solid base catalyst. For estimating adsorbed on the Pd (110) surface. In elastically tun-
the amount of active site in noble metal supported cat- neled electrons from the tip of a STM proceed the
alyst, total exposure area of noble metal particles is reaction and the trans-2-butene molecules changed
measured by CO adsorption method. The surface of into 1,3-butadiene molecule under the STM observa-
noble metal can adsorb CO molecule selectively. CO- tion (Fig. 6.8.1) [9].
pulse adsorption method was standardized by Catalyst Next, reactors for catalyst activity test are
Society of Japan [6]. It is explained as follows. Noble explained. They are roughly classified into three
metal in a catalyst is reduced at 400 C for 15min in types, pulse reactor, closed reactor and flow reactor.
hydrogen atmosphere at the beginning. After cooling In the pulse reactor, reactant is injected as a pulse into
to room temperature, CO is injected into the catalyst the reactor. Merits of the pulse reactor are high sensi-
like a pulse of a prescribed amount, and simultane- tivity for minute amounts of reactant or product, and
ously, the quantity of the CO which is not adsorbed is its device has a simple configuration. On the other
measured. The amount of CO adsorption can reach the hand, the reaction in pulse reactor is unsteady state,

378