Experimental Observation of Critical Depletion:  Nitrous Oxide Adsorption on Silica Gel

Abstract

The adsorption of nitrous oxide (N2O) on silica gel has been studied using a gravimetric apparatus under near critical conditions. In gravimetric measurements, the determination of the excess amount adsorbed depends on a buoyancy correction factor which accounts for the volumes of the adsorbent and the solid parts in the cell. The accurate measurement of the volume of the sorbent and the metal parts in the measuring cell plays an important role in the precise measurement of the excess adsorbed amount especially at high densities. To this aim, a new protocol has been proposed which accounts for non-negligible helium adsorption and thermal expansion of the sorbent and the solid parts of the balance. Helium isochores under moderate temperature and densities have been experimentally measured. The adsorption behavior of helium has been characterized using a nonlinear mobile adsorption model. The proposed protocol has then been applied for the measurement of the excess adsorbed amount of supercritical N2O on silica gel very close to the critical point. Clear experimental evidence of critical depletion, i.e., the decrease of the excess adsorption along an isochore with decreasing temperature, has been obtained. Unlike the previous observation, where critical depletion was seen only in the direct vicinity of the critical isochore, in the present study this is observed on isochores in the range of 0.74 ≤ ρr ≤ 1.15.

Reliable measurement of near-critical adsorption by gravimetric method

“…An exception to the general trend is observed near the critical temperature and the critical density. At reduced density values between 0.9 and 1.2, the measured values of the excess amount adsorbed at 31.4 • C are in fact smaller than or equal to those at 36.3 • C. Similar effects have been observed previously for other experimental systems, and have been called "critical depletion" (Machin , 1999;Rajendran et al, 2002;Thommes et al, 1994Thommes et al, , 1995. Such phenomenon has always eluded a convincing theoretical explanation.…”

“…In the past, several effects were observed in our laboratory when measuring near-critical isotherms by the gravimetric method, namely critical adsorption (Hocker et al, 2003) and critical depletion (Rajendran et al, 2002). However, anomalous behaviors in the near-critical region were also observed by other authors.…”

“…To evaluate the adsorption excess then, it is very important that the conditions in the different locations are the very same in terms of temperature and density. More information about the measuring procedure can be found else where (Di Giovanni et al, 2001;Rajendran et al, 2002).…”

“…In this study a device based on the gravimetric technique is used. It is a magnetic suspension balance from Rubotherm (Bochum, Germany), that has been used for high-pressure adsorption measurements of several gases on different adsorbents in our laboratory (Di Giovanni et al, 2001;Hocker et al, 2003;Rajendran et al, 2002). This instrument allows to measure the weight of a suspended basket containing the adsorbent and the density of the fluid bulk phase with the help of a calibrated sinker (Dreisbach and Lösch, 2000).…”

Adsorption of pure carbon dioxide and methane on dry coal from the sulcis coal province (SW Sardinia, Italy)

“…In our laboratory, we have been successfully measuring supercritical adsorption of different adsorbates on standard adsorbents using a gravimetric magnetic suspension balance, for example CO 2 and N 2 O on silica gel and 13X zeolite [20][21][22]. The observed excess adsorption isotherms all increase to reach a maximum, and then decrease regularly with increasing density.…”