James A. Schwarz

Estimation of the point of zero charge of simple oxides by mass titration

Abstract A method is proposed to estimate the point of zero charge (pzc) of pure compounds which can be described accurately by the surface ionization model of amphoteric oxides. It is demonstrated that under the limiting conditions of “infinite” mass/volume ratio, the pH of the system will approach pH ∞ = 1 2 ( p K 1 + p K 2 )  pzc . Experimental values of the pH of the oxides of titanium, aluminum, a silicon suspended in a closed aqueous system exhibited the existence of a constant pH at high mass fractions. This limiting pH is found to be a reasonable estimate for the point of zero charge of each oxide.

Effect of HNO3 treatment on the surface acidity of activated carbons

Abstract A homologous series of activated carbon samples was prepared from a low ash source carbon by controlled oxidation using nitric acid. The physicochemical properties (specifically, BET surface area, ash content, acid/base neutralization, and ammonia adsorption) were determined. Each property value changed systematically with increasing oxidation treatment; the more oxidized sample showed higher surface acidities without significant changes in BET area or ash content. The point of zero charge (PZC) of each sample was determined by mass and acid/base potentiometric titration. The PZC values also changed systematically with the extent of oxidation; the more oxidized the carbon, the lower its PZC. It is concluded that the PZC can be used as a convenient index for correlating changes in surface acidity of carbons.

Temperature-Programmed Desorption and Reaction: Applications to Supported Catalysts

Abstract In a typical temperature-programmed desorption (TPD) experiment on a supported metal catalyst, a small amount of catalyst (10–200 mg) is contained in a reactor that can be heated by a furnace. An inert gas, usually helium at atmospheric pressure, flows over the catalyst. Following pretreatment to obtain a reduced catalyst, a gas is adsorbed on the surface, usually by pulse injections of the adsorbate into the carrier gas upstream from the reactor. After the excess gas is flushed out, the catalyst is heated to create a linear rise in temperature with time. A small thermocouple inserted in the catalyst measures the temperature and a detector downstream measures the change in the inert gas stream. The ideal detector is a mass spectrometer which measures the composition of the effluent stream as a function of catalyst temperature. Because of the high carrier-gas flow rate, the detector response is proportional to the rate of desorption if diffusion and readsorption are not limiting.

The zero point of charge of silica-alumina oxide suspensions

The pHzpc (pH at the zero point of charge) of a series of seven silica—alumina oxide supports, previously heat treated to 500°C, were determined by potentiometric titrations. Rigorous experimental procedures in support preparation and standardization have allowed us to compare the pHzpc of each of the oxides with the weight fraction of the pure components. The pHzpc of this homologous series varied linearly with the weight fraction of the pure components. These results are in direct agreement with a mass weighting model suggested by G. A. Parks (Advances in Chemical Series 67, ACS, Washington, D. C.) for calculating the pHzpc for any composition of a mixed oxide system.

Surface functionality and porosity of activated carbons obtained from chemical activation of wood

a `´ ´ ´ ´ Abstract Mixtures of wood with either phosphoric acid or diammonium hydrogen phosphate ((NH ) HPO ) were heated under 42 4 nitrogen or steam / nitrogen flows at temperatures between 300 and 500 8C. As a result of these processes, activated carbons with various pore size distribution and surface properties were obtained. The samples were characterised using potentiometric titration and sorption of nitrogen at its boiling point. The results showed the significant influence of the nature of the activating agent and the atmosphere on the final properties of the materials. It was demonstrated that steam inhibits the incorporation of heteroatoms into the carbon matrix. Activation of wood in the presence of phosphoric acid together with 2 steam gives carbon of high surface area (|1800 m / g) with well-developed mesoporosity and an almost neutral surface.

Surface acidity of carbons characterized by their continuous pK distribution and Boehm titration

The surface acidity of several activated carbons was evaluated using the Boehm and potentiometric titration methods. Experimental conditions (particle size, rate of titration) were varied and several procedures to collect raw data for proton binding isotherms (slow continuous addition of titrant, incremental addition conditioned by a pH stability criterion and batch experiments) were compared. Proton binding isotherms were analyzed at two levels. First, continuous pK distributions were derived and the inventory of surface functionalities was characterized in terms of amount and pK values. Then these data afforded calculations of the base neutralizing capacity of the carbons studied at any pH level. The results were compared with those from Boehm titration of the same samples. Best results were found for titration of finely powdered carbons. Continuous titration denatures the binding isotherms and damps significant features in the pK distribution. On the other hand, titration by incremental addition with a pH stability criterion provides reliable results which can be compared to those retrieved from Boehm titration provided the proper common metric is used. The agreement was quantitative when the equilibrium pH of the supernatant of the Boehm bases was taken as the endpoint for counting neutralized sites. Advantages of analyzing potentiometric titration data in terms of its acidity distribution function with respect to providing a comprehensive characterization of the inventory of surface functionalities as well as its predictive power to assess a carbons base neutralizing capacity under different experimental conditions are discussed.

Analysis of high pressure adsorption of gases on activated carbon by potential theory

Analysis of high pressure adsorption isotherms on activated carbon at a number of temperatures for methane, ethane, ethylene, propane, carbon dioxide and nitrogen was carried out. Isotherms were obtained both above and below the critical temperature (Tc) for ethane, ethylene, and carbon dioxide, below (Tc) for propane and above (Tc) for methane and nitrogen and up to 60 atm of pressure. A comprehensive evaluation of the various methods proposed to apply potential theory to the adsorption data is presented. Temperature-independent characteristic curves for each adsorbate could be obtained if 1. (1) adsorbed phase molar volumes, necessary to obtain the characteristic curves, are evaluated by Dubinins method and 2. (2) if the adsorption potential is defined as RT ln(fsf), where fs was calculated from Ps = (TTc)2Pc. The Dubinin-Astakhov equation gave the most suitable representation of the data.

Hydrogen storage on superactivated carbon at refrigeration temperatures

Abstract This paper presents a comparison of hydrogen storage on superactivated carbon (AX-31M) at refrigeration temperatures (SA-M) to that of more traditional alternatives: compressed gas storage, liquefaction and metallic hydride systems. It specifically reports investigations on the performance characteristics of this new SA-M system. Based on the experimental data obtained, an economic evaluation relative to the other alternatives is made. The SA-M system is demonstrated to be an alternative hydrogen storage method with a great future potential.

A modified approach for estimating pseudo-vapor pressures in the application of the Dubinin-Astakhov equation

High pressure adsorption data of methane and hydrogen on four microporous carbons were obtained at supercritical temperatures. The data were analyzed on the basis of the Potential Theory using the Dubinin-Astakhov (D-A) equation. The method for estimating pseudo-vapor pressures proposed by Dubinin resulted in characteristic curves for methane but failed for the hydrogen data. A more general form of Dubinins method for estimating pseudo-vapor pressures is proposed. This approach introduces a parameter, k, that accounts for interactions in the adsorbate-adsorbent systems under consideration. It was found that the modified approach to the use of the D-A equation leads to reliable prediction of both methane and hydrogen data.

Hydrogen storage systems using activated carbon

Abstract This paper reviews state-of-the-art technology in the development of secondary stationary and mobile hydrogen storage systems, in particular the use of activated carbons. A new concept called Modification/Metal Assisted Cold Storage (MACS) is described. This includes the improvement in the efficiency of activated carbons in two ways: one is modification of the acidity of activated carbon surfaces and the other is modification by metal impregnation on the activated carbon surface. Evaluation of systems performance and cost of the four major alternatives for hydrogen storage: pressurized cylinder, metal hydride, liquefaction, and storage on activated carbons is presented. The MACS system is demonstrated to be an attractive method for advanced hydrogen energy storage.