Low-Temperature Generation of Basic Carbon Surfaces by Hydrogen Spillover J. Angel Mene ´ ndez and Ljubisa R. Radovic Fuel Science Program, Department of Materials Science and Engineering, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 Bo Xia and Jonathan Phillips* Department of Chemical Engineering, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: April 30, 1996; In Final Form: August 26, 1996 X It is demonstrated that stable basic carbons, which will not adsorb oxygen in ambient laboratory conditions, can be created via a relatively low-temperature process. These highly basic carbons are created by treating mixtures of carbons and platinum (in the form of particles supported on a high surface area material) in hydrogen at 500 °C, or even at lower temperatures in some cases. In the absence of platinum, creation of highly basic and stable surfaces with the same starting material requires hydrogen treatments at far higher temperatures (ca. 900 °C). Evidence is presented to support the hypothesis that the role played by platinum (or any noble metal) is to produce atomic hydrogen, which spills over onto the carbon surface. This atomic hydrogen hydrogasifies the most reactive, unsaturated carbon atoms at far lower temperatures than molecular hydrogen, thus leading to surface stabilization at relatively low temperatures. Introduction Modern industry requires activated carbons with not only optimum physical properties (e.g., surface area, porosity, pore size distribution, hardness, etc.) but also specified chemical properties. By use of different treatments, the chemical proper- ties of a carbon can be tailored 1,2 for specific applications. For example, there are a variety of oxidative treatments designed to produce acidic activated carbons with different distributions of oxygen-containing acidic groups. 2-7 In contrast, methods for creating activated carbons with basic properties, which are stable in ambient conditions, are still under investigation. Such carbons are of great interest in numerous applications. 8-13 Recently, we demonstrated one method for creating a stable basic carbon. 14 Activated carbons treated in hydrogen at high temperature (only 900 °C or higher) were found to be basic in character and stable in ambient laboratory conditions for periods of months. This treatment was effective in removing oxygen in the form of CO and CO 2 , thus removing the acidic functional groups (e.g., carboxyl, lactone, phenol) from the surface. Moreover, the most reactive carbon atoms, which were left behind by oxygen removal (as carbon oxides), were subse- quently removed by hydrogen, thus greatly diminishing the tendency of the carbon to readsorb oxygen and reacidify. The focus of the present work is on testing a new hypothesis regarding the creation of stable, basic carbon surfaces. To wit: In the presence of hydrogen atoms, stable basic carbon surfaces can be created at far lower temperatures than is possible in the presence of molecular hydrogen. Experimentally, it was shown that in the presence of noble metals, capable of creating and “spilling over” hydrogen atoms, stable basic carbon surfaces could be created at 500 °C or higher. That is, in the presence of noble metals, stable basic carbons can be created at temperatures at least 400 °C lower than is possible in their absence. Experimental Section Sample Preparation. Two different carbons were employed in the experiments described below. One material is a com- mercial activated carbon, Norit C-Granular (Nc), prepared from a wood precursor by phosphoric acid activation. It is of very high purity and has a high BET surface area (1378 m 2 /g). This material was in some cases mixed with a commercially available carbon (Alfa Aesar, Johnson-Matthey, Stk#38343, 1% Pt) containing 1 wt % platinum in highly dispersed form (PtC). The particle sizes of the carbons were very different (PtC: 4 × 8 mesh, N c < 70 mesh), allowing them to be separated by sieving at the completion of all treatments and prior to any measurements. Samples were prepared in quartz boats placed within a quartz reactor (5 cm, i.d.). In all cases, samples were prepared in 2 g batches. Table 1 is a list of all samples treated, giving the treatment conditions used (gaseous atmosphere, time, and temperature) as well as the values for the point of zero charge (PZC), which are discussed in detail below. The nitrogen- treated sample (N950) was first conditioned in flowing nitrogen (150 mL/min) for 1 h at ambient temperature. Next, the sample temperature was raised to 950 °C at a rate of 25 °C/min. The sample was held in flowing nitrogen for 3 h at 950 °C, then rapidly cooled to room temperature and held at that temperature for another hour. The sample was then removed from the reactor and a fraction (0.3 g) prepared immediately for the determination of PZC. Thus, the PZC was determined after a very brief exposure (<5 min) to ambient laboratory atmosphere. All other samples were treated in a similar fashion. The hydrogen-treated samples (H150-H950) were treated in the same way except that (i) hydrogen was substituted for nitrogen, (ii) the maximum treatment temperatures ranged from 150 to 950 °C, and (iii) once cooled to room temperature, nitrogen was flowed over the samples for 1 h before the samples were removed and exposed to ambient atmosphere. The third step was performed to ensure that all hydrogen was flushed from the chamber/sample prior to oxygen exposure. In one group of experiments, the commercial PtC was pretreated in an acid solution (PtC-Ox). That is, the material * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, October 1, 1996. 17243 J. Phys. Chem. 1996, 100, 17243-17248 S0022-3654(96)01243-9 CCC: $12.00 © 1996 American Chemical Society