Activated carbons heat treatment temperature effects

It has been demonstrated that a pyrolysis step is necessary and critical in improving both the activity and the stability of Fe- and Co-Nx ORR electrocatalysts, van Veen et al. discussed four models in an effort to explain this pyrolysis effect (1) improving the dispersion of the supported chelate (2) catalyzing the formation of a special type of carbon, which is actually the active phase (3) generating the M-Nx species and (4) promoting a reaction between chelate and subjacent carbon in such a way as to modify the electronic structure of the central metal ion with retention of its N4 coordinated environment. Actually, the active sites should be M—N4 or M—N2 units, depending on the heat treatment temperature (see Figure 3.8). 

The temperature range used to heat-treat the precursors was at first relatively wide, from 300 to 1,000 °C. Wiesener et al. studied the effect of heat-treatment temperature on the ORR current density at catalysts derived from N4 macrocycles at a potential of 0.70 V (vs. RHE) in 2.25 M H2SO4 (Fig. 8.1) [23]. They found that the optimum temperature for heat-treating cobalt tetraazaannulene (CoTAA) on activated carbons P33 and P33p was aroimd 650 °C. 

Activated carbons have a spread of pore sizes. Consequently the possibility that they can show a partial molecular sieve effect cannot be overlooked when the components of the binary solution are not of the similar molecular dimensions. This factor would add a degree of preferential adsorption of the components of smaller size molecules irrespective of the competitive adsorption due to other factors. The composite isotherms would, therefore, be of the type obtained on heterogeneous surfaces. This competitive adsorption effect wiU be more prominent and visible when carbons are produced from the same source raw materials by different procedure or post preparation treatments. For example, carbons that have been produced after varying degrees of activation or carbons that are heat treated at varying temperatures after activation will have different porosities and pore size distribution. The extremely fine micropores get partially blocked as the final heat treatment temperature exceeds 800°C to 900°C, due to the calcinations of the pores. This will produce molecular sieve effect depending upon the heat treatment temperature. 

Okada et al. investigated the binary system Pt-Co(MPQH) (where MPQH mono-8-quinolyl-o-phenylenediamine) [89]. The catalysts were heat treated in Ar atmosphere for 2 h. The optimum heat treatment temperature was 873 K. The mass specific catalytic activity of the binary catalysts supported on carbon was 5 A gn compared to 40-60 A gpt for PtRu. It was also shown that the nature and structure of the ligand coordinating the Co had an effect on performance. Co(NH3)6Cl3 did not exert any catalytic effect when combined with Pt, while porphyrin based complexes of Co heat treated at 1073 K yielded approximately 20 to 30 mV lower overpotentials compared to catalyst formulatiorrs using Co(MPQH). 

Some recent results have revealed that the optimal activity for N4-chelate catalysts is normally obtained at a heat-treatment temperature range of 500-700 °C [30-32], However, it has also been discovered that a higher pyrolysis temperature (> 800 °C) is necessary in order to achieve stable performance in a PEM fuel cell enviromnent. A deleterious effect on electrode performance was observed at temperatures higher than 1100 °C [33], Even for some carbon-supported Fe- and Co-phthalocyanines, stability can also be considerably improved. For example, an almost 50 times greater enhancement in electrocatalytic activity was achieved at an electrode potential of 700 mV (vs. NHE) when carbon-supported Co-phthalocyanine was heat-treated in an environment of N2 or Ar at 700-800 °C [34]. Furthermore, in experiments with carbon-supported Ru-phthalocyanine, heat treatment at 650 °C could increase the catalytic activity by 20 times at 800 mV (vs. NHE). Unfortunately, there was no insignificant improvement in catalyst stability. Not all heat-treated carbon-supported metal phthalocyanines gave positive results. For example, the activities and stabilities of Zn- and Mn-phthalocyanines were not affected by heat treatment [34]. The duration of heat treatment for these complexes is usually around 0.3 5 hrs. 

FIGURE 3.10 Effect of heat-treatment temperature versus specific corrosion rates of Vulcan XC-72R. Corrosion rates of the carbon black were tested at 1.0 V in H3PO4 at 180°C. (Reprinted from Journal of Power Sources, 173, Bezerra, C. W. B. et al. A review of heat-treatment effects on activity and stability of PEM fuel cell catalysts for oxygen reduction reaction, 891-908, Copyright (2007), with permission from Elsevier.)... 

Another part of our investigation deals with the effect of heat treatment on the leaching behavior of palladium on activated carbon catalysts. Heat treatment is a known technique to increase the performance of catalysts. (3) Therefore, standard carbon supported palladium catalysts were exposed to different temperatures ranging from 100 to 400 °C under nitrogen. The catalysts were characterized by metal leaching, hydrogenation activity and CO-chemisorption. 

K [1,3], which would result in the drastic increase of the mesopore in the activated carbons synthesized in 100% Hj. Based on these mechanisms of the mesopore production, control of the particle size of the Fe compound should be important to produce mesopores. The size of the particles can be controllable through adjustment of the introduced amount of Fe in the precursor, dispersion of Fe at the preparation stage of the precursor and the condition of heat-treatment such as heating rate and treatment temperature. The conditions should be optimized depending on precursors. We also confirmed the effectiveness of the present method in selective increase of mesopores of activated carbons using used coffee beans and tea leaves wastes. The results will be presented in the next paper. 

On the other hands, it becomes obvious that SOx and NOx in flue gas can be removed at room temperature by using active carbon fibers (ACF) subjected to surface treatment such as heat treatment [1, 2], The flue gas treatment technology using ACF is a semidry oxidation type de-SOx method which is effective even around room temperature. In addition, this technology enables by-products such as sulfuric acid, sulfates, nitric acid, and various nitrates to be recovered, and is applicable in the field of flue gas treatment to which the conventional de-SOx method, such as the limestone gypsum method, could not be applied for economical reasons. 

Macias-Garcia et al. [208] did not cite any of these studies and thus came to very confusing conclusions, but they did analyze an intriguing set of chemically modified activated carbon samples (exposure to N at 900°C, exposure to H S at 900°C. exposure to SO2 and then H2S at 30°C followed by treatment in N2 at 200°C). Their results are summarized in Table 11. The characterization of the surface chemistry after various treatments is limited to statements about the formation of surface sulfur and S=0 groups [which] may belong to the species SO or SOj-. The authors then simply conclude that the most effective [method] to increase the adsorption capacity of AC [is] the heat treatment in N , which they attributed to the development of porosity in this sample (even though their own data—see Table 11—do not really support such an explanation). The authors do not comment on the fact that the uptake at low pH is electrostatically unfavorable and is expected to be even more so after the removal of acidic surface groups (unless these are reconstituted upon exposure of the sample treated in N at 900°C to room-temperature air see Refs. 82 and 84). Finally, a... 

---