Synthesis of microporous eskolaite from Cr(VI) using activated carbon as a reductant and template A. Cruz-Espinoza a , V. Ibarra-Galván a , A. López-Valdivieso b,⇑ , J. González-González a a Universidad de Colima, Facultad de Ciencias Químicas, Carretera Colima-Coquimatlán km. 9.5, Coquimatlán, Colima 28400, Mexico b Universidad Autónoma de San Luis Potosí, Instituto de Metalurgia, Av. Sierra Leona 550, San Luis Potosí 78210, Mexico article info Article history: Received 17 September 2011 Accepted 27 January 2012 Available online 8 February 2012 Keywords: Chromium oxide Eskolaite Activated carbon Synthesis Adsorption Dichromate abstract A novel technique has been devised for the synthesis of microporous a-Cr 2 O 3 (eskolaite). The technique was based on the formation of amorphous-Cr 2 O 3 onto microporous activated carbon through adsorption– reduction of dichromate ions ðCr 2 O 2 7 Þ at the activated carbon/aqueous solution interface. Then, the Cr 2 O 3 -loaded carbon was thermally processed under oxidizing conditions to remove the carbon and recover the chromium oxide as a-Cr 2 O 3 . Both the Cr 2 O 3 -loaded carbon and the synthetic product were characterized by XRD, SEM, surface area and pore volume measurements. The synthetic eskolaite assayed 97.3% Cr 2 O 3 and its specific surface area was 15.48 m 2 /g and pore size of 16.1 Å. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Chromium oxide (Cr 2 O 3 ) is an important refractory material due to its high melting temperature and oxidation resistance. Ultrafine Cr 2 O 3 (eskolaite) is used as a catalyst in oxidation reactions, hydro- genation reactions, isomerization of olefins, dehydrogenation of al- kanes and pigment production [1–3,13]. Cr 2 O 3 particles with nanostructure and micropores are becoming important in many fields such as catalysis, pigment production, enhancement of lith- ium-battery electrochemical performance and hydrogen absorp- tion material [4–6]. Chevalier et al. [7] have shown that nanoparticles of Cr 2 O 3 and Nd 2 O 3 on stainless steel protect this material against corrosion at high temperature. Grinding nanosize particles of Cr 2 O 3 together with magnesium gives rise to a material which is very effective for hydrogen absorption [4]. Anhydrous chromic oxide is produced commercially from chro- mium hydroxide, dry ammonium dichromate, or sodium dichro- mate by heating with sulfur. There are various routes to prepare nanoparticles of Cr 2 O 3 such as the precipitation–gelation reaction of Cr(III) followed by calcinations [8], precipitation of Cr(III) at the yeast/aqueous solution interface followed by calcinations [6], hydrothermal reduction of CrO 3 with HCHO [9], combustion of (NH 4 ) 2 Cr 2 O 7 together with glycine or urea [2], thermal decomposi- tion of chromium nitrate mixed with cetyltrimethylammonium bromide [10]. Chevalier et al. [7] deposited Cr 2 O 3 nanoparticles on stainless steel by metal–organic chemical vapor deposition using chromium acetylacetonato. Zhong et al. [11] used laser assisted deposition from solution to make Cr 2 O 3 nanoparticles on Si wafers or soda glass from CrCl 2 and Cr(CO) 6 dissolved in mix- tures of methanol, cyclohexane, tetrahydrofuran and diethylether. Wang et al. [12] followed the hard-templating pathway to deposit Cr 2 O 3 nanoparticles on mesoporous silica using chromium nitrate as the precursor. This work aims at presenting a novel technique for the synthesis of ultrafine a-Cr 2 O 3 using K 2 Cr 2 O 7 as a precursor and microporous activated carbon as a reductant and template. The chromium- loaded activated carbon was thermally treated under oxidizing conditions to remove the carbon and produced the a-Cr 2 O 3 . 2. Experimental section Activated carbon was obtained from Calgon Company in granu- lar form. It was prepared from coconut shell and physically activated. This carbon contained 97% C and 3% inorganic compounds with SiO 2 , Al 2 O 3 , CaO and P 2 O 5 . The carbon was ground in a ceramic mill with alumina rods as grinding media. The ground powder was wet-screened to collect the 60 + 325 mesh (250 + 45 lm) size fraction for chromium adsorption. This size fraction was dried at 110 °C for 24 h and kept in a desiccator. Analytical grade potassium dichromate (K 2 Cr 2 O 7 ) was used as the source of Cr(VI). All aqueous solutions were prepared with deionized water, which was produced by passing distilled water through a Barnstead E-Pure system. Dilute solutions of sodium hydroxide and sulfuric acid were used to adjust the pH in the adsorption tests. 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2012.01.059 ⇑ Corresponding author. Fax: +52 444 8255004x108. E-mail address: alopez@uaslp.mx (A. López-Valdivieso). Journal of Colloid and Interface Science 374 (2012) 321–324 Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis