Preparation of carbon supported cobalt by electrostatic adsorption of [Co(NH3)6]Cl3

Our previous paper [L. D'Souza, L. Jiao, J.R. Regalbuto, J.T. Miller, A.J. Kropf, J. Catal. 248 (2007) 165] presented the synthesis of cobalt catalysts on carbon (Timrex) and silica supports by strong electrostatic adsorption (SEA), using a cobalt hexaamine chloride ([Co(NH3)6]Cl3, CoHA) precursor. The CoHA undergoes reductive deammination in an uncontrolled manner in the presence of NaOH and adsorbs as Co3O4 on carbon with broad size distribution. The present paper extends these studies toward the end of synthesizing well-dispersed Co oxide particles in a narrow size range on carbon supports using NH4OH. Cobalt uptake versus pH was determined in NH4OH and NaOH basified solutions over a number of carbons with varying point of zero charge (PZC). The resulting materials were characterized by ICP, powder XRD, XAS, TPR and STEM. CoHA in the presence of NH4OH adsorbs as well dispersed as CoO, Co3O4 and Co(OH)2−4 depending upon the pH of the adsorption solution. These phases were undetectable by powder XRD and STEM Z-contrast imaging, but could be identified by XAS. Additionally, non-adsorbed CoHA complexes underwent transformation to [Co(NH3)5Cl]Cl2 at pH>11pH>11 in solution. After calcinations of 250 °C, particle sizes of Co3O4 range from 20–50 Å from NH4OH and 50–200 Å from NaOH. Maximum metal uptake was approximately 3.3 and 2.7 μmol/m2 in presence of NaOH and NH4OH, respectively. The SEA method of preparation was compared with incipient wetness impregnation (IWI) of Co(NO3)2⋅6H2O; this method yields Co3O4 particles after 250 °C calcinations which are almost as small or in one case, smaller than the calcined SEA samples. Higher metal loadings can be achieved by the SEA method by successive adsorption steps with a little variation in particle size and distribution. However, the main advantage of SEA is in forming mono- or submonolayer of different Co oxide phases on carbon surface.