The aim of this work is to understand the role of oxygen surface groups during the preparation, activation and reaction of heterogeneous ruthenium catalysts supported on activated carbon materials. Hence, non-promoted and sodium promoted ruthenium catalysts supported on two different activated carbon materials, with and without oxygen surface groups, were prepared by successive incipient wetness impregnation and tested in the ammonia decomposition reaction. The catalysts were characterised with a multi-technique approach that involves; nitrogen adsorption isotherms at - 196 degrees C (BET and BJH methods), temperature programed oxidation (TPO), scanning electron microscope (SEM), temperature programed desorption (TPD), transmission electron microscopy (TEM), in-situ X-ray absorption near edge structure (XANES), temperature programed reduction (TPR) and microcalorimetry of hydrogen chemisorption. The performance of the different ruthenium supported catalysts during the ammonia decomposition reaction was determined in a constant flow fixed-bed reactor at 1 atm, in the temperature range from 350 degrees C to 450 degrees C. This work shows how the oxygen surface groups of the activated carbon can be used to control/influence (i) the final oxidation state of ruthenium particles, (ii) ruthenium particles size, (iii) selective deposition of the sodium promotor. We believe that such use of the presence of surface groups on the activated carbon surface could potentially be employed to improve the catalytic performance of next generation heterogeneous catalysts.