Methanol synthesis from CO2 + H-2 was studied at mild reaction conditions (140-250 degrees C and 7 bar) over Cu/ZnO catalysts prepared by citrate method. The copper content and calcination temperatures were varied so as to obtain a wide range in copper particle size (2-12 nm). Methanol formation rates vary between 0.84 and 2.98 x 10(-3) s(-1) at 180 degrees C. Methanol selectivity can attain 100% at temperatures lower than 160 degrees C. At higher temperatures, CO formation by reverse water gas shift reaction is highly favored. The apparent activation energy for methanol formation is in the range 8-10 kcal/mol, whereas that of CO formation is much higher (29-32 kcal/mol). The methanol formation rates depend linearly on the amount of exposed copper atoms, whereas for CO formation there is no clear correlation, showing that special surface sites are responsible for CO formation. The size of copper particles greatly influences the selectivity to methanol at constant CO2 conversion. Larger particles (10-12 nm) are much more selective than smaller ones (2-3 nm), this effect is enhanced when CO2 conversion is low. Catalysts prepared by citrate method are much more active than a reference catalyst prepared by coprecipitation. The activity of CuO + ZnO catalysts prepared by mechanically mixing of CuO and ZnO present activity comparable with that of Cu/ZnO catalysts prepared by citrate method, whereas pure copper and ZnO are not active at these conditions. That strongly suggests that activity is defined by a good contact between CuO and ZnO, whereas the selectivity depends on the morphology of copper nanoparticles. (C) 2012 Elsevier B.V. All rights reserved.