The effects of CO2 on the selective formation of i-C-4 hydrocarbons (isobutene and isobutane) from CO2/CO/H-2 reactant mixtures were studied. Three ZrO-based catalysts including unmodified ZrO2, 8.6%Y2O3-ZrO2, and 15.3%Al2O3-0.5%K2O-ZrO2, were used in this study. The catalysts were characterized by N-2 adsorption, X-ray diffraction (XRD), Raman, spectra, temperature-programmed desorption (TPD) of ammonia and carbon dioxide, and temperature-programmed reduction (TPR). The influences of CO2 on the catalytic performances of the catalysts were investigated by varying the content of CO2 in the feed up to 20%. It was found that the addition of CO2 in the synthesis gas (CO/H-2) significantly reduced the net formation of CO2, but did not affect the formation of hydrocarbons. For the distribution of hydrocarbons, the selectivity to i-C-4 in total hydrocarbons decreased with increasing the content of CO2 in the feed, while the selectivities to C-1-C-3 hydrocarbons increased. However, the i-C-4 selectivities in all products were enhanced significantly because of the inhibition of CO2 formation with CO2 adding in the feed. The predominant products in CO2 hydrogenation on the ZrO2-based catalysts were CO and H2O at 648-723 K, indicating that the ZrO2-based catalysts were not active in the reactions towards hydrocarbons from CO2/H-2. A 5-6% yield of i-C-4 hydrocarbons with similar to 62% selectivity in the products was achieved on 15.3%Al2O3-0.5%K2O-ZrO2 catalyst at 8-9% CO conversion with 20% CO2 adding in the feed at 698 K. The yield of CO2 was only 0.5% (similar to 5% CO2 selectivity in the products). Our results would suggest one potential way of using the recycle of CO2 formed to selectively synthesize i-C-4 hydrocarbons from coal or natural gas-derived syngas (CO + H-2) with high carbon efficiency (with free or very low CO2 emission). (C) 2007 Elsevier B.V. All rights reserved.