Sol immobilization method by using a ligand protected gold colloid precursor, Au-n(SC12H25)(m), was applied for the deposition of gold on Nb2O5 which is a typical solid acid. The effects of the thiolate ligand on the preparation of Au/Nb2O5 and on the catalytic activity for CO oxidation were investigated. Although most of the thiolate ligands were removed during the calcination at 300 degrees C, a small amount of sulfur species still remained on the catalyst, which played an important role for preventing the aggregation of the gold nanoparticles to give ca. 2.8 nm in a mean diameter with the gold loading amounts in the range from 0.9 wt% to 4.2 wt%. Calcination at a lower temperature (250 degrees C) gave a larger amount of the sulfur species than did the calcination at 300 degrees C, lowering catalytic activity for CO oxidation. The temperature for 50% CO conversion (T-50%) was 3 degrees C for 0.9 wt% Au/Nb2O5 under 20,000 mL h(-1) g(cat)(-1), using 1 vol% CO in air. With an increase in the loading amount of gold, the catalytic activity became higher. The T-50% of 4.2 wt% Au/Nb2O5 was -41 degrees C and turnover frequency (TOF) based on a number of the perimeter gold atom was calculated to be 8.7 s(-1) at 20 degrees C. The TOF value obtained by Au/Nb2O5 prepared by deposition-reduction without thiolate ligands was 5.0 s(-1). This result suggests that the sulfur remained on the catalyst did not inhibit CO oxidation. (C) 2020 Elsevier Inc. All rights reserved.