A hydrothermal method of preparing uniform, monodisperse ceramic lanthanum hydroxyl carbonate (LaCO3OH) having cherry-blossom-like nanogears and/or nanocubes is described. The method produced a hexagonal crystal with a crystal lattice in which at least on lanthanum ion is substituted with calcium ion. The ceramic nanoparticles produced by the method are good catalyst for the reduction of nitrogen oxides with a hydrocarbon. A method of reducing exhaust gases is described.