For a surface-catalyzed decomposition reaction, the rate depends on the fraction of the catalyst surface covered by the reactant molecules. At low pressures, the surface is sparsely covered, and the surface coverage is directly proportional to the partial pressure of the reactant gas. Since the rate is proportional to the surface coverage, it becomes proportional to the pressure of the gas, making it a first-order reaction. Conversely, at high pressures, the surface becomes fully saturated (completely covered), and the rate becomes independent of pressure, resulting in zero-order kinetics.