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Abstract The chemical decomposition of H 2 O 2 to H 2 O and O 2 catalyzed by platinum nanocatalysts is important in many technologies such as steam propulsion, biosensors, fuel cells, and synthetic chemistry. Surface chemisorbed oxygen strongly impacts the kinetics of this reaction. However, the effect of this surface species has not been quantified in the context of the reaction mechanism. This study determined the rate constants of the elementary reaction steps of H 2 O 2 decomposition on platinum nanocatalysts. For that, hydrogen peroxide decomposition rate measurements were analyzed on samples with variable surface chemisorbed oxygen (Pt(O)) abundance. The order of reaction in terms of surface Pt(O) abundance is 0.83, which indicates a nearly first order effect on the rate of H 2 O 2 decomposition. This result is consistent with a reaction mechanism for H 2 O 2 decomposition on platinum that involves two cyclic steps, where step 1 is the rate limiting step. $$\begin{array}{*{20}l} {{\text{Step}}\;1} \hfill & {{\text{Pt}} + {\text{H}}_{2} {\text{O}}_{2 } \xrightarrow{k = 0.0028} {\text{H}}_{2} {\text{O}} + {\text{ Pt}}({\text{O}})} \hfill \\ {{\text{Step}}\;2} \hfill & {\underline{{{\text{Pt}}({\text{O}}) + {\text{H}}_{2} {\text{O}}_{2} \xrightarrow{k = 0.038}{\text{Pt}} + {\text{O}}_{2} + {\text{H}}_{2} {\text{O}}}} } \hfill \\ {{\text{Overall}}} \hfill & {2{\text{H}}_{2} {\text{O}}_{2 } \xrightarrow{k = 0.037}{\text{O}}_{2 } + 2{\text{H}}_{2} {\text{O}}} \hfill \\ \end{array}$$ Step 1 Pt + H 2 O 2 → k = 0.0028 H 2 O + Pt ( O ) Step 2 Pt ( O ) + H 2 O 2 → k = 0.038 Pt + O 2 + H 2 O ̲ Overall 2 H 2 O 2 → k = 0.037 O 2 + 2 H 2 O The rate constant $$(k)$$ ( k ) of step 2 is 14 times higher than that of step 1 at a reaction temperature T = 295 K. This is consistent with a 4.4 time larger activation energy for step 1. Reaction rates are initially faster on samples with more surface Pt(O) sites because these sites decompose H 2 O 2 through step 2 in the first cycle of the reaction. The relative rates of step 1 and step 2 become smaller at higher temperature. The method presented in this study can be used for determining the rate constants of the intermediate reaction steps of many other chemical reactions that are important for numerous scientific and technological applications. Graphic Abstract