Catalytic reaction characterization using micromachined nanocalorimeters

Nanocalorimeters based on a micromachined hotplate platform were characterized for potential applications in catalyst screening and reaction mechanism studies. The nanocalorimeters were designed to limit heat loss to the surroundings, resulting in a small heat capacity (50 nJ K−1) and thermal time constant (1 ms). This greatly increases the sensitivity of the device, allowing reaction heat flows on the order of <1 μW to be measured in air. The Pt catalyzed combustion of hydrogen in air was used as a model reaction. A relatively-simple heat and mass transfer model was developed which accurately described the data from the microscale system, and the rate-limited and mass transfer-limited regimes of the ignition traces were clearly distinguished. An average activation energy of 48 ± 4 kJ mol−1 was determined for the Pt catalyzed combustion of hydrogen in air. However, unique rapid temperature programmed calorimetry methods convincingly demonstrate that the activation energy is influenced by kinetic phase transitions which also result in reaction rate bistabilities and oscillations.

Figure optionsDownload high-quality image (60 K)Download as PowerPoint slideResearch highlights▶ 4-Element nanocalorimeter arrays with 50 nJ/K sensitivity have been evaluated as catalyst screening platforms. ▶ Combined heat and mass transfer models using both a 2-D finite element method and a 1-D mathematical method were used to model calorimeter response to reaction power. ▶ Reaction rate for catalytic hydrogen oxidation was measured using ramped temperature programming and hysteresis and oscillations in reaction rates were observed. ▶ The 1-D model was used to determine model parameters for mass transfer and reaction kinetics for a range of hydrogen concentrations. ▶ Unique millisecond pulsed temperature programming method suggests that the hysteresis and oscillations result from kinetic surface coverage phase transitions. ▶ Results were used to discuss the utility of the method for catalyst screening.