测量来自非固定催化剂的瞬态反应速率
Measuring transient reaction rates from non-stationary catalysts
摘要：
到目前为止，可用于测量在多相催化剂上发生的反应的速率常数的方法要求催化剂在长的测量时间内是稳定的。但催化剂通常是非固定的，它们可能会在反应条件下被活化或在使用过程中中毒。因此，需要开发具有高动力学数据采集速率的方法，以便可以在非固定催化剂上测量瞬态速率。在这项工作中，我们提出了速度分辨动力学使用高重复频率脉冲激光电离和高速离子成像检测。该反应是由入射在表面的分子束脉冲引发的，并且通过高重复率的激光脉冲序列来观察产物形成速率。离子成像提供了作为每个激光脉冲的反应时间的函数的解吸产物通量（反应速率）。我们演示了使用10 Hz脉冲CO分子束脉冲训练来启动CO从Pd（332）解吸的方法-通过使用1 kHz Ti：Sapphire激光器的非共振多光子电离每毫秒检测解吸的CO。这种方法克服了过去实验中所需的CO和激光脉冲之间延迟的耗时扫描，并提供了10-1000倍的数据采集速率。我们也将这种方法应用于CO在Pd（332）上的氧化--当CO束从O-饱和表面上照射氧原子时，我们记录了CO $_2$形成的动力学轨迹。这在单个实验中提供了作为O覆盖率的函数的反应速率。我们利用这一点来生产受控但不均匀混合的反应物样品，用于在扩散控制条件下测量反应速率。
英文摘要：
Up to now, the methods available for measuring the rate constants of reactions taking place on heterogeneous catalysts require that the catalyst be stable over long measurement times. But catalyst are often non-stationary, they may become activated under reaction conditions or become poisoned through use. It is therefore desirable to develop methods with high data acquisition rates for kinetics, so that transient rates can be measured on non-stationary catalysts. In this work, we present velocity resolved kinetics using high repetition rate pulsed laser ionization and high-speed ion imaging detection. The reaction is initiated by molecular beam pulses incident at the surface and the product formation rate is observed by a sequence of laser pulses at a high repetition rate. Ion imaging provides the desorbing product flux (reaction rate) as a function of reaction time for each laser pulse. We demonstrate the method using a 10 Hz pulsed CO molecular beam pulse train to initiate CO desorption from Pd(332) - desorbing CO is detected every millisecond by non-resonant multiphoton ionization using a 1-kHz Ti:Sapphire laser. This approach overcomes the time-consuming scanning of the delay between CO and laser pulses needed in past experiments and delivers a data acquisition rate that is 10-1000 times higher. We also apply this method to CO oxidation on Pd(332) - we record kinetic traces of CO$_2$ formation while a CO beam titrates oxygen atoms from an O-saturated surface. This provides the reaction rate as a function of O-coverage in a single experiment. We exploit this to produce controlled yet inhomogeneously mixed reactant samples for measurements of reaction rates under diffusion-controlled conditions.
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