Fischer-Wolfarth Jan-Henrik, Hartmann Jens, Farmer Jason A, Flores-Camacho J Manuel, Campbell Charles T, Schauermann Swetlana, Freund Hans-Joachim
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany.
Rev Sci Instrum. 2011 Feb;82(2):024102. doi: 10.1063/1.3544020.
A new ultrahigh vacuum microcalorimeter for measuring heats of adsorption and adsorption-induced surface reactions on complex single crystal-based model surfaces is described. It has been specifically designed to study the interaction of gaseous molecules with well-defined model catalysts consisting of metal nanoparticles supported on single crystal surfaces or epitaxial thin oxide films grown on single crystals. The detection principle is based on the previously described measurement of the temperature rise upon adsorption of gaseous molecules by use of a pyroelectric polymer ribbon, which is brought into mechanical∕thermal contact with the back side of the thin single crystal. The instrument includes (i) a preparation chamber providing the required equipment to prepare supported model catalysts involving well-defined nanoparticles on clean single crystal surfaces and to characterize them using surface analysis techniques and in situ reflectivity measurements and (ii) the adsorption∕reaction chamber containing a molecular beam, a pyroelectric heat detector, and calibration tools for determining the absolute reactant fluxes and adsorption heats. The molecular beam is produced by a differentially pumped source based on a multichannel array capable of providing variable fluxes of both high and low vapor pressure gaseous molecules in the range of 0.005-1.5 × 10(15) molecules cm(-2) s(-1) and is modulated by means of the computer-controlled chopper with the shortest pulse length of 150 ms. The calorimetric measurements of adsorption and reaction heats can be performed in a broad temperature range from 100 to 300 K. A novel vibrational isolation method for the pyroelectric detector is introduced for the reduction of acoustic noise. The detector shows a pulse-to-pulse standard deviation ≤15 nJ when heat pulses in the range of 190-3600 nJ are applied to the sample surface with a chopped laser. Particularly for CO adsorption on Pt(111), the energy input of 15 nJ (or 120 nJ cm(-2)) corresponds to the detection limit for adsorption of less than 1.5 × 10(12) CO molecules cm(-2) or less than 0.1% of the monolayer coverage (with respect to the 1.5 × 10(15) surface Pt atoms cm(-2)). The absolute accuracy in energy is within ∼7%-9%. As a test of the new calorimeter, the adsorption heats of CO on Pt(111) at different temperatures were measured and compared to previously obtained calorimetric data at 300 K.
本文描述了一种新型的超高真空微量热计，用于测量复杂单晶基模型表面上的吸附热和吸附诱导的表面反应热。它专门设计用于研究气态分子与由负载在单晶表面上的金属纳米颗粒或生长在单晶上的外延氧化物薄膜组成的明确定义的模型催化剂之间的相互作用。检测原理基于先前所述的利用热释电聚合物带测量气态分子吸附时的温度升高，该热释电聚合物带与薄单晶的背面进行机械/热接触。该仪器包括：（i）一个制备室，提供所需设备，用于在清洁的单晶表面上制备包含明确定义的纳米颗粒的负载型模型催化剂，并使用表面分析技术和原位反射率测量对其进行表征；（ii）吸附/反应室，包含分子束、热释电热探测器以及用于确定绝对反应物通量和吸附热的校准工具。分子束由基于多通道阵列的差动泵浦源产生，能够提供0.005 - 1.5×10¹⁵分子·cm⁻²·s⁻¹范围内的高、低蒸气压气态分子的可变通量，并通过计算机控制的斩波器进行调制，最短脉冲长度为150 ms。吸附热和反应热的量热测量可以在100至300 K的宽温度范围内进行。引入了一种用于热释电探测器的新型振动隔离方法以降低声学噪声。当用斩波激光向样品表面施加190 - 3600 nJ范围内的热脉冲时，该探测器显示脉冲间标准偏差≤15 nJ。特别是对于CO在Pt(111)上的吸附，15 nJ（或120 nJ·cm⁻²）的能量输入对应于小于1.5×10¹²个CO分子·cm⁻²的吸附检测限，或小于单层覆盖度的0.1%（相对于1.5×10¹⁵个表面Pt原子·cm⁻²）。能量的绝对精度在~7% - 9%以内。作为对新微量热计的测试，测量了不同温度下CO在Pt(111)上的吸附热，并与先前在300 K时获得的量热数据进行了比较。
