High pressure scanning tunneling microscopy studies of adsorbate structure and mobility during catalytic reactions. Novel design of an ultra high pressure, high temperature scanning tunneling microscope system for probing catalytic conversions.

摘要

The aim of the work presented therein is to take advantage of scanning tunneling microscope's (STM) capability for operation under a variety of environments under real time and at atomic resolution to monitor adsorbate structures and mobility under high pressures, as well as to design a new generation of STM systems that allow imaging in situ at both higher pressures (> 30 atm) and temperatures (> 300 °C).; The design of a high pressure, high temperature scanning tunneling microscope system, that is capable of monitoring reactions in situ at conditions from UHV and ambient temperature up to 1 atm and 250 °C, is briefly presented. Extensive vibrational and thermal analysis of the system is presented, as this system serves as a template to improve upon during the design of the new ultra high pressure, high temperature STM.; Using this existing high pressure scanning tunneling microscope we monitored the co-adsorption of hydrogen, ethylene and carbon dioxide on platinum (111) and rhodium (111) crystal faces in the mTorr pressure range at 300 K in equilibrium with the gas phase. During the catalytic hydrogenation of ethylene to ethane in the absence of CO the metal surfaces are covered by an adsorbate layer that is very mobile on the time scale of STM imaging. We found that the addition of CO poisons the hydrogenation reaction and induces ordered structures on the single crystal surfaces. Several ordered structures were observed upon CO addition to the surfaces pre-covered with hydrogen and ethylene: a rotated (√19 x √19)R23.4° on Pt(111), and domains of c(4 x 2)-CO+C 2H3, previously unobserved (4 x 2)-CO+3C2H 3, and (2 x 2)-3CO on Rh(111). A mechanism for CO poisoning of ethylene hydrogenation on the metal single crystals was proposed, in which CO blocks surface metal sites and reduces adsorbate mobility to limit adsorption and reaction rate of ethylene and hydrogen.; In order to observe heterogeneous catalytic reactions that occur well at high pressure and temperature that more closely resemble industrial settings, a custom STM motor has been designed and constructed in-house. The new STM design features a much reduced size and a rigid coupling to the sample, and has been tested to show considerably higher resonance frequency than conventional tripod designs, providing the ability to image faster and yielding smaller susceptibility to noise. A flow reactor cell of much reduced volume for pressures up to 30 atmospheres has also been designed and constructed to house the new STM. The small volume reduces gas consumption and sensitivity to impurities in high pressure gases, as well as maximizes product concentration and reduces response time. (Abstract shortened by UMI.)