ARTICLE pubs.acs.org/JPCC
New Model for a Pd-Doped SnO2-Based CO Gas Sensor and Catalyst Studied by Online in-Situ X-ray Photoelectron Spectroscopy Wancheng Li,† Chunsheng Shen,† Guoguang Wu,† Yan Ma,† Zhongmin Gao,‡ Xiaochuan Xia,† and Guotong Du*,† †
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering and ‡State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Jilin University, Changchun 130012, China ABSTRACT: The functional mechanism of oxide-based gas sensors and the gas catalysis reaction are controversial topics. The traditional static photoelectron data collection technique is obviously not satisfied for this study. However, after appropriate remodification of the data collection system we propose a new, online XPS in-situ data collection technique for overcoming this difficulty. This online photoelectron collection technique was employed for the first time on this kind of study with SnO2-based gas sensors. The results we found are as follows: (1) For rutile SnO2 in powder form, its O1s peak of 531.8 eV would come from the lattice oxygen in the surface area, not from chemisorbed oxygen as proposed conventionally; (2) we observed that in the adsorptive process of CO onto Pd-doped SnO2, Pd, CO, and lattice oxygen in the surface area of SnO2 could form an intermediate product [Pd(CO)4]O4, from which comes a unified new model for illustrating the mechanism of this kind gas sensor, on one hand, and the oxidation catalysis process, on the other; (3) we noticed a transient magnetic phenomenon during the adsorption process of CO onto SnO2. This could open a door for studying spin sensors.
’ INTRODUCTION SnO2-based gas sensors have the advantage of having a small size and being lightweight and simple fabrication procedures. Thus, they are widely utilized for detecting toxic gases or contamination gases in the atmosphere, such as CO, NOX, SO2, and CO2, as well as combustible gases, like H2, CH4, and C2H5OH. By doping with noble metals such as Pd, Pt, and Au, the sensitivity and selectivity of this kind of gas sensors can be elevated.1 On the other hand, SnO2 and other metal oxides doped with Pd,2 Pt,3 and Au4 are employed for catalytic oxidation reactions with these kinds of gases as just cited. Noble metals as catalyst could promote catalytic efficiency and lower reaction temperature. This means the functional mechanism of gas sensors and catalysis could be interexplainable. A Pd-doped SnO2-based CO gas sensor is a typical example among gas sensors based on metal oxides, whereas oxidation of CO constitutes a major model for illustrating the process involved in the related catalytic reaction. This means how does CO + O = CO2? Thus, first we have to figure out how oxygen is transformed in the reaction process involved. However, an argument has been associated in understanding the problem either for the working mechanism related to the study of gas sensors or for investigation of the catalytic process involved with gaseous oxidation. For sensors related to combustive gases, it is generally regarded that the oxygen source involved in the reactive process comes from chemisorbed oxygen.5 On the other hand, for the study of the catalytic process involved, acceptable reductionoxidation models assume the oxygen source comes from the lattice oxygen of related catalytic reagents or oxides.6 The investigation technique for a solid surface, such as XPS (X-ray photoelectron spectroscopy), has special advantages, such r 2011 American Chemical Society
as a very shallow layer for sample data collection (
