Szanyi János, Kwak Ja Hun
Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352, USA.
Phys Chem Chem Phys. 2014 Aug 7;16(29):15126-38. doi: 10.1039/c4cp00617h.
Alumina supported Pd catalysts with metal loadings of 0.5, 2.5 and 10 wt% were investigated by in situ FTIR spectroscopy in order to understand the nature of adsorbed species formed during their exposure to CO2 and CO. Exposing the annealed samples to CO2 at 295 K resulted in the formation of alumina support-bound surface species only: linear adsorbed CO2, bidentate carbonates and bicarbonates. Room temperature exposure of all three samples to CO produced IR features characteristic of both ionic and metallic Pd, as well as bands we observed upon CO2 adsorption (alumina support-bound species). Low temperature (100 K) adsorption of CO on the three samples provided information about the state of Pd after oxidation and reduction. Oxidized samples contained exclusively ionic Pd, while mostly metallic Pd was present in the reduced samples. Subsequent annealing of the CO-saturated samples revealed the facile (low temperature) reduction of PdO(x) species by adsorbed CO. This process was evidenced by the variations in IR bands characteristic of ionic and metallic Pd-bound CO, as well as by the appearance of IR bands associated with CO2 adsorption as a function of annealing temperature. Samples containing oxidized Pd species (oxidized, annealed or reduced) always produced CO2 upon their exposure to CO, while no CO2-related surface entities were observed on samples having only fully reduced (metallic) Pd.
通过原位傅里叶变换红外光谱对金属负载量分别为0.5 wt%、2.5 wt%和10 wt%的氧化铝负载钯催化剂进行了研究，以了解其在暴露于二氧化碳和一氧化碳过程中形成的吸附物种的性质。在295 K下将退火后的样品暴露于二氧化碳中，仅形成了与氧化铝载体结合的表面物种：线性吸附的二氧化碳、双齿碳酸盐和碳酸氢盐。所有三个样品在室温下暴露于一氧化碳中，产生了离子态和金属态钯的特征红外特征，以及我们在二氧化碳吸附时观察到的谱带（与氧化铝载体结合的物种）。在100 K下对三个样品进行一氧化碳吸附，提供了氧化和还原后钯状态的信息。氧化后的样品仅含有离子态钯，而还原后的样品中主要存在金属态钯。随后对一氧化碳饱和的样品进行退火，结果表明吸附的一氧化碳能使PdO(x)物种容易地（在低温下）还原。离子态和金属态钯结合的一氧化碳的红外谱带变化以及与二氧化碳吸附相关的红外谱带随退火温度的出现证明了这一过程。含有氧化钯物种（氧化、退火或还原）的样品在暴露于一氧化碳时总是会产生二氧化碳，而在仅具有完全还原（金属）钯的样品上未观察到与二氧化碳相关的表面实体。
