Noble metals, e.g. Pd, are often made into hybrid or composite catalysts (with less expensive materials) to oxidize industry-source emitted volatile organic compounds (VOCs) at low temperatures. In general, the loadings of these metals should be optimized to reduce costs whilst maintaining activity. There exists the possibility to obtain highly active catalysts with low loadings of noble metals by properly tuning the valence state of the metal(s). However, the relationship between the valence state and its effect on catalyst performance is still a matter of debate. In this article, we used supercritical water (sc-H₂O), in the presence of oxidizing or reducing gases, as a feasible reaction medium to synthesize Pd/NiO hybrid nano-catalysts and in situ modify the valence state of Pd. After subjecting the catalysts to a range of analytical techniques, including XRD, H₂-TPR, DRIFT, TPSR, etc., we unveiled that Pd⁰ is more active than PdO_x and metal oxides in the catalytic oxidation of toluene. This is mainly because the stabilized Pd⁰ is capable of activating gaseous oxygen (and toluene) at low temperatures and returning to the original state by toluene even with excess oxygen. Although PdO_x could desorb active oxygen under a reducing atmosphere and might assist in the oxygen spillover from NiO, it is difficult to convert into Pd⁰ in an oxygen-rich environment. The developed Pd⁰-dominated catalyst was found to be robust and highly active after an ageing test with and without water vapour.

中文翻译：

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超临界水中Pd / NiO纳米催化剂对甲苯氧化的原位价修饰†

通常将贵金属（例如Pd）制成混合催化剂或复合催化剂（使用较便宜的材料），以在低温下氧化工业源排放的挥发性有机化合物（VOC）。通常，应优化这些金属的负载量，以降低成本，同时保持活性。通过适当调节一种或多种金属的化合价态，可以获得具有低负载量的贵金属的高活性催化剂。然而，价态及其对催化剂性能的影响之间的关系仍然是一个争论的问题。在本文中，我们使用超临界水（sc-H ₂ O）在存在氧化性或还原性气体的情况下，作为可行的反应介质来原位合成Pd / NiO杂化纳米催化剂。修改Pd的价态。在对催化剂进行了一系列分析技术（包括XRD，H ₂ -TPR，DRIFT，TPSR等）后，我们揭示了Pd ⁰在甲苯的催化氧化中比PdO _x和金属氧化物更具活性。这主要是因为稳定化的Pd ⁰能够在低温下活化气态氧（和甲苯），并且即使有过量的氧也能被甲苯恢复到原始状态。尽管PdO _x可以在还原性气氛下解吸活性氧，并可能有助于从NiO溢出氧，但在富氧环境中很难将其转化为Pd ⁰。已开发的Pd ⁰在有和没有水蒸气的老化试验之后，发现以铜为主要成分的催化剂是坚固的和高活性的。