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Fe-doped MnO 2 (Fe-MnO 2 ) generally exhibits higher NO oxidation activity than MnO 2 , which is widely attributed to the promoting effect of surface chemisorbed oxygen species (O ads ) adsorbed on surface oxygen vacancies (SOVs) generated upon Fe doping. Whether the higher content of SOVs and thus higher content of O ads can be achieved by increasing the content of Fe precursor in synthesis and whether higher content of SOVs in MnO 2 can lead to higher NO oxidation activity remain to be investigated. Herein, Fe-MnO 2 catalysts with different Mn/Fe molar ratios were synthesized by a simple coprecipitation method based on the comproportionation reaction between Mn 7+ and Mn 2+ by adjusting the Mn/Fe molar ratio in precursor. Physicochemical characterization shows that Fe doping can not only increase the content of SOVs but also significantly influence the other physicochemical properties of MnO 2 , such as crystalline phase, textural property, and redox property, but increasing the content of Fe cannot monotonically increase the content of SOVs in Fe-MnO 2 catalyst. NO oxidation test shows that the Fe-MnO 2 catalyst with higher content of SOVs exhibits higher NO oxidation activity. The enhanced NO oxidation activity observed on MnO 2 after Fe doping should result from its higher content of SOVs and O ads , improved oxygen mobility, and more adsorption sites resulting from its larger specific surface area and pore volume as well as the additional adsorption sites provided by Fe ions.

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The steering vector mismatch causes signal self-nulling for adaptive beamforming when the training data contain the desired signal component. To prevent signal self-nulling, many beamformers use robust technology, which is usually equivalent to the diagonal loading approach. Unfortunately, the diagonal loading approach achieves better signal enhancement at the cost of losing its interference suppression capability, especially at high input signal-to-noise ratio. In this paper, a novel robust adaptive beamforming method is developed to improve the interference suppression capability. The proposed beamformer is based on the worst-case performance optimization technology with a new estimated steering vector and a special set parameter. Firstly, a subspace which is orthogonal to the interference's steering vector is obtained by using the interference-plus-noise covariance matrix; then a new steering vector which is orthogonal to each interference's steering vector is estimated; finally, the beamformer's weight is solved with the worst-case performance optimization technology with a special set parameter. Theoretical analysis of the interference suppression principle is analyzed in detail, and some simulation results are presented to evaluate the performance of the proposed beamformer.