120.S. Guo, K.N. Heck, S. Kasiraju, H. Qian, Z. Zhao, L.C. Grabow, J.T. Miller, and M.S. Wong, "Insights into Nitrate Reduction over Indium-Decorated Palladium Nanoparticle Catalysts" ACS Catalysis, 8, 503-515 (2017) DOI:10.1021/acscatal.7b01371
Nitrate (NO3-) is an ubiquitous groundwater contaminant and is detrimental to human health. Bimetallic palladium-based catalysts have been found to be promising for treating nitrate (and nitrite, NO2-) contaminated waters. Those containing indium (In) are unusually active, but the mechanistic explanation for catalyst performance remains largely unproven. We report that In deposited on Pd nanoparticles (NPs) (“In-on-Pd NPs”) shows room-temperature nitrate catalytic reduction activity that varies with volcano-shape dependence on In surface coverage. The most active catalyst had an In surface coverage of 40%, with a pseudo-first order normalized rate constant of kcat ∼ 7.6 L gsurface-metal-1 min-1, whereas monometallic Pd NPs and In2O3 have nondetectible activity for nitrate reduction. X-ray absorption spectroscopy (XAS) results indicated that In is in oxidized form in the as-synthesized catalyst; it reduces to zerovalent metal in the presence of H2 and reoxidizes following NO3- contact. Selectivity in excess of 95% to nontoxic N2 was observed for all the catalysts. Density functional theory (DFT) simulations suggest that submonolayer coverage amounts of metallic In provide strong binding sites for nitrate adsorption and they lower the activation barrier for the nitrate-to-nitrite reduction step. This improved understanding of the In active site expands the prospects of improved denitrification using metal-on-metal catalysts.
119.Wu Zhou, Nikolaos Soultanidis, Hui Xu, Michael S. Wong, Matthew Neurock, Christopher J. Kiely and Israel E. Wachs, "Nature of Catalytically Active Sites in the Supported WO3/ZrO2 Solid Acid System: A Current Perspective" ACS Catalysis, 7(3), 2181-2198 (2017)DOI: 10.1021/acscatal.6b03697
Tungstated zirconia (WO3/ZrO2) is one of the most well-studied solid acid catalyst systems and continues to attract the attention of both academia and industry. Understanding and controlling the properties of WO3/ZrO2 catalysts has been a topic of considerable interest over almost the past three decades, with a particular focus on discovering the relationship between catalytic activity and the molecular structure of the surface acid site. Amorphous tungsten oxide (WOx) species on ZrO2 surfaces were previously proposed to be very active for different acidic reactions such as alcohol dehydration and alkane isomerization. Recent developments in electron optical characterization and in situ spectroscopy techniques have allowed researchers to isolate the size, structure, and composition of the most active catalytic species, which are shown to be three-dimensional distorted Zr-WOx clusters (0.8-1.0 nm). Complementary theoretical calculations of the Bronsted acidity of these Zr-WOx clusters have confirmed that they possess the lowest deprotonation energy values. This new insight provides a foundation for the future characterization and theory of acidic supported metal oxide catalytic materials that will, hopefully, lead to the design of more active and selective catalysts. This perspective presents an up-to-date, comprehensive summary of the leading models of WO3/ZrO2 solid acid catalysts.