Aerogels; a new class of materials for catalytic purposes
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- Institutt for kjemi 
Silica aerogels containing copper(II) cations were prepared by adding the metal precursor during the sol-gel stage. The gel surfaces were made hydrophobic by applying the ambient-pressure-drying (APD) method, and various approaches to the surface modification were tested: Using tetramethyl-chlorosilane (TMCS, Cl-Si(CH3)3) as a silylation agent resulted in leaching of copper from the gel and into the pores during the formation of copper(II) chloride. Hexadimethylsilazane (HDMZ, NH-(Si(CH3)3)2) therefore was best suited as the modifying agent in copper-containing gels, seen by high metal uptake while maintaining the hydrophobic nature. The most suited approach for surface modification with HDMZ was the co-precursor approach, which minimize the consumption of expensive surface modification agents. Gels modified with HDMZ were chosen for further studies. The presence of copper in the sol led to significantly shortened gelation times compared to plain sols. In addition, copper altered the APD-aerogel architecture, seen by considerable expansion of the average pore size and higher porosity. Copper species in both silica ADP-aerogels and xerogel analogue are divalent and present in a tetragonally distorted octahedral environment. XAS confirms incorporation of the guest cation in the gel framework coordinated to siloxy/silanol groups, seen by several Cu…Si pairs rather than Cu…Cu interaction before and after annealing at 450 °C. This confirms that copper is present in thermally stable single-site configuration for copper loadings up to 11 wt %. The copper cations were still available at the surface, seen by extra-framework ammine groups during annealing with in situ DRIFTS, exclusively in copper gels. The ammine groups depart in the temperature range 200 – 250 °C, accompanied by a color change from blue to green, attributed to an exchange of coordination from ammine to silanol or siloxy groups. Further structural studies with XAS and DRIFTS were conducted in situ to obtain a fundamental understanding on the guest nature of copper, the aerogel carrier, and their interaction during the different reaction stages. During hydrogen reduction, XAS show the formation of ultra-small copper metal clusters, their dimensions not exceeding 1.6 nm in silica aerogels and the xerogel reference. Correlating with the reduction of copper and formation of metal clusters, was the formation of Brønsted acidic polyhydrogen bonded silanols (silanol clusters) exclusively seen in copper-containing gels by DRIFTS. The silanol clusters were most likely created upon the removal of the guest cation from the network, leaving silanol/siloxy rich vacancies on the gel surfaces. Cluster dynamics are thoroughly investigated by EXAFS and correlated with theoretical calculations using Density Functional Theory (DFT) calculations. EXAFS show a significant decrease in Cu-Cu shell multiplicities and distances in the temperature range 350-450 °C, which suggest cluster flattening. XAS confirms re-oxidation and complete redispersion of the copper clusters to their initial isolated surroundings during a subsequent treatment in nitric oxide/oxygen. The silanol clusters exhibit Brønsted acidity with several-fold strength compared to mono-hydrogen bonded silanols, and constitute a possible driving force for cluster flattening and re-dispersion of the copper cations. Gold and gold-copper bimetallic gels were prepared by adding the tetraammine gold(III) precursor in the sol-gel stage and modifying the gels using the co-precursor approach. The formation of gold metal clusters in silica APD-aerogels was studied in situ using XAS to determine the cluster sizes in hydrogen. Au-Au shell multiplicities suggest the formation of trimer clusters (Au3) up to 450 °C and tetramers (Au4) after cooling in nitric oxide. Preliminary results on a parallel study on the formation of bimetallic Au-Cu clusters in silica APD-aerogels show a synergetic electronic effect between the metals. The single-site copper containing silica APD-aerogels were highly active for the selective catalytic reduction of NOx with propene (C3H6) as reducing agent (SCR-HCdeNOx) in realistic conditions (15 % H2O), reaching conversions up to 67 % at 450 °C. In contrast to reported behavior of zeolite type matrixes, water in the feed had a promoting effect on the activity. XAS and DRIFTS were conducted in situ to elucidate the origin of deNOx activity in dry vs. wet feed. During redox cycling in dry and wet cycles, the copper-containing aerogel shows superior stability in wet feed regarding the state of copper and surroundings. The possible origins of the promoting effect of water in the catalytic measurements were provided by DRIFTS; Exclusive to Cu-aerogels was the formation of Brønsted acidic silanol cluster surrounding copper by multi-hydroxyl interaction with water. These silanol clusters are capable of storing reaction components and key intermediates, which we believe is responsible for enhancing the catalytic removal of NOx in copper-containing silica APD-aerogels.