Bulk and Surface Properties of Dispersed CuO Phases in Relation with Activity of NO x Reduction
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Catalysis Letters Vol. 98, No. 4, December 2004 (Ó 2004)
Bulk and surface properties of dispersed CuO phases in relation with activity of NOx reduction Simona Bennici, Paolo Carniti, and Antonella Gervasini* Dipartimento di Chimica Fisica ed Elettrochimica, Universita` degli Studi di Milano, via Camillo Golgi, 19, I-20133 Milano, Italy
Received 7 June 2004; accepted 10 September 2004
Synthesized silicas modified with alumina, titania, and zirconia (about 13% wt) were used as supports for dispersing nanosized 2 1 CuO phase. All the prepared catalysts, containing about 1 mmolcu g1 cat , possessed high surface areas (230–430 m gcat ) and homogeneous coverage of the relevant support, as revealed by SEM-EDS analysis. The nature of the support and its acidity directed the CuO deposition modifying the dimensions of the CuO aggregates and the ratio between highly and scarcely interacting copper species with support, as revealed by complementary analyses. The redox character of the CuO phase was studied realizing cycles of programmed temperature reduction/oxidation (TPR-TPO) which gave the extent of CuO reduction and Cu(O) reoxidation. Deconvolution of the reduction profiles permitted identifying different copper species which presence depended on the support nature. Attempts were made to individuate relations between the properties of the CuO species and catalytic activity in NOx reduction with ethene (HC-SCR process) in highly oxidant atmosphere. The CuO phase deposited on the most acidic supports showed the best activity and selectivity in the NOx reduction. KEY WORDS: copper oxide; silica supports; temperature-programmed-reduction; temperature-programmed-oxidation; NOx reduction; HC-SCR.
1. Introduction Copper oxide is among the most active transitional metal oxide catalysts for emission control reactions [1–5]. Catalysts based on finely divided copper oxide have displayed interesting activity towards total oxidation of CO, [6–8], hydrocarbons [6,9] and alcohols [10,11], and NOx , and SO2 reduction reactions [3–16]. In the case of selective catalytic reduction of nitrogen oxides (NOx ) by hydrocarbons in an oxygen-rich atmosphere, (HC-SCR process), various catalytic systems based on CuO dispersed with different methods on supports of different nature have been successfully reported in the literature [17–21]. To achieve high catalytic activity, it is necessary to highly disperse CuO particles in small aggregates on a support. By conventional methods for catalyst preparation, such as impregnation, inhomogeneous agglomeration of copper oxide species occurs especially at high copper content, and large-sized particles result [22,23]. Ion-exchange method, even if it is able to stabilize copper species assuring good dispersion, cannot be widely used because of the low loading of copper obtained due to the limit terminal –OH groups on the oxide surfaces. To obtain higher copper loading and stability of the copper species, grafting or chemisorption methods of deposition could be pursued [19,24]. Recently, our research group h
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