Size-Dependent Hydrogenation Selectivity of Nitrate on Pd−Cu/TiO₂ Catalysts

Abstract

Catalytic hydrogenation of nitrate (NO3 −) in water on Pd−Cu ensembles has been denoted as a promising denitridation method, but its hydrogenation selectivity remains challenging. In this study, the hydrogenation selectivity of nitrate on the Pd−Cu/TiO2 systems was discussed mainly concerning the size effect of Pd−Cu ensembles in a gas−liquid cocurrent flow system. Demonstrated by their TEM images, homogeneous morphologies as well as narrow size distributions of Pd−Cu ensembles on titania have been prepared by a facile photodeposition process, and the size of the ensembles was controlled and varied with the total metal loadings. The different XRD patterns and XPS spectra of Pd−Cu/TiO2 catalysts from their corresponding monometallic counterparts suggested the formation of Pd−Cu complex on the surface of TiO2. It is first indicated that the hydrogenation selectivity of nitrate generally depends on the size of active phase with critical size of approximately 3.5 nm, below which NO2 − becomes the predominant product instead of nitrogen. Ammonium production was increasing slowly throughout the reaction, but this can be efficiently restrained by bubbling CO2. The optimal catalytic activity and nitrogen selectivity of 99.9% and 98.3% respectively could be achieved on the Pd−Cu/TiO2 catalyst with average size of 4.22 nm under the modification of CO2 after approximately 30 min reduction. The catalytic activities of nitrite on several Pd−Cu bimetallic catalysts were examined to strongly depend on the size of active metal, as is well responsible for the observed distinct hydrogenation selectivity of nitrate.

The effect of support porosity on the selectivity of Pd–In/support catalysts in nitrate reduction

“…This is probably the main reason for the low activity of our 5%Pd/TiO 2 catalyst in the reduction of nitrates. Similar results connected with the activity of monometallic palladium catalysts supported on titanium oxide were obtained by Zhang et al [19]. The catalytic properties of selected 5%Pd-2%In/support (support = Al 2 O 3 , SiO 2 , TiO 2 ) systems were studied in the hydrogenation of nitrates (100 mg/dm 3 NO 3 -) at room temperature and under atmospheric pressure.…”

“…The accepted mechanism for the reduction of nitrates to nitrites requires promotion of the noble metal by an addition of a second metal like Cu, Ag or In [19,[30][31][32][33][34]39]. From the literature data, it also is known that some supports like TiO 2 , CeO 2 and SnO 2 in the monometallic palladium catalysts participate in the reduction of nitrates to nitrogen [8][9][10][11].…”

“…The dispersion of 5%Pd/support and 5%Pd-2%In/support catalysts was estimated on the basis of chemisorptions of CO and by the XRD technique (Table 1). On the basis of literature data, [19,31,34,44] the selectivity to N 2 is higher for the catalysts with lower dispersion. We observed the same tendency, but only for the monometallic palladium catalysts (Table 1).…”

“…However, they show high activity in the reduction of nitrites [3][4][5][12][13][14]. Among the studied bimetallic systems, palladium systems promoted with Cu, Sn, Ag or Fe supported on different kinds of oxide carriers [14][15][16][17][18][19][20], polymers [21,22], zeolites [23], active carbons [24][25][26], resin [27][28][29], showed the best catalytic characteristics. The mechanism of these systems has not been explained unequivocally yet.…”

“…The mechanism of these systems has not been explained unequivocally yet. The one which is the most frequently discussed in the literature is the bifunctional mechanism, according to which nitrates are reduced to nitrites in the redox process on the metallic promoter, during the second stage nitrites are reduced to nitrogen or ammonia on a noble metal [19,[30][31][32][33][34]. In this mechanism, the role of possible intermetallic interaction, which may occur on the surface of bimetallic supported catalysts, was not taken into account.…”