J.M. García-Cortés

On the Structure Sensitivity of deNOx HC-SCR over Pt-Beta Catalysts

Abstract In this paper, the integration of activity results for the selective catalytic reduction of NO x with C 3 H 6 with a thorough characterization of Pt-beta catalysts with different platinum dispersion has been accomplished. The parent zeolite NH 4 -beta (Si/Al = 11.4) was ion-exchanged with a Pt(II) precursor and activated by calcination and reduction in H 2 before reaction. The evolution and nature of the Pt phase during the different preparation stages and after catalytic tests were investigated by CO chemisorption, TEM, XRD, and XPS. By using different heating rates during activation of the ion-exchanged material and upon the deNO x HC-SCR, the average particle size of platinum in the final catalyst was varied in the range of 2–25 nm. It has been clearly shown that deNO x HC-SCR conditions produce a decrease in the metal dispersion of the catalysts by sintering of Pt particles. The increase in average particle size has a positive effect on the activity of the catalysts. Thus, the larger the platinum particles, the higher the NO x conversion and the lower the operation temperature. The XPS results show that both Pt(0) and Pt(II) species are present in the calcined samples, after H 2 reduction and during reaction. Coke deposits, formed during reaction on the zeolite support, were studied by XPS, DRIFT, and TPO-TPD/MS. The structure sensitivity of the lean deNO x reaction toward the platinum phase has been confirmed by the direct correlation established between platinum particle size and TOF. Based on previous results on a single crystal, it seems that the key steps are the NO dissociation on Pt(100) planes and PtO clean off which will be easily performed on large Pt particles. On the other hand, the independence of Pt(100)/Pt(111) ratios with particle size explains the similar N 2 and N 2 O selectivity values presented along all the samples.

Effect of the Support in de-NOx HC-SCR Over Transition Metal Catalysts

Several catalysts based on transition metals (Cu, Co, Fe) and different supports (ZSM-5, activated carbon, Al2O3) have been tested by Temperature-Programmed Reaction (TPR) experiments for the selective catalytic reduction of NOx with propene in the presence of excess oxygen, simulating lean-burn conditions. The activity order with respect to the metal was Cu∼Fe>Co for all supports used. ZSM-5 catalysts have a superior behavior over Al2O3, as observed for noble metal catalysts. Application of activated carbon as a support is not practical due to its consumption at the reaction temperatures. The selectivity to N2 of the catalysts was also independent of the support, being higher than 95% in all the cases.

Reduction of NO by Propene Over Pt, Pd and Rh-Based ZSM-5 Under Lean-Burn Conditions

Selective catalytic reduction of NO by propene has been investigated over noble metal (Pt, Pd, Rh)-based ZSM-5 catalysts. These samples were tested in a gas mixture system in the presence of excess oxygen, simulating lean-burn exhaust gases. The sequence in activity for NO reduction was Pt > Rh ∼ Pd. Regarding the selectivity of the reaction to N2, an opposite trend was observed: Rh > Pd ∼ Pt. The catalytic systems have presented stable operation under isothermal conditions during time-on-stream experiments.

Selective catalytic reduction of NOx with C3H6 under lean-burn conditions on activated carbon-supported metals

The Selective Catalytic Reduction of NO x with C 3 H 6 under excess of oxygen has been investigated using different transition metals supported on a carbonaceous material. For this purpose, 1 wt % Pt, Pd, Fe, Co, Ni and Cu supported on an activated carbon, ROXN, have been tested by Temperature Programme Reaction (TPR). A as mixture containing 1000 ppm of NO, 1500 ppm of C 3 H 6 and 5 vol. % of O 2 (SV = 3600 h -1 ) has been used to carry out the experiments. In terms of NOx conversion and N 2 selectivity, a diversity of behaviours have been obtained. Pt/ROXN presents the highest NO x conversion, almost 100%, but exhibits a moderate N 2 selectivity. On the other hand, Fe/ROXN presents a 100% N 2 selectivity, but exhibits low NO x conversions. The results have been discussed attending to the characteristics of the active phase and to the behaviour of the catalysts for C 3 H 6 combustion. Conventional alumina-supported catalysts have been used for comparative purposes: an identical activity trend has been obtained for the studied metals. Nevertheless, the activated carbon-supported metal catalysts show two advantages; i) higher NO x conversions at lower temperatures and ii) the achievement of their maximum values for N 2 selectivities, which are similar to the obtained for the alumina-supported catalysts, at lower temperatures.

Dual-Bed Catalytic System for Removal of NOx-N2O in Lean-Burn Engine Exhausts

Platinum-based catalysts exhibit a high activity and stability for the selective catalytic reduction of NOx using hydrocarbons (HC-SCR) at low temperatures (450–600 K), withstanding the presence of H2O and SO2 in the feed mixture. Therefore, they are attractive for NOxemission control of exhaust gases of lean-burn engines (lean deNOx). The high N2O selectivity (∼70%) of these catalysts is a serious drawback, since N2O contributes to the greenhouse effect and to stratospheric ozone depletion. The influence of various parameters on the N2 selectivity of platinum-based catalysts was studied: (i) support (zeolites, conventional metal oxides, activated carbon), (ii) platinum dispersion (20–45%), (iii) platinum loading (0.3–3 wt.%), and (iv) addition of sodium to the catalyst formulation (0–8 wt.%). However, no satisfactory improvements of intrinsic N2 selectivity could be accomplished under conventional testing conditions. An alternative strategy is the implementation of an additional catalytic function serving for the decomposition of N2O generated in the deNOx process over the Pt-catalyst. The dual-bed catalytic system (deNOx catalyst in series with deN2O catalyst) provides promising results. FeZSM-5 turned out to be the most suitable catalyst for the deN2O function (direct N2O decomposition) showing a stable activity even when SO2 and H2O were present in the feed. The deN2O activity over FeZSM-5 can be increased by secondary propene injection (selective N2O reduction). In an optimal situation, ∼90% of NOx was converted over Pt-USY (deNOx bed) at 475 K, and ∼90% of N2O was converted over FeZSM-5 (deN2O bed) at 700 K. Finally, the implementation of such system into mobile sources of NOx was evaluated.