NO Reduction by H2 on Pt

The same experimental procedure used in Fig. 4.15 is followed here. The Pt surface is initially (t - 1 min) cleaned from Na via application of a positive potential (Uwr=0.2 V) using the reverse of reaction (4.23). The potentiostat is then disconnected (1=0, t=-lmin) andUWR relaxes to 0 V, i.e. to the value imposed by the gaseous composition and corresponding surface coverages of NO and H. Similar to the steady-state results depicted in Fig. 4.18 this decrease in catalyst potential from 0.2 to 0 V causes a sixfold enhancement in the rate, rN2, of N2 production and a 50% increase in the rate of N20 production. Then at t=0 the galvanostat is used to impose a constant current I=-20 pA Na+ is now pumped to the Pt catalyst surface at a 

It is important to observe that the electrochemically promoted Pt surface (Uwr O V) gives SN2 selectivity values above 70% vs 35% on the unpromoted surface (Uwr 0 V). The Pt surface is thus made as selective as a Rh surface would be under similar conditions. The ability of electrochemical promotion to alter the product selectivity of catalyst surfaces is one of its most attractive features for practical applications. 

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Figure 4.17. NO reduction by H2 on Pt/p"-AI203.52 Transient effect of applied constant negative current (Na supply to the catalyst) on catalyst potential (a) under reaction conditions (solid line) and in a He atmosphere (dashed line) and on the rates of formation of N2 and N20 (b). Potentiostatic restoration of the initial rates see text for discussion. Reprinted with permission from Academic Press.

Figure 4.18. NO reduction by H2 on Pt/p"-Al2Oj. Effect of catalyst potential on the rates of formation of N2 and N20 and on the selectivity to N2.52 Reprinted with permission from Academic Press.

Figure 4.27 presents steady-state potentiostatic r vs 0Na results during NO reduction by H2 on Pt/p"-Al203f2 PInb values well in excess of 4000 are obtained for 0Na values below 0.002. This is due to the tremendous propensity of Na to induce NO dissociation on transition metal surfaces. Since Plj is often found to be strongly dependent on 0, (Figs. 4.26 and 4.27), it is also useful to define a differential promotion index pij from ... 

Oscillations connected with adsorbate-induced surface restructuring were studied also in [29]. The model used was aimed at mimicking oscillations in NO reduction by H2 on a mesoscopic Pt particle containing two catalytically active (100) areas connected by an inactive (111) area that only adsorbed NO reversibly. NO diffusion on and between facets was much faster than other steps. The results obtained show that the coupling of the catalytically active sublattices may synchronize nearly harmonic oscillations observed on these sublattices and also may result in the appearance of aperiodic partially synchronized oscillations. The spatiotemporal patterns corresponding to these regimes are nontrivial. In particular, the model predicts that, due to phase separation, the reaction may be accompanied by the formation of narrow NO-covered zones on the (100) sublattices near the (lOO)-(lll) boundaries. These zones partly prevent NO supply from the (111) sublattice to the (100) sublattices. 

Pt-Mo/y-AljO catalyst (1) lowers the activity but increases the selectivity of the Pt catalyst for the NO reduction by H2, and (2) increases the activity for the NO reduction by CO and the activity and selectivity for the NO reduction by CO + H2> Based on the TPR and IR data, we attribute these results to a strong interaction between Pt and Mo oxides on the y-A170, support. Such interaction facilitates the removal of the surface... 

When they studied the CO oxidation reaction, Meunier et al. found that Co " ions in Pt/CoOyAl203 catalysts dissociate oxygen more easily than Co [30]. Partially reduced CoOx is apparently necessary for O2 dissociation. Pande and Bell [31] foimd a high activity of Rh/Ti02 for the NO reduction by H2. The authors attributed the promoting effect of TiO, to the presence of catalytically active TiO centres on the support as well as on the sur ce of the supported Rh crystallites. 

S. (2009) Reduction of NO by H2 on Pt/ WO3/Z1O2 catalysts in oxygen-rich exhaust Appl Catal B Environ, 87, 18-29. 

Sodium also enhances NO dissociation on the Pt surface this is a consequence of the above observation and is apparent from the dramatic enhancement in selectivity towards N2 with increasing Na coverage, an affect that is observed in all EP studies involving NO reduction on Pt. In frict, increases from 15% to 70% and from 40% to 80% have been reported for the cases of NO reduction by CO [11] and by H2 [20], respectively. In present study an increase in N2 selectivity from 60% to 80% was observed. 

Figure 4.17 shows another galvanostatic transient obtained on Pt/p"-AI2O3 at 375°C. The reaction under study is the reduction of NO by H2, a reaction of significant technological interest52 ... 

Identification of the Reaction Network During Reduction of Stored NOxbyH2 To explain the formation of ammonia in the reduction of stored NO, a series of experiments were performed in which NO were stored on Pt-Ba/Al2O3 (1 20 100 w/ w) at 350 °C and then reduced by H2 at different temperatures in the range 150-350 °C [135]. 

On the basis of these data, the following mechanism for reduction by hydrogen can be suggested. H2, activated over the Pt sites according to the Pt-catalyzed pathway discussed previously, reduces the stored nitrates directly to ammonia or, more likely, induces the decomposition of nitrates to gaseous NO, which are then reduced by H2 to NH3 over the Pt sites [overall reaction (13.47)]. Once ammonia has been formed, it can react with adsorbed nitrates and this reaction is very selective towards nitrogen. It is worth noting that the reaction of ammonia with NOx obeys the stoichiometry of reaction (13.49), which is different from that of the well-known NH3-NO SCR reaction because it implies the participation of nitrates. 

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