H2-SCR

It has long been known that NO is reduced by H2 in low concentrations of O2, which is one of the main reactions taking place in three-way catalytic converters for gasoline engines (i.e., air/fuel 14.7, O2 0.5 %) [96]. AU noble metals have 

a desirable catalyst would have high activities for Reaction (6.8) and minimized activities for the other two reactions particularly. Reaction (6.10). Excess H2 (H2/NO 2) is typically needed. The mechanism of the H2-SCR reaction is far less understood than that for NH3-SCR. In this part, we will summarize the main catalysts that have been investigated for H2-SCR. 

Support Surface area (m g ) Temperature (°C) No conversion (%) Selectivity (%) 

1 Pt/Si02, Pt/Al203, and Pt on Other Metal-Oxide Supports 

Besides the silica support aforementioned, another type of sihca-mesoporous silica, with ordered pore structure and large surface area, was recently studied as Pt catalyst support. Wu et al. studied the H2-SCR activities of Pt/Si-MCM-41 and Pt/ Al-MCM-41 and found that the introduction of Al species to MCM-41 greatly promoted the H2-SCR performance of Pt/MCM-41 [101]. The Pt/Al-MCM-41 with a Si/Al ratio of 10 exhibited 80 % NOx conversion as well as 85 % N2 selectivity at 140 °C (GHSV = 80,000 h ). In-situ DRIFT spectra showed that Brpnsted acid sites on Al-MCM-41 changed the H2-SCR reaction pathway and the NH4 species adsorbed on acid sites were the key intermediates for N2 production. 

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The cationic complex [Rh(DiPFc)(COD)]OTf was discussed earlier as being an excellent catalyst for the hydrogenation of aldehydes under mild conditions. Under similarly mild conditions (25 °C, 4 bar H2, SCR 450, 4h, TOF 110h 1), a range of ketones was hydrogenated quantitatively (Fig. 15.9) [13]. 

Low-Temperature Selective Catalytic Reduction (SCR) of NOx with NH3 Over Zeolites and Metal Oxide-Based Catalysts and Recent Developments of H2-SCR... 

The mechanism of H2-SCR is far less understood than that of NH3-SCR, partly due to the competing reaction of H2 combustion. Because of the tremendous potential for future apphcations of H2-SCR, it merits more R D efforts. 

In this chapter, an attempt is made to present important information regarding the contribution of perovskite-t)q)e materials toward the development of new lean-burn NO , control catalytic technologies within the technical fields of H2-SCR and diesel engine emissions control (NO , and soot). 

Figure 26.1 Location of a current NH3-SCR (a) and future H2-SCR unit (b) in an industrial furnace used in olefin production plants.

Figure 26.7 In situ DRIFTS spectra recorded over the 0.1 wt% Pt/Lao sCeo sMnOa (a) and 0.1 wt% Pt/Si02 catalysts (b) after 30 min of H2-SCR at 140 °C. Inset graph deconvolution...

To gather information for the role of O2 in the H2-SCR mechanism, similar SSITKA experiments with the use of 02 were conducted at 140 °C. Figure 26.8 presents the transient response curves of N2 0, N2 0, and Ar obtained on Pt/Lao.sCeo.sMnOs (Figure 26.8a) and Pt/Si02 (Figure 26.8b) catalysts after the switch N0/H2/ 02/Ar/He N0/H2/ 02/He was made at 140 °C. As seen in Figure 26.8, the concentration of N2 0 produced by both catalysts is reduced after the isotopic switch, whereas the continuous evolution of N2 0 is noticed. The sum of the steady-state concentrations of N2 0 and N2 0 formed under H2-SCR in the isotopic gas mixture is the same as the steady-state concentration... 

There is a proven close relationship between the electronic configuration of the transition metal ion (M) of the B-site in a perovskite and its catalytic oxidation activity and oxygen reduction. However, in multicomponent de-NO reaction systems, such as H2/NO/O2/H2O/CO2 (practical H2-SCR) or HCs/NO/ O2/H2O/CO2 (HC-SCR) the chemical potential of the gas phase might be cru-ciaL This shoidd lead to future further investigations. 

Transient isotopic techniques (e.g., SSITKA-DRIFTS, SSITKA-MS, 2, NO, and N 0-pulsing) were used only to a small extent toward a better understanding of important but complex mechanistic issues of lean-burn deNO catalytic systems. The need of use of such techniques shoidd be enhanced as illustrated in the present case of H2-SCR for stationary sources. 

The influence of reaction temperature on the chemical structure and surface concentration of active NO, in H2-SCR over Pt/MgO-Ce02 SSITKA-DRIETS and transient mass spectrometry studies. 

Efstathiou, A.M. and Olympiou, G.G. (2010) Catalyst consisting of platinum supported on chemically promoted magnesium oxide and cerium dioxide towards H2-SCR. EP 2010/003494 Gulf Countries No. 2010-16072. 

Polychronopoulou, K. and Efstathiou, A.M. (2012) NO control via H2-selective catalytic reduction (H2-SCR) technology for stationary and mobile applications. Recent Patents Mater. Scl, 5, 87-104. 

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