PROX reactions

The experimental apparatus is consists of reformed gas feeding sections, CO PrOx reaction section in the reactor, and the analysis section with a gas chromatograph system. Simulated reformed gas composition was 75 vol.% H2, 24 vol.% CO2 and 1.0 vol.% CO. The dry reformed feed stream was fed with O2 (A.=l) into the microchannel reactor by MFC (Brooks 5850E). Water vapor (10vol.% of reformed gas) was also fed into the reactor by a s)ninge pump. 

The CO PrOx reaction was conducted in the temperature range of 100"C to 240"C and the reaction temperature was controlled by electrical heater. 

Depending on the reason for converting the produced gas from biomass gasification into synthesis gas, for applications requiring different H2/CO ratios, the reformed gas may be ducted to the water-gas shift (WGS, Reaction 4) and preferential oxidation (PROX, Reaction 5) unit to obtain the H2 purity required for fuel cells, or directly to applications requiring a H2/CO ratio close to 2, i.e., the production of dimethyl ether (DME), methanol, Fischer-Tropsch (F-T) Diesel (Reaction 6) (Fig. 7.6). 

The main unit is the catalytic primaiy process reactor for gross production, based on the ATR of biodiesel. After the primary step, secondary units for both the CO clean-up process and the simultaneous increase of the concentration are employed the content from the reformated gas can be increased through the water-gas shift (WGS) reaction by converting the CO with steam to CO and H. The high thermal shift (HTS) reactor is operating at 575-625 K followed by a low thermal shift (LTS) reactor operating at 475-535 K (Ruettinger et al., 2003). A preferential oxidation (PROX) step is required to completely remove the CO by oxidation to COj on a noble metal catalyst. The PROX reaction is assumed to take place in an isothermal bed reactor at 425 K after the last shift step (Rosso et al., 2004). 

Active and Selective Catalysis of Pt Nanowires/FSM-16 in the PROX Reaction... 

As an application of Pt nanowires in heterogeneous catalysis, we performed preferential oxidation (PROX) of CO as a test reaction [32]. The PROX reaction is useful for PEM fuel cells for the selective removal of contaminating CO from hydrogen gas, because CO works as a strong catalyst poison for Pt electrode catalysts (Figure 15.24). H2 produced in steam-reforming and the water-gas shift reaction needs further to be purified in the PROX reaction to selectively oxidize a few% CO towards inert CO2 in a H 2-rich atmosphere, to reduce the CO content to <10ppm. Under the PROX conditions, the facile oxidation of H2 to H2O may also occur, thus the catalyst selectivity for CO oxidation over H2 oxidation is an... 

To study the promotion mechanism of Pt wire/FSM-16 in the PROX reaction, the Pt nanowires were extracted by HF/EtOH treatment from FSM-16, and the wires were again deposited on the external surface of FSM-16 from the ethanol solution. We found that the resulting external Pt wire/FSM-16 catalyst gave low TOFs (>35) and lower CO selectivity (>30%) in the PROX reaction [32]. This implies that the encapsulation of Pt wires in the silica channels of FSM-16 is a key to promote the selective CO oxidation in the PROX reaction. Furthermore, from the structural characterization by XANES, XPS and IR in CO chemisorption... 

Figure 15.25 Turnover frequency and selectivity of CO oxidation at 333 K in the PROX reaction, (a) Turnover frequency of CO oxidation at 33 K in the PROX reaction on various Pt catalysts (b) CO oxidation selectivity at 333 K in the PROX reaction on various Pt catalysts.

Figure 15.26 Pictorial representation of proposed mechanism for selective CO oxidation in PROX reaction through the carboxyl intermediates (COOH) on Pt nanowires and particles supported on FSM-16 and HMM-1 with active OH groups.

We have observed similar IR bands (1520, 1352 and 1295 cm ) on the Pt wire/ FSM-16 sample in an in situ IR study of the PROX reaction. From these results, we propose that the selective CO oxidation in the PROX on Pt wire/FSM-16 proceeds through the reaction of a carboxyl intermediate (COOH) on Pt nanowires (and particles) supported on FSM-16 with active OH groups (Figure 15.26). CO reacts with an active silica surface OH of FSM-16 to convert the HCOO intermediate on Pt wires and particles into CO2, thereby leading to selective CO oxidation. The subsequent H2/O2 chemisorption generates active surface OH groups near the Pt wires and particles on FSM-16. Smaller HCOO intermediates due to the smaller OH interaction on Pt particle/HMM-1 and Pt necklace wire/HMM-1 may reflect in their lower TOFs and lower CO selectivity in the PROX reaction (Figure 15.25a and b). 

The metallic Cu clusters on Mo02 (denoted Cu/Mo-CTAB) were completely inactive for CO PROX at 90 °C (Table 2.2). Metallic Cu clusters on ZnO (Cu/Zn-CTAB) and Si02 (Cu/Si-CTAB) were active for the methanol dehydrogenation but they were inactive for the PROX reaction. Similarly prepared Cu/Zr-CTAB, Cu/Fe-CTAB and Cu/Al-CTAB catalysts were also inactive for the PROX reaction. In contrast, the new Cu/Ce-CTAB catalyst exhibited tremendous activity with the feed C0/02/H2/He = 1 1 50 48 (mol.%) (Table 2.2), whereas the activities of conventional impregnated Cu/Ce02 and Cu/Ce203 catalysts and co-precipitated Cu-Ce catalysts were much lower. 

EXAFS analysis provided structural parameters (bond distance and coordination number) for Cu-O and Cu-Cu. The small coordination numbers of the Cu-Cu (0.9) and Cu-O bonds (2.4) indicate that the hydrothermal synthesis prohibits the growth of Cu species and produced small Cu + -oxide clusters, which did not significantly change in size after the PROX reaction [90]. CO, 5.75 x 10 4 mol adsorbed on 1 g of Cu/Ce-CTAB (0.49 CO/Cu) was present, but no C02 formation was observed. The results indicate that neither the water-gas shift reaction nor CO oxidation with lattice oxygen proceeded on the Cu/Ce-CTAB catalyst. In contrast, with 2.40 x 10 4 mol 02 adsorbed on 1 g of the fresh Cu/Ce-CTAB catalyst (0.20 02/Cu) a stoichiometric amount of C02 (0.39 C02 per Cu) was produced when this surface was subsequently exposed to CO, which suggests the high oxidation activity of the Cu+-oxide cluster species on the Ce02 surface. XRF analysis showed that the small amount... 

Table 2.2 shows the catalytic performances of various Cu and Ce catalysts for CO PROX reactions in excess H2 at 90 °C. Ce oxides, Ce-CTAB and Cu-CTAB were completely inactive for CO oxidation at 90 °C. In contrast, the hydrothermally-prepared Cu/Ce-CTAB catalyst (7.5 wt%Cu) exhibited good catalytic performance for the CO PROX, with 91.9-96.1% CO conversion and 99.4—99.8% 02 selectivity at 90°C in a feed of C0/02/H2= 1 1 50 (Table 2.2). Table 2.3 summarizes the performance of the Cu/Ce-CTAB catalyst under various reaction conditions, different W/F, reaction temperatures and feed compositions. Notably, high CO conversions and 02 selectivities were also achieved in reactant feeds containing substantial amounts of H20 and C02. The CO conversions and 02 selectivities at W/F = 2.24 gcath mol-1 and 90 °C were 85.7% and 98.7%, respectively when H20 (10%) existed, and 81.4% and 98.2%, respectively when H20 (10%) and C02 (20%) co-existed. 

Preferential Carbon Monoxide Oxidation 1 [PrOx 1] MEMS-like Reactor Applied to Studies of the PrOx Reaction in Micro Channels... 

Table 2.8 Substrate, specific mass, composition and thickness of the catalyst layer of micro structured foils used for PrOx reaction [83],...

Figure 2.59 Experimental results for CO conversion of the PrOx reaction vs. reaction temperature for an average residence time of 14 ms.

Au/TiC>2 catalysts made by deposition-precipitation have been examined for the PROX reaction 27 they show some differences from Au/Fe203 catalysts. Hydrogen now interferes with the oxidation of carbon monoxide, perhaps by competing for the same adsorption sites this was shown by a marked increase in the order of reaction when hydrogen was present, but not in the oxygen order (see Table 7.2). However, there is no reason to expect that gold particles on these two supports differ in any fundamental... 

Benefiting from the assistance of oxygen to simultaneously remove CO by the Prox reaction, oxygen-assisted WGS on bimetallic Cu-Pd/Ce02 reported by Bickford et al.33 have achieved an increased CO conversion from 95% (without oxygen) to more than 99.7% (with oxygen). The results were reported for a feed composition of 4 vol% CO, 2% 02, 40% H20, 10% C02, and 42% H2 at 210 °C and a GHSV of 17,760 IT1. 

Coupling between the WGS and Prox reactions was suggested in a few instances.33,43 Since Au/ceria is a recognized CO partial oxidation catalyst, such an approach would enhance at least CO conversion with oxygen. As shown above, this approach has proven itself feasible with bimetallic Cu-Pd deposited on nanosized ceria. 

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