Recent progress in selective CO removal in a H2-rich stream

In this review, recent works related to the selective CO removal in a H₂-rich stream for the application of the low-temperature fuel cell are discussed. The membrane separation, the selective CO hydrogenation, and the preferential CO oxidation (PROX) have been generally studied to meet the requirement for the polymer electrolyte membrane fuel cell (PEMFC) where the CO concentration should be controlled to be less than 10 ppm not to degrade the electrochemical performance of Pt anode. For the membrane separation, the thin layer of Pd-based alloy metal on the porous ceramic material coupled with the catalytic purification is the most advanced method at present. For PROX catalysts, supported Ru catalysts and Pt-based alloy catalysts have been successfully developed so far. The combination of highly selective PROX catalysts and the CO methanation catalyst can provide the extended temperature range to achieve the acceptable CO removal. Because each method has presently its own weak points, the further advance is still in need. The non-noble metal-based membrane requiring smaller pressure differentials is highly plausible in the membrane separation. The highly selective catalyst for CO methanation in the presence of excess CO₂ and H₂O can simplify the CO removal step. The PROX catalyst should be operative over a wide reaction temperature as well as at low temperatures not to cause the reverse water–gas shift reaction. During the development of these catalysts, the progress on the high-temperature PEM fuel cell or the CO-tolerant anode should be carefully evaluated.     

Influence of temperature and relative humidity on performance and CO tolerance of PEM fuel cells with Nafion-Teflon-Zr(HPO4)2 higher temperature composite membranes

The carbon monoxide (CO) poisoning effect on carbon supported catalysts (Pt-Ru/C and Pt/C) in polymer electrolyte membrane (PEM) fuel cells has been investigated at higher temperatures (T > 100 °C) under different relative humidity (RH) conditions. To reduce the IR losses in higher temperature/lower relative humidity, Nafion^®-Teflon^®-Zr(HPO₄)₂ composite membranes were applied as the cell electrolytes. Fuel cell polarization investigation as well as CO stripping voltammetry measurements was carried out at three cell temperatures (80, 105 and 120 °C), with various inlet anode relative humidity (35%, 58% and 100%). CO concentrations in hydrogen varied from 10 ppm to 2%. The fuel cell performance loss due to CO poisoning was significantly alleviated at higher temperature/lower RH due to the lower CO adsorption coverage on the catalytic sites, in spite that the anode catalyst utilization was lower at such conditions due to higher ionic resistance in the electrode. Increasing the anode inlet relative humidity at the higher temperature also alleviated the fuel cell performance losses, which could be attributed to the combination effects of suppressing CO adsorption, increasing anode catalyst utilization and favoring OH_ads group generation for easier CO oxidation.     

CO tolerance and CO oxidation at Pt and Pt-Ru anode catalysts in fuel cell with polybenzimidazole-H3PO4 membrane

CO tolerance of H₂-air single cell with phosphoric acid doped polybenzidazole (PA-PBI) membrane was studied in the temperature range 140-180 °C using either dry or humidified fuel. Fuel composition was varied from neat hydrogen to 67% (vol.) H₂-33% CO mixtures. It was found that poisoning by CO of Pt/C and Pt-Ru/C hydrogen oxidation catalysts is mitigated by fuel humidification. Electrochemical hydrogen oxidation at Pt/C and Pt-Ru/C catalysts in the presence of up to 50% CO in dry or humidified H₂-CO mixtures was studied in a cell driven mode at 180 °C. High CO tolerance of Pt/C and Pt-Ru/C catalysts in FC with PA-PBI membrane at 180 °C can be ascribed to combined action of two factors—reduced energy of CO adsorption at high temperature and removal of adsorbed CO from the catalyst surface by oxidation. Rate of electrochemical CO oxidation at Pt/C and Pt-Ru/C catalysts was measured in a cell driven mode in the temperature range 120-180 °C. Electrochemical CO oxidation might proceed via one of the reaction paths—direct electrochemical CO oxidation and water-gas shift reaction at the catalyst surface followed by electrochemical hydrogen oxidation stage. Steady state CO oxidation at Pt-Ru/C catalyst was demonstrated using CO-air single cell with Pt-Ru/C anode. At 180 °C maximum CO-air single cell power density was 17 mW cm⁻² at cell voltage U = 0.18 V.     

Heterogeneous Catalytic Aziridination of Styrene Using Transition-Metal-Exchanged Zeolite Y

Hydrogen for fuel cells can be produced by reforming hydrocarbons. The H₂-rich reformate typically contains about 1 mol% CO which will poison the anode of polymer electrolyte fuel cells. The CO concentration can be reduced by preferential oxidation (PROX) using near-stoichiometric amounts of O₂. The conversion of CO should be over 99% while minimizing oxidation of H₂. Supported Pt catalysts with and without promotion by Ce were compared for the catalytic oxidation of CO by O₂ in a H₂ stream. With unsupported Pt catalysts, selectivity (to CO₂ as opposed to H₂O) was highest at low temperatures and low O₂/CO ratios, however conversion was low. Addition of Ce significantly improved CO conversion under these conditions.     

Improved electrochemical CO removal via potential oscillations in serially connected PEM fuel cells with PtRu anodes

The polarization performance of two PEM fuel cells (with anode PtRu/C catalyst) connected either in parallel or serial, was compared to the performance of a single PEM fuel cell in galvanostatic operation using CO-free H₂ or 200 ppm CO-containing H₂ stream as anode feed at ambient temperature. Spontaneous potential oscillations were observed experimentally for the coupled configuration with two cells connected in serial or parallel using CO-containing H₂ feed at various current densities applied. The potential oscillations are ascribed by the dynamic CO adsorption and subsequent electrochemical CO oxidation on the anode. The measured anode outlet CO concentration was found to decrease with the order: single cell > parallel cells > serial cells at various current densities and anodic flow rates. The low anode outlet CO concentration (<10 ppm) at high current densities applied showed that CO in the anode feed was removed efficiently by the electrochemical CO oxidation occurring on the PtRu anode. The anode outlet CO concentration decreased as follows: a single cell > the parallel cells > the serial cells at broad range of current densities and anodic flow rates. The highest CO conversion and the highest average power output at equal hydrogen recovery degree were obtained with serially coupled fuel cells.     

Electrocatalysis of the hydrogen oxidation in the presence of CO on RhO2/C-supported Pt nanoparticles

This work presents a study on the kinetics of the hydrogen oxidation reaction (HOR) in the absence and in the presence of CO in ultra thin porous layer and in PEM fuel cell electrodes formed with Pt supported on RhO₂/C substrates. Together with the electrochemical measurements, the structural and electronic properties of these catalysts were characterized, enabling to correlate their structural and electronic properties with the HOR kinetics. The results show that the presence of Rh oxides leads to an emptying of the Pt 5d band and a consequent reduction of the back-donation of electrons from Pt to CO, weakening the Pt-CO bond and diminishing the CO degree of coverage on Pt, leaving more sites available to HOR. These changes in the electronic spectra do not lead to any perceptible change in the kinetics or the reaction of pure hydrogen. Also, the formation of CO₂ monitored by the MS experiments in the fuel cell anode outlet indicates that the bifunctional mechanism is also operative, but the major CO tolerance is achieved by the electronic effect induced by the RhO₂ support.     

Effect of hydrogen partial pressure on a polymer electrolyte fuel cell performance

We first investigated the effect of partial pressure of hydrogen (H₂) on the performance of polymer electrolyte fuel cells (PEFCs) by controlling the ratio of hydrogen and nitrogen (N₂). The cell performance with Pt/C anode was significantly decreased with reduction of the partial pressure of H₂ in the presence of carbon monoxide (CO), while the performance variation was negligible in the absence of CO. Severe CO poisoning on Pt/C electrode at low partial pressure of H₂ might be attributed to the hindering effect by N₂ and CO. On the other hand, PtRu/C anode showed consistent power performance even at low partial pressure of H₂.     

Analysis of sulfur poisoning on a PEM fuel cell electrode

The extent of irreversible deactivation of Pt towards hydrogen oxidation reaction (HOR) due to sulfur adsorption and subsequent electrochemical oxidation is quantified in a functional polymer electrolyte membrane (PEM) fuel cell. At 70 °C, sequential hydrogen sulfide (H₂S) exposure and electrochemical oxidation experiments indicate that as much as 6% of total Pt sites are deactivated per monolayer sulfur adsorption at open-circuit potential of a PEM fuel cell followed by its removal. The extent of such deactivation is much higher when the electrode is exposed to H₂S while the fuel cell is operating at a finite load, and is dependent on the local overpotential as well as the duration of exposure. Regardless of this deactivation, the H₂/O₂ polarization curves obtained on post-recovery electrodes do not show performance losses suggesting that such performance curves alone cannot be used to assess the extent of recovery due to sulfur poisoning. A concise mechanism for the adsorption and electro-oxidation of H₂S on Pt anode is presented. H₂S dissociatively adsorbs onto Pt as two different sulfur species and at intermediate oxidation potentials, undergoes electro-oxidation to sulfur and then to sulfur dioxide. This mechanism is validated by charge balances between hydrogen desorption and sulfur electro-oxidation on Pt. The ignition potential for sulfur oxidation decreases with increase in temperature, which coupled with faster electro-oxidation kinetics result in the easier removal of adsorbed sulfur at higher temperatures. Furthermore, the adsorption potential is found to influence sulfur coverage of an electrode exposed to H₂S. As an implication, the local potential of a PEM fuel cell anode exposed to H₂S contaminated fuel should be kept below the equilibrium potential for sulfur oxidation to prevent irreversible loss of Pt sites.     

The influence of oxygen additions to hydrogen in their electrode reactions at Pt/Nafion interface

The influence of oxygen gas added to hydrogen in their electrode reactions at the Pt/Nafion interface was investigated using ac impedance method. The electrochemical cell was arranged in either electrolytic (hydrogen enrichment) or galvanic (fuel cell) mode. The impedance spectra of the electrode reaction of a H₂/O₂ gas mixture were taken in each mode as a function of the gas composition, electrode surface roughness and the cell potential. The spectrum taken for the anodic reaction of electrolytic arrangement confirmed the anodic oxygen reduction reaction (AOR, the local consumption of hydrogen by the added oxygen) by showing an independent arc distinguishable from that for hydrogen oxidation. But the independent arc was not revealed in the spectrum taken on a smooth (low surface area) electrode or on a Pt/C anode of the galvanic cell. At any cell current density, the electrolytic mode showed its anodic overpotential much higher (nearly three times higher at the current density of 100 mA cm⁻²) than the potential registered in galvanic mode implying that the oxygen gas in the mixture engages more active and independent AOR at the anode of the electrolytic cell.     

The Mechanism for the Selective Oxidation of CO Enhanced by H2O on a Novel PROC Catalyst

Preferential oxidation (PROX) reaction of CO in H₂ catalyzed by a new catalyst of FeO _x /Pt/TiO₂ (Fe: Pt: TiO₂ = 100: 1: 100) was studied by dynamic in-situ DRIFT-IR spectroscopy. The oxidation of CO is markedly enhanced by H₂ and H₂O, and the enhancement by H₂/D₂ and H₂O/D₂O takes a common hydrogen isotope. Dynamics of DRIFT-IR spectroscopy suggests that the oxidation of CO with O₂ in the absence of H₂ proceeds via bicarbonate intermediate. In contrast, rapid oxidation of CO in the presence of H₂ proceeds via HCOO intermediate and the subsequent oxidation of HCOO by the reaction with OH, that is, CO + OH→ HCOO and HCOO + OH → CO₂ + H₂O. The latter reaction is a rate determining step being responsible for a common hydrogen isotope effect by H₂/D₂ and H₂O/D₂O.     

Steam reforming of liquid hydrocarbon fuels for micro-fuel cells. Pre-reforming of model jet fuels over supported metal catalysts

The present work deals with pre-reforming of logistic hydrocarbon fuel (jet fuel) as a part of an integrated approach to developing an on-board fuel reformer for use in a micro-solid-oxide fuel cell system. The purpose of doing pre-reforming is to ensure carbon-free reformulation of JP-8 jet fuel into hydrogen and carbon monoxide for use in a micro-solid-oxide fuel cell. Several model jet fuels have been tested for the pre-reforming at low temperature (450–550 °C) in a lab-scale reforming reactor. Proper temperature control and pre-mixing of feed fuels and steam have been found to be important for the prevention of coke formation prior to pre-reforming. Both noble metal and base-metal catalysts have been prepared and tested. As compared with an Al₂O₃-supported Ni catalyst, supported Rh catalysts show not only high activity but also high resistance to deactivation due to carbon formation. Removal of residual Cl⁻ from Rh/CeO₂–Al₂O₃ improves the metal dispersion and the pre-reforming activity. The reformates from the current pre-reformer contain mainly CH₄, CO, H₂, in which CH₄ can be further converted to H₂ and CO by subsequent main-reforming.     

Isotope effect of H2/D2 and H2O/D2O for the PROX reaction of CO on the FeOx/Pt/TiO2 catalyst

The oxidation reaction of CO with O₂ on the FeOx/Pt/TiO₂ catalyst is markedly enhanced by H₂ and/or H₂O, but no such enhancement occurs on the Pt/TiO₂ catalyst. Isotope effects were studied by H₂/D₂ and H₂O/D₂O on the FeOx/Pt/TiO₂ catalyst, and almost the same magnitude of isotope effect of ca. 1.4 was observed for the enhancement of the CO conversion by H₂/D₂ as well as by H₂O/D₂O at 60 °C. This result suggests that the oxidation of CO with O₂ via such intermediates as formate or bicarbonate in the presence of H₂O, in which H₂O or D₂O acts as a molecular catalyst to promote the oxidation of CO as described below.      

Selective CO oxidation in the presence of hydrogen over supported Pt catalysts promoted with transition metals

Selective CO oxidation in the presence of excess hydrogen was studied over supported Pt catalysts promoted with various transition metal compounds such as Cr, Mn, Fe, Co, Ni, Cu, Zn, and Zr. CO chemisorption, XRD, TPR, and TPO were conducted to characterize active catalysts. Among them, Pt-Ni/γ-Al₂O₃ showed high CO conversions over wide reaction temperatures. For supported Pt-Ni catalysts, Alumina was superior to TiO₂ and ZrO₂ as a support. The catalytic activity at low temperatures increased with increasing the molar ratio of Ni/Pt. This accompanied the TPR peak shift to lower temperatures. The optimum molar ratio between Ni and Pt was determined to be 5. This Pt-Ni/γ A1₂O₃ showed no decrease in CO conversion and CO₂ selectivity for the selective CO oxidation in the presence of 2 vol% H₂O and 20 vol% CO₂. The bimetallic phase of Pt-Ni seems to give rise to stable activity with high CO₂ selectivity in selective oxidation of CO in H₂-rich stream.     

On-line deactivation of Au/TiO2 for CO oxidation in H2-rich gas streams

Au-based catalysts, known for ambient temperature CO oxidation, have to provide stable performance of up to 5000 h in order to be commercially applicable in automotive fuel cells. In this report, the on-line deactivation characteristics of Au/TiO₂ in unconventional PROX conditions are discussed. As opposed to CO removal from air, results in this report suggests that carbonates have a minor effect on deactivation of Au/TiO₂ in dry H₂-rich conditions. Also, no conclusive correlation between surface hydration and deactivation was observed. Rather, deactivation appeared to have occurred as a result of an intrinsic transformation in the oxidation state of the active species in the reducing operating conditions; a process which was reversible in an oxidizing atmosphere.     

Hydrogen Production Via CH4 and CO Assisted Steam Electrolysis

Porous composite anodes consisting of a yttria-stabilized zirconia (YSZ) backbone that was impregnated with CeO₂ and various amounts of metallic components including Cu, Co and Pd were fabricated. The performance of these anodes was then tested in a solid oxide water electrolysis cell under conditions where the anode was exposed to the reducing gasses H₂, CH₄ and CO. The reducing gasses were used to decrease the electrochemical potential of the cell and increase overall efficiency. The results of this study show that Cu–CeO₂–YSZ anodes have low catalytic activity for the oxidation of CO and CH₄ and are not very effective in lowering the cell potential while operating in the reducing gas assisted mode. The addition of Co to the Cu–CeO₂–YSZ anode resulted in a modest increase in the catalytic activity and enhanced the thermal stability of the anode. A Pd–C–CeO₂–YSZ anode was found to have the highest catalytic activity of those tested and gave the largest reductions in the operating potential of the solid oxide electrolysis cell.     

Poisoning of the adsorption of CO, O2 and D2 on Pt(111) by preadsorbed H2O

We have measured the influence of adsorbed H₂O on the sticking coefficients and saturation coverages of CO, O₂ and D₂ on Pt(111) at ∼100 K. Strong poisoning is observed for all three gases. For O₂ and D₂, the surface is essentially totally poisoned at 1 monolayer (ML) water coverage. For CO, the effect is weaker, with some CO adsorption still occurring at 2–3 ML H₂O. The influence of these results on the kinetics of the CO and H₂ oxidation reactions are discussed briefly. It is concluded that the influence of water must be included in kinetics simulations, at least at low temperatures, when significant humidity levels are present in inlet gas mixtures, or produced by the reactions themselves. This revised version was published online in July 2006 with corrections to the Cover Date.     

Significant enhancement of the oxidation of CO by H2 and/or H2O on a FeO x /Pt/TiO2 catalyst

Oxidation of CO on the FeO _x /Pt/TiO₂ catalyst is markedly enhanced by H₂ and/or H₂O at 60 °C, but no such enhancement is observed on the Pt/TiO₂ catalyst, but shift reaction (CO + H₂O → H₂ + CO₂) does not occur on the FeO _x /Pt/TiO₂ catalyst at 60 °C. DRIFT-IR spectroscopy reveals that the fraction of bridge bonded CO increases while that of linearly bonded CO decreases on the FeO _x loaded Pt/TiO₂ catalyst. The in-situ DRIFT IR spectra proved that the bridged CO is more reactive than the linearly bonded CO with respect to O₂, and the reaction of the bridge-bonded CO with O₂ as well as of the linearly bonded CO is markedly enhanced by adding H₂ to a flow of CO + O₂. From these results, we deduced that the promoting effect of H₂ and/or H₂O is responsible for the preferential oxidation (PROX) reaction of CO on the FeO _x /Pt/TiO₂ catalyst, and a following new mechanism via the hydroxyl carbonyl or bicarbonate intermediate is proposed for the oxidation of CO in the presence of H₂O.      

Molten V2O5/Cs0.9K0.9Na0.2S2O7 and V2O5/K2S2O7 catalysts as electrolytesin an electrocatalytic membrane separation device for SO2 removal

Bench scale fuel cell tests have been carried out on the SO₂ oxidation catalyst systems V₂O₅/M₂S₂O₇ (M = alkali) used as electrolytes in a standard molten carbonate fuel cell (MCFC) fuel cell setup for removal of SO₂ from power plant flue gases. Porous Li _x Ni_(1–x)O electrodes were used both as anode and cathode. The cleaning cell removes SO₂ when a potential is applied across the membrane, potentially providing cheap and ecological viable means for regeneration of SO₂ from off-gases into high quality H₂SO₄. Results show that successful removal of up to 80% SO₂ at 450 °C can be achieved at approximately 5 mAcm^–2. However, the data obtained during the experiments explain the current limitations of the process, especially in terms of electrolyte wetting capability and acid/base chemistry of the electrolyte.     

Synergism Between Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Au/TiO<Subscript>2</Subscript> in the Low Temperature Oxidation of Propene

CO impedes the low temperature (<170 °C) oxidation of C₃H₆ on supported Pt. Supported Au catalysts are very effective in the removal of CO by oxidation, although it has little propene oxidation activity under these conditions. Addition of Au/TiO₂ to Pt/Al₂O₃ either as a physical mixture or as a pre-catalyst removes the CO and lowers the light-off temperature (T ₅₀) for C₃H₆ oxidation compared with Pt catalyst alone by ~54 °C in a feed of 1% CO, 400 ppm C₃H₆, 14% O₂, 2% H₂O.     

The electro-oxidation of carbon monoxide, hydrogen/carbon monoxide and methanol in acid medium on Pt-Sn catalysts for low-temperature fuel cells: A comparative review of the effect of Pt-Sn structural characteristics

The electrocatalytic activity for CO, H₂/CO and CH₃OH oxidation of Pt-Sn catalysts has been extensively investigated for a possible use as anode materials for low-temperature fuel cells. This paper presents an overview of the relationship between the structural characteristics of the catalysts (catalyst composition, degree of alloying, presence of oxides) and their electrocatalytic activity for the oxidation of the different fuels.