Water-gas shift reaction over magnesia-modified Pt/CeO2 catalysts

Fuel cells have risen as a clean technology for power generation and much effort has been done for converting renewable feedstock in hydrogen. The water-gas shift reaction (WGS) can be applied aiming at reducing the CO concentration in the reformate. As Pt/CeO₂ catalysts have been pointed out as an alternative to the industrial WGS catalysts, the modification of such systems with magnesium was investigated in this work. It was shown that the addition of MgO to Pt/CeO₂ increased the activity and stability of the catalyst irrespective of the preparation method used, either impregnation or co-precipitation. Based on TPR and IR spectroscopy experiments, it was seen that the presence of magnesium improved ceria reduction favoring the creation of OH groups, which are considered the active sites for the WGS reaction. The evolution of the surface species formed under reaction conditions (CO, H₂O, H₂) observed by DRIFTS evidenced that the formation of formate species and the generation of CO₂ is closely attached to each other; under a reaction stream containing hydrogen the presence of formate species showed to be more relevant while the CO₂ formation was hindered. It is suggested that the addition of MgO favors the formate decomposition and lower the carbonate concentration on the catalyst surface during WGS reaction.     

Pt/ZrO2 catalyst for a single-stage water-gas shift reaction: Ti addition effect

Ti modified Pt/ZrO₂ catalysts were prepared to improve the catalytic activity of Pt/ZrO₂ catalyst for a single-stage WGS reaction and the Ti addition effect on ZrO₂ was discussed based on its characterization and WGS reaction test. Ti impregnation into ZrO₂ increased the surface area of the support and the Pt dispersion. The reducibility of the catalyst was enhanced in the controlled Ti impregnation (∼20 wt.%) over Pt/ZrO₂ by the Pt-catalysed reduction of supports, particularly, at the interface between ZrO₂ and TiO₂. The significant CO₂ gas band in the DRIFTS results of Pt/Ti[20]/ZrO₂ indicated that the Ti addition made the formate decomposition rate faster than the Pt/ZrO₂ catalyst, linked with the enhanced Pt dispersion and reducibility of the catalyst. Consequently, Ti impregnation over the ZrO₂ support led to a remarkably enhanced CO conversion and the reaction rate of Pt/Ti[20]/ZrO₂ increased by a factor of about 3 from the bare Pt/ZrO₂ catalyst.     

Si-modified Pt/CeO2 catalyst for a single-stage water-gas shift reaction

Si-modified Pt/CeO₂ catalysts were prepared for a water-gas shift (WGS) reaction and the effects of this silica addition on the textural and structural characteristics, reducibility and WGS reaction performance of Pt/CeO₂ were investigated. The surface areas of the prepared catalysts increased and both interplanar spacing and average crystalline size of ceria gradually decreased with Si content, resulting in less crystalline and smaller particles. Si addition up to 20 wt. % facilitated the bulk reduction of ceria by inducing significant hydrogen consumption. The oxygen defects in the support, associated with lower valence state cerium, increased with the Si addition. These modifications offer a promising potential to increase the density of hydroxyl groups on the surface of the ceria and consequently increase the concentration of surface intermediate species. The addition of Si to ceria improved the catalytic performance for the WGS reaction, in spite of its irreducible nature. Pt catalysts supported on Si-modified ceria, with a Si content of 5-10 wt.%, exhibited a 2.5-fold increase in reaction rate and turnover frequency (TOF) compared to that of Pt/CeO₂.     

Effects of Pt precursors on Pt/CeO2 to water-gas shift (WGS) reaction activity with Langmuir-Hinshelwood model-based kinetics

The crystallite size effects of Pt nanoparticles on the CeO₂ (Pt/CeO₂) prepared with four different Pt precursors were investigated in terms of their thermal stability and catalytic activity for a water-gas shift (WGS) reaction using the compositions of reformates after a typical steam reforming of propane. The Pt/CeO₂ prepared with a diamine dinitroplatinum (Pt(NO₂)₂(NH₃)₃) precursor, which forms the cationic Pt(NH₃)₂²⁺ species on the negatively-charged CeO₂ surfaces, revealed a superior catalytic activity and thermal stability by forming the partially oxidized smaller Pt nanoparticles decorated with metallic Pt surfaces as well as by forming the strongly interacted PtOx-CeO₂ interfaces. The stable preservation of the pristine smaller Pt nanoparticles with small aggregations even under the hysteresis test from 250 to 400 °C was mainly attributed to the strong metal-support interactions. The optimized Pt/CeO₂ was further studied to obtain kinetic equations derived by Langmuir-Hinshelwood (LH) model, and the optimal operating conditions of WGS reaction were found to be ~280 °C and H₂O/CO molar ratio of 9 with the activation energy of ~78.4 kJ/mol.     

Mechanistic aspects of the low temperature steam reforming of ethanol over supported Pt catalysts

The influence of the support of Pt catalysts for the reaction of steam reforming of ethanol at low temperatures has been investigated on Al₂O₃, ZrO₂ and CeO₂. It was found that the conversion of ethanol is significantly higher when Pt is dispersed on Al₂O₃ or ZrO₂, compared to CeO₂. Selectivity toward H₂ is higher over ZrO₂-supported catalyst, which is also able to decrease CO production via the water-gas shift reaction. Depending on catalyst employed, interaction of the reaction mixture with the catalyst surface results in the development of a variety of bands attributed to ethoxy, acetate and formate/carbonate species associated with the support, as well as by bands attributed to carbonyl species adsorbed on platinum sites. The oxidation state of Pt seems to affect catalytic activity, which was found to decrease with increasing the population of adsorbed CO species on partially oxidized (Pt^δ+) sites. Evidence is provided that the main reaction pathway ethanol dehydrogenation, through the formation of surface ethoxy species and subsequently acetaldehyde, which is decomposed toward methane, hydrogen and carbon oxides. The population of adsorbed surface species, as well as product distribution in the gas phase varies significantly depending on catalyst reactivity towards the WGS reaction.     

Reactivity of Pt/Ni supported on CeO2-nanorods on methanol steam reforming for H2 production: Steady state and DRIFTS studies

The reactivity of the PtNi supported on CeO₂-nanorods was performance on methanol steam reforming (MSR). CO_ads revealed that outer of the PtNi-catalyst could be mainly Pt-terminated and, CO_ads was slightly attenuated on the surface of the CeO₂-R. The catalytic performance of the bimetallic PtNi/CeO₂-NR catalyst exhibited better methanol conversion and H₂ selectivity than the monometallic samples. The surface species associated with the reaction mechanism from TPD-MSR-DRIFTS identified on the CeO₂-NR sample showed stronger bands associated at the methoxy species complemented with stretching C–H bands, while on the Pt/CeO₂-NR catalyst, the methoxy groups diminish indicating that it decomposes to CO and hydrogen and, new peaks of formate (HCOO^?) groups emerge. This finding suggests that the methoxy groups interacted with the surface oxygen of the support during the reaction to yield formate species and the Pt had important role to promote it as intermediary of the reaction.     

Exploring the role of promoters (Au,Cu and Re) in the performance of Ni–Al layered double hydroxides for water-gas shift reaction

Water-gas shift (WGS) reaction is well-known industrial process targeting hydrogen production. Designing well-performing and economically profitable WGS catalysts is the key toward production of pure hydrogen for application in fuel cell processing systems. Promotional role of Au, Cu, or Re in the WGS performance of Ni–Al formulations derived from hydrotalcite precursor was analysed on the basis of deep characterization by BET, XRD, UV–Vis, XPS, and TPR measurements of as-prepared and WGS-tested samples. Additionally, modification by ceria was examined. WGS results revealed that catalyst behaviour was strongly dependent on promoter type. The best performance exhibited gold-promoted Ni–Al layer double hydroxide modified with ceria. This system showed a superior catalytic activity as 99.7% CO conversion at 220 °C that correlated well with significantly enhanced reducibility of support. Although Au-containing CeO₂-modified Ni–Al catalyst outperformed WGS activity of Cu- and Re-promoted analogues, stability of Re-containing sample and enhanced activity of Cu-based sample after tests at different reaction conditions manifested promising results. They leave open space for future investigations addressing improved catalyst performance by tuning Re⁴⁺/Re⁷⁺ redox structures or optimizing catalyst composition.     

Photodeposition of Pt nanoparticles onto TiO2@CNT as high-performance electrocatalyst for oxygen reduction reaction

The commonly used Pt/C catalyst has low durability for oxygen reduction reaction (ORR). In this work, CNT-supported TiO₂ nanoparticles, which synergistically combines the merits of TiO₂ (high stability and strong interactions with the supported Pt nanoparticles) and CNT (high specific surface area and large electrical conductivity), are prepared by a sol-gel process coupled with an annealing process and used as the support for Pt nanoparticles, which are anchored around TiO₂ nanoparticles by a photodeposition technique. The as-synthesized Pt/TiO₂@CNT catalyst exhibits a mass activity 5.3 times as large as that of the commercial Pt/C catalyst (0.358 A mg_Pt⁻¹ vs. 0.067 A mg_Pt⁻¹ at 0.9 V) and an excellent stability (no activity loss after 10000 potential cycles) for ORR, which can be mainly attributed to the lower oxygen adsorption energy of Pt, resulting from the strong metal-support interaction induced by the deposition of Pt nanoparticles around the well-dispersed TiO₂ nanoparticles on CNT.     

Defect engineering by steam treatment over Pt/CeO2 catalyst promoting C–H activation in partial oxidation of methane

Reducible oxides such as ceria have been certified to significantly enhance the catalytic activity of redox reactions. In this study, Pt/CeO₂ catalyst was treated with steam, a good deal of hydroxyl groups came into being on surface to form Pt–OH species, thus achieving a high degree of Pt dispersion. The results of O₂-TPD, XPS, Raman and EPR indicated that the dissociation of steam at surface vacancy was in favor of activation and removal of lattice oxygen and generation of oxygen vacancies. DFT calculation and in situ DRIFTS results showed that the oxygen vacancy was favorable for activating C–H bond in methane. The Pt/CeO₂–H₂O catalyst with abundant oxygen vacancies presented outstanding catalytic activity than that of Pt/CeO₂, the CH₄ conversion rate, CO and H₂ selectivity was 62%, 60% and 58%, respectively. What's more, defective engineering promoting methane partial oxidation can be extended to all kinds of redox reactions.     

CeO2-promoted Ni/activated carbon catalysts for the water–gas shift (WGS) reaction

The low temperature water–gas shift (WGS) reaction has been studied over carbon-supported nickel catalysts promoted by ceria. To this end, cerium oxide has been dispersed (at different loadings: 10, 20, 30 and 40 wt.%) on the activated carbon surface with the aim of obtaining small ceria particles and a highly available surface area. Furthermore, carbon- and ceria-supported nickel catalysts have also been studied as references. A combination of N₂ adsorption analysis, powder X-ray diffraction, temperature-programmed reduction with H₂, X-ray photoelectron spectroscopy and TEM analysis were used to characterize the Ni–CeO₂ interactions and the CeO₂ dispersion over the activated carbon support. Catalysts were tested in the low temperature WGS reaction with two different feed gas mixtures: the idealized one (with only CO and H₂O) and a slightly harder one (with CO, CO₂, H₂, and H₂O). The obtained results show that there is a clear effect of the ceria loading on the catalytic activity. In both cases, catalysts with 20 and 10 wt.% CeO₂ were the most active materials at low temperature. On the other hand, Ni/C shows a lower activity, this assessing the determinant role of ceria in this reaction. Methane, a product of side reactions, was observed in very low amounts, when CO₂ and H₂ were included in the WGS feed. Nevertheless, our data indicate that the methanation process is mainly due to CO₂, and no CO consumption via methanation takes place at the relevant WGS temperatures. Finally, a stability test was carried out, obtaining CO conversions greater than 40% after 150 h of reaction.     

Effect of oxygen addition on the water–gas shift reaction over Pt/CeO2 catalysts in microchannels – Results from catalyst testing and reactor performance in the kW scale

Wash-coated Pt/CeO₂, Pt/CeO₂/ZrO₂ and Pt/Cu/CeO₂ and Pt/CeO₂/Al₂O₃ based formulations were tested in sandwich type microreactors for water–gas shift (WGS) activity. At low reaction temperature of 260 °C, low conversion of carbon monoxide was initially observed which increased considerably upon the addition of air, a behaviour which was observed even after multiple cycles of start-up, operation with and without air and shut-down. At a higher reaction temperature of 400 °C air addition did not further improve the performance of the catalysts, which converted the carbon monoxide already close to equilibrium. One of the catalysts was incorporated into a larger reactor of kW scale and tested for its performance under conditions of WGS and oxygen enhanced WGS. The carbon monoxide conversion was increased by the air addition also on the larger reactor.     

Hydrogen purification for fuel cell using CuO/CeO2-Al2O3 catalyst

CuO/CeO₂, CuO/Al₂O₃ and CuO/CeO₂-Al₂O₃ catalysts, with CuO loading varying from 1 to 5 wt.%, were prepared by the citrate method and applied to the preferential oxidation of carbon monoxide in a reaction medium containing large amounts of hydrogen (PROX-CO). The compounds were characterized ex situ by X-ray diffraction, specific surface area measurements, temperature-programmed reduction and temperature-programmed reduction of oxidized surfaces; XANES-PROX in situ experiments were also carried out to study the copper oxidation state under PROX-CO conditions. These analyses showed that in the reaction medium the Cu⁰ is present as dispersed particles. On the ceria, these metallic particles are smaller and more finely dispersed, resulting in a stronger metal-support interaction than in CuO/Al₂O₃ or CuO/CeO₂-Al₂O₃ catalysts, providing higher PROX-CO activity and better selectivity in the conversion of CO to CO₂ despite the greater BET area presented by samples supported on alumina. It is also shown that the lower CuO content, the higher metal dispersion and consequently the catalytic activity. The redox properties of the ceria support also contributed to catalytic performance.     

Effect of the CeO2 synthesis method on the behaviour of Pt/CeO2 catalysis for the water-gas shift reaction

A comparative study of three different ceria synthesis procedures (template- and MW- assisted hydrothermal synthesis and urea homogeneous precipitation) is reported in this paper. The obtained materials were employed as supports for Pt nanoparticles, and the Pt/CeO₂ catalysts were evaluated in the WGS reaction under model and realistic conditions. The influence of the support, e.g., its morphology and electronic properties, has been studied in detail by means of XRD, H₂-TPR, XPS, UV–Vis spectroscopy and toluene hydrogenation (for metal dispersion assessment). The catalytic performance of the samples is directly correlated with the modification of the electronic properties, as a result of the preparation method used. The conventional homogeneous precipitation method with urea resulted to be the best option, leading to enhanced ceria reducibility and adequate Pt dispersion, which in turns resulted in a very efficient WGS catalyst.     

Water-gas-shift reaction over Au single-atom catalysts with reversible oxide supports: A density functional theory study

The low-temperature water-gas-shift (LT-WGS) reaction has shown remarkable activity for Au single-atom supported on reducible oxide catalysts. The water dissociation step of the WGS reaction is calculated using density functional theory (DFT) over four single Au atom (Au₁)-pristine reversible oxides support systems Au₁/M_aO_b-O_v (Au₁/TiO_2-x, Au₁/ZrO_2-x, Au₁/CeO_2-x and Au₁/Co₃O_4-x) system and four Au₁-supports with O vacancy (O_v) systems Au₁/M_aO_b (Au₁/TiO₂, Au₁/ZrO₂, Au₁/CeO₂ and Au₁/Co₃O₄). According to its greatest H₂O adsorption energy, lowest water dissociation barrier, and slightest structural distortion, Au₁/CeO_2-x is chosen as the most beneficial catalyst for the WGS process. Afterwards, for Au₁/CeO_2-x, three reaction pathways (redox path, formate path and carboxyl path) are calculated. The predominant reaction pathway is the carboxyl pathway, and hydrogen production is the rate-determining step (RDS). For the purpose of designing single metal-support catalysts for the LT-WGS reaction, this paper gives information on the strong metal-support interaction (SMSI).     

Oxygen-enhanced water gas shift over ceria-supported Au–Cu bimetallic catalysts prepared by wet impregnation and deposition–precipitation

Au–Cu/ceria bimetallic catalysts were prepared incorporating Au by incipient wetness impregnation (IWI) and deposition-precipitation (DP) methods (with loadings of 1 wt.% and 7 wt.% of Au and Cu, respectively). The as-prepared catalysts were characterized by techniques such as BET, XRD, Raman, XPS, H₂-TPR, CO-TPD and Oxygen Storage Capacity (OSC) measurements. The results indicated a good dispersion of gold and copper for copper ceria catalyst and Au–Cu bimetallic catalysts. Addition of Au to CuO/CeO₂ increases highly the capacity to release lattice oxygen to oxidized CO at low temperatures compared to pure CuO/CeO₂. Au/CeO₂ and Au–CuO/CeO₂ catalyst prepared by DP show higher OSC value than counterparts prepared by IWI, either at 120 and 250 °C. Also, gold-containing catalysts prepared by DP show lower temperature of reduction that the samples prepared by IWI as a consequence of the higher dispersion of gold in the former samples. The presence of gold at different oxidation states was observed by XPS analysis. Preparation method strongly affects to the atom ratio of Au and Au + Cu with respect to surface ceria. The gold incorporation method was a key factor that enhances the redox properties and activity in both WGS and OWGS reactions. The present study shows the gas phase oxygen enhanced the activity of monometallic CuO/ceria and bimetallic Au–Cu/ceria prepared by IWI and DP methods in both WGS and OWGS reactions. AuCC catalyst prepared by DP shows higher hydrogen yield and also higher CO conversion than other prepared by IWI during OWGS reaction.     

Preferential oxidation of CO in H2 stream on Au/CeO2–TiO2 catalysts

A series of Au catalysts supported on CeO₂–TiO₂ with various CeO₂ contents were prepared. CeO₂–TiO₂ was prepared by incipient-wetness impregnation with aqueous solution of Ce(NO₃)₃ on TiO₂. Gold catalysts were prepared by deposition–precipitation method at pH 7 and 65 °C. The catalysts were characterized by XRD, TEM and XPS. The preferential oxidation of CO in hydrogen stream was carried out in a fixed bed reactor. The catalyst mainly had metallic gold species and small amount of oxidic Au species. The average gold particle size was 2.5 nm. Adding suitable amount of CeO₂ on Au/TiO₂ catalyst could enhance CO oxidation and suppress H₂ oxidation at high reaction temperature (>50 °C). Additives such as La₂O₃, Co₃O₄ and CuO were added to Au/CeO₂–TiO₂ catalyst and tested for the preferential oxidation of CO in hydrogen stream. The addition of CuO on Au/CeO₂–TiO₂ catalyst increased the CO conversion and CO selectivity effectively. Au/CuO–CeO₂–TiO₂ with molar ratio of Cu:Ce:Ti = 0.5:1:9 demonstrated very high CO conversion when the temperature was higher than 65 °C and the CO selectivity also improved substantially. Thus the additive CuO along with the promoter and amorphous oxide ceria and titania not only enhances the electronic interaction, but also stabilizes the nanosize gold particles and thereby enhancing the catalytic activity for PROX reaction to a greater extent.     

Hierarchically organized nanostructured TiO2/Pt on microfibrous carbon paper substrate for ethanol fuel cell reaction

Titanium dioxide is emerging as new class of catalyst support for fuel cell reactions. Via pulsed laser deposition, we prepare hierarchically organized and vertically aligned TiO₂/Pt nanostructured films on microfibrous carbon paper (CP) substrate at room temperature. Microstructural analyses reveal a metal-support interaction between TiO₂ and Pt. Namely; (i) the particles size of Pt on TiO₂ is smaller than those of Pt grown onto carbon paper substrate under similar condition of synthesis and (ii) Pt metallic state is transformed to ionized Pt²⁺ and Pt⁴⁺. Compared to CP/Pt electrocatalyst, the CP/TiO₂/Pt electrocatalysts (i) oxidize ethanol at much lower potentials, (ii) display superior current peak densities of more than 2.5 times higher by voltammetry, and (iii) steady state current density during long-term stability up to 8 times greater. For portable electronic applications, the binder-free and hierarchical structures of the CP/TiO₂/Pt electrocatalyst with its planar deposition make it very attractive as anode for direct ethanol fuel cells.     

Cooperative effect of Pt and Cu on CeO2 for the CO-PROX reaction under CO2–H2O feed stream

Catalyst improvement for the preferential oxidation of CO (CO-PROX) is essential in developing efficient fuel cell technologies. Here, we investigate the promotion of the Cu/CeO₂ system with Pt, prepared by impregnation and alcohol-reduction methods, in the CO-PROX reaction under ideal and realistic feed compositions. The high Pt dispersion in PtCu/CeO₂ prepared by impregnation led to a CO conversion of 62% and CO₂ selectivity of 83% at 50 °C under a feed stream composed of H₂/CO/O₂, while monometallic Cu/CeO₂ and Pt/CeO₂ showed negligible activity at these conditions. By adding CO₂–H₂O to the feed stream, PtCu/CeO₂ catalysts prepared by both methods presented similar activity. The maximum CO conversion temperature was shifted to 100 °C. Under these conditions, Cu/CeO₂ was inactive, and Pt/CeO₂ showed identical conversion but lower CO₂ selectivity. In-situ XANES revealed that fast oxidation of Cu species at low temperatures is responsible for Cu/CeO₂ deactivation, while preferential adsorption of CO on Pt⁰ sites in PtCu/CeO₂ avoided deactivation. The use of deactivation-resistant Pt sites as complimentary sites for CO activation associated with improved oxygen mobility over Cu–CeO₂ surface proved to be an effective strategy for CO-PROX under H₂O/CO₂ feed stream at low temperatures.     

Defect configuration of ceria for Pt anchoring toward efficient methanol oxidation

As a potential next-generation power source for portable electronic devices, commercialization process of direct methanol fuel cell (DMFC) technology is hindered by the high dependence of anode methanol oxidation reaction (MOR) on precious Pt catalyst. In order to improve the efficiency of Pt toward MOR catalysis, a Ni doping strategy is proposed for defect engineering on ceria substrate to achieve uniform dispersion of Pt nanoparticles. Besides, Ni could also act as electron donor for Pt and hence favor the removal of CO intermediate on Pt and act as a co-catalyst toward MOR. Superior MOR activity and great stability is therefore achieved for the as-prepared Pt/CeO₂@Ni catalyst with 3 times higher peak MOR current density compared with Pt/C catalyst. Due to the evenly anchored Pt and enhanced CO oxidation ability caused from Ni doped ceria substrate, Pt utilization of the Pt/CeO₂@Ni catalyst is calculated to be 3.24 times higher than that of the commercial Pt/C catalyst. By considering the significantly improved stability, the Pt/CeO₂@Ni catalyst has the potential for application in DMFC devices.     

Effect of precipitation sequence on physicochemical properties of CeO2 support for hydrogen production from low-temperature water-gas shift reaction

A one-step reverse precipitation method has been developed to prepare nano-sized ceria (CeO₂) support with controlled physicochemical properties for low temperature water-gas shift (LT-WGS) reaction. The nano-sized CeO₂ support prepared by reverse precipitation method has a high Brunauer-Emmett-Teller (BET) surface area of 162.8 m²/g. To compare catalytic activity with that of CeO₂ prepared by normal precipitation method, 5 wt% Cu was employed as the active metal, coupled to the CeO₂ support. The catalytic activity of CeO₂ supported Cu catalyst prepared by reverse precipitation method was evaluated for the first time in LT-WGS reaction. Notably, the CeO₂ – R supported Cu catalyst, prepared by reverse precipitation method, showed higher CO conversion and turnover frequency (TOF) values than CeO₂–N supported Cu catalyst prepared by normal precipitation method.