Syngas by catalytic partial oxidation of methane on rhodium: Mechanistic conclusions from spatially resolved measurements and numerical simulations

doi:10.1016/j.jcat.2006.05.008

Journal of Catalysis

Volume 242, Issue 1, 15 August 2006, Pages 92-102

https://doi.org/10.1016/j.jcat.2006.05.008 Get rights and content

Abstract

The mechanism for the catalytic partial oxidation of CH₄ on Rh-coated α-Al₂O₃ foam monoliths was investigated by measuring species and temperature profiles along the catalyst axis and comparing them with numerical simulations. A thin quartz capillary connected to a quadrupole mass spectrometer was moved through the catalyst with a spatial resolution of ∼0.3 mm. Profiles were measured under autothermal operation for C/O ratios of 0.7, 1.0 and 1.3. The influence of the flow rate (5 vs. 10 l min⁻¹) was studied for syngas stoichiometry (C/O = 1). Numerical simulations were performed with a 38 step surface mechanism using both a porous 2D-model with mass and heat transfer and a simple plug-flow model. The experimental profiles reveal complete O₂ conversion within 2 mm of the catalyst entrance for all C/O ratios and flows. H₂ and CO are formed partly in the oxidation zone and partly after O₂ is fully converted by steam reforming. CO₂ is formed in small amounts in the oxidation zone and remains constant thereafter, except for C/O = 0.7, where some water gas shift is observed. CO₂ reforming does not occur under the experimental conditions. Based on the experimental findings, a two-zone picture of the reaction mechanism is proposed. The 2D numerical simulations and the measured profiles agree qualitatively for all experimental conditions. Quantitative agreement is best for syngas stoichiometry (C/O = 1.0) at 5 and 10 l min⁻¹ flow rate. Some quantitative differences are observed for C/O = 0.7 and 1.3. The plug flow model is for all conditions inferior to the 2D model. The importance of spatial profiles for mechanism and reactor model validation is highlighted.

Introduction

The conversion of methane into hydrogen, liquid fuels, or chemicals is of growing economic importance. Natural gas, which consists mainly of methane, is available in similar amounts on earth as oil, but it is far less exploited. According to recent assessments, oil production will peak before the year 2040 [1]. Strong efforts in science and technology are underway to convert natural gas into liquids to open up a large petroleum resource that can compensate potential oil shortfalls [2].

The first step in the transformation of methane to liquids is the production of syngas. The catalytic partial oxidation (CPO) of methane (Eq. (1)) on Rh-coated foam monoliths is an efficient way to achieve this transformation [3]. In contrast to the highly endothermic steam reforming (Eq. (2)), the CPO reaction (1) is slightly exothermic. ${CH}_{4} + (1 / 2) O_{2} \to CO + 2 H_{2}, Δ H_{r}^{○} = - 36 kJ {mol}^{−1},$ ${CH}_{4} + H_{2} O \to CO + 3 H_{2}, Δ H_{r}^{○} = + 206 kJ {mol}^{−1} .$ Steam reforming on Ni requires contact times in the range of 1 s to achieve sufficient CH₄ conversion. For the CPO reaction on Rh, CH₄ conversion close to 100% and >90% selectivities to H₂ and CO can be achieved in a few ms [4]. CPO reactors can be operated autothermally and with a much higher gas hourly space velocity than a steam reformer, making them attractive for remote gas field applications. For process integration, methane CPO supplies a H₂/CO ratio of 2 that is more favorable for downstream chemistry (methanol, Fischer–Tropsch synthesis) than the higher ratio obtained by steam reforming.

There is an ongoing debate about the mechanism of the methane CPO. The present literature review focuses on results on Rh, but similar arguments may hold for other catalysts. A direct mechanism (pyrolysis–oxidation) and an indirect (combustion–reforming) mechanism are discussed in the literature [5]. The pyrolysis–oxidation mechanism assumes that H₂ and CO are primary reaction products formed in the oxidation zone at the catalyst entrance. After methane pyrolysis (CH₄ → C_s + 4H_s), surface carbon reacts with surface oxygen to CO (C_s + O_s → CO) and surface hydrogen atoms combine to H₂ (H_s + H_s → H₂). In contrast, the combustion-reforming mechanism postulates a two-zone model with a CH₄ combustion zone at the catalyst entrance (CH₄ + 2O₂ → CO₂ + 2H₂O), and H₂ and CO production in a reforming zone downstream (CH₄ + H₂O → CO + 3H₂, CH₄ + CO₂ → 2CO + 2H₂, respectively).

Because the mechanisms postulate different reaction zones, spatially resolved measurements would help verify the assumptions; however, only one paper reporting intracatalyst species and temperature profiles (at high pressures, 0.2–0.8 MPa) has been published so far. Lyubovsky et al. [6] used a stack of Microlith^® screens and measured species and temperature behind each screen. They argued for mainly direct formation of CO and indirect formation of H₂ by steam reforming. This conclusion was drawn by extrapolating CO and H₂ selectivities to zero CH₄ conversion. But because the spatial resolution was only 2 mm, and the CH₄ and O₂ conversion at the first point were already 40% and >80%, respectively, extrapolation over such a large range could be imprecise. With the capillary technique used in the present paper, 0.3 mm resolution has been achieved.

Nonspatially resolved investigations into the mechanism are numerous in the literature. Among these are some supporting the direct mechanism [7], [8], [9], [10], [11], [12], some supporting the indirect mechanism [13], [14], [15], and others supporting a mixed mechanism [6], [16]. In some cases, the mechanistic conclusions contradict each other. However, the literature results indicate a dependence of the mechanism on (i) temperature [10], [17], [18], [19], (ii) pressure [13], (iii) Rh oxidation state [8], [11], [12], [13], [14], [16], [18], [19], (iv) catalyst loading [11], [17], and (v) nature of the support [8], [14], [15], [18], [20]. Therefore, contradictions may arise because the mechanistic conclusions cannot be extrapolated beyond the conditions of the particular experiment.

Without discussing individual results, an overall picture of the reaction can be extracted from the studies cited above. It seems that reduced Rh sites (Rh metal) are active for syngas production with CH₄ dissociation as the rate-limiting step. Oxidized Rh sites (Rh_xO_y) lead to the formation of total oxidation products (H₂O, CO₂). The oxidation state of the Rh surface depends on temperature and gas atmosphere. Many studies report CO₂ and H₂O formation after the reaction was started on an oxidized Rh catalyst, but the selectivities turn to syngas formation after a short operational time, because reduced Rh sites are formed. At high temperatures, the overall gas atmosphere is sufficiently reducing to restore metallic Rh sites quickly even if gas-phase oxygen is present. Chemisorbed oxygen can lead to total oxidation if the surface temperature is low (e.g., 500 °C). In this case H₂ and CO are not desorbing quickly enough and become oxidized. The higher the temperature, the higher the selectivity to H₂ and CO. Most published studies agree that CO₂ reforming is unimportant in the reaction network. It seems that the support is important, especially at low Rh loadings. Some oxides can serve as an oxygen source for the Rh surface (inverse spillover of OH or H₂O), even though the intrinsic catalytic activity of the support is low.

As with the experimental studies, the numerical studies published differ in their conclusions about the reaction mechanism. The original papers from Hickman and Schmidt assumed the direct mechanism, because a high-temperature surface model (19 reactions) was developed that described the experimental conversion and selectivity data reasonably well [4], [21]. In this model, CH₄ dissociation on metallic Rh sites was lumped into a single step, and H₂ and CO were formed according to the pyrolysis–oxidation mechanism. Except for CO₂ adsorption, all adsorption–desorption steps in this original surface mechanism were reversible. Therefore, steam reforming was not excluded by definition, but CO₂ reforming was excluded (low sticking coefficient of CO₂ on Rh). A 2D simulation using that mechanism showed no contribution of steam reforming [22]; it was too slow under the given conditions. A plug-flow study using the 19 step surface model and a gas-phase mechanism with 227 reversible reactions (GRI-Mech 2.11) showed that gas-phase reactions were insignificant for atmospheric pressure but become important at elevated pressures (>5 bar) [22], [23], [24]. Experimental results corroborate this [13]. Gas-phase reactions are usually not considered in atmospheric pressure simulations.

The mechanism was further improved by including steam reforming and water–gas shift (38 reactions) [25]. CO₂ readsorption was also included. These refinements are in better agreement with experiments showing that steam reforming on Rh is possible in ms contact times [25], [26], [27]. The 38 step surface mechanism has been validated against integral steady-state [28], [29] and transient experimental data [30], [31]. Coverage-dependent desorption energies for CO and O₂ were included [30], because they are important for light-off agreement. The 38 surface reaction step mechanism without coverage dependencies [28] was used in the present publication to compare numerical simulations with experimental spatially resolved profile measurements.

Another, much more extensive C1 mechanism on Rh (104 reactions) was recently reported by Mhadeshwar and Vlachos [32]. All activation energies in this mechanism are coverage-dependent, temperature-dependent, or both. Besides methane CPO, this mechanism also considers methane reformation by water and CO₂ and decomposition of oxygenates on Rh. For methane CPO, this mechanism predicts distinct oxidation (CO₂ and H₂O are products) and reforming (CO and H₂ are products) zones. According to Mhadeshwar and Vlachos, there is no H₂ formation in the oxidation zone for experiments closely matching the present work.

Our experiments and simulations were motivated by the following conclusions from the literature:

(i)
Mass transport, heat transport, and chemistry are strongly coupled in this reaction. Mechanistic conclusions from low-pressure studies (e.g., temporal analysis of products) and from studies with highly diluted reactants or catalysts have to be verified for higher pressures and higher reactant concentrations ⇒ Experimental data must be measured as close to technically relevant conditions as possible.
(ii)
Integral conversion and selectivity data can be described equally well by different mechanisms ⇒ Numerical simulations must be compared with experiments with higher information content-here, spatially resolved data.

Section snippets

Reactor configuration and capillary sampling

The reactor setup and capillary sampling system are shown schematically in Fig. 1. The reaction was carried out in a quartz tube (i.d. = 19 mm, length = 40 cm). High-purity reactants CH₄, O₂; and the internal standard Ar were fed through calibrated mass flow controllers through a side port at the bottom of the tube and left the reactor from the top for incineration. An injection needle inserted through a septum at an end port at the bottom of the tube enabled guided movement of the capillary

Species and temperature profiles

According to literature results, the mechanism for methane CPO depends most strongly on the Rh oxidation state and temperature. To study the influence of these parameters, profile measurements were performed for two different flow rates (5 and 10 l min⁻¹) and three different C/O ratios ( $0.7, 1.0, 1.3$ ).

Experimental results

Fig. 6 compares the experimental profiles for the reactants CH₄ and O₂ and the desired products H₂ and CO for all C/O ratios studied ( $0.7, 1.0, 1.3$ ) and a total flow of 5 l min⁻¹. All species profiles, including those measured for C/O = 1.0 at 10 l min⁻¹, show that H₂ and CO are formed partly in the presence of oxygen and partly after total oxygen consumption by steam reforming. CO₂ reforming does not contribute to the syngas formation (cf. Fig. 3, Fig. 4). A very small increase in CO₂ for C/O = 0.7

Conclusions

The species and temperature profiles presented in this work are the first profiles published for methane CPO on Rh-coated foams. A spatial resolution of (0.3 mm) was reached. The profiles clearly show that under technically relevant conditions (autothermal operation, up to 10 l min⁻¹ flow), syngas is formed by a combination of direct partial oxidation and steam reforming. Numerical simulations using the 38 surface reaction mechanism with a 2D porous reactor model including heat and mass

Acknowledgements

R.H. acknowledges funding through an Alexander von Humboldt Foundation Feodor Lynen Fellowship. The authors acknowledge financial support by the Department of Energy. K.A.W. thanks the Minnesota Supercomputer Institute for the generous computational resources and partial support of this work and A.B. Mhadeshwar and D.G. Vlachos for sharing their C1 mechanism on Rh.

References (44)

D.J. Wilhelm et al.
Fuel Process. Technol.
(2001)
S. Freni et al.
J. Power Sources
(2000)
E.P.J. Mallens et al.
J. Catal.
(1997)
W.Z. Weng et al.
Catal. Today
(2000)
L. Basini et al.
Catal. Today
(2001)
C.T. Au et al.
J. Catal.
(1997)
D. Wang et al.
J. Catal.
(1996)
K. Nakagawa et al.
J. Catal.
(1999)
S. Rabe et al.
Appl. Catal. A Gen.
(2005)
E. Ruckenstein et al.
J. Catal.
(1999)

C.T. Goralski et al.

Chem. Eng. Sci.

(2000)

G. Veser et al.

Chem. Eng. Sci.

(2000)

E.J. Klein et al.

Stud. Surf. Sci. Catal.

(2001)

J. Wei et al.

J. Catal.

(2004)

O. Deutschmann et al.

Stud. Surf. Sci. Catal.

(2001)

R. Schwiedernoch et al.

Chem. Eng. Sci.

(2003)

M.V. Twigg et al.

Chem. Eng. Res. Des.

(2002)

J.T. Richardson et al.

Appl. Catal. A Gen.

(2000)

C.Y. Zhao et al.

Mater. Sci. Eng. A Sruct. A

(2004)

K.L. Hohn et al.

Appl. Catal. A Gen.

(2001)

M.J. Stutz et al.

Chem. Eng. Sci.

(2005)

T.S. Ahlbrandt

Int. Geol. Rev.

(2002)

Cited by (210)

Effect of noble gases on the transport and thermodynamic characteristics of microchannel reforming reactors for hydrogen production
2024, International Journal of Hydrogen Energy
Noble gases can offer the advantages of high selectivity in steam reforming processes with especially great potential for hydrogen production, but the underlying cause for process intensification has not yet been identified. Calculations are performed to investigate the effect of helium or argon gases on the transport and thermodynamic characteristics of a microchannel reactor for hydrogen production. Dimensionless Nusselt and Sherwood number analyses are carried out and the thermodynamic properties of the reaction mixture are determined in order to effectively control the heat and mass transport processes with increasing precision. The heat and reaction phenomena in the reactor system are studied in order to understand the underlying cause for the improvements in transport performance. Enthalpy analysis is performed and the evolution of energy in the chemical processes is discussed based upon reaction heat flux. The results indicate that the noble gas offers the advantages of process intensification due to enhanced heat and mass transport. Additionally, the noble gas can be utilized to enhance conversion behavior while maintaining the reaction rate. Furthermore, the transport properties can be effectively controlled and manipulated, thereby serving as a means for improving the effectiveness of convection at the catalyst surface. Additional advantages include the ability to avoid the problem inherent in thermal management, thereby providing nearly isothermal operation and thus suppressing undesirable side reactions. Finally, the reforming process is characterized for purposes of demonstration or explanation by thermodynamic parameters, including temperature, specific heat, specific enthalpy, sensible enthalpy, and mean molecular weight.
Temperature distribution in a gas-solid fixed bed probed by rapid magnetic resonance imaging
2023, Chemical Engineering Science
Controlling the temperature distribution inside catalytic fixed bed reactors is crucial for yield optimization and process stability. Yet, in situ temperature measurements with spatial and temporal resolution are still challenging. In this work, we perform temperature measurements in a cylindrical fixed bed reactor by combining the capabilities of real-time magnetic resonance imaging (MRI) with the temperature-dependent proton resonance frequency (PRF) shift of water. Three-dimensional (3D) temperature maps are acquired while heating the bed from room temperature to 60 $°$ C using hot air. The obtained results show a clear temperature gradient along the axial and radial dimensions and agree with optical temperature probe measurements with an average error of $\pm$ 1.5 $°$ C. We believe that the MR thermometry methodology presented here opens new perspectives for the fundamental study of mass and heat transfer in gas–solid fixed beds and in the future might be extended to the study of reactive gas–solid systems.
Tri-reforming of methane for syngas production using Ni catalysts: Current status and future outlook
2023, Catalysis Today
Tri-reforming of methane (TRM) emerged as an attractive approach for syngas production that combines the methane steam reforming (SMR), dry reforming (DRM) and partial oxidation (POX) reactions in a single process. The key merits of the TRM process are the ability to use flue gas directly in the process feedstock, the great flexibility to control the H₂/CO ratio for downstream processes, and the relatively lower energy demand and carbon footprint. Considerable progress was achieved over the past decade in catalyst development, process modeling and optimization for syngas production using the TRM process. In this work a critical review of the literature pertaining to Ni-based catalyst development, TRM reaction kinetics, process development, and economics is presented. The use of Ni-based catalysts has received significant attention in recent years; hence this work aims to provide a summary and a critical analysis of the advancement achieved in the design of Ni-based catalysts. Developing active Ni-based catalysts with enhanced resistance to coke formation for the TRM process is anticipated to pave the way for further commercialization ventures. Ni catalysts supported on a wide range of support materials such as Al₂O₃, CeO₂, ZrO₂, TiO₂, MgO and their mixed oxides are the most widely used catalysts for TRM processes. This review provides a critical analysis of the different attempts made to tune the catalyst properties such as the use of different supports, promotors, and synthesis protocols. Catalyst properties such as Ni dispersion, basicity, Ni particle size, thermal stability is evaluated for various catalytic systems to develop structure-property relationship in TRM processes. Moreover, studies on TRM reaction kinetics, process modeling, and economics are also reviewed. Future outlook in catalysts and process development is also presented in this review to shed lights on the potential pathways to develop TRM processes in an environmentally friendly, and cost-effective manner.
Syngas production via chemical looping reforming using methane-based feed and NiO/Al<inf>2</inf>O<inf>3</inf> oxygen carrier
2022, Energy
Syngas production based on chemical looping reforming was experimentally studied using a fixed-bead reactor. The 15 wt% NiO/Al₂O₃ was used as the oxygen carrier. The reaction temperature was fixed as 800 °C. For reactant containing CH₄ only, it was termed as chemical looping partial oxidation of methane (CL-POM). With five CL-POM cycles, no decay in oxygen carrier activity was found. A high H₂/CO ratio resulted due to dominant H₂ production reactions in the reduction stage. Carbon deposition on the oxygen carrier surface can be identified from the CO and CO₂ formations in the oxidation stage. With CO₂ added in the reactant in addition to CH₄, this was referred to as the chemical looping dry reforming of methane (CL-DRM). Near theoretical amounts of H₂ and CO yields were obtained. With both H₂O and CO₂ added in the reactant in addition to CH₄, this was referred to as the chemical looping tri-reforming of methane (CL-TRM). Due to coupled steam reforming of methane (SRM), POM, and DRM, higher CH₄ conversion, H₂ and CO yields resulted compared with the CL-POM and CL-DRM cases. The average H₂/CO ratio of 1.2 was obtained with reactant with molar ratio of CH₄/CO₂/H₂O = 1/1/1.
Resolving gradients in an ammonia oxidation reactor under industrial conditions: A combined experimental and simulation study
2022, Chemical Engineering Journal
The industrial production of nitric acid by the Ostwald process is a major emission source of the potent green-house gas nitrous oxide that is formed during the catalytic oxidation of ammonia. A systematic knowledge-based optimization of industrial ammonia oxidation, e.g. with respect to minimizing nitrous oxide formation, is hindered by the limited fundamental process understanding resulting from a lack of experimental insight into the reaction at industrial conditions. Here we bridge this gap of knowledge by resolving concentration and temperature profiles in a bench-scale reactor setup under industrial conditions using capillary profile measurements in combination with detailed CFD simulations. The profile measurement technique has a spatial resolution sufficient to resolve the steep gradients around single catalyst gauze wires. The influence of the sampling orifice diameter and reaction conditions on the sampling region and the resulting spatial resolution is discussed. The importance of the sampling orifice to gauze positioning in order to measure well averaged concentration profiles is shown. A comparison of simulated and experimental profiles demonstrates that the most widely used surface mechanism in CFD simulations of ammonia oxidation, the Kraehnert and Baerns model, shows the correct trends but cannot capture the product distribution quantitatively. The approach presented in this article provides detailed experimental insight into ammonia oxidation under industrial conditions required to develop and validate future kinetic models for knowledge-based optimization of industrial ammonia oxidation.
Partial oxidation process for syngas production
2022, Advances in Synthesis Gas: Methods, Technologies and Applications: Syngas Production and Preparation: Volume 1
Syngas, a mixture of CO and H₂, can be used for many processes such as methanol and Fischer–Tropsch synthesis. It can be produced through several ways such as steam reforming, dry reforming, trireforming, and partial oxidation. Catalytic partial oxidation has some benefits over other methods, for example, lower reaction temperature levels. Different fuels, including light hydrocarbons, high hydrocarbons, oxygenated hydrocarbons, and biofuels, can be used as a feedstock for partial oxidation; however, methane is the favorite. Knowing the reaction pathway, kinetics, and catalyst properties can help further experiments and research studies. This chapter discusses the catalytic partial oxidation of various types of hydrocarbons focusing on methane and its reaction kinetics. A brief review of two categories of catalysts, containing noble and non-noble catalysts, is also presented.

View all citing articles on Scopus

View full text

Syngas by catalytic partial oxidation of methane on rhodium: Mechanistic conclusions from spatially resolved measurements and numerical simulations

Abstract

Introduction

Section snippets

Reactor configuration and capillary sampling

Species and temperature profiles

Experimental results

Conclusions

Acknowledgements

Fuel Process. Technol.

J. Power Sources

J. Catal.

Catal. Today

Catal. Today

J. Catal.

J. Catal.

J. Catal.

Appl. Catal. A Gen.

J. Catal.

Chem. Eng. Sci.

Chem. Eng. Sci.

Stud. Surf. Sci. Catal.

J. Catal.

Stud. Surf. Sci. Catal.

Chem. Eng. Sci.

Chem. Eng. Res. Des.

Appl. Catal. A Gen.

Mater. Sci. Eng. A Sruct. A

Appl. Catal. A Gen.

Chem. Eng. Sci.

Int. Geol. Rev.