Elsevier

Combustion and Flame

Volume 211, January 2020, Pages 270-280
Combustion and Flame

Kinetic interactions between H2 and CO in catalytic oxidation over PdO

https://doi.org/10.1016/j.combustflame.2019.09.035Get rights and content

Abstract

Kinetic interactions between H2 and CO over PdO, a widely used catalyst in combustion systems, were studied experimentally and numerically. Global reaction parameters of H2 and CO oxidation over PdO were extracted from wire microcalorimetry experiments at atmospheric pressure in the temperature range 380–800 K, based on which a full catalytic reaction mechanism was developed. Comparison of ignition temperatures and heat release rates of different H2/CO blends along with density functional theory (DFT) simulations revealed complex physicochemical coupling of the H2 and CO catalytic oxidation pathways. The coupling evolves from an inhibiting effect of one fuel component onto the other due to their competition for surface adsorption sites and a direct repelling mechanism between the co-adsorbed H(s) and CO(s), to a promoting effect at sufficiently high temperatures caused by alleviated O(s) surface blocking. Implications of the H2single bondCO kinetic coupling to the operation of practical power generation systems are outlined.

Introduction

Combustion of syngas, a gas mixture primarily consisting of hydrogen and carbon monoxide, has drawn much attention in recent years as an environmentally clean fuel that can facilitate reduced emissions in gas turbines of power generation systems [1], [2], [3], [4]. Specifically, greenhouse gas emissions can be controlled via pre-combustion CO2 capture, accomplished by reforming of natural gas or gasification of solid biomasses to syngas, subsequently converting large part of the produced CO to CO2 in a water–gas-shift reactor (WGS) and finally capturing the formed CO2 prior to combustion [5,6]. Another method of mitigating CO2 emissions in gas turbines is the post-combustion capture of CO2, which is typically accomplished with flue gas recycle (FGR); in this case the addition of high-hydrogen content syngas is very beneficial, as it increases the reactivity of the heavily-diluted reactant stream [7]. Syngas fuels are also suited for the enhancement of combustion stability and emission reduction in automotive internal combustion engines [8] and micro-scale portable power generation systems [9].

Of particular interest for the present investigation are the hybrid catalytic/gaseous combustion methodologies, as they can mitigate flame flashback of the very reactive high-hydrogen content syngas fuels inside the channels of typical honeycomb catalytic reactors [10]. In both pre- and post-combustion CO2 capture methods, catalytic reactions can be utilized with the catalytically stabilized thermal combustion (CST) concept, in which fractional fuel conversion is achieved heterogeneously in a reactor that is typically coated with a noble metal, while the remaining fuel is subsequently consumed homogeneously in a gaseous combustion zone [11]. Furthermore, CO addition is of special benefit in H2-fueled catalytic reactors as it moderates the surface superadiabaticity induced by the diffusional imbalance of hydrogen [9,12] thus facilitating reactor design and thermal management [1,13].

Palladium is a widely used noble metal catalyst for methane oxidation [13], [14], [15], [16]. Furthermore, because of its superior low-temperature activity in methane oxidation, palladium-based catalysts have been prime choices as front-face catalysts in natural gas fueled turbine combustors as well as microreactors [14,17,18]. Recently, a non-monotonic pressure dependence of the methane catalytic reactivity on PdO was observed [18,19], which is contrary to the monotonically increasing pressure dependence of the methane reactivity on other noble metals (Pt, Rh) [20,21], while a microkinetic model was developed for methane oxidation over PdO that captures the hysteresis in PdO/Pd decomposition and re-oxidation [22].

In view of the practical importance of the catalytic oxidation of syngas over palladium, and given that several surface reaction mechanisms for the oxidation of simple fuels over palladium have been reported [23], [24], [25], [26], [27], it is timely to conduct dedicated kinetic studies of syngas mixtures on palladium. Such studies are complicated by the intricate interplay between the two fuel components, H2 and CO, and the complex temperature and pressure dependent phase change (Pd/PdOx) of palladium-based catalysts [28,29]. Consequently, the primary objective of the present work is to investigate the heterogeneous kinetics of syngas over palladium-based catalysts and the associated physicochemical interactions between the H2 and CO fuel components.

This paper is organized as follows. First, the adopted experimental and numerical methodologies are introduced in Sections 2 and 3, respectively. Next, the global and detailed reaction parameters for H2 oxidation on PdO, extracted from wire microcalorimetry experiments, are reported in Section 4.1. In Section 4.2 the CO-poisoning effects on the PdO catalyst and the determination of separate H2 and CO reactions over the poisoned catalyst are presented. The combustion of H2/CO mixtures and the kinetic coupling between H2 and CO on PdO surfaces are subsequently discussed in Section 4.3, wherein a detailed syngas reaction mechanism is formulated. The main results are summarized in Section 5.

Section snippets

Experimental

The experiments employed the wire microcalorimetry technique that has been detailed previously [26,30] and is briefly described next. The experimental setup is schematically illustrated in Fig. 1(a). A pre-oxidized polycrystalline PdO wire, 100 µm in diameter and 100 mm in length, is positioned horizontally in the center of a fixed-pressure closed rectangular chamber (with horizontal cross-stream area of 200 × 200 mm2 and height of 200 mm). The gaseous environment is perpetually replenished

Numerical

In the numerical part, two investigations have been pursued. First, 2D simulations over half the y-z domain (schematic of the computational domain is shown in Fig. 1(b) and details in [32]) with the Fluent code were carried out; the code has been validated against past wire microcalorimetry experiments [26,27,32,33] in simulations with detailed methane catalytic and gas-phase reaction mechanisms. Gravity in the y-direction was included in the simulations. In the present work, a detailed

Reaction parameters of H2 oxidation on PdO

Global reaction parameters of H2 oxidation over PdO were first extracted from the heat release measurements. To ensure negligible changes in the heat transfer properties of the reactive mixtures and air [40] and to avoid distortion from the moisture accumulated in the PdO porous structure, only up to 1.0% vol. H2 was doped into air. Self-inhibiting kinetic effect [23] of H2 on the catalytic ignition over PdO was observed, with measured ignition temperatures 380 K, 385 K and 389 K for H2

Conclusions

Using fine-tuned wire microcalorimetry experiments and simulations, global and elementary reaction parameters were derived for the oxidation of separate H2 and CO fuels as well as H2single bondCO fuel blends over PdO catalysts, leading to a detailed catalytic reaction mechanism for syngas oxidation on PdO. The PdO surface was partially deactivated due to CO-poisoning, resulting in reduced H2 catalytic reactivity. The kinetic coupling of H2 and CO oxidation developed from an inhibiting two-sided effect due

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors acknowledge the assistance in micro-GC measurements from Prof. Yiguang Ju's group at Princeton University. RS is supported by the Swiss National Science Foundation EPM fellowship (Award Number 178619). LZ acknowledges the use of the computer time allocation at Extreme Science and Engineering Discovery Environment (XSEDE) supported through the US National Science Foundation (Award Number CHE160084).

References (55)

  • D. Ciuparu et al.

    Support and water effects on palladium based methane combustion catalysts

    Appl. Catal. A – Gen.

    (2001)
  • R. Sui et al.

    Hetero-/homogeneous combustion of fuel-lean CH4/O2/N2 mixtures over PdO at elevated pressures

    Proc. Combust. Inst.

    (2019)
  • M. Reinke et al.

    High-pressure catalytic combustion of methane over platinum: in situ experiments and detailed numerical predictions

    Combust. Flame

    (2004)
  • R. Sui et al.

    Hetero-/homogeneous combustion of fuel-lean methane/oxygen/nitrogen mixtures over rhodium at pressures up to 12bar

    Proc. Combust. Inst.

    (2017)
  • H. Stotz et al.

    Surface reaction kinetics of methane oxidation over PdO

    J. Catal.

    (2019)
  • O. Deutschmann et al.

    Numerical modeling of catalytic ignition

    Symp. (Int.) Combust.

    (1996)
  • R.W. Sidwell et al.

    Catalytic combustion of premixed methane-air on a palladium-substituted hexaluminate stagnation surface

    Proc. Combust. Inst.

    (2002)
  • T. Shimizu et al.

    Temperature-dependent gas-surface chemical kinetic model for methane ignition catalyzed by in situ generated palladium nanoparticles

    Proc. Combust. Inst.

    (2011)
  • Y.X. Xin et al.

    Kinetics of catalytic oxidation of methane, ethane and propane over palladium oxide

    Combust. Flame

    (2014)
  • M.M. Wolf et al.

    Kinetic model for polycrystalline Pd/PdOx in oxidation/reduction cycles

    Appl. Catal. A – Gen.

    (2003)
  • T.C. Zhang et al.

    A wire microcalorimetric study of catalytic ignition of methane-air mixtures over palladium oxide

    Proc. Combust. Inst.

    (2011)
  • P. Cho et al.

    Catalytic ignition of fuel/oxygen/nitrogen mixtures over platinum

    Combust. Flame

    (1986)
  • T.C. Zhang et al.

    Catalytic oxidation of methane over PdO in wire microcalorimetry

    Combust. Flame

    (2013)
  • Y.X. Xin et al.

    Kinetics of catalytic oxidation of ethylene over palladium oxide

    Proc. Combust. Inst.

    (2015)
  • S. Zheng et al.

    The effect of different Hitran databases on the accuracy of the snb and snbck calculations

    Int. J. Heat Mass Transf.

    (2019)
  • C. Appel et al.

    An experimental and numerical investigation of homogeneous ignition in catalytically stabilized combustion of hydrogen/air mixtures over platinum

    Combust. Flame

    (2002)
  • R. Sui et al.

    H2 and CO heterogeneous kinetic coupling during combustion of H2/CO/O2/N2 mixtures over rhodium

    Combust. Flame

    (2019)
  • Cited by (17)

    View all citing articles on Scopus
    View full text