Elsevier

Journal of Catalysis

Volume 247, Issue 2, 25 April 2007, Pages 277-287
Journal of Catalysis

Quantitative analysis of the reactivity of formate species seen by DRIFTS over a Au/Ce(La)O2 water–gas shift catalyst: First unambiguous evidence of the minority role of formates as reaction intermediates

https://doi.org/10.1016/j.jcat.2007.02.013Get rights and content

Abstract

The reactivity of the species formed at the surface of a Au/Ce(La)O2 catalyst during the water–gas shift (WGS) reaction were investigated by operando diffuse reflectance Fourier transform spectroscopy (DRIFTS) at the chemical steady state during isotopic transient kinetic analyses (SSITKA). The exchanges of the reaction product CO2 and of formate and carbonate surface species were followed during an isotopic exchange of the reactant CO using a DRIFTS cell as a single reactor. The DRIFTS cell was a modified commercial cell that yielded identical reaction rates to that measured over a quartz plug-flow reactor. The DRIFTS signal was used to quantify the relative concentrations of the surface species and CO2. The analysis of the formate exchange curves between 428 and 493 K showed that at least two levels of reactivity were present. “Slow formates” displayed an exchange rate constant 10- to 20-fold slower than that of the reaction product CO2. “Fast formates” were exchanged on a time scale similar to that of CO2. Multiple nonreactive readsorption of CO2 took place, accounting for the kinetics of the exchange of CO2(g) and making it impossible to determine the number of active sites through the SSITKA technique. The concentration (in mol g−1) of formates on the catalyst was determined through a calibration curve and allowed calculation of the specific rate of formate decomposition. The rate of CO2 formation was more than an order of magnitude higher than the rate of decomposition of formates (slow + fast species), indicating that all of the formates detected by DRIFTS could not be the main reaction intermediates in the production of CO2. This work stresses the importance of full quantitative analyses (measuring both rate constants and adsorbate concentrations) when investigating the role of adsorbates as potential reaction intermediates, and illustrates how even reactive species seen by DRIFTS may be unimportant in the overall reaction scheme.

Introduction

Ceria-supported noble metal-based materials are currently receiving much interest as promising oxidation-resistant and durable low-temperature water–gas shift (WGS; CO + H2O → CO2 + H2) catalysts [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. The role of formates as potential reaction intermediates has been much discussed, often as an alternative route to a redox mechanism [13], [14], [15], [16], [17], [18], [19], [20]. Burch recently reviewed the field of WGS over Au-based catalysts and proposed a unified WGS mechanism in which the redox route would derive from other routes at the higher temperatures, at which adsorbate surface coverage becomes low [21]. An important feature of this model is the recognition that the dominant mechanism will be a function of the choice of catalyst and the experimental conditions.

Following earlier work by Shido and Iwasawa [3], [22], Jacobs et al. [23], [24], [25], [26], [27] proposed that the formates generated by the reaction of CO with bridging OH groups associated with the Ce3+ defects were main reaction intermediates. The formates were thought to decompose to CO2(g) via surface carbonate species. Kinetic isotope effect and isotopic tracer studies suggested that the rate-limiting step of the forward formate decomposition was the C–H bond rupture, aided by the noble metal [25]. Behm et al. [28] carried out a quantitative analysis of the rate of formate surface decomposition over a Au/CeO2 catalyst and proposed that formates could account for about half of the CO2 produced. This calculation was based on the nonsteady-state measurement of the rate constant of formate decomposition (i.e., during desorption under 2% H2O in N2) and an estimate of the formate surface coverage determined via TPD under N2 of a catalyst pretreated under WGS conditions. Meunier et al. [29] recently proposed that formates can potentially be reaction intermediates over Pt/CeO2 above 473 K, whereas the formate species seen by DRIFTS were merely reaction spectators below this temperature. It is important to stress that in none of the aforementioned studies, including the one carried out by some of us [29], were the formate decomposition rate constant and surface coverage determined simultaneously at the chemical steady state under WGS reaction conditions.

Meunier et al. showed that the reactivity of surface species over Pt/CeO2 can dramatically depend on the experimental procedure used [30]. Based on DRIFTS analyses combined with the utilization of isotopic tracers, steady-state experiments showed that formates were less reactive than carbonyl and carbonate species under steady-state conditions, whereas the reverse trend was observed during the desorption-type nonsteady-state experiments carried out in an inert purge gas. The operando DRIFTS–SSITKA method used in the present study uses a single catalytic bed, which allows DRIFTS characterization of the surface of the very same catalyst that is responsible for the catalytic activity measured at the cell exit by gas chromatography or mass spectrometry [8]. This methodology is a powerful tool for an in-depth investigation of catalysts under reaction conditions, similar to the method developed earlier for transmission FTIR by Chuang et al. [31], [32].

The present paper deals with reactivity in terms of isotopic exchange of the surface species formed over an Au/Ce(La)O2 catalyst under forward WGS conditions (i.e., 2% CO + 7% H2O). The main objective was to investigate the reactivity of the surface formates species observed under reaction conditions. A fully quantitative analysis was carried out to determine both the rate constant of formate decomposition and the surface coverage of formate under steady-state reaction conditions. This methodology allowed us to compare the specific rate of CO2 formation to that of formate decomposition and to conclude that the formates detected by DRIFTS are not important reaction intermediates in CO2 formation.

Section snippets

Experimental

The catalyst used in this study was a 0.6 at% Au + 7.3 at% La/CeO2 (abbreviated 0.6AuCL) prepared by NaCN leaching of a higher Au-loading parent material [7]. The bulk composition was determined by ICP. The concentration of La near the surface was determined by XPS to be 19.1 at%, much higher than the bulk content of 7.3 at% La, due to La enrichment of the surface during the calcination step at 673 K. The BET specific surface area was 161 m2 g−1. The purity of the gases used (H2, Kr, CO, and

Sample activity and DRIFTS reactor evaluation

The WGS activity of the 0.6AuCL was measured between 398 and 473 K in our modified DRIFTS cell (in Belfast) and compared with that evaluated in a traditional quartz plug-flow reactor (in Medford) using the same feed, that is, 2% CO + 7% H2O in Ar (Fig. 1). Only the rates of CO2 formation for which the CO conversion was <15% (i.e., essentially differential conditions) are reported in Fig. 1. A perfect agreement between the rates measured over the two different reactors was observed. The apparent

Ceria oxidation state and nature of the surface species

The combination of the in situ DRIFTS and the SSITKA technique facilitated the assignment of the bands observed under WGS conditions over the 0.6% Au/Ce(La)O2 catalyst. In particular, the band observed here at 2132 cm−1 (located at 2125 cm−1 before baseline correction) appears to be solely due to the normally forbidden electronic transition of reduced ceria [39] and not to any CO(ads), because no band shift was observed on CO(g) isotopic exchange (Fig. 3). Note that a band at ca. 2130–2140 cm−1

Conclusion

The following conclusions can be drawn from the results of our in situ DRIFTS–SSITKA study of the WGS reaction over a 0.6% Au/Ce(La)O2 catalyst between 428 and 493 K:

  • 1.

    The importance of fully quantifying not only the rate of exchange, but also the absolute amounts of surface species has been established, showing that otherwise very misleading conclusions about possible reaction intermediates can be drawn.

  • 2.

    The specific rate of CO2 formation was ca. 60 times higher than the rate of formate

Acknowledgements

This work was partly supported by the EPSRC, under the CARMAC project and the European Social Funds (D.R.). W.D. and M.F.-S. acknowledge the support of this work by the U.S. National Science Foundation, NIRT Grant #0304515; and by the U.S. Department of Energy, Basic Energy Sciences, Hydrogen Fuel Initiative Grant #DE-FG02-05ER15730.

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