Article pubs.acs.org/JPCC
Reducible Support Effects in the Gas Phase Hydrogenation of p‑Chloronitrobenzene over Gold Xiaodong Wang,† Noémie Perret,† Juan J. Delgado,‡ Ginesa Blanco,‡ Xiaowei Chen,‡ Carol M. Olmos,‡ Serafin Bernal,‡ and Mark A. Keane*,† †
Chemical Engineering, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, Scotland Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Facultad de Ciencias, Universidad de Cádiz Campus Rio San Pedro, 11510-Puerto Real, Cádiz, Spain
‡
ABSTRACT: The use of nonreducible (Al2O3) and reducible (Ce0.62Zr0.38O2, CZ) carriers to support nanoscale Au has been studied in gas phase p-chloronitrobenzene hydrogenation. Reaction over Au/Al2O3 generated p-chloroaniline as the sole product, whereas Au/CZ catalyzed nitro-group reduction and dechlorination to aniline. A parallel/consecutive kinetic model has been applied to quantify selectivity for Au/CZ. Catalyst characterization has included temperature programmed reduction (TPR)/desorption (TPD), XPS, HAADF-STEM, CO adsorption-FTIR, and oxygen storage capacity (OSC) measurements. The incorporation of Au with CZ promoted reduction of the support with the generation of surface hydrogen and oxygen vacancies, where the latter was facilitated at higher reduction temperature (from 393 to 973 K). Strong Au−CZ interactions enhanced Au dispersion with a narrow size distribution (mean = 1.8−1.9 nm) and influenced adsorptive and catalytic properties. Sintering of Au (from 5.7 to 8.8 nm mean) on Al2O3 was observed with increasing reduction temperature (473−673 K). A higher H2 content in the reacting gas elevated hydrogenation (action of supported Au), whereas dechlorination (action of oxygen vacancies) over Au/CZ was favored under H2 lean conditions. The contribution of spillover hydrogen to increase selective hydrogenation rate is demonstrated. A temporal irreversible loss of activity is established and linked to Cl poisoning of oxygen vacancies.
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INTRODUCTION 1,2
Zirconia (ZrO2) is widely used as a heterogeneous catalyst support due to its adjustable (via variation in synthesis, precursor, and use of dopants) structural and chemical characteristics.25 Gold supported on ZrO2 has been applied in low temperature WGS,26 CO oxidation,27 and the selective hydrogenation of 1,3-butadiene, acrolein, and crotonaldehyde.28,29 Of particular relevance to this study, He et al. achieved high selectivities in the liquid phase hydrogenation of p- and o-chloronitrobenzene and 2,5-dichloronitrobenzene over Au/ZrO2, limiting the degree of C−Cl bond scission.30 Ceria supported Au has been the subject of a significant corpus of published work,31−34 with again a particular focus on CO oxidation31,33 and WGS.32,34 Incorporation of Zr4+ into the CeO2 lattice increases the oxygen storage capacity (OSC) of the solid solution and improves textural properties (resistance to sintering).35,36 The coordination number of the smaller Zr ion is lowered in the mixed oxide, increasing oxygen mobility, which can contribute to the creation of oxygen vacancies.37 These vacancies serve to promote CO oxidation,38 WGS,34 and hydrogenation reactions.39 In previous work,40−43 we estab-
3
Following the landmark work of Haruta and Hutchings establishing a catalytic response for nanoscale Au, the development of Au catalysts is attracting appreciable research. While bulk Au exhibits negligible catalytic properties,4 Au dispersed on metal oxide supports has been used to promote a range of processes of commercial and environmental remediation importance,5 notably CO oxidation,6 NOx treatment,7 and the water gas shift (WGS) reaction.8 The use of Au in hydrogenation reactions has been studied to a lesser extent,9 and the available literature has been compiled and assessed in reviews by Claus,10 Hashmi,11 and McEwan et al.12 Work to date has largely focused on the conversion of carbon oxides, alkenes, alkynes, and α,β-unsaturated aldehydes and ketones.10,13 Catalyst performance has shown a dependence on synthesis,14 Au particle size/shape,15 and support characteristics.16 Taking the latter, a range of catalytic responses has been observed for Au on different oxides.10,17−20 The support can affect Au morphology and dispersion via surface interactions21,22 where smaller (≤10 nm) Au particles exhibit electronic properties distinct from bulk Au.23 Moreover, the incorporation of Au is known to influence support reducibility, notably in the case of ceria (CeO2).24 © 2012 American Chemical Society
Received: September 21, 2012 Revised: December 11, 2012 Published: December 18, 2012 994
dx.doi.org/10.1021/jp3093836 | J. Phys. Chem. C 2013, 117, 994−1005
The Journal of Physical Chemistry C
Article
oxidation at 523 K, Au/CZ was reduced for 1 h in flowing 5% v/v H2/Ar or pure H2 at a series of successive isothermal steps over the temperature range 393−973 K. The OSC values were determined from the weight loss recorded at each of the isothermal steps. Temperature programmed reduction (TPR) of Au/Al2O3 and temperature programmed desorption (TPD) from Au/Al2O3, Au/Al2O3 + Al2O3, Au/CZ, and Au/CZ + CZ were recorded using the commercial CHEMBET 3000 (Quantachrome) unit with data acquisition using the TPR Win software. Fourier transform infrared (FTIR) spectroscopy studies were performed on a Bruker (Vertex 70) spectrometer using a DTGS detector, taking an average of 100 scans. Transmission FTIR spectra of adsorbed CO (at PCO = 40 Torr and 298 K) were recorded using a quartz cell (with CaF2 windows) attached to a metallic high vacuum manifold equipped with a turbo molecular pump (residual pressure
