Surface spectroscopy on UHV-grown and technological Ni–ZrO2 reforming catalysts: from UHV to operando conditions

Abstract

Ni nanoparticles supported on ZrO2 are a prototypical system for reforming catalysis converting methane to synthesis gas. Herein, we examine this catalyst on a fundamental level using a 2-fold approach employing industrial-grade catalysts as well as surface science based model catalysts. In both cases we examine the atomic (HRTEM/XRD/LEED) and electronic (XPS) structure, as well as the adsorption properties (FTIR/PM-IRAS), with emphasis on in situ/operando studies under atmospheric pressure conditions. For technological Ni–ZrO2 the rather large Ni nanoparticles (about 20 nm diameter) were evenly distributed over the monoclinic zirconia support. In situ FTIR spectroscopy and ex situ XRD revealed that even upon H2 exposure at 673 K no full reduction of the nickel surface was achieved. CO adsorbed reversibly on metallic and oxidic Ni sites but no CO dissociation was observed at room temperature, most likely because the Ni particle edges/steps comprised Ni oxide. CO desorption temperatures were in line with single crystal data, due to the large size of the nanoparticles. During methane dry reforming at 873 K carbon species were deposited on the Ni surface within the first 3 h but the CH4 and CO2 conversion hardly changed even during 24 h. Post reaction TEM and TPO suggest the formation of graphitic and whisker-type carbon that do not significantly block the Ni surface but rather physically block the tube reactor. Reverse water gas shift decreased the H2/CO ratio. Operando studies of methane steam reforming, simultaneously recording FTIR and MS data, detected activated CH4 (CH3 and CH2), activated water (OH), as well as different bidentate (bi)carbonate species, with the latter being involved in the water gas shift side reaction. Surface science Ni–ZrO2 model catalysts were prepared by first growing an ultrathin “trilayer” (O–Zr–O) ZrO2 support on an Pd3Zr alloy substrate, and subsequently depositing Ni, with the process being monitored by XPS and LEED. Apart from the trilayer oxide, there is a small fraction of ZrO2 clusters with more bulk-like properties. When CO was adsorbed on the (fully metallic) Ni particles at pressures up to 100 mbar, both PM-IRAS and XPS indicated CO dissociation around room temperature and blocking of the Ni surface by carbon (note that on the partially oxidized technological Ni particles, CO dissociation was absent). The Ni nanoparticles were stable up to 550 K but annealing to higher temperatures induced Ni migration through the ultrathin ZrO2 support into the Pd3Zr alloy. Both approaches have their benefits and limitations but enable us to address specific questions on a molecular level.