XPS/SIMS studies of the promoter action in methanol synthesis over silica-supported Pd catalysts C. Sellmer and R. Prins Laboratory for Technical Chemistry, Federal Institute of Technology, CH-8092 Zurich, Switzerland N. Kruse  Chimie-Physique des Surfaces et Catalyse H et erog ene, Universit e Libre de Bruxelles, Campus Plaine, CP 243, B-1050 Bruxelles, Belgium Received 1 May 1997; accepted 28 June 1997 Pd-based catalysts prepared from ultra-pure SiO 2 and doped with 5000 ppm Li, Ca and La were subjected to CO hydrogenation (H 2 /CO  2, p  9 bar, T  553 K) in a flow reactor directly attached to an ultra-high vacuum chamber with facilities for X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The catalysts exhibited high selectivity (usually above 99%) for methanol synthesis and their activities were found to increase in the order Li < Ca < La. Subsequent analysis by SIMS revealed the presence of surface formate and methoxy on the silica support. No such species were found to be bound to the Pd particle surface. These conclusions are based on the detection of COO(H) and SiOCH 3 ions while respective Pd-based species were absent from the mass spectra. This indicates a mechanism to be in operation in which hydrogen is activated on Pd metal and subse- quently spilled over to the support in order to react with formate to methanol. XPS studies of SiO 2 samples containing 10 wt% of Ca, in the absence of Pd metal, demonstrated Si 2p and O 1s line shifts to lower binding energies as compared to pure SiO 2 . The spectra bear strong similarities with those of wollastonite, ``CaSiO 3 ''. A comparable behavior was found for Mg-doped SiO 2 but not for K- or Cs-doped SiO 2 , which show low activity in methanol formation. Keywords: promoted Pd/SiO 2 catalysts, CO hydrogenation, methanol synthesis, promoter^support interaction, secondary ion mass spectrometry, X-ray photoelectron spectroscopy 1. Introduction Conversion of synthesis gas to methanol is one of the large-scale industrial processes of heterogeneous cat- alysis. Catalysts applied in the low-pressure ICI process are based on Cu/ZnO/Al 2 O 3 . Also formulations with Pt group metals have been found active in oxygenate pro- duction. For example, promoted Pd/SiO 2 catalysts are highly selective in methanol formation [1^9], and repre- sent interesting model systems, in particular for studying the mechanism and microkinetics of this reaction. Initial studies of CO hydrogenation over supported Pd catalysts brought forth contradictory results: while Poutsma et al. [1] reported high selectivities towards methanol, Fajula et al. [7] found mainly methane forma- tion. It seems clear by now that the main reason for this contradiction was the use of silica supports (Davison grade 57 versus grade 01) of different purity. In fact, Nonneman et al. [8], in a study with Rh catalysts, demonstrated Na and Fe contaminations in the commer- cial silica support to be responsible for the considerable activity towards ethanol. On the other hand, using pure SiO 2 at otherwise identical reaction conditions caused the disappearance of ethanol and the formation of hydrocarbons along with some actetaldehyde in the product spectrum. A recent study by Gotti and Prins [9] worked out the effect of Ca doping on Pd catalysts sup- ported by pure silica. They found the selectivity to increase to > 99% methanol after doping with 1000 ppm. This increase was associated with a gain in activity by more than a factor of ten. Recent studies with La- doped catalysts led to further increases of the activity. On the other hand, methane formation appeared to be strongly suppressed meaning that CO dissociation is lar- gely impeded on these promoted catalysts. Furthermore, as the authors found a clear correlation between the amount of Ca, the noble metal surface and the methanol activity, the hypothesis may be advanced that the forma- tion of methanol occurs through formate intermediates on the support surface or in the interface between the support and the Pd particles. This scenario considers the Pd metal as the supplier for dissociative hydrogen which moves in a spillover process onto the support in order to subsequently react with formate to methanol. There are a number of open questions as to the details of such a mechanism. For details see, e.g., ref. [9]. The present paper presents SIMS results (secondary ion mass spectrometry) performed in order to identify the rele- vant surface species occurring during methanol syn- thesis and to locate them either on the Pd metal or the support surface. Emphasis has been laid at demonstrat- ing by means of XPS (X-ray photoelectron spectros- Catalysis Letters 47 (1997) 83^89 83 * To whom correspondence should be addressed. Ä J.C. Baltzer AG, Science Publishers