Production of hydrogen from oxidative steam reforming of methanolI. Preparation and characterization of Cu/ZnO/Al2O3 catalysts from a hydrotalcite-like LDH precursor

Abstract

Catalysts for oxidative methanol reforming were prepared by thermal and H 2 -reduction treatments of a precursor consisting mainly of a hydrotalcite-like Cu–Zn–Al hydroxycarbonate and a Zn-rich paratacamite. The precursor was obtained by homogeneous precipitation of metal cations with a properly modified urea method. XRPD and quantitative Rietveld analysis of the precursor material revealed the presence of hydrotalcite and paratacamite phases in a weight ratio 3:1. Treatment in situ of the precursor produced Cu/ZnO/Al 2 O 3 (Cu = 18%, Zn = 33%, Al = 49% mol) catalysts. Chemical properties of the precursor and of the catalysts were studied by TPR, TPO, NH 3 -TPD, N 2 O chemisorption , FTIR, and UV–vis techniques. Physical characterization was carried out by the SEM, EDS, XRPD, TG/DTA, and N 2 adsorption techniques. The influence of heating rate of the precursor was investigated. Heat-treated samples contained CuO, ZnO, and amorphous Al 2 O 3 , and probably ZnAl 2 O 4 , as suggested by XRPD and FTIR measurements. The crystallinity of oxide phases was higher for the sample treated with a lower heating rate that also showed lower surface area and lower Cu dispersion. The presence of Cu 2+ in octahedral sites of alumina was suggested by UV–vis spectra. CO adsorption gave evidence of easy reduction of Cu(II) to Cu(I) and Cu(0) and the formation of stable complexes with Cu(I). NH 3 -TPD and FTIR measurements showed the presence of surface acid sites of the Lewis type with wide strength distributions, mainly due to Zn 2+ and Al 3+ cations. Cu(II) was able to oxidize NH 3 , while Cu(0) activated NH 3 decomposition. TPR and TPO measurements indicated that Cu species are easily reduced and reoxidized and Cu(I) species are intermediate for both processes. The redox properties appeared to be influenced by the rate of the previous heat treatment.