Beware of beam damage under reaction conditions: X-ray induced photochemical reduction of supported VOx catalysts during in situ XAS experiments

In situ X-ray absorption spectroscopy (XAS) is a powerful technique for the investigation of heterogeneous catalysts and electrocatalysts. The obtained XAS spectra are usually interpreted from the point of view of the investigated chemical processes, thereby sometimes omitting the fact that intense X-ray irradiation may induce additional transformations in metal speciation and, thus, in the corresponding XAS spectra. In this work, we report on X-ray induced photochemical reduction of vanadium in supported vanadia (VOx) catalysts under reaction conditions, detected at a synchrotron beamline. While this process was not observed in an inert atmosphere and in the presence of water vapor, it occurred at room temperature in the presence of a reducing agent (ethanol or hydrogen) alone or mixed with oxygen. Temperature programmed experiments have shown that X-ray induced reduction of VOx species appeared very clear at 30–100 °C but was not detected at higher temperatures, where the thermocatalytic ethanol oxidative hydrogenation (ODH) takes place. Similar to other studies on X-ray induced effects, we suggest approaches, which can help to mitigate vanadium photoreduction, including defocusing of the X-ray beam and attenuation of the X-ray beam intensity by filters. To recognize beam damage under in situ/operando conditions, we suggest performing X-ray beam switching (on and off) tests at different beam intensities under in situ conditions.


In situ DR-Vis measurements
The in situ diffuse reflectance UV-Visible (DR UV-Vis) spectroscopy study was performed on an Agilent Carry 4000 spectrometer equipped with a Praying Mantis mirror unit (Harrick). The commercial Harrick cell was equipped with a homemade CaF2 window (d = 25 mm, thickness 2 mm; Crystran) in place of the commercial dome. The catalyst sample (ca. 25 mg) was placed in the sample cup. The pre-treatment and temperature-programmed experiments were performed following the protocol described for the V K-edge XAS experiments. Spectra were recorded only in 350 -800 nm interval in order to exclude the signal saturation at 250 -350 nm ( Figure S6) and to enhance the signal intensity in the d-d transition region.

Experiments with aluminum filters
To reduce the intensity of the incident beam, aluminum filters with different thickness were utilized. On the Figure S1 b, the thickness of Al-foil which provides e times diminished beam versus incident energy is shown. On Figure S1 c, the thickness of used filters and corresponding intensities are marked with "x".

V pre-edge analysis
For more accurate calculation of the pre-edge area and pre-edge center of mass, cumulative distribution function (CDF, S1) was used as a baseline to correct the edge rising ( Figure S2).
The edge jump was fitted with CDF with the use of least square method (implemented on the base of Python). The resulting peak was integrated. In the case if an edge arise contained an additional shoulder (B-peak in the Figure S3), the resulting after subtraction peaks were fitted with multiple pseudo-Voight functions to identify and subtract the contribution of the shoulder ( Figure S3 c). The detailed fits of all used in this work references could be found in [1].
Where x is the energy scale, μ is a fitted parameter, which defines the x position of fCDF and σ is a fitted parameter, which defines the slope of fCDF.

Quantitate analysis of the products by IR spectrometry
The spectra of ethanol, acetaldehyde and other possible products are shown in Figure S3.      Table S1). For more details refer to [1]. At 160 o C, the thermocatalytic processes (oxidation and reduction of vanadium involved in catalytic transformation of ethanol into acetaldehyde) become significantly faster than the photocatalytic reduction of V 5+ . For this reason, the observed trends are rather independent on the X-ray brilliance.

Supplementary Figures and Tables
. Figure S9.  Figure 12).   Percentage numbers indicate the intensity of the beam relative to the beam without filter (intensity of transmitted beam). Figure

DR UV-Vis experiments
Electronic spectroscopy is also sensitive to oxidation state changes in this type of catalysts [2].
Therefore, we performed similar experiments using diffuse reflectance UV-Vis spectroscopy (DR UV-Vis; Figure S13). First, we performed ethanol TPE (1.6 vol% EtOH in He) and ethanol-oxygen TPE (1.6 vol% EtOH, 6.4 vol% O2 in He) experiments in the temperature range 50-400 o C as shown in Figure S13 a and b, respectively. The spectra recorded in ethanol feed demonstrated a significant increase in the spectral range between 400 and 800 nm centered at around 558 nm that is characteristic of d-d transitions of V 4+ /V 3+ ions [3]. In the ethanol-oxygen feed, the change was much less evident.
The in situ DR UV-Vis measurement in ethanol feed at 50 °C is shown in Figure S13

Effect of the support
The bilayered 5 wt% V2O5/15 wt% TiO2/SiO2 catalyst, as was mentioned in the experimental part, serves as a model of VOx/TiO2 catalyst, which demonstrates outstanding catalytic activity in a number of chemical reactions. For instance, the rate of acetaldehyde production over titania-supported vanadia is ca. 2 orders of magnitude higher than over silica-supported vanadia [1,2]. Using density functional theory (DFT), it was shown that the ability of titania to form oxygen vacancies in close proximity to surface vanadia species may facilitate vanadium reduction during catalytic reactions and, therefore, increase its activity [4][5][6]. Besides, titania itself is a well-known photocatalyst and prone to generate electron-hole pairs upon light exposure [7]. In this regard, we had to test, whether titania-support is facilitating X-ray beaminduced vanadium reduction. For this, we performed ethanol TPE over titania-free 8 wt% V2O5/SiO2 catalyst. The activity of this catalyst in ethanol ODH is significantly lower, (Figure S14 b); the acetaldehyde production curve is shifted by ca. 90 °C in comparison to one of 5 wt% V2O5/ 15 wt% TiO2/ SiO2 catalyst. In Figure S14 a, we plotted the pre-edge height of V K-edge spectra measured during ethanol TPE over 5 wt% V2O5/ 15 wt% TiO2/ SiO2 and 8 wt% V2O5/SiO2 catalysts. These pre-edge height profiles could be conventionally divided into two parts, low temperature (30-110 °C) and high temperature (110-400 °C). Whereas the vanadium reduction in the high-temperature part is related to ethanol ODH and depends on the support (Figure S14 b), the reduction in the low-temperature part is related to X-ray induced reduction and seems to be very similar on different catalysts. It suggests that photochemical process is not strongly influenced by the presence of titania. Additionally, we performed the ethanol-feeding experiment over VOx/SiO2 catalyst at 30 °C, the pre-edge intensity and the edge position profiles of both catalysts are shown in Figure S14 c and d. Similarly to bilayered VOx/TiO2/SiO2 catalyst, titania-free VOx/SiO2 catalyst demonstrates strong vanadium reduction at 30 °C in the presence of ethanol. Moreover, the rate of vanadium reduction is faster on VOx/SiO2 catalyst, which could be explained by partial X-ray absorption by TiOx support in VOx/TiOx/SiO2 catalyst. The ability of the catalyst to be reduced by X-ray beam does not correlate with the chemical activity of the catalyst in ethanol oxidation and is not facilitated by photoactive titania-support.  Figure S12). The beam size is 400 x 200 μm 2 .